Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–Nx moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–Nx moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–Nx moieties and it provides predictive guidelines for the rational design of selective carbon-based CO2 reduction catalysts.

In this work, we propose four non‐coupled binding energies of intermediates as descriptors, or “genes”, for predicting the product distribution in CO2 electroreduction. Simple reactions can be understood by the Sabatier principle (catalytic activity vs. one descriptor), while more complex reactions tend to give multiple very different products and consequently the product selectivity is a more complex property to understand. We approach this, as a logistical classification problem, by grouping metals according to their major experimental product from CO2 electroreduction: H2, CO, formic acid and beyond CO* (hydrocarbons or alcohols). We compare the groups in terms of multiple binding energies of intermediates calculated by density functional theory. Here, we find three descriptors to explain the grouping: the adsorption energies of H*, COOH*, and CO*. To further classify products beyond CO*, we carry out formaldehyde experiments on Cu, Ag, and Au and combine these results with the literature to group and differentiate alcohol or hydrocarbon products. We find that the oxygen binding (adsorption energy of CH3O*) is an additional descriptor to explain the alcohol formation in reduction processes. Finally, the adsorption energy of the four intermediates, H*, COOH*, CO*, and CH3O*, can be used to differentiate, group, and explain products in electrochemical reduction processes involving CO2, CO, and carbon–oxygen compounds.

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 .

This study explores the kinetics, mechanism, and active sites of the CO2 electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon-based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO2 to CO/H2 mixtures outperforming a low-area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low-cost, abundant electrocatalysts for synthetic fuel production from CO2 .

We demonstrate the direct electrochemical conversion of CO2 to CO using solid state Ni–N–C carbon catalysts characterized by a coordinative molecular Ni–Nx active moiety at industrial current densities of up to 700 mA cm−2 with faradaic efficiencies superior to those of the state-of-the-art AgOx electrocatalysts.

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...

In the present study we demonstrate that the activity and selectivity of copper during CO2 electrochemical reduction can be tuned by simply adding halides to the electrolyte. Comparing the production rate and Faradaic selectivity of the major products as a function the working potential in the presence of Cl–, Br–, and I–, we show that the activity and selectivity of Cu depends on the concentration and nature of the added halide. We find that the addition Cl– and Br– results in an increased CO selectivity. On the contrary, in the presence of I– the selectivity toward CO drops down and instead methane formation is enhanced up to 6 times compared with the halide-free electrolyte. Even though Br– and I– can induce morphology changes of the surface, the modification in the catalytic performance of Cu is mainly attributed to halides adsorption on the Cu surface. We hypothesizes that the adsorption of halides alters the catalytic performance of Cu by increasing the negative charge on the surface according to the following order: Cl– < Br– < I–. In the case of adsorbed I–, the induced negative charge has a remarkably positive effect favoring the protonation of CO. These results present an easy way to enhance CH4 production during the CO2RR on Cu. Furthermore, understanding this effect can contribute to the design of new and more efficient catalysts.

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