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Electrochemical conversion of carbon dioxide (CO2) to chemicals or fuels can effectively promote carbon capture and utilization, and reduce greenhouse gas emission but a serious impediment to the process is to find highly active electrocatalysts that can selectively produce desired products. Herein, we have established the design principles based on the density functional theory calculations to screen the most promising catalysts from the family of coordinately unsaturated/saturated transition metal (TM) embedded into covalent organic frameworks (TM-COFs). An intrinsic descriptor has been discovered to correlate the molecular structures of the active centers with both the activity and selectivity of the catalysts. Among all the catalysts, the coordinately unsaturated Ni-doped covalent triazine framework (Ni-CTF) is identified as one of the best electrocatalysts with the lowest overpotential (0.34 V) for CO2 reduction toward CO while inhibiting the formation of the side products, H2 and formic acid. Compared with coordinately saturated TM-COFs and noble metals (e.g. Au and Ag), TM-CTFs exhibit higher catalytic activity and stronger inhibition of side products. The predictions are supported by previous experimental results. This study provides an effective strategy and predictive tool for developing desired catalysts with high activity and selectivity.

The development of catalysts with high activity and selectivity for electrochemical reduction of CO2 could provide renewable energy sources and improved environmental remediation. However, most current electrocatalysts have large energy barriers that lower their activity and selectivity. Here, we designed a class of electrocatalysts with 3d transition bimetallic active sites embedded into covalent organic frameworks (COFs) and discovered that they were more efficient than noble metals and single-atom electrocatalysts in CO2 reduction reaction to CO. To facilitate rational design and quick screening of desired catalysts, an intrinsic descriptor was identified for the catalysts, which correlates the catalytic activity with the topological, bonding, and electronic structures. Among the catalysts, Fe/Cu-, Ni/Zn-COFs could be excellent electrocatalysts, with the low overpotential (~0.25 V) while suppressing side reactions. The excellent activity and selectivity of the catalysts stems from their unique bimetallic sites that break the constraint of scaling relations, leading to much more adsorption modes and facilitating intermediates to achieve the ideal states.

Proton exchange fuel cells (PEFCs) are one of the most popular and promising energy conversion devices because of their highly stable and efficient membranes in acidic media, but there is a lack of durable non-noble metal electrocatalysts suitable for acidic environments. Herein, we designed a new type of electrocatalysts consisting of transition metal halide molecules covered by graphene sheets, which is supported by experiments. To rapidly screen the best catalysts from numerous candidate materials, the electronic structures, reaction free energies and overpotentials of those graphene-covered halide catalysts were studied by the first-principles calculations to predict the catalytic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). An intrinsic descriptor, the electrostatic force induced by the metallic ions, was found to well describe the catalytic activities and provide a better understanding of the local electrical field effects on catalytic activities. The spin-down d-band center was also introduced to describe catalytic activities of the catalysts. The results demonstrate that the graphene-covered CrBr2 shows the best bifunctional catalytic activities for fuel cells while graphene-covered CoF2 could well facilitate H2O2 production. These catalysts are better than the best commercial noble metal catalysts (e.g., Pt and RuO2) in terms of overpotentials and activities. This work provides a theoretical base for rationally designing durable electrocatalysts with excellent catalytic activities.

Coordination-driven self-assembly has been extensively employed to construct a variety of discrete structures as a bottom-up strategy. However, mechanistic understanding regarding whether self-assembly is under kinetic or thermodynamic control is less explored. To date, such mechanistic investigation has been limited to distinct, assembled structures. It still remains a formidable challenge to study the kinetic and thermodynamic behavior of self-assembly systems with multiple assembled isomers due to the lack of characterization methods. Herein, we use a stepwise strategy which combined self-recognition and self-assembly processes to construct giant metallo-supramolecules with 8 positional isomers in solution. With the help of ultrahigh-vacuum, low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy, we were able to unambiguously differentiate 14 isomers on the substrate which correspond to 8 isomers in solution. Through measurement of 162 structures, the experimental probability of each isomer was obtained and compared with the theoretical probability. Such a comparison along with density functional theory (DFT) calculation suggested that although both kinetic and thermodynamic control existed in this self-assembly, the increased experimental probabilities of isomers compared to theoretical probabilities should be attributed to thermodynamic control.

Hydrogen-bonded frameworks (HOFs) with permanent microporosity are still hard to obtain, due to the inherent nature of easy-to-collapse of HOF skeleton on solvent removal. On the other hand, tuning the electronic structure of HOFs by incorporating redox-active units for expanding the functionality of this class of material presents a waiting-for unveiled and highly desired field. In this work, the redox-active imide unit was utilized to construct HOF. The resultant HOF (namely ECUT-HOF-30) enables robust microporous structure of ca. 4.0 Å, performing effective separation of C2H2/CO2 via the unique molecular recognition from imide oxygen atoms. Whilst exciting optoelectronic active properties such as photochromism and electrochromism were observed in this HOF. The proof-of-concept results open up a gate for fundamental design of advanced HOFs with redox-active unit and versatility.

Heteroatom-doped carbon based catalysts have been demonstrated as one type of the most promising electrocatalysts to replace traditional noble metal catalyst, such as Pt for oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, experimental results have shown that the carbon based catalysts exhibit inferior catalytic activities in acidic than that in alkaline mediums. As the catalytic mechanisms is unclear, there is no effective strategy to design and synthesize highly efficient carbon based catalysts working in acidic medium. In this work, the density functional theory (DFT) methods was applied to understand the inferior performance of doped graphene in acid. Our results show that the excellent performance of doped graphene is downgraded by protonation of dopants and the adsorption of acidic anions. The ORR overpotentials were calculated to be higher due to the protonation and the aggregation extent of acid anion on the graphene surface. To enhance the catalytic activities, the adverse effects of protonation and acid anion should be minimized as much as possible. These insights provide a direction to boost the catalytic efficiency and stability of metal-free carbon based catalysts for clean energy conversions and storages.

Nitrogen-coordinated single-atom catalysts (SACs) catalyzed electrochemical reduction of CO2 (CO2RR) to CO has emerged as a promising strategy in the management of the global carbon cycle. Herein, we carried out density functional theory (DFT) calculations to investigate the role of possible Ni-Nx and Ni–C4 coordinations in CO2RR catalysis. We discover that the free energy change for CO2RR is lowered with a decrease in Ni-Nx coordination number, with Ni–C4 displaying the lowest overpotential for CO2RR. Using these findings, we develop an effective strategy to transform Ni–N4 to Ni–C4 active sites by removing N moieties within Ni embedded in a hollow nitrogen-doped carbon shell (Ni@NCH). We demonstrate an improvement in CO selectivity with this transformation of active sites and the optimized Ni@NCH-1000 catalyst is capable of converting CO2 to CO with high Faradaic efficiency for CO (FECO) of 96% and a current density (j) of −35 mA cm−2 at an applied potential of −1 V vs Reversible Hydrogen Electrode (RHE). When adopted in a high-throughput gas diffusion electrolyzer, the newly-developed superhydrophobic catalyst is capable of maintaining CO selectivity >95% over a wide range of applied cell voltages from 2.4 V to 3 V with high current densities (~100 mA cm−2 at 3 V). Our insights and findings with active site transformation in Ni–N–C SACs can serve as guidelines for designing highly active SACs for large-scale CO2RR systems.

Carbon nanomaterials are promising metal-free catalysts for energy conversion and storage, but the catalysts are usually developed via traditional trial-and-error methods. To rationally design and accelerate the search for the highly efficient catalysts, it is necessary to establish design principles for the carbon-based catalysts. Here, theoretical analysis and material design of metal-free carbon nanomaterials as efficient photo-/electrocatalysts to facilitate the critical chemical reactions in clean and sustainable energy technologies are reviewed. These reactions include the oxygen reduction reaction in fuel cells, the oxygen evolution reaction in metal-air batteries, the iodine reduction reaction in dye-sensitized solar cells, the hydrogen evolution reaction in water splitting, and the carbon dioxide reduction in artificial photosynthesis. Basic catalytic principles, computationally guided design approaches and intrinsic descriptors, catalytic material design strategies, and future directions are discussed for the rational design and synthesis of highly efficient carbon-based catalysts for clean energy technologies.

A highly efficient cathode catalyst for rechargeable Li-CO2 batteries is successfully synthesized by implanting single iron atoms into 3D porous carbon architectures, consisting of interconnected N,S-codoped holey graphene (HG) sheets. The unique porous 3D hierarchical architecture of the catalyst with a large surface area and sufficient space within the interconnected HG framework can not only facilitate electron transport and CO2 /Li+ diffusion, but also allow for a high uptake of Li2 CO3 to ensure a high capacity. Consequently, the resultant rechargeable Li-CO2 batteries exhibit a low potential gap of ≈1.17 V at 100 mA g-1 and can be repeatedly charged and discharged for over 200 cycles with a cut-off capacity of 1000 mAh g-1 at a high current density of 1 A g-1 . Density functional theory calculations are performed and the observed appealing catalytic performance is correlated with the hierarchical structure of the carbon catalyst. This work provides an effective approach to the development of highly efficient cathode catalysts for metal-CO2 batteries and beyond.

Solar fuels produced from sunlight via photocatalytic water splitting and artificial photosynthesis represent one of the most promising clean energy sources. Various semiconductors, including graphitic carbon nitride (g-C3N4) and transition metal oxides, have been considered as photocatalysts for the fuel production, but there lacks of the design principles for rapid screening of the best photocatalysts from numerous candidate materials. Here, we demonstrate, for the first time, a universal guiding principle that governs the photocatalytic activities of p-orbital element-doped C3N4-based photocatalysts for photocatalytic water splitting, which is also applicable to other doped semiconductors including ZnO and TiO2 for water splitting and carbon dioxide reduction. An activity indictor is introduced to determine the efficiency of non-metal-doped semiconductor photocatalysts for the solar fuel production. These predictions are supported by experimental results. This generalized principle could guide a broad photochemical production of solar fuels including hydrogen, hydrocarbon, and other green chemicals.