Abstract
Mixed nickel-iron (Ni-Fe) compounds have recently emerged as promising non-precious electrocatalysts for alkaline water splitting. The understanding of the charge-transfer mechanism involved in the multi-step Faradic reaction, however, is still limited for the overall electrochemical process. In this paper, electrochemical impedance spectroscopy (EIS) measurements of Fe incorporated Ni oxide nanosheets were used to study the reaction kinetics for both hydrogen (HER) and oxygen (OER) evolution reactions in alkaline media. Our results showed that Fe incorporation improves the catalytic property of NiO nanosheets because of the lower reaction resistance and faster intermediate transformations. Detailed EIS modeling enables a separation of the surface coverage relaxation from the charge transfer resistance, with an inductive behavior observed in the low-frequency range for HER, holding important information on the dominating reaction mechanism. For OER, the good agreement between the EIS experimental results and a model with an inductance loop indicated that similar inductive behavior would be determining the EIS response at very low frequencies. The physical significance of the elementary steps gives insight into the governing reaction mechanisms involved in the electron and hole charge transfer, as well as the inherent properties of catalysts and their surface coverage relaxation.
Highlights
Electrochemical water splitting is a promising approach to produce hydrogen (H2 )for clean energy applications powered by renewable but intermittent energy sources, e.g., solar, tidal, and wind [1,2,3,4]
Based on an extension of the equivalent circuits (ECs) proposed by Harrington and Conway (HC) [26], we provide an investigation of this effect under operation and elucidate an approach that can rationalize the phenomena involved in the Fe-NiO system, thereby providing the rate-limiting reaction mechanism and kinetic properties for overall alkaline water splitting
The catalytic electrodes in this work were synthesized by a chemical bath deposition (CBD) method described in the following
Summary
Electrochemical water splitting is a promising approach to produce hydrogen (H2 ). Electrochemical impedance spectroscopy (EIS) is used to investigate the influence of Fe on the intrinsic charge-transfer kinetic of NiO nanosheets, leading to enhanced activities for overall water splitting in alkaline media. The ability of EIS to cover a wide range of frequencies at different direct current (DC) potentials allows us to investigate and understand the electrochemical behavior of multistep processes that influence the catalytic property [24,25]. Based on an extension of the equivalent circuits (ECs) proposed by Harrington and Conway (HC) [26], we provide an investigation of this effect under operation and elucidate an approach that can rationalize the phenomena involved in the Fe-NiO system, thereby providing the rate-limiting reaction mechanism and kinetic properties for overall alkaline water splitting
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