Templating-Induced Enhancement Of The Electrocatalytic Activity Of Pt-free Carbons For Oxygen Reduction Reaction

For decades scientific research has been devoted to develop efficient systems to produce electrical energy by means of fuel cells. The engineering of such devices is one of the greatest challenges of the present time, however with ensuing drawbacks due to their relatively short lifetime, prohibitive cost of many involved processes and materials. Focusing on the cathodic oxygen reduction reaction (ORR), it is kinetically hindered and catalysts are compulsory. Among them platinum and platinum alloys are nowadays the best credited ones, nevertheless there are many problems arising from platinum availability and cost. Platinum-free catalysts are highly desirable and actively searched for by worldwide investigation. The aim of the work was not simply to develop carbon-based, platinum-free catalysts for ORR, but mostly to investigate the effects that any variation in the templating procedure may introduce on their final ORR behaviour. A constant reactant mixture (comprising a given sugar and a nitrogen-containing base) was 900°C heat-treated following a number of different templating techniques. Shape-imprinting materials (e.g. high surface area silica and Black Pearls 2000) were used with the aim of obtaining a replica of template porosity and surface features in the synthesis products. Some auto-templating processes (e.g. freeze-drying and in-situ generation of gas) were also experimented. Reaction products were characterized by physico-chemical methods comprising BET surface area and cyclic voltammetry for ORR by rotating disk electrode. An accurate analysis of these results show that the catalyst activity depends deeply on its structural and morphological features. ORR improves with increasing sample surface area, which is a general effect related to the increased number of available reacting centres. Other interesting aspects may have more specific origins. The first one is related to the relative abundance of mesopores with sizes between 5 and 20 nm, facilitating mass transport processes from/to recessed active sites. Whereas the last distinctive feature of samples with a satisfying ORR behavior is imprinted in their nitrogen adsorption isotherm: bad catalysts isotherms have the typical shape of bottle-neck pores. The above considerations let us infer that a smart choice of the proper templating procedure, besides the obvious compositional aspects, might lead to a final sample with an enhanced catalytic activity towards ORR deriving from an optimized merging of surface properties and pores shape.