Production of Hydrogen and Chemicals from the Electroreforming of Renewable Alcohols
Both Alkaline and PEM electrolyzers exploit water splitting, evolving hydrogen at the cathode and oxygen at the anode. The standard reaction potential for this process is 1.23 V, meaning that water splitting is a strongly uphill reaction. In practice to get current densities in the range of 1 A cm-2 the cell potential usually ranges between 1.6 and 2 V. Considering 1.8 V as a reasonable average, we conclude that 68.3% of the energy input is consumed by thermodynamics, while kinetic factors account for only 31.7%.
Replacing anodic oxygen evolution with the oxidation of much more readily oxidizable species leads to a significant reduction of the potential required to produce hydrogen. Following this strategy compounds such as ammonia, methanol, ethanol, glycerol and urea have been tested. These electrolytic processes that lead also to the concomitant generation of chemicals at the anode and hydrogen at the cathode are often indicated as “Electrochemical Reforming”.
While undoubtedly promising, these approaches suffer the limitation of delivering current densities well below 1 A cm-2, and as such are unpractical for technological exploitation. Here we propose to overcome such limitations using a range of nanostructured anode electrocatalysts which combine fast electrode kinetics for alcohol electrooxidation with tunable selectivity for the formation of partially oxidised intermediates of industrial interest. We demonstrate that electrochemical reforming can occur at current densities comparable to that of state of the art PEM water electrolysis. Furthermore we report a net energy analysis of hydrogen production by bio-ethanol electrochemical reforming. Particularly, we define the energy cost break-even point, expressed in terms of the Energy Return of the Invested Energy of bio-ethanol production, at which electrochemical reforming becomes more energy efficient than water electrolysis.