Symposium EC1 : Redox Activity on the Molecular Level—Fundamental Studies and Applications

In the search for high density electrochemical energy storage, non-aqueous rechargeable metal-O₂ batteries are very attractive owing to their reliance on molecular oxygen which is electrochemically reduced to form oxides on discharge, and in turn releases oxygen on charge. Much work has focused on aprotic Li–O₂ cells, although the Na-O₂ system is also of interest owing to its more reversible chemistry. Although solution growth of large crystallites of A-O₂ (either Li₂O₂ or NaO₂) on discharge increases cell capacity, it makes charging of the cells challenging. There is a need to oxidize large electrically insulating alkali oxide particles in the pores of the cathode distant from the electrode surface. Soluble oxidation mediators in the electrolyte – either classic redox-active molecules or phase transfer catalysts - can address this problem, and also greatly facilitate redox chemistry on discharge. This presentation will focus on the complex chemistry of these novel systems, highlighting our very recent studies that elucidate the mechanism of mediation in both Li-O₂ and Na-O₂ batteries, and also covering the latest developments from other labs. Characterization techniques ranging from electron spin resonance spectroscopy spectroscopy combined with electron microscopy; surface spectroscopy; and operando electrochemical mass spectrometry will be presented, which have been utilized to investigate the electrochemistry. These methods have identified the HO₂ radical in solution and its reactivity, uncovered new stable molecular mediators, and resulted in a deeper understanding of the critical parameters for positive electrodes in aprotic A-O₂ batteries. Recent developments in molten salt electrolytes will also be presented.

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the capacity of Li-ion batteries. We must explore alternatives to Li-ion if we are to have any hope of meeting the long-term needs for energy storage. One such alternative is the Li-air (O₂) battery; its theoretical specific energy exceeds that of Li-ion, but many hurdles face its realization.
Potentially disruptive technologies, such as Li-O₂, almost by definition lack understanding of their underpinning fundamental processes. This was certainly true of the reactions at the O₂ (positive) electrode. Work to understand the fundamentals has highlighted the importance of carrying out the reaction O₂ + 2e^- + 2Li⁺ = Li₂O₂ in solution instead of on the electrode surface. Although solution phase discharge can be promoted by electrolyte solutions that dissolve the intermediate in the discharge reaction, LiO₂, often such solutions are more susceptible to attack by the reduced oxygen species. Here we discuss the use of molecular mediators on discharge and charge, the former changes the pathway of the reduction reaction avoiding the reactive LiO₂ intermediate. Using the duel mediator approach decouples the storage reaction (formation and decomposition of Li₂O₂) from the surface electrochemistry and permits the reaction in relatively stable ether based electrolytes. It enables high capacities, relatively high rates and sustained cycling.

Ionic liquids can be made intrinsically electroactive by modifying the structure of its ionic constituents with various redox moieties. These phases, which can be liquid around room temperature, provide new opportunities to study and exploit electron transfer reactions as they can behave differently from conventional solute-in-solvent systems. Our research aims at studying the structure-properties relationships of electroactive ionic liquids and orient their design to develop novel redox systems in energy storage devices.
In this contribution, we will demonstrate several design and synthetic strategies to the modification of imidazolium cations and bis(trifluoromethane)sulfonimide anions with ferrocenyl moieties. The electron transfer kinetics and transport properties of the modified ions were studied in diluted solutions of the ionic liquids. Lower heterogeneous rate constants were attributed to factors such as size, ion asymmetry and a higher internal reorganization energy imparted by the addition of a large charged substituent on the ferrocenyl unit. This latter effect is more important in the case of the ferrocenylsulfonyl-(trifluoromethylsulfonyl)-imide (FcNTf) due to the permanent substituent negative charge which provides a zwitterion upon oxidation of the ferrocenyl. A dramatic change in electrochemical behaviour of FcNTf-based ionic liquids is observed for highly concentrated solutions and for the pure liquid phase. Under such conditions, the oxidized zwitterion deposits on the electrode surface as a film which can be removed by reduction. We will reveal that this property, combined with other properties inherent to the nature of ionic liquids, make electroactive ionic liquids relevant for a use as electrolytes in several redox devices. Examples will be provided on energy storage devices like lithium-ion batteries and supercapacitors as well as for self-bleaching electrochromic devices.

Redox-active electrolytes and additives offer a large selection of available species serving for supercapacitor energy enhancement. The first group refers to typical organic compounds like hydroquinone with isomeric forms. There are also several reports on methylene blue and indigo carmine compounds tested on multi-walled carbon nanotubes. Nevertheless, these organic admixtures have limited solubility in water, and/or their bulkiness restricts adsorption at the interface. Apart from organic molecules, another group of electroactive species includes halide ions. Initially, the iodide/iodine and bromine/bromide redox couples were introduced to supercapacitors. To the best of our knowledge, this is the first report on the employment of alkali metal and ammonium thiocyanates (SCN^-) aqueous solutions serving as an electrolyte for supercapacitors. The investigation presents a comprehensive study including the effect of salt concentration, the impact of counter anion type (K⁺, Na⁺, Li⁺ and NH₄⁺), the influence of applied current collectors as well as the effectiveness of asymmetric configuration. Our research proves that symmetric AC/AC system can easily operate up to 1.6 V with high capacitance and great cyclability. The thiocyanate redox shuttle with formal redox potential at +0.77 V vs. SHE preserves the positive electrode against oxidation and contrarily to iodides or bromides, oxidized SCN^- specimen (e.g. polythiocyanogen) in a polymeric form does not evaporate from the system (unlike I₂ or Br₂).
A comparative study on the effect of salt concentration and dependence on the type of current collectors (gold, titanium, and stainless steel) was performed using the KSCN aqueous solutions at various concentrations (1-7 mol/L). It has been demonstrated that the highest concentration of SCN^- specimen (7 mol/L) preserved the best performance whatever the current collector used. However, on Ti and stainless steel current collectors, the activity of SCN^- seems to be slightly reduced and more ‘capacitive’ profile has been observed.
More in-depth studies demonstrated that the system based on 7 mol/L KSCN aqueous solution preserved high capacitance values even at 10 A/g current load and retained high energy density (~30 Wh/kg) up to 10 kW/kg of power rate. Furthermore, we observed that kind of current collector strongly influences the cycle life of the system. Although the gold current collectors provided the highest capacitance during first 1000 cycles, their cyclability is limited to 2 000 cycles. For stainless steel-based systems, the capacitance retention reached 10 000 cycles at 1.6V voltage range.

Redox active polymers (RAPs) are emerging as attractive charge storage medium for applications in novel size-exclusion flow batteries.^[1] In these devices, supporting electrolyte crosses freely through a porous separator while solution-phase RAPs store charge reversibly and remain in their compartment. Detailed studies linking polymer structure and their charging mechanisms in solution to electrode reactivity are still incipient. Here, we discuss the rate limiting steps obtained through a battery of electroanalytical methods, as well as the applicability and translation of charge transfer principles to RAPs.
We will discuss the application of hydrodynamic techniques and scanning electrochemical microscopy to the characterization of viologen, ferrocene and nitrostyrene RAPs in solution. We will focus on polymers in the range from 20 kDa to 318 kDa, and sizes starting at 4 nm and larger. From these studies, in conjunction with simulation analysis, we have found that the reactivity of viologen and ferrocene-based RAPs follows a “CE” mechanism in which several chemical equilibria preceding or concurrent with electron transfer dominate the observed electrochemical kinetics. ^[2] In nitrostyrene, these mechanistic traits are influenced by electrolyte identity and composition. ^[3] Finally, by tuning the synthetic scheme of RAPs to specifically place redox pendants within close proximity on the polymer backbone, we have begun to exploit predictions from Marcus-Hush theory to enhance the reactivity of RAPs. Here, electron self-exchange between neighboring pendants are strongly modulated by small structural differences which critically impact the solution-phase reversibility. Developing new probes and tools for exploring charge transfer and transport in RAPs allows us to swiftly develop working principles for emerging non-aqueous flow batteries.

[1] Nagarjuna, G.; Hui, J.; Cheng, K.; Lichtenstein, T.; Shen, M.; Moore, J. S.; Rodriguez-Lopez, J. J. Am. Chem. Soc. (2014), 136, 46, 16309-16316.
[2] Burgess, M.; Hernández-Burgos, K.; Simpson, B. H.; Lichtenstein, T.; Avetian, S.; Nagarjuna, G.; Cheng, K. J.; Moore, J. S.; Rodríguez-López, J. J. Electrochem. Soc. (2016), 163, 4, H3006-H3013.
[3] Burgess, M.; Hernández-Burgos, K.; Cheng, K. J.; Moore, J. S.; Rodríguez-López, J. J. Analyst (2016), 141, 3842-3850.

Li-S batteries have drawn strong interest as the battery technology likely to succeed the ubiquitous Li-ion batteries. Current development is however hampered by some tenacious issues of the sulfur cathode; necessitating the use of excess conducting additive or operations in a smaller voltage window (where the active sulfur species are soluble). The battery energy density is significantly reduced by these compromises. We have previously demonstrated a viable flow battery alternative based on the principle of redox-targeting by using two redox mediators in tandem. This study presents a more advanced design which uses only one redox mediator; and the chemical and electrochemical charging and discharging of the sulfur cathode to enable the flow battery to operate close to the full potential of the Li-S chemistry without a conducting additive or a soluble cathode. This new solution strategy is generic and may be applied to other high energy density flow batteries.

Transition from fossil fuels to renewable energy sources requires energy storage. This is due to the variable output of carbon-neutral resources such as wind, solar and tidal power. Non-aqueous redox-flow batteries (NRFB) are a promising technology to meet this need, possessing many of the benefits of their aqueous counterparts, a wider electrochemical window, and an improved range of operating temperature. Currently, NRFB are limited by poor stability of solution-phase, charge-carrying metal complexes. A major mechanism of decomposition of these redox electrolytes is substitution of ligands that have weak interactions with substitution-labile metal ions. Presented herein is an approach to mitigate these thermodynamic and kinetic challenges with a naturally occurring compound that is produced biologically. Mushrooms of the genus Amanita synthesize a molecule known as Amavadin, in which a vanadium ion is chelated by a pair of 2,2’-(hydroxyimino)dipropionic acid ligands yielding an eight-coordinate first-row transition metal complex. Millions of generations of evolution have optimized the stability of this molecule. As a result, under physiological conditions in which most vanadium compounds would decompose to form vanadyl species, ligand substitution is suppressed.
We will present the electrochemical properties of redox molecules based on Amavadin as NRFB electrolytes. This will include data on electrochemical reversibility and stability to deep redox-cycling under various conditions. In addition, we will present the results of computational investigations that have guided ligand-design efforts to optimize the properties of these compounds for application in energy storage devices without losing their extraordinary ability to bind vanadium ions. In this context we will discuss the fundamental aspects of coordination chemistry and electronic structure that underpin the remarkable stability exhibited by these molecules. We will discuss strategies for optimizing these complexes as redox electrolytes by altering the counter ions and through covalent modification of the ligands.

The integration of redox flow battery (RFB) technology into automobile application demands high energy density energy storage systems due to the limited space in vehicles. Aqueous based inorganic redox couples have been widely researched in RFBs, their low energy density, however, limits their application to mainly stationary applications. Recently, a type of highly concentrated catholyte made by the combining of plasticizing salt, LiTFSI, and organic active material, 4-MeO-2,2,6,6-tetra-methypiperidine 1-oxyl , has been reported and demonstrated a very promising energy density to as high as 200 Wh/L, when mixing with 17w.t % water. To further boost liquid electrolyte energy density, three parameters can be played with that are the discharge voltage (V), active material concentration (C), and valence number (n, electrons that allow transferring per redox species). Thus some other alternative active materials, such as 2-Azaadamantane-N-oxyl and its derivatives are examined by applying the same reported strategy.

Homogenous liquids were acquired for the all of the organic active materials tested in this work, which proves the universality of this method. The resulted catholyte solutions possess the nature of supercooled liquid and can be recognized as “good” solvated ionic liquid, as shown from their Walden plot. Cyclic voltammetry technique was performed on the solutions, and 2 electron transfer mechanism was observed in all the tested liquid. The performance of the solutions are further evaluated with Li-ion-conducting architecture batteries, and catholyte made by one of the active materials achieved to the energy density as high as 431.2 Wh/L calculated by multiplying its discharge capacity and average voltage. It is, to the best of our knowledge, the highest reported energy density in the world by far. Good charge-discharge cyclability up to 50 times was also demonstrated using this catholyte.

Non-aqueous Redox Flow Batteries (NRFBs) are emerging devices for electric grid storage applications. A major challenge in redox flow batteries is enhancing the ionic conductivity across the membrane while preventing the crossover of the anolyte and catholyte .¹ Recently, we reported that using redox active polymers (RAPs) in combination with size exclusion membranes, in contrast to ion selective membranes, provides a viable alternative that addresses these challenges and enables NRFBs.¹
The electron transfer kinetics of large polymer redox mediators with hundreds to thousands of redox active centers in solution are not well understood.^2-3 In addition, the existence of a preceding chemical step that mediates the redox chemistry of viologen- and ferrocene- based polymer particles in solution highlights the importance of 3-dimensional charge hopping on these polymer nanostructures.² In order to gain a better understanding of the electrochemical properties of polymer particles, we have evaluated their reactivity as single entities and well-ordered monolayers. Because of their discrete nature, polymer particles can be self-assembled into compact arrays on conductive substrates. Here, we report on the electrochemical evaluation of polymer monolayers through advanced analytical tools such as scanning electrochemical microscopy, bulk electrolysis, spectroelectrochemistry and rotating disk electrode voltammetry. We report on the redox properties, and the mechanism of charge transfer of these films. New methodologies based on micro- and nano-electrode measurements are introduced to calculate the propagation of charge and parameters related to charge transport. Our studies provide the required morphological control and electroanalytical tools for understanding charge transfer and mobility effects that are important for the performance of RAPs for better performing energy storage flow devices.

1. Nagarjuna, G.; Hui, J.; Cheng, K.; Lichtenstein, T.; Shen, M.; Moore, J. S.; Rodriguez-Lopez, J., J. Am. Chem. Soc. 2014, 136 (46), 16309-16316.
2. Burgess, M.; Hernández-Burgos, K.; Simpson, B. H.; Lichtenstein, T.; Avetian, S.; Nagarjuna, G.; Cheng, K. J.; Moore, J. S.; Rodríguez-López, J., J. Electrochem. Soc. 2016, 163 (4), H3006-H3013.
3. Burgess, M.; Hernandez-Burgos, K.; Cheng, K.J.; Moore, J.S.; Rodríguez-López, J., Analyst, 2016, 141, 3842-3850.

We describe an electrochemically mediated interaction between Li⁺ and a promising active material for nonaqueous redox flow batteries (RFBs), 1,2,3,4-tetrahydro-6,7-dimethoxy-1,1,4,4-tetramethylnaphthalene (TDT), and the impact of this structural interaction on material stability during voltammetric cycling. TDT could be an advantageous organic positive electrolyte material for nonaqueous RFBs due to its high oxidation potential, 4.21 V vs Li/Li⁺, and solubility of at least 1.0 M in select electrolytes. Although results from voltammetry suggest TDT displays Nernstian reversibility in many nonaqueous electrolyte solutions, bulk electrolysis reveals significant degradation in all electrolytes studied, the extent of which depends on the electrolyte solution composition. Results of subtractively normalized in situ Fourier transform infrared spectroscopy (SNIFTIRS) confirm that TDT undergoes reversible structural changes during cyclic voltammetry in propylene carbonate and 1,2-dimethoxyethane solutions containing Li+ electrolytes, but irreversible degradation occurs when tetrabutylammonium (TBA⁺) replaces Li⁺ as the electrolyte cation in these solutions. By combining the results from SNIFTIRS experiments with calculations from density functional theory, solution-phase active species structure and potential-dependent interactions can be determined. We find that Li⁺ coordinates to the Lewis basic methoxy groups of neutral TDT and, upon electrochemical oxidation, this complex dissociates into the radical cation TDT⁺ and Li⁺. The improved cycling stability in the presence of Li⁺ relative to TBA⁺ suggests that the structural interaction reported herein may be advantageous to the design of energy storage materials based on organic molecules.

Redox flow batteries (RFBs) offer several advantages over enclosed batteries in grid-scale applications such as decoupled power rating (reactor size) and energy capacity (tank size), high active-to-inactive materials ratio (especially at long durations), and improved safety characteristics.¹ Recent literature has focused on emerging aqueous organic RFBs (AORFBs) that employ abundant redox active organic species dissolved in inexpensive acidic electrolytes. Anthraquinone disulfonic acid (AQDS) is considered a benchmark compound for AORFBs^2–5 due to its good charge retention (days), low crossover rates (through Nafion), high solubility (>1 M), and low redox potential (~0.2 V vs RHE). Some studies report idealized electrochemical behavior of AQDS at low concentrations (1 mM),⁶ however, at higher concentrations AQDS deviates from ideal behavior and undergoes intramolecular reactions, which contribute to changes in storage capacity, redox potential, and solubility.

Here, we examine the complexities of AQDS in aqueous electrolytes utilizing cyclic voltammetry (CV), bulk electrolysis (BE), chemical titration, and nuclear magnetic resonance (NMR) spectroscopy. Depending on the AQDS concentration and electrolyte choice, the observed electrochemical behavior can vary indicating that different mechanistic pathways are accessed. To elucidate these reaction mechanisms, we synthesize various derivatives of AQDS and examine their electrochemistry, via CV and BE, as well as their chemical structures via NMR. Furthermore, we correlate the chemical structure of AQDS in solution with the observed electrochemistry to understand variations in reversible capacity and to provide guidelines for maximizing storage performance. The complex, solution-phase interactions experienced by AQDS suggest that other novel organic compounds may display similar, or more convoluted, phenomena. To this end, the suite of chemical and electrochemical methods presented here are portable and can be applied to other candidate materials for RFBs to better understand inconsistencies in storage capacity, solubility, and stability.

Acknowledgments
We thank the Joint Center for Energy Storage Research for financial support. This research was conducted with Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a.

References
1. Su, L., et al. Rechargeable Batteries (eds. Zhang, Z. & Zhang, S. S.) 673–712 (Springer International Publishing, 2015).
2. Huskinson, B. et al. Nature 505, 195–198 (2014).
3. Yang, B., et al. J. Electrochem. Soc. 161, A1371–A1380 (2014).
4. Lin, K. et al. Science 349, 1529–1532 (2015).
5. Yang, B. et al. J. Electrochem. Soc. 163, A1442–A1449 (2016).
6. Batchelor-McAuley, C. et al. J. Phys. Chem. B 114, 4094–4100 (2010).

Redox flow batteries have shown great prospects in grid-scale stationary energy storage applications. The decoupling of the energy and power allows independent scaling of tank volume and stack size, which offers tremendous design flexibility to meet different energy/power ratio requirements. Traditional flow batteries are based on inorganic redox materials but generally suffer limited solubility, low electrochemical reactivity and/or high cost. On the contrary, electrochemically active organic compounds are promising alternative material candidates because of their molecular diversity, structural tailorability, environmental benignity, and potentially low cost. There has merged a gradual shift to organics-based redox flow chemistries to push the boundaries of grid storage technology development.^1,2
Here we report a series of organic redox flow chemistries in nonaqueous electrolytes.³ Promising redox material candidates have been identified with desirable redox potentials, high solubilities, and good electrochemical properties. Flow cells of these systems resulted in remarkable cell efficiencies, high rate performance, and great cycling stability. Rational molecular engineering has been demonstrated as a general strategy to improve the battery performance and durability. Moreover, we have developed FTIR-based diagnostics of the state of charge (SOC) of a nonaqueous symmetric organic flow battery.⁴
References
(1) K. Gong, Q. Fang, S. Gu, S. Li, and Y. Yan, Energ. Environ. Sci. 2015, DOI: 10.1039/C5EE02341F.
(2) W. Wang and V. Sprenkle, Nat. Chem. 2016, 8 (3), 204.
(3) X. Wei, W. Xu, J. Huang, L. Zhang, E. Walter, C. Lawrence, M. Vijayakumar, W. A. Henderson, T. Liu, L. Cosimbescu, B. Li, V. Sprenkle, and W. Wang, Angew. Chem. Int. Ed. 2015, 54, 8684.
(4) W. Duan, R. S. Vemuri, J. D. Milshtein, S. Laramie, R. D. Dmello, J. Huang, L. Zhang, D. Hu, M. Vijayakumar, W. Wang, J. Liu, R. M. Darling, L. Thompson, K. Smith, J. S. Moore, F. R. Brushett, and X. Wei J. Mater. Chem. A, 2016, 4, 5448 – 5456.

Nonaqueous redox flow batteries (NAqRFBs) are an emerging class of grid-scale, electrochemical energy storage devices that promise high cell potentials and a broad materials space. Although NAqRFBs could achieve future prices approaching $100 kWh^-1, current prices are substantially higher, and chemical cost contributions must decrease significantly to reach commercial viability [1]. Several studies (e.g., [2]) sought to increase the charge stored per unit mass of active species as a pathway to decrease cost. To date, however, no studies have considered designing a supporting electrolyte to reduce chemical costs. Typical fluorinated salts for NAqRFBs will likely have higher costs than the active species [1], and increased salt concentrations can suppress active species solubility, limiting energy density [3]. Reducing salt content, while maintaining electroneutrality and conductivity, is an overlooked challenge. Here, we analyze the importance of common-ion cell configurations, which enable lean electrolytes, for NAqRFBs.
The majority of reported NAqRFBs employ salt-splitting cell configurations, where, upon charging, the positive active species oxidizes to a cation and the negative active species reduces to an anion. This charge distribution necessitates that salt cations and anions migrate to the negative and positive electrolytes, respectively. Salt-splitting cells require a minimum 1:1 molar ratio of salt-to-active species to maintain electroneutrality, and ion-exchange membranes are typically not designed to transfer both cations and anions. Contrastingly, common-ion RFBs use at least one ionic active species at 0% state-of-charge (SOC), and the active species charges do not change sign during cycling. The active species counter ion affords charge balance, and the supporting salt merely imparts ionic conductivity on the electrolyte, lowering salt requirements. This study employs iron tris(2,2’-bipyridine) tetrafluoroborate (Fe(bpy)₃(BF₄)) and ferrocenylmethyl dimethyl ethyl ammonium tetrafluoroborate (Fc1N112-BF₄) as ionic active species to show the benefits of common-ion exchange. We extend this concept further by studying active species that remain as ions across all SOCs, and show that ionic active species, such as Fe(bpy)₃(BF₄) and Fc1N112-BF₄, can exhibit higher conductivities than typical supporting salts (e.g., LiBF₄). Combining these ionic active species in a common-ion configuration yields a NAqRFB devoid of any supporting salt, operating at moderate current densities (20 mA cm^-2) and greater than 85% current and voltaic efficiencies.
Acknowledgments
We acknowledge the Joint Center for Energy Storage Research and National Science Foundation Graduate Research Fellowship Program for financial support.
References
[1] R.M. Darling et al., Energy Environ. Sci. 7 (2014) 3459–3477.
[2] C.S. Sevov et al., J. Am. Chem. Soc. 137 (2015) 14465–14472.
[3] A.A. Shinkle et al., J. Power Sources. 248 (2014) 1299–1305.

Alternative energy sources such as solar and wind provide clean energy into the electrical grid, yet their inherent intermittency requires the development of energy storage systems to provide continuous supply. Non-aqueous redox flow batteries (RFBs) promise efficient grid-scale energy storage owing to their straightforward scalability and ample choice for active materials¹. However, currently used ion-exchange membranes (IEMs) in RFBs show low ionic conductivity in organic media.² An alternative to the use of IEMs is a size-exclusion approach that incorporates redox active polymers (RAPs) as storage media with porous separators.³ RAPs exhibit fast charge transfer, chemical reversibility, and high solubility in nonaqueous systems, making them attractive for flow applications. Here, we evaluate the performance of this approach under flow conditions using a model non-aqueous redox system.
Size-exclusion principles based on RAP size and membrane pore radius were used to suppress crossover while allowing high ionic conductivity. We will present on the development and characterization of RAPs that can be paired and cycled reversibly. These RAPs individually show promising electrochemical properties in voltammetric and electrolysis experiments, with Coulombic efficiency values consistently higher than 95%, high voltage efficiency and rate capabilities above 1C, as well as minimal redox active material crossover. Flow cell testing demonstrates the performance of these polymers under varying rate capabilities and material loadings for enabling a new generation of flow battery research based on non-aqueous redox systems.
[1] Energy Environ. Sci. 2014, 7(11), 3459. [2] J. Electrochem. Soc. 2016, 163(1), A5253. [3] J. Am. Chem. Soc. 2014, 136(46), 16309.

In advanced electrical grids of the future, electrochemically rechargeable fluids of high energy density could capture the power generated from intermittent sources like solar and wind. To meet this outstanding technological demand there is a need to understand the fundamental limits and interplay of electrochemical potential, stability, and solubility in low-weight redox-active molecules, among which, stability is the crucial factor for determining the real cycling performance. ^1,2
The derivatives of 1,4-dimethoxybenzene are thus far the best performing catholyte molecules and have been intensively studied for non-aqueous redox flow batteries (NRFBs). However, most of these molecules have been found by trial and error and the structural factors that determine durability of their radical cations and even the nature of reactions controlling the life times of these radical cations are insufficiently understood. In this report, an exploratory study focusing on the correlation between molecular symmetry and electrochemical stability was conducted. As shown in Fig 1, we examined quite a few previous reported stable redox molecules^3-5 for the root causes for radical cation stability. By comparing their cycling performance, we observed that symmetric molecules are more likely stable than the relative asymmetric ones. Evidenced by DFT calculations, we rationalize this connection through the effect of this symmetry on fragmentation in the stability of a conformationally frozen radical cation; in particular, on the rates of radical deprotonation to fragmentation reactions. In asymmetrical radical cations, the extra positive charge residing on some functional groups becomes greater than on other such groups, causing their faster fragmentation. Following this observation, we design and synthesize several new structures that are expected to be even more stable redox molecules.


(1) Wei, X.; Xu, W.; Huang, J.; Zhang, L.; Walter, E.; Lawrence, D.; Vijayakumar, M.; Henderson, W. A.; Liu, T.; Cosimbescu, L.; Li, B.; Sprenkle, V.; Wang, W. Angewandte Chemie International Edition 2015, 54, 8684.
(2) Wei, X.; Cosimbescu, L.; Xu, W.; Hu, J. Z.; Vijayakumar, M.; Feng, J.; Hu, M. Y.; Deng, X.; Xiao, J.; Liu, J.; Sprenkle, V.; Wang, W. Adv. Energy Mater. 2015, 5, 10.1002/aenm.201570002.
(3) Huang, J.; Cheng, L.; Assary, R. S.; Wang, P.; Xue, Z.; Burrell, A. K.; Curtiss, L. A.; Zhang, L. Adv. Energy Mater. 2015, 5, 1401782.
(4) Zhang, L.; Zhang, Z.; Redfern, P. C.; Curtiss, L. A.; Amine, K. Energy & Environ. Sci. 2012, 5, 8204.
(5) Zhang, Z.; Zhang, L.; Schlueter, J. A.; Redfern, P. C.; Curtiss, L.; Amine, K. J. Power Sources 2010, 195, 4957.

The ability to store large amounts of electrical energy is of increasing importance with the growing fraction of electricity generation from intermittent renewable sources such as wind and solar. Solid-electrode batteries are drained far too soon, when discharged at their rated power, to economically fill the gaps in photovoltaic or wind temporal power profiles. Flow batteries show promise because the designer can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all electro-active species in fluids. Wide-scale utilization of flow batteries is limited by the abundance and cost of these materials, particularly those utilizing redox-active metals such as vanadium or precious metal electrocatalysts. We have developed [1-5] high performance flow batteries based on the aqueous redox behavior of small organic molecules such as quinones. The redox active materials can be very inexpensive and sometimes exhibit rapid redox kinetics and long lifetimes. This new approach should enable massive electrical energy storage at greatly reduced cost. Relevant materials and molecular issues will be discussed.

[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, "A metal-free organic-inorganic aqueous flow battery", Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909
[2] Q. Chen, M.R. Gerhardt, L. Hartle, and M.J. Aziz, "A quinone-bromide battery with 1 W/cm² power density", J. Electrochem. Soc. 163, A5010 (2016), doi: 10.1149/2.0021601jes
[3] Q. Chen. L. Eisenach, and M.J. Aziz, "Cycling analysis of a quinone-bromide redox flow battery", J. Electrochem. Soc. 163, A5057 (2016), doi: 10.1149/2.0081601jes
[4] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz and M.P. Marshak, "Alkaline Quinone Flow Battery", Science 349, 1529 (2015) http://dx.doi.org/10.1126/science.aab3033
[5] K. Lin, R. Gómez-Bombarelli, E.S. Beh, L. Tong, Q. Chen, A.W. Valle, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, Nature Energy, in press (2016).

Aqueous quinone/hydroquinone couples exhibit rapid redox kinetics, require no electrocatalyst, and are potentially very inexpensive, making them attractive candidates for large-scale energy storage devices such as flow batteries [1-3]. We evaluate four anthraquinone derivatives for use as the negolyte in aqueous flow batteries. These anthraquinones are shown to undergo reversible two-electron reduction and oxidation in acidic aqueous solution, with tunable reduction potentials. When paired with a posolyte containing bromine and hydrobromic acid, three of these quinone derivatives exhibit cell voltages above 1 V. These derivatives are evaluated for chemical stability against bromination and stability upon electrochemical reduction and oxidation. DFT calculations paired with bromine exposure experiments demonstrate each derivative’s susceptibility, or lack thereof, to bromination in the oxidized and reduced forms. Electrochemical reduction and oxidation mechanisms of these quinones are explored via cyclic voltammetric studies. Two anthraquinone derivatives apparently stable to bromination and electrochemical reduction are cycled in a flow cell, showing greater than 98% current efficiency at 0.25 A/cm² cycling current. Mechanisms for capacity loss and energy inefficiency are discussed to guide further research.

[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, “A metal-free organic-inorganic aqueous flow battery”, Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909
[2] B. Huskinson, M.P. Marshak, M.R. Gerhardt and M.J. Aziz, “Cycling of a quinone-bromide flow battery for large-scale electrochemical energy storage”, ECS Trans. 61, 27 (2014)
[3] B. Yang, L. Hoober-Burkhardt, F. Wang, G.K. Surya Prakash, and S.R. Narayanan, J. Electrochem. Soc. 161, A1371 (2014)

Quinone and its hydroquinone counterpart can form quinhydrone dimer through dispersion interactions, with the best-known case being the quinhydrone reference electrode based on p-benzoquinone. Similar phenomena may occur in quinone-based flow batteries, as the reduced and oxidized forms of quinone coexist in all states of charge except 0 and 100%. We use UV-Vis spectroscopy to confirm the quinhydrone formation of the 9,10-anthraquinone-2,7-disulfonic acid (AQDS) molecule. An in-line UV-Vis spectrometer is also constructed to conduct such an observation in a flow cell during charge and discharge processes. These measurements help us quantify the equilibrium constant, and provide insights into the formation mechanisms corroborated by density functional theory calculation. We will also discuss how the quinhydrone formation, as well as the tribromide formation in the bromine/bromide electrolyte, impacts the cell voltage of the AQDS-bromide flow battery.

Redox activity in molecule-based materials is attracting much attention in the research of rechargeable batteries, in which the structural robustness of these materials in electrochemical reactions would be the key to opening their practical application. From this perspective, we have been examining various transition-metal cluster complexes (molecular clusters), metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) as promising materials to realize high energy-density and high power-density electrical energy storage. In this presentation, we report the performance of the molecular clusters and MOFs as cathode active materials for rechargeable Li batteries. Various in operando measurements, developed in our group, revealed the following: (i) nano-hybridization between polyoxometalates (POMs) and nano-carbons induces a synergetic effect between the redox activity of POMs and the capacitance of the nano-carbons, enhancing the battery capacity significantly [1]; (ii) coexistence of redox activity on the metal ions and organic ligands in a MOF realizes a “bipolar charging” mechanism in Li batteries [2]. In addition, we also report on Li-S batteries stabilized through the use of MOFs and COFs.

[1] H. Wang, S. Hamanaka, Y. Nishimoto, S. Irle, T, Yokoyama, H. Yoshikawa, and K. Awaga, J. Am. Chem. Soc., 2012, 134, 4918; K. Kume, N. Kawasaki, H. Wang, T. Yamada, H. Yoshikawa, K. Awaga, J. Mater. Chem. A, 2014, 2, 3801.
[2] Z. Zhang, H. Yoshikawa, K. Awaga, J. Am. Chem. Soc., 2014, 136, 16112; Z. Zhang, H. Yoshikawa, K. Awaga, Chem. Mater., 2016, 28, 1298.

Self-assembled materials based on perylene bisimide (PBI) derivatives, e.g. hydrogels and organogels, have many fascinating properties that result from the ease with which the perylene core can be reduced and/or oxidized. Amongst other things, such materials can be photoconductive with long lifetimes of the generated radical anions [1], photocatalytically active for the reduction of protons to hydrogen [2,3], as well as structurally respond when electrochemically reduced [4]. Sadly enough most of these materials are amorphous on the molecular scale and can be hence difficult to fully characterize by experiment alone.
To shed light on the origin of the redox properties of these materials and how this is related to both the chemical derivatisation of the PBI units and their intermolecular arrangement, we perform electronic structure calculations on structural models for multitude of possible structures and compare the results to both experimental electrochemistry and optical experiments. In my contribution I will discuss computational insight into the link between the redox and the electronic and mechanical properties of PBI-based self-assembled materials, highlighting where possible the subtle link between intermolecular structure and material performance. Time provided I will also touch upon the computational challenges of predicting the redox properties of organic amorphous materials.

[1] E.R. Draper, J.J. Walsh, T.O. McDonald, M.A. Zwijnenburg, P. Cameron, A.J. Cowan, D.J. Adams, J. Mater. Chem. C, 2, 5570, 2014.
[2] AS. Weingarten, R.V. Kazantsev, L.C. Palmer, M. McClendon, A.R. Koltonow, A.P.S. Samuel, D.J. Kiebala, M.R. Wasielewski, S.I. Stupp, Nature Chem. 6, 964, 2014.
[3] AS. Weingarten, R.V. Kazantsev, L.C. Palmer, D.J. Fairfield, A.R. Koltonow, S.I. Stupp, J. Am. Chem. Soc. 137, 15241, 2015.
[4] F. Schlosser, M. Moos, C. Lambert, F. Wurthner, Adv. Mater. 25, 410, 2013.

Chemical modification of conductive surfaces with composite molecular assemblies brings about new and useful properties. This work focuses on understanding and controlling electron transfer in molecular materials, which is demonstrated by the formation of electrochemical rectifiers, photoelectrochemical cells, and photovoltaic devices. We use redox-active metal-polypyridyl complexes and organic chromophores to construct our composite materials utilizing Pd(II)-pyridine coordination chemistry. These materials can pass electrons selectively in directions that are determined by their molecular composition and nano-scale arrangement. Unidirectionality is achieved and controlled by generating well-separated and homogeneous layers of complexes having different redox potentials that thermodynamically allow either oxidation or reduction. Combining the individual components of these materials into one film, bidirectional current flow through different pathways was demonstrated.¹ Furthermore, we introduce a method to structurally modify our assemblies using light and by that, modify the nature of the electron transfer processes. In this process, we conduct a photochemical reaction that results in a reduced degree of conjugation through the assemblies and an increased porosity. Higher porosity allows a more efficient mass transport within the assemblies, thereby, enhancing the electrochemical communication.
The ability to form composite molecular materials allows us to incorporate different functional elements and obtain a new function. As an example, a dual-component molecular assembly, composed of a Ru(II)-based electron donor and a Co(III)-based electron acceptor, was shown to have a programmable photoelectrochemical response that can be turned on and off as a function of the applied potential. In a related work, we have shown that self-assembled monolayers of our metal complexes can be used as energetically tunable electron transporting layers for organic photovoltaic cells. The fundamental science behind the design of such cells lies in the formation of energy level gradients for efficient charge separation and collection. Tuning the energy levels at the device electrodes by the right choice of the components is a key requirement for achieving enhanced characteristics. We found that changing the metal center of the complexes results in evident shifts of the HOMO and LUMO energy levels and the work functions of the corresponding monolayers, which has a prominent effect on the device performance.²
By using pyridine coordination chemistry with metal ions, we have shown that different and functional molecular assemblies can be formed. The discussed assemblies allow for interface engineering and provide great control of the resulting properties.

¹ Balgley, R.; Shankar, S.; Lahav, M.; van der Boom, M. E. Angew. Chem., Int. Ed., 2015, 54, 12457.
² Balgley, R.; Drees, M.; Bendikov, T.; Lahav, M.; Facchetti, A.; van der Boom, M. E. J. Mater. Chem. C, 2016, 4, 4634.

Metal-organic frameworks (MOFs) are porous ordered arrays of inorganic clusters supported by organic linking units. They have attracted attention for gas storage, separation and catalysis, which rely on weak interaction with an absorbate. The recent focus has shifted to their physical properties including unique magnetic, optical and ferroelectric responses. The control of electrical conductivity and redox activity would open up a new dimension of applications for MOFs. In this talk, I will discuss the absolute band energies (ionisation potentials) in comparison to standard semiconductors and electrical contacts[1]. I will show how the optical response can be tuned by chemical modification of the organic and inorganic building blocks[2,3]. Finally, the impact of stress and strain for redox engineering in devices will be touched upon[4]. The combination of chemical diversity, mechanical flexibility and electronic control in a single family of compounds could make metal-organic frameworks the semiconductors of the future.

1. Butler, K. T., Hendon, C. H. & Walsh, A. Electronic chemical potentials of porous metal-organic frameworks. J. Am. Chem. Soc. 136, 2703 (2014).
2. Butler, K. T., Hendon, C. H. & Walsh, A. Electronic structure modulation of metal-organic frameworks for hybrid devices. ACS Appl. Mater. Interfaces 6, 22044 (2014).
3. Hendon, C. H. et al. Engineering the optical response of the titanium-MIL-125 metal-organic framework through ligand functionalisation. J. Am. Chem. Soc. 135, 10942 (2013).
4. Hendon, C. H. & Walsh, A. Chemical principles underpinning the performance of the metal-organic framework HKUST-1. Chem. Sci. 6, 3674 (2015).

Lithium-sulfur batteries (LSBs) have attracted great interest in recent years because of their high theoretical capacity (1675 mAh g^-1) and high theoretical energy density (2500 Wh kg^-1). Therefore, LSBs are attractive systems that might become future replacements for typical lithium-ion batteries(LIBs).The shuttling process involving lithium polysulfides is one of the major factors responsible for the degradation in capacity of LSBs. Two approaches have been employed to overcome this defect: one is to load sulfur into high porous materials, and another is to combine sulfur and to form a cross-linked polymers. We supposed that a combination of the two approaches using covalent organic framework-graft-polysulfide (COF-graft-PS) would be an efficient methodology. A COF-graft-PS was synthesized by graft co-polymerization of elemental sulfur on to the skeleton of a functionalized COF. The polysulfide can be robustly impregnated into the pores through the strong covalent bonds between COF and sulfur. The COF serves as the host as well as the cross-links of polysulfides. The COF-graft-PS was utilized as the cathode for LSBs and the electrochemical properties were tested. The COF-graft-PS based LSBs show high capacity and great cycling performance. The stable cycling performance is due to the robust sulfur cathode structure in which sulfur is homogeneously distributed throughout the regular pores within the framework together with the C-S covalent links. The chemical sulfur impregnation within the pores of COF effectively suppresses the dissolution of polysulfides into the electrolyte.

Electroactive polymers undergo reversible reduction-oxidation reactions and have the potential to broadly impact applications including energy storage, sensing, and electronics. In this talk, the redox mechanisms of conjugated polymers, organic radical polymers, and conjugated organic radical polymers will be presented. While the redox mechanism in conjugated polymers such as polyaniline is fairly well established, we present unique routes to manipulate the mechanism so as to obtain extremely high doping levels. Conversely, organic radical polymers switch between a radical and charged state with unusually fast kinetics. We present electrochemical quartz crystal microbalance with dissipation monitoring (EQCMD) as a precise means to monitor elucidate mass and electron transport during electrochemical interrogation. This provides information on how dopant ions move in and out of the organic radical polymer for every electron passed. Finally, we will discuss the unusual reduction-oxidation activity of conjugated radical polymers, which consist of a conjugated backbone and pendant radical groups. These polymers exhibit dual redox activity and internal electron transfer between the conjugated and radical groups. With a deep understanding of the redox mechanisms in electroactive polymers, the future for custom design of novel polymers for advanced technological applications broadens.

Conducting polymers have been demonstrated to be promising electrode material for its electronic conductivity, however, existing conducting polymers for this application still suffer from low theoretical capacities, due to their inability to exchange more than one electron, and, often, cumbersome synthesis procedures. Previously, PEDOT and PEDOT-(redox-active-pendant)-derivatized molecules have shown reversible redox behaviors and stability applicable for energy storage application. In an effort to dramatically increase the capacity and electronic conductivity of organic electrodes for electrochemical/electrical energy storage applications, polypyrrole (PPy) has been used as an anchoring backbone to form a family of dimethox- ybenzene- (DMB-), N, N, N’, N’-tetramethylphenylenediamine- (TMPD-), and N, N, N’, N’-tetramethylbenzidine- (TMB- )decorated redox-active polymers. PPy was designed to provide electronic conductivity as well as immobilization of the polymers onto current collectors. The redox-active pendants (RAPs) dramatically increase the capacities of the materials by the reversible exchange of multiple electrons. The electropolymerization of the pyrrole-anchored RAP monomers was achieved via cyclic volt- ammetry. The stability of the electropolymerized films was highly dependent upon the redox potential of the RAPs. The effect of charge density during the electropolymerization process was studied by varying the structure of the monomers, and by the systematic characterization of the electrochemical properties of the synthesized films. The electrochemical behavior of these polymers provides validation of our targeted design to maximize energy density of organic electrode materials for electrical energy storage.

Conjugated polymers bearing pendant nitroxide radicals have been an intriguing research topic in energy storage due to their dual electroactive nature with different redox potentials within the same polymer backbone. Recently, we demonstrated polythiophenes bearing pendant nitroxide radical groups such as the TEMPO radical (2, 2, 6, 6-tetramethyl-1-piperidinyloxy). Polythiophenes possess a high oxidation potential with tunable electronic conductivity, and the organic radicals possess a very high electron transfer rate. However, these polymers exhibited worse battery performance than that of either PTMA (the polymeric form of TEMPO radical) or polythiophenes alone. One reason accountable for this phenomenon is that conjugated polymers and nitroxide radicals are both redox active and that intra- and inter-charge transfer is possible. As such, it is relevant to investigate the charge transfer pathways in such a system. In this study, we report the systematic study of polythiophenes carrying pendant TEMPO radicals. By adjusting the alkyl chain spacer length between the thiophene ring and TEMPO radical, we tune the electronic and electrochemical properties of these electroactive polymers. In addition, the charge storage mechanism during the galvanostatic charge/discharge process is also interrogated and the respective contributions from each redox moiety are discussed. Evidence of internal charge transfer is observed using a potentiostatic pulse technique. These results provide a path forward in the design of conjugated radical polymers for energy storage.

Organic radical batteries (ORBs), in which the organic radical polymer is used as the cathode and/or anode, have been considered as a promising alternative to conventional lithium-ion batteries, as the organic cathodes allow rapid charge transport and high cycling stability. One commonly studied stable nitroxide radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), is capable of a two-electron transfer process between its n-doped and p-doped states. The reported specific capacity of PTMA cathodes is between 77 to 220 mAh/g, depending on the charge/discharge conditions. Most previous research on PTMA is aimed at improving the electrode capacity and cyclability either by adding highly conductive materials or by optimizing the radical polymer synthesis. However, the charge storage process and the electrode/electrolyte interface in such systems are not understood on the molecular level. Here, we present the first application of in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) towards understanding the charge storage process in PTMA cathodes. EQCM-D monitors changes in frequency and dissipation of a PTMA-coated quartz crystal during controlled electrochemical interrogation (cyclic voltammetry). The change of mass and shear viscosity can be further obtained from the viscoelastic modeling of the raw data, leading to a quantitative view of mass transport associated with the doping process. Here, we specifically investigate the effect of different lithium electrolyte salts on the charge transfer process. Each salt gives different charge storage behavior, mass transport, and hysteresis, which may be attributed to varied polymer-dopant and dopant-solvent interactions.

Radical polymers (i.e., macromolecules composed of an aliphatic backbone and with pendant groups that bear open-shell chemical functionalities) are an emerging class of oxidation-reduction-active (redox-active) materials for myriad energy conversion and energy storage applications. To date, the primary focus in the community has centered on the design of these materials for electrolyte-supported (e.g., redox flow batteries) systems, and the design of radical polymers for these applications has been of great success. Despite being less-studied, the redox-active nature of the pendant groups of radical polymers allows them to transport charge in the solid state, if the radical groups populate the thin film in a relatively high density. This could be of great interest as the non-conjugated nature of the radical polymer backones causes the macromolecules to be very weak absorbers in the visible portion of the spectrum; thus, they could be of utility as transparent, conducting plastics. Here, we will discuss the design rules regarding the solid-state transport ability of radical polymers with specific attention focusd on: (1) the radical density; (2) the stability of the radical pendant groups with respect to exposure to electrical bias and environmental conditions; (3) the ability to dope radical polymers; and (4) next-generation considerations for improved solid-state device performance.

Specifically, we will demonstrate that a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), is capable of achieving electrical conductivity and space charge-limited hole mobility values on par with common conjugated polymer semiconductors [e.g., poly(3-hexylthiophene) (P3HT)] despite the fact that the material is completely amorphous (i.e., is a glass) in the thin film state. Moreover, the transport in radical polymer thin films is independent of temperature, which suggests that the limiting step to macroscopic electrical conductivity increases is not limited by the redox reactions but, instead, is limited by the proximity of radical groups to each other within the thin film. Through selctive doping of PTMA with a redox-active small molecule salt, the electrical conductivity of the PTMA thin film is increased by a factor of 5 at optimal salt loading conditions. Tranisent current-voltage data and temperature-dependent measurements support the idea that this increase in conductivity is due mainly to increased electrical conductivity with only a very small amount of ionic conductivity present, despite the presence of ionic salts within the radical polymer matrix. Finally, we demonstrate that the PTMA thin films are rather stable with respect to both electrical bias stressing and upon exposure to humid and oxygen-filled environments. These combined data demonstrate the fundamental electrochemical processes within radical polymers and highlight the potential of radical polymers to play a significant role in organic elecronic devices.

We have begun to unify heterogeneous and homogeneous catalysis at the molecular level by defining the redox mechanisms of cobaltate clusters as linchpins between extended cobalt oxides and molecular cobalt oxides. A dogma of heterogeneous systems is that redox chemistry at “edges” matter in promoting catalytic transformations. We provide a rationale for such dogma by showing that oxygen evolution reaction (OER) in cobaltic oxides likely occurs at a dimensionally reduced dicobalt edge site. Redox chemistry at edge sites is clearly revealed by isotope distributions as determined by differential electrochemical mass spectrometry. To more deeply address the nature of edge site redox chemistry, we have prepared a dicobalt complexes wherein a diamond Co₂(OH)₄ core is stabilized by multidentate ligands. The kinetics of anion binding to dicobalt centers is a critical determinant of the redox rate constants, thus controlling overall OER activity. This chemistry at the molecular level directly translates to the OER function of heterogeneous cobalt-oxide catalysts. This interplay between the redox chemistry of the homogenous and heterogeneous systems will be presented.

Erwin Reisner
Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
E-mail: [email protected]; Website: http://www-reisner.ch.cam.ac.uk/
Solar fuel synthesis requires the finely tuned combination of solar light absorption, charge separation and coupling of two multi-electron half-reactions. This demanding task is highly controlled in natural photosynthesis, but much less so in artificial systems. A drawback in many synthetic approaches is the reliance on sacrificial redox reagents, which does not only prevent truly energy-storing chemistry to proceed but also introduces misleading free radical chemistry. Another limitation in most artificial systems is their reliance on expensive materials, which precludes the development of ultimately scalable devices. This presentation will give an overview about our recent progress to perform controlled full redox cycle solar fuel catalysis with enzymatic and synthetic 3d transition metal catalysts.
We have recently achieved photoelectrochemical (PEC) full water splitting through three different approaches. In a semi-artificial system through the wiring of the water oxidation enzyme Photosystem II to a H₂ producing enzyme known as hydrogenase,^1,2 in a molecular PEC cell based on a H₂-evolving Ni catalyst^3,4 and a water-oxidising Fe catalyst,⁵ and an all-solid-state tandem PEC water splitting cell, which was constructed through single-source precursor chemistry.⁶ These approaches can also be extended to solar CO₂ splitting.⁷ PEC approaches are ideally suited for the separation of individual half reactions on different electrodes and compartments, but are rather difficult and expensive to assemble and scale. Progress in the assembly of photocatalytic molecule-nanoparticle hybrid systems, which operate in a single compartment, will also be discussed.^8-10
References
(1) D. Mersch, C.-Y. Lee, J.Z. Zhang, K. Brinkert, J.C. Fontecilla-Camps, A.W. Rutherford, E. Reisner, J. Am. Chem. Soc., 2015, 137, 8541.
(2) C.-Y. Lee, H.S. Park, J.C. Fontecilla-Camps, E. Reisner, Angew. Chem. Int. Ed., 2016, 55, 5971.
(3) M.A. Gross, A. Reynal, J.R. Durrant, E. Reisner, J. Am. Chem. Soc., 2014, 136, 356.
(4) M.A. Gross, C.E. Creissen, K.L. Orchard, E. Reisner, Chem. Sci., 2016, DOI: 10.1039/C6SC00715E.
(5) T.E. Rosser, M.A. Gross, Y.-H. Lai, E. Reisner, Chem. Sci., 2016, DOI: 10.1039/C5SC04863J.
(6) Y.-H. Lai, D.W. Palm, E. Reisner, Adv. Energy Mater., 2015, 5, 1501668.
(7) T.E. Rosser, C.D. Windle, E. Reisner, E. Angew. Chem. Int. Ed., 2016, DOI: 10.1002/anie.201601038.
(8) J. Willkomm, K.L. Orchard, A. Reynal, E. Pastor, J.R. Durrant, E. Reisner, Chem. Soc. Rev., 2016, 45, 9.
(9) C.A. Caputo, M.A. Gross, V.W. Lau, C. Cavazza, B. Lotsch, E. Reisner, Angew. Chem. Int. Ed., 2014, 53, 11538.
(10) B.C.M. Martindale, G.A.M. Hutton, C.A. Caputo, E. Reisner, J. Am. Chem. Soc., 2015, 137, 6018.

Ring-opening metathesis polymerization (ROMP) is an incredibly powerful method for preparation of diverse polymers and materials. Traditionally, ROMP is mediated by well-defined transition metal complexes. Inspired by earlier work on electrochemical cross metathesis, we explored the possibility of electro-mediated ROMP that could circumvent reliance on metal-based reagents. Our method has been adapted to enable metal-free ROMP via photoredox-mediated oxidation of electron rich vinyl ether initiators. Moreover, the ability to use benign visible light to drive the polymerization provides operational advantages and temporal control over polymer growth. We will present our most recent developments which have targeted an expanded scope of monomers, systems capable of bulk curing and crosslinking, block copolymer synthesis, and application of electrochemical mediators.

Heterogeneous redox catalysis generally involves an electron mediator, which engages in a reversible redox couple that is initiated by reacting with one reactant and followed by a reaction of interest. In this way the catalytic employment of the electron transfer mediator is possible and reagent waste along with difficult separation procedures can be avoided. Herein, we introduce a combination of morphological design with redox chemistry involving two multivalent metal (Cu and Mn) for oxidative alkyne coupling reaction. We synthesized a set of mesoporous copper supported manganese oxide materials (meso Cu/MnO_x) catalysts using an inverse micelle templated method. The material was composed by aggregation of rounded nanoparticles with high surface area (as high as 270 m²g^-1) and uniform mesoporous size (3.0 – 3.4 nm) distribution. The alignment of copper over the manganese oxide was shown by PXRD refinement showing a copper (II) oxide (CuO) phase at higher copper doping. Elemental mapping analysis by TEM-EDX confirmed a uniform distribution of copper oxide over the manganese oxide. Broad substrate scope and excellent functional group tolerability were demonstrated for the oxidative homo and cross-coupling of terminal alkynes. Our mechanistic studies coupled with DFT computational modelling supported that both a redox copper mechanism coupled with manganese multivalency contributes to the catalytic cycle. The lattice oxygen of the material was proposed to deprotonate the alkyne to form Cu(II) acetylide species, which dimerize to the diyne and generate Cu(I) which is then oxidized to Cu(II) by Mn³⁺ /Mn⁴⁺. Air (oxygen) was the terminal oxidant. The manganese oxide acted not only as support, but also an electron mediator to generate an electron transfer pathway between alkyne and dioxygen. This combined experimental and theoretical study can serve as a benchmark for the cooperative redox catalytic reactions and opens up a new avenue to design and identify supported heterogeneous catalysts for potential applications in other complex redox catalysis.

The redox flow battery (RFB) has been considered for green grid applications as it is safe, low cost, pollution free and reliable (1-3). While in RFB both the electrolytes are under free flow during operation, a semi redox flow battery uses one stationary electrolyte. In this category Zinc-iron redox flow battery is actively considered (4,5). We wish to report here the construction and performance of semiflow redox cell involving Zinc-graphene quantum dot (ZnGQD)/Zn²⁺ and Fe²⁺/Fe³⁺ as a redox flow cell having nanoparticles of 5 nm to 100 nm (GQD) having a large surface area (1-3) in acid electrolytes. The cyclic voltammetric features of ZnGQD electrode shows a reversible peak for Zn²⁺ reduction at E_pc=-1.05 V vs SCE. The composite prevents HER reaction and dendritic growth. The reduction of Fe³⁺ in GQD bath is faster and occurs at about E_pc=0.39 V. Due to the continuous movement of the quantum dots in the bath, it produces higher currents. This has been accelerated by using external magnetic flux. The semi flow redox battery produces a high voltage with a theoretical energy density of 776 Wh/kg. The advantage of Fe²⁺/Fe³⁺ in GQD bath is in the charge storage and recovery.
-------------------------------------------------------------

1. Z. Yang, J. Zhang, M. C. W. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon and J. Liu, Chem. Rev., 111, 3577–3613 (2011).
2. K.S.V. Santhanam and G. Lein, Encyclopedia of Nanoscience and Nanotechnology, Vol. 24, p.249, (2011).
3. Y.D. Chen, K.S.V. Santhanam and A.J. Bard, J. Electrochem. Soc., 128, 1460 (1981).
4. K. Gong, X. Ma, K.M. Conforti, K.J. Kuttler, J.B. Grunewald, K.L. Yeager, M.Z. Bazant, S. Gu and Y. Yan, Energy Environ. Sci., 8, 2941 (2015).
5. Z. Protich, K. S. V. Santhanam, A. Jaikumar, S. G. Kandlikar and P. Wong, Journal of The Electrochemical Society, 163 (6) E166-E172 (2016).

Supported nanoparticles can be used as model catalysts to gain a fundamental understanding of the electrocatalytic properties at the interface. The preparation of unprotected nanoparticles in alkaline ethylene glycol is a powerful approach allowing for the synthesis of stable particles with a narrow size distribution in the subnanometer regime.
The stability of these wet-chemically synthesized platinum particles on a bare gold as well as on a self-assembled monolayer on gold is investigated by electrochemical scanning tunneling microscopy. A loss in the electrochemical active surface area of supported particles on Au(111) is found during potential cycling and could be attributed to a two-step degradation mechanism of the particles. A single rate is obtained for the change in electrochemical active surface area for particles on the SAM covered gold support. To validate the underlying degradation processes of the particles on the respective surface, a kinetic Monte Carlo model is implemented.

Gas-to-solid electrochemical reactions involving redox of dissolved molecules are of increasing scientific interest for nonaqueous energy storage and gas conversion systems such as alkali metal-O₂, -CO₂, and -SO₂ batteries and capture devices. Here we report a new system involving the thermodynamically downhill, multi-electron transfer reduction of a dissolved gas, SF₆, and its reaction with Li ions in a primary Li battery configuration, forming solid phases including LiF upon discharge and delivering useful electrical work. Owing to its relevance as an industrially emitted greenhouse gas, as well as its more fundamental behavior as a classically inert molecule, SF₆ reactivity has previously been sought after using chemical approaches, which have included reactions of SF₆ with alkali metals at high temperature, or with transition metal organometallic complexes at more moderate temperatures. However, mechanistic understanding is still needed of how such “inert” yet energetically rich molecules can be activated, and accessed for subsequent energy conversion or destruction reactions, in various reaction environments.

In this work, we study the conversion of SF₆ upon electrochemical reduction in Li cells using both galvanostatic discharge and cyclic voltammetry approaches. Despite its widely reported inertness, our results indicate that in a closed battery cell containing nonaqueous battery electrolytes (0.3M LiClO₄ in TEGDME or DMSO) and a carbon positive electrode, the electrochemical reaction of SF₆ with Li occurs at moderate voltages of ~ 2.2 V vs Li and to capacities in excess of 1000 mAh/g_carbon at low rates ( < 50 mA/g_carbon). The use of a nonaqueous electrolyte allows the reaction to be driven with large kinetic overpotentials (~1 V), which is uniquely afforded by the wide voltage stability window. Here, we present an analysis of the discharge phases present in the battery electrode in order to develop understanding of the reactions occurring during discharge. Additionally, we employ a Rotating Disk Electrode (RDE) setup incorporating planar glassy carbon and metal electrodes in an electrolysis cell to study the sensitivity of reaction kinetics to electrode surface composition. Based on our results, we discuss the possibility of heterogeneous catalysis in activating the inert SF₆ molecule with improved kinetics for energy delivery in primary Li cells.

A bis(imino)acenapthene (BIAN) ligand can participate in electron transfer reactions with coordinated transition metal ions and this behavior can be applied to beyond lithium battery technologies. The ligand has been shown to coordinate and solubilize metals such as Mg, Zn, and Ga to give the corresponding (BIAN)₂M compounds. The behaviors of the gallium based compounds were investigated structurally and electrochemically to determine the viability of the BIAN ligand to act as an electro-active ligand for a gallium based flow batteries. The study reveals interplay of electron transfer driving coordination behavior in terms of stabilizing the lewis acidic metal center.

Safety is one of the most crucial problems faced by the lithium-ion battery (LIBs) industry. In this work we propose a strategy to avoid overcharging of a battery via the application of a solid-state combined cathode. The goal of this research is to produce LIBs with overcharge self-regulated capabilities. In order to achieve self-regulated functionality, 1,4-di-tert-butyl-2,5-dimethoxybenzene (DBB) is added to as-synthesized LiFePO_4, post synthesis. The DBB has a trigger voltage of 3.75 V and when this voltage is reached, DBB forms a reduced ion that is released into the electrolyte from the cathode side. The DBB ion transfers to the anode side where it oxidizes and transfers back to the cathode side. This process forms a redox shuttle and consumes the extra charges keeping the voltage at a safe level (i.e. 3.75 V). The DBB redox shuttle protects the LiFePO₄-based LIBs with working voltage between 3.4 and 3.5 V. The cycleability of assembled batteries are tested at 200% overcharge using an Arbin Tester. The battery performance after multiple cycles of overcharging is also tested to demonstrate that battery performance is maintained after the overcharging process, and the redox shuttle can successfully protect the battery.

Platinum nanoparticles on a Polymer Electrolyte Membrane Fuel Cell (PEMFC) catalyze the oxidation of hydrogen gas, generating electricity. However, PEMFCs suffer from low power output and platinum’s high cost and susceptibility to carbon monoxide (CO) poisoning. This research created a new catalyst of iron (Fe) nanoparticles on partially reduced graphene oxide (prGO). Precursors of Fe⁺², Fe⁺³, and Fe⁰ were tested to see which is optimal. Trace amounts of gold (Au) and platinum (Pt) were also tested in combinations with Fe. Characterizations by Raman, Mössbauer, Scanning and Transmission Electron Microscopies, and Vibrating Sample Magnetometry displayed and characterized Fe nanoparticles, ~2.5 nm in diameter, uniformly distributed on the prGO sheet.
Coating PEMFC’s electrodes and membrane with test solutions resulted in significant increases of power output and resistance to CO poisoning, suggesting that iron can be incorporated as an efficient catalyst to improve PEMFC’s durability and performance while reducing its cost. In addition, PEMFC enhancement is produced using noble metal alloy nanoparticles, where it is shown that the combined effect is larger than from either metal alone.

Powerful reductants that react via simple one-electron transfer are inevitably highly air sensitive. More easily handled, yet still highly reducing, species, are possible if the electron-transfer process can be coupled to a chemical reaction; such species are potentially useful as n-dopants for organic semiconductors, as surface modifiers to tune the properties of inorganic electrode materials and of two-dimensional materials such as graphene, as reductants for generating reduced species for spectroscopic study, and as reagents for organic synthesis. This presentation will describe efforts to understand the solution and solid-state redox behavior of one such class of reductants: the dimers (D₂) formed by certain highly reducing odd-electron species (D^.), including dimers of both 19-electron organometallic sandwich compounds and of organic radicals. The thermodynamic reducing strength of these species (the potential E(D⁺/0.5D₂)) depends on both E(D⁺/D^.) and on the free energy of dissociation of the dimer, ΔG_diss(D₂). ΔG_diss(D₂) and E(D₂^.+/D₂) will determine the kinetic behavior of the reductants, including the mechanism by which they react with a given acceptor. Values of E(D⁺/D^.) and E(D₂^.+/D₂) can easily be determined electrochemically, but experimental determination of ΔG_diss(D₂), and, therefore, E(D⁺/0.5D₂), is less straightforward and has only been performed for a few examples; however, these results are in reasonable agreement with density functional theory (DFT) values. Interestingly, values of ΔG_diss(D₂) show poor correlation with DFT and crystallographic values of the central C-C bond lengths in the dimers, apparently depending more strongly on factors that stabilize D^. than on the orbital overlap and steric interaction in D₂. Although there is considerable variation in E(D⁺/D^.) and in DFT values of ΔG_diss(D₂), for most of the dimers these variations more-or-less cancel to result in values of E(D⁺/0.5D₂) that are ca. –2.0 V vs. the FeCp₂^.+/0 couple. Possible approaches to obtaining stronger reductants that still retain moderate air stability will be outlined. Finally an approach will be discussed whereby the thermodynamic limit can be “cheated” to allow n-doping of an organic semiconductor with a solution reduction potential of –2.2 V and a solid-state electron affinity of 2.2 eV.

While a variety of stable molecular p-dopants have been developed and successfully deployed in devices during the past decade, n-dopants suitable for low electron affinity (EA) materials (EA < 3 eV), which usually are used as electron-transport materials (ETMs) in OLEDs, still present a set of challenging issues. If operating as simple one-electron reductants, n-dopants must have an ionization energy (IE) that is close to or lower than the EA of the host material, increasing their susceptibility to oxidation. To satisfy requirements of dopant ambient stability and efficient reduction of low EA materials, Guo et al. [1] recently introduced a new class of air-stable dopants, i.e. dimers of highly reducing organometallic or organic species, whereby the air-stable dimer reacts to yield two monomer cations and two electrons, to n-dope low EA materials like pentacene (EA = 2.8 eV). In this work, we investigate the ultraviolet (UV) light activation of n-doping of a low electron affinity (~2.2 eV) electron-transport material (ETM) with ruthenium-based organometallic dimers. To study the impact of UV irradiation, we use a combination of electron spectroscopy, contact potential, current-voltage, and optical absorption spectroscopy measurements. With seconds of UV (375 nm) irradiation, the Fermi level of the doped ETM is pinned close to edge of the lowest unoccupied molecular level and the film conductivity increases by 3 to 6 orders of magnitude. Importantly, doping persists long-term following activation, in spite of the fact that the effective reducing strength of the dimers is less than required for the EA of the ETM, indicating that the thermodynamic limit was beaten by kinetics. OLEDs comprising this low EA ETM were fabricated and demonstrate that doping significantly improves the performance of the device.

[1] Guo, S. et al., Adv. Mater., 24: 699–703 (2012).

The application of conjugated polymers as semiconductors in organic electronics, such as solar cells, field-effect transistors or light emitting diodes is well established, more recently they have attracted attention in the fields of energy storage and thermoelectrics. While hole conducting materials are frequently studied in literature, significantly fewer reports can be found on electron conducting polymers.
One of these electron conducing polymers is poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} P(NDI2OD-T2). This polymer exhibits very promising properties for rechargeable Li batteries and organic thermoelectric generators.^[1]
In this contribution we present an extensive investigation of the electrochemical charging behavior of P(NDI2OD-T2) in solution and thin films.^[2] Cyclic voltammetry and in-situ spectroelectrochemistry measurements reveal a twofold reversible n-doping of the polymer. The direct comparison between the doping behavior of the polymeric and the monomeric system leads to the observation that charges seem to be strongly localized on the repeat units. Each repeat unit can be doped twice, which allows higher doping levels than for other conjugated polymers. From an electrochemical point of view, P(NDI2OD-T2) can be considered rather as redox polymer than as conjugated polymer.^[3]
In addition we investigated the influence of morphology on the first voltammetric cycle. We found that changes in the morphology have a significant influence on the shape of the first voltammetric cycle, i.e. the reduction signals are shifted towards more negative potentials. A deeper understanding of the charge transport in P(NDI2OD-T2) thin films was achieved by in-situ conductance measurements using organic electrochemical transistors. It was found that electron transport occurs via hopping of charges localized on the naphthalene diimide moieties, which can be explained by the mixed valence conductivity model.^[4]

References
[1] Y. Liang, Z. Chen, Y. Jing, Y. Rong, A. Facchetti, Y. Yao, J. Am. Chem. Soc. 2015, 137, 4956. R. A. Schlitz, F. G. Brunetti, A. M. Glaudell, P. L. Miller, M. A. Brady, C. J. Takacs, C. J. Hawker,
M. L. Chabinyc, Adv. Mater. 2014, 26, 2825.
[2] D. Trefz, A. Ruff, R. Tkachov, M. Wieland, M. Goll, A. Kiriy, S. Ludwigs, J. Phys. Chem. C 2015, 119, 22760.
[3] J. Heinze, B. A. Frontana-Uribe, S. Ludwigs, Chem. Rev. 2010, 110, 4724.
[4] O. Yurchenko, J. Heinze, S. Ludwigs, ChemPhysChem 2010, 11, 1637.

Pi-conjugated polymers currently employed for cutting-edge device applications are typically constructed through careful bandgap engineering whereby the electronic properties of the constituent monomers are built into the material target, thus leading to a desired optical bandgap, planarity, etc. This talk will highlight recent studies to transiently engineer high degrees of polarizability and delocalization into organic semiconductors. We take advantage of two specific molecular modifications that enable the realization of photon-triggered electrocyclizations and open-shell diradical characters that are postulated to invoke unusual aromatic topologies beyond the classical Hückel notion. The ability to manipulate local oligomer or polymer electronics could allow for a new direction to bandgap engineering within organic semiconductors.

B. C. Streifel, J. L. Zafra, G. L. Espejo, C. J. Gómez-García, J. Casado, J. D. Tovar, “An unusually small singlet-triplet gap in a quinoidal 1,6-methano[10]annulene due to Baird’s 4n pi-electron triplet stabilization,” Angewandte Chemie International Edition, Vol. 54 (2015), pp. 5888-5893.

Quantum-chemical approaches are widely used to study molecule-centered redox events, with the knowledge derived from these techniques providing fundamental details of the physicochemical processes that take place in a number of emerging technologies. In this presentation we will discuss how studies of electron-transfer processes in organic mixed-valence systems and semiconductor materials can inform the investigation and development of new generations of redox shuttles envisioned for use in high energy density batteries. Understanding the parallels of electron-transfer events that take place across a diverse set of systems, along with the essential limitations of the computational methods, can deliver critical insight to move forward paradigms of computer-based materials design.

Many of the electrochemical energy-storage systems under consideration for large-scale power storage and allocation rely on stable electroactive materials to shuttle and/or store charge. Here we seek to design a new class of organic compounds, based on the electroactive molecule phenothiazine, for use as (a) redox-shuttle additives to mitigate excess charge in overcharging lithium-ion batteries, and (b) catholyte materials for use in redox flow batteries. This presentation will include the results of density functional theory calculations that are used to model the physicochemical effects of substituent identity and placement on phenothiazine by introducing electron-donating and/or -withdrawing substituents at strategic positions around the heterocyclic core. Substituent crowding is shown to effect changes in oxidation potential at odds with those anticipated from Hammett constants, as well as changes in energetically optimal conformations, consistent with more-restricted relaxation pathways afforded by the additional steric strain. A subset of the compounds under consideration were subsequently selected for synthesis and electrochemical analysis. The results of this testing suggest that, unlike prior methods of increasing oxidation potentials using electron-withdrawing groups, strategic placement of substituents can be exploited to raise oxidation potentials without raising reduction potentials, thereby preventing access to reduction decomposition pathways. The results of this analysis reveal strategies for designing and tuning the properties of new electroactive compounds for energy-storage applications and beyond.

Because of their low cost, potential sustainability, environmental friendliness, and most importantly their large structural diversity, which make it conceivably usable as both positive and negative electrodes, organic-based electrode materials become one of the most promising candidates to replace scarce inorganic materials coming from geological resources for the new generation of “greener lithium ion batteries”.^1,2,3 Conjugated carbonyls have thus been early recognized as interesting electroactive materials characterized by high energy/power density and high cycling stability, but their implementation is still limited to Li metal cells with the serious safety issue caused by Li dendrites.² To be successful the Li-ion organic battery technology needs first, the implementation of a lithiated positive electrode able of competing with LiCoO₂ or LiFePO₄ and second, a negative electrode material operating at the target potentials (0.5-1.0 V) providing high voltage as well as safety.
Seeking for such efficient organic materials, our research group has reported recently, for positive electrode application, two organic salts able to reversibly deintercalate Li ions with promising electrochemical performance.^4,5 However their operating redox potential remain still too low which makes them unstable in ambient environment (difficult to store) and reduce the final output voltage. Concerning the negative electrode application, we proposed π-extended carboxylate core unit thus improving the cycling rate capability.^6,7
Herein we report on a substitution approach around the organic backbone allowing tuning of the lithium storage potential in order to increase the output voltage. Firstly we present an upshift of the working potential of a dilithiumoxyquinone-derivative as a lithiated organic positive electrode stable to air (>3.0 V vs. Li⁺/Li⁰). Secondly and following the same approach, we present a downshift of the working potential of a carboxylate-based structure as an organic negative electrode able to use aluminum-based current collector (<0.8 V vs. Li⁺/Li⁰). The matching of both materials will pave the way toward a novel n-type all-organic Li-ion cell with an attractive working voltage greater than 2 Volts.

1. Poizot, P.; Dolhem, F. Energy Environ. Sci. 2011, 4, 2003−2019.
2. Song, Z.; Zhou, H. Energy Environ. Sci. 2013, 6, 2280−2301
3. Liang, Y.; Tao, Z.; Chen, J. Adv. Energy Mater. 2012, 2, 742−769
4. Renault, S.; Gottis, S.; Courty, M.; Chauvet, O.; Dolhem, F.;Poizot, P. Energy Environ. Sci. 2013, 6, 2124−2133.
5. Gottis, S.; Barres A-L.; Dolhem, F.;Poizot, P. ACS Appl. Mater. Interfaces. 2014, 6, 10870−10876.
6. Fedele, L.; Sauvage, F.; Bois, J.; Tarascon, J.-M.; Becuwe, M. Electrochem. Soc. 2014, 161, A46−A52
7. Fedele, L.; Sauvage, F.; Becuwe, M. J. Mater. Chem. 2014, 6, 18225−18228.

Organic redox-active compounds are considered to be very promising alternative to conventional inorganic intercalation compounds for the next-generation secondary batteries because of their potential for high energy and power density, low-cost, and environmental friendliness. Many organic compounds typically undergo reversible multi-electron redox reactions, which can facilitate the batteries to have high specific capacity. The p-benzoquinone (BQ) is one of the simplest π-conjugated organic molecule possessing the lowest molecular weight (MW = 108.10 g/mol) that undergoes reversible reduction with two electrons. The theoretical specific capacity of the BQ as electrode-active material is calculated to be as high as 496 mAh/g. However, its high solubility in polar solvents and rather high vapor pressure at room temperature (P_{BQ, 25°C} = 0.1 mmHg) significantly limit its practical use as electrode material in electrochemical energy storage devices such as lithium ion batteries.
Here, we present a series of triptycene derivatives containing multiple quinone units in a rigid tripod structure for use in lithium ion battery as a cathode-active material. Through theoretical calculation by DFT method and electrochemical analysis by cyclic voltammetry, it is revealed that the triptycene molecule TT bearing three BQs can store and release up to five electrons reversibly, yielding a theoretical capacity of 389 mAh/g. It is also found that the first reduction potential of TT is 3.0 V vs Li/Li+ which is 0.2 V higher than that of BQ attributed to the three dimensionally π-conjugated structure of the TT molecule. In the 2032-type coin cell, the two-step charge/discharge profile is clearly observed from the TT electrode. The charge plateaus are 2.5 and 3.0 V, similar to its discharge plateaus at 2.4 and 2.9 V, respectively. The TT electrode can deliver a discharge capacity of 348 mAh/g, corresponding to 90% of the theoretical capacity, and an energy density of 933 Wh/kg at 0.1 C-rate. It can be expected that the three-dimensional arrangement of the redox-active quinone units of the TT molecule favors stable and highly reversible access to a large number of redox states. It should be also noted that ether-based electrolytes with 2 M LiTFSI and 1 wt% LiNO₃ additive effectively enhance its cycle performance.

Viologens have been in focus for increasing attention due to their unique properties. The special colour change and the stable redox property are the two main features which made the viologens work as a frequently used functional material in various applications, such as electrochromic devices, sensors and fuel cells. The viologen can undergo two redox processes with three redox forms: dication form, radical cation form and neutral form. The dication form is the most stable one, the radical cation form is usually colourful and the neutral form has strong reductive property. In dication form, the two pyridine rings are non-parallel to each other at certain angles. However, when the viologen is reduced into its radical cation form, the two pyridine rings will switch to planar. This configuration is maintained also in the neutral form. In the reduction process from the dication to radical cation form, the unique colors of viologens will appear. It is one of the reasons why viologens are frequently utilized as electrochromic materials. The colors and the redox potentials of viologens can be tuned and verified by changing the substituent groups correspondingly.

In order to enhanced the electrochemical properties and extended the application of viologens, some other functional materials are introduce to synthesize the polyviologen-based composite films. Composite assemblies are introduced where the redox active polymers are blended with electrically conducting materials, thereby strengthening the electrochemical reversibility of the redox polymer. For example, the reduced graphene oxide is utilized to prepare a composite film with polyviologens. In the last years, a series of polyviologen-based composite films were electrochemically synthesized from different cyanopyridine based precursors and other functional materials in our laboratory. These viologen materials have been characterized by several electrochemical, physicochemical and imaging techniques, which have revealed unique redox properties of the composite materials that have huge potential to be applied in different fields of applications.

Three alkylated dipyridinium thiazolo[5,4-d]thiazole derivatives are reported which exhibit strong blue-green fluorescence with high quantum yields between 0.6 – 0.7. The dioctyl, dimethyl, and dibenzyl derivatives all show distinctive and reversible electrochromism at reduction potentials that are 200 mV less than methyl viologen. These materials demonstrate yellow to dark blue electrochromism with an applied voltage of -250 mV. The fused bicyclic thiazolo[5,4-]thiazole allows the alkylated pyridinium groups to remain planar, strongly affecting their electrochemical properties. The fluorescence emission intensity is greatly enhanced with quaternarization of the peripheral pyridine groups while PL decay lifetimes were observed between 2.2 - 2.4 ns in solution. The new thiazolothiazole viologens have been characterized using cyclic voltammetry, UV-visible absorbance and fluorescence spectroscopy, spectroelectrochemistry, and time-resolved photoluminescence. The good electrochromic reversibility observed in solution along with strong fluorescent emission properties make these materials attractive for advanced applications where materials with multi-functional optoelectronic properties are required.

Microelectrode arrays are potentially powerful tools for analyzing small molecule libraries because they contain thousands of spatially isolated, individually addressable electrodes. If a molecular library is placed or synthesized on the array such that each unique member of the library is placed by a unique, addressable electrode or set of electrodes in the array, then the electrodes can be used to monitor interactions between the molecules in the library and biological targets as the events happen. But how does one build molecules proximal to individually addressable electrodes in a microelectrode array having a density of over twelve thousand electrodes/cm²? The answer to this question involves the use of designed, photo-crosslinkable diblock copolymers to coat the arrays, and then site-selective chemical reactions that can be used to conduct transformations on that polymer proximal to specific electrodes in the array. These synthetic methods take advantage of the electrodes themselves to both mediate the synthetic reactions and to recover molecules from the array so that the members of the library can be characterized. The result of these efforts is a synthetic toolbox that allows for the controlled synthesis of complex, addressable, and characterizable molecular surfaces. In the talk to be presented, the nature of the polymers employed, a general synthetic strategy for site-selective manipulation of the polymer, and examples of the resulting functionalized arrays can be utilized will be presented.

The I₃^–/I^– redox couple has been the predominant redox shuttle in DSSCs partially due to its ability to achieve both quantitative dye regeneration and charge collection when used with conventional ruthenium based sensitizers such as N3. Efficient dye regeneration with I₃^–/I^–, however, comes at an energy penalty of >500 meV, and it is not generally compatible with all sensitizers. Outersphere redox shuttles have recently shown promise in reducing the dye regeneration energy penalty, with some systems also allowing efficient charge collection with Zn porphyrin sensitizers. Our aim is to elucidate the interplay of dye regeneration and recombination as a function of redox shuttle property in order to develop design rules of redox shuttles for next generation high efficiency DSSCs.

Recent results comparing the dye regeneration efficiency and the electron diffusion length, which is a function of recombination, for DSSCs employing new cobalt-based outersphere redox shuttles will be presented. Ligand sets were utilized to control the Co(II) spin state for Co(III/II) redox couples, as well as a new example of a Co(IV/III) redox shuttle. Self-exchange rate constants of these redox shuttles were determined from cross-exchange measurements. Results will be presented that show we are able to tune the electron-transfer self-exchange rate constant by nine orders of magnitude with cobalt redox shuttles through such variation of the ligand framework. Application of Marcus theory allowed the difference in self-exchange rate constants to quantitatively account for differences in regeneration efficiency and electron diffusion length of the redox shuttles measured. These results point to a new energy and kinetic space of redox shuttle which have the capability of achieving high efficiencies in next-generation DSSCs, which will be discussed.

Interfacial electron transfer is a central process in numerous electrochemical and photoelectrochemical applications with a number of device-relevant platforms considering redox-active self-assembled monolayers on metal oxides. Sub-populations can result due to local steric and attachment site chemistries and configurations, each with unique energetic and kinetic properties. These sub-populations alter the potential-dependent response of the electrode, with the majority of the measured current flux funneled through electrical “hot spots,” or localized regions of lowest charge transfer resistance. Such heterogeneity can greatly convolute interpretation in a macroscale current-only measurement and have been attributed to over-potential requirements for many fundamental electrochemical reactions.

The ability to measure potential-dependent kinetics can yield enhanced insight into underlying mechanistic phenomena driving electron transfer at interfaces. Resolution of sub-population voltage-dependent kinetics is incredibly difficult using linear response circuit elements and classic electrochemical techniques including impedance-based methods, which only consider an average representation of the electrode. Optical-based spectroelectrochemical methods overcome these limitations, while simultaneously are independent of non-faradaic processes. Yet there has been very little evaluation of the effective rate of electron transfer, as a function of applied driving force, i.e. applied potential.

In this work, we show that by using a combination of optical and electrochemical characterization techniques, we are able to probe different sub-population kinetics of a complex, heterogeneous system comprised of a monolayer of zinc phthalocyanine functionalized with phosphonic acid (ZnPcPA) self-assembled onto an indium tin oxide (ITO). We also demonstrate within this work that we are able to (i) first measure the voltage-dependent electron transfer coefficient and (ii) show how rate coefficients vary for the different sub-populations using classic and quantum models. Importantly, our ZnPcPA model system represents a broader class of optically and electrochemically relevant monolayers that could include proteins, polymers, small molecules, and metal oxide sol-gels; the results presented here are expected to be applicable to numerous energy conversion systems relevant to both energy and environmental sustainability.

The proposal to create molecular analogs of circuit components dates back to the work of Aviram and Ratner from 1974,¹ where they suggested using a single molecule as a diode circuit element in giving birth to the field of molecular electronics. This field has advanced tremendously since then; nanoscale single-molecule devices are now also used as test beds for understanding and controlling electron transfer across metal/organic interfaces. Despite the long-standing interest in creating molecular diodes, their experimental realization has been difficult, with only a handful of studies showing rectification at the single molecule level. This is because most designs, such as the Aviram-Ratner model of rectification¹, rely on the complex interplay between many variables, such as the level alignments of the molecular components and the Fermi level of the metal electrodes. In this talk, I will review the scanning tunneling microscope break-junction technique we use to measure conductance through single molecule junctions² and then present our results illustrating different mechanisms to achieve rectification in single-molecule junctions. I will focus on different mechanisms focusing on the molecular contacts³, altering the electrode ⁴ and showing that quantum interference effects can be used to create asymmetrically coupled molecular junctions that rectify⁵. Finally, I will present new results that illustrate how a high on/off ratio can be created using an environmental gating technique.⁶

1. Aviram, A. & Ratner, M.A., Molecular Rectifiers. Chem. Phys. Lett. 29 (2), 277-283 (1974).
2. Venkataraman, L., Klare, J.E., Nuckolls, C., Hybertsen, M.S., & Steigerwald, M.L., Dependence of single-molecule junction conductance on molecular conformation. Nature 442 (7105), 904-907 (2006).
3. Batra, A. et al., Tuning Rectification in Single-Molecular Diodes. Nano Lett. 13 (12), 6233-6237 (2013).
4. Kim, T. et al., Determination of energy level alignment and coupling strength in 4,4'-bipyridine single-molecule junctions. Nano Lett 14 (2), 794-798 (2014).
5. Batra, A. et al., Molecular diodes enabled by quantum interference. Faraday Discussions 174 (0), 79-89 (2014).
6. Capozzi, B. et al., Single-molecule diodes with high rectification ratios through environmental control. Nat. Nano. 10 (6), 522-527 (2015).