Carbon Aerogels Doped with NItrogen and Abundant Metals for Efficient and Sustainable Membrane Electrode Assemblies – ANIMA

In the ANIMA project, we propose to synthesize 3D carbon aerogels doped with N and Fe or Co using a «one-pot« method, to characterize them using classical and advanced electron microscopy, X-ray, gamma-ray and electrochemical techniques and then to transfer them to automotive PEMFC devices. The metallic, nitrogenous and carbonaceous precursors are chosen for (i) their compatibility with the sol-gel reaction at the basis of the aerogel chemistry, (ii) to facilitate the formation of micropores (seat of active sites for the electroreduction of molecular oxygen, ORR), mesopores and macropores (allowing the Fickian diffusion of oxygen), (ii) to promote the formation of pyridinic and pyrrolic nitrogen functions, source of exacerbated electrocatalytic activity for ORR. We will also take advantage of the porous texture of carbonaceous aerogels by functionalizing them with Fe/Co atoms or metal-organic-frameworks (MOFs) in order to combine the optimal O2 transport properties (porous structure of aerogels) and the enhanced ORR activity of Metal-N-C catalysts derived from MOFs. After physicochemical characterization, the performance of Metal-N-C catalysts derived from aerogels are evaluated using a rotating disc-ring electrode, a floating electrode and a 5 cm2 PEMFC single cell. Our target mass activity for ORR is 10 A g-1 at 0.8 V, a 2-fold enhancement over that of a MOF-derived Fe-N-C catalyst and a Fe-N-C catalyst commercialized by Pajarito Powder. The most promising MEAs identified at ICGM will then be tested under the conditions of a PEMFC embedded in an electric vehicle (SYMBIO, 25 and 75 cm2 cells). Accelerated stress tests will also be performed to unravel the degradation mechanisms of these materials and to propose mitigation strategies.

ARMINES has developed an original synthesis method allowing the one-pot synthesis of Metal-N-C aerogels. Our efforts were first focused on Fe-N-C materials, the most active towards the oxygen electroreduction reaction (ORR). Several metallic precursors, several carbon (C) and nitrogen (N) sources have been used to synthesize a library of materials with different textures, different amounts of N (comprised between 2.5 and 3.5 wt. %) and different levels of Metal (up to 1.5 wt. %). We are currently achieving initial ORR mass activities comparable to that of the reference catalyst (Pajarito Powder) in model conditions. Accelerated degradation tests (LEPMI) have also allowed to demonstrate the role of (i) the texture of the carbon matrix, (ii) the N doping and (iii) the free radicals produced during the ORR on the long term stability of the materials. We have shed fundamental light on a previously ignored degradation mechanism: the mobility and agglomeration of atomically-dispersed Fe atoms (ACS Catal. 11 (2021) 484-494). We also questioned the conditions of accelerated degradation tests recommended by the US Department of Energy. Indeed, the latter were classically performed under neutral atmosphere and at near-ambient temperature, leading to low drop in ORR mass activity. Our results (Angew. Chem. Int. Ed. 59 (2020) 3235-3243) showed that these losses are more pronounced when the electrolyte temperature is close to 80°C or in the presence of oxygen, these conditions being closer to the cathode application of PEMFC and leading to the formation of free radicals between hydrogen peroxide, by-product of ORR and Fe2+/Fe3+ ions. SYMBIO, the industrial partner, has started a benchmarking step on a Fe-N-C catalyst from ICGM and on the reference Fe-N-C catalyst.

Outstanding features
ANIMA has already led to several highlights listed below (in italics the associated reference).
- Development of an original one-pot synthesis method allowing the elaboration of Metal-N-C aerogels;
- Constitution of a library of 18 Fe-N-C materials with different textural, structural and chemical characteristics;
- Correlation between catalytic activity for ORR and atomic/nature percentage of Fe metal precursor;
- Correlation between performance loss of Fe-N-C aerogels and load-cycling stability in liquid electrolyte saturated with neutral gas or oxygen (Angewandte Chemie. International Edition 59 (2020) 3235-3243);
- First evidence of the agglomeration of Fe atoms during load-cycling leading to a drop in ORR performance (ACS Catalysis 11 (2021) 484-494).

Perspectives
In the second part of the project, the contribution of in-situ/operando methods (mass spectrometry, Mössbauer spectroscopy) should allow major scientific advances as well as the improvement of the initial and long-term ORR activity of the materials. We are also engaged in industrial work to develop membrane-electrode assemblies incorporating these promising new materials.

1. R. Sgarbi, K. Kumar, F. Jaouen, A. Zitolo, E. Ticianelli, F. Maillard, “Oxygen Reduction Reaction Mechanism and Kinetics on M-NxCy and M@N-C Active Sites Present in Model M-N-C Catalysts Under Alkaline and Acidic Conditions”, Journal of Solid State Electrochemistry 25 (2021) 45-56. DOI: 10.1007/s10008-019-04436-w.

2. K. Kumar, L. Dubau, M. Mermoux, J. Li. A. Zitolo, J. Nelayah, F. Jaouen, F. Maillard, “On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts”, Angewandte Chemie. International Edition 59 (2020) 3235-3243. DOI: 10.1002/anie.201912451.

3. K. Kumar, T. Asset, X. Li, Y. Liu, X. Yan, Y. Chen, M. Mermoux, X. Pan, P. Atanassov, F. Maillard, L. Dubau, “Fe-N-C Electrocatalysts’ Durability: Effects of Single Atoms’ Mobility and Clustering”, ACS Catalysis 11 (2021) 484-494. DOI: 10.1021/acscatal.0c04625.

The world energy demand is rapidly rising with fossil fuels meeting most of the increase and renewables seeing impressive gains. However, the intermittent production of renewables calls for the development of electrochemical energy storage and conversion systems. Water electrolysers and fuel cells are perfectly suited to this goal as they provide means to store electricity as hydrogen (H2) and to re-produce electricity from it later, on demand. The ANIMA project concerns electric power generation from H2 with low-temperature proton-exchange membrane fuel cells (PEMFCs). One bottleneck still hampering the sustainable development of this technology is the high cost/low electrocatalytic performance of the cathode where the oxygen reduction reaction (ORR) occurs. Attempts to tackle this issue consist in developing inexpensive Metal-nitrogen-carbon catalysts (Metal-N-C) with Metal = iron or cobalt. To date, the electrocatalytic performance of the most promising Metal-N-C catalysts approaches that of Pt/C in rotating ring disk electrode (RRDE) set-up. However, transferring successfully the RRDE performance to durable membrane electrode assemblies (MEAs) designed to operate in H2/air PEMFC at = 1 A cm-2 requires (i) high surface area Metal-N-C catalysts featuring large pore volume and 3D interconnected pores of controllable size (to limit mass-transport issues), (ii) increasing the active site density (ASD) and their turnover frequency (TOF) values, (iii) increasing their resistance to electrochemical corrosion in PEMFC cathode operating conditions.

To simultaneously meet these requirements, innovative 3D C aerogels doped with N and Fe or Co atoms will be synthesized and pyrolysed at ARMINES, characterized at ICGM and LEPMI before being transferred to real PEMFC devices (SYMBIO). The metal, N and C precursors will be chosen to (i) be compatible with the sol-gel reaction, (ii) lead to a texture combining micropores (hosting the active sites), mesopores and macropores (allowing Fickian diffusion of O2 in real PEMFC operating conditions), (iii) promote the formation of highly-active pyridinic and pyrrolic N species. We also want to take advantage of the highly porous carbon texture of carbon aerogels, and functionalize them with Fe or Co-doped metal-organic framework (MOFs) prior to pyrolysis to combine improved O2-transport properties (due to open structure of carbon aerogels) with state-of-art ORR activity of MOF-derived Metal-N-C catalysts. The Metal-N-C catalysts will be then characterized using conventional and advanced electron microscopy, X-ray and gamma-ray based techniques before/after operation in automotive PEMFC conditions. Their performance for the ORR will be evaluated using RRDE (LEPMI, ARMINES) and floating electrode set-ups (LEPMI) and in single-cell PEMFC with 5 cm2 membrane electrode assemblies (ICGM). The targeted ORR mass activity of Metal-N-C catalysts derived from aerogels is 10 A g-1 at 0.8 V vs. the reversible hydrogen electrode (RHE) i.e. a 2-fold enhancement relative to a reference Fe-N-C catalyst derived from MOFs synthesized at ICGM and to a commercial Fe-N-C catalyst produced by Pajarito Powder. The most promising MEAs identified at ICGM will be tested in automotive PEMFC conditions by SYMBIO (25 and 75 cm2 cells). Accelerated stress tests will also be carried out to unravel degradation mechanisms and propose mitigation strategies. Also, for the first time, operando Mössbauer spectroscopy will be applied to distinguish surface FeNx sites from bulk sites through their different dependence on electrochemical potential. This technique will give unprecedented insights into the amount of surface active sites remaining after long-term PEMFC operation; therefore, the expected benefits of ANIMA are both advanced technological progresses and beyond state-of-art insights of the highest possible academic level.