Abstract
The electrode kinetic parameters for hydrogen and oxygen evolution were determined at temperatures of 80°, 150°, 208°, and 264°C on nickel electrodes in solutions. Improvements in the exchange current density with increasing temperature were more significant for the oxygen evolution reaction than for the hydrogen evolution reaction. A favorable change in the Tafel slope for the oxygen evolution reaction occurs between 150° and 264°C which corresponds to an increase in the transfer coefficient from 0 67 to 3 3. This result supports the concept that a change in the reaction mechanism occurs for the oxygen electrode reaction near the Neel temperature of nickel oxide. At the higher experimental temperatures, the Tafel slope for the hydrogen evolution reaction changes from at low overpotentials to about at high overpotentials suggesting a slow electrochemical desorption mechanism. The present study indicates that significant reductions in cell voltage for water electrolysis can be obtained by higher operating temperatures At temperatures of about 150°C, it should be possible to approach a 100% energy efficiency (based on ) at current densities commonly used in commercial water electrolyzers.
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