Templated iron single atoms in nitrogen-doped carbon O 2 reduction electrocatalysts: towards 100% active site utilisation? Angus Pedersen 1 , Jesus Barrio-Hermida 1, Saurav Ch. Sarma 1 , Silvia Favero 1 , Mengjun Gong 3 , Chang-Xin Zhao,Hui Luo 4 , Alain You Li 1 , Anthony Kucernak 1 ,
Qiang Zhang 3 , Maria-Magdalena Titirici 4 , Ifan E. L. Stephens 2 1 Department of Chemical Engineering, Imperial College London, UK 2 Royal School of Mines, Imperial College London, UK 3 Molecular Sciences Research Hub, Imperial College London, UK 4 Tsinghua University, P.R. China
Atomic Fe in N-doped C (Fe-NC) electrocatalysts for O 2 reduction at the cathode of proton exchange membrane fuel cells (PEMFCs) are the most promising alternative to expensive and low accessibility Platinum-group- metal catalysts. Recent progress of atomic Fe-NC O 2 reduction has achieved initial activity comparable to Pt nanoparticles on carbon; 1 however, their facile controlled synthesis and stability for practical applications remains challenging. To selectively form high site density atomic iron sites and avoid undesired Fe-induced carbothermal reactions, the high temperature pyrolytic step must be decoupled from the Fe loading, by using a suitable N x site template. 1–3 While such two-step approach has led to significant advances in terms of Fe-loading and mass activity, the Fe utilisation typically remains <10%, 4 owing to the difficulty of building scaffolds with sufficient porosity that electrochemically expose the active sites. Herein, we addressed this issue by coordinating Fe in a highly porous nitrogen doped carbon support, prepared by pyrolysis of inexpensive 2,4,6-Triaminopyrimidine and a Mg 2+ salt which acts an active site template and porogen. The catalyst pyrolysed at 900ºC demonstrates initial O 2 reduction kinetic mass activity of 4.0 A g carbon -1 at 0.8 V RHE, iR-free in acid electrolyte and it retains 85% of this activity after 1,500 cycles from 0.8-0.4 V RHE in an accelerated stress test under O 2 -saturated conditions. The Fe single atoms are characterised pre- and post- electrochemical accelerated stress testing by aberration corrected high-angle annular dark field scanning transmission electron microscopy. Moreover, ex-situ X-ray absorption spectroscopy (XAS) suggests the presence of Fe-N 5 sites, potentially enabling the stability of the active site for O 2 reduction. The high specific surface area (3,295 m 2 g -1 ) and mesoporous structure results in a high active site density from in-situ nitrite stripping of 2.54×10 19 sites g -1 and a record Fe utilisation of 45%. The presented work highlights how high active site utilisation can be achieved. References 1. Jiao, L. et al. Chemical vapour deposition of Fe–N–C oxygen reduction catalysts with full utilization of dense Fe–N4 sites. Nat. Mater. 20 , 1385–1391 (2021). 2. Mehmood, A. et al. Facile Metal Coordination of Active Site Imprinted Nitrogen Doped Carbons for the Conservative Preparation of Non-Noble Metal Oxygen Reduction Electrocatalysts. Adv. Energy Mater. 8 , 1701771 (2018). 3. Mehmood, A. et al. High loading of single atomic iron sites in Fe–NC oxygen reduction catalysts for proton exchange membrane fuel cells. Nat. Catal. 5 , 311–323 (2022). 4. Primbs, M. et al. Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells. Energy Environ. Sci. 13 , 2480–2500 (2020).
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