Functionalized mesoporous carbons as non-precious metal fuel cell catalysts

Doctoral thesis, 2016

Fuel cells allow for clean and efficient chemical to electrical energy conversion. However, their high price significantly limits the market viability of the technology. This is to a large part due to the high cost of the precious metals used in most fuel cell catalysts. Therefore development of new, cost efficient, highly active and durable catalyst materials is necessary in order to make fuel cells competitive. In this work a new synthesis approach for cost efficient non-precious metal (NPM) catalysts for oxygen reduction in a proton exchange membrane fuel cell is developed. The catalysts were synthesised via a silica template synthesis route for highly ordered mesoporous carbons. The use of nitrogen-containing carbon precursors in combination with transition metal salts resulted in the formation of active sites bound to the electronically conducting and mesoporous carbon matrix. Advantages of this synthesis approach are the high specific surface area of the resulting NPM material and the high tunability of the synthesis route. Detailed studies of the nature and local atomic structure of the active site in the new NPM catalysts were performed. Additionally a literature study of the state of NPM catalyst research in general was conducted and several key areas for improvements were identified. These included open questions regarding the active site in NPM catalysts and the need for NPM catalysts with improved mass transport properties. Finally, we demonstrate that the mass transport in the NPM catalysts developed here can be enhanced very efficiently by changes of the NPM catalyst mesostructure. Electrochemical evaluation of the NPM catalysts was done in a single cell fuel cell setup. Catalytic activity, mass transport properties and degradation behaviour were studied. Results showed that the developed catalysts reach an activity performance about 1/3 of commercial high platinum loaded catalysts tested under the same conditions. Catalyst stability and degradation were tested for several hundred hours showing promising durability performance. The results presented here illustrate that it is possible to synthesize metal-chelating ordered mesoporous carbon NPM catalysts in a one-pot approach. It was also shown that it is possible to vary the material structure on the mesoscale without disturbing the atomic-scale active site structure. This allows for efficient tuning of the surface area and pore volume of the catalyst.