PUBLICATIONS

High loading of single atomic iron sites in Fe–NC oxygen reduction catalysts for proton exchange membrane fuel cells, A. Mehmood, M. Gong, F. Jaouen, A. Roy, A. Zitolo, A. Khan, M.-T. Sougrati, M. Primbs, A. M. Bonastre, D. Fongalland, G. Dražič, P. Strasser and A. Kucernak Nat Catal 5, 311–323 (2022) - DOI https://doi.org/10.1038/s41929-022-00772-9

Abstract: Non-precious iron-based catalysts (Fe–NCs) require high active site density to meet the performance targets as cathode catalysts in proton exchange membrane fuel cells. Site density is generally limited to that achieved at a 1–3 wt%(Fe) loading due to the undesired formation of iron-containing nanoparticles at higher loadings. Here we show that by preforming a carbon–nitrogen matrix using a sacrificial metal (Zn) in the initial synthesis step and then exchanging iron into this preformed matrix we achieve 7 wt% iron coordinated solely as single-atom Fe–N₄ sites, as identified by ⁵⁷Fe cryogenic Mössbauer spectroscopy and X-ray absorption spectroscopy. Site density values measured by in situ nitrite stripping and ex situ CO chemisorption methods are 4.7 × 10¹⁹ and 7.8 × 10¹⁹ sites g⁻¹, with a turnover frequency of 5.4 electrons sites⁻¹ s⁻¹ at 0.80 V in a 0.5 M H₂SO₄ electrolyte. The catalyst delivers an excellent proton exchange membrane fuel cell performance with current densities of 41.3 mA cm⁻² at 0.90 V_iR-free using H₂–O₂ and 145 mA cm⁻² at 0.80 V (199 mA cm⁻² at 0.80 V_iR-free) using H₂–air.

Advancements in cathode catalyst and cathode layer design for proton exchange membrane fuel cell, Y. Sun, S. Polani, F. Luo, S. Ott, P. Strasser & F. Dionigi, Nat Commun. 12, 5984 (2021) - DOI 10.1038/s41467-021-25911-x - OPEN ACCESS

Abstract: Proton exchange membrane fuel cells have been recently developed at an increasing pace as clean energy conversion devices for stationary and transport sector applications. High platinum cathode loadings contribute significantly to costs. This is why improved catalyst and support materials as well as catalyst layer design are critically needed. Recent advances in nanotechnologies and material sciences have led to the discoveries of several highly promising families of materials. These include platinum-based alloys with shape-selected nanostructures, platinum-group-metal-free catalysts such as metal-nitrogen-doped carbon materials and modification of the carbon support to control surface properties and ionomer/catalyst interactions. Furthermore, the development of advanced characterization techniques allows a deeper understanding of the catalyst evolution under different conditions. This review focuses on all these recent developments and it closes with a discussion of future research directions in the field..

Sulfur Doping versus Hierarchical Pore Structure: The Dominating Effect on the Fe-N-C Site Density, Activity, and Selectivity in Oxygen Reduction Reaction Electrocatalysis, G. Daniel, M. Mazzucato, R. Brandiele, L. De Lazzari, D. Badocco, P. Pastore, T. Kosmala, G. Granozzi, and C. Durante, ACS Appl. Mater. Interfaces 2021, 13, 36, 42693–42705 - DOI 10.1021/acsami.1c09659 - OPEN ACCESS

Abstract: Nitrogen doping has been always regarded as one of the major factors responsible for the increased catalytic activity of Fe–N–C catalysts in the oxygen reduction reaction, and recently, sulfur has emerged as a co-doping element capable of increasing the catalytic activity even more because of electronic effects, which modify the d-band center of the Fe–N–C catalysts or because of its capability to increase the Fe–Nx site density (SD). Herein, we investigate in detail the effect of sulfur doping of carbon support on the Fe–Nx site formation and on the textural properties (micro- and mesopore surface area and volume) in the resulting Fe–N–C catalysts. The Fe–N–C catalysts were prepared from mesoporous carbon with tunable sulfur doping (0–16 wt %), which was achieved by the modulation of the relative amount of sucrose/dibenzothiophene precursors. The carbon with the highest sulfur content was also activated through steam treatment at 800 °C for different durations, which allowed us to modulate the carbon pore volume and surface area (1296–1726 m² g^–1). The resulting catalysts were tested in O₂-saturated 0.5 M H₂SO₄ electrolyte, and the site density (SD) was determined using the NO-stripping technique. Here, we demonstrate that sulfur doping has a porogenic effect increasing the microporosity of the carbon support, and it also facilitates the nitrogen fixation on the carbon support as well as the formation of Fe–Nx sites. It was found that the Fe–N–C catalytic activity [E1/2 ranges between 0.609 and 0.731 V vs reversible hydrogen electrode (RHE)] does not directly depend on sulfur content, but rather on the microporous surface and therefore any electronic effect appears not to be determinant as confirmed by X-ray photoemission spectroscopy (XPS). The graph reporting Fe–Nx SD versus sulfur content assumes a volcano-like shape, where the maximum value is obtained for a sulfur/iron ratio close to 18, i.e., a too high or too low sulfur doping has a detrimental effect on Fe–Nx formation. However, it was highlighted that the increase of Fe–Nx SD is a necessary but not sufficient condition for increasing the catalytic activity of the material, unless the textural properties are also optimized, i.e., there must be an optimized hierarchical porosity that facilitates the mass transport to the active sites.

Highly Graphitized Fe-N-C Electrocatalysts Prepared from Chitosan Hydrogel Frameworks, G. Daniel, T. Kosmala, F. Brombin, M. Mazzucato, A. Facchin, M. C. Dalconi, D. Badocco, P. Pastore, G. Granozzi, and C. Durante, Catalysts 2021, 11(3), 390- DOI 10.3390/catal11030390 - OPEN ACCESS

Abstract: The development of platinum group metal-free (PGM-free) electrocatalysts derived from cheap and environmentally friendly biomasses for oxygen reduction reaction (ORR) is a topic of relevant interest, particularly from the point of view of sustainability. Fe-nitrogen-doped carbon materials (Fe-N-C) have attracted particular interest as alternative to Pt-based materials, due to the high activity and selectivity of Fe-Nx active sites, the high availability and good tolerance to poisoning. Recently, many studies focused on developing synthetic strategies, which could transform N-containing biomasses into N-doped carbons. In this paper, chitosan was employed as a suitable N-containing biomass for preparing Fe-N-C catalyst in virtue of its high N content (7.1%) and unique chemical structure. Moreover, the major application of chitosan is based on its ability to strongly coordinate metal ions, a precondition for the formation of Fe-Nx active sites. The synthesis of Fe-N-C consists in a double step thermochemical conversion of a dried chitosan hydrogel. In acidic aqueous solution, the preparation of physical cross-linked hydrogel allows to obtain sophisticated organization, which assure an optimal mesoporosity before and after the pyrolysis. After the second thermal treatment at 900 °C, a highly graphitized material was obtained, which has been fully characterized in terms of textural, morphological and chemical properties. RRDE technique was used for understanding the activity and the selectivity of the material versus the ORR in 0.5 M H2SO4 electrolyte. Special attention was put in the determination of the active site density according to nitrite electrochemical reduction measurements. It was clearly established that the catalytic activity expressed as half wave potential linearly scales with the number of Fe-Nx sites. It was also established that the addition of the iron precursor after the first pyrolysis step leads to an increased activity due to both an increased number of active sites and of a hierarchical structure, which improves the access to active sites. At the same time, the increased graphitization degree, and a reduced density of pyrrolic nitrogen groups are helpful to increase the selectivity toward the 4e- ORR pathway.

Effect of induced Micro- and Meso-porosity on the formation and activity of Fe-N-C active sites for Oxygen Reduction Reaction, M. Mazzucato, G. Daniel, A. Mehmood, T. Kosmala, G. Granozzi, A. Kucernak, C. Durante, Applied Catalysis B: Environmental, 291, 15 August 2021 , - DOI 10.1016/j.apcatb.2021.120068 - OPEN ACCESS

Abstract: Fe-N-C have emerged as one of the best non-PGM alternatives to Pt/C catalysts for the electrochemical reduction of O₂ in fuel cells. In this work, we explore the effect of steam and CO₂ treatments at high temperatures on the nanometric porous structure of a commercial carbon black. Using those support materials, we synthesize different Fe-N-C catalysts to achieve a better understanding on the role of micro- and mesopores of the support towards catalytic site formation and site activity. Different time and temperature of treatments result in an almost linear increment of surface area and microporous volume, which allows better nitrogen functionalization. Site density evaluation, performed using a recently described NO-stripping technique, showed an increase in site density and TOF which correlates well with the morphology variation. The percentage of active iron increases from 2.65 % to 14.74 % in activated catalysts confirming a better access of electrolyte to the iron sites.

Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: gas sorption, physical and electrochemical investigation using NO and O₂, P. Boldrin, D. Malko, A. Mehmood, U. I. Kramm, S. Paul, N. Weidler, A. Kucernak, Applied Catalysis B: Environmental, Volume 292, 5 September 2021, 120169, - DOI 10.1016/j.apcatb.2021.120169 - SPIRAL: https://spiral.imperial.ac.uk/handle/10044/1/87903

Abstract: We show that gaseous nitric oxide (NO) and oxygen (O₂) are useful molecular probes to uncover complex surface processes in Fe-N/C catalysts. We unravel the difference between using gaseous NO in a temperature programmed desorption experiment and using NO (and progenitors) in an electrochemical experiment. Gas phase O₂ adsorption is almost exclusively desorbed as CO₂, and continued exposure to oxygen increases the amount of chemisorbed oxygen species on the surface. The oxidation state of the carbon surface is an important activity determining factor, and under normal “electrochemical” conditions many of the active sites are blocked. Only by treatment at 600 °C in Ar can we free those sites for oxygen adsorption, however under atmospheric storage, and especially during the oxygen reduction reaction (ORR), the surface quickly becomes deactivated with chemisorbed oxygen species and water. We demonstrate that the material can be super-activated by reductive electrochemical treatment, both in an electrochemical three electrode cell and in a fuel cell. The energy gained following the treatment is significantly larger than the energetic cost.

Impact of ionomer structuration on the performance of bio-inspired noble- metal-free fuel cell anodes, N. Coutard, T. Ngoc Huan, F. Valentino, R. T. Jane, S. Gentil, E. S. Andreiadis, A. Le Goff, T. Asset F. Maillard, B. Jousselme, A. Morozan, S. Lyonnard, V. Artero and P. Chenevier, Chem. Catalysis – Online 28 Janauary 2021, - DOI 10.1016/j.checat.2021.01.001 - HAL: https://hal-cnrs.archives-ouvertes.fr/hal-03126604 - OPEN ACCESS

Abstract: Molecular-engineered bio-inspired catalysts hold promise for the next generation of proton-exchange membrane fuel cells (PEMFCs). Yet, their implementation in catalytic layers with Nafionionomer faces nanocomposite formulation issues. Here, we use various DuBois nickel catalysts immobilized on carbon nanotubes to exemplify how self-assembly at the mesoscale affects H₂ oxidation anode performance. We exploited the reversible activity of these catalysts together with potential-step chronoamperometry to locally produce H₂ and probe mass transport within the catalytic layer. Small-angle neutron scattering studies serve to build amodel describing how the surface functionalization drives the structuration of the ionomer and affects the diffusion of protons and gas from and to catalytic centers. This study thus demonstrates that implementation of unconventional catalysts in catalytic layers requires the redesign of the whole system of materials. On the basis of such information, catalytic layer formulation was optimized, allowing order-of-magnitude performance enhancement of noble-metal-free PEMFCs.

Identification of Durable and Non-Durable FeNx Sites in Fe-N-C Materials for Proton Exchange Membrane Fuel Cells, J. Li M.-T. Sougrati, A. Zitolo, J. Ablett, I. C. Oguz, T. Mineva, I. Matanovic, P. Atanassov, A. di Cicco, K. Kumar, L. Dubau, F. Maillard, F. Jaouen, Nature Catalysis 2020 - DOI 10.1038/s41929-020-00545-2,  ChemrXiv: 10.26434/chemrxiv.11842431.v1

Abstract: While Fe–N–C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells, limited understanding of their operando degradation restricts rational approaches towards improved durability. Here we show that Fe–N–C catalysts initially comprising two distinct FeN_x sites (S1 and S2) degrade via the transformation of S1 into iron oxides while the structure and number of S2 were unmodified. Structure–activity correlations drawn from end-of-test ⁵⁷Fe Mössbauer spectroscopy reveal that both sites initially contribute to the oxygen reduction reaction activity but only S2 substantially contributes after 50 h of operation. From in situ ⁵⁷Fe Mössbauer spectroscopy in inert gas coupled to calculations of the Mössbauer signature of FeN_x moieties in different electronic states, we identify S1 to be a high-spin FeN₄C₁₂ moiety and S2 a low- or intermediate-spin FeN₄C₁₀ moiety. These insights lay the groundwork for rational approaches towards Fe–N–C cathodes with improved durability in acidic fuel cells.

Upcycling of polyurethane into iron-nitrogen-carbon electrocatalysts active for oxygen reduction reaction, G. Daniel, T. Kosmala, M. C. Dalconi, L. Nodari, D. Badocco, P. Pastore, A. Lorenzetti, G. Granozzi, and C. Durante, Electrochimica Acta Volume 362, 1 December 2020, 137200 - DOI 10.1016/j.electacta.2020.137200

Abstract: World plastic production has increased since industrial-scale production began in the 1940s and while large amount of thermoplastic polymers can be effectively recycled and re-used, undifferentiated polymers or thermoset polymers cannot, and as a result, most of these raw materials end up in landfill or energy recovery in incinerators. The synthesis of carbon nanomaterials from conversion of waste polymers is an alternative, promising approach owing to the high added-value of these products. In particular, novel carbon materials, could translate into interesting and cheap material for the catalytic reduction of oxygen (ORR), a fundamental reaction for the production of H₂O₂ or in fuel cell and metal air batteries.
This paper presents the synthesis of iron-nitrogen-carbon (Fe-N-C) electrocatalysts starting from a mix of polyethylene (PE) and polyurethane (PU) by adding FeCl₃ as an iron source for promoting Fe-N_x sites formation. The two polymers were mixed according to a solvent assisted process employing p-xylene or with a solvent free process using a Brabender plastograph. The blend of thermoplastic and thermoset polymers was converted into Fe-N-C materials in a two-step pyrolysis, where the temperature of second pyrolysis showed to sensitively affect the chemical and textural properties of the resulting material. Depending on the temperature, and initial PE content, the surface area ranges between 195 and 479 m² g⁻¹, with a preferential formation of micropores. XPS analysis confirmed that the employment of PU leads to the formation of nitrogen functional groups and Fe-N_x sites, while XRD investigation in heating chamber allowed to follow the formation Fe₃C, Fe₂O₃, γ-Fe and α-Fe with temperature rises between 700–1000°C. The new Fe-N-C catalysts were characterized by linear sweep voltammetry at ring disk electrode showing interesting activity for the ORR in both acid and alkaline electrolyte and a selectivity for H₂O₂ which deeply depends on the type of electrolyte as well as on nitrogen content and on the surface area of the samples.

A comparative perspective of electrochemical and photochemical approaches for catalytic H₂O₂ production, Y. Sun, L. Han and P. Strasser, Chem. Soc. Rev., 2020, Advance Article, - DOI 10.1039/D0CS00257G - OPEN ACCESS

Abstract: Hydrogen peroxide (H₂O₂) has a wide range of important applications in various fields including chemical industry, environmental remediation, and sustainable energy conversion/storage. Nevertheless, the stark disconnect between today’s huge market demand and the historical unsustainability of the currently-used industrial anthraquinone-based production process is promoting extensive research on the development of efficient, energy-saving and sustainable methods for H₂O₂ production. Among several sustainable strategies, H₂O₂ production via electrochemical and photochemical routes has shown particular appeal, because only water, O₂, and solar energy/electricity are involved during the whole process. In the past few years, considerable efforts have been devoted to the development of advanced electrocatalysts and photocatalysts for efficient and scalable H₂O₂ production with high efficiency and stability. In this review, we compare and contrast the two distinct yet inherently closely linked catalytic processes, before we detail recent advances in the design, preparation, and applications of different H₂O₂ catalyst systems from the viewpoint of electrochemical and photochemical approaches. We close with a balanced perspective on remaining future scientific and technical challenges and opportunities.

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells, M. Primbs, Y. Sun, A. Roy, D. Malko, As. Mehmood, M.-T. Sougrati, P.-Y. Blanchard, G. Granozzi, T. Kosmala, G. Daniel, P. Atanassov, J. Sharman, C. Durante, A. Kucernak, D. Jones, F. Jaouen and P. Strasser, Energy Environ. Sci., 2020, Advance Article, - DOI 10.1039/D0EE01013H - OPEN ACCESS

Abstract: We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four of today's most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts (Fe–N–Cs). Our analysis reaches far beyond previous such attempts in linking kinetic performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activity with previously elusive kinetic metrics such as the active metal site density (SD) and the catalytic turnover frequency (TOF). Kinetic ORR activities, SD and TOF values were evaluated using in situ electrochemical NO₂⁻ reduction as well as an ex situ gaseous CO cryo chemisorption. Experimental ex situ and in situ Fe surface site densities displayed remarkable quantitative congruence. Plots of SD versus TOF (“reactivity maps”) are utilized as new analytical tools to deconvolute ORR reactivities and thus enabling rational catalyst developments. A microporous catalyst showed large SD values paired with low TOF, while mesoporous catalysts displayed the opposite. Trends in Fe surface site density were linked to molecular nitrogen and Fe moieties (D1 and D2 from ⁵⁷Fe Mössbauer spectroscopy), from which pore locations of catalytically active D1 and D2 sites were established. This cross-laboratory analysis, its employed experimental practices and analytical methodologies are expected to serve as a widely accepted reference for future, knowledge-based research into improved PGM-free fuel cell cathode catalysts.

Stable, Active and Methanol Tolerant PGM-free Surface in Acidic Medium: Electron Tunneling at Play in Pt/FeNC Hybrid Catalysts for Direct Methanol Fuel Cell Cathode, T. Kosmala, N. Bibent, M.-T. Sougrati, G. Drazic, S. Agnoli, F. Jaouen, and G. Granozzi, ACS Catal. Online June 10, 2020, - DOI 10.1021/acscatal.0c01288

Abstract: PGM-free catalysts have high initial activity for O₂ reduction reaction, but suffer from low stability in acid medium in PEMFC and DMFC. Here, we shed light on the atomic-scale structure of hybrid Pt/FeNC catalysts (1-2 wt% of Pt), revealing by STEM and EDXS the presence of Pt@FeOx particles. The absence of exposed Pt on the surface is confirmed by the suppression of methanol oxidation reaction and CO stripping experiments. The promising application of such Pt/FeNC catalysts, comprising FeNx sites and Pt@FeOx particles, is demonstrated at the cathode of DMFC. To gain fundamental understanding on the stability in acid medium and on the intrinsic ORR activity of Pt@FeOx, we constructed model surfaces by depositing FeOx films with controlled thickness (from 1.0 to 6.4 nm), fully covering the Pt(111) surface, which resulted stable in acid medium in the potential range 0.45 – 1.05 V vs. RHE. The specific ORR activity of Fe2O3/Pt(111) increases exponentially with decreasing overlayer thickness, which is explained by the tunneling of Pt electrons through Fe₂O₃. This special phenomenon sheds light onto recently reported excellent durability of Pt/FeNC composites in PEMFC and identify a promising core@shell strategy leading to stable PGM-free surfaces in acid medium, and tolerant to methanol.

Noncovalent Integration of a Bioinspired Ni Catalyst to Graphene Acid for Reversible Electrocatalytic Hydrogen Oxidation, B. Reuillard, M. Blanco, L. Calvillo, N. Coutard, A. Ghedjatti, P. Chenevier, S. Agnoli, M. Otyepka, G. Granozzi and V. Artero, ACS Appl. Mater. Interfaces 2020 - DOI 10.1021/acsami.9b18922- OPEN ACCESS

Abstract: Efficient heterogeneous catalysis of hydrogen oxidation reaction (HOR) by platinum group metal (PGM)-free catalysts in proton-exchange membrane (PEM) fuel cells represents a significant challenge toward the development of a sustainable hydrogen economy. Here, we show that graphene acid (GA) can be used as an electrode scaffold for the noncovalent immobilization of a bioinspired nickel bis-diphosphine HOR catalyst. The highly functionalized structure of this material and optimization of the electrode-catalyst assembly sets new benchmark electrocatalytic performances for heterogeneous molecular HOR, with current densities above 30 mA cm^–2 at 0.4 V versus reversible hydrogen electrode in acidic aqueous conditions and at room temperature. This study also shows the great potential of GA for catalyst loading improvement and porosity management within nanostructured electrodes toward achieving high current densities with a noble-metal free molecular catalyst.

Accurate Evaluation of Active-Site Density (SD) and Turnover Frequency (TOF) of PGM-Free Metal−Nitrogen-Doped Carbon (MNC) Electrocatalysts using CO Cryo Adsorption, F. Luo, C. H. Choi, M. J.M. Primbs, W. Ju, S. Li, N. D. Leonard, A. Thomas, F. Jaouen and P. Strasser, ACS Catal. 2019, 9, 4841−4852 - DOI 10.1021/acscatal.9b00588 - HAL repository

Abstract: The number of catalytically active sites (site density, SD) and the catalytic turnover frequency (TOF) are critical for meaningful comparisons between catalytic materials and their rational improvement. SD and TOF numbers have remained elusive for PGM-free, metal/nitrogen-doped porous carbon electrocatalysts (MNC), in particular, FeNC materials that are now intensively investigated and widely utilized to catalyze the oxygen reduction reaction (ORR) in fuel cell cathodes. Here, we apply CO cryo sorption and desorption to evaluate SD and TOF numbers of a state-of-art FeNC ORR electrocatalyst with atomically dispersed coordinative FeN_x (x ≤ 4) sites in acid and alkaline conditions. More specifically, we study the impact of thermal pretreatment conditions prior to assessing the number of sorption-active FeN_x sites. We show that the pretreatment temperature sensitively affects the CO sorption uptake through a progressive thermal removal of airborne adsorbates, which, in turn, controls the resulting catalytic SD numbers. We correlate CO uptake with CO desorption and analyze the observed temperature-programmed desorption characteristics. The CO uptakes increased from 45 nmol·mg^–1_catalyst at 300 °C cleaning to 65 nmol·mg^–1_catalyst at 600 °C cleaning, where it leveled off. These values were converted into apparent SD values of 2.7 × 10¹⁹ to 3.8 × 10¹⁹ surface sites per gram catalyst. Because of similar ORR activity of the pristine catalyst and of the sample after 600 °C cleaning step, we conclude that the nature and number of surface FeN_x sites remained largely unaffected up to 600 °C and that cleaning to less than 600 °C was insufficient to free the sites from previously adsorbed species, completely or partially impeding CO adsorption. Cleaning beyond that temperature, however, led to undesired chemical modifications of the FeN_x moieties, resulting in higher TOF. In all, this study identifies and recommends a practical and useful protocol for the accurate evaluation of catalytic SD and TOF parameters of PGM-free ORR electrocatalyst, which enables a more rational future catalyst development and improvement.
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Understanding Active Sites in Pyrolyzed Fe−N−C Catalysts for Fuel Cell Cathodes by Bridging Density Functional Theory Calculations and ⁵⁷Fe Mössbauer Spectroscopy, T. Mineva, I. Matanovic, P. Atanassov, M.-T. Sougrati, L. Stievano, M. Clémancey, A. Kochem, J.-M. Latour, and F. Jaouen,  ACS Catalysis 2019 9 (10), 9359-9371 - DOI 10.1021/acscatal.9b02586 - HAL repository

Abstract: Pyrolyzed Fe–N–C materials are promising platinum-group-metal-free catalysts for proton-exchange membrane fuel cell cathodes. However, the detailed structure, oxidation, and spin states of their active sites are still undetermined. ⁵⁷Fe Mössbauer spectroscopy has identified FeN_x moieties as the most active sites, with their fingerprint being a doublet in low-temperature Mössbauer spectra. However, the interpretation of the doublets for such materials has lacked theoretical basis. Here, we applied density functional theory to calculate the quadrupole splitting energy of doublets (ΔE_QS) for a range of FeN_x structures in different oxidation and spin states. The calculated and experimental values are then compared for a reference Fe–N–C catalyst, whereas further information on the Fe oxidation and spin states was obtained from electron paramagnetic resonance, superconducting quantum interference device, and ⁵⁷Fe Mössbauer spectroscopy under external magnetic field. The combined theoretical and experimental results identify the main presence of FeN_x moieties in Fe(II) low-spin and Fe(III) high-spin states, whereas a minor fraction of sites could exist in the Fe(II) S = 1 state. From the analysis of the ⁵⁷Fe Mössbauer spectrum under the external magnetic field and the comparison of calculated and measured ΔE_QS values, we assign the experimental doublet D1 with a mean ΔE_QS value of around 0.9 mm·s^–1 to Fe(III)N₄C₁₂ moieties in high-spin state and the experimental doublet D2 with a mean ΔE_QS value of around 2.3 mm·s^–1 to Fe(II)N₄C₁₀ moieties in low and medium spin. These conclusions indicate that D1 corresponds to surface-exposed sites, whereas D2 may correspond either to bulk sites that are inaccessible to O₂ or to surface sites that bind O₂ weaker than D1.

Toward Platinum Group Metal-Free Catalysts for Hydrogen/Air Proton-Exchange Membrane Fuel Cells, Jaouen, F.; Jones, D.; Coutard, N.; Artero, V.; Strasser, P.; Kucernak, A.  Johnson Matthey Technology Review, Volume 62, Number 2, 1 April 2018, pp. 231-255(25) - DOI 10.1595/205651318X6968288 - HAL repository - OPEN ACCESS

Abstract: The status, concepts and challenges toward catalysts free of platinum group metal (pgm) elements for proton-exchange membrane fuel cells (PEMFC) are reviewed. Due to the limited reserves of noble metals in the Earth’s crust, a major challenge for the worldwide development of PEMFC technology is to replace Pt with pgm-free catalysts with sufficient activity and stability. The priority target is the substitution of cathode catalysts (oxygen reduction) that account for more than 80% of pgms in current PEMFCs. Regarding hydrogen oxidation at the anode, ultralow Pt content electrodes have demonstrated good performance, but alternative non-pgm anode catalysts are desirable to increase fuel cell robustness, decrease the H₂ purity requirements and ease the transition from H₂ derived from natural gas to H₂ produced from water and renewable energy sources.
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