Cyclic voltammetry was used to determine the electrocatalytic activities of 31 metals for the hydrogen and oxygen evolution reaction in 0.1M H~SO4 at 80~ For the hydrogen evolution reaction, properties of the metals such as electronic structures, work functions, and metal-hydrogen bond strengths tend to correlate with the observed electrocatalytic activities. The best electrocatalysts for the hydrogen evolution reaction in the order of decreasing catalytic activity are Fd > Pt --~ Rh > Ir > Re > Os ~ Ru > Ni. For the oxygen evolution reaction, the order of catalytic activity is Ir ~-Ru > Pd > Rh > Pt > Au > Nb. Most other metals undergo anodic corrosion and cannot be used for the evolution of oxygen in acid solutions. The potentials at which corrosion is observed are presented for these metals.Most commercial water electrolyzers operate at 70 ~ 90~ in 25-35 weight percent (w/o) potassium hydroxide solutions using nickel anodes (1, 2). A possible improvement in water electrolysis as a route for hydrogen production is the use of solid polymer electrolytes such as General Electric's perfluorinated sulfonic acid polymer. During electrolysis, the hydrogen ions produced by the oxidation of water move across the solid polymer electrolyte and are reduced to form hydrogen at the cathode. One disadvantage of such a system is the acid environment which develops at the anode causing corrosion of metals such as nickel. In this study, various metals are investigated for possible use as electrocatalysts for water electrolysis in an acid medium at 80~ A previous study in alkaline solutions has shown that cyclic voltammetry is a convenient method for evaluating electrocatalysts for water electrolysis (3).Experimental The cyclic voltammetric measurements were made with a PAR Model 170 instrument using a potential sweep rate of 50 mV/sec. Most electrodes consisted of high purity wires which were spot-we:ded to nickel or copper leads and sealed into glass tubing with clear epoxy. These electrodes were mechanically polished with emery paper prior to use. The ruthenium and osmium electrodes were formed by electroplating the metal onto a platinum wire electrode. A ruthenium oxide electrode formed on titanium (RuO2/TiO.2) similar to those used in the chlor-alkali industry (4) was also tested. The hydrogen evolution reaction was investigated first in order to minimize surface changes due to electrode oxidation or oxide film formations.Reagent sulfuric acid (Baker) and distilled water were used to prepare the 0.1M H2SO4 solution. Measurements were made in a beaker-type glass cell equipped with a cap, a cylindrical platinum screen counterelectrode, a saturated calomel reference electrode (SCE) with a ceramic junction which gives a negligible leak rate, and the test electrode. The temperature was controlled at 80 ~ • 2~ in all experiments.
ResultsThe cyclic voltammetric traces for the iridium electrode in 0.1H H2SO4 at 80~ are shown in Fig. 1. The short potential cycles at each end show the results for studies of the hydrogen and oxygen evo...
The electrode kinetic parameters for hydrogen and oxygen evolution were determined at temperatures of 80°, 150°, 208°, and 264°C on nickel electrodes in
50 normalweight per centKOH
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
2RT/3boldF
at low overpotentials to about
2RT/boldF
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
normalΔH
) at current densities commonly used in commercial water electrolyzers.
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