Review ArticleElectrocatalytic CO2 reduction on nanostructured metal-based materials: Challenges and constraints for a sustainable pathway to decarbonization
Introduction
The concentration of carbon dioxide in the atmosphere is increasing very fast, due to anthropogenic activities, such as combustion of fossil fuels from industrial processes and transportation. CO2 greatly contributes to greenhouse gas emissions, being the main driver of global warming. To promote the sustainable development of the planet, the Intergovernmental Panel on Climate Change recently recommended to limit global warming to 1.5 degrees Celsius, rather than the previous threshold of 2, requiring governments to take actions to limit carbon emission [1,2]. As a key objective of the European Green Deal and in line with the EU’s commitment to global climate action under the Paris Agreement, the EU aims to be climate-neutral by 2050, adopting an economy with net-zero greenhouse gas emissions [3]. The carbon capture and storage (CCS) is considered as a viable strategy to decrease CO2 emissions. Despite the extensive global efforts in the past two decades, large-scale development of CCS is slow, due to long-term-storage issues related to gaseous CO2 leakage [4]. The carbon dioxide reduction reaction (CO2RR) that transforms CO2 into different value-added carbon-based compounds is an alternative strategy to CCS, not only allowing the mitigation of CO2 emission but also producing useful chemicals, such as CH3OH, CH4, CO, and HCOOH which can be used as fuels [[5], [6], [7], [8], [9], [10]].
CO2RR can be achieved by using different approaches, such as biochemical, thermochemical, photochemical, and electrochemical methods, achieving high energy efficiency, high reaction rates, and high value products [[11], [12], [13], [14]]. The efforts of the academic community on improving productivity, stability, and environmental friendliness of processes while reducing costs have gradually increased since mid-2010, leading to an exponential increase of the number of publications dealing with CO2RR, as indicated in Fig. 1.
Among these methods, electrochemical CO2 reduction reaction (E−CO2RR) technique provides several advantages, including controllable reaction steps, relatively mild conditions, and good conversion efficiency. In addition, the drive power of E−CO2RR can be efficiently and sustainably harvested from renewable energy sources, such as wind, solar, and hydroelectric energy. This could potentially close the carbon loop and simultaneously address the issues of global warming and energy crisis. Exhaustive reviews addressing E−CO2RR process from a both mechanistic/theoretical perspective and technology development have been published in 2020–2021 [[15], [16], [17], [18], [19], [20], [21]].
Although the faradaic efficiency can exceed 90 %, a poor long-term stability is a bottleneck of this technology, which can be addressed by fabricating highly efficient catalysts with high stability for industrial application [22,23]. Nanostructured metal-based electrocatalysts with tuned morphology, controlled composition, and tailored active sites have achieved satisfactory catalytic activity for CO2 reduction; however, stability issues are still challenging and the real active sites and key factors governing the catalytic performance and the obtained reduction products need to be fully understood.
In this review article we summarize the recent advances in designing various nanostructured metal-based heterogeneous electrocatalysts for E−CO2RR, discussing the reaction mechanism and structure–performance relationship in detail. Challenges and constraints toward controlled synthesis of advanced electrocatalysts are proposed for promoting the development of E−CO2RR as a sustainable path for decarbonization of energy system.
Section snippets
Electrochemical Carbon dioxide reduction reaction (E−CO2RR): fundamental reaction pathways
The electrochemical reduction of carbon dioxide is a powerful strategy for reducing CO2 levels in atmosphere and obtaining fuels using renewable energy [24]. The most common products derived from carbon dioxide reduction in aqueous media are carbon monoxide and formic acid [25], while multi-carbon hydrocarbons and oxygenates (like ethylene, isobutane, ethanol, methanol, acetate, and n-propanol) are more desirable due their higher energy density and wider applicability. However, the commercial
Metal bulk catalysts
Pioneering studies related to CO2 electroreduction report the reaction product distribution achieved by electrocatalysts based on metals such as Hg, Pb, Zn, Cd, Sn, In [42,43], Au, Ag, Cu, Ni and Fe in hydrogencarbonate solution [[43], [44], [45]]. The metals can be divided into five groups, based on their selectivity toward a specific product: i) Hg, Pb, Zn, Cd, Sn, and In predominantly lead to formate (HCO2−), ii) Zn leads to formate and CO, iii) Au and Ag yield CO, iv) Cu mostly yields a
Structure-activity relationship of M-N-C heterogeneous catalysts
Recent works provided experimental and theoretical elucidation on the effect of chemical surface, structure, and morphology of M-N-C heterogeneous catalysts on their activity and selectivity towards CO2RR. One of the key features of M-N-C materials which improves selectivity toward CO production as compared to metal bulk catalysts is their nanostructure. As reported by Bagger et al. [135], HER is disadvantaged in this type of material since the atomicaly dispersed active sites are distant,
Conclusions and outlook
The existing challenges for a sustainable pathway to decarbonization has caused a continuing interest in catalyst development for electrochemical reduction of carbon dioxide (E−CO2RR). Among the metal bulk catalysts, Cu has been found the most active metal in CO2 conversion: CO2 can be efficiently reduced to CO or HCOOH, while the formation of C2+ products remains challenging, as highlighted by very recent reviews [[165], [166], [167]]. Rds for CO2 reduction to C2 products is the formation of a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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