Electrocatalytic conversion of CO2 to hydrocarbon and alcohol products: Realities and prospects of Cu-based materials
Graphical abstract
Introduction
Humanity consumes non-renewable energy derived from fossil fuels, biomass, wind, hydropower and solar. Most developed countries such as India, China, US, etc. suffer from high increase energy and electrical power demands such that new power plants needed to be constructed to meet such intensive demands [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Reducing the use of fossil fuels that release the carbon (IV) oxides (CO2) to the atmosphere is crucial to have a sustainable environment, because CO2 is extremely notorious greenhouse gas (GHG). The problems associated with the emission of CO2 have raised serious concerns over the past two decades with lots of associated investments therein. In order to realize eco-friendly, stable environmental and transitions that are favorable towards societal sustainability, the CO2 gas produced from the earth's crust is expected to be the same as the amount of CO2 gas consumed to maintain the unchanged or balanced CO2 concentrations in the atmosphere. Therefore, reducing the emission of CO2 and its further regeneration to chemicals and fuels is paramount and expected to be excellent techniques to alleviate the global demands for fossil energy that has a high affinity for pollution and threatens global peace. This could be achieved through mimicking the green plants' photosynthetic process, thereby providing essential assets for major applications in the industries [6,[10], [11], [12], [13]].
Numerous methods have been employed for converting CO2 to various chemicals e.g. biological and chemical transformations [6,14,15], photocatalytic [[16], [17], [18], [19], [20], [21]] and electrocatalytic reductions [1,[5], [6],12,[22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]], hydrogenation and reforming methods [1,27,28,29,37,[39], [40], [41], [42], [43], [44]]. Converting CO2 to various hydrocarbons has been documented as a notable and valuable approach than geological sequestration due to its highly efficient utilization and carbon source recycling abilities. Though efforts made to achieve this through CO2 absorptions, conversion, and activation processes have resulted in certain shortcomings, including high energy demands involve in transferring molecules of CO2 to the active sites, and also lower conversion rates for obtaining the higher value of hydro-chemicals. Among all methods available, electrochemical reduction of CO2 has gained much attention and realized as the unique method owing to several advantages [1,5,6,512,[22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38],[45], [46], [47], [48], [49], [50], [51]], such as practical applications of the system, ability to operate the CO2 electroreduction process under mild, moderate and controllable conditions. In addition, electrochemical reduction products could be adjusted by tuning the reaction/operating parameters (i.e. temperature, redox potential, electrolyte), electrocatalysts can be optimized, minimization of CO2 electroreduction by-products at lower contents can be achieved, and also possibility of attaining electrical power (as a drive force) with some additional renewable energy sources (including solar, wind) without generating more CO2.
CO2 is an inert and very stable molecule since its carbon atom has the highest oxidation state. This, therefore, necessitates the development of electrocatalysts which are efficient to promote the sluggish kinetic CO2 reduction processes. The electrochemical routes for reducing CO2 involve multiple electrons transfer in an aqueous media using suitable electrocatalysts [52,53]. Various products of CO2 electroreduction via different pathways include formic acid, carbon monoxide, methanol, methane, etc. as illustrated in Fig. 1 [54]. Considering the thermodynamic studies for CO2 reduction [55,56], it is highly possible to form a mixture of liquid products (such as formic acid and methanol) and gaseous products (e.g. carbon monoxide and methane) than the single product in an electrochemical cell. Table 1 presents a variety of half-reactions with the electrode potential versus standard hydrogen electrodes (SHE) [52,53].
The electrocatalysts used and electrode potential applied are fundamental factors in determining the efficiency and selectivity of CO2 electroreduction. The associated challenges here are the capital cost, lower efficiency and/or product selectivity, inadequate stability, degradation of electrocatalytic activities [[57], [58], [59], [60], [61], [62], [63], [64], [65]] which could be categorized into mechanical, thermal and chemical/electrochemical [66]. These challenges experienced by the existing electrocatalysts have made the electrocatalytic conversion technique to be unable to meet the large-scale industrial application requirements. Over the past years, as the clean energy demands increases globally, so also the electroreduction of CO2 progressively increases such that numerous attentions have been made to enhance the electrocatalysts' performance and reaction conditions to overcome the limitations associated with CO2 reduction [5,29,32,[34], [35], [36], [37],[45], [46], [47], [48], [49],[51], [52], [67], [68]].
Considering, all these shortcoming and inherent problems; associated with the other metal electrodes and the superior ability of Cu to produce alcohols, aldehydes, and hydrocarbons which bypass other metal electrodes. These have attracted the attention of numerous researchers to gain better insight and understanding of the Cu reactivity, stability, tuneability to obtain better selectivity and efficiency. Thus leading to numerous studies on the different classes of Cu-based electrocatalysts including Cu-nanocrystals, nanostructures or nanowires, Cu oxides, mixed oxide and oxide derived Cu, polycrystalline Cu, Cu thin-film, bimetallic Cu, etc. Therefore, this review is mainly focused on Cu-based electrodes. We discussed some realized advancements reported on the classes of different Cu-based electrocatalytic materials. In addition, some improvement on the pathways for CO2 reduction to C1-C3 products, new mechanistic insights on the Cu electrodes for the electroreduction of CO2 to C3 and C4 and the roles of the catalyst supports for the realization of more useful products and efficient systems. Additionally, for the furtherance of the effective and practicability of Cu-based electrocatalysts, major frequently unnoticed factors were proposed as the challenges and prospects to foster better improvements of these electrocatalyst materials for CO2 reduction.
Section snippets
Environmental impact of CO2 and recent efforts to its utilization
Since the last centuries, the universe has witnessed the unparalleled energy consumption growths, being stimulated by the increased populations and technological advances. Meeting these increasing growing demands have made the planet turned to the combustion of fossil fuels such as coal, petroleum, wool, and natural gases. Notable successes have been established through this means in term of quality of energy produced at low-cost but their uses are unavoidable and more so, they release many
Current efforts to chemically utilize CO2
Utilization of CO2 for industrial feedstocks to produce the appropriate reaction precursor. Direct application of organic material remains the potentially long-term resources that are sustainable in the use of the energy of renewable sources (from the wind, solar or geothermal) as the only energy input for various processes. Interestingly, scientists have embarked on the applicability and feasibility of CO2 as sources of producing organic chemicals of industrial relevance. Several authors have
Uniqueness of Cu over the conventional catalysts for CO2 electroreduction
As far back as the 1980s, the research in the field electroreduction has been gaining widest popularity on investigating the activities of different catalysts especially bulk metal electrocatalysts [98,99,[108], [109], [110], [111], [112], [113], [114], [115],[100], [101], [102], [103], [104], [105], [106], [107]]. So, it is no more new that a single elemental (metal) electrode such as bulk polycrystalline electrocatalyst has been utilized for CO2 electroreduction [[116], [117], [118], [119],
The chemistry of CO2 reduction: Expectation and realities of the mechanism on Cu electrodes
The combustion of the organic molecule produces CO2 as the final carbon products, and which is very linear and stable from the thermodynamic point of view. The CO2 reduction final products are usually determined by the cathodic electrocatalysts used in the electrochemical reactions. Generally, this is multistep reaction processes which involve adsorption of CO2 onto the electrocatalyst surface, transfer of protons and electrons, and desorption of CO2 from the surface of the electrocatalyst. It
Pathway for CO2 reduction to C1 products
The CO2 reduction to C1 products includes CH3OH, HCOOH, HCHO, CO and CH4. HCOOH may be formed via 3 different pathways: firstly, by CO2 insertion into metal–hydrogen bond or via direct CO2 protonation with H+ from the solution, where the intermediate formed binds with the electrocatalyst through the atom in a monodentate or bidentate manner as shown in Fig. 7A. The second pathway involves the reactions between the protons and the CO2* radicals as presented in Fig. 7B. Thirdly, the reactions
Cu- nanoparticles/nanowires
The generation of carbon multi-products from CO2 electrocatalytic conversion processes has attracted a lot of interest in Cu electrodes. Numerous hydrocarbon products are generated from Cu electrode, the selectivity of C−C coupled products in relation to H2 and CH4 have always remained barriers. The Cu electrodes are known for mixtures of hydrocarbon products typically methane, ethylene, and alcohol. The formation of C2H4 on a rough Cu-electrode was reported by Kas et al. [205]. Covering of
New mechanistic insights on the Cu electrodes
Numerous benchmark activities of Cu-based electrodes have been reported on C1 C4 products since the advent of CO2 electroduction on Cu by Hori group [156]. The products formed and respective FE include:- Hori et al., 2002: C2H4 (46%), C2H5OH (19%) and C3H7OH (2%) [157], Li and Kanan 2012: CO (45%) and HCOO− (33%) [118], Manthiram et al., 2014: CH4 (76%) [332], Rasul et al., 2015: CO (85%) [191], Kas et al., 2016: CO (73%) [333], Mistry et al., 2016: C2H4 (60%) [217], Bertheussen et al., 2016: C2
Roles of the catalyst supports
The role of electrocatalyst supports on the Cu-based materials is an important aspect to achieve a significant and better activity as well as the selectivity of the products of the CO2RR. Over the past years, numerous efforts have been devoted to the models that would further enhance the support materials, this in turn has helped to design and optimize some novel electrocatalyst systems [188]. Example of common supporting materials includes carbon black [346,347], CNT [231,348,349], inorganic
Challenges and future prospect
Globally, CO2 remains limitless resources having the potentiality of producing different useful chemicals and fuels including urea, methanol, formic acid, etc. To date, only scanty technologies are available at the doorstep of industrial recognitions. This means that extensive, deep-insight and realistic researches are seriously required to meet the technological utilization; such that CO2 is not only to be reduced on the lab scale, but also on the industrial scale. This would not only
Conclusions
Globally, seeking green and sustainable energy sources and energy carriers that satisfy economic development demands is an urgent need. Fossil fuels from energy sources: natural gas, coal, and petroleum are not sustainable due to their severe reserved depletions and serious environmental damages triggered by their usages. As revealed in this article, several parameters are needed to be simultaneously optimized to achieve the reality of efficient continuous flow of CO2 electroreduction not only
Declaration of competing interest
There are no conflicts to declare.
Acknowledgements
Doctoral scholarship award given to the K. A. Adegoke by National Research Foundation-the World Academy of Sciences (UID: 105453 & Reference: SFH160618172220) and the National Research Foundation S&F-Extended Support for Scholarships and Fellowships (Reference: MND190603441389, UID: 121108) are greatly acknowledged. Supports obtained from LAUTECH 2016 TET Fund Institution Based Research Intervention (TETFUND/DESS/UNI/OGBOMOSO/RP/VOL. IX) given to the O. S. Bello are all acknowledged.
References (375)
- et al.
Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes
J. Mol. Catal. A Chem.
(2003) - et al.
Sustainable hydrocarbon fuels by recycling CO2and H2O with renewable or nuclear energy
Renew. Sust. Energ. Rev.
(2011) - et al.
The technical and energetic challenges of separating (photo)electrochemical carbon dioxide reduction products
Joule
(2018) Conversion of carbon dioxide into methanol – a potential liquid fuel: fundamental challenges and opportunities (a review)
Renew. Sust. Energ. Rev.
(2014)- et al.
Highly selective electrochemical reduction of carbon dioxide using Cu based metal organic framework as an electrocatalyst
Electrochem. Commun.
(2012) - et al.
Cu2O-tipped ZnO nanorods with enhanced photoelectrochemical performance for CO2photoreduction
Appl. Surf. Sci.
(2018) - et al.
Influence of dilute feed and pH on electrochemical reduction of CO2 to CO on Ag in a continuous flow electrolyzer
Electrochim. Acta
(2015) - et al.
A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper
J. Electroanal. Chem.
(2006) - et al.
Electrochemical membrane reactors for the utilisation of carbon dioxide
Chem. Eng. J.
(2016) - et al.
Opportunities and prospects in the chemical recycling of carbon dioxide to fuels
Catal. Today
(2009)