Hydrogen-chlorine fuel cell for production of hydrochloric acid and electric power : chlorine kinetics and cell design

Abstract

This thesis work is the continuation and final part of a joint project between the Department of Materials Technology, NTNU and Norsk Hydro Research Center in Porsgrunn, looking at the possibility of using fuel cells for production of hydrogen chloride and electric power. The experimental work encompass an evaluation of three hydrogen - chlorine fuel cell design concepts, development and implementation of a mathematical fuel cell model and a kinetic study of the chlorine reduction reaction.

The evaluated fuel cell designs consisted of a) a conventional PEM fuel cell applying a Nafion membrane, b) a composite system applying an aqueous HCl electrolyte and Nafion membrane and c) a phosphoric acid doped PBI membrane fuel cell operating at intermediate temperatures of 150 - 175 ◦C. From the evaluation it was found that the chlorine reduction kinetics are much faster than the corresponding oxygen reduction reaction, leading to low activation losses on the fuel cell cathode. However, the nature of the reactant, chlorine, and the product, HCl, places strict demands on the corrosion resistance of the construction materials and drastically increases the difficulties related to water management in the cells. Due to these effects, none of the investigated systems were able to demonstrate stable operation under the conditions used in this study. The PBI cell showed best potential and seems to be the system in which the humidification and corrosion difficulties easiest can be remedied. The first design criteria for such a system should be the minimisation of the existence of liquid water, ideally a hydrogen - chlorine fuel cell system should operate in totally water free environment and consist of a high temperature proton conductor.

A two dimensional, isothermal mathematical model of a hydrogen - chlorine single fuel cell with an aqueous HCl electrolyte is presented. The model focuses on the electrode reactions in the chlorine cathode and also includes the mass and momentum balances for the electrolyte and cathode gas diffusion layer. There is good agreement between the model predictions and experimental results. Distributions of physical parameters such as reactant and product concentrations, solution and solid phase potentials and local current densities and overpotentials as a function of cell voltage are presented. Effects of varying the initial electrolyte concentration and operating pressure are analysed. It was found that an electrolyte inlet concentration of 6 mol dm−3 gave the best cell performance and that an increase of operating pressure gave a steady increase of the fuel cell performance.

The rate and mechanism of the electroreduction of chlorine on electrochemically oxidised Pt and Ru electrodes has been investigated relative to the state of oxide formation. Current/potential curves for the reduction process in 1 mol dm−3 HCl solution saturated with Cl2 have been obtained for electrode surfaces in various states of preoxidation with the use of the rotating disc electrode technique (RDE). In the case of chlorine reduction on platinum, the results indicate that adsorption of chlorine molecules with a subsequent rate determining electrochemical adsorption step is the dominant mechanism. The exchange current density seems to decrease linearly with the logarithm of the amount of surface oxide. Chlorine reduction on ruthenium is best described by a Heyrovsky-Volmer mechanism with the first charge transfer reaction as the rate determining step. The Krishtalik mechanism incorporating adsorbed O•Cl+ intermediates is also able to describe the reaction successfully. The reaction order is constant for all oxide coverages while the exchange current density apparently moves through a maximum at intermediate oxide coverages (∼100 mC cm−2). The results show that the electrocatalysis of the cathodic reduction of chlorine is very sensitive to the state of the oxidation of the electrode surface.

The rate and mechanism of the electroreduction of chlorine on electrooxidised ruthenium has further been investigated with focus on the effect of solution pH. Current/potential curves for the reduction process in solutions with constant chloride concentration of 1.0 mol dm−3 and varying H+ concentration have been obtained with the use of the rotating disk electrode technique (RDE). It was found that the chlorine reduction rate is highly inhibited in solutions with high H+ concentrations and that it can be satisfactorily described by the Erenburgh mechanism, previously suggested for the chlorine evolution on RuO2 and ruthenium titanium oxides (RTO). The expression of the kinetic current as a function of chlorine and H+ concentration was obtained by solving the elementary rate equations of the kinetic mechanism. The kinetic constants obtained from the correlation of the kinetic current expression to the experimental data were used to simulate the dependence of the surface coverages and elementary reaction rates on overpotential.