Qiu, K; (2017) Cost-effective and durable graphene based oxygen electro-catalysts in alkaline media. Doctoral thesis , UCL (University College London).

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Abstract

Development of cost-effective and durable catalysts for sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is imperative to promote critical energy conversion and storage technologies such as fuel cells and metal-air batteries. Low-cost non-precious metal (NPM) and metal-free (MF) catalysts are promising choices to replace commercial noble metal counterparts. However, there exist crucial challenges in developing such catalysts with comparable or superior performance to the noble metals, while ensuring effective cost reduction. Hence, the primary goal of this thesis is to design and develop low-cost graphene-based ORR / OER electrocatalysts in alkaline media, either in the form of MF catalysts or as a support / co-catalyst to NPMs. Three types of graphene-based systems were designed and evaluated.  The first system involves “MF for ORR” – Intercalated graphene/graphitic carbon nitride (GCN): It was synthesised through intercalation of graphene by GCN to enhance the electrical conductivity while maintain sufficient catalytic sites. Utilisation of hierarchically porous structures can further increase the accessible active sites and improve mass transfer. The optimised structure shows comparable ORR activity and superior durability to commercial Pt/C.  The second system is based on “NPM for ORR” – Maghemite embedded N-doped graphene framework (γ-Fe2O3/N-GF): The hierarchical N-GF substrate was firstly optimised to achieve high pore volume for rapid mass transfer, and then incorporated by an appropriate amount of γ-Fe2O3, via “one-pot” synthesis, to boost reduction potentials without compromising active site accessibility. The optimised structure outperforms Pt/C for ORR, both in activity and durability.  The third system focuses on “MF for bifunctional ORR/OER” – A P,N co-doped graphene framework (PNGF) shows almost identical activity and greater durability than commercial Pt/C for ORR and Ir/C for OER, respectively. Closely coupled first-principle simulations and experiment design show that the OER performance is strongly correlated with P-N bonds, while the ORR activity with N-doped moieties.

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