Analyzing Production Rate and Carbon Utilization Trade-offs in CO₂RR Electrolyzers

Abstract

Carbon utilization is a crucially important performance characteristic of CO2 electrolyzers; however, applications of simple and accurate descriptive models to experimental data on this topic are currently lacking. Here, we apply a simple analytical reactor model to parameterize single-pass conversion as a function of feed-gas flow rate and show that it captures a wide body of experimental data in the literature exceptionally well. In doing so, we demonstrate that this simple conceptual approach can characterize progress and capture recent innovations in reactor design. To validate the extracted parameters, we employ a well-established comprehensive model to confirm that the results agree with physical expectations. Finally, we explore the implications of the descriptive model by examining the inherent trade-off between single-pass conversion efficiency and reactor productivity. Understanding such trade-offs is crucially important for advancing this electrolyzer technology toward commercial applications.

Zero-Gap Electrochemical CO2 Reduction Cells: Challenges and Operational Strategies for Prevention of Salt Precipitation

“…In order to avoid potassium carbonate precipitation in a strongly alkaline system, the concentrations of both CO 3 2– and K + must be kept below 7.93 and 15.86 M, respectively. Although these concentrations are much higher than the ∼1 M K + of typical CO 2 RR electrolytes, the substantial production of hydroxide and carbonate at elevated current densities creates such an environment, as was computationally hypothesized by several catalyst layer concentration models. ,, …”

“…Although these concentrations are much higher than the ∼1 M K + of typical CO 2 RR electrolytes, the substantial production of hydroxide and carbonate at elevated current densities creates such an environment, as was computationally hypothesized by several catalyst layer concentration models. 17,38,39 The experiences of rapid salt formation at industrially relevant current densities (e.g., 50 min for a 2 M KOH anolyte operating at 100 mA cm −2 ) 40 indicate that the migration term of cations toward the cathode is larger than the diffusion term in eq 4. Once salting out conditions are met, nucleation occurs and rapid growth of crystals is observed into the cathode pores and flow field until salts block gas flow altogether.…”

“…Although these concentrations are much higher than the ∼1 M K + of typical CO 2 RR electrolytes, the substantial production of hydroxide and carbonate at elevated current densities creates such an environment, as was computationally hypothesized by several catalyst layer concentration models. 17 , 38 , 39 …”

Techno-economic Assessment of CO2 Electrolysis: How Interdependencies between Model Variables Propagate Across Different Modeling Scales

“…Therefore, experimental studies on electrolyser design have mostly been accompanied by modeling efforts, to capture the interdependencies at the channel scale. − These models are used to resolve local effects in the electrolyser, for example, to understand concentration gradients and mass transfer limitations due to the change in pH near the catalyst layer. , While these models can provide relevant insights into the interdependencies of the performance variables, they so far have not been translated into techno-economic analyses. Channel models can additionally account for concentration gradients along the flow channel, taking into account their effect on single-pass conversion. − For example, the study of Kas et al showed an increased loss of CO 2 to carbonate formation at high current densities due to the limited buffer capacities of the electrolyte. This insight reveals a trade-off between current density and conversion, one of the interdependencies commonly neglected when using fixed performance variables for techno-economic analyses.…”

Pilot-Scale CO2 Electrolysis Enables a Semi-empirical Electrolyzer Model

“…Demonstrations of CO 2 electrolysis have been limited to the lab-scale, with most cell areas <10 cm 2 . As the cell size increases, discrepancies can develop across the active area in electrode composition, , electrode compression, CO 2 concentration, − pressure, − temperature, and current density/voltage, , all of which impact CO 2 electrolysis selectivity and efficiency. , For CO 2 electrolysis into multi-carbon products, the largest single cell reported is 25 cm 2 , , and the largest stack is 30 cm 2 (3 × 10 cm 2 cell stack) . Although informative, these demonstrations of CO 2 electrolysis are several orders of magnitude too small to inform on operation at industrial scale (i.e., electrolyzer area >100 m 2 ) …”