Hydrogen has many ‘colours’. One of the most tempting is green, as it stands for the production of this gas using electrolysis driven by the renewable energy sources. Whenever green hydrogen production is considered, high costs related to its production are expected. These include capital costs as well as operational expenditures. In the first case, mainly expensive catalysts are necessary (based on noble metals), as they enable overcoming the energy barrier for water splitting. Of course, the electrolysis can run also without the catalysts, but the energy demand would be considerably higher.
A possible alternative includes a list of more abundant and affordable materials to serve as catalysts. The research on non-noble metal catalysts is conducted all over the world and many examples of the developed materials have been already put forward. Although the results show good catalytic performance of these materials, still they indicate higher overpotential towards hydrogen evolution reaction then, for example, platinum. However, at certain conditions (especially at high current densities) the performance of noble and non-noble metal catalyst can become comparable. This is possible owing to kinetic parameters and diffusion limitations which are governing the reaction rate to the highest extent. To be able to benefit from this phenomenon, a careful non-noble catalyst engineering is vital. It includes the deep understanding of the adsorption of hydrogen ions or water molecules on the heterogeneous surface and a proper management of intermediate products.
Figure 1 Metal oxide catalyst activated in-situ
The most interesting non-noble catalyst include transition metal oxides. These materials effectively reduce the overpotential towards hydrogen evolution by offering suitable adsorption sites for the substrates on their surface, especially in the vicinity of transient metal oxidation state. This property makes the electron energy and density favourable to crack the bonding, modify and fuse two hydrogen adatoms together to produce hydrogen gas molecule. In MacGhyver Work Package 2 we aim at investigating this phenomenon to approach the performance of noble metal in terms of overpotential reduction. The specific methodology involves the use of rotating disc experiment enriched with density functional theory to reveal its mechanistic pathway. This will contribute to better understanding of the electrode/electrolyte interface.
Operational expenditures are inevitably related to reduced overpotential as the energy demand for hydrogen production is the product of charge passed through the electrolyser and voltage. It has been already demonstrated in MacGhyver that almost double reduction of energy expenses is possible owing to the use of non-noble metal catalyst (from 120 kWh/kg to 62 kWh/kg). However, there is still room for improvement and all the necessary steps will be undertaken as the goal of the project is to reduce the energy consumption to a value below 45 kWh/kg.