Metal Oxide Clusters on Nitrogen-Doped Carbon are Highly Selective for CO2Electroreduction to CO

The electrochemical reduction of CO2 (eCO2RR) using renewable energy is an effective approach to pursue carbon neutrality. The eCO2RR to CO is indispensable in promoting C-C coupling through bifunctional catalysis and achieving cascade conversion from CO2 to C2+. This work investigates a series of M/N-C (M = Mn, Fe, Co, Ni, Cu, and Zn) catalysts, for which the metal precursor interacted with the nitrogen-doped carbon support (N-C) at room temperature, resulting in the metal being present as (sub)nanosized metal oxide clusters under ex situ conditions, except for Cu/N-C and Zn/N-C. A volcano trend in their activity toward CO as a function of the group of the transition metal is revealed, with Co/N-C exhibiting the highest activity at -0.5 V versus RHE, while Ni/N-C shows both appreciable activity and selectivity. Operando X-ray absorption spectroscopy shows that the majority of Cu atoms in Cu/N-C form Cu0 clusters during eCO2RR, while Mn/, Fe/, Co/, and Ni/N-C catalysts maintain the metal hydroxide structures, with a minor amount of M0 formed in Fe/, Co/, and Ni/N-C. The superior activity of Fe/, Co/, and Ni/N-C is ascribed to the phase contraction and the HCO3- insertion into the layered structure of metal hydroxides. Our work provides a facile synthetic approach toward highly active and selective electrocatalysts to convert CO2 into CO. Coupled with state-of-the-art NiFe-based anodes in a full-cell device, Ni/N-C exhibits >80% Faradaic efficiency toward CO at 100 mA cm-2.The research leading to these results has received funding from the A-LEAF Project, which is funded by the European Union’s H2020 Programme under grant agreement no. 732840. ICN2 and ICIQ acknowledge funding from the FEDER/Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación (projects ENE2017-85087-C3 and RTI2018-095618-B-I00) and the Generalitat de Catalunya (2017 SGR 327 and 2017- SGR-1406) and by the CERCA Programme / Generalitat de Catalunya. ICN2 and ICIQ are supported by the Severo Ochoa program from Spanish MINECO (grants no. SEV-2017-0706 and CEX2019-000925-S)

Particules creuses peu onéreuses, durables et actives pour la réduction de l'oxygène dans le cadre d'une application en pile à combustible à membrane échangeuse de protons

This Ph.D. thesis investigates the mechanisms driving the formation, the enhanced activity for the oxygen reduction reaction (ORR) and the durability of porous hollow PtM/C nanoparticles (NPs) for proton ex-change membrane fuel cell (PEMFC) applications. The formation and growth of the NPs, synthesized by a ‘one-pot’ process, were discussed in the light of microscopic, operando X-ray and electron-based tech-niques, unveiling the different intermediate steps of the synthesis. The synthe-sis process was extended to different non-noble metals (M = Ni, Co, Cu, Zn and Fe) and to different carbon supports. The enhanced activity for the ORR resulted from (i) the contraction of the lattice pa-rameter by the non-noble metal (the final NPs contains ca. 15 – 20 at. % of M), (ii) the open porosity and (iii) the density of structural defects at the surface of the NPs, which was semi-quantitativey estimated by COads stripping measurements and Rietveld analysis of wide-angle X-ray scattering patterns. The non-noble metal was found to be annealed (dissolved) faster than the structural defects during the accelerated stress tests

Particules creuses peu expensives, durables et actives pour la réduction de l'oxygène dans le cadre d'une application en pile à combustible à membrane échangeuse de protons

This thesis investigates the mechanisms driving the formation, the en-hanced activity for the oxygen reduction reaction (ORR) and the dura-bility of porous hollow PtM/C nanoparticles (NPs) for proton exchange membrane fuel cell (PEMFC) applications. The formation and growth of the NPs, synthesized by a ‘one-pot’ process, were discussed in the light of microscopic, in operando X-ray and electronic measurements, unveiling the different intermediate steps of the synthesis. The synthe-sis process was extended to different non-noble metals (M = Ni, Co, Cu, Zn and Fe) and to different carbon supports. The enhanced activity for the ORR resulted from (i) the contraction of the lattice parameter by the non-noble metal (the final NPs contains ca. 15 – 20 at. % of M), (ii) the open porosity and (iii) the density of structural defects at the surface of the NPs, rationalized by COads stripping measurments and Rietveld analysis. The non-noble metal was found to segregate faster than the structural defects during the accelerated stress tests.Cette thèse a pour but de discuter les mécanismes à l’origine de la for-mation, de l’activité pour la réduction de l’oxygène (ORR) et de la sta-bilité de nanoparticules (NPs) creuses à base de Pt supportées sur car-bone pour des applications en pile à combustible à membrane échan-geuse de protons (PEMFC). La formation et la croissance des NPs, syn-thétisées par une méthode dite ‘one-pot’, ont été étudiées par l’inter-médiaire de mesures microscopiques, électroniques et de diffraction, nous permettant ainsi de mettre en évidence les différentes étapes de la synthèse. Le procédé de synthèse a été étendu à différents métaux non nobles (M = Ni, Co, Cu, Zn and Fe) et à différents supports carbonés. L’activité supérieure des NPs pour l’ORR résulte (i) de la contraction du paramètre de maille induit par la présence du métal non-noble, (ii) de leur porosité ouverte et (iii) de la densité de défauts structuraux à la surface des NPs (rationalisée via COads stripping et analyse de Riet-veld). Les défauts structuraux se seront montrés plus stables que le mé-tal non-noble durant les tests de vieillissement accéléré

Hollow nanoparticles for low cost, high oxygen reduction reaction activity and durability for proton exchange membrane fuel cell application

Cette thèse a pour but de discuter les mécanismes à l’origine de la for-mation, de l’activité pour la réduction de l’oxygène (ORR) et de la sta-bilité de nanoparticules (NPs) creuses à base de Pt supportées sur car-bone pour des applications en pile à combustible à membrane échan-geuse de protons (PEMFC). La formation et la croissance des NPs, syn-thétisées par une méthode dite ‘one-pot’, ont été étudiées par l’inter-médiaire de mesures microscopiques, électroniques et de diffraction, nous permettant ainsi de mettre en évidence les différentes étapes de la synthèse. Le procédé de synthèse a été étendu à différents métaux non nobles (M = Ni, Co, Cu, Zn and Fe) et à différents supports carbonés. L’activité supérieure des NPs pour l’ORR résulte (i) de la contraction du paramètre de maille induit par la présence du métal non-noble, (ii) de leur porosité ouverte et (iii) de la densité de défauts structuraux à la surface des NPs (rationalisée via COads stripping et analyse de Riet-veld). Les défauts structuraux se seront montrés plus stables que le mé-tal non-noble durant les tests de vieillissement accéléré.This thesis investigates the mechanisms driving the formation, the en-hanced activity for the oxygen reduction reaction (ORR) and the dura-bility of porous hollow PtM/C nanoparticles (NPs) for proton exchange membrane fuel cell (PEMFC) applications. The formation and growth of the NPs, synthesized by a ‘one-pot’ process, were discussed in the light of microscopic, in operando X-ray and electronic measurements, unveiling the different intermediate steps of the synthesis. The synthe-sis process was extended to different non-noble metals (M = Ni, Co, Cu, Zn and Fe) and to different carbon supports. The enhanced activity for the ORR resulted from (i) the contraction of the lattice parameter by the non-noble metal (the final NPs contains ca. 15 – 20 at. % of M), (ii) the open porosity and (iii) the density of structural defects at the surface of the NPs, rationalized by COads stripping measurments and Rietveld analysis. The non-noble metal was found to segregate faster than the structural defects during the accelerated stress tests

Small-angle scattering by supported nanoparticles: exact results and useful approximations

In functional materials, nanoparticles are often dispersed in a porous support for the purpose of stabilizing them. This makes their characterization by small-angle scattering challenging because the signal comprises contributions from the nanoparticles of interest, from the inert support and from their crosscorrelation. Exact analytical expressions for all three contributions are derived in the case of a Gaussian-field model of the porous support, with nanoparticles randomly distributed over the surface. For low nanoparticle loading, the expressions simplify to the addition of properly scaled support and particle scattering. For higher loadings, however, the cross-correlation cannot be ignored. Two approximations are introduced, which capture correlation effects in cases where the pores of the support are much larger or only slightly larger than the nanoparticles. The methods of the paper are illustrated with the smallangle X-ray scattering analysis of hollow metallic nanoparticles supported on porous carbon

Electrocatalysis with single metal atom sites in doped carbon matrices

International audienceWhile supported metal nanoparticles cannot achieve full electrochemical utilisation of metal atoms, catalysts featuring single metal atom sites may offer this possibility, along with advantages in selectivity. However, the passage from nanometric to atomic dimension is not without consequences. It first raises the question of efficient and robust synthesis methods, and underlines the need of cutting-edge characterization techniques that can target single metal atoms. These analytical tools are also pivotal to gain insights into the structure of the active sites, and establish atomic structure-catalytic activity-selectivity-stability relationships. Herein, we illustrate these topics for electrocatalysis, with a particular focus on metal-nitrogen-carbon single metal atom catalysts, for which a fantastic leap forward has been achieved in the last 15 years, triggered by the growing interest in sustainable energy storage and conversion systems

Electrocatalysis with single metal atom sites in doped carbon matrices

International audienceWhile supported metal nanoparticles cannot achieve full electrochemical utilisation of metal atoms, catalysts featuring single metal atom sites may offer this possibility, along with advantages in selectivity. However, the passage from nanometric to atomic dimension is not without consequences. It first raises the question of efficient and robust synthesis methods, and underlines the need of cutting-edge characterization techniques that can target single metal atoms. These analytical tools are also pivotal to gain insights into the structure of the active sites, and establish atomic structure-catalytic activity-selectivity-stability relationships. Herein, we illustrate these topics for electrocatalysis, with a particular focus on metal-nitrogen-carbon single metal atom catalysts, for which a fantastic leap forward has been achieved in the last 15 years, triggered by the growing interest in sustainable energy storage and conversion systems