Tuning Ni-MoO2 Catalyst-Ionomer and Electrolyte Interaction for Water Electrolyzers with Anion Exchange Membranes

Abstract

Tailoring catalyst-ionomer and electrolyte interaction is crucial for the development of anion exchange membrane (AEM) water electrolysis. In this work, the interaction of Ni-MoO₂ nanosheets with ionomers and electrolyte cations was investigated. The activity of Ni-MoO₂ nanosheets for the hydrogen evolution reaction (HER) increased when tested in 1 M NaOH compared to 1 M KOH; however, it decreased when tested in 0.01 M KOH compared to 1 M KOH electrolyte. The capacitance minimum associated with the potential of zero free charge (pzfc) was shifted negatively from 0.5 to 0.4 V versus RHE when KOH concentration increased from 0.1 mM to 1 M KOH, suggesting a softening of the water in the double-layer to facilitate the OH^- transport and faster kinetics of the Volmer step that lead to improved HER activity. The catalyst interaction with cationic moieties in the anion ionomer (or organic electrolytes) can also be rationalized based on the capacitance minimum, because the latter indicates a negatively charged catalyst during the HER, attracting the cationic moieties leading to the blocking of the catalytic sites and lower HER performance. The HER activity of Ni-MoO₂ nanosheets is lower in benzyltrimethylammonium hydroxide (BTMAOH) than in tetramethylammonium hydroxide (TMAOH). Anion fumion ionomer and electrolytes with organic cations with benzyl group adsorption (such as BTMAOH) lead to decreased HER activity in comparison with TMAOH and Nafion. By utilizing Ni-MoO₂ nanosheet electrodes as a cathode in a full non-platinum group metal (PGM) AEM electrolyzer, a current density of 1.15 A/cm² at 2 V cell voltage in 1 M KOH at 50 °C was achieved. The electrolyzer showed exceptional stability in 0.1 M KOH for 65 h at 0.5 A/cm².

Figure 1

Schematic representation of the AEM water electrolyzer setup including the membrane electrode assembly used in this work. Figure reproduced with permission from ref (12). Copyright 2021, Elsevier.

Figure 2

(a) SEM image and (b,c) STEM images at different magnifications of Ni–MoO₂ nanosheets.

Figure 3

(a) XRD patterns of Ni–MoO₂, and Ni nanosheets, (b) XPS survey spectrum of Ni–MoO₂ nanosheets, and the corresponding high-resolution XPS spectrum of (c) Ni 2p, (d) Mo 3d, (e) O 1S, and (f) B 1s.

Figure 4

(a) LSVs and (b) Tafel plots for Ni–MoO₂ and Ni nanosheets catalysts compared to Pt/C in 1 M KOH using Nafion ionomer, (c) Comparison of the mass activity of Ni–MoO₂ nanosheets with literature data as summarized by Kibsgaard et al. Reprinted with permission from ref (28). Copyright 2019, Springer Nature. (d) LSVs of Ni–MoO₂ nanosheets in purified and nonpurified 1 M KOH using Nafion ionomer, (e) chronoamperometry at −0.35 V for 1800 min (30 h) of Ni–MoO₂ nanosheets and Pt/C in 1 M KOH using Nafion ionomer, (f) LSV of Ni–MoO₂ nanosheets before and after 5000 cycles in 1 M KOH using Nafion ionomer.

Figure 5

(a) LSVs of Ni–MoO₂ nanosheets in 1, 0.1, and 0.01 M KOH concentrations, (b) LSVs of Ni–MoO₂ nanosheets in 1 M KOH and 1 M NaOH, (c) LSVs Ni–MoO₂ nanosheets in 1 M KOH using Fumion and Nafion ionomers and catalyst ionomer-free electrodes in organic cationic electrolytes (TMAOH and BTMAOH), (d) corresponding impedance complex plane plot at −0.25 V versus RHE at 1600 rpm rotation rate, (e) LSVs of Pt/C in 1 M KOH using Fumion and Nafion ionomers and catalyst ionomer-free electrodes in organic cationic electrolytes (TMAOH and BTMAOH), and (f) loss in current density at −0.2 V versus RHE versus BTMAOH concentration in [Y M KOH + (1 – Y) M BTMAOH] electrolyte for Ni–MoO₂ nanosheets and Pt/C.

Figure 6

(a) Capacitance–potential curves gathered from the impedance measurements of Ni–MoO₂ nanosheets in various KOH concentrations (1 M to 0.1 mM). (b) Magnified view of the capacitance–potential curves displaying the changes in potential for capacitance minimum at different concentrations of KOH electrolyte.

Figure 7

Schematic of the double layer during HER in the presence of quaternary ammonium species and KOH electrolyte.

Figure 8

(a) Raw and HFR corrected polarization curves of Ni–MoO₂ nanosheets/Ni_0.6Co_0.2Fe_0.2 AEM electrolyzer in various electrolytes. (b) The Ni–MoO₂ nanosheets/Ni_0.6Co_0.2Fe_0.2 electrolyzer activity comparison with literature data. Published with permission from ref (62). Copyright 2019, Royal Society of Chemistry. (c) The AEM electrolyzer stability profile for 65 h, (d) EIS complex plane plot at the beginning and end of stability test (BOT and EOT) in 0.1 M KOH for 65 h recorded at 0.5 A/cm² of Ni–MoO₂ nanosheets/Ni_0.6Co_0.2Fe_0.2 AEM electrolyzer.