Oxygen can quickly poison experimental molecular catalysts that might be used in fuel cells where a common metal has been used to mimic an enzyme active site as an alternative to costly noble metal catalysts. Now, a team from the Ruhr-Universität Bochum (RUB), the Max-Planck-Institute for Energy Conversion in Mülheim in league with colleagues at the Pacific Northwest National Laboratories, Washington, USA, have found the oxygen antidote for their catalysts. [A. A. Oughli et al, Nature Commun (2018); DOI: 10.1038/s41467-018-03011-7]
Currently, platinum and other precious metals are used as the catalysts for hydrogen/oxygen fuel cells. But, rarity and cost mean the economics of a future hydrogen economy might be stymied by our reliance on such metals. Molecular catalysts that use nickel and/or iron and mimic the active centre of hydrogenase enzymes and other biocatalyst could drive fuel cell research down new avenues. Unfortunately, oxygen damage has put the brakes on this route to some extent.
One interesting class of molecular catalyst is the DuBois type complexes. Their active center comprises a central nickel-atom that is coordinated by pendant bases. These catalysts are very active, almost on a par with the hydrogenases they seek to mimic. Their pendant ligands can also be fine-tuned to allow aqueous activity and to attach them to electrode surfaces, which is critical for immobilization and enhancing performance. High oxygen sensitivity remains a serious hindrance. Of course, the active site in a hydrogenase enzyme is protected by the surrounding protein chains and sheets. By analogy the reducing environment of a polymer matrix might be used to mimic the protective protein and save a DuBois catalyst from degradation.
To this end, the researchers have introduced a hydrophobic and redox-inactive polymer as immobilization matrix for their nickel-complex based catalyst. The polymer matrix in two layers allows the team to maintain the active site close to the electrode surface while protecting it at the interface between polymer and oxidizing electrolyte . The first layer facilitates efficient conversion of hydrogen at the electrode surface and the second layer removes incoming oxygen.
The polymer also blocks the transfer of electrons from the active hydrogen oxidation layer at the electrode surface to the protection layer. The team has demonstrated that their cat protection scheme endows the system with excellent long-term stability and remarkable current densities. Such characteristics are essential for a useable fuel cell.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".