Structural engineering of atomic catalysts for electrocatalysis.

As a burgeoning category of heterogeneous catalysts, atomic catalysts have been extensively researched in the field of electrocatalysis. To satisfy different electrocatalytic reactions, single-atom catalysts (SACs), diatomic catalysts (DACs) and triatomic catalysts (TACs) have been successfully designed and synthesized, in which microenvironment structure regulation is the core to achieve high-efficiency catalytic activity and selectivity. In this review, the effect of the geometric and electronic structure of metal active centers on catalytic performance is systematically introduced, including substrates, central metal atoms, and the coordination environment. Then theoretical understanding of atomic catalysts for electrocatalysis is innovatively discussed, including synergistic effects, defect coupled spin state change and crystal field distortion spin state change. In addition, we propose the challenges to optimize atomic catalysts for electrocatalysis applications, including controlled synthesis, increasing the density of active sites, enhancing intrinsic activity, and improving the stability. Moreover, the structure-function relationships of atomic catalysts in the CO2 reduction reaction, nitrogen reduction reaction, oxygen reduction reaction, hydrogen evolution reaction, and oxygen evolution reaction are highlighted. To facilitate the development of high-performance atomic catalysts, several technical challenges and research orientations are put forward.

Nanoflower-Like High-Entropy Co-Fe-Cr-Mo-Mn Spinel for Oxygen Evolution.

Oxygen evolution reaction (OER) is the key anode reaction of electrolytic water. To improve the slow OER kinetics, we synthesize nanoflower-like Co-Fe-Cr-Mo-Mn high-entropy spinel (HES) nanosheets on nickel foam (NF) by one-step solvothermal method, which exhibit an overpotential (η10) of only 188 mV at 10 mA cm-2, much lower than bimetallic CoFeOx/NF (233 mV), trimetallic CoFeCrOx/NF (211 mV), and tetrametallic CoFeCrMoOx/NF (200 mV). The OER overpotential decreases with the increase of the number of metals, indicating that the formation of HES has a positive effect on the improvement of electrocatalytic performance, since the synergistic effect between different metals enhances the charge transfer rate and decreases reaction barrier. In-situ Raman spectra demonstrate that the formation of γ-NiOOH on the HES surface is a crucial active species for the OER. This work demonstrates a simple and efficient synthesis method to prepare nanoflower-like high-entropy electrocatalysts for efficient OER electrocatalysis.

Optimization strategies of high-entropy alloys for electrocatalytic applications.

High-entropy alloys (HEAs) are expected to become one of the most promising functional materials in the field of electrocatalysis due to their site-occupancy disorder and lattice order. The chemical complexity and component tunability make it possible for them to obtain a nearly continuous distribution of adsorption energy curve, which means that the optimal adsorption strength and maximum activity can be obtained by a multi-alloying strategy. In the last decade, a great deal of research has been performed on the synthesis, element selection and catalytic applications of HEAs. In this review, we focus on the analysis and summary of the advantages, design ideas and optimization strategies of HEAs in electrocatalysis. Combined with experiments and theories, the advantages of high activity and high stability of HEAs are explored in depth. According to the classification of catalytic reactions, how to design high-performance HEA catalysts is proposed. More importantly, efficient strategies for optimizing HEA catalysts are provided, including element regulation, defect regulation and strain engineering. Finally, we point out the challenges that HEAs will face in the future, and put forward some personal proposals. This work provides a deep understanding and important reference for electrocatalytic applications of HEAs.

Achievements and challenges of copper-based single-atom catalysts for the reduction of carbon dioxide to C2+ products.

Copper is the only metal that can convert CO2 into C2 and C2+ in electrocatalytic carbon dioxide reduction (CO2RR). However, the Faraday efficiency of CO2 conversion to C2 and C2+ products at high current densities is still low, which cannot meet the actual industrial demand. Here, the design methods of single-atom copper catalysts (including regulating the coordination environment of single-atom copper, modifying the carbon base surface and constructing diatomic Cu catalysts) are reviewed, and the current limitations and future research directions of copper-based single-atom catalysts are proposed, providing directions for the industrial conversion of CO2 into C2 and C2+ products.

In situ synthesis of rosette-like Co-doped FeNiOOH/NF for seawater oxidation.

The development of high activity and strong resistance to seawater corrosion oxygen evolution reaction (OER) electrocatalysts for seawater electrolysis has broad application prospects. Herein, we prepare Co-doped FeNiOOH rosette-like nanoflowers on nickel foam (NF) with different Co dosages by one-step solvothermal method. The Co0.2-FeNiOOH/NF exhibits a low overpotential (η10) of 185 mV and Tafel slope of 30 mV dec-1 in 1 M KOH. Moreover, it shows a low η10 of 244 mV in alkaline seawater electrolyte. The remarkable OER performance of Co0.2-FeNiOOH/NF is ascribed to the fact that the introduction of Co regulates the morphology and electron structure of the material, which provides abundant active sites for the reaction and promotes charge transfer. In situ Raman results demonstrate that NiOOH and γ-FeOOH are the key active species for the OER. This study provides a feasible basis for seawater electrolysis over transition metal (oxy)hydroxides.

Nanoflower-like high-entropy Ni-Fe-Cr-Mn-Co (oxy)hydroxides for oxygen evolution.

High-entropy materials (HEMs) have potential application value in electrocatalytic water splitting because of their unique alloy design concept and significant mixed entropy effect. Here, we synthesize a high-entropy Ni-Fe-Cr-Mn-Co (oxy)hydroxide on nickel foam (NF) by a solvothermal method. The flower-like structure of FeNiCrMnCoOOH/NF can provide abundant active sites, thus improving the oxygen evolution reaction (OER) activity. In 1 M KOH, the FeNiCrMnCoOOH/NF shows an ultra-low overpotential (η10) of 201 mV for the OER, superior to FeNiCrMnAlOOH/NF, FeNiCrMnCuOOH/NF, FeNiCrMnMoOOH/NF, and FeNiCrMnCeOOH/NF. In addition, it exhibits a low η10 of 223 mV in 0.5 M NaCl + 1 M KOH and excellent stability. Electrochemical impedance spectroscopy measurements indicate that the synergistic effect between multiple metals accelerates charge transfer, while in situ Raman measurements reveal that NiOOH is a key active species for the OER. This work is of great significance for the construction of high-entropy (oxy)hydroxides for seawater electrolysis.

Electron coupling effect-triggered monatomic copper laccase-mimicking nanozyme for the degradation and detection of guaiacol produced by Alicyclobacillus acidoterrestris.

The generation of guaiacol by Alicyclobacillus acidoterrestris (A. acidoterrestris) in fruit juices negatively affects public health and causes severe environmental pollution. Therefore, the sensitive detection and efficient degradation of guaiacol in real samples are crucial. Here, we develop an electrochemical sensor utilizing a copper single-atom nanozyme (CuN4-G) to detect and degrade guaiacol at the picomolar level. Density functional theory (DFT) calculations verify that the bonding electron coupling effect in the CuN4-G facilitates rapid electron transfer, enhances electrical conductivity, and provides abundant active sites, thereby leading to exceptional catalytic performance. Moreover, CuN4-G demonstrates a Km value similar to that of natural laccase but a higher Vmax, highlighting its potential as a highly efficient biocatalyst. The CuN4-G-based electrochemical sensor achieves a detection from 5 to 50,000 pM for guaiacol, with a 1.2 pM (S/N = 3) detection limit. Additionally, CuN4-G-modified electrodes display high selectivity and excellent stability. CuN4-G nanozyme can keep its activity in conditions of pH (3-9), temperature (30-90 °C), ionic strength (0-400 mM), and organic solvent (0-50% (v/v)), overcoming the deficiencies of natural enzymes. Furthermore, our electrochemical sensor can not only accurately detect guaiacol, but also degrade it in actual fruit juice samples infected by A. acidoterrestris, demonstrating its potential applications in food and environmental monitoring.

Single-atom nanozyme-based electrochemical sensors for health and food safety monitoring.

Electrochemical sensors and biosensors play an important role in many fields, including biology, clinical trials, and food industry. For health and food safety monitoring, accurate and quantitative sensing is needed to ensure that there is no significantly negative impact on human health. It is difficult for traditional sensors to meet these requirements. In recent years, single-atom nanozymes (SANs) have been successfully used in electrochemical sensors due to their high electrochemical activity, good stability, excellent selectivity and high sensitivity. Here, we first summarize the detection principle of SAN-based electrochemical sensors. Then, we review the detection performances of small molecules on SAN-based electrochemical sensors, including H2O2, dopamine (DA), uric acid (UA), glucose, H2S, NO, and O2. Subsequently, we put forward the optimization strategies to promote the development of SAN-based electrochemical sensors. Finally, the challenges and prospects of SAN-based sensors are proposed.

Nanozyme colorimetric sensor array based on monatomic cobalt for the discrimination of sulfur-containing metal salts.

The identification of sulfur-containing metal salts (SCMs) is of great interest because they play an important role in many biological processes and diseases. Here, we constructed a ternary channel colorimetric sensor array to detect multiple SCMs simultaneously, relying on monatomic Co embedded in nitrogen-doped graphene nanozyme (CoN4-G). Due to the unique structure, CoN4-G exhibits activity similar to native oxidases, capable of catalysing directly the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) by O2 molecules independent of H2O2. Density functional theory (DFT) calculations suggest that CoN4-G has no potential barrier in the whole reaction route, thus presenting higher oxidase-like catalytic activity. Based on different degrees of TMB oxidation, different colorimetric response changes are obtained as "fingerprints" on the sensor array. The sensor array can discriminate different concentrations of unitary, binary, ternary, and quaternary SCMs and has been successfully applied to detect six real samples (soil, milk, red wine and egg white). To advance the field detection of the above four types of SCMs, we creatively propose a smartphone-based autonomous detection platform with a linear range of 1.6-320 µM and a limit of detection of 0.0778-0.218 µM, which demonstrates the potential use of sensor arrays in the application of disease diagnosis and food and environment monitoring.

Effect of Metal Complexing on Mn-Fe/TS-1 Catalysts for Selective Catalytic Reduction of NO with NH3.

TS-1 zeolite with desirable pore structure, an abundance of acidic sites, and good thermal stability promising as a support for the selective catalytic reduction of NO with NH3 (NH3-SCR). Herein, a series of Mn-Fe/TS-1 catalysts have been synthesized, adopting tetraethylenepentamine (TEPA) as a metal complexing agent using the one-pot hydrothermal method. The introduced TEPA can not only increase the loading of active components but also prompts the formation of a hierarchical structure through decreasing the size of TS-1 nanocrystals to produce intercrystalline mesopores during the hydrothermal crystallization process. The optimized Mn-Fe/TS-1(R-2) catalyst shows remarkable NH3-SCR performance. Moreover, it exhibits excellent resistance to H2O and SO2 at low temperatures. The characterization results indicate that Mn-Fe/TS-1(R-2) possesses abundant surface Mn4+ and Fe2+ and chemisorbed oxygen, strong reducibility, and a high Brønsted acid amount. For comparison, Mn-Fe/TiO2 displays a narrower active temperature window due to its poor thermostability.

Unveiling the role of defects in iron oxyhydroxide for oxygen evolution.

Development of earth-abundant and robust oxygen evolution reaction (OER) catalysts is imperative for cost-effective hydrogen production via water electrolysis. Herein, we report ultrafine iron (oxy)hydroxide nanoparticles with average particle size of 2.6 nm and abundant surface defects homogeneously supported on oleum-treated graphite (FeOx(n)@HG-T), providing abundant active sites for the OER. The optimal FeOx(0.03)@HG-110 exhibits high electrocatalytic OER activity and excellent stability. Electrochemical testing results and theoretical calculations reveal that the outstanding OER activity of FeOx(0.03)@HG-110 is due to its stronger charge transfer ability and lower OER energy barrier than defect-free FeOx nanoparticles. This work demonstrates that the OER performance of oxyhydroxide-based electrocatalysts can be improved by surface defect engineering.

Electrocatalysts for Oxygen/Hydrogen-Involved Reactions.

Oxygen/hydrogen-involved reactions are key reactions in many energy-related technologies, such as electrolytic water, electrocatalytic carbon dioxide reduction, electrochemical ammonia synthesis, rechargeable metal-air batteries, and renewable fuel cells [...].

Enhanced oxygen evolution activity on mesoporous cobalt-iron oxides.

To solve the energy crisis and environmental pollution problems, the use of clean and renewable energy to replace fossil energy has become a top priority. The oxygen evolution reaction (OER) is the core of many renewable energy technologies. Developing low-cost and high-performance OER electrocatalysts is the key to implementing efficient energy conversion processes. Here, we synthesize ordered mesoporous iron-cobalt oxides using a hard template strategy. As a mesoporous oxide catalyst, meso-CoFe0.05Ox exhibits low OER overpotentials of 280 and 373 mV at current densities of 10 and 100 mA cm-2, respectively, and does not show deactivation for at least 18 hours at 100 mA cm-2. The introduction of iron can change the electronic structure of Co, and the orbital electrons are easily transferred from cobalt to iron. The enhanced OER performance can be attributed to concerted catalysis between the iron and cobalt sites that lowers the OER energy barrier, and the large specific surface area of the porous oxide providing efficient active sites for the reaction.

A binuclear Co-based metal-organic framework towards efficient oxygen evolution reaction.

The search for low-cost and high-performance electrocatalysts for oxygen evolution reaction (OER) has aroused enormous research interest in the last few years. Reported herein is the topotactic construction of a binuclear Co-based metal-organic framework (Co2-tzpa) using a solvothermal reaction. Prominently, as a porous catalyst, Co2-tzpa holds its activity for at least 25 hours and exhibits low OER overpotentials of 336 and 396 mV to achieve the current density of 10 mA cm-2 in 1 M KOH and 0.1 M KOH, respectively. The excellent OER performance should be attributed to each cobalt site coordinated with two tetrazolate N atoms.

Promotion of the water oxidation activity of iridium oxide by a nitrogen coordination strategy.

The water oxidation reaction is the pivotal half-reaction for photo-/electro-catalytic water splitting. Fabrication of high-efficiency and robust water oxidation is essential to realize wide-scale artificial photosynthesis. Here, we report an efficient strategy to improve the water oxidation activity of iridium oxide by a nitrogen-coordination method. Due to the coordination effect, the iridium oxide can be well dispersed to generate ultra-small nanoparticles and the intrinsic activity can be improved for the water oxidation reaction. This study suggests that high-performance water oxidation catalysts can be constructed based on a nitrogen-coordination strategy.

Enhancing Methane Aromatization Performance by Reducing the Particle Size of Molybdenum Oxide.

Efficient use of natural gas to produce aromatics is an attractive subject; the process requires catalysts that possess high-performance active sites to activate stable C-H bonds. Here, we report a facile synthetic strategy to modify HMCM-49 with small molybdenum oxide nanoparticles. Due to the higher sublimability of nano-MoO3 particles than commercial MoO3, they more easily enter into the channels of HMCM-49 and associate with Brønsted acid sites to form active MoCx-type species under calcination and reaction conditions. Compared with commercial MoO3 modified MCM-49, nano-MoO3 modified MCM-49 exhibits higher methane conversion (13.2%), higher aromatics yield (9.1%), and better stability for the methane aromatization reaction.

Recent advancement in the electrocatalytic synthesis of ammonia.

Ammonia can not only be used as an active nitrogen component of nitrogen fertilizers, fibers, explosives, etc., but also provides a high energy density and carbon free energy carrier. Currently, ammonia is industrially synthesized by the Haber Bosch process at high temperature and high pressure, which results in high energy loss and a serious greenhouse effect. The electrocatalytic nitrogen reduction reaction (NRR) is a sustainable and environmentally friendly strategy for the synthesis of ammonia. Although lots of electrocatalysts have been developed for this reaction, further breakthroughs are needed in catalytic activity, selectivity and Faraday efficiency to meet the large-scale commercial demand. In this review, the recent advance in NRR electrocatalysis is thoroughly commented on. Different kinds of electrocatalysts used in ammonia synthesis (including single atom catalysts, metal oxide catalysts, nanocomposite catalysts, and metal free catalysts) are introduced. The reaction mechanism of the NRR is discussed in detail. The structure-function relationship and efficient strategies to improve the ammonia yield are clearly discussed. The effect of the electronic structure and morphology of catalysts on the selectivity of the NRR is highlighted. The research directions and perspectives on the further development of more efficient electrocatalysts for the NRR are provided.

Atomic cobalt catalysts for the oxygen evolution reaction.

Development of high-efficiency single-atom catalysts (SACs) for the oxygen evolution reaction (OER) is challenging. Herein, we report a facile annealing strategy to construct an atomically dispersed cobalt- and nitrogen-codoped graphene catalyst for high-efficiency OER. The as-prepared 0.7-Co@NG-750 electrocatalyst requires an ultra-low onset overpotential of approximately 210 mV on a glassy carbon electrode in both 1.0 M KOH and 0.1 M KOH solutions. The density functional theory (DFT) calculations reveal that the theoretical overpotential on Co-N4 and Co-N2C2 sites is 0.41 and 0.53 V, respectively, and these might be efficient active sites for electrocatalytic water oxidation.

Mono-/Multinuclear Water Oxidation Catalysts.

Water splitting, in which water molecules can be transformed into hydrogen and oxygen, is an appealing energy conversion and transformation strategy to address the environmental and energy crisis. The oxygen evolution reaction (OER) is dynamically slow, which limits energy conversion efficiency during the water-splitting process and requires high-efficiency water oxidation catalysts (WOCs) to overcome the OER energy barrier. It is generally accepted that multinuclear WOCs possess superior OER performances, as demonstrated by the CaMn4 O5 cluster in photosystem II (PSII), which can catalyze the OER efficiently with a very low overpotential. Inspired by the CaMn4 O5 cluster in PSII, some multinuclear WOCs were synthesized that could catalyze water oxidation. In addition, some mononuclear molecular WOCs also show high water oxidation activity. However, it cannot be excluded that the high activity arises from the formation of dimeric species. Recently, some mononuclear heterogeneous WOCs showed a high water oxidation activity, which testified that mononuclear active sites with suitable coordination surroundings could also catalyze water oxidation efficiently. This Review focuses on recent progress in the development of mono-/multinuclear homo- and heterogeneous catalysts for water oxidation. The active sites and possible catalytic mechanisms for water oxidation on the mono-/multinuclear WOCs are provided.

Progress in Nonmetal-Doped Graphene Electrocatalysts for the Oxygen Reduction Reaction.

Owing to energy shortages and environmental pollution, green energy sources such as polymer electrolyte fuel cells and metal-air batteries play a more and more important role, whereby the oxygen reduction reaction (ORR) is the rate-determining step. Development of high-efficiency and stable catalysts to facilitate the ORR is of importance. Graphene is a new type of material with two-dimensional structure and high surface area, which has wide-ranging applications in many fields. However, graphene with zero band gap shows low electrocatalytic activity toward the ORR. Introduction of nonmetal atoms can change the electronic arrangement, generate active sites, and further improve the catalytic activity of graphene. Some nonmetal-doping strategies (e.g., N, S, and P doping) can promote ORR activity. Herein, the recent development of nonmetal-doped graphene catalysts for ORR is reviewed. Some common synthetic methods for nonmetal-doped graphene materials are summarized, and the active sites and possible reaction mechanisms for ORR on various nonmetal-doped graphene catalysts are discussed.