Membrane-free Electrocatalysis of CO2 to C2 on CuO/CeO2 Nanocomposites

Introduction

Carbon dioxide electroreduction reaction (CO2RR) has attracted worldwide attention because of its promising application in suppressing carbon dioxide emission (Gong et al., 2019; Fan et al., 2020; Li H et al., 2021; Sun et al., 2021; Zhao et al., 2021; Li et al., 2022). However, it still needs high performance catalysts to accelerate the conversion of carbon dioxide and control the selectivity of products for the industrial application.

On different catalysts, various products may be obtained, such as carbon monoxide, formic acid, methanol, methane, ethylene, ethanol, oxalic acid, and acetic acid. Among them, the C2+ products were desired because of their much higher added value and more important theoretical significance of understanding C-C bond formation (Ma et al., 2020; Li Z et al., 2021; Zhang et al., 2021). Among all the elements, copper is considered as the only candidate as a high selective catalyst for C2+ products. In order to achieve better catalytic performance of copper-based catalysts, tremendous works have been carried out from the aspect of crystal faces (Suen et al., 2019), size (Wang et al., 2019), morphology (De Luna et al., 2018), defects (Gu et al., 2021), valence state (Mistry et al., 2016), and surface modification (Li F et al., 2020). Unfortunately, nearly all the pioneer works are executed in a two-chamber cell system, which need a cation or anion exchange membrane to separate the anode and cathode chambers. From a practical and industrial point of view, membranes make the CO2RR reactor more complex, increase cost, and limits large-scale application. Therefore, the design of a membrane-free electroreduction process is of great significance. Though with more concise and compact structure, the membrane-free cell still faces several critical challenges, such as the separation of products and the coupling with anodic reaction. For instance, the water oxidation reaction is inevitably on the anode when using aqueous electrolyte. In this condition, the oxygen produced and dissolved in the electrolyte may oxidize the active sites and lead to the decomposition of products in the liquid phase. In order to improve the oxygen tolerance of catalysts for CO2RR, several interesting works have been reported (Lu et al., 2019; He et al., 2020). For example, Sun et al. (Li P et al., 2020) developed a CO2-selective layer by introducing aniline into the pores of a microporosity polymer to permeate CO2 from O2 mixture. It was found that the acid-base interaction between CO2 and aniline enhances CO2 separation from O2. Even if the polymer coating is a feasible strategy to protect the active sites from interacting with oxygen, it is not conducive to the effective exposure of active sites. Therefore, new strategies need to be developed to achieve oxygen tolerance and full exposure of active sites simultaneously.

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In addition to oxidation, the reduction of active sites in the CO2RR process also needs to be considered, whether there is a membrane or not. Especially for the oxide-derived copper (OD-Cu) catalysts, the Cu+ has been proposed as the most important active site in the formation of C-C bond, while its stability has always been a great challenge due to its easy reduction under negative applied potentials in the CO2RR procedure (Yuan et al., 2018). The strong metal-support interactions have been proved a promising way to improve catalytic activity of metal active sites, which is also applied to upgrade the catalytic performance of copper-based catalysts for the CO2RR (Geioushy et al., 2017). For example, Lee et al. (2019) sintered the CuO nanocrystals onto CeO2 and found that the ceria is proposed to weaken the hydrogen binding energy of adjacent Cu sites to stabilize the *OCCO intermediate via an additional chemical interaction with an oxygen atom in the *OCCO. Zheng et al. (Wang et al., 2018) designed single-atomic Cu-substituted mesoporous CeO2 nanorods with multiple oxygen vacancy bond. They found each Cu atom substituted in the CeO2(110) surface can be stabilized by coordinating with three oxygen vacancies, yielding a highly effective catalytic center for CO2 adsorption and activation to methane. On the contrary, Qiao et al. (Wu et al., 2018) proved the isolated cuprous ions doped in ceria nanorods exhibit strong capability to capture CO2 and CO molecules and decrease the energy barrier for producing C2 products, leading to a high Faradaic efficiency (FE) of 47.6% toward ethylene. On the other hand, the stabilization effects of ceria substrate toward cuprous ions are responsible for the long-term durability. Sun et al. (Chu et al., 2020a) found that the selectivity and activity of the CO2RR on Cu/CeO2 composites are depended strongly on the exposed crystal facets of CeO2. By tuning the CuO/CeO2 interfacial interaction, they further improved the selectivity of ethylene by stabilizing Cu+ at the CuO-CeO2 interface (Chu et al., 2020b). Han et al. (Yan et al., 2021) modified copper oxide with cerium oxide to enhanced water activation and accelerate the formation rate of *CHO, thus enhance the selectivity and activity of C2+ products by promoting the hydrogenation of *CHO. Though the existing works have proved that CeO2 is a promising candidate as the supports of copper in the traditional H-cell (with a membrane to separate two champers) for the CO2RR process, few works have been reported achieving a high selectivity toward C2 products in a membrane-free cell.

Herein, we synthesized a series of novel CuO/CeO2 composites by dispersing CuO nanoparticles (NPs) on CeO2 nonocubes (NCs), which is then used as catalysts for the CO2RR in a single chamber cell without membrane. Surprisingly, the Faradaic efficiency of C2 was found exceeding 62% for the first time. This new finding may set off a new research climax in a simple membrane-free cell for industrial perspective of CO2 electrocatalysis.

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