Int. J. Hydrogen Energy, Vol. 7, No. 12, pp. 933-937, 1982. Printed in Great Britain. 0360-3199/82/120933435 $03,00/0 Pergamon Press Ltd. © 1982 International Associationfor Hydrogen Energy. OPERATION OF A STEADY-STATE pH-DIFFERENTIAL WATER ELECTROLYSIS CELL O. TESCHKE and M. G. ZWANZ1GER Instituto de Fisica, UNICAMP, Campinas, 13100, SP Brazil (Received for publication 24 March 1982) Abstract--The reversible potential for conventional water electrolysis is rather high, 1.23 V at 25°C. In this paper we present a new water electrolysis process that uses a steady-state pH-differential maintained by a heat source. We built and tested a cell that operates under these conditions and that consumes considerably less electricity than the conventional method for electrolytic hydrogen production. INTRODUCTION Hydrogen attracts considerable attention as an energy storage medium and raw material for the chemical industry [1]. Although various methods have been pro- posed for the large scale production of hydrogen from water, water electrolysis remains the simplest method [21. It is possible to increase the efficiency of water elec- trolysis significantly by an appropriate choice of anode and cathode materials. A route, other than finding better electrocatalysts to reduce activation overpoten- tials in electrolysis cells, is to decrease the water-splitting thermoneutral voltage under dynamic conditions. The minimum voltage required for water electrolysis at 25°C and 1 atm. is V = 1.23 V (reversible voltage), with heat added. The thermoneutral potential at 25°C is V'= 1.4.7 V. 1.5 1.0 0.5 Z x o~-o.s ~ -'-" -I.0 I I J I I 0 2 4 6 8 I0 12 pH Fig. 1. Thermodynamic voltage vs pH. 14 MODEL OF A STEADY-STATE pH- DIFFERENTIAL ELECTROLYSIS CELL Fig. 1 is a diagram of the thermodynamic reversible voltages for the evolution of hydrogen and oxygen in liquid phase water electrolysis showing the reversible potentials of the oxygen and hydrogen electrodes as a function of hydrogen ion concentration in the pH range 0-14. The thermodynamic voltage for the electrolysis of water is approximately V = 1.23 V and is independent of pH when both the cathode and anode are operated at identical pH (symmetric electrolysis). From this diagram we observe that oxygen evolution occurs 0.8 V less anodic at pH 14 than at pH 0. It can also be seen that hydrogen evolution has its lower voltage in an acidic pH solution. It is possible then, in principle, to build a cell in which the pH at the cathode (hydrogen evolution) is acid and the anode (oxygen evolution) is basic, and obtain water decomposition with a minimum theoretical voltage of approximately V'= 0.4V at 25°C and 1 atm. Electric energy input would be substantially decreased relative to symmetric electrolysis. The above condition clearly is not obtained under thermodynamic equilibrium. Schematically, the steady-state pH-differential elec- H20 2 H 2 ACIDIC AQUEOUS SOLUTION I CATHOD E l ¢ COMPARTMENT , ..., ..... ' VXAA/VV~ 'NAAAAA/V + [ ANODE COMPARTMENT I BASIC AQUEOUS SOLUTION 02 I --____j CATION FLUX ------ ~ WATER FLUX 1/"////,,/l CATION CONDUCTOR FVVXf~'XA POROUS ELECTRODE ANION IMPERMEABLE MEMBRANE Fig. 2. Schematic diagram of the steady-state pH-differential water electrolysis cell for a pH equal to 0 at the cathode and equal to 14 at the anode. 933