Synthesis of Novel Melilite-Type Iron/Cobalt Oxides and Their Oxygen Evolution Reaction Electrocatalytic Activity

Abstract

Here, we report the synthesis and oxygen evolution reaction (OER) electrocatalytic activity of A 2 Fe 2−2x Co x Ge 1+x O 7 (A = Sr and Ba) with a melilite-type structure. The iron/cobalt-mixed melilites were successfully synthesized for the first time and found to exhibit high OER activity in highly alkaline media. In particular, the OER performance of Sr 2 Fe 0.67 Co 0.67 Ge 1.66 O 7 (x = 0.67) is comparable to, or even superior to, that of a precious-metal catalyst, RuO 2 . The remarkable OER activity of the melilites seems to originate from a synergetic effect of Fe/Co coexistence as well as the structural feature of melilites, where "coordinatively unsaturated" Fe/ Co sites effectively promote the formation of catalytically active surface layers through the adsorption of OH − ions in highly alkaline media.

Highly Durable Bifunctional Gas Diffusion Electrodes Fabricated with Melilite-Type Fe/Co/Ni-Mixed Oxide Electrocatalysts

“…A similar phenomenon was also reported for CFCO, where the metal–oxygen framework undergoes an immediate modification under OER conditions, resulting in the formation of edge-sharing (Co, Fe)­O 6 octahedral catalytic centers . We thus believe that the TM-enriched surface layer in the Fe/Co- (ref ) and Fe/Co/Ni-mixed melilites also works as an OER-active site. Considering the phenomenological similarity, the Fe and Co components in the present BFCNG catalyst are suggested to transform into oxyhydroxides (Fe 3+ OOH and Co 3+ OOH), while the Ni component exists as a hydroxide (Ni 2+ (OH) 2 ).…”

“…The end-member Ba 2 Fe 2 GeO 7 is essentially OER-inactive, while its counterpart Ba 2 CoGe 2 O 7 shows relatively high activity. The OER current density of the Fe/Co-mixed (Ni-free) melilites is higher than that for the noble metal catalyst RuO 2 , reaching J 1.7V = 150 mA cm –2 . The Co/Ni-mixed melilites show similar activity to Ba 2 CoGe 2 O 7 .…”

“…In fact, Fe/Co-mixed melilites A 2 Fe 2−2x Co x Ge 1+x O 7 (A = Sr and Ba) were found to be highly OER-active in alkaline media. 27 Typically, the Sr 2 Fe 0.67 Co 0.67 Ge 1.66 O 7 (A = Sr, x = 0.67) catalyst exhibited 80 mA cm −2 (normalized by the geometric surface area of the disc electrode) at 1.60 V vs RHE with a Tafel slope of 39 mV dec −1 . The remarkable OER activity of the melilites is believed to originate from their crystallographic feature, where tetrahedral Fe/Co sites effectively promote the formation of catalytically active surface layers (presumably as oxyhydroxides) through the adsorption of OH − ions in highly alkaline media.…”

“…Compounds with such “coordinative unsaturation” cations would be promising because of their capability of adsorbing molecules/ions at the grain surface. In fact, Fe/Co-mixed melilites A 2 Fe 2–2 x Co x Ge 1+ x O 7 (A = Sr and Ba) were found to be highly OER-active in alkaline media . Typically, the Sr 2 Fe 0.67 Co 0.67 Ge 1.66 O 7 (A = Sr, x = 0.67) catalyst exhibited 80 mA cm –2 (normalized by the geometric surface area of the disc electrode) at 1.60 V vs RHE with a Tafel slope of 39 mV dec –1 .…”

The Pivotal Role of s‐, p‐, and f‐Block Metals in Water Electrolysis: Status Quo and Perspectives

“…Since the effects of Te have also been discussed in previous literature, [232,233] herein we focus on B, Si, Ge, As, and Sb as a component. Actually, the purpose of a metalloid in electrocatalysts for water splitting is somewhat related to the role of a lean metal: [234][235][236][237][238][239][240][241][242] i) the metalloid is a key component to form intermetallics, which transform into a core-shell architecture for efficient catalysis during the OER process; ii) doping of the catalyst with a metalloid can induce an oxygen vacancy, which then alters the catalytic mechanism and/or regulates the free adsorption ability during catalysis; iii) some of the metalloid-based compounds can function as an effective support to confine specific cluster formation and single atoms; iv) some of the metalloids can also be prepared in the form of single atoms and can serve as the catalytic active sites to catalyze the HER; v) some metalloids possess a unique photothermal effect, i.e., it can effectively absorb the light and lead to the increase of temperature for the local part of electrocatalyst), which can boost the catalytic kinetics. In consideration of the above points, Driess and co-workers recently synthesized a series of intermetallics consisting of Fe or Ni and metalloids (Si, Ge, As) for OER using the single-source molecular precursor approach and arc-melting method.…”