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

Abstract

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-ns2 with modifications. An aqueous solution of metal salts (NiCl2 and FeCl3 Ni/Fe = 9) was mixed with that of tris(hydroxymethyl)aminomethane (Tris-NH2), followed by heating at 90 °C. The concentrations of metal salts and Tris-NH2 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 105–10–1 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 salts and Tris-NH2 were 1.0 M for the synthesis of NiFe-ns. When the concentration of metal salts is too low or that of Tris-NH2 is too high, NiFe-ns could not be collected by filtration because it is too small. Too high concentration of metal salts or too low concentration of Tris-NH2 caused aggregation. XRD indicated that the formation of randomly stacked hybrid nanosheets. Ni/Fe ratio of the product was 1.45, indicating Fe was preferentially precipitated. During the pretreatment using NiFe-ns, the area of the redox peak due to Ni2+/Ni3+ at 1.41 and 1.37 V vs. RHE increased along with the processing cycles, which implies that NiFe-ns was simultaneously deposited (Fig. 2). η 100 was decreased from 330 mV to 308 mV, whereas η 100 using no catalyst and Co-ns indicated 395 mV (initial activity) and 360 mV. NiFe-ns showed much higher OER activity than Co-ns. The ADT results using no catalyst presented 90 mV increase in η 100 after the 30000th cycle (Fig. 3). Those using Co-ns exhibited only 9 mV increase after the 40000th cycle. η 100 using NiFe-ns decreased gradually during the first 20000 cycles from 309 mV to 277 mV. η 100 was at approximately 275 mV up to the 40000th cycle. The area of the redox peaks due to Ni2+/Ni3+ increased along with the decrease in η 100 (Fig. 4). The deposition of NiFe-ns was slower than that of Co-ns. The quite high durability is consistent with the self-repairing behavior of highly dispersed hybrid nanosheets.2 In conclusion, a novel NiFe-ns was synthesized with high dispersibility in the electrolyte. NiFe-ns could be used as self-repairing anode catalyst with much higher OER activity than Co-ns. References Y. Uchino, T. Kobayashi, S. Hasegawa, I. Nagashima, Y. Sunada, A. Manabe, Y. Nishiki, S. Mitsushima, Electrocatalysis 9 (2018) 67. Y. Kuroda, T. Nishimoto, S. Mitsushima, Electrochim. Acta 323 (2019) 134812. S. Fujita, I. Nagashima, Y. Sunada, Y. Nishiki, S. Mitsushima, Electrocatalysis 8 (2017) 422. Figure 1