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 .

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.

We present an approach for a probabilistic and unbiased discovery of selective and active catalysts for the carbon dioxide (CO 2 ) and carbon monoxide (CO) reduction reactions on high-entropy alloys (HEAs). By combining density functional theory (DFT) with supervised machine learning, we predict the CO and hydrogen (H) adsorption energies of all surface sites on the (111) surfaces of the disordered CoCuGaNiZn and AgAuCuPdPt HEAs. This allows an optimization for the HEA compositions with increased likelihood for sites with weak hydrogen adsorption to suppress the formation of molecular hydrogen and with strong CO adsorption to favor the reduction of CO. As opposed to the construction of specific arrangements of surface atoms, our approach makes the desired surface sites more frequent through an increase in their probability. This leads to the unbiased discovery of several catalyst candidates for which selectivity toward highly reduced carbon compounds is expected and of which some have been verified in the literature.

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The electrochemical CO2 reduction reaction (CO2RR) is a promising technology for converting waste CO2 into chemicals which could be used as feedstock for the chemical industry or as synthetic fuels. The technological viability of this process, however, is contingent on finding affordable and efficient catalysts. Recently, carbon-based solid state catalyst materials containing small amounts of nitrogen and transition metals (MNC) have emerged as a selective and cost-efficient alternative to noble metal catalysts for the direct electrochemical reduction of CO2 into CO. In addition, other products have also been reported, including formic acid and methane. In this Perspective, we offer a focused discussion of recent advances in the field of MNC catalysts for the CO2RR. The different factors which control the catalytic performance of MNC toward the CO2RR are discussed in this Perspective. We focus on density functional theory-guided experimental studies aiming to elucidate key experimental parameters and molecular descriptors that control the activity and selectivity of this class of materials. We close addressing the remaining challenges and take a look forward into future studies.

In the past decades, the surplus of atmospheric CO2 concentration has drawn tremendous political and scientific attention for its negative impacts, such as the greenhouse effect and ocean carbonation. To mitigate such CO2 issues, a combination of various strategies is required. The electrochemical CO2 reduction reaction (CO2RR) is a promising alternative to convert CO2 into carbon-based chemicals and fuels, and electricity generated from the renewable sources (solar and wind) could be employed to sustain this transformation. At the current moment, the technological viability of this process is still contingent on finding affordable and efficient catalysts. In this thesis, a family of catalyst materials composed of abundant elements, in particular, non-precious metals, nitrogen, and carbon, typically referred to as precious group metal (PGM)-free "M-N-C" catalysts, were synthesized and mechanistically investigated-both experimentally and computationally-as catalyst candidates for the CO2RR. MNC catalysts feature hemoglobin-like single-site metallated porphyrin moieties with great impact on the catalytic reactivity and selectivity of the CO2RR. Among our studied M-N-C catalysts, the Ni-functionalized one exhibits great efficiency for CO yielding at high potentials and current densities. In particular, employment of Ni-N-C-based gas diffusion electrodes (GDE) combined with micro flow cells, allowed high CO evolution that could exceed 80% faradaic efficiency at 250 mA cm-2 current density, outperforming the industry commonly used Ag benchmark. By coupling our experimental observation and density functional theory (DFT) simulation, the reaction path from CO2 to CO over this sort single site catalyst could be deduced. Unlike the Ni-N-C catalyst, the Fe-N-C shows selective CO production only at low potentials. Further, due to relatively strong interaction with CO*, it opens the chance for hydrocarbons formation, yet showing little selectivity. To understand the mechanism behind this kind of selectivity, we carried out a series of studies, discussing catalytic tests, in-operando spectroscopic analysis, and computational modeling. Towards material research, operando-XAFS measurements identified an unusual Fe-N3, possibly a Fe I-N3 state, which appears to enable CH4 evolution. Further mechanistic studies included the electrocatalytic reduction of CO and CH2O as possible reactive intermediates for CH4 production. By combining the experimental and computational results, we suggest a reaction network for CO2 reduction into a variety of carbon-based products over the Fe-N-C catalyst. This contributes to the overall mechanistic understanding of CO2RR over the M-N-C catalysts and delivers perspectives to evolve and design novel catalysts to produce hydrocarbons of high value.