Ni1-xFexOOH thin films prepared via cathodic electrodeposition have been demonstrated to be highly active catalysts for the oxygen evolution reaction (OER) in basic media. Integration of these catalysts with light-absorbing semiconductors is required for photoelectrochemical fuel generation. However, the application of cathodic potentials required for typical electrochemical catalyst deposition limits the library of compatible photoanode materials. Sputter deposition of catalysts circumvents this limitation by enabling facile catalyst layering without cathodic potentials. In this work, we compare the structure and OER activity of sputter-deposited and electrodeposited Ni1-xFexOOH thin films. Electrochemical cycling converts sputtered Ni1-xFex metallic films to the desired oxides/(oxy)hydroxides. Both film preparation methods give catalysts with similar electrochemical behavior across all compositions. Additionally, OER activity is comparable between the deposition methods, with maximum activity for films with ∼20% Fe content (320 mV overpotential at j = 10 mA cm-2 geometric). Electrochemical cycling to convert sputtered metallic Ni1-xFex films to metal oxides/(oxy)hydroxides is found to lower the Fe/Ni ratio, while the electrodeposited films exhibit comparable Fe/Ni ratios before and after electrochemical cycling and characterization. Structurally, Fe is found to incorporate within the Ni(OH)2/NiOOH lattice for films formed through both sputter-deposition and electrodeposition. Layered films were also compared to codeposited 1:1 Fe/Ni films. It is found that, for layered films, an Fe top layer inhibits the electrochemical conversion of metallic Ni to Ni(OH)2/NiOOH, thus reducing the amount of Ni1-xFexOOH OER-active phase formed. In contrast, migration of metals within Ni-on-top films occurs readily during electrochemical cycling, resulting in films that are structurally and electrochemically indistinguishable from codeposited Ni1-xFexOOH. These findings enable direct application of Ni1-xFexOOH sputtered films to a wider library of photoanodes for light-driven water-splitting applications.

Iron-doped nickel (oxy)hydroxide catalysts (FexNi1-xOOH) exhibit high electrocatalytic behavior for the oxygen evolution reaction in base. Recent findings suggest that the incorporation of Fe³⁺ into a NiOOH lattice leads to nearly optimal adsorption energies for OER intermediates on active Fe sites. Utilizing electrochemical impedance spectroscopy and activation energy measurements, we find that pure NiOOH and FeOOH catalysts exhibit exceedingly high Faradaic resistances and activation energies 40-50 kJ/mol^-1 higher than those of the most active FexNi1-xOOH catalysts. Furthermore, the most active FexNi1-xOOH catalysts in this study exhibit activation energies that approach those previously reported for IrO2 OER catalysts.

Ni (oxy)hydroxide based materials are promising earth abundant catalysts for electrochemical water oxidation in basic media. Recent findings demonstrate that incorporation of trace Fe impurities from commonly used KOH electrolytes significantly improves oxygen evolution reaction (OER) activity over NiOOH electrocatalysts. Because nearly all previous studies detailing structural differences between α Ni(OH)2/γ NiOOH and β Ni(OH)2/β NiOOH were completed in unpurified electrolytes, it is unclear whether these structural changes are unique to the aging phase transition in the Ni (oxy)hydroxide matrix or if they arise fully or in part from inadvertent Fe incorporation. Here, we report an investigation of the effects of Fe incorporation on structure activity relationships in Ni (oxy)hydroxide. Electrochemical, in situ Raman, X ray photoelectron spectroscopy, and electrochemical quartz crystal microbalance measurements were employed to investigate Ni(OH)2 thin films aged in Fe free and unpurified (reagent grade) 1 M KOH (<1 ppm Fe). We find that Ni films aged in unpurified electrolyte can incorporate ≥20% Fe after 5 weeks of aging, and the maximum catalyst activity is comparable to that reported for optimized Ni1 xFexOOH catalysts. Conversely, Fe free Ni(OH)2 films exhibit a substantially lower activity and higher Tafel slope for the OER. Films aged in Fe free electrolyte are predominantly disordered β Ni(OH)2/β NiOOH if maintained below 0.7 V vs Hg/HgO in 1 M KOH and will "overcharge" to form a mixture of γ and β NiOOH above this potential. Fe containing Ni(OH)2 films evidence a lesser extent of β Ni(OH)2 formation and instead exhibit NiOOH structural changes in accordance with the formation of a NiFe layered double hydroxide phase. Furthermore, turnover frequency calculations indicate that Fe is the active site within this phase, and above ∼11% Fe content, a separate, Fe rich phase forms. These findings are the first to demonstrate the in situ changes in the catalyst structure resulting from the incorporation of Fe electrolyte impurities within Ni (oxy)hydroxide, providing direct evidence that a Ni Fe layered double (oxy)hydroxide (LDH) phase is critical for high OER activity.

Addition of Fe to Ni- and Co-based (oxy)hydroxides has been shown to enhance the activity of these materials for electrochemical oxygen evolution. Here we show that Fe cations bound to the surface of oxidized Au exhibit enhanced oxygen evolution reaction (OER) activity. We find that the OER activity increases with increasing surface concentration of Fe. Density functional theory analysis of the OER energetics reveals that oxygen evolution over Fe cations bound to a hydroxyl-terminated oxidized Au (Fe-Au2O3) occurs at an overpotential ∼0.3V lower than over hydroxylated Au2O3 (0.82V). This finding agrees well with experimental observations and is a consequence of the more optimal binding energetics of OER reaction intermediates at Fe cations bound to the surface of Au2O3. These findings suggest that the enhanced OER activity reported recently upon low-potential cycling of Au may be due to surface Fe impurities rather than to "superactive" AuIII surfaquo species.

In this paper, we propose that a p-type transparent conducting oxide (p-TCO) can function as a selective hole contact and corrosion protection layer on photoanodes used for light-driven water oxidation. To prove the concept, NiCo2O4 was used as the p-TCO for n-Si protection in alkaline condition, and we show that this material has the requisite electronic structure, stability, transparency, and hole conductivity to achieve sustained and efficient solar water oxidation. The photoelectrochemical performance demonstrates the attractive combination of transparency and low-resistance hole conductivity in the NiCo2O4. Long-term testing indicates multi-day stability with minimal decrease in performance or observable corrosion of the Si photoanode. This works suggests that p-TCOs are promising as corrosion protection layers for stable water oxidation photoanodes.

Achieving stable operation of photoanodes used as components of solar water splitting devices is critical to realizing the promise of this renewable energy technology. It is shown that p-type transparent conducting oxides (p-TCOs) can function both as a selective hole contact and corrosion protection layer for photoanodes used in light-driven water oxidation. Using NiCo2O4 as the p-TCO and n-type Si as a prototypical light absorber, a rectifying heterojunction capable of light driven water oxidation was created. By placing the charge separating junction in the Si using a np(+) structure and by incorporating a highly active heterogeneous Ni-Fe oxygen evolution catalyst, efficient light-driven water oxidation can be achieved. In this structure, oxygen evolution under AM1.5G illumination occurs at 0.95 V vs RHE, and the current density at the reversible potential for water oxidation (1.23 V vs RHE) is >25 mA cm(-2). Stable operation was confirmed by observing a constant current density over 72 h and by sensitive measurements of corrosion products in the electrolyte. In situ Raman spectroscopy was employed to investigate structural transformation of NiCo2O4 during electrochemical oxidation. The interface between the light absorber and p-TCO is crucial to produce selective hole conduction to the surface under illumination. For example, annealing to produce more crystalline NiCo2O4 produces only small changes in its hole conductivity, while a thicker SiOx layer is formed at the n-Si/p-NiCo2O4 interface, greatly reducing the PEC performance. The generality of the p-TCO protection approach is demonstrated by multihour, stable, water oxidation with n-InP/p-NiCo2O4 heterojunction photoanodes.