###### Abstract  
Electrocatalysis has become an important topic in various areas of research, including chemical catalysis, environmental research, and chemical engineering. There have been a multitude of different catalysts used in the electrocatalytic reduction of CO\({}_{2}\), which include large classes of materials such as transition metal oxide nanoparticles (TMO), transition metal nanoparticles (TMNp), carbon-based nanomaterials, and transition metal sulfides (TMS), as well as porphyrins and phthalocyanine molecules. This review is focused on the CO\({}_{2}\) reduction reaction (CO\({}_{2}\)RR) and the main products produced using TMS nanomaterials. The main reaction products of the CO\({}_{2}\)RR include carbon monoxide (CO), formate/formic acid (HCOO\({}^{-}\)/HCOOH), methanol (CH\({}_{3}\)OH), ethanol (CH\({}_{3}\)CH\({}_{2}\)OH), methane (CH\({}_{4}\)), and ethene (C\({}_{2}\)H\({}_{4}\)). The products of the CO\({}_{2}\)RR have been linked to the type of transition metal-sulfide catalyst used in the reaction. The TMS has been shown to control the intermediate products and thus the reaction pathway. Both experimental and computational methods have been utilized to determine the CO\({}_{2}\) binding and chemically reduced intermediates, which drive the reaction pathways for the CO\({}_{2}\)RR and are discussed in this review. electrocatalysis; electrocatalytic CO\({}_{2}\) reduction reaction; transition metal sulfide; nanomaterials; composite nanomaterials +
Footnote †: journal: Applied Sciences  
reaction (CO\({}_{2}\)RR), methanol oxidation reaction (MOR), and nitrate reduction/ammonia production reactions [20; 21; 22; 23; 24; 25]. These reactions are essential for fuel production, electricity generation in fuel cells, as well as environmental remediation. These reactions are similar to those observed in photocatalysis, where reactive oxygen species are generated at a catalytic surface which in turn oxidize the organic pollutant. In aqueous-based systems, the HER and OER are extremely important for environmental remediation. The reaction in electrocatalyils can be initially based on an adsorption mechanism [26; 27]. The adsorption is controlled by the attraction forces generated typically by an electrostatic interaction. The adsorption mechanism is followed by either oxidation or a reduction reaction (electron transfer reactions) followed by the desorption of the product molecule from the surface [28; 29; 30]. These steps are interdependent upon each other and sometimes the release of the bound molecule from the surface is slow and can inhibit the reaction progression. Furthermore, electrocatalytic reactions occur at the interface between the charged electrode surface and the electrolyte supplying a counter charge in solution. The surface charge is dependent on the electrode material, the potential applied to the electrode, the electrolyte, and the entire cell [31]. The electrocatalytic reduction mechanism of CO\({}_{2}\) involves multiple electron transfer steps as well as adsorption processes [32]. There has been much interest in the conversion of CO\({}_{2}\) into value-added products, which include formaldehyde, formic acid, and methanol, among other products [33; 34; 35]. Research has led to the production of ethanol, ethylene, methane, and syngas [34]. These types of products have been achieved using different catalysts; for example, InO\({}_{x}\), \(\beta\)-Bi\({}_{2}\)O\({}_{3}\), Cu nanoparticles (NPs), Ag/Cu (NPs), Cu-Ce(OH)\({}_{x}\), Cu/nanodiamonds, CuO, Pd/B/C, Pd nanosheets, Pd/Mg nanospheres, Pd/Ag alloys, and AuPd-MOFs [35; 36; 37]. Other examples of nanomaterials used in CO\({}_{2}\) reduction include monometallic nanoparticles, Au, Ag, Zn, Fe, Ni, Pt, Sn, In, and PD [48; 49; 50; 51; 52; 53; 54; 55; 56].