A general solvent-dependent protocol directly influencing the oxygen reduction reaction (ORR) in metal oxide/graphene nanohybrids has been demonstrated. We conducted the two-step synthesis of cobalt oxide/N-doped graphene nanohybrids (CNG) with solvents of water, ethanol, and dimethylformamide (DMF), representing tree typical categories of aqueous, polar organic, and organic N-containing solvents commonly adopted for graphene nanocomposites preparation. The superior ORR performance of the DMF-hybrids can be attributed to the high nitrogen-doping, aggregation-free hybridization, and unique graphene porous structures. As DMF is the more effective N-source, the spectroscopic results support a catalytic nitrogenation potentially mediated by cobalt-DMF coordination complexes. The wide-distribution of porosity (covering micro-, meso-, to macro-pore) and micron-void assembly of graphene may further enhance the diffusion kinetics for ORR. As the results, CNG by DMF-synthesis exhibits the high ORR activities close to Pt/C (i.e. only 8 mV difference of half-wave potential with electron transfer number of 3.96) with the better durability in the alkaline condition. Additional graphene hybrids comprised of iron and manganese oxides also show the superior ORR activities by DMF-synthesis, confirming the general solvent-dependent protocol to achieve enhanced ORR activities.
The interchangeable operation of
alkaline oxygen evolution and
reduction using bifunctional electrocatalysts in devices consolidates
the commercialization milestone of energy storage in hydrogen, and
yet, it is usually limited by issues of carbon corrosion in electrocatalysts
and inhomogeneous electrode fabrication. Here, we demonstrate a synthetic
route toward carbon-free ternary rock salt oxide (i.e., NiO/CoO/FeO)
hollow spheres with silver decoration on the surface for durable operation
in bifunctional cells. These Ag-decorated ternary oxides exhibit an
overall bifunctional potential gap (ΔE = E
j10, OER – E
jhalf, ORR) of 0.89 V. Operando Raman studies show that
the rock salt structure shows the phenomenon of a highly reversible
local environment during the charge–discharge OER cycles, a
key characteristic for high durability in bifunctional devices. At
equivalent content between Ni and Co, NiOOH is the main OER-active
species with CoOOH and/or CoO2 as the cocatalyst, where
the presence of the FeO dopant facilitates structural activation and
reversibility. With the proof-of-concept performance in anion-exchange
membrane (AEM) devices, the catalysts achieve a durable cyclic operation
with a high current density (1000 mA cm–2) at smaller
potentials (2.03 V) than RuO2 (2.16 V) under the electrolyzer
mode, while it can yield two times greater power density (96.98 mW
cm–2) than Pt/C (53.58 mW cm–2) in the fuel cell mode.
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