The Advanced Energy Materials Laboratory at the Colorado School of Mines focuses on the characterization and deployment of nanostructured materials to improve energy conversion technologies including fuel cells and solar cells. This presentation will discuss two aspects of our recent research with particular relevance for fuel cell and fuel reforming applications: 1) doped carbon-supports for enhanced catalytic applications and 2) high temperature steam permeation in co-ionic ceramic materials. 1) Doped carbon supports for enhanced catalysis. Recent research suggests that catalytic activity of Pt nanoparticles towards a variety of low-temperature fuel-cell relevant reactions is significantly enhanced (by 3-10X) when a nitrogen-doped carbon support is used in place of a standard carbon support. This level of enhancement brings the technological promise of low-temperature fuel cells much closer to reality. Furthermore, the basic idea of catalyst-support doping opens the door to a game-changing new method for catalyst design, with applications not only in fuel cells, but also in industrial chemistry, biology, energy, and even medicine. While the reason for this surprising dopant-induced enhancement effect have so far remained uncertain, in this presentation we provide both experimental and theoretical results that clarify the situation by using highly ordered pyrolytic graphite (HOPG) as a model support. Our experimental results provide clear and compelling evidence that N-doping can be used to substantially enhance the catalytic activity (by a factor of 3-6 fold) and durability (by a factor of 10 fold) of carbon-suppported Pt catalysts. Supporting experimental evidence and theoretical calculations to show that the activity enhancement is due to a chemical doping effect, where N-HOPG donates charge to Pt clusters, changing the d-band electronic structure, and therefore the catalytic activity. Our theoretical calculations suggest that B-doping at low levels will have a similar impact as high-doses of N-doping. Therefore, a predictive model has been proposed, and experimental confirmation is ongoing. 2) High temperature steam permeation in co-ionic ceramics. Selective steam transport in dense proton-conducting ceramic membranes has been recently been hypothesized, but has so far never conclusively demonstrated experimentally. If verified, high-temperature ceramic steam-permeation membranes (SPMs) would have the potential to significantly improve the efficiency of a variety of energy conversion technologies including solid oxide fuel cells, membrane reformers, and gasification. In this presentation, we provide direct experimental evidence for steam permeation in yttrium-doped barium zirconate (BZY20) and discuss the mechanism of steam permeation in this material based on cell permeation and hydration dilatometry measurements over a temperature range of 20° - 1100°C.

Introduction: Considerable attention is being focused on the synthesis and characterisation of new electrolyte materials with high proton conductivities at temperatures in the range 200-600°C. The lack of suitable materials currently limits the development of intermediate temperature range fuel cells [1]. The development and improvement of material conduction properties necessitates an understanding of the proton transport mechanism. Several models have been suggested to rational the correlation between structure and conductivity of perovskite oxides. Recently, Imashuku et al. [2] showed that proton conductivity can be improved by co-doping at B-site. However, this study was carried out on lightly doped systems. The concentration of protons in the material depends on the doping level of acceptor dopant. In this context we are aiming to investigate the crystal structure, microstructure and proton conductivity of heavily doped perovskite oxides, BaZr_0.5In_0.25Yb_0.25O_3-δ. The material was prepared via a traditional solid state sintering route. The results of x-ray powder diffraction, scanning electron microscopy (SEM), dynamic thermogravimetric analysis (TGA), and impedance spectroscopy will be discussed.Results:The unit cell parameter a, was determined to be 4.1991(5) Å with average P m-3m symmetry. Dynamic TGA on pre-protoneted sample shows that mass loss occurred beyond 300°C. The mass loss was 1.18‰ which implies that 78‰ of the protonic defects (OH^●) were filled during hydration. The proton conductivity in this sample was investigated by using 2- probe AC impedance spectroscopy. Nyqvist plot of pre-protonated sample showed that grain-boundary conductivity is significantly higher than the bulk conductivity, which is typical for ziconate systems. References:[1] T. Norby, Solid State Ionics 125(1999) 1-11.[2] S. Imashuku et al., J. Electrochem. Soc., 155 (2008) B 581-B586.

The effects of the anode functional layer (AFL) for improving the performance of solid oxide fuel cells have been examined by inserting the NiO/YSZ-YSZ functional layer between the NiO-YSZ anode and YSZ electrolyte. The dual NiO/YSZ-YSZ composite powders synthesized by the polymerizable-complex method could be well described as the composite of nano-sized NiO and YSZ particles adhered to core YSZ particles, and their mean particle size and surface area were 0.4 µm and 39.0 m2/g, respectively. The AFL was fabricated by dip-coating the anode substrate with the slurry of composite powders, and the anode supported YSZ electrolyte was also prepared by dip-coating the composite anode with the YSZ slurry, followed by sintering at 1400°c. It was concluded that the composite AFL could minimize the possibility of defect occurrence between anode and electrolyte layers during processing as well as increase the number of the three phase boundary(TPB) considerably. The unit cell with the AFL of 8 µm thickness in conjunction with the LSM-YSZ cathode showed the maximum power density of 1.58 W/cm2 at 800°c in hydrogen (3% H2O), which is about 3 times higher than that of the unit cell without the AFL layer, and an excellent cell durability.

As it is well know that the dimension and the types of materials affect the performance of the SOFC. Hence a different anode material Ti:YSZ (anode) compatible thermal expansion coefficient with YSZ (electrolyte) was synthesized by emulsion precipitation followed by heat treatment. Nano-set ball milling of the powder was carried out in ethanol media. The phase, morphology and the porosity of the powder were analyzed by XRD, SEM, TEM and BET. YSZ pellet were prepared (99% of theoretical density) by cold iso-static uniaxial pressing with varying thickness, Ti: YSZ were coated on the pellets by screen printing process. Impedance spectroscopy was carried out at different temperatures of the symmetry cell and the half cell. Impedance measurement data were collected using different suitable circuit elements and plotted for analysis. The effect of relative thickness of the YSZ and the Ti: YSZ to the cell performance at intermediate temperature has been discussed in detail.

Metallic alloys are massively employed in energy generation devices subjected to high temperatures such as gas turbines or more recently interconnects in solid oxide fuel cells (SOFC). However, an oxide scale inevitably grows at the oxygen-exposed surface, accompanied in most cases by the development of stresses and non-flat interfaces. While these phenomena limit the lifetime of such systems, the underlying mechanisms are still poorly understood which prevents material optimization.This work investigates the critical oxide/metal interface evolution. A framework has been developed along with a finite element-based numerical code for the propagation of the phase boundary coupled with stress development. The model considers a diffusion-driven process with mass consumption and it incorporates the large volumetric strain associated with phase transformation. A continuum mechanics description of a dissipative sharp interface propagation is derived to include the influence of the stress state and accommodation work on the phase boundary motion kinetics. The implementation of this complex formulation and its framework required the development of an original numerical scheme allowing a field-dependent propagation of a sharp interface. A finite element method is used for the strain/stress and concentration fields resolution, coupled with an external routine for calculating the interface composition, the effects of fast diffusion and the phase boundary propagation. The oxidation of a chromia-forming SOFC interconnect material is simulated and stress and morphological developments are investigated, with a particular focus on the influence of the known reactive element effects. The mechanisms likely leading to mechanical failure are discussed.

We report the use of Environmental Scanning Electron Microscopy, Laser Ablation Mass Spectrometry (ICP-MS), and high resolution X-ray Computed Tomography (XCT) to observe internal changes in the catalyst/ionomer morphology and catalyst distribution in the membrane and electrode assemblies (MEAs)/gas diffusion electrodes (GDE) after they have been subjected to various operating conditions and forms of testing. (e.g such as drive cycle testing) Membrane and electrode assemblies consisting of both conventional Pt-carbon catalyst mixtures and various non-precious metal catalysts were studied before and after fuel cell testing. Voltage cycling through potentials of 0.8 V causes Pt electrocatalyst sintering and consequently losses in electro-active surface area. Carbon corrosion is greatly accelerated when the FC is subjected to harsh start-up/shut-down conditions. In the case of non-precious catalysts, we are probing the catalyst/ionomer migration into the gas diffusion layer at various hot pressing temperatures. XCT imaging has the advantages of being non-invasive, requiring little sample preparation, not requiring the sample to be conductive, and not disturbing the microstructure of the sample when compared to conventional SEM or TEM. Elemental analysis via ICP-MS and X-ray analysis from the ESEM will be used to study catalyst and impurity distributions within MEAs at various stages of fuel cell lifecycle testing procedures.

The United States department of energy has placed hydrogen fuel cell research and development at the heart of its drive for energy independence. The current key barriers preventing commercialization of automotive-scale fuel cells are cost and durability of materials. Current materials used for catalyst support for electrocatalytic applications, such as carbon black, are continuously exposed to highly oxidative environments, which lead to carbon corrosion. Ceramic materials are highly stable in corrosive environments in comparison to carbon materials; however making inexpensive, conductive ceramic materials with adequate electronic conductivity, and controlled pore structure remains a major challenge. Corrosion-resistant mesoporous ceramic/carbon nanocomposites can be designed and fabricated by incorporating the conductivity of low-cost carbon materials with the chemical robustness of ceramic inorganic materials, thus creating a family of conductive, corrosion-resistant, chemically stable, porous nanocomposites which can be used for the next generation of durable electrocatalyst supports.
Random disorder porous nanocomposites have been synthesized using a simple self-assembly process with a ceramic cluster (hydrolyzed tetraethylorthosilicate), a carbon precursor (sucrose), and porogens (zinc chloride). Pore size control was achieved by tuning the concentration of activation reagent, in which increasing the amount of ZnCl2 resulted in higher porosity (pore volume 0.1-0.3 cm3/g) and larger pore size (2-3nm). In addition, carbon/ceramic nanocomposites with ordered crystalline pore structures were constructed from a sol gel route using a surfactant (pluronic F127), inorganic cluster (hydrolyzed tetraethylorthosilicate), and oligomeric assembling blocks (phenol). Subsequent carbonization at high temperature produces both disordered and ordered mesoporous nanocomposites with controlled composition and pore structure. Preliminary conductivity studies concluded that an increase in carbon content leads to higher conductivity. When the carbon content is higher than 56%, the conductivity of carbon/silica nanocomposite can reach 6.6 S/cm, which is adequate for current fuel cell device application. Promising corrosion resistance was obtained from the carbon/silica nanocomposites. The weight loss under oxidative environment is reduced by ten times with the addition of silica while the nanocomposites is able to retain a high enough electrical conductivity. The fabrication of these nanocomposites provides a new family of porous nanocomposites for many potential applications, such as adsorbents, catalyst supports and hydrogen storage.

Many researchers have investigated to develop a suitable anode catalyst for low temperature fuel cells such as DMFC and PEMFC. A large amount of precious noble metals particularly platinum is used for electrooxidation which makes the cost of DMFC and PEMFC prohibitively very high. Considering this fact, tungsten carbides have been considered as an alternating electrocatalyst at the anode and cathode electrode of low temperature fuel cells. Because they have shown high activities for electro-oxidation of hydrogen, methanol with minimal loading of noble metals or without them due to their platinum-like behaviors in catalysis. We have fabricated the various structures and phases of tungsten carbides preserved the excellent physicochemical properties and electro-activities via polymer induced carburization method. All electrocatalysts were characterized by XRD, PSD, SEM, HRTEM, LSV, CV and Single cell test so on. The fabricated tungsten carbides were corresponded to microporous and mesoporous W2C microspheres and mesoporous WC nanoparticle. Small amount of noble metals (less 10wt%) was loaded on these tungsten carbides via conventional chemical reduction and polyol method. Regarding the electrochemical evaluation, these electrocatalysts showed the higher electrochemical active surface area (EAS) values represented the hydrogen oxidation ability, and electro-activity for methanol oxidation reaction (MOR) than commercial Pt/C and PtRu/C electrocatalysts, respectively. Especially, 7wt% Pt loaded tungsten carbides resulted in the higher EAS values by factors of 3.0-3.5 than that of commercial 20wt% Pt/C (E-Tek) catalyst. And, they also displayed the high CO tolerance superior to the commercial Pt/C in 1% CO dissolved electrolyte. In addition, mass activity (mA/mg of Pt taken at 0.75V – Ag/AgCl) of all 7wt% Pt loaded tungsten carbide based catalysts showed the 2.5-3.42 times higher activity for methanol oxidation than commercial 20wt% PtRu/C (E-Tek) catalyst. Particularly, the single cell performances of small amount of noble metals loaded tungsten carbides for methanol oxidation will be firstly demonstrated in this presentation. These enhanced electro-activities may be originated from the synergistic effect between platinum and tungsten carbides including the intrinsic properties of tungsten carbides. Tungsten carbides could become a noble metal economic electrocatalyst for low temperature fuel cells.

The proton exchange membrane (PEM) fuel cell has become a central focus within the alternative energy industry. Its high power density and efficiency, along with environmental benefits and relatively low operation temperature make the PEM fuel cell an especially ideal power source for transportation applications. However, this promising technology is still not economically feasible for use in automobiles, being significantly limited by the PEM material itself, which degrades under the acidic operating conditions. To meet commercialization targets set by the Department of Energy for 2010, a radical new approach is needed for locating materials with markedly improved durability and performance-to-price characteristics. The extensive selection of low-cost polymers and polymer combinations, coupled with the broad effects of polymer processing conditions present an engineering challenge that is well-suited for high-throughput screening. In this work we describe the development and implementation of an efficient and thorough approach for screening these variables using high-throughput characterization of key fuel cell performance indicators. These include mechanical strength, proton conductivity, water transport, and chemical resistance. By incorporating combinatorial film synthesis techniques to produce 1-D and 2-D sample libraries, we have shown that we are able to screen and optimize processing properties such as composition ratio, thickness, annealing temperature, and cross-linking density with respect to the key fuel cell performance indicators in a cost-effective manner. From this screening, we can determine potential PEM candidates suitable for more detailed performance analysis. Of particular interest to this study are polyimide blends which are low cost and can be tuned for mechanical strength and durability through cross-linking density.

Approach to targeted synthesis of promising superionic materials on the base of complex phosphates of Group III elements (In and Sc) was developed. New phases of high ionic conductivity were obtained using results of the systematic study on binary and ternary systems and crystal chemistry analysis of ionic transport in complex phosphates with NASICON-like three-dimensional framework. Heterovalent substitution into phosphates cation sublattice realized according to scheme M³⁺→M⁴⁺+v (where v-is a vacancy) allows increasing conducting cation (Li, Na) mobility and tracing structure motif evolution.Samples were prepared through every 2-10 mol%, annealed at 800, 850, 900 and 1050°C during 200-400 h, quenched in liquid nitrogen and then were subject of powder X-ray diffraction analysis. Temperature dependence of ionic conductivity of NASICON-like Li₃In₂(PO₄)₃ solid solutions and heterovalent Zr-substituted Na₃Sc₂(PO₄)₃ samples was studied by impedance spectroscopy.Ionic conductivity of a solid solutions sample Li_3-xIn_2+1/3x(PO₄)₃ (x = 0.8) was calculated to be 0.44x10^-2 S/cm at 300°C. This value is comparable to that of well-known NASICON-like compounds such as Li₃Fe₂(PO₄)₃. Heterovalent Zr-substitution of Na₃Sc₂(PO₄)₃ according to scheme Sc³⁺→Zr⁴⁺+v leads to solid solutions formation up to 10 mol% Zr, while further substitution up to 20 mol% results in formation of a secondary phase. Ionic conductivity of Na₃Sc₂(PO₄)₃ (1.6x10^-2 S/cm at 300°C) was considered to be the best among NASICON-like complex phosphates. However, that of 10 mol% Zr-substituted sample obtained in this work was calculated to be 3.18x10^-1 S/cm at 300°C, which is one order of magnitude higher than ionic conductivity of pure Na₃Sc₂(PO₄)₃.Summarizing, as-obtained heterovalent substituted Na₃Sc₂(PO₄)₃ phase can be proposed as a promising solid-state electrolyte for applications working at elevated temperature.