Protection of inorganic semiconductors for sustained, efficient photoelectrochemical water oxidation

Abstract

Small-band-gap (E g < 2 eV) semiconductors must be stabilized for use in integrated devices that convert solar energy into the bonding energy of a reduced fuel, specifically H 2 (g) or a reduced-carbon species such as CH 3 OH or CH 4 . To sustainably and scalably complete the fuel cycle, electrons must be liberated through the oxidation of water to O 2 (g). Strongly acidic or strongly alkaline electrolytes are needed to enable efficient and intrinsically safe operation of a full solar-driven water-splitting system. However, under water-oxidation conditions, the smallband-gap semiconductors required for efficient cell operation are unstable, either dissolving or forming insulating surface oxides. We describe herein recent progress in the protection of semiconductor photoanodes under such operational conditions. We specifically describe the properties of two protective overlayers, TiO 2 /Ni and NiO x , both of which have demonstrated the ability to protect otherwise unstable semiconductors for >100 h of continuous solar-driven water oxidation when in contact with a highly alkaline aqueous electrolyte (1.0 M KOH(aq)). The 2 stabilization of various semiconductor photoanodes is reviewed in the context of the electronic characteristics and a mechanistic analysis of the TiO 2 films, along with a discussion of the optical, catalytic, and electronic nature of NiO x films for stabilization of semiconductor photoanodes for water oxidation.

Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO2–RuO2 Alloy Schottky Contacts for Silicon Photoanodes

“…1,2,[7][8][9][10][11][12][13] Incorporation of such protection layers into the MIS junction, however, has compromised the photovoltages commonly reported for these devices. An ideal MIS junction using silicon has a theoretical maximum open circuit voltage of 700-800 mV.…”

Operando Analyses of Solar Fuels Light Absorbers and Catalysts

“…Designs using large numbers of micro-and/or nano-dimensioned wires connected electrically in parallel hold the promise of improved stability because failure of an individual wire will not compromise the entire electrode. Recent efforts have beneficially improved the stability of technologically advanced semiconductors such as Si, GaAs, InP and GaP [11,13,23] in such solar fuels generator systems.…”

Kinetic Competition between Water‐Splitting and Photocorrosion Reactions in Photoelectrochemical Devices

“…For this reason, the applicationo fp rotectivef ilms, nanoparticles, and fusiblea lloys at the SC-electrolytei nterface are often used to enhance the durability and robustness of SCs that are prone to photocorrosion. [8][9][10][11] Generally,t he photocorrosion of SCs immersed in liquid electrolytes is studied on the basis of thermodynamic aspects, by comparing the quasi-Fermi levels of chargec arrierst ot he decomposition potentials of the SC. [12,13] Pourbaixd iagrams can also be used to investigate the stability and phase composition of the SCs at differentp otentials and pH.…”