Discovery and Characterization of a Pourbaix-Stable, 1.8 eV Direct Gap Bismuth Manganate Photoanode

Abstract

Solar-driven oxygen evolution is a critical technology for renewably synthesizing hydrogen-and carboncontaining fuels in solar fuel generators. New photoanode materials are needed to meet efficiency and stability requirements, motivating materials explorations for semiconductors with (i) band-gap energy in the visible spectrum and (ii) stable operation in aqueous electrolyte at the electrochemical potential needed to evolve oxygen from water. Motivated by the oxygen evolution competency of many Mn-based oxides, the existence of several Bi-containing ternary oxide photoanode materials, and the variety of known oxide materials combining these elements with Sm, we explore the Bi−Mn−Sm oxide system for new photoanodes. Through the use of a ferri/ferrocyanide redox couple in high-throughput screening, BiMn 2 O 5 and its alloy with Sm are identified as photoanode materials with a nearideal optical band gap of 1.8 eV. Using density functional theory-based calculations of the mullite BiO 5 phase, we identify electronic analogues to the well-known BiVO 4 photoanode and demonstrate excellent Pourbaix stability above the oxygen evolution Nernstian potential from pH 4.5 to 15. Our suite of experimental and computational characterization indicates that BiMn 2 O 5 is a complex oxide with the necessary optical and chemical properties to be an efficient, stable solar fuel photoanode. ■ INTRODUCTIONTo photoelectrochemically synthesize fuel, solar fuel generators couple the oxygen evolution reaction (OER) to a fuel-forming reaction, typically hydrogen evolution or carbon dioxide reduction.1 Solar fuel generation comprises a promising renewable energy technology and is particularly desirable among renewable energy technologies due to the high-energydensity storage and transportation of chemical fuels. While efficient solar fuel generators can be created by use of tandem photoelectrochemical (PEC) cells, which combine an OER photoanode and a fuel-forming photocathode, widespread deployment of this technology is impeded by several technological challenges, most notably the development of an efficient and stable photoanode. 2−6While substantial technology development has proceeded through utilization of highly stable metal oxide photoanodes such as TiO 2 and WO 3 , the optical absorption of these semiconductors is limited to the ultraviolet spectrum, substantially limiting solar conversion efficiency. Substantial effort has also been placed in the development of α-Fe 2 O 3 , which is remarkable for its combination of high stability under OER photoanode operating conditions and relatively low band-gap energy near 2 eV. 7,8 While the performance of α-Fe 2 O 3 is limited by intrinsic charge-transport properties, it remains a fixture in solar fuel research because, until the present work, no other material has been shown to rival its stability and desirable band-gap energy. By combining high-throughput experimentation and materials theory, we identify BiMn 2 O 5 as a photoanode that meets these stringent requirements. With Pou...

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