Highly Active Self-Repairing Anode Catalyst for Alkaline Water Electrolysis Using Ni-Based Hybrid Nanosheets

Alkaline water electrolysis (AWE) is an established technology for hydrogen production from electricity. Recent demands on the hydrogen production from renewable energy raised the issue of electrode degradation in AWE because of reverse current that is caused during the suspended period of AWE. 1 We have recently reported that the cobalt hydroxide nanosheet modified with organic tripodal ligand (hybrid cobalt hydroxide nanosheet: Co-ns) is dispersed in an electrolyte and behaves as self-repairing catalyst. 2 Co-ns is deposited on a nickel anode during electrolysis, which has been utilized as repairing process of a catalyst layer. Because high dispersibility is required for self-repairing catalysts, the change in the composition to improve the oxygen evolution reaction (OER) activity was limited. Here, we report the synthesis of a novel NiFe-based hybrid nanosheet (NiFe-ns) with high dispersibility (Fig. 1) and its use as self-repairing anode catalyst with high OER activity. NiFe-ns was synthesized according to that of Co-ns 2 with modifications. An aqueous solution of metal salts (NiCl 2 and FeCl 3 Ni/Fe = 9) was mixed with that of tris(hydroxymethyl)aminomethane (Tris-NH 2 ), followed by heating at 90 °C. The concentrations of metal salts and Tris-NH 2 were varied to obtain highly dispersed NiFe-ns. The electrochemical test was performed in a three-electrode cell made of PFA. 3 Nickel wire and coil were used as working and counter electrodes, respectively. Reversible hydrogen electrode was used as a reference. 1 M KOH aq. was used as an electrolyte. No catalyst, Co-ns, or NiFe-ns was added in the electrolyte (approximately 40 ppm). As a pretreatment to deposit the catalyst on the nickel anode, the following processes were repeated for 8 times. Chronopotentiometry (CP) at 800 mA cm –2 for 30 min and potential sweep between 0.5 and 2.0 V vs. RHE at 5 mV s –1 . The solution resistance was corrected at each cycle by the AC impedance technique at 10 5 –10 –1 Hz. The accelerated durability test (ADT) was performed by repeating the following processes for 20 cycles. (1) CP at 800 mA cm –2 for 30 min, (2) cyclic voltammetry (CV) at 0.5–2.0 V vs. RHE for 2 cycles at 5 mV s –1 , (3) CV at 0.5–1.6 V vs. RHE for 2 cycles at 50 mV s –1 , and (4) CV at 0.5–1.8 V vs. RHE for 2000 cycles at 500 mV s –1 . The overpotential ( η 100 ) of OER at 100 mA cm –2 was plotted as a function of the cycles ( n ) of the CV at 500 mV s –1 . The optimum concentrations of both metal salt