Professor Zhengxiao Guo, Departments of Chemistry and Mech Engineering, The University of Hong Kong; HKU Zhejiang Institute of Research and Innovation, Hangzhou, China;

University College London, 20 Gordon Street, London WC1H 0AJ,UK (zxguo@hku.hk; z.x.guo@ucl.ac.uk)

Catalysts underpin efficiency and economics of industrial processes, where structure, density and durability of active sites are critically important.  Graphene and its derivatives offer great potential for such purposes, as active functionalities can be effectively tuned by means of atomic doping, defect control, inter-layer spacing, porosity architecturing, and hybridisation with other nanostructures. The focus here is to demonstrate how those approaches can be effectively engineered to the development of storage materials for hydrogen, methane and CO2, and of electrochemical catalysts for oxygen reduction and/or evolution reactions (ORR or OER), which are important for rechargeable metal–air batteries and regenerative fuel cells – the energy conversion / storage technologies for portable devices, electric vehicles and the smart grid. Currently, the commercial noble metal catalysts, such as Pt/C and Ir/C, only exhibit mono-functional activity for either ORR or OER. Non-noble metal or metal-free materials are increasingly considered as cost-effective alternatives, but their catalytic activities, especially OER performance, are yet to match their metallic counterparts. Our systematic development firstly demonstrates the enrichment of N-doping and graphene / graphitic carbon-nitride intercalation are effectively for enabling rapid four-electron transfer process in ORR, and then switching of ORR and OER by single heat-treatment of a metal-organic-framework. Finally by closely coupling theory and experiment, we show the most effective catalytic sites in phosphorus-nitrogen co-doped graphene frameworks (PNGF), and then engineered the synthetic formulations to enrich such sites. The developed electrocatalysts show highly efficient bifunctionality for both ORR and OER. The ORR/OER potential gap is reduced successively from the initial 1.252 mV, to 1.037 mV with P,N co-doping, then to 795 mV after PNGF optimisation, and finally to 705 mV after purposeful enrichment of the active P–N sites. This design strategy, synthesis approach and the efficient catalysts offer great opportunities for the development of highly cost-effective energy storage technologies on a large scale.

References: Jijia Xie et al., Efficient Visible Light-Driven Water Oxidation and Proton Reduction by an Ordered Covalent Triazine-Based Framework, Energy & Env. Sci 11 (2018)1617-1624; G. Srinivas and Z.X. Guo, “Graphene-based materials: Synthesis and gas sorption, storage and separation”, Progress in Materials Science, 69 (2015) 1-60; J.Gu, M.X. Gao, H.G. Pan, Y.F. Liu, B.Li, Y.J. Yang, C. Liang, H.L.Fu, and Z.X. Guo, “Improved Hydrogen Storage Performance of Ca(BH4)2: A Synergetic Effect of Porous Morphology and In-Situ Formed TiO2”, Energy & Env. Sci, 6(2013) 847-858; S.A. Shevlin and Z.X. Guo, “Density functional theory simulations of complex hydride and carbon-based hydrogen storage materials”, Chem. Soc. Rev., 38 (2009) 211-225.

Professor Zhengxiao Guo, Departments of Chemistry and Mech Engineering, The University of Hong Kong; HKU Zhejiang Institute of Research and Innovation, Hangzhou, China;

University College London, 20 Gordon Street, London WC1H 0AJ,UK (zxguo@hku.hk; z.x.guo@ucl.ac.uk)

Catalysts underpin efficiency and economics of industrial processes, where structure, density and durability of active sites are critically important.  Graphene and its derivatives offer great potential for such purposes, as active functionalities can be effectively tuned by means of atomic doping, defect control, inter-layer spacing, porosity architecturing, and hybridisation with other nanostructures. The focus here is to demonstrate how those approaches can be effectively engineered to the development of storage materials for hydrogen, methane and CO2, and of electrochemical catalysts for oxygen reduction and/or evolution reactions (ORR or OER), which are important for rechargeable metal–air batteries and regenerative fuel cells – the energy conversion / storage technologies for portable devices, electric vehicles and the smart grid. Currently, the commercial noble metal catalysts, such as Pt/C and Ir/C, only exhibit mono-functional activity for either ORR or OER. Non-noble metal or metal-free materials are increasingly considered as cost-effective alternatives, but their catalytic activities, especially OER performance, are yet to match their metallic counterparts. Our systematic development firstly demonstrates the enrichment of N-doping and graphene / graphitic carbon-nitride intercalation are effectively for enabling rapid four-electron transfer process in ORR, and then switching of ORR and OER by single heat-treatment of a metal-organic-framework. Finally by closely coupling theory and experiment, we show the most effective catalytic sites in phosphorus-nitrogen co-doped graphene frameworks (PNGF), and then engineered the synthetic formulations to enrich such sites. The developed electrocatalysts show highly efficient bifunctionality for both ORR and OER. The ORR/OER potential gap is reduced successively from the initial 1.252 mV, to 1.037 mV with P,N co-doping, then to 795 mV after PNGF optimisation, and finally to 705 mV after purposeful enrichment of the active P–N sites. This design strategy, synthesis approach and the efficient catalysts offer great opportunities for the development of highly cost-effective energy storage technologies on a large scale.

References: Jijia Xie et al., Efficient Visible Light-Driven Water Oxidation and Proton Reduction by an Ordered Covalent Triazine-Based Framework, Energy & Env. Sci 11 (2018)1617-1624; G. Srinivas and Z.X. Guo, “Graphene-based materials: Synthesis and gas sorption, storage and separation”, Progress in Materials Science, 69 (2015) 1-60; J.Gu, M.X. Gao, H.G. Pan, Y.F. Liu, B.Li, Y.J. Yang, C. Liang, H.L.Fu, and Z.X. Guo, “Improved Hydrogen Storage Performance of Ca(BH4)2: A Synergetic Effect of Porous Morphology and In-Situ Formed TiO2”, Energy & Env. Sci, 6(2013) 847-858; S.A. Shevlin and Z.X. Guo, “Density functional theory simulations of complex hydride and carbon-based hydrogen storage materials”, Chem. Soc. Rev., 38 (2009) 211-225.