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Introduction: A water-based electrochemical system (WES) is attracting attention as an innovative technology for overcoming the global-warming and fossil-fuel crises. Water-based electrochemical reactions smoothly occur by using various catalysts. And then, their reactions are designed, proceeding on a gas-diffusion electrode (GDE) working on three-phase boundaries of gas//liquid//solid interfaces.
Oxygen evolution reaction (OER), 4-electron oxidation of two water molecules , is indispensable for the construction of WES combined with electrochemical reductions (ORR and CO2RR) of the ubiquitous gases O2 and CO2 (Fig. 1). Toward the development of alternative catalysts using rare metals such as Pr and Ir, advanced catalysts composed of earth-abundant and low-cost 3d transition metals have comprehensively explored. FeNi-layered double hydroxides (LDHs) of binary metal OER catalysts are benchmark materials, where a synergistic mechanism via the binary metal atoms [2,3]. Note that inherent catalytic activities can be totally evaluated based on four parameters of overpotential, Tafel slope, turnover frequency, and mass activity .
Here, we aimed to prepare nanoparticulate OER catalysts composed of FeNi-based hydroxides via FeNi-Prussian blue analog (FeNi-PBA, Fe1-xNix[Fe(CN)6]0.67), whose metal composition ratios are systematically controlled. Carbon paper (CP) is a commercially available GDE, employed for the deposition of the nanoparticulate OER catalysts.
Experimental section: FeNi-PBA nanoparticles (NPs) were synthesized by mixing of an aqueous mixture of FeSO4·7H2O and Ni(NO3)2·6H2O and an aqueous solution of K3[Fe(CN)6] in difference metal composition ration of x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, and 1 of Fe1-xNix[Fe(CN)6]0.67 . FeNi-PBAs were hydrolyzed to Fe1-xNixOH. For control microgram-scale mass-loading amounts of catalysts, we prepared FeNi-PBA NP aqueous dispersion solutions via surface modification of [Fe(CN)6]4-, and the as-prepared solutions were drop-casted on CP. After the hydrolysis in a KOH solution, OER activities were measured in an alkali condition using the three-electrode method.
Results and discussion: FeNi-PBA NP solids were decomposed to Fe1-xNixOH with release of [Fe(CN)6]n- in a KOH solution from X-ray diffraction (XRD) patterns, FT-IR spectra, and X-ray fluorescence analyses. In x = 0.4 – 0.8, the FeNi-PBA NPs was transformed to FeNi-layered double hydroxide (FeNi-LDH) nanoflakes with sheet-domain sizes between 10 and 20 nm (Fig. 2a).
A surface-modified FeNi PBA NPs immobilized on CP were directly transformed to Fe1-xNixOH by a similar hydrolysis reaction, based on X-ray photoelectron spectroscopy (XPS). The OER overpotentials, η10, normalized at a catalytic current of 10 mA cm−2 were minimized in the case of FeNi-LDH nanoflakes (x = 0.6). Depending on loading amounts of FeNi-PBA (= FeNi-LDH), the OER overpotentials were changed. In an optimized condition, the iR-corrected OER overpotential of FeNi-LDH (x = 0.6) was estimated to be η10 = 269 mV. To the best of our knowledge, the as-prepared FeNi-LDH nanoflake immobilized on CP showed the highest OER activities based on the lowest Tafel slope (15.1 mV dec−1) and the highest values of turnover frequency (1.58 s−1) and mass activity (10,600 A g−1) at a low overpotential of 300 mV (Fig.2b) . The inherently high catalytic activity is derived from the downsizing effect of a sheet dimension (= increasing OER-active sites of FeNi-LDHs) and the controlled microgram-scale mass loading procedure of FeNi-LDH nanoflakes (= inhibition of catalyst aggregation).
How to Cite
Oxgen evolution reaction, Prussian blue, FeNi-layered double hydroide
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