Selective, Stable, Bias-Free, and Efficient Solar Hydrogen Peroxide Production on Inorganic Layered Materials

Hydrogen peroxide (H₂O₂) is one of the most significant chemicals and is in high industrial demand. Further, H₂O₂ is a potential energy carrier. Indeed, H₂O₂ has a comparable energy density (3.0 MJ L^-1 for 60% aqueous H₂O₂) to that of compressed hydrogen (H₂) gas (2.8 MJ L^-1 at 35 Mpa). Moreover, it is easily transported, owing to its high solubility in water, and can be produced in a centralized container on a very large scale. Currently, the anthraquinone process has been predominantly applied in industry to supply H₂O₂. However, this process is environmentally harmful and requires multi-step reactions with organic solvents, rare metal catalysts, and high energy inputs for hydrogenation and further oxidation of an alkylanthraquinone.<br/>Alternatively, electrochemical catalysis of metal complexes has been extensively investigated for eco-friendly production of H₂O₂. The key challenge here is the development of oxygen reduction reaction (ORR) catalysts that exhibit high selectivity and activity for a two-electron (2e^-) pathway instead of a four-electron (4e^-) pathway. Co complexes have an intrinsic advantage of catalyzing the 2e^- reduction of O₂ to H₂O₂. This is because Co has high <i>d</i>-electron counts and their single Co atom configurations can suppress 4e^- ORR, which requires multi-atom sites. Typically, Co porphyrin structures have been used as redox catalytic agents for 2e^- ORR. These Co-macrocycles have attracted attention because of their good activity and notable selectivity towards the 2e^-ORR. However, these molecular catalysts have the disadvantage of nitrogen ligand degradation in the presence of H₂O₂. Carbon-based materials have also been widely investigated as another type of 2e^- ORR catalysts. However, they are limited by their unstable long-term stability, owing to carbon corrosion at high overpotential. On the other hand, metal catalysts such as Pt–Hg, Pd–Hg, and Au–Pd nanoparticles provide adequate stability under harsh reaction conditions, In H₂O₂ generation reactions, metal catalysts with engineered active sites have demonstrated excellent catalytic activity and selectivity. However, the processes are difficult to scale up to an industrial level because of the high cost of the catalysts.<br/>Layered double hydroxides (LDHs) have been extensively investigated as promising electrocatalysts, owing to their unique two-dimensional lamellar structures and abundant constituent cations and anions. LDHs comprise a class of inorganic solids with metal hydroxide layers in which two or more types of metal cations are immobilized by hydroxide anion arrays and exchangeable interlayer anions. These inorganic solids are easily and economically synthesized in high purity and yield. Further, they are characterized by compositional variety, precise control of cation ratios, and the interspersion (rather than segregation) of cations within the hydroxide layers. Therefore, LDHs have great potential to tune active sites in the atomic scale for high catalytic efficiency.<br/>In this study, for the first time, we report a highly 2e^- ORR pathway-selective cobalt-containing MgAl [(Co)-MgAl] LDH-based ORR catalyst (near-100% 2e^- ORR selectivity), wherein the single atomic Co configuration can be realized by simply controlling the precursor ratios in the synthesis. Moreover, to use an abundant energy source, sunlight for H₂O₂ production, we designed a compartmented photo-electrochemical cell with a membrane separator, in which two catalytic systems (an LDH-based electrocatalyst for H₂O₂ production and an LDH-passivated organic-semiconductor-based photoanode for photovoltage generation) are integrated. The cell generated a concentration of H₂O₂ (~108.2 mM cm^-2_photoanode in 24 h) and a solar-to-chemical conversion efficiency (~3.24%) through sunlight illumination without any external bias. The concentration and solar-to-chemical conversion efficiency are the highest among all reported systems to the best of our knowledge.