Is maximising current density always the optimum strategy in electrolyser design for electrochemical CO2 conversion to chemicals?

Abstract

Electrochemical conversion of CO2 to chemicals and fuels can potentially play a role in reducing CO2 emissions from industrial processes and providing non-fossil fuel routes to important chemical feedstocks. Most of the recent research on electrocatalysts for CO2 reduction (CO2R) focuses on achieving maximum selectivity for desired products at the highest possible current density. This approach assumes that maximising current density leads to the lowest cost of CO2R (e.g. $·kg-1 CO2 converted) because it requires the lowest catalyst loadings and electrode area per kg of CO2 treated and thus minimising the electrolyser equipment cost. Using a techno-economic analysis (TEA) model with experimental data from a two-cell vapor fed electrolyser, we show this assumption is not valid for CO2 conversion to CO if the process model accounts for relationships between current density, selectivity, cell voltage, ohmic losses, and product separation costs. Instead, our model predicts the lowest CO production costs at current densities from 500 – 700 A·m-2. At current densities above 1000 A·m-2, growing ohmic losses in the electrolyser lead to increasing power costs that become much larger than any capital savings related to reduced electrode area at the higher current density. Further, we investigate different opportunities that could bring down the CO production cost, however, in all the cases, the lowest CO production cost was found at current densities between 600 – 1400 A·m-2. This work also provides insights that can help identify feasible design spaces for both catalysts and electrolysers to develop CO2 conversion technologies that could soon compete on a cost basis with the natural reforming technologies to produce CO (0.60 $·kg-1 market price).

Urea-Functionalized Silver Catalyst toward Efficient and Robust CO2 Electrolysis with Relieved Reliance on Alkali Cations

“…We also performed a long-term stability test with the untreated Ag GDE in our other work 49 (also shown in Figure S19) and found that the selectivity of CO was only around 62%, much less than what we observed with the Ag-U-mixed GDE (85%). In addition, the electrocatalytic performance S3.…”

Integrating a Top-Gas Recycling and CO2 Electrolysis Process for H2-Rich Gas Injection and Reduce CO2 Emissions from an Ironmaking Blast Furnace

“…Assumptions for the material and energy balance in the CO 2 conversion unit were based on laboratory demonstration data with additional inputs from literature sources. Briefly, the model was based on multiple two-cell vapour fed electrolyser stacks with the capacity to treat 50 tCO 2 per day; further details can be found in our other report [ 33 ]. The current density of the electrolyser was altered from 2.68 V at 0 A/m 2 to 3.59 V at 1862 A/m 2 to produce the H 2 -rich gas with different H 2 /CO compositions.…”

“…The electricity consumption for CO 2 conversion is proportional to the H 2 generation, which can be estimated as in Equation (2) [ 33 ]:…”

“…However, it provides an added opportunity for a BF because hydrogen is coproduced in the electrolyser, which can be used for iron-ore reduction. In low-temperature electrolysis cells, CO 2 reduction is carried out in aqueous solutions, and different levels of current density can be used to produce hydrogen at various concentrations [31][32][33]. As an additional benefit, a pure oxygen stream is generated as another byproduct during electrolysis, which can be used directly for oxygen enrichment to the BF [34].…”

Insights into electrolyte flooding in flexible gas diffusion electrodes for CO2 electrolysis: from mechanisms to effective mitigation strategies

“…5 A much larger electrode area is required to process about 50 t day −1 of CO 2 to achieve competitive cost compared to petrochemical processes. 6 Although the selectivity to desired products such as CO and C 2 H 4 and current density achieved in bench-scale experiments approach likely industrial requirements, 7 the present energy efficiency and stability of the lab electrolysers are far from the targets set by Sargent et al 8 The challenges in attaining a scalable performance primarily stem from operational stability of a gas diffusion electrode (GDE) as the cathode. The GDE not only exhibits exceptional mechanical resilience due to its exibility but, more importantly, effectively addresses the mass transport issue with planar electrodes in static electrolysers.…”