While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe-N-C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe-N-C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe-N-C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe-N-C materials. These new insights open the path to bottom-up synthesis approaches and studies on site-support interactions.

Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M−N−C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic activity of M−N−C materials toward four-electron oxygen reduction reaction (ORR) to H 2 O is a mainstream line of research for replacing platinumgroup-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H 2 O 2 , a future green "dream" process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H 2 O 2 production over a series of M−N−C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M−N x sites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M−N−C catalysts on the electrocatalytic activity/selectivity for ORR (H 2 O 2 and H 2 O products) and H 2 O 2 reduction reaction (H 2 O 2 RR). Co−N−C catalyst was uncovered with outstanding H 2 O 2 productivity considering its high ORR activity, highest H 2 O 2 selectivity, and lowest H 2 O 2 RR activity. The activity−selectivity trend over M−N−C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four-and two-electron ORR. The predicted binding energy of HO* intermediate over Co−N−C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H 2 O 2 productivity over Co−N−C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide g catalyst −1 h −1 at a current density of 50 mA cm −2 .

No abstract

Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co–N–C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoN4C12, CoN3C10,porp and CoN2C5. The O2 electro-reduction and operando X-ray absorption response are measured in acidic medium on Co–N–C and compared to those of a Fe–N–C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between redox potential and O2-adsorption strength, we conclude that cobalt-based moieties bind O2 too weakly for efficient O2 reduction.

It is generally believed that CO2 electroreduction to multicarbon products such as ethanol or ethylene may be catalyzed with significant yield only on metallic copper surfaces, implying large ensembles of copper atoms. Here, we report on an inexpensive Cu-N-C material prepared via a simple pyrolytic route that exclusively feature single copper atoms with a CuN4 coordination environment, atomically dispersed in a nitrogen-doped conductive carbon matrix. This material achieves aqueous CO2 electroreduction to ethanol at a Faradaic yield of 55% under optimized conditions (electrolyte: 0.1 M CsHCO3 , potential:-1.2V vs. RHE and gas-phase recycling set up), as well as CO electroreduction to C2-products (ethanol and ethylene) with a Faradaic yield of 80%. During electrolysis the isolated sites transiently convert into metallic copper nanoparticles, as shown by operando XAS analysis, which are likely to be the catalytically active species. Remarkably, this process is reversible and the initial material is recovered intact after electrolysis.

Selective electrochemical reduction of CO 2 into energy-dense organic compounds is a promising strategy for using CO 2 as a carbon source. Herein, we investigate a series of iron-based catalysts synthesized by pyrolysis of Fe-, N-and C-containing precursors for the electroreduction of CO 2 to CO in aqueous conditions and demonstrate that the selectivity of these materials for CO 2 reduction over proton reduction is governed by the ratio of isolated FeN 4 sites vs. Fe-based nanoparticles. This ratio can be synthetically tuned to generate electrocatalysts producing controlled CO/H 2 ratios. It notably allows preparing materials containing only FeN 4 sites, which are able to selectively reduce CO 2 to CO in aqueous solution with Faradaic yields over 90% and at low overpotential. KEYWORDS. CO 2 reduction-electrocatalysis-iron-Fe-N-C materials-Structure-selectivity relationship.

This contribution reports the discovery and analysis of the first PGM-free, p-block Sn-based single metal and nitrogen-doped carbon (MNC) catalysts for the electroreduction of molecular oxygen (ORR) in acidic conditions at fuel cell cathodes. The prepared SnNC catalysts meet and exceed state of art FeNC catalysts in terms of intrinsic catalytic turn over frequency (TOF) and hydrogenair fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Added benefits include a high favorable selectivity for the 4-electron reduction pathway and a Fenton-inactive character of Sn.A range of analytical techniques, combined with DFT calculations indicate that stannic Sn(IV)-Nx single metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytic active moieties. The superior PEMFC performance of SnNC cathode catalysts under realistic, hydrogen-air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising replacement of today's state-of-art Fe-based catalysts. 4Growing concerns over fossil energy and the environment are incentives to develop new energy technologies. Low-temperature hydrogen/air proton-exchange membrane fuel cell (PEMFC) is one such technology, converting hydrogen into electrical energy 1, 2 . For catalyzing the oxygen reduction reaction (ORR) and hydrogen oxidation reaction at the electrodes, PEMFCs rely however on precious, in particular platinum-based catalysts 3, 4 , a scarce and expensive metal.Research to replace precious group metals (PGMs) has led to a class of bio-inspired catalysts, labelled MNC, that involve non-precious 3d transition metal cations stabilized by nitrogen atoms (Metal-Nx moieties), themselves incorporated in conductive carbon matrices. Fe, Co and Mn are hitherto the only three metals that result in ORR-active Metal-Nx moieties in acidic reaction environments 5,6,7,8,9,10 . While the number and utilization of such moieties embedded in carbon are being improved 9 , the fundamental nature of such sites is not so new. Indeed, the large body of experimental research on pyrolyzed FeNC and CoNC materials identifies Metal-N4 motifs as the most active sites for catalyzing ORR in acid 11,12,13 . Such sites are akin to square-planar Metal-N4 sites in Fe or Co macrocycles, identified in 1964 to be ORR active 14 Here, we report the discovery of the first p-block single metal site catalyst, SnNC, exhibiting catalytic ORR reactivities in acidic environments that meet and exceed all state-of-art PGM-free catalyst concepts, while adding important benefits in terms of catalyst stability. The catalytically active single-metal SnNx moieties embedded in the surface of the SnNC catalyst were characterized by high-resolution scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), extended X-ray absorption fine structure (EXAFS), Xray photoelectron spectroscopy (XPS) and 119 Sn Mössbauer spectroscopy, com...

A new set of ionic radii in aqueous solution has been derived for lanthanoid(III) cations starting from a very accurate experimental determination of the ion-water distances obtained from extended X-ray absorption fine structure (EXAFS) data. At variance with previous results, a very regular trend has been obtained, as expected for this series of elements. A general procedure to compute ionic radii in solution by combining the EXAFS technique and molecular dynamics (MD) structural data has been developed. This method can be applied to other ions allowing one to determine ionic radii in solution with an accuracy comparable to that of the Shannon crystal ionic radii.