Cobalt oxides and (oxy)hydroxides have been widely studied as electrocatalysts for the oxygen evolution reaction (OER). For related Ni-based materials, the addition of Fe dramatically enhances OER activity. The role of Fe in Co-based materials is not well-documented. We show that the intrinsic OER activity of Co(1-x)Fe(x)(OOH) is ∼100-fold higher for x ≈ 0.6-0.7 than for x = 0 on a per-metal turnover frequency basis. Fe-free CoOOH absorbs Fe from electrolyte impurities if the electrolyte is not rigorously purified. Fe incorporation and increased activity correlate with an anodic shift in the nominally Co(2+/3+) redox wave, indicating strong electronic interactions between the two elements and likely substitutional doping of Fe for Co. In situ electrical measurements show that Co(1-x)Fe(x)(OOH) is conductive under OER conditions (∼0.7-4 mS cm(-1) at ∼300 mV overpotential), but that FeOOH is an insulator with measurable conductivity (2.2 × 10(-2) mS cm(-1)) only at high overpotentials >400 mV. The apparent OER activity of FeOOH is thus limited by low conductivity. Microbalance measurements show that films with x ≥ 0.54 (i.e., Fe-rich) dissolve in 1 M KOH electrolyte under OER conditions. For x < 0.54, the films appear chemically stable, but the OER activity decreases by 16-62% over 2 h, likely due to conversion into denser, oxide-like phases. We thus hypothesize that Fe is the most-active site in the catalyst, while CoOOH primarily provides a conductive, high-surface area, chemically stabilizing host. These results are important as Fe-containing Co- and Ni-(oxy)hydroxides are the fastest OER catalysts known.

Fe cations dramatically enhance oxygen evolution reaction (OER) activity when incorporated substitutionally into Ni or Co (oxy)hydroxides, serving as possible OER active sites. Pure Fe (oxy)hydroxides, however, are typically thought to be poor OER catalysts and are not well-understood. Here, we report a systematic investigation of Fe (oxy)hydroxide OER catalysis in alkaline media. At low overpotentials of ∼350 mV, the catalyst dissolution rate is low, the activity is dramatically enhanced by an AuO x /Au substrate, and the geometric OER current density is largely independent of mass loading. At higher overpotentials of ∼450 mV, the dissolution rate is high, the activity is largely independent of substrate choice, and the geometric current density depends linearly on loading. These observations, along with previously reported in situ conductivity measurements, suggest a new model for OER catalysis on Fe (oxy)hydroxide. At low overpotentials, only the first monolayer of the electrolyte-permeable Fe (oxy)hydroxide, which is in direct contact with the conductive support, is OER-active due to electrical conductivity limitations. On Au substrates, Fe cations interact with AuO x after redox cycling, leading to enhanced intrinsic activity over FeOOH on Pt substrates. At higher overpotentials, the conductivity of Fe (oxy)hydroxide increases, leading to a larger fraction of the electrolyte-permeable catalyst film participating in catalysis. Comparing the apparent activity of the putative Fe active sites in/on different hosts/surfaces supports a possible connection between OER activity and local structure.

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Protecting Si photocathodes from corrosion is important for developing tandem water-splitting devices operating in basic media. We show that textured commercial Si-pn(+) photovoltaics protected by solution-processed semiconducting/conducting oxides (plausibly suitable for scalable manufacturing) and coupled to thin layers of Ir yield high-performance H2-evolving photocathodes in base. They also serve as excellent test structures to understand corrosion mechanisms and optimize interfacial electrical contacts between various functional layers. Solution-deposited TiO2 protects Si-pn(+) junctions from corrosion for ∼24 h in base, whereas junctions protected by F:SnO2 fail after only 1 h of electrochemical cycling. Interface layers consisting of Ti metal and/or the highly doped F:SnO2 between the Si and TiO2 reduce Si-emitter/oxide/catalyst contact resistance and thus increase fill factor and efficiency. Controlling the oxide thickness led to record photocurrents near 35 mA cm(-2) at 0 V vs RHE and photocathode efficiencies up to 10.9% in the best cells. Degradation, however, was not completely suppressed. We demonstrate that performance degrades by two mechanisms, (1) deposition of impurities onto the thin catalyst layers, even from high-purity base, and (2) catastrophic failure via pinholes in the oxide layers after several days of operation. These results provide insight into the design of hydrogen-evolving photoelectrodes in basic conditions, and highlight challenges.

Thin films with tunable and homogeneous composition are required for many applications. We report the synthesis and characterization of a new class of compositionally homogeneous thin films that are amorphous solid solutions of AlO and transition metal oxides (TMO) including VO, CrO, MnO, FeO, CoO, NiO, CuO, and ZnO. The synthesis is enabled by the rapid decomposition of molecular transition-metal nitrates TM(NO) at low temperature along with precondensed oligomeric Al(OH)(NO) cluster species, both of which can be processed from aq solution. The films are dense, ultrasmooth (R < 1 nm, near 0.1 nm in many cases), and atomically mixed amorphous metal-oxide alloys over a large composition range. We assess the chemical principles that favor the formation of amorphous homogeneous films over rougher phase-segregated nanocrystalline films. The synthesis is easily extended to other compositions of transition and main-group metal oxides. To demonstrate versatility, we synthesized amorphous VCrMnFeZnAlO and VCrFeAlO with R ≈ 0.1 nm and uniform composition. The combination of ideal physical properties (dense, smooth, uniform) and broad composition tunability provides a platform for film synthesis that can be used to study fundamental phenomena when the effects of transition metal cation identity, solid-state concentration of d-electrons or d-states, and/or crystallinity need to be controlled. The new platform has broad potential use in controlling interfacial phenomena such as electron transfer in solar-cell contacts or surface reactivity in heterogeneous catalysis.