(585bi) N-Doped Carbon Aerogel Supported Cobalt Catalysts By Supercritical Deposition for Oxygen Reduction Reaction

Hydrogen fuel cells are promising devices for electricity generation due their high efficiencies and low emissions as compared to internal combustion engines. One of the major obstacles towards the commercialization of fuel cells is the high cost of membrane electrode assembly which contains high amounts of Pt as the catalyst for the oxygen reduction reaction (ORR) which has very sluggish kinetics. In order to overcome this drawback, development of low-platinum, platinum-free and even metal free carbon based nanomaterials for ORR have been attracting increased attention. Among these, transition metal containing (usually Co and Fe) nitrogen-doped carbon materials are the most promising candidates. Different carbon materials such as carbon blacks, carbon nanotubes and graphene doped with nitrogen have shown promising activities. Nanostructured carbon aerogels (CAs) have been receiving increased attention due their unique properties such as high surface area, low mass density, high porosity and high electrical conductivity. Carbon aerogels are prepared by pyrolysis of organic aerogels which are prepared by polycondensation reactions of resorcinol and formaldehyde in an aqueous solution followed by supercritical drying. CAs have also advantages of a tunable three-dimensional hierarchical morphology achievable by changing the process parameters during the sol-gel polymerization, the gelation, the curing and the pyrolysis steps. Therefore, CAs with such a tunable morphological diversity are attractive alternatives to conventional N-doped carbons in preparation of cobalt or iron based ORR catalysts. There are several conventional techniques to prepare supported metal catalysts such as wet impregnation, co-precipitation and sol–gel methods. However, these techniques have some limitations and they are not suitable for highly porous materials such as aerogels because of the high surface tension of the liquids which can cause the collapse of the nano-sized pores of the aerogels. An alternative route for the preparation of aerogel supported metal nanoparticles is supercritical deposition (SCD). The use of scCO₂ for the preparation of aerogel catalysts is advantageous since the low surface tension prevents the collapse of nano-sized pores. SCD consists of the dissolution of a metal complex in a supercritical fluid with subsequent adsorption on a porous support followed by the conversion of the metal complex to its metal form. In this study, we prepared highly active N-doped CA supported Co catalysts using SCD combined with ammonia treatment. Firstly, resorcinol formaldehyde aerogels were contacted with a solution of cobalt acetylacetonate (Co(acac)₃) in scCO₂ at 20 MPa and 353 K. The cobalt precursor was found to adsorb and react on the surface of the organic aerogel. Subsequently, the resulting sample was subjected to pyrolysis performed under NH₃ flow at various temperatures ranging from 700 °C to 900 °C. After pyrolysis, a substantial amount of cobalt in the form of both metallic and non-metallic form was detected on the N-doped catalysts. The cobalt loadings were measured by ICP. For comparison, N-doped CA samples without Co were also prepared at the same pyrolysis temperatures. The higher pyrolysis temperatures resulted in lower nitrogen content in the catalyst. Surface area and pore volumes of the prepared aerogels were measured via N₂ adsorption and detailed chemical nature of the surface species were investigated using XPS.Different types of C-N peaks which were pyridinic, pyrrolic, graphitic and oxidized N as well as Co-N and metallic Co peaks were detected from the N1s and Co2p spectra of the samples in XPS. The electrochemical characterization of the catalysts was carried out using cyclic voltammetry (CV) and Rotating Disk Electrode (RDE) measurements with the non-precious catalyst loading on the RDE keeping as constant at 0.30 mg/cm² .The Co containing sample pyrolyzed at 800 °C had the highest ORR activity which was 11.03 mA/mg_catalyst as determined from the current density at 0.8 V vs RHE in 0.1 M KOH solution. To the best of our knowledge, this is the highest value reported in the literature so far. The lower activity of samples pyrolyzed at temperatures higher than 800 °C can be attributed to the decrease in the concentration of Co-N, pyridinic and graphitic nitrogen active sites on the catalyst surface which have been suggested as the active sites for oxygen electro-reduction. The activity of all Co-containing N-doped CAs for the ORR were higher than that of the N-doped CAs and the cobalt loaded catalyst treated with NH₃ at 800 °C had very close onset potential which was 1.01 V vs RHE compared to Pt/CA (1.04 V vs RHE), where the cathodic current reaches 0.1 mA/ cm² in alkaline medium.