Given the global increase in energy demand and serious environmental pollution, hydrogen fuel is a promising energy carrier to replace traditional fossil fuels due to its zero gas emissions and high energy density by weight. Electrochemical water electrolysis with non-precious metal catalysts offers a simple and cost-effective way for high purity and large-scale hydrogen generation. The realization of hydrogen evolution, however, is hampered by the large sustainable driving potential needed above the thermodynamic requirements. Here, we report dynamically crystallinity-enhanced (DCE) NiFe layered double hydroxide (LDH) ultrathin nanosheets, leading to faster electron transfer, smooth gas release ability, and more active surface areas, resulting in markedly improved catalytic efficiency. Compared with untreated NiFe LDH, DCE NiFe LDH exhibits much lower overpotential for the cathode reaction. Under 1 M KOH aqueous electrolyte, the bi-functional DCE catalysts require only 1.48 V and 1.29 V to reach 10 and 1 mA cm-2 in two-electrode measurements without iR-compensation, corresponding to 83% and 95% electricity-to-fuel conversion efficiency with respect to the lower heating value of hydrogen. In-situ Raman spectro-electrochemistry was carried out to obtain insight into the active catalyst phases, revealing the role of Fe and Ni and their function for OER and HER, respectively. The transformation from Ni(OH)2 to γ-NiOOH was clearly observed by in-situ Raman spectroscopy under OER operation. While, the Raman features of Ni(OH)2 and FeOOH were shown under HER process. It means the function of Ni and Fe is different under OER and HER, but it is noticeable that the observed Ni and Fe species at the different applied overpotential are dominant contribution to the catalytic activity. Our results shed light on the full understanding of overall water splitting in NiFe LDH ultrathin nanosheets and developing more efficient catalysts.

Direct observation of active catalyst redox states and the effect of dynamically increased crystallinity on efficient alkaline water splitting

2019 Spring National Meeting

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