A Low-Cost Carbon-Based Oxygen Electrode for Polymer Membrane Fuel Cells

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1 A Low-Cost Carbon-Based Oxygen Electrode for Polymer Membrane Fuel Cells Dr. Qiong Cai Department of Chemical and Process Engineering, University of Surrey

2 Challenges of Oxygen Reduction Reaction (ORR) Multi-electron transfer Various electron transfer processes via 2e - or 4e - Slow kinetics Cathode ORR is six or more orders of magnitude slower than the anode hydrogen oxidation reaction (HOR) High Pt loading Typical amount is mg Pt cm -2 Poison by CO Activity and durability degradation [1] M.K. Debe, Nature 486 (2012)

3 Metal-Free Carbon Catalysts for ORR Carbon nanotube C 60 Graphene Graphite 3D graphene- CNT hybrid Diamond [2] L.M. Dai, Y.H. Xue, L.T. Qu, H.J. Choi, J.B. Baek, Chem Rev 115 (2015)

4 Heteroatom Doping N-doped Attract electrons from the adjacent C atom Modulate O 2 chemisorption energy Weaken the O-O bonding Facilitate the direct oxygen reduction via a 4e - pathway [2] L.M. Dai, Y.H. Xue, L.T. Qu, H.J. Choi, J.B. Baek, Chem Rev 115 (2015) [3] C. Zhu, H. Li, S. Fu, D. Du, Y. Lin, Chem Soc Rev 45 (2016)

5 Structural Effects Surface area High surface area to accommodate adequate active sites; Graphitization degree Structural Effects Morphology Unique morphology to enhance the exposure of catalytic centers thus improving the utilization efficiency; Hierarchical porous structure to facilitate the diffusion and adsorption of oxygen molecules; Porosity High graphitization degree to avoid electrochemical oxidation and the loss of surface catalytic sites. [4] W.H. He, Y. Wang, C.H. Jiang, L.H. Lu, Chem Soc Rev 45 (2016)

6 Controlled Synthesis of Porous Carbon Carbonization and activation of carbon precursors containing pore formers Sol gel process to synthesize mesoporous carbon gels or aerogels Carbonization of carbon precursor in confined nanospaces of template and subsequent dissolving template feasible to adjust the morphology, pore structure and surface area [5] A.P. Wang, F.Y. Kang, Z.H. Huang, Z.C. Guo, X.Y. Chuan, Micropor Mesopor Mat 108 (2008)

7 Halloysite Template Aluminosilicate clay mineral One alumina octahedron sheet and one silica tetrahedron sheet (1:1) Hollow tubular structure and rich mesopores Inner diameters of nm and outer diameters of nm Specific surface area is m 2 /g [6] X.P. Wu, C. Liu, H.J. Qi, X.L. Zhang, J.J. Dai, Q.X. Zhang, L. Zhang, Y.C. Wu, X.H. Peng, Appl Clay Sci 119 (2016)

8 Project Objectives To synthesize N-doped carbonaceous catalysts by using two different carbon sources; To direct the structure by using tubular halloysite templates through different synthesis processes; To identify the structural effects of the morphology, surface area, porosity, nitrogen doping level and species on the ORR performance; To evaluate the catalytic performance by single fuel cell measurements. 8

9 Experiment Vacuum infiltration & Stirred Heating Characterization STEM, XPS, BET 9

10 Microstructure GU FU a b e f c d g h Flaky morphology was obtained with glucose and urea Tubular morphology was obtained with furfural and urea 10

11 XPS Analysis Catalysts N Pyridinic-N Pyrrolic-N Graphitic N Pyridinic N + - O - GU 5.18% 40.58% 4.59% 53.51% 1.32% FU 6.31% 33.79% 2.97% 56.73% 6.51% 11

12 BET Analysis Catalysts S BET (m 2 g -1 ) V p (cm 3 g -1 ) D p (nm) GU nm FU nm, 3.62 nm Both has a hysteresis phenomena, indicating the formation of mesopores FU catalysts have higher surface area and larger pore volume with both micro- and meso- pores 12

13 Electrochemical Measurement Ink preparation Carbon catalyst: 3 mg Isoproponal (IPA)=200 µl H 2 O=770 µl Nafion (5 wt%)=30 µl Ink concentration: 3 mg/ml Sonication for 1h Drop 5 µl to 3 mm RDE electrode Drop 13.8 µl to 5 mm RRDE electrode Electrochemical measurement Electrolyte: 0.1 M KOH CV in O 2 LSV in O 2 at 1600 rpm LSV in O 2 at different rotating speeds from 400 to 2400 rpm Testing with RRDE to calculate n and %H 2 O 2 Catalyst loading: 196 µg/cm 2 13

14 Cyclic Voltammogram (CV) and Linear Sweep Voltammogram (LSV) CV in 0.1 M KOH saturated with O 2 LSV in 0.1 M KOH saturated with O 2 at 1600 rpm FU shows more positive O2 reduction peak More positive onset potential and half-wave potential Larger limiting diffusion current density 14

15 Koutechy-Levich Plot 1 J 1 J L 1 J K 1 B 1/ 2 B=0.62nFC 0 (D 0 ) 2/3 ν -1/6 GU: n=3.48 FU: n=3.82 J 1 k 15

16 Electron Transfer Number and H 2 O 2 Yield----RRDE Catalyst Diffusion limiting current density / (ma cm -2 ) Onset potential / (V vs. RHE) Half-wave potential / (V vs. RHE) n %H 2 O 2 GU FU

17 Anion Exchange Membrane Fuel Cells (AEMFCs) Testing MEA Fabrication: Anode: Pt/C (40%) with Pt loading=0.5 mg cm -2 Cathode: FU catalysts with loading=3 mg cm -2 Anion Membrane Exchange Testing Conditions: T cell =60, H 2 /air: 1000 ml/min Back pressure: 0 Bar Relative humidity: 100% Catalyst : ionomer=4:1 Fuel Cell Testing is still being optimised. 17

18 Summary By using different carbon precursors and through different synthesis processes flaky and tubular carbonaceous catalysts were obtained; The nitrogen species and corresponding contents of the two catalysts are very similar; Tubular FU catalysts have a higher surface area, larger pore volume and diverse pore types compared with flaky GU catalysts; The better ORR activity of FU catalysts may benefit from its unique structure. 19

19 Future Work Investigation of other types of morphology, e.g. porous carbon spheres----in collaboration with Dr Bahman Amini (Surrey) Synthesis of hybrid porous carbons with non-precious metals, e.g. Fe, Co, to further enhance the ORR catalytic activity Optimisation of fuel cell testing to achieve higher power density 19

20 Acknowledgements Dr. Yaxiang Lu Professor John R. Varcoe Dr. Lianqin Wang Professor Maria-Magdalena Titirici Kathrin Preuss 21

21 Thank you for your attention! Any Questions?