Electrodes for generation of hydrogen and oxygen from seawater

Abstract

Direct electrolysis of seawater in non-diaphragm cells results in the cathodic evolution of hydrogen at high current efficiency. Large volumes of chlorine in the form of hypochlorite solution are normally evolved at the anode, however, and this presents a major environmental problem. Relative chlorine and oxygen efficiency was determined in seawater at a variety of temperatures and salinities, and using a number of typical anode materials. Oxygen is evolved exclusively below the theoretical voltage for chlorine evolution, but current density is too low (< 1 mA/cm 2) at this point to be of practical importance. The limiting current density for chlorine evolution in seawater is variable depending on salinity, temperature, cell design, and flow considerations, but for practical cell operation will range from 100 to 1000 mA/cm 2. It can be concluded from these data that the use of conventional electrode materials at any practical conditions of electrolysis will result in significant evolution of chlorine in the form of sodium hypochlorite. Experimentally measured exchange currents and Tafel slopes for both oxygen and chlorine evolution at the DSA are used to explain the relationship between the two competing reactions under a variety of conditions. Under normal conditions of seawater electrolysis, mass transfer limitations and reaction kinetics combine to make chlorine evolution the dominant anodic reaction. Following this basic study, electrodes were prepared to specifically favor the evolution of oxygen from brine solutions. Unique manganese dioxide coatings are capable of evolving oxygen from seawater at 99% efficiency, and from saturated sodium chloride brine at 95% efficiency. A mechanism is proposed in which the surface coating acts as a diaphragm, causing the chlorine evolution reaction to more rapidly become mass transfer limited. Limiting current density of chloride oxidation can be lowered 2–3 orders of magnitude in this way, making oxygen the dominant anodic reaction at all reasonable conditions of electrolysis.