We report a scalable, solution-processing method for synthesizing low-dimensional hyperthin FeS 2 nanostructures, and we show that 2D FeS 2 disc nanostructures are an efficient and stable hydrogen evolution electrocatalyst. By changing the Fe:S ratio in the precursor solution, we were able to preferentially synthesize either 1D wire or 2D disc nanostructures. The 2D FeS 2 disc structure has the highest electrocatalytic activity for the hydrogen evolution reaction, comparable to platinum in neutral pH conditions. The ability of the FeS 2 nanostructures to generate hydrogen was confirmed by scanning electrochemical microscopy, and the 2D disc nanostructures were able to generate hydrogen for over 125 h.

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

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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 .