Grafting Catalysts for Hydrogen production/uptake on nanostructured electrodes
Incorporating the 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 above biomimetic molecular approach 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. X-ray absorption spectroscopy measurements confirm that the catalysts are stable and retain their structure during turnover.
Such newly developed bio-inspired molecular H2 oxidation catalysts could be implemented in a noble metal-free PEMFC fuel cell, with 0.74 V open circuit voltage and 23 µW×cm-2 output power under technologically relevant conditions.
Figure 1: Schematic representation of the structure and reactivity of the material of the catalyst obtained by grafting bio-inspired nickel based on carbon nanotubes. Electrons are exchanged between the nanotubes and the complex that catalyzes the reduction of protons to hydrogen or oxidation of the latter with the amine functions.
This material incorporated in a membrane-electrode assembly indeed evolves H2 from aqueous sulphuric acid solution without overvoltage and proves exceptionally stable (> 100.000 turnovers). This Pt-free catalyst is also very efficient for hydrogen oxidation under the same conditions with current densities similar to those observed for hydrogenase-based materials. In addition the catalytic activity for H2 uptake displayed by this novel molecular engineered electrocatalytic material is sustained 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 also recently developed a straightforward and highly convenient preparation of these new electrocatalytic materials compatible with standard deposition/printing of an ink containing the electro-active material with tunable catalyst loading.
The same methodology has been used to stabilize a diimine-dioxime cobalt catalyst at the surface of an electrode made of carbon nanotubes (Figure 2). Again, this yields a very active electrocatalytic cathode material, mediating H2 generation with a turnover number of 55.000 TON in 7 h from pure aqueous solutions (acetate buffer pH 4.5) at relatively low overpotentials (onset potential for H2 evolution is -350 mV vs RHE). Scanning electron microscopy was used to exclude the formation of any particle at the surface of carbon nanotubes and X-ray photoelectron spectroscopy confirmed the preservation of the electronic structure of the grafted molecular complex after extensive catalytic turnover. This material is thus remarkably stable, allowing extensive cycling without degradation, which contrasts with the limited stability of the same molecular catalyst under bulk electrolysis conditions. This clearly indicates that grafting provides a largely increased protection of these cobalt catalysts against modification and allows preserving their molecular structure and the related activity. Importantly, these immobilized catalysts proved tolerant to the presence of O2 molecules in the media.
This research project has been initiated through the NanoSciences transversal program of the CEA and was carried out in collaboration with the Laboratory of Chemistry of Surfaces and Interfaces (Serge Palacin and Bruno Jousselme, Matter Science Division of the CEA, Iramis, SPCSI), the Laboratory for Innovation in Energetic Technologies and Nanomaterials (Nicolas Guillet and Laure Guetaz Liten/DEHT/LCPEM) and the Laboratory of Electronics and Information Technology (Muriel Matheron, Leti/DTBS/SBSC/LCMI).
Figure 2: Schematic representation of the structure of the catalytic material obtained by grafting of the complex of cobalt diimine-dioxime of carbon nanotubes.