PUBLICATIONS

2019
 

Accurate Evaluation of Active-Site Density (SD) and Turnover Frequency (TOF) of PGM-Free Metal−Nitrogen-Doped Carbon (MNC) Electrocatalysts using CO Cryo Adsorption, Fang Luo, Chang Hyuck Choi, Mathias J.M. Primbs, Wen Ju, Shuang Li, Nathaniel D. Leonard, Arne Thomas, Frédéric Jaouen and Peter 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 FeNx (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 FeNx 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–1catalyst at 300 °C cleaning to 65 nmol·mg–1catalyst 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 FeNx 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 FeNx 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 FeNx 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 (ΔEQS) for a range of FeNx 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 FeNx 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 ΔEQS values, we assign the experimental doublet D1 with a mean ΔEQS 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 ΔEQS 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.

 
2018
 

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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