Highly Durable Oxygen Evolution Reaction Catalyst: Amorphous Oxyhydroxide Derived from Brownmillerite-Type Ca₂FeCoO₅

Abstract

Brownmillerite-type Ca2FeCoO5 (CFCO) is a highly active electrocatalyst for the oxygen evolution reaction (OER). In this study, we identified the actual catalytically active phase of this oxide formed via the long-term OER and, moreover, demonstrated that the active phase can persist during the OER for four weeks without significant loss of electrocatalytic activity. The long-term durability tests were carried out on CFCO via continuous galvanostatic OER in 4 mol dm -3 KOH aqueous solution for periods ranging from a few hours to one month, and the specimens submitted to the tests were characterized by means of electrochemical measurements and structural analysis using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, Auger electron spectroscopy, and X-ray absorption fluorescence spectroscopy. CFCO was readily converted to amorphous cobalt oxyhydroxides with 10% of Fe substituents through the OER process, and these compounds had a similar local rearrangement to the layered γ-CoOOH-type structure. This transformation involved large morphological changes of the oxide particles because of the extensive dissolution of Ca and Fe, yielding skeletal grains made of oxyhydroxide nanosheet aggregates. The extended durability studies with total polarization charge density of the order of 10 5 C cm -2 revealed that (Co, Fe)OOH like compound is the actual electrocatalytic phase.

Spinel-Type Metal Oxide Nanoparticles Supported on Platelet-Type Carbon Nanofibers as a Bifunctional Catalyst for Oxygen Evolution Reaction and Oxygen Reduction Reaction

“…The onset potential defined at a current density of 10 mA cm ¹2 is higher than the Ca 2 FeCoO 5 +carbon electrode which is one of the state-of-art OER electrodes in 4.0 mol dm ¹3 KOH electrolyte (Table 3); however, the current density at 1.7 V vs. RHE was over 100 mA cm ¹2 disk and this can be comparable with Ca 2 FeCoO 5 +carbon electrode. [29][30][31] The overpotential of pCNF/MCO at 100 mA cm ¹2 is comparable to the well-known RuO 2 OER active electrocatalyst (Fig. 7).…”

“…Nafion was added as a binder and neutralized before dispersion to prevent MCO from undergoing acidic dissolution due to the lower pH of the Nafion binder. 9,[29][30][31][32][33] Physically mixed samples of carbon materials and MCO were designated as DB+MCO and pCNF+MCO. The RuO 2 and Pt/C electrode were prepared based on the previous report.…”

“…The electron transfer number during ORR was evaluated by the RRDE method under the ring electrode potential of 1.4 V vs. RHE. 34 The potential was converted from the Hg/HgO/4 mol dm ¹3 KOH reference scale to the RHE using the following equation: [29][30][31][32] E vs: RHE ¼ E vs: Hg=HgO=4 mol dm À3 KOH þ 0:9260 ð3Þ…”

Improvement of Photoelectrocatalytic Activity and Stability of WO3 for Oxygen Photoevolution Reaction by Loading of Brownmillerite‐Type Ca2FeCoO5 as a Cocatalyst

“…As is well known, especially in the field of rechargeable metal−air batteries, perovskite‐type [ 23–26 ] oxides ABO 3 , where A is a rare‐earth or alkaline‐earth metal and B is a transition metal, and its derivative structured, for example, Ruddlesden−Popper‐type‐layered perovskite‐type, [ 27 ] double‐perovskite‐type, [ 28 ] quadruple‐perovskite‐type, [ 29 ] and brownmillerite‐type, [ 30–33 ] oxides containing a number of transition metals such as cobalt, iron, manganese, and nickel are quite of interest due to the high catalytic activity for OER compared with those of noble metal oxides. For example, perovskite‐type Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ (BSCF), which is one of the most famous OER catalysts, has been widely investigated.…”

Simple Construction of Amorphous Monometallic Cobalt‐Based Selenite Nanoparticles using Ball Milling for Highly Efficient Oxygen Evolution Reaction

“…To reduce OER overpotential, many methods and strategies have been reported, such as heteroatom doping to improve the inherent activity by adjusting their electronic structure [6–8] or preparation of materials with amorphous structures to increase the number of active sites and accelerate ionic diffusion [9–10] . Recent investigations have pointed out that the amorphous phases associated with anisotropy and defect could be beneficial to achieve effective water oxidation owing to their more catalytically active sites [11–17] . Thence, more and more strategies for preparing electrocatalysts with amorphous structures have been reported.…”