Axis II : Nanohybrid systems
Incorporating molecular catalysts in devices for hydrogen production/uptake is a major goal in the perspective of their implementation into technological devices such as fuel-cells and electrolyzers. As stated in the introduction, the covalent grafting onto an electrode material such as carbon or the incorporation of catalyst into a polymer coating an electrode would lead to new electrocatalytic materials for hydrogen production or uptake. We have developed an original strategy, combining the biomimetic molecular approach developed in Axis I with nanochemical tools and shown that the covalent attachment of a nickel-bisdiphosphine mimic of the active site of hydrogenase enzymes on carbon nanotubes results in a nickel-based cathode nanomaterial (Figure 1) with remarkable performances under the strongly acidic conditions required in the expanding proton exchange membrane (PEM) technology.
These materials are unique catalysts for hydrogen oxidation under highly acidic conditions with current densities reaching >20 mA/cm2 even in the presence of carbon monoxide (CO), a major impurity in H2 fuels derived from reformed hydrocarbons or biomass. This constitutes a major breakthrough for Nafion-based PEM fuel cells technology since CO poisoning limits the commercialization of devices based on Pt electrocatalysts. We demonstrated their possible integration in noble metal-free PEMFC fuel cell. Our program combines formulation studies as well as detailed physico-chemical characterization to progress towards educated and rational optimization of their performances with the objective of reaching current densities compatible with technological integration.
The same methodologies can be applied to elaborate molecular–engineered H2 evolution catalytic materials. Grafting indeed provides a largely increased protection of these catalysts against degradation. some of these immobilized catalysts prove tolerant to the presence of oxygen in the media.
As an alternative approach to the grafting of molecular catalysts onto a suitable electrode substrate, we also investigate the preparation of novel solid-state electrocatalytic materials. Our program specifically targets metal sulfides that mimic some of the catalytic properties of the active sites of metalloenzymes such as hydrogenases, CO-dehydrogenases or nitrogenases. Such metal sulfide materials prove quite active as H2 evolution catalysts from acidic solutions and we formulate them in catalytic layers of proton-exchange membrane electrolyzers.