(Invited) A Critical Review on the Expressions of Energy Efficiency Used in Low Temperature Water Electrolysis

Abstract

The accurate knowledge of the energy efficiency coefficient of low temperature water electrolysis (Alkaline, PEM or AEM Electrolyzer) is a critical-point to estimate the total cost of electrolytic hydrogen production. After describing the fundamentals of these three processes and presenting the main data of water electro-dissociation under given temperature and pressure conditions, the energy efficiency coefficient can be calculated at the Laboratory scale by taking the ratio of the theoretical energy (ΔH) to decompose water under equilibrium conditions (no current) to the real energy used (ΔH + energy losses) to electrolyze water at a given current intensity. Different approximations of this coefficient are discussed and compared to data from literature. Conversely in the industry this coefficient can be calculated by taking the ratio of the total energy of hydrogen collected at the exhaust of the electrolyzer to the total energy (electricity + heat) supplied to the complete system. It is demonstrated that both definitions are consistent and similar if all the energy involved is taken into account. It is also shown that some approximations, in particular those neglecting the thermal energy in the denominator of this ratio, give efficiency coefficients larger than unity for cell voltages lower than the thermo-neutral voltage, which is a thermodynamic non-sense.The next figure shows the plot of the theoretical energy efficiency coefficient ε cell for p = 1 bar under the STP conditions (U0 tn = 1.48 V and U 0 rev = 1.23 V) up to Ucell = 2.0 V. There is a continuous decrease of ε cell,th vs. Ucell from 1.0 to 0.85 when Ucell = Utn = 1.48 V under STP conditions. Figure 1