Mechanistic Insights into Enhanced Alkaline Oxygen Evolution on Zn-Al Alloy Electrodes

Abstract

Electrochemical water electrolysis, which produces clean energy carriers to mitigate carbon emissions, lacks suitable, low-cost electrodes for efficient oxygen evolution reaction (OER) in alkaline water splitting. To address this challenge, we developed Zn-Al alloy electrodes with varying Al contents up to 20 wt.% via powder metallurgy method and conducted electrochemical measurements of the OER in alkaline solution to investigate their catalytic performance. We also performed first-principles calculations to examine their thermodynamic phase stability and electronic structures. Both theoretical and experimental results indicated that incorporating $\geq 20$ wt.% Al into Zn led to thermodynamic phase instability and secondary-phase segregation in Al-rich regions, limiting reaction kinetics and reducing catalytic efficiency. Although the Al content of 5 wt.% into Zn exhibited favorable thermodynamic and electronic characteristics, but its electrochemical performance was inefficient and poor due to inadequate reaction active sites on the surface. In contrast, the 10 wt.% and 15 wt.% Al into Zn showed approximately three- and two-fold increases in anodic exchange current density relative to pure Zn, respectively. Additionally, the anodic overpotential losses ($η_{0,a}$) measured at a current density of 12 mAcm$^{-2}$ were 0.240 V for Zn$_{0.9}$Al$_{0.1}$ and 0.5603 V for Zn$_{0.85}$Al$_{0.15}$, significantly lower than that of pure Zn ($η_{0,a} = 1.086$ V). While Zn$_{0.9}$Al$_{0.1}$ and Zn$_{0.85}$Al$_{0.15}$ showed similar charge transfer resistance ($R_{\rm CT}$), Zn$_{0.9}$Al$_{0.1}$ demonstrated superior reaction kinetics and lower $η_{0,a}$ across all samples tested. Furthermore, the improved kinetics and reduced overpotential of the Zn-Al alloys favorably compare with those of other transition-metal-based catalysts, including Fe-Co-Ni-Mo alloys and Fe-doped CuO.

Figures (9)

• Figure 1: Hexagonal crystal structure of (a) pure Zn and (b) Zn-Al alloy electrodes.
• Figure 2: Schematic illustration of the synthesis process for Zn-Al alloy electrodes.
• Figure 3: Electronic density of states (DOS) of (a) pure Zn using a $2 \times 2 \times 2$ supercell and (b) pure Al modeled with its unit cell crystal structure. For both of them, the Fermi level is set to zero.
• Figure 4: Electronic density of states (DOS) of (a) 5% Al into Zn, (b) 10% Al into Zn, (c) 15% Al into Zn, and (d) 20% Al into Zn electrodes using $2 \times 2 \times 2$ supercell crystal structure. For all of them, the Fermi level is set to zero.
• Figure 5: X-Ray Diffraction (XRD) pattern for pure Zn and 10 wt.% Al into Zn alloy (Zn$_{0.9}$Al$_{0.1}$) electrodes.