Electrocatalysts design for selective oxidation of organic molecules in high-performance fuel cell
In the long run, electrocatalysts should be rationally designed instead of being developed by random trial and error approaches. The present Ph.D. research work is to push forward toward this ultimate goal by establishing some detailed strategies of electrocatalysts design for selective oxidation of organic molecules such as alcohols (methanol, ethanol, ethylene glycol, and glycerol), aldehydes (formaldehyde, acetaldehyde, glycolaldehyde, glyceraldehyde and glyoxylate), and carbohydrazide, for fuel cell applications.
The first strategy is designing the morphology of catalysts (e.g. roughness, 3D structure) with optimized particle size and dispersion. With surface area increasing, surface dealloyed PtCo nanoparticles supported on carbon nanotube (SD-PtCo/CNT) were prepared by ex-situ method and used for crude glycerol oxidation. SD-PtCo/CNT anode catalyst based AEMFC with a 0.5 mgPt cm-2 achieved peak power densities of 268.5 mW cm-2 (88% crude glycerol/O2) and 284.6 mW cm-2 (high purity (99.9%) glycerol/O2) at 80 oC and ambient pressure. 3D graphene, a carbon-only catalyst prepared by reduction of carbon monoxide with lithium oxide, is found to electrochemically catalyze carbohydrazide oxidation reaction efficiently. Prototypes of anode metal catalyst free and completely metal catalyst free anion exchange membrane fuel cell with 3D graphene anode catalyst generate peak power density of 75.1 mW cm-2 and 24.9 mW cm-2, respectively. With carbon nanotubes as the anode catalyst (3D catalyst layer), anode metal-catalyst-free and completely metal-catalyst-free direct carbohydrazide anion exchange membrane fuel cells have demonstrated to be able to generate a peak power density of 77.5 mW cm-2 and 26.5 mW cm-2, respectively.
The second strategy is comparing different monometallic catalysts’ activity towards different reaction intermediates through experiments, and rationally combining them to facilitate entire reaction network by taking advantage of the synergetic effect. For electrocatalytic oxidation of glycerol, it is found Pd can re-adsorb the desorbed glycerate in the bulk electrolyte and further oxidize it to tartronate while Au can alleviate C-C bond cleavage of C3 species. Therefore, combining Pd and Au takes advantage of these effects synergistically, maximizing the yield of tartronate in electrocatalytic glycerol oxidation reaction. PdAu alloy also increases glycerol and glycerate reaction rate, so that higher power output and yield of tartronate in shorter reaction time can be achieved in anion exchange membrane fuel cell.
The third strategy is screening elements with quantum theory and calculation for the activity towards certain reaction intermediates. After experimental confirmation of theoretical prediction results, combining the well-defined active sites to obtain the catalyst with maximum activity towards the entire reaction network. Orbital energy difference matching is proposed as a new descriptor for screening electrocatalysts with respect to the charge transfer process, during which the homogeneous reaction can be electrochemically heterogenized with polarized electrocatalytic materials. Combining different catalytic active sites targeting at original substrate or reaction intermediates is proposed as a new strategy of designing multi-functional catalysts to take advantage of spillover effect in heterogeneous catalysis. Herein, electrocatalytic oxidation of aldehyde and alcohol serves as an example, the range of electrocatalyst candidates for intermediate aldehyde oxidation was firstly narrowed to Lewis soft acids via hard and soft acids and bases (HSAB) theory from an adsorption aspect, followed by targeting at Ag, Au and Pd with the descriptor of orbital energy difference matching from a charge transfer aspect. After confirming the high electrocatalytic activity of Ag towards aldehyde (formaldehyde, acetaldehyde and glyoxylate) oxidation with cyclic voltammetry, Pd for hydroxyl group deprotonation and Ag for aldehyde intermediates oxidation were combined for the highest activity towards the whole alcohol oxidation reaction network. The bimetallic PdAg supported on carbon nanotube (PdAg/CNT) was prepared via a wet-chemistry method, and then applied as a highly active anode catalyst (0.5 mgPd cm-2) for direct alcohol fuel cells (DAFCs) with peak power densities of 135.1 mW cm-2, 202.3 mW cm-2, 245.2 mW cm-2, 276.2 mW cm-2 when using methanol, ethanol, ethylene glycol and glycerol as fuel respectively, which are much higher than monometallic Pd anode catalyst-based DAFCs.