Emerging single-atom catalysts for efficient electrocatalytic CO2 reduction and water splitting: Recent advances

Exploring the generation of efficient and long‐lasting bifunctional electrocatalysts obtained from low‐cost transition metal oxides is crucial to the optimal production of hydrogen and oxygen by electrocatalytic water splitting. This study aims to demonstrate the applicability of layered TiO2 nanosheets as support for designing electrocatalysts. We have demonstrated the performance by decorating the TiO2 support with NiCo2O4 nanoparticles (NiCo2O4/TiO2) as catalysts for electrocatalytic overall water splitting. Moreover, the corrosion effect of usually used carbon‐based supporting materials can decrease the working efficiency and, thus, the overall performance of the catalysts. In this aspect, TiO2 can be a better alternative to carbon‐based systems. Layered TiO2 was synthesized at room temperature, and a simple heat treatment protocol was employed for the large‐scale synthesis of NiCo2O4/TiO2. TiO2 facilitated the formation of smaller NiCo2O4 nanoparticles, also improving the dispersion. This bifunctional electrocatalyst exhibits high OER and HER performance with a low overpotential of 309 mV and 185 mV respectively, at a current density of 10 mA cm−2. TiO2 supported catalyst also exhibits other advantages like remarkable durability in the alkaline medium along with high turnover frequency (TOF) values. This inexpensive catalyst can deliver a current density of 10 mA cm−2 at only 1.64 V with a steady performance for more than 12 h for overall water splitting. Thus, this homemade system provides a proficient and low‐cost alternative to the more expensive systems such as RuO2, IrO2 or Pt for the electrochemical water splitting applications.

Modulating ternary Mo–Ni–P by electronic reconfiguration and morphology engineering for boosting all-pH electrocatalytic overall water splitting

Continuous consumption of fossil energy and excessive CO2 emission severely restrict human society. Sustainable carbon cycle is a promising technology to simultaneously relieve greenhouse effect and energy crisis based on electrocatalysis and photocatalysis. However, the energy conversion efficiency is confined by the poor carriers utilization and insufficient reactive sites. Single‐atom catalysts (SACs) display outstanding performance in effectively overcoming the aforementioned problems. Herein, recent advances of SACs for enhancing the efficiency, selectivity, and long‐range stability of CO2 reduction are provided. First, the characteristics of SACs have been introduced in detail to provide rational design for SACs based on the relationship between structure and performance, including type, structure, and synthesis of SACs. Then, the high performance of SACs in electrocatalytic, photocatalytic, and thermocatalytic CO2 reduction has been discussed for disclosing reaction mechanism, such as charge transfer, activation barriers, and reaction pathway. In particular, the strategies of enhancing CO2 reduction performance have been summarized to provide deep insight into designing and developing more efficient SACs. Finally, an outlook on the current challenges and perspectives of SACs for electrocatalytic, photocatalytic, and thermocatalytic CO2 reduction is proposed. This review aims to provide a systematic reference for developing SACs in advanced CO2 catalytic conversion.

Crystalline Ni5P4/amorphous CePO4 core/shell heterostructure arrays for highly-efficient electrocatalytic overall water splitting

Electrocatalytic CO2 reduction reaction (CO2RR) has been extensively considered as a potential carbon cycle technology because it can use CO2 as resources to produce high value-added fuels and chemicals under mild conditions. Therefore, developing highly efficient CO2RR catalysts is extremely significant. Here, we designed a series of TM-N2O2Cx (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) single-atom catalysts and investigated their catalytic activities for CO2RR by using the density functional theory. Interestingly, these TM-N2O2Cx catalysts exhibit excellent CO2RR selectivity towards three kinds of different products, including CH4, CO, and HCOOH at relatively low overpotentials, where the CO2RR product of Sc-N2O2Cx, Mn-N2O2Cx, and Zn-N2O2Cx is CO, and that of V-N2O2Cx is CH4, while the other six catalysts would produce HCOOH. Moreover, the best pathway for CH4 formation is also unveiled, that is, * + CO2 → C*OOH → C*O + H2O → C*HO → C*HOH → C*H + H2O → C*H2 → C*H3 → * + CH4. In short, TM-N2O2Cx catalysts exhibit high catalytic activity for electrocatalytic CO2RR, which provides useful information for experimental scientists to synthesize these promising CO2RR catalysts.

Harnessing bimetallic spinel cobaltite with hematite for electrocatalytic overall water splitting: A comprehensive experimental & theoretical study.

High-throughput calculation-based rational design of Fe-doped MoS2 nanosheets for electrocatalytic pH-universal overall water splitting

Recent developments in the use of single-atom catalysts for water splitting

Benefitting from the maximum atom utilization efficiency, special size quantum effects and tailored active sites, single-atom catalysts (SACs) have been promising candidates for bifunctional catalysts toward water splitting. Besides, due to the unique structure and properties, some amorphous materials have been found to possess better performance than their crystalline counterparts in electrocatalytic water splitting. Herein, by combining the advantages of ruthenium (Ru) single atoms and amorphous substrates, amorphous molybdenum-based oxide stabilized single-atomic-site Ru (Ru SAs-MoO3- x /NF) catalysts are conceived as a self-supported electrode. By virtue of the large surface area, enhanced intrinsic activity and fast reaction kinetics, the as-prepared Ru SAs-MoO3- x /NF electrode effectively drives both oxygen evolution reaction (209mV @ 10mA cm-2 ) and hydrogen evolution reaction (36mV @ 10mA cm-2 ) in alkaline media. Impressively, the assembled electrolyzer merely requires an ultralow cell voltage of 1.487V to deliver the current density of 10mA cm-2 . Furthermore, such an electrode also exhibits a great application potential in alkaline seawater electrolysis, achieving a current density of 100mA cm-2 at a low cell voltage of 1.759V. In addition, Ru SAs-MoO3- x /NF only has very small current density decay in the long-term constant current water splitting test.

Introduction In recent years, there has been growing interest in converting carbon dioxide (CO2), a greenhouse gas, into a valuable resource. The electrolytic reduction method, which uses only water and electricity, offers a particularly clean approach to CO2 conversion. Its high conversion efficiency at ambient temperature and pressure, facilitated by gas diffusion electrodes (GDE), makes it a promising option for widespread adoption. Electrocatalytic CO2 reduction using GDE selectively yields carbon monoxide, ethylene, and formic acid with Ag, Cu, or Sn catalysts, respectively 1). These catalysts are often supported on conductive carbon black (CB), valued for its high electrical conductivity and porosity. However, CB-based catalysts encounter challenges like redox-induced degradation and high CO2 overvoltage. So, we turned to plasma in liquid as a new method for making electrocatalysts. Plasma in liquid is generated by subjecting a metal in solution to high-frequency, high-voltage conditions. This process enables the sputtering of metal onto powder samples dispersed in solution, with the metal surface instantly heated to high temperatures. This process can improve electrochemical properties like oxygen deficiency 2). Consequently, plasma treatment in liquid presents an opportunity to utilize highly durable and chemically resistant metal oxides as catalyst base materials. The objective of this study was to synthesize a new catalyst for electrocatalytic CO2 reduction using titanium dioxide as the catalyst base via in-liquid plasma. Titanium dioxide offers advantages of affordability, durability, and chemical resistance. Moreover, oxygen-deficient TiO2 with enhanced conductivity finds application as a base material for batteries.Experimental In the experiments, high-frequency high voltage (4 kV, 100 kHz, 0.5 µs-Pulse Width) was applied under the same conditions to various metal electrodes in a 5.0 wt%-NH4 solution (200 mL) containing dispersed TiO2 (P25: 0.5 g) to generate plasma in the liquid. Common plasma electrodes included tungsten (W), silver (Ag), and copper (Cu). Each gas diffusion electrode (GDE) was prepared by washing the powder after in-liquid plasma treatment with pure water, followed by spray coating on a gas diffusion layer with Nafion dispersion as a binder. Electrocatalytic CO2 reduction was evaluated in a conventional three-chamber gas diffusion cell at 500 mA/cm2 for 10 minutes. The CO2 supplied to the gas chamber was collected in a Tedlar bag, and the CO2 reduction products were analyzed by gas chromatography. Results and Discussion Figure shows the CO2 reduction products and Faraday efficiency of P25 treated with various liquid plasma electrodes. In pristine P25, small amounts of CO and CH4 were detected due to the redox cycle of Ti4+ and Ti3+. The use of tungsten (W), silver (Ag), and copper (Cu) as in-liquid plasma electrodes increased the production of H2, CO, and CH4, respectively. The difference in conversion can be attributed to the fact that the amount of etched Cu is less than that of Ag. Additionally, black nanoparticles were dispersed in the filtered solution when Ag was used, while the solution turned a clear blue color with Cu, suggesting the formation of a complex with ammonia. Generally, oxidized metal nanoparticles are synthesized when plasma is generated in solution. In this study, since an alkaline solution was used, it can be considered that the oxidized Ag nanoparticle surfaces were reduced, resulting in highly dispersed reduced Ag nanoparticles being deposited. In contrast, the generated Cu nanoparticles impregnated TiO2 by dissolving in ammonia and exhibited catalytic activity. CH4 significantly affects Faraday efficiency relative to the amount produced because CO and H are two-electron reactions, while CH4 is an eight-electron reaction, requiring four times as many electrons. Although C2H4 is produced by Cu nanoparticles at sufficient current density, the present results show increased CH4 production. This may be due to TiO2 promoting the hydrogenation of CO2 before Cu forms the C-C bond. Further investigation is required. Conclusions In this study, TiO2 underwent treatment with various in-liquid plasma electrodes for electrocatalytic CO2 reduction. As a result, changing the electrode allowed for successful support of metal nanoparticles on TiO2, resulting in selectivity of CO2 reduction products. During the presentation, we will discuss the comparison with conductive carbon black (CB) and the changes in electrocatalytic CO2 reduction characteristics under different plasma conditions in liquid. Compared to pristine P25, the production of CO increased by 9.5-fold from 40 to 380 μmol, and CH4 increased by 2.0-fold from 20 to 40 μmol.Reference1) Sassenburg et al., ACS Appl. Energy Mater., 5, 5, 5983 (2022).2) Takagi et al., Sci. Total Environ., 902 (13), 166018 (2023). Figure 1

Research progress and surface/interfacial regulation methods for electrophotocatalytic hydrogen production from water splitting

Electrocatalytic CO2 reduction is one of the most promising pathways for addressing environmental and green energy concerns while converting CO2 into added value chemicals and fuels. For this purpose, single-atom catalysts (SACs) have emerged as highly active and selective classes of materials toward electrochemical CO2 reduction (CO2RR) due to their unique electronic properties, exposed active centers, and tunable coordination environment. Herein, a critical assessment of the recent development of SACs for CO2RR is presented. Rational design and synthetic strategies of SACs have been summarized. The interaction of ligands and modulation of both activity and selectivity with extensive analysis on local atomic structure and different SAC types is discussed. The reaction mechanisms of SACs based CO2RR and synergistic effect of SACs with nanoparticles and nanoclusters are also highlighted, emphasizing enhanced catalytic performance due to improved charge transfer, optimized binding of intermediates, and improved accessibility of the active site. Finally, the future perspective of SACs based CO2RR is provided.

The electrochemical CO2 reduction reaction (CO2RR) is a promising approach to alleviating global warming and emerging energy crises. Yet, the CO2RR efficiency is impeded by the need for electrocatalysts with good selectivity and efficiency. Recently, single-atom catalysts (SACs) have attracted much attention in electrocatalysis and are more efficient than traditional metal-based catalysts. In this study, we modeled a Cu single atom embedded on MoX2 (X = Se and Te) monolayer with a single chalcogen (X) vacancy as SAC. Employing the dispersion-corrected density functional theory (DFT-D3) method, the electrocatalytic CO2RR activity of the Cu-MoX2 SACs is systematically investigated through significant descriptors, such as the Gibbs free energy change, charge density difference, and COHP analysis. The stability of SACs, CO2 adsorption configurations, and all possible reaction pathways for the formation of C1 products (HCOOH, CO, CH3OH, and CH4) were examined. All of the Cu-MoX2 SACs are stable and show high catalytic selectivity for the CO2RR by significantly suppressing the hydrogen evolution reaction (HER). We found that the catalytic activity is mainly due to the level of antibonding states filling between the Cu atom and *OCHOH intermediate. Among the C1 products, CH4 is selectively produced in all three SACs. Notably, there is a decrease in the limiting potential (UL) when X changes from S to Te in Cu-MoX2. Among these three SACs, Cu-MoTe2 SAC is the most promising catalyst for reducing CO2 to CH4, with as low as UL of -0.34 V vs RHE. Our results demonstrate that the local coordination environment in SACs has a significant impact on the catalytic activity of CO2RR.

Here, we studied a simple, scalable, and in situ hydrothermal method to prepare an Fe–Mn-doped ZnO nanowire–nanosheet on a three-dimensional (3D) Ni-foam substrate for electrocatalytic overall water splitting. Attractively, the doping of Fe and Mn in ZnO plays a significant role in mobilizing the electron from Fe and Mn toward ZnO in the Fe–Mn-doped ZnO nanowire–nanosheet due to different vacuum levels of Fe, Mn, and ZnO, facilitating the development of more active sites on the surface of the catalyst, which plays a crucial role in improving the catalytic performances during overall water splitting. Consequently, the Fe–Mn-doped ZnO nanowire–nanosheet shows a lowermost overpotential of 230 mV and a lowermost Tafel slope of 115.2 mV dec–1 during the hydrogen evolution reaction (HER) and 248 mV overpotential and a short Tafel slope of 109.1 mV dec–1 during the oxygen evolution reaction (OER) in a 1.0 M KOH electrolyte. Besides, the Fe–Mn-doped ZnO nanowire–nanosheet depicts low charge transfer and series resistances of 3.7 and 0.41 Ω during the HER and 0.36 and 1.66 Ω during the OER, respectively. Also, it elucidates outstanding durability at −10 mA cm–2 for 12 h (HER) and 10 mA cm–2 for 12 h (OER) using chronopotentiometry and 1000 cycles. In addition, the Fe–Mn–ZnO||Fe–Mn–ZnO nanowire–nanosheet cell shows a lower potential of 1.74 V and outstanding stability over 24 h to deliver 10 mA cm–2 in electrocatalytic overall water splitting. Besides, the staircase stability of the Fe–Mn–ZnO||Fe–Mn–ZnO nanowire–nanosheet cell also suggests outstanding stability over 8.2 h at different current densities. Captivatingly, the concept of energy band modulation in the bimetallic doped Fe–Mn–ZnO nanowire–nanosheet catalyst is envisaged to explore insights into the mechanisms of the evolution of hydrogen and oxygen.

Electron distribution regulating of nonmetal doped monolayer g-GaN for enhanced electrocatalytic CO2 reduction