Pressurized CO2 Electrochemical Conversion into Value-added Chemicals

The decarbonization of the world economy and energy’s storage and production from alternative C-based sources is considered a relevant topic. The electrochemical conversion of CO2 (CO2CR) has been widely investigated since the 1870s as a promising strategy to convert waste-CO2 into value-added chemicals [1- 2]. Among the several emerging technologies for CO2 conversion to value-added products on an applicative scale, electrochemical technologies are the closest to commercialization due to the numerous start-ups and established companies being invested in this area (e.g., Opus-12, Dioxide Material, and Carbon Recycling International) [3]. According to the literature [3], the process’ performances still fall short of meeting the requirements at the application scale. To be suitable for the commercialisation, the CO2CR should reach simultaneously high current density (j > 100 mA cm−2), faradaic efficiency (FE > 95 %), high product concentration as well as limited cost and long-term stability of cathodes [3]. One of the main handles of the CO2CR in aqueous electrolyte is the low CO2 solubility, limiting the performances of the process [4]. Several strategies have been studied to overcome this issue. Recently, increasing attention was focused on the cathodic reduction of pressurized CO2 (PrCO2CR) to produce various chemicals, such as formic acid or formate (FA), carbon monoxide (CO), synthesis gas and higher hydrocarbons. The objective of this work is to show how far this technology is from being implemented on an industrial scale and to critically discuss the main strategies to improve the process for the synthesis of formic acid/formate or carbon monoxide. In detail, the PrCO2CR process in aqueous electrolytes was investigated using various cathodes, including Sn, Bi, Ag and Ni, in an undivided cell to evaluate the role of both the P (ranged from 1 to 56 bar) and j (ranged from –50 to 230 mA cm-2) on the FE and total productivity (rTOT). In line with the literature, it was observed that the cathode’s nature strongly affects the selectivity of the process: Sn and Bi favor the production of FA, Ag favors the CO synthesis, while the hydrogen evolution prevails using Ni under all adopted operative conditions. Additionally, the selection of proper operative conditions (such as 40 bar and –150 mA cm-2) allows to achieve both high FE (close to 80-90 %) and productivity rTOT (close to 7 mmol s-1 m-2) for all adopted electrodes except for Ni; in the case of Ni, at –50 mA cm-2 the increase of P from 1 to 55 bar allowed to increase FE from about 0 to 40 %. To conclude, the effect of P and j on the economic figures of the process using Sn, Ag and Bi will be discussed highlighting the main factors that affect the scalability of the process on an industrial scale. Overall, the most appealing economic figures were obtained under very similar operative conditions, such as relatively high P of 30 - 56 bar and high j ranged from –150 and –190 mA cm-2, since under these adopted conditions very similar values of FE and rTOT were reached.