Nickel–iron oxyhydroxides (Ni1−xFexOOH) are non-precious metal electrocatalysts for the oxygen evolution reaction (OER) that have high efficiency in alkaline media.

Layered double hydroxide (LDH) and amorphous nickel−iron (oxy)hydroxides (Ni 1−x Fe x OOH) are emerging catalysts for the electrochemical oxygen evolution reaction (OER). It is still unresolved if the layered twodimensional (2D) structure allows for active catalytic sites to exist below the traditional electrode/electrolyte interface. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) to directly measure active site densities in situ. We determined that Ni 0.8 Fe 0.2 OOH LDH showed a 10-fold increase in the active site density compared to rock salt Ni 0.8 :Fe 0.2 oxide, giving direct evidence that water and hydroxide in the interlayer are able to create stable Ni IV /Fe IV active species at layers below the electrode/ electrolyte interface. This result suggests that electrolyte permeability of the 2D LDH structure is a major contributor for its increased catalytic activity. Amorphous Ni 0.8 :Fe 0.2 oxide also exhibits an anomalously high active site density and higher activity than Ni 0.8 Fe 0.2 OOH LDH.

The renewable production of green hydrogen powered by water electrolysis will be an important step in the electrification of the chemical industry. However, to make water-splitting more sustainable and practical, earth-abundant catalysts need to be developed, which can both be synthesized using the principles of green chemistry and have high performance specifically at high hydrogen production rates. In this work, we report four main findings to help contribute toward this goal. First, we report a "green" synthesis method for producing a mixed-metal oxide catalyst that uses only water as the solvent and no harsh oxidizing or reducing agents. Second, we show that this synthesis method can enable an amorphous nickel−iron oxide/(oxy)hydroxide catalyst with a 1:1 Fe/ Ni ratio. This increased iron content further improves the performance over the conventional 1:4 Fe/Ni ratio. Third, we show that these catalysts can be easily deposited on a 3D porous Ni-foam electrode and achieve current densities up to 1 A cm −2 and an overpotential of 245 mV at 100 mA cm −2 for oxygen evolution reaction (OER) and an overpotential of 422 mV at 100 mA cm −2 for hydrogen evolution reaction (HER). Finally, we show that combining both HER and OER catalysts, synthesized with our method, in a flow-through water electrolyzer achieves an overpotential of 140 mV at 100 mA cm −2 at 80 °C. In addition, this electrolyzer can achieve 76% efficiency at 1 A cm −2 and 70% efficiency at 2 A cm −2 .

It is experimentally challenging to deconvolute the potential‐dependent adsorption of the different intermediates that occur during the hydrogen evolution reaction (HER) in alkaline media. This difficulty has limited our understanding regarding why the HER kinetics are more sluggish in alkaline media compared to acidic media. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI‐SECM) to investigate the surface adsorbed species that form during the HER in alkaline media on polycrystalline platinum, Pt(poly). To deconvolute the different adsorbed intermediates, we developed a detailed COMSOL‐based kinetic model to rapidly simulate the SI‐SECM titration reactions under our experimental conditions. Utilization of this rapid‐kinetic model overcomes the limitation of SI‐SECM not having the ability to simultaneously resolve multiple surface adsorbed intermediates. We demonstrate that these numerical simulations can separate the potential dependant formation of the underpotential deposition of hydrogen (H(UPD)) from the overpotential deposition (H(OPD)) of hydrogen. In addition, our simulations show that a spectator species may also exist on the surface during HER potentials. Our simulations also show that at full H2‐producing potentials, the surface of Pt(poly) is fully saturated with intermediates. Comparison between the potential‐dependent adsorption of H(OPD) and Tafel analysis reveal that the Heyrovsky step is likely rate‐determining in alkaline media. However, in alkaline media the Heyrovsky step transitions from first‐order in H(OPD) at low H(OPD) coverage to zero‐order at high H(OPD) coverage, due to surface saturation of adsorbed intermediates. Tafel analysis in acidic media shows that the Heyrovsky step is likely rate‐determining, but remains first‐order in Had over a larger potential range. These fundamental insights reveal that a sluggish Heyrovsky step is a major contributor to the attenuated kinetics of the HER in alkaline media.

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