Engineering Aspects for the Design of a Bicarbonate Zero-Gap Flow Electrolyzer for the Conversion of CO₂ to Formate

Abstract

CO2 electrolyzers require gaseous CO2 or saturated CO2 solutions to achieve high energy efficiency (EE) in flow reactors. However, CO2 capture and delivery to electrolyzers are in most cases responsible for the inefficiency of the technology. Recently, bicarbonate zero-gap flow electrolyzers have proven to convert CO2 directly from bicarbonate solutions, thus mimicking a CO2 capture medium, obtaining high Faradaic efficiency (FE) and partial current density (CD) toward carbon products. However, since bicarbonate electrolyzers use a bipolar membrane (BPM) as a separator, the cell voltage (VCell) is high, and the system becomes less efficient compared to analogous CO2 electrolyzers. Due to the role of the bicarbonate both as a carbon donor and proton donor (in contrast to gas-fed CO2 electrolyzers), optimization by using know-how from conventional gas-fed CO2 electrolyzers is not valid. In this study, we have investigated how different engineering aspects, widely studied for upscaling gas-fed CO2 electrolyzers, influence the performance of bicarbonate zero-gap flow electrolyzers when converting CO2 to formate. The temperature, flow rate, and concentration of the electrolyte are evaluated in terms of FE, productivity, V Cell, and EE in a broad range of current densities (10–400 mA cm–2). A CD of 50 mA cm–2, room temperature, high flow rate (5 mL cm–2) of the electrolyte, and high carbon load (KHCO3 3 M) are proposed as potentially optimal parameters to benchmark a design to achieve the highest EE (27% is obtained this way), one of the most important criteria when upscaling and evaluating carbon capture and conversion technologies. On the other hand, at high CD (>300 mA cm–2), low flow rate (0.5 mL cm–2) has the highest interest for downstream processing (>40 g L–1 formate is obtained this way) at the cost of a low EE (<10%).

Electrochemical Conversion of CO2 from Direct Air Capture Solutions

“…The experimental conditions were set based on the optimization study for bicarbonate electrolysis performed recently by our group . From this study, the most optimal flow rate, temperature, and CD to achieve the highest energy efficiency were chosen.…”

“…Although direct electrochemical conversion of the bicarbonate anion has been reported, it has been a topic of discussion within the community whether bicarbonate was the substrate of the electrochemical reaction or solely played the role of carbon donor by providing CO 2 (derived from the equilibrium with water) to the surface of the electrode (or both). , Albeit neither confirming nor discarding any line of thought, several more recent studies showed how promoting the release of CO 2 from bicarbonate in situ the electrolyzer improved to a great extent both the FE and partial CD of the bicarbonate electrolysis. − It was found that this release could be promoted by using a bipolar membrane (BPM) in combination with a zero-gap flow electrolyzer. Indeed, experimental results demonstrated that bicarbonate is more efficiently converted to CO 2 thanks to the water dissociation (1) occurring in the BPM once the potential is applied (H + is released toward the catholyte, and OH – is released toward the anolyte) .…”

“…Due to the zero-gap configuration, CO 2 is generated close to the surface of the cathode (2), where it is readily reduced to carbon products (3) . Through this strategy, Lees and co-workers converted CO 2 from a 3 M potassium bicarbonate solution at 100 mA cm –2 obtaining a FE of 64% and 37% toward formate and CO, respectively. − However, due to the three-membrane configuration of the BPM (cation exchange, interface, and anion exchange layers), a higher ohmic drop than with typical anion- or cation-exchange membranes was observed, thereby increasing to a great extent the cell voltage needed to apply the desired current density and thus lowering the energy efficiency. , For instance, in our previous study, we observed how, although a FE over 35% toward formate was obtained, the cell voltage was 6.4 V at 400 mA cm –2 , which significantly decreased the energy efficiency of the system . Nevertheless, at a lower CD such as 50 mA cm –2 , higher FE (58%) and lower cell voltage (3.5 V) were obtained (Figure S1), thus increasing the feasibility of the technology (although further optimization is still needed). normalH 2 normalO ⇄ normalH + + OH − HCO 3 − + normalH + ⇄ CO 2 + normalH 2 normalO CO 2 + normalH 2 normalO + 2 normale − → CO + 2 OH − …”

Bifunctional Artificial Carbonic Anhydrase for the Integrated Capture and Electrochemical Conversion of CO2

“…In addition, after conversion, the alkalinity of the solution is regenerated such that it can be reused as a capture solution . However, bicarbonate electrolyzers are still far from being optimized and cannot yet compete in terms of energy efficiency with pure CO 2 electrolyzers. , The main reason is the high proton donor ability of bicarbonate leading to undesired coreactions like the hydrogen evolution reaction (HER) and the mass transfer limitations in the electrode–electrolyte interface since CO 2 needs to be delivered from the equilibrium of bicarbonate with water. , In this context, there is increasing interest in optimizing this combined capture and conversion process to make it more feasible . For instance, a promising option to optimize the capture step is to decrease the operational time to capture a certain amount of CO 2 (g CO 2 h –1 ) in the form of bicarbonate .…”

“…33,34 In this context, there is increasing interest in optimizing this combined capture and conversion process to make it more feasible. 35 For instance, a promising option to optimize the capture step is to decrease the operational time to capture a certain amount of CO 2 (g CO 2 h −1 ) in the form of bicarbonate. 36 On the other hand, a strategy to increase the energy efficiency of the bicarbonate electrolyzer is to inhibit the proton donor ability of bicarbonate and the undesired HER.…”