Ni-(oxy)hydroxide-based materials are promising earthabundant 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 Ni 1−x Fe x OOH 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.

Iron-doped nickel (oxy)hydroxide catalysts (Fe x Ni 1−x OOH) exhibit high electrocatalytic behavior for the oxygen evolution reaction in base. Recent findings suggest that the incorporation of Fe 3+ 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 Fe x Ni 1−x OOH catalysts. Furthermore, the most active Fe x Ni 1−x OOH catalysts in this study exhibit activation energies that approach those previously reported for IrO 2 OER catalysts.

Efficient electrochemical splitting of H 2 O to O 2 and H 2 fuels has become an important goal in the quest for a renewable source of energy.[1] A major source of the inefficiency of this process is the significant overpotential associated with the anodic oxygen evolution reaction (OER). Understanding the mechanism of the OER could help in identifying the elementary processes contributing to OER overpotential and their relationship to the anode composition and morphology. While the OER has been investigated both experimentally and theoretically for over 50 years, its mechanism and the identity of the chemical intermediates involved remain uncertain. [2][3][4][5][6][7] Two principal pathways have been postulated for the OER on metal surfaces, such as gold (Au). The first involves a direct recombination of oxygen atoms to give O 2 , as shown in Equations (1)- (4):The second mechanism consists of a sequence of four consecutive one-electron oxidations, the first two of which are identical to those of the first mechanism, and the next two are as shown in Equations (5) and (6):In this case, the oxygen coupling step produces a hydroperoxy species (MÀOOH), which then dissociates to produce O 2 . Recent theoretical studies indicate that the second mechanism for oxygen coupling should be favored because it has a lower activation barrier. [4,5] Hydroperoxy species have also been suggested as key intermediates in the electrochemical reduction of O 2 (ORR), and initial experimental evidence for their presence has been presented. [8,9] It is notable nonetheless, that while OOH species have been envisioned to be critical for OER, the species have not been observed under electrochemical conditions.Herein we report the first spectroscopic identification of surface-bound OOH as intermediates of oxygen evolution reaction occurring on the surface of a gold catalyst. The presence of OOH species was observed by in situ electrochemical surfaceenhanced Raman spectroscopy (SERS) in both acidic and basic electrolytes. Roughened gold, rather than a more active catalyst such as platinum, was chosen for investigation because it is an excellent SERS substrate.[10] It was also anticipated that the decomposition of hydroperoxy species on Au might be slower than on more active metals, which would result in a higher accumulation of OOH species and facilitate their spectroscopic detection.A confocal Raman microscope coupled with a high numerical aperture water-immersion objective and 633 nm excitation was used to record these spectra. Real-time SER spectra of a Au electrode in 1 m HClO 4 during a linear voltammetry sweep from 1.0 to 1.65 V are shown in Figure 1. At 1.0 V, the Au surface is reduced, as can be seen from its relatively featureless SER spectrum. The peak at 934 cm À1 is assigned to the symmetric stretching mode of ClO 4 À .[11] The elevated spectral background is associated with high SERS activity exhibited by a metal surface, and has been previously assigned to photons emitted during the annihilation of inelastically scattered locali...

Water drops on Nafion films caused the surface to switch from being hydrophobic to being hydrophilic. Contact angle hysteresis of >70 degrees between advancing and receding values were obtained by the Wilhelmy plate technique. Sessile drop measurements were consistent with the advancing contact angle; the sessile drop contact angle was 108 degrees . Water drop adhesion, as measured by the detachment angle on an inclined plane, showed much stronger water adhesion on Nafion than Teflon. Sessile water and methanol drops caused dry Nafion films to deflect. The flexure went through a maximum with time. Flexure increased with contact area of the drop, but was insensitive to the film thickness. Methanol drops spread more on Nafion and caused larger film flexure than water. The results suggest that the Nafion surface was initially hydrophobic but water and methanol drops caused hydrophilic sulfonic acid domains to be drawn to the Nafion surface. Local swelling of the film beneath the water drop caused the film to buckle. The maximum flexure is suggested to result from motion of a water swelling front through the Nafion film.

Ni 1−x Fe x OOH 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 Ni 1−x Fe x OOH thin films. Electrochemical cycling converts sputtered Ni 1−x Fe x 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 Ni 1−x Fe x 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 Ni 1−x Fe x OOH 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 Ni 1−x Fe x OOH. These findings enable direct application of Ni 1−x Fe x OOH sputtered films to a wider library of photoanodes for light-driven water-splitting applications.

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.

Addition of Fe to Ni-and Co-based (oxy)hydroxides significantly enhances the activity of these materials for electrochemical oxygen evolution reaction (OER). Here, we show that Fe cations bound to the surface of oxidized Au enhance its OER activity, the OER activity increasing with increasing surface concentration of Fe. Density functional theory analysis of the energetics of the OER revealed that oxygen evolution over Fe cations bound to a hydroxylterminated oxidized Au surface (Fe-Au2O3) occurs at an overpotential 0.43 V lower than that at which the OER occurs on hydroxylated Au2O3 (0.86 V). This finding agrees very well with experimental observation and is a consequence of the more optimal binding energetics for the 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" Au(III) surfaquo species.3

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, NiCo 2 O 4 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 NiCo 2 O 4 . 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.