Hydrogen is a bulk chemical produced on the 107 ton/year scale but could also be a fuel of the future. Today this hydrogen is produced from methane in steam reforming. Switching the hydrogen production to a renewable resource is a challenge, but some electrolyzers produce hydrogen on a commercial scale already from electricity (electrochemically). Today’s electrolyzers use harsh alkaline or acidic conditions, both of which are costly and increase the environmental impact of the technology. We are developing hydrogen evolution catalysts for benign neutral conditions in order to reduce hydrogen costs to a level competitive with conventional fuels. We sought inspiration in nature’s nitrogenase enzymes, which contain nickel, iron, sulfur, and nitrogen, and saw that heterogeneous analogs based on nickel and phosphide have impressive activity and stability.
1. Development of earth-Abundant Catalysts for Hydrogen Evolution
See: Laursen, AB, Patraju KR, Whitaker M, Retuerto M, Sarkar T, Yao N, Ramanujachary KV, Greenblatt M, Dismukes GC. 2015. Nanocrystalline Ni5P4: A hydrogen evolution electrocatalyst of exceptional efficiency in both alkaline and acidic media. Energy and Environmental Science. 8:1027-1034.
Producing hydrogen (H2) by splitting water with fossil-free electricity is considered a grand challenge for developing sustainable energy systems and a carbon dioxide-free source of renewable H2. Renewable H2 may be produced from water by electrolysis with either low-efficiency alkaline electrolyzers that suffer 50–65% losses, or by more efficient acidic electrolyzers with rare platinum group metal catalysts (Pt). Consequently, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements. Crystalline Ni5P4 evolves H2 with geometric electrical to chemical conversion efficiency on par with Platinum in strong acid (33 mV dec−1 Tafel slope and −62 mV overpotential at −100 mA cm−2 in 1 M H2SO4). The conductivity of Ni5P4 microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degradation is seen in short-term studies at current densities of −10 mA cm−2, equivalent to ∼10% solar photoelectrical conversion efficiency. The realization of a noble metal-free catalyst performing on par with Pt in both strong acid and base offers a key step towards industrially relevant electrolyzers competing with conventional H2 sources.