Cobalt–Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism

Abstract

Cobalt oxides and (oxy)hydroxides have been widely studied as electrocatalysts for the oxygen evolution reaction (OER). For related Ni-based materials, the addition of Fe dramatically enhances OER activity. The role of Fe in Co-based materials is not well-documented. We show that the intrinsic OER activity of Co(1-x)Fe(x)(OOH) is ∼100-fold higher for x ≈ 0.6-0.7 than for x = 0 on a per-metal turnover frequency basis. Fe-free CoOOH absorbs Fe from electrolyte impurities if the electrolyte is not rigorously purified. Fe incorporation and increased activity correlate with an anodic shift in the nominally Co(2+/3+) redox wave, indicating strong electronic interactions between the two elements and likely substitutional doping of Fe for Co. In situ electrical measurements show that Co(1-x)Fe(x)(OOH) is conductive under OER conditions (∼0.7-4 mS cm(-1) at ∼300 mV overpotential), but that FeOOH is an insulator with measurable conductivity (2.2 × 10(-2) mS cm(-1)) only at high overpotentials >400 mV. The apparent OER activity of FeOOH is thus limited by low conductivity. Microbalance measurements show that films with x ≥ 0.54 (i.e., Fe-rich) dissolve in 1 M KOH electrolyte under OER conditions. For x < 0.54, the films appear chemically stable, but the OER activity decreases by 16-62% over 2 h, likely due to conversion into denser, oxide-like phases. We thus hypothesize that Fe is the most-active site in the catalyst, while CoOOH primarily provides a conductive, high-surface area, chemically stabilizing host. These results are important as Fe-containing Co- and Ni-(oxy)hydroxides are the fastest OER catalysts known.

Electrocatalytic activity of CoFe2O4 thin films prepared by AACVD towards the oxygen evolution reaction in alkaline media

“…The smaller Tafel slope of CoFe 2 O 4 compared with the F:SnO 2 substrate shows the superior electrocatalytic activity of CoFe 2 O 4 towards the OER. It is generally understood that a Tafel slope close to 60 mV dec − 1 is associated with a rate-limiting chemical step following the first electron transfer [5,22]. It appears that the Tafel slope of CoFe 2 O 4 might be influenced by a small oxidation peak occurring just before/on the onset of oxygen evolution.…”

“…Commercial electrolysers are becoming widely available due to rapidly increasing demand for hydrogen and clean water [3,4]. Unfortunately, the major obstacle to achieving efficient water electrolysis is the large overpotential required for the oxygen evolution reaction (OER) [5]. This is therefore the most energy intensive step in water electrolysis.…”

“…This is therefore the most energy intensive step in water electrolysis. A low-cost and efficient electrocatalyst is thus required to minimize the energy needed in this step [5]. In terms of long-term stability of practical devices, water electrolysis in alkaline media is becoming more attractive [5].…”

“…It has been generally established that transition metal oxides often form (oxy)hydroxides at their surfaces in alkaline conditions. A recent report has found that in CoFe oxyhydroxides, Fe is the most active site, whilst the CoOOH provides a conductive support, resulting in a synergistic effect towards catalysing the OER [5].…”

ORR Electrocatalysis on Cr3+, Fe2+, Co2+-Doped Manganese(IV) Oxides

“…Co and Fe-based materials have been shown to be excellent OER catalysts and are stable in alkaline solutions [29][30][31].…”