The use of renewable (CO₂-free) energy is one of the most critical issues toward sustainability. Hydrogen production by alkaline water electrolysis (AWE) is promising for storage and transport of unevenly distributed renewable energy in terms of time and place; however, high overpotential and degradation of electrodes under potential fluctuation and/or daily shutdown significantly limit the use of AWE with the connection to renewable energy. Here, the degradation of anode would contain two types of mechanisms. One is the consumption of electrocatalysts to decrease activity,¹ and the other is the corrosion of electrocatalyst and/or base substrate to increase electronic resistivity.² We here focus on the self-assembly of catalyst on an electrode by adding catalytst/precatalyst in the electrolyte to form self-healing and corrosion-resistant catalyst layer. There are reports on in-situ activation of an anode with cobalt salt³ and catalyst nanoparticles,⁴ though their effects on the durability against frequent potential change have not been clarified to date. Here, we report a novel hybrid cobalt hydroxide nanosheets (CoNS; Figure 1(a))⁵ as an active and durable OER catalyst material that is deposited on a Ni electrode during the AWE process to form a self-assembled catalyst layer.

To synthesize CoNS, cobalt chloride and tris(hydroxymethyl)aminomethane (Tris) were mixed in water and heated at 80°C for 24 h. ³ The electrochemical tests were performed in a three-electrode electrochemical cell made of PFA at 30°C. A Ni wire and a Ni coil were used as working and counter electrodes, respectively. The Ni wire was etched in a boiling 20% HCl for 6 min prior to the measurement. A reversible hydrogen electrode (RHE) was used as a reference. All the following potentials are referred to the RHE. 300 mL of 1 M KOH was used as an electrolyte under N₂ bubbling. An aqueous dispersion of 50 mg/mL CoNS (0.2 mL) was injected in the electrolyte solution. Cyclic voltammetry (0.5–1.5 V, 200 mV/s, 200 cycles) was performed as a pretreatment. For the stability test, potentiostatic electrolysis at 1.8 V for 1 h, 3 cycles of cycle voltammetry (0.5–1.8 V, 5 mV/s), and 2,000 cycles of potential cycling (0.5–1.8 V, 1 V/s) were repeated up to 40,000 potential cycling. The above setup is denoted as Ni-CoNS. Similar experiments with Co(NO₃)₂ instead of CoNS (denoted as Ni-Co(NO₃)₂) and without Co species (denoted as Ni-noCo) were also performed.

CoNS consists of a brucite layer on which Tris molecules are covalently immobilized. The modification with Tris enhances the ability of layered cobalt hydroxides for exfoliation.³ The thickness was ca. 1.3 nm and the lateral size was varied in 10–100 nm (Figure 1(b)). In the IR-corrected cyclic voltammograms before (Figure 2(a)) and after 10,000 cycles of potential cycling (Figure 2(b)), the redox peaks appeared at 1.2–1.4 V after potential cycling, indicating the deposition of cobalt and/or nickel hydroxides, which would activate oxygen evolution reaction (OER). An intense peak at 1.35 V, possibly attributable to CoNS, was observed only in the cyclic voltammogram of Ni-CoNS. After 10,000 cycles, peaks at 1.55–1.60 V due to corroded hydrous nickel oxides⁶ were observed for only Ni-noCo and Ni-Co(NO₃)₂. The CoNS catalytic layer is suggested to form anticorrosion film on the Ni electrode. The OER current densities at 1.60 V as a function of the number of potential cycling is shown in Figure 3. The Ni-noCo exhibited significant deterioration in activity after 5,000 cycles. The current density of Ni-CoNS increased gradually before 10,000 cycles and then retained at around 90 mA cm^–2, suggesting self-assembly of catalytic layer with excellent durability. Ni-Co(NO₃)₂ did not show such an activity; thus, the use of preformed CoNS is quite important.

In conclusions, CoNS is deposited during electrolysis to form a catalytic layer. If the catalytic layer is deteriorated by potential cycling, it is easily reformed under the operation conditions. Therefore, CoNS is useful as a self-assembled catalyst with excellent durability for AWE.

References

1. S. Cherevko et al., Catal. Today, 262 (2016) 170.
2. A. Manabe et al., Electrochim. Acta, 100 (2013) 249.
3. H. Wendt et al., Angew. Chem. Int. Ed., 56 (2017) 8573.
4. Y. Kuroda et al., Chem. Eur. J., 23 (2017) 5023.
5. M. E. G. Lyons et al., J. Electrochem. Soc., 159 (2012) H932.