Diamond Annual Review 2021/22

86 87 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Biomass-derived low cost and scalable catalysts for oxygen reduction Related publication: Feng, J., Cai, R., Magliocca, E., Luo, H., Higgins, L., Romario, G. L. F., Liang, X., Pedersen, A., Xu, Z., Guo, Z., Periasamy, A., Brett, D., Miller, T. S., Haigh, S. J., Mishra, B., &Titirici, M. Iron, Nitrogen Co-doped carbon spheres as low cost, scalable electrocatalysts for the oxygen reduction reaction. Advanced Functional Materials 31, 2102974 (2021). DOI: 10.1002/adfm.202102974 Publication keywords: Oxygen reduction reaction, Non-PGM catalysts, Anion exchange membrane fuel cell H ydrogen and fuel cells play a vital role toward reaching 2050 net-zero carbon emissions targets. However, the oxygen reduction reaction (ORR) occurring at the fuel cell cathode is slow, reducing energy conversion efficiencies, and the platinum-based catalysts currently in use are expensive. Significant progress has beenmade in the development of non-preciousmetal catalysts for ORRelectrocatalysis. Catalysts based on iron atoms coordinated to nitrogen and supported on carbon offer promising performance in the ORR. A teamof scientists usedDiamond Light Source’s Core EXAFSbeamline (B18) to investigate the local coordinationof ironwithin the free-standing carbon electrodes and confirm nitrogen-iron catalytic active sites. The bending magnet source at B18 allows for high sample throughput and a wide range of detection capabilities, ideal for the Extended X-ray Absorption Fine Structure (EXAFS) studies required in this investigation. Furthermore, the remoteoperation capabilities of B18and thegenerous support of thebeamline staffallowed this experiment tobeperformed remotely during the COVID-19 pandemic. The quality of the data from B18 allowed for the successful determination of the Fe-N 4 active site on the electrode. It also highlighted the presence of iron carbide nanoparticles, whichwere later confirmed using X-ray Photoelectron Spectroscopy (XPS) and High-Angle Annular Dark- Field Scanning Transmission ElectronMicroscop y (HAADF-STEM). The study revealed the local coordination environment and the oxidation state aroundthe ironcentres. Italsoshowedthat theoxidationstateof the ironprecursor influences theoxidationstateof theFe-Nsite intheobtained catalysts, which in turn influences the catalytic activity. This work provides a newperspective on understanding the catalytic active sites. Great progress has been made to date in designing non-precious metal- based catalysts for fuel cell cathodes, specifically for the sluggish oxygen reduction reaction (ORR) based on Fe, Co, Mnwith acceptable yet inferior catalytic performance to the noble-metal catalysts 1-3 . The M-N-C catalysts (where M is a transition metal single atom) are among the most promising electrocatalysts for ORR. The key characteristic of these catalysts is the presence of M-N x , which has shown high stability and high catalytic activity 4 . Fe-N-C catalysts have shown superior performance among various metals, which has attracted considerable recent attention from the research community. Various aspects, including the local structure of the Fe-N sites, synergetic performance of different iron species, stability of the Fe-N coordination structures, active site evolution from raw precursors into the final carbon materials during carbonisation, and the oxygen reduction reaction (ORR) pathways have been investigated. Although great progress has been made, there are still challenges in establishing the exact structure-to-property correlation in such catalysts, which is essential for the rational design and synthesis of new catalysts with tailored activities for wide ranges of electrocatalytic processes 4 . In this work, a facile route to construct scalable, low-cost iron nitrogen- doped carbon spheres as high-performance ORR catalysts were prepared. Firstly, hydrothermal carbon spheres with abundant oxygen functionalities were synthesised. The obtained hydrothermal carbon spheres were then impregnated with iron precursors (FeCl 2 or FeCl 3 ) and a nitrogen precursor (melamine), followed by two-step carbonisation under inert N 2 gas. Iron and nitrogen were hybridised into the carbon support during the carbonisation process, followed by acid treatment to remove any free metallic iron species formed on the surface, allowing only Fe-N x complexes to remain (Fe@NCS-A, where A represents acid). Samples impregnated with FeCl 2 or FeCl 3 are denoted as Fe 2+ @ NCS-A and Fe 3+ @ NCS-A, respectively, where -A refers to acid wash. Gram scale of Fe@NCS catalyst (≈51 wt% yields) can be easily obtained in one batch, demonstrating the scalability of this reaction. To obtain an overview of the morphology for the obtained electrocatalysts, high-resolution High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) was employed. As shown in Fig. 1a-b, both obtained electrocatalysts showed a size range of 200-500 nm. Most of the formed carbon spheres have uniformbrightness, with only a small portion showing the presence of the nanoparticles. Energy-Dispersive X-ray Spectroscopy (EDS) was performed to map the presence of Fe species, where most carbon spheres showed a well- distributed Fe signal, suggesting that Fe may exist in the form of isolated sites within the carbonmatrix. X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) data were collected on the B18 to study the chemical configuration of Fe-N single sites. To reveal the local structure of catalysts, XANES signals were compared with theoretical XANES spectrums reported by Zitolo et al . 5 , where FeN 4 without additional ligand gave the best match for both catalysts. Moreover, the best fit values for EXAFS fittings (Fig. 2a-b) of Fe 2+ @NCS-A give an average coordination number of 4.0 ± 0.4 for Fe-N at 2.10 ± 0.01 Å and 2.4 ± 0.4 Fe-C at 1.92 ± 0.01 Å. Meanwhile, the average coordination numbers for Fe 3+ @NCS-A were found to be 3.4 ± 0.1 Fe-N at 2.08 ± 0.01 Å and 1.2 ± 0.3 Fe-C at 1.90 ± 0.01 Å. In addition, Fe-Fe bonding with a coordination number of 0.5 ± 0.1 was required to fit Fe 3+ @NCS-A. The Fe-C and Fe-Fe signal was found to be due to the presence of Fe 3 C in these samples, as later confirmed by HAADF-STEM and EDS analyses. The absence of the Fe-Fe signal in Fe 2+ @NCS could be either due tominimal Fe 3 C content or amuch smaller average particle size of Fe 3 C. Therefore, we propose that the iron existed primarily as Fe- N x on the carbon substrate for Fe 2+ @NCS-A and Fe 3+ @NCS-A, with Fe 2+ @NCS-A being a fully stoichiometric Fe-N 4 species. The deconvoluted N1s spectra from X-ray Photoelectron Spectroscopy (XPS) (Fig. 2c) show four types of nitrogen species for both samples, in which the peak at 399.9 eV can be assigned to FeN x complexes. Further, Electron Paramagnetic Resonance (EPR) was performed to investigate the electronic structure of Fe 3+ @ NCS-A and Fe 2+ @NCS-A. EPR results shown in Fig. 2d reveal that Fe 3+ @NCS-A has a stronger response than Fe 2+ @NCS-A, suggesting greater Fe 3+ content in Fe 3+ @NCS-A. Fe content was analysed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), XPS and EDS, where Fe content in Fe 2+ @NCS-A is higher than that in Fe 3+ @NCS-A. Thus themissing Fe signal in EPR suggests a higher Fe 2+ content in the Fe 2+ @NCS-A than Fe 3+ @NCS-A. This difference in iron oxidation states is due to the iron precursors (FeCl 2 /FeCl 3 ) that could be further responsible for a different catalytic activity. Linear Sweep Voltammetry (LSV) was carried out to assess the performance of the obtained electrocatalysts (Fig. 3a). The Fe 2+ @NCS-A shows the most promising ORR activity due to its more electroactive iron species. LSV (Fig. 3a) curves of Fe 2+ @ NCS-A show a pronounced peak for oxygen reduction, whose onset potential (0.94 V, the potential at −0.1 mA cm –2 ) is more positive than Fe 3+ @NCS-A (0.91 V). Fe 2+ @NCS-A also displays a half-wave potential of 0.79 V, which is 30 mV more positive than Fe 3+ @NCS-A (0.76 V), suggesting a better catalytic activity in Fe 2+ @NCS-A. Chronoamperometry testing was performed to check the stability of Fe 2+ @NCS-A. After 10 000 s at 0.7V, due to the Fe-N singles sites, nearly 85% of the current was retained for Fe 2+ @NCS-A, whilst Pt/C only showed 76% current retention (Fig. 3b). To further explore the performance of Fe 2+ @NCS-A in real operating devices, the electrochemical performance in Anion Exchange Membrane Fuel Cells (AEMFCs) was also tested (Fig. 3c-d). The operatingAEMFCswere fedwithH 2 andO 2 gases.The Fe 2+ @NCS-catalyst achieved high open-circuit potentials at 0.96 V in the operating AEMFC compared to other M-N-C catalysts. Although it is still lower than Pt-based catalysts tested under similar conditions, it shows great potential after further electrode optimisation. Toconclude, the researchersdemonstratedscalableelectrocatalysts synthesis, applied them in AEMFC, and determined the Fe in the obtained electrocatalysts exist mainly in Fe-N 4 sites with the help of synchrotron techniques. The influence of iron precursors was also studied; however, more experiments are needed to reveal themechanismof howthe Feoxidation state influences theelectrocatalysis steps. References: 1. Lefèvre, M. et al. Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science 324, 71–74 (2009). DOI: 10.1126/science.1170051 2. Pedersen, A. et al. Dual-metal atom electrocatalysts: theory, synthesis, characterization, and applications. Advanced Energy Materials 12, 2102715 (2022). DOI: 10.1002/aenm.202102715 3. Xie, X. et al. Performance enhancement and degradationmechanism identification of a single-atom Co–N–C catalyst for proton exchange membrane fuel cells. Nature Catalysis 3, 1044–1054 (2020). DOI: 10.1038/ s41929-020-00546-1 4. Fei, H. et al. General synthesis and definitive structural identification of MN 4 C 4 single-atom catalysts with tunable electrocatalytic activities. Nature Catalysis 1, 63–72 (2018). DOI: 10.1038/s41929-017-0008-y 5. Zitolo, A. et al. Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature Materials 14, 937–942 (2015). DOI: 10.1038/nmat4367 Funding acknowledgement: The authors thank Giannantonio Cibin and Nitya Ramanan from the B18 (SP26201) and Andy Smith from the I22 beamlines (SM27900) at Diamond Light Source to support and assist the measurements. T.S.M. thanks the EPSRC for support via EP/P023851/1 and EP/S01800X/1. J.Y.F., Z.X., and Z.Y.G. thank the support from Chinese Scholarship Council. R.C. and S.J.H. thank EPSRC EP/ P009050/1. The authors want to thank Prof. JohnVarcoe’s group for providing the membrane and the ionomer. Corresponding author: Prof Maria-Magdalena Titirici, Imperial College London, m.titirici@imperial.ac.uk Spectroscopy Group Beamline B18 (and Soft CondensedMatter Group Beamline I22) Figure 1: a) top HAADF-STEM image of Fe 2+ @NCS-A, bottom EDS mapping results of Fe 2+ @ NCS-A; b) top HAADF-STEM image of Fe 3+ @NCS-A, bottom EDS mapping results of Fe 3+ @ NCS-A. Figure 2: The magnitude of EXAFS FT k2-weight Fe K-edge spectra and fitting curve of a) Fe 2+ @NCS-A; b) Fe 3+ @NCS-A; c) high resolution N1s XPS spectrum of Fe 2+ @NCS-A (top) and Fe 3+ @NCS-A (bottom); d) X-band EPR of Fe 3+ @NCS-A and Fe 2+ @NCS-A. Figure 3: a) LSV curves at 1600 rpm, 10 mV s –1 scan rate; b) chronoamperometric responses of Fe 2+ @NCS-A and Pt/C at 0.7 V and 1600 rpm. RDE/RRDE tests were performed in O 2 -saturated 0.1 M KOH, background N 2 current was extracted. Reference electrode: Hg/HgO, counter electrode: graphite rod. Catalyst loading: 0.28 mg cm –2 , Pt loading 0.021 mg cm –2 ; c) Photo image of AEMFC under testing; d) AEMFC performance of Fe 2+ @NCS-A cathode (red line corresponds to power density, pink line corresponds to cell potential).