Electrochemical Activation of NiFe₂O₄ for the Oxygen Evolution Reaction in Alkaline Media

Electrochemical H 2 production via low temperature H 2 O electrolysis is a promising strategy to facilitate decarbonization across sectors; 1 however, current high-performing proton exchange membrane (PEM) electrolyzers require the use of expensive and rare platinum-group metals (PGMs) for catalysts and hardware, limiting scale up feasibility. 2 More recently, anion exchange membrane (AEM) electrolyzers have emerged as an alternative strategy, combining the zero-gap approach employed by PEM with operation in an alkaline environment where many non-PGMs are thermodynamically stable. 3 In fact, oxides of first row transition metals including Ni, Fe, and Co have been proposed as promising anode catalyst alternatives to IrO 2 in AEM electrolyzers. 4–7 However, these materials suffer from high overpotentials (> 300 mV @ 10 mA/cm 2 ) 8 and kinetic improvements are needed to facilitate the deployment of this technology. Ni-Fe oxides have gained particular attention due to their high ex-situ activities and typically low material criticality compared to IrO 2 or other non-PGMs such as Co. 9 Ni and Fe are known to have a synergistic effect, with trace amounts of Fe in Ni catalysts leading to increased OER activity. 10 However, the mechanism behind the improved performance and the optimal Fe content is heavily debated in the literature; 11 for example, this activity enhancement has been attributed to increased electrical conductivity with increasing Fe content, 10 stabilization of the Ni 4+ state, 12 and a shift of the Ni (II/III) redox couple. 13 Furthermore, these studies have evaluated Ni-Fe catalysts in their metallic or oxy(hydroxide) forms, although device-level conditioning may passivate near-surfaces and minimize performance differences between catalysts with difference ex-situ oxide content. 14 Uncovering how Fe content changes over time-on-stream for nanoparticle oxides and correlating this to changes in activity is needed to facilitate optimization of these materials at the device level. To this end, this work evaluated the stability of NiFe 2 O 4 nanoparticles (30 nm) in alkaline environments, correlating changes in Fe composition to changes in activity. All tests were performed in a rotating disc electrode cell with Au working electrode (0.193 cm 2 ), Au counter electrode, and reversible hydrogen reference electrode. Tests were performed in 0.1 M NaOH electrolyte at room temperature with a catalyst loading of 17.8 μg M /cm 2 . We found that the activity of NiFe 2 O 4 improved over time-on-stream (+ 80 % in current at 1.65 V after 13.5 h) concurrent with a dissolution of Fe (2 – 8 wt% Fe loss). We hypothesize that the Fe content in NiFe 2 O 4 (48 wt% Fe) was prohibitively high, and that dissolution of Fe shifted this value closer to optimum, resulting in higher activity. For NiFe (oxy)hydroxide materials, optimal Fe content has been reported at 15 - 25 wt% Fe. 10 However, this value likely changes depending on the structure of the material and, relatedly, the relative abundances of Ni and Fe on the surface. We further looked at the effect of longer time on stream at 1.8 V (13.5 h – 27 h) and potentiodynamic cycling (1.4 - 1.8 V, 1.4 – 2.0 V, 0.0 – 2.0 V; 13.5 h) on NiFe 2 O 4 stability. The results, in turn, showed that catalyst reactivity for OER improved over time-on-stream by 50 – 80% for potentiostatic holds and by upwards 500% for cycling tests (Fig. 1). This activity improvement was found to be concurrent with Fe dissolution (determined from ICP-MS), which ranged from 2 – 8 wt% Fe loss for the potentiostatic stress test to upwards of 40 wt% Fe loss for the cycling tests. Increased time on stream (13.5 – 27 h) was found to not significantly impact the activity enhancement. These results show a correlation between Fe dissolution and activity improvement and suggest that the activity of nanoparticle NiFe 2 O 4 oxides may be electrochemical activated via this method, providing new insights into the viability of NiFe oxides as alternatives to IrO 2 for OER. [1] Borup et al., Electrochem. Soc. Interface 2021 , [2] IRENA Green Hydrogen Cost Reduction - Scaling up Electrolyzers to Meet the 1.5C Climate Goal 2020 , [3] Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions , 1974 , [4] Suen et al., Chem. Soc. Rev . 2017 , [5] Plevová et al., J. Power Sources 2021, [6] Burke et al., Chem. Mater. 2015 , [7] Anderson et al., J. Electrochem. Soc. 2020, [8] McCrory et al., J. Am. Chem. Soc 2015 , [9] European Commission, Report on Critical Raw Materials for the EU 2018 , [10] Trotochaud et al., J. Am. Chem. Soc . 2014 , [11] Anantharaj et al., Nano Energy 2021, [12] Li et al., Proc. Natl. Acad. Sci. 2017 , [13] Görlin et al., J. Am. Chem. Soc. 2016, [14] Alia et al., J. Electrochem. Soc . 2019 Figure 1