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Green & Sustainable Science & Technology
Electrocatalytic CO2 reduction to valued products is a promising way to mitigate the greenhouse effect, as this reaction makes use of the excess CO2 in the atmosphere and at the same time forms valued fuels to partially fulfill the energy demand for human beings. Methane, which is among the products of CO2RR, is regarded as a high-value commodity due to its high energy density of 55.5 MJ/kg. Moreover, the methane produced by the electrochemical reduction of CO2 is not emitted into the atmosphere and contributes to the greenhouse effect as the well-established infrastructure for gas pipelines, allowing for the seamless storage, consumption, and distribution of methane, rendering it a widely utilized component of natural gas.
- carbon dioxide reduction
- methane
- electrocatalysts
1. Introduction
Anthropogenic activities, particularly the extensive utilization of fossil fuels, have caused a significant increase in atmospheric CO2 concentrations, exacerbating the ongoing climate change crisis. Increased levels of CO2 lead to a heightened greenhouse effect, resulting in several adverse impacts on the global climate, including an increase in temperatures and sea level, altered precipitation patterns, and an increased frequency and intensity of natural disasters. These phenomena, in turn, have far-reaching ecological, social, and economic consequences, such as habitat destruction, species extinction, displacement of populations, and loss of biodiversity. Therefore, it is imperative to adopt sustainable energy sources and practices that reduce our dependence on fossil fuels to mitigate the detrimental effects of climate change and ensure a sustainable future for the planet and its inhabitants [1,2,3,4,5,6,7]. Thus, the reduction of CO2 to carbon-containing fuels is a promising technology for reducing CO2 emissions and achieving a sustainable future. This approach allows the conversion of intermittent renewable energy into high-energy fuels, providing a pathway to reduce our reliance on fossil fuels. Additionally, integrating CO2 into the global energy cycle through hydrocarbon synthesis allows us to achieve true global carbon neutrality [8,9,10,11,12,13]. At present, the main technologies aimed at reducing CO2 emissions include photo-, electro-, bio-, thermal, and their synergistic catalyses [14,15,16,17,18,19]. Each of these methods has its own set of advantages and limitations. For instance, photocatalysis is easy to perform and has a broad range of applications; however, it suffers from poor catalyst stability and lifespan [20]. Biocatalysis involves using biological enzymes to catalyze CO2 in mild-reaction conditions with good selectivity; however, yields are often low and catalyst deactivation is a common issue [21].
Methane, which is among the products of CO2RR, is regarded as a high-value commodity due to its high energy density of 55.5 MJ/kg. Moreover, the methane produced by the electrochemical reduction of CO2 is not emitted into the atmosphere and contributes to the greenhouse effect as the well-established infrastructure for gas pipelines, allowing for the seamless storage, consumption, and distribution of methane, rendering it a widely utilized component of natural gas. With a composition of 21.4% of total primary energy, methane boasts a high abundance and is an attractive candidate for various energy applications [30,31,32,33,34]. Concomitantly, contemporary technology offers the potential to convert CH4 into fundamental chemicals through various routes, such as the oxidative conversion into syngas or direct conversion into other chemical compounds [35,36,37,38]. Even after the inevitable transition to thermonuclear energy in the distant future, methane remains the most portable, easily stored, and transported fuel and general-purpose chemical raw material [39]. Most importantly, the next generation of rocket fuel will be liquid methane; the in situ production of methane as rocket fuel on alien planets, such as Mars, will become a key technology for human interstellar navigation. Therefore, the development of CO2RR technology to prepare high amounts of CH4 is necessary for this application.
2. Reaction Mechanism
Many research efforts have attempted to discover the mechanism of the electrochemical reduction of CO2 to CH4 from both experimental [46,47,48,49] and computational points of view [49,50,51,52,53,54]. From the pure-thermodynamics point of view, it is possible to reduce carbon dioxide to CH4 at a potential of + 0.17 V vs. reversible hydrogen electrode (RHE) [55] (Table 1). However, numerous studies have shown that CO2 electrochemical reduction to CH4 consists of multiple elementary steps [47,52]. CO2 is firstly absorbed on the catalyst and hydrogenated into *COOH via an electron transfer–proton coupling process [51]. Then, the *COOH is further evolved into *CO, which is the main branch point to determine whether or not to produce oxygen-containing products. In path I, *CO goes through a CHO* intermediate, with the overall path proceeding as:
CO2* → COOH* → CO* → CHO* → CH2O* → CH3O* → CH4 + O* or CH3OH*
(the H+ + e− reactants and H2O product formed were left off).
Table 1. Half reactions and potentials of CO2 electrochemical-reduction reactions.
| Half-Reactions Formula | Electrode Potential/V (vs. RHE) |
|---|---|
| CO2 + H2O + 2e− → CO + 2OH− | −0.10 |
| CO2 + 2H2O + 2e− → HCOOH + 2OH− | −0.20 (pH < 4); −0.20 + 0.059 (pH > 4) |
| CO2 + 3H2O + 4e− → HCHO + 4OH− | −0.07 |
| CO2 + 5H2O + 6e− → CH3OH + 6OH− | 0.02 |
| CO2 + 6H2O + 8e− → CH4 + 8OH− | 0.17 |
| 2CO2 + 8H2O + 12e− → C2H4 + 12OH− | 0.08 |
| 2CO2 + 9H2O + 12e− → CH3CH2OH + 12OH− | 0.09 |
The step to determine the selectivity is the final CH3O* reduction step. It was found that the production of CH4 had a more favorable reaction-free energy. In path II, *CO goes through a COH* intermediate with the overall path proceeding as:
CO* → COH* → C* → CH* → CH2* → CH3* → CH4*
(the H+ + e− reactants and H2O products formed were left off).
Notably, from Peterson et al.’s work, it is known that the absorption energy of the intermediate is crucial for product distributions [56]. For example, the metals (Au, Ag, and Zn) with weak *CO-bound energy produce little methane because CO experiences priority desorption before any further reductions occur [57,58,59]. Additionally, metals (Pt, Pd, and Ni) with strong *CO-bound energy cannot remove the *CO from the surface because of the highly unfavorable thermodynamic conditions. Thus, CO2 can only be reduced further to CH4 with an exceedingly low Faraday efficiency (FE) on these electrodes [60]. In contrast, the metal Cu is located near the top of the volcano curve of the limit potential. This means that the *CO-adsorption intensity of Cu is suitable for CH4 production via the CO2RR process.
Notably, some details of the reactions may be slightly different from what was mentioned above over different CO2RR catalysts. Dong et al. [61] reported that *CO protonated through a similar bridge configuration on the Cu2O/Cu interface. This conclusion was confirmed by the density functional theory (DFT) calculation. Interestingly, they also found that the Cu2O/Cu interface formed during the electrochemical reaction process played a crucial role in determining the selectivity of methane formation, which may indicate that the crystal plane is not the key factor for the CO2RR to CH4 formation process on reconstructed Cu2O microparticles.
As for the surface-reaction mechanism, two hypotheses were proposed, which are the Eley–Rideal (H comes from the solution) and Langmuir–Hinshelwood (H comes from the surface-adsorbed hydrogen (*H)) mechanisms, respectively. Yogesh and coworkers [62] studied the mechanism of electrochemical CO2 reduced to CH4 on the surface of Cu. They found that the methane production rate was significantly suppressed when increasing the pressure of CO. However, for the Eley–Rideal mechanism, the reaction rate should be positively correlated with the pressure of CO, which was inconsistent with the experimental phenomena. The experimental result thus excludes the Eley–Rideal mechanism and strongly supports the Langmuir–Hinshelwood mechanism, where COads and Hads are in competition with each other for surface sites. The result was also confirmed by Asthagiri and coworkers’ works with the DFT calculation [51].
This entry is adapted from the peer-reviewed paper 10.3390/methane2020012
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