**5. Summary and outlook**

**4.2. CO2 reduction reaction**

Royal Society of Chemistry.

Despite the tremendous efforts being made to implement renewable energy sources, there remains a need in the longer term to be able to sustainably generate liquid fuels for applications including aviation and mining [9, 11]. Electrochemical CO2 reduction, recycling CO2 back to fuels, and commodity chemicals utilizing renewable energy as a power source could poten‐ tially provide a solution to this problem [99]. However, CO2 is very stable under environmental conditions and HER often prevails over CO2 reduction in aqueous electrolytes under cathodic polarization [100, 101], making it essential to find a suitable catalyst to achieve cost‐effective CO2 reduction with high efficiency and selectivity. Metals and especially nanostructured metals derived from metal oxide have been widely studied as electrocatalysts for CO2 reduction [100, 102–106]; however, these systems generally show low activities and/or selectivity for a solo product (such as CO, formate, methanol, methane, ethylene, and ethanol)

**Figure 4.** TMDs as electrocatalysts for electrochemical reduction of CO2. (a) Binding energies Eb(COOH) vs. Eb(CO) for transition metals and Mo and S edges of MoS2. Reproduced with permission from Shi et al. [108]; copyright 2014 Royal Society of Chemistry, (b) raw greyscale HAADF and false‐colour low‐angle annular dark‐field (LAADF) image (inset) of MoS2 edges (scale bar, 5 nm), (c) cyclic voltammetric (CV) curves for bulk MoS2, Ag nanoparticles (Ag NPs), and bulk Ag in CO2 environment. The electrolyte is a mixture of 96 mol% water and 4 mol% EMIM‐BF4, (d) CO and H2 Faradaic efficiency (FE) at different applied potentials. Reproduced with permission from Asadi et al. [109]; copyright 2014 Nature Publishing Group, (e) CVs of rGO‐PEI‐MoS2‐modified GCE in N2‐saturated (black curve) and CO2‐saturat‐ ed (red curve) 0.5 M aqueous NaHCO3 solution. Scan rate was 50 mV s-1, (f) Faradaic efficiency for CO (red bars) and H2 (blue bars) as a function of potential, (g) amount and Faradaic efficiency of H2 (circles) and CO (squares). Potentio‐ static electrolysis at -0.4 V in CO2‐saturated 0.5 M aqueous NaHCO3 solution and (h) Tafel plot of CO production parti‐ al current density vs. overpotential on rGO‐PEI‐MoS2. Reproduced with permission from Li et al. [110]; copyright 2016

Recently, Nørskov et al. demonstrated theoretically that MoS2 or MoSe2 could possibly be electrocatalysts for CO2 reduction by DFT calculation [107, 108]. Their results indicate the edge site of MoS2 or MoSe2 is active for electrochemical CO2 reduction due to the different scaling relationships of adsorption energies between key reaction intermediates (\*CO and \*COOH) on the edges of MoS2 or MoSe2 compared to transition metals (**Figure 4a**). Experimental results of MoS2 as electrocatalyst for CO2 reduction were firstly reported by Asadi et al. [109]

or need nonaqueous solvents which may limit practical application.

72 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Energy issue is one of the most urgent and critical topics in our modern society. Recently, there is increasing demand for cost‐effective, efficient, and environmental‐friendly energy conver‐ sion and storage devices to reduce the excessive reliance on nonrenewable fossil fuels. Due to the unique physicochemical properties of 2D TMDs, they have shown enormous potential for wide‐ranging and diversified fundamental and technological applications, which include intensive research on electrocatalytic energy conversion applications, especially hydrogen evolution reaction and CO2 reduction reaction. In these electrocatalytic reactions, the maximi‐ zation of active edges and the conductivity are identified as the core issues for further development of TMD‐based catalysts. A large number of synthetic strategies have been focused on maximizing the exposure of edge sites; phase structure tuning has also been as a potential tool for enhancing the electrical transport properties of TMDs.

Overall, the rich chemistry of TMDs builds an extensive platform for the study of fundamental and practical scientific phenomena in the development of real electrocatalysts for energy conversion applications. There is still much room to further improve the electrocatalytic performance of TMDs. Specifically, the fine tune of band structure and Fermi level could provide as powerful tools. Hence, a combination of theoretical, fundamental, and electroca‐ talysis‐based applications should be explored in order to make a guidance to the developing directions. Furthermore, the mass‐productive synthesis of high‐quality TMDs should emerge as an urgent issue to adapt to the widely application of them in an industry level.
