Temperature and doping-tuned coordination environments around electroactive centers in Fe-doped α(β)-Ni(OH)2 for excellent water splitting

In this study, Fe-doped beta-Ni(OH)(2) exhibited an unpredictable hydrogen evolution reaction (HER) performance, which is much better than that of Fe-doped alpha-Ni(OH)(2). Meanwhile, Fe-doped alpha-Ni(OH)(2) showed a much better oxygen evolution reaction (OER) than Fe-doped beta-Ni(OH)(2). At 120 and 240 degrees C, Fe-doped alpha-Ni(OH)(2) and Fe-doped beta-Ni(OH)(2) are synthesized, respectively, by the addition of Fe3+ into a hydrothermal solution immersed with nickel foam. The higher valence of Ni and Fe in the alpha-phase benefits the OER performance. In contrast to the alpha-phase case, the enrichment of the electron cloud around Fe in the beta-phase due to surrounding coordination environment is conducive to the HER, which can be verified consistently from XPS, XAFS, XANES and DFT. By taking Fe as the active sites, DFT calculations have confirmed that the energy barrier for each step of the HER is much lower than that corresponding to the Ni site. Only a 53.8 mV overpotential for Fe-doped beta-Ni(OH)(2)-240 is needed to achieve a current density of 10 mA cm(-2), which is comparable with most of the active HER electrocatalysts. The electrolyzer cell employing Fe-doped beta-Ni(OH)(2)-240 as the cathode and Fe-doped alpha-Ni(OH)(2)-120 as the anode demonstrates superior performance for overall water splitting with excellent stability.