| Summary: | Cobalt oxide (Co<sub>3</sub>O<sub>4</sub>) serves as a promising electrocatalyst for oxygen evolution reactions (OER) in water-electrolytic hydrogen production. For more practical applications, advances in dry-deposition processes for the high-throughput fabrication of such Co<sub>3</sub>O<sub>4</sub> electrocatalysts are needed. In this work, a thermal metal–organic deposition (MOD) technique is developed to form Co<sub>3</sub>O<sub>4</sub> deposits on microscale-diameter carbon fibers constituting a carbon fiber paper (CFP) substrate for high-efficiency OER electrocatalyst applications. The Co<sub>3</sub>O<sub>4</sub> electrocatalysts are deposited while uniformly covering the surface of individual carbon fibers in the reaction temperature range from 400 to 800 °C under an ambient Ar atmosphere. It is found that the microstructure of deposits is dependent on the reaction temperature. The Co<sub>3</sub>O<sub>4</sub> electrocatalysts prepared at 500 °C and over exhibit values of 355–384 mV in overpotential (η<sub>10</sub>) required to reach a current density of 10 mA cm<sup>−2</sup> and 70–79 mV dec<sup>−1</sup> in Tafel slope, measured in 1 M KOH aqueous solution. As a result, it is highlighted that the improved crystallinity of the Co<sub>3</sub>O<sub>4</sub> electrocatalyst with the increased reaction temperature leads to an enhancement in electrode-level OER activity with the high electrochemically active surface area (ECSA), low charge transfer resistance (R<sub>ct</sub>), and low η<sub>10</sub>, due to the enhanced electrical conductivity. On the other hand, it is found that the inherent catalytic activity of the surface sites of the Co<sub>3</sub>O<sub>4</sub>, represented by the turnover frequency (TOF), decreases with reaction temperature due to the high-temperature sintering effect. This work provides the groundwork for the high-throughput fabrication and rational design of high-performance electrocatalysts.
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