**1. Introduction**

Nowadays, global warming and CO2 emissions as well as atmospheric CO2 concentration are central topics in politics and scientific debate. The global energy supply based on fossil fuels has reached an unprecedented scale leading to excess anthropogenic CO2 emission. CO2 accumulates in the atmosphere and its concentration has surpassed 409 ppm in 2019 much higher than the 270 ppm during the preindustrial era [1]. As a well-known greenhouse gas, accumulated CO2 traps more infrared radiation, breaking the energy balance on the earth's surface. Using CO2 as feedstock to produce valuable carbon-based chemicals is considered to be a feasible approach to close the carbon cycle and mitigate the climate change. Many strategies have been developed for CO2 valorisation, including thermochemical, photochemical, electrochemical and biological approaches [2–5]. Among these methods, electrochemical conversion presents several advantages. Firstly, this method can use green chemicals as electrolytes and electricity from renewable energy sources, thus not contributing to new CO2 emissions while transforming it [6]. Secondly, the products and conversion rates can be tuned by utilizing different catalysts and applying various potentials [7, 8]. Finally, the electrolyzer and electrolysis process for CO2 conversion can be developed based on the already existing technologies

such as water electrolyzers, polymer electrolyte membrane fuel cells, solid oxide fuel cells and so on [9]. However, the CO2 reduction reaction (CO2RR) involves several proton-assisted multiple-electron-transfer processes with similar standard potentials (V *vs* the reversible hydrogen electrode (RHE), Reactions (1)-(6) [10], leading to the formation of carbon monoxide (CO), formic acid (HCOOH), methane (CH4), ethylene (C2H4), methanol (CH3OH) and ethanol (C2H5OH). Moreover, hydrogen (H2) evolution is the competing reaction in aqueous solution (Reaction (7)). Therefore, it is a challenge to control the selectivity of the CO2RR from the thermodynamic view.

$$\text{CO}\_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{CO} + \text{H}\_2\text{O} \text{E} = -0.105\text{ V} \tag{1}$$

$$\text{HCO}\_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{HCOOH} \\ \text{E} = -0.169 \text{ V} \tag{2}$$

$$\text{C}\text{O}\_2 + \text{8H}^+ + \text{8e}^- \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O} \text{E} = +0.169\,\text{V} \tag{3}$$

$$\text{2CO}\_2 + \text{12H}^\* + \text{12e}^- \rightarrow \text{C}\_2\text{H}\_4 + 4\text{H}\_2\text{O} \text{E} = +0.079\,\text{V} \tag{4}$$

$$\text{C}\text{O}\_2 + \text{6H}^+ + \text{6e}^- \rightarrow \text{CH}\_3\text{OH} + \text{H}\_2\text{O} \text{E} = + \text{0.017 V} \tag{5}$$

$$\text{2CO}\_2 + \text{12H}^+ + \text{12e}^- \rightarrow \text{C}\_2\text{H}\_3\text{OH} + \text{3H}\_2\text{O} \text{E} = + \text{0.08 V} \tag{6}$$

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_2\text{ E} = 0.000\text{ V} \tag{7}$$

From the kinetic point of view, it is even more challenging to form chemical bonds for the complex and energetic molecule products [11]. Transferring one electron to the adsorbed CO2 molecule to activate it (generating the radical CO2\* − ) is believed to be the rate-determining step of the CO2RR on transition metalbased catalysts because of the high activation barrier needed for this step [12]. Consequently, much more negative potentials than the standard ones are needed to drive the CO2RR. Therefore, an appropriately designed catalyst is essential in order to activate the CO2 molecules. Once CO2\* − forms on the catalyst's surface, its reactivity in this state controls the distribution of final products. Both early and later studies [13–15] of electrochemical CO2RR on various metal-based electrodes found that the radical CO2\* − interacts with the surface of the catalyst in different ways, depending on the intrinsic electronic surface's properties of the material. Hence, a suitable catalyst is necessary in order to selectively drive the CO2RR and to obtain a specific product. In the present chapter, numerous electrocatalysts are classified based on the CO2RR product, involving the reaction pathways and mechanism study.

#### **2. CO-selective catalysts**

CO is an important product from the reduction of CO2 since it has high relevance for the chemical industry [16]. It is considered the most important C1-building block and is intensively used in large industrial processes such as Fischer-Tropsch synthesis of hydrocarbons and Monsanto/Cativa acetic acid synthesis. By a techno-economic analysis that takes into consideration the costs of CO2, electricity, separation, capital and maintenance, operation and the known product selectivity

#### *Heterogeneous Electrocatalysts for CO2 Reduction to Value Added Products DOI: http://dx.doi.org/10.5772/intechopen.97274*

and outputs the levelized cost of the chemical produced, CO is one of the most economically viable and atom-economic targets [17].

In recent years, great efforts have been dedicated to the study of electrocatalysts for the electrochemical CO2RR to CO. **Table 1** summarizes the most widely investigated types.


#### **Table 1.**

*CO-selective catalysts for the CO2RR.*

### **2.1 Metals and bimetallic materials**

From both experimental and theoretical studies, Au, Ag and Zn are the most selective metals for CO formation. The CO2RR on Au and Ag is characterized by low overpotentials, excellent selectivity and high activity [18–20, 30, 44]. On contrast, Zn shows relatively higher overpotentials, lower activity and moderate-to-high selectivity [8, 21].

Many bimetallic materials are demonstrated to selectively catalyze the CO2RR to CO, including Cu-Sn [22–24], Cu-Zn [25], Cu-Sb [26], Cu-Ag [27], Cu-Au [28] and so on. Among all these materials, Cu-Sn catalysts have attracted the most intensive attention due to the high selectivity, good activity and outstanding repeatability. In addition, compared with others, Cu and Sn are relatively more abundant and more cost-effective, making Cu-Sn catalysts more suitable for the large-scale implementation. Hence, further study on the Cu-Sn catalysts is expected to bring benefits to both the academic and industrial sectors related to the CO2 valorization.

#### **2.2 Single metal atom supported on N-doped carbon**

Single-metal-atom catalysts supported on porous N-doped carbon represent a class of catalysts with high atom efficiency. After introduced in 2015 by Varela et al. [30], it has gained much attention for CO2 reduction. Ni supported on N-C, in contrast to Ni nanoparticles that are known to be effective in the HER, is reported to be an efficient electrocatalyst for the CO2RR to CO [29, 31, 32]. Various types of Fe-N active sites have been identified and demonstrated to selectively promote the CO formation at very low overpotentials [29, 33–35]. Compared to the metallic Zn and ZnO, single atom Zn sites show much lower overpotentials where excellent CO selectivity has obtained [36, 37]. Sb atomic sites, compared to bulk Sb, Sb2O3, and Sb nanoparticles that exhibit poor activity and selectivity for the CO2RR, enable the CO formation with good selectivity at relatively high overpotentials [38]. Isolated diatomic Ni-Fe sites anchored on nitrogenated carbon are also studied as an electrocatalyst for CO2 reduction [39]. The catalyst exhibits high selectivity with CO Faradaic efficiency above 90% over a wide potential range from −0.5 to −0.9 V (98% at −0.7 V, vs. RHE), and robust durability.

Single atoms of selected transition metals anchored in N-doped carbon have emerged as unique and promising electrocatalysts because of the maximal atom utilization and high efficiency. Most of them perform differently from their bulk metal or oxide species, due to the metal–matrix interfacial interaction that leads to the manipulation of the electronic structures of the materials and to the emergence of additional active sites. Despite the big progress made in the recent years, many challenges remain in the development of the single atom catalysts. For example, the loading of metals is usually low, leading to relatively low geometric current density and thus limitations for practical applications. In addition, big efforts have to focus on **both** the synthetic front and structural characterizations and these necessitate the development of effective computational methods and characterization tools.

#### **2.3 Immobilized molecular catalysts**

Homogeneous electrocatalysis constitutes an efficient way of converting CO2 to various products but some distinct challenges persist [44]. For example, the catalyst stability and recyclability are usually poor; only a small portion of the catalyst molecules at the reaction interface is active, while most of them are passive; some catalysts have poor solubility; product separation could be difficult. To overcome these disadvantages, great efforts have been dedicated to the immobilization of

*Heterogeneous Electrocatalysts for CO2 Reduction to Value Added Products DOI: http://dx.doi.org/10.5772/intechopen.97274*

**Figure 1.** *Schematic of possible pathways for CO production.*

molecular catalysts on electrode surfaces for the heterogeneous CO2RR. Being fixed on carbon supports, the porphyrin- and phthalocyanine-based catalysts with Fe and Co centers are very selective for CO formation at relatively low overpotentials [41–43, 45]. The catalytic performance can be affected by both the intrinsic properties of the catalysts such as the structure and the metal center, and the extrinsic factors such as the catalyst immobilization methods, the support material and the catalyst loading. A deeper understanding of those intrinsic and extrinsic factors can enable the optimization of supported molecular catalysts in order to achieve the CO2RR performance as high as that of the nanostructured metals, metal alloys and single atom catalysts supported on N-carbon materials [16].

The mechanism study of CO2RR on metal-based materials is widely studied, in combination of in-situ spectroscopic analyses and DFT calculations [45, 46]. As shown in **Figure 1**, it is suggested that the CO2RR to CO process on metallic Zn or Ag surface includes four elementary reaction steps: (1) one electron transfers to CO2 to form CO2 \*−; (2) one proton transfers to CO2 \*− to obtain COOH\* intermediate; (3) an electron and a proton transfer to COOH\* to form CO\* ; (4) CO\* desorbs to produce CO. Another possible pathway is supposed to include three main steps: (1) an electron coupled with a proton transfers to CO2 to form COOH\* intermediate; (2) another electron coupled with a proton transfers to COOH\* to form CO\* ; (3) CO\* desorbs to produce CO.

## **3. Formate-selective catalysts**

Due to the large storage and safety requirements for CO during carbon sequestration and storage (CCS), the production of liquid formic acid is becoming a more attractive solution. Formic acid could be directly used as a feedstock for fuel cells and as a precursor for manufacturing value-added chemicals such as formate esters, methanol, and other carboxylic acids and derivatives [47]. Some heavy metals, including Pb, Hg, In, Cd, and Tl, are efficient electrocatalysts for converting CO2 into formate/formic acid. However, the defects of the high toxicity and/ or high cost are standing in the way for their large-scale applications [48]. Other earth abundant metals like Sn, Cu and Bi gained a lot of attentions in recent years. **Table 2** has summarized some of the important results for formic acid production through electrocatalysis of CO2.

#### **3.1 Metal and metal oxides**

From the pioneer work of *Hori* eta al. [57], the metals Pb, Hg, In, Sn, Cd and Tl are selective for HCOOH formation. Among them, Sn and SnOx catalysts have become the most interesting one due to the high selectivity and their non-noble, eco-friendly

*Electrocatalysis and Electrocatalysts for a Cleaner Environment - Fundamentals...*


#### **Table 2.**

*Formate-selective catalysts for the CO2RR.*

and low-cost characteristics [56]. Recently, Pd demonstrates to be an appealing catalyst for HCOOH formation, showing high activity and good stability at extremely low overpotentials [19, 49]. Bismuth (Bi), as an HER inert metal [58], is also widely studied for the CO2RR in recent years. The Bi-based catalysts are demonstrated to be a selective and active for the HCOOH production [48]. Due to the low cost and low toxicity, Bi becomes as important as Sn and is to be used in large-scale CO2RR to HCOOH [59].

#### **3.2 Metal sulfides**

In very recent years, sulfur-modified metals have been explored as electrocatalysts, showing promising catalytic performance for the CO2RR. CuxS is one of the most intensively studied sulfides, which can selectively produce HCOOH [50]. SnSx [51], PbSx [56], BiSx [48] and InSx [55] are also demonstrated to be effective catalysts for the CO2RR to HCOOH. Even though the promising performance, the role of S in the electrochemical performance is not clear until now. In order to design catalysts with higher activity, selectivity and stability, it is necessary to acquire a deeper understanding of how S functions during CO2RR by performing both insitu/operando experiments and theoretical studies.

#### **3.3 Bimetallic catalysts**

Compared with the pure metals, bimetallic catalysts with tuned electronic and structural properties are of particular interest. Early studies by *Hori* et al. [57] have shown that the modification of metallic surface with foreign atoms can tune the selectivity for CO or HCOO<sup>−</sup> production. Sn-based bimetallic materials are the most studied type, probably due to the high performance of Sn alone for the CO2RR to HCOOH. It is worth to note that most of the Sn bimetallic materials show good HCOOH selectivity at very positive potentials, with much lower overpotentials with respect to those at the Sn/SnOx electrodes [56]. Particularly, a Cu-Au catalyst shows good selectivity and activity for the HCOOH production at even more positive potentials [54]. Until now, many studies suggested that the combination of different types of metals provide the opportunity to modulate the surface chemical

*Heterogeneous Electrocatalysts for CO2 Reduction to Value Added Products DOI: http://dx.doi.org/10.5772/intechopen.97274*

**Figure 2.** *Schematic of possible pathways for formic acid production.*

environment and the relative binding with different intermediates, tuning the electrochemical performance of the multi-metallic catalysts in the CO2RR.

In recent years, many works have been dedicated to understand the mechanism of the CO2RR to HCOOH, including computational, electrokinetic and in situ analysis) [60–62]. As depicted in **Figure 2**, the formation of formate generally goes through the following pathway: 1) CO2· − radical anion is firstly formed via a oneelectron transfer and bonded to the electrode surface through O atom, 2) protonation of CO2· − on the carbon atom leads to the formation of a HCOO· intermediate and 3) a second electron transfer and protonation step results in the HCOOH product [63].
