8.1 Hydrogen Production

The theoretical ('reversible') voltage required to split water at 25 C (298.15 K) and 101.325 kPa (1 atm) by electrolysis is 1.229 V; see Box 8.1 for further explanation. Hydrogen is produced at the negative electrode and oxygen at the positive electrode. The convention used to distinguish the two types of electrode polarity in electrochemical cells is discussed in Box 8.2 (Figure 8.2). The voltage for water decomposition decreases almost linearly to 1.088 V at 200 C (473 K) (Figure 8.3). The decrease in ?G with increasing temperature is largely offset by an increase in the entropy term T ?S, so that the enthalpy of the reaction, ? H, is almost independent of temperature. Since electrolyzers are essentially adiabatic ( i.e. little heat is absorbed from the surroundings), the energy corresponding to the entropy term is also supplied electrically. At 25 C, this increases the minimum cell voltage for water electrolysis to 1.47 V, and the electrical energy consumed in the reversible reaction is almost temperature-independent. In other words, the voltage is 'thermo-neutral' (upper line in Figure 8.3). Many electrolyzers operate at enhanced pressure in order to increase the production rate of hydrogen from a given size of unit, and to avoid the necessity of employing compressors subsequent to electrolysis (without compression, the storage volumes would be enormous). Raising the operating pressure from, for example, 0.1 to 2.5 MPa (1 25 atm) leads to a further increase of about 0.7 V in the reversible voltage.