Selective Electroreduction of CO₂ to n‐Propanol in Two‐Step Tandem Catalytic System

Abstract

Electrocatalytic conversion of CO 2 to chemical fuels serves a two-fold purpose of energy storage and carbon offset. Among those, the efficiency of electrocatalytic reduction of CO 2 to one-carbon (C 1 ) products including CO [1][2][3] and formate [4][5][6] is approaching commercialization, while the transformation of CO 2 to C 2 products such as ethylene [7][8][9][10] and ethanol [11,12] is developing rapidly. In comparison, research on the conversion of CO 2 to high-energy C 3 fuels like n-propanol [13][14][15][16][17][18][19] is seriously lagging behind despite the unique merits of products, e.g., high added value and easy management.Metallic Cu is known to be the only efficient catalyst by far for the electroreduction of CO 2 to n-propanol. Since the pioneering report on electroreduction of CO 2 to n-propanol at Cu electrode in 1988, [14] several strategies of modifying copper-based catalysts, including structural reconstruction, [15] two-step activation, [16] metal ion cycling, [17] core-shell structure [18] and double sulfur vacancies, [19] have been developed, pushing the FE of n-propanol to 15.4% and the partial current density to 10.2 mA cm -2 . Nevertheless, complex design on Cu catalysts are needed, because n-propanol formation involves multiple active sites that are responsible for the reduction of CO 2 to CO intermediate and subsequent carbon-carbon coupling, respectively. Such drawback largely limits the selection of Cu-based catalysts, thus impeding commercial production of n-propanol from CO 2 electroreduction.We notice that direct electrocatalytic reduction of CO (CORR) could considerably promote n-propanol generation. [20,21] Early work on CORR obtained a FE in excess of 20% for n-propanol at gas diffusion electrode in a flow cell. [21][22][23][24] Furthermore, an Ag-Ru-Cu alloy catalyst endowed FE of 36% for propanol at the current density of 300 mA cm -2 . [23] Unfortunately, owing to the risk of high-pressure storage of CO and high equipment maintenance cost, it is not realistic to achieve either industrial or decentralized multi-carbon products using CO as a feedstock.Distinct from CO, CO 2 is a safe, cheap, and easily liquefied gas. Hence, we propose that a tandem catalytic CO 2 RR system consisting of two independent sections of CO 2 to CO and CO to n-propanol may easily solve the problem of highpressure CO storage. [25] Since the input chemicals account for the largest proportion of cost in CORR industrialization, [23] Electrocatalytic reduction of CO 2 (CO 2 RR) to high-energy-density C 3 fuels like n-propanol has been widely recognized as one of the core technologies for both renewable energy storage and carbon neutrality. Unfortunately, exceptional catalytic sites that are able to simultaneously implement efficient conversion to C 1 and C 2 products are difficult to design for single electrolyzers, leading to poor activity and selectivity toward n-propanol. Alternatively, electroreduction of CO (CORR) has shown good performance toward C 3 fuels, but the cost and safety of gas storage h...

Competition of CO and Acetaldehyde Adsorption and Reduction on Copper Electrodes and Its Impact on n-Propanol Formation

“…18 The strategy to use CO instead CO 2 , as applied by Wang et al, 18 has led to the idea to apply a two-step cascade process in which the first step consists of converting CO 2 into CO followed by its conversion into n-propanol in a second step. Wu et al 19 showed that a tandem system consisting of two electrolyzers for converting CO 2 to CO and CO to n-propanol, resulted in a FE of 15.9% for n-propanol. Similarly, Romero Cuellar et al 20 showed that the total FE toward multi-carbon products of a one-step system using the Cu gas diffusion electrode as a working electrode was limited to 20% at a total current density of −470 mA cm −2 while the two-step configuration [using a Ag-gas diffusion electrode (GDE) as an electrode for the first step followed by Cu-GDE in the second step] led to a total FE toward C 2 and C 3 products of 62% (with about 18% of n-propanol) at a total current density of −300 mA cm −2 .…”

A rational design of functional porous frameworks for electrocatalytic CO2reduction reaction

“…86,87 To deeply exploit the potentiality of Ni-based functional porous frameworks, a series of technology combination strategies was proposed, including the photocoupled ECO 2 RR 167 and tandem catalysis of the ECO 2 RR and ECORR. 142 ) were successfully obtained with a similar 2D skeleton structure except for the Ni-O 4 and Ni-N 4 linking units (Fig. 15a and d), respectively.…”

“…9). 142 Beyond the reported tandem electrodes and one-pot tandem catalysts, precise voltage control of tandem electrolyzers enables the optimized conversion of both the ECO 2 RR to CO and ECORR towards n -propanol. Specifically, the tandem electrolyzers achieved an n -propanol FE of 15.9% together with the corresponding half-cell power conversion efficiency (CPE) of 19.3%.…”

“…86,87 To deeply exploit the potentiality of Ni-based functional porous frameworks, a series of technology combination strategies was proposed, including the photo-coupled ECO 2 RR 167 and tandem catalysis of the ECO 2 RR and ECORR. 142…”

Enriching Reaction Intermediates in Multishell Structured Copper Catalysts for Boosted Propanol Electrosynthesis from Carbon Monoxide

“…Although a high percentage of C 2+ product is achieved in ECOR, most of the products are only limited to C 2 such as ethylene and acetic acid. Most importantly, high-energy-density C 3 products, such as propanol, remain difficult to obtain. − Throughout the existing reports on ECOR, the selectivity of propanol is usually below 10%, which hinders further applications in industry. , Currently, the best performance of propanol electrosynthesis in ECOR is achieved by noble-metal-doped Cu-based catalysts such as Ag-doped Cu and Ag–Ru-doped Cu. , Nevertheless, considering the scarcity of precious metals and the effect of doping amount, the strict doping ratio and high cost of catalysts may limit their large-scale synthesis and use. − Therefore, other cost-effective strategies for catalyst design are worth considering. C 1 –C 2 coupling is considered a crucial route for the formation of C 3 products based on previous studies. − Moreover, many catalysts have an issue of insufficient coverage of *CO caused by morphology and/or structure, thus resulting no further coupling to C 3 products. ,− Therefore, enriching the local concentration of C 1 and C 2 intermediates by altering their diffusion kinetics can provide the possibility of promoting C 3 production. ,− However, achieving such a target remains a grand challenge, such as the controlled nanoscale adjustment of the same series of catalysts.…”