Size-modified Poisson–Nernst–Planck approach for modeling a local electrode environment in CO2 electrolysis

Electrochemical reduction of CO2 heavily depends on the reaction conditions found near the electrode surface. These local conditions are affected by phenomena such as electric double layer formation and steric effects of the solution species, which in turn impact the passage of CO2 molecules to the catalytic surface. Most models for CO2 reduction ignore these effects, leading to an incomplete understanding of the local electrode environment. In this work, we present a modeling approach consisting of a set of size-modified Poisson–Nernst–Planck equations and the Frumkin interpretation of Tafel kinetics. We introduce a modification to the steric effects inside the transport equations which results in more realistic concentration profiles. We also show how the modification lends the model numerical stability without adopting any separate stabilization technique. The model can replicate experimental current densities and faradaic efficiencies till −1.5 vs. SHE/V of applied electrode potential. We also show the utility of this approach for systems operating at elevated CO2 pressures. Using Frumkin-corrected kinetics gels well with the theoretical understanding of the double layer. Hence, this work provides a sound mechanistic understanding of the CO2 reduction process, from which new insights on key performance controlling parameters can be obtained.

Esaar Naeem Butt,  Johan T. Padding and Remco Hartkamp

An analytical multiphase flow model for parallel plate electrolyzers

Membraneless parallel-plate electrolyzers use electrolyte flow to avoid product crossover. Using a mixture model neglecting inertia, and assuming an exponential gas fraction profile, we derive approximate analytical expressions for the velocity profile and pressure drop for thin plumes. We verify these expressions using numerical solutions obtained with COMSOL and validate them using experimental data from the literature. We find that the wall gas fraction increases rapidly at small heights, but becomes fairly constant at larger heights. These expressions serve as a guiding framework for designing a membraneless parallel-plate electrolyzer by quantifying the maximum possible height. We find that buoyancy driven membraneless parallel-plate electrolyzers with a 3 mm gap can be designed with a maximum height of around 7.6 cm at 1000 A/m2 for operation with 98% product purity at atmospheric pressure. For a forced flow at Re=1000, the same electrolyzer can be made around 17.6 cm tall at 1000 A/m2. These limits can be further improved with smaller bubbles or higher pressure.

A. Rajora and J.W. Haverkort

Energy comparison of sequential and integrated CO2 capture and electrochemical conversion

Integrating carbon dioxide (CO2) electrolysis with CO2 capture provides exciting new opportunities for energy reductions by simultaneously removing the energy-demanding regeneration step in CO2 capture and avoiding critical issues faced by CO2 gas-fed electrolysers. However, understanding the potential energy advantages of an integrated process is not straightforward due to the interconnected processes which require knowledge of both capture and electrochemical conversion processes. Here, we identify the upper limits of the integrated process from an energy perspective by comparing the working principles and performance of integrated and sequential approaches. Our high-level energy analyses unveil that an integrated electrolyser must show similar performance to the gas-fed electrolyser to ensure an energy benefit of up to 44% versus the sequential route. However, such energy benefits diminish if future gas-fed electrolysers resolve the CO2 utilisation issue and if an integrated electrolyser shows lower conversion efficiencies than the gas-fed system.

Mengran Li, Erdem Irtem, Hugo-Pieter Iglesias van Montfort, Maryam Abdinejad & Thomas Burdyny

Electrochemical Reduction of CO2 in Tubular Flow Cells under Gas–Liquid Taylor Flow

Electrochemical reduction of COusing renewable energy is a promising avenue for sustainable production of bulk chemicals. However, COelectrolysis in aqueous systems is severely limited by mass transfer, leading to low reactor performance insufficient for industrial application. This paper shows that structured reactors operated under gas–liquid Taylor flow can overcome these limitations and significantly improve the reactor performance. This is achieved by reducing the boundary layer for mass transfer to the thin liquid film between the CO2 bubbles and the electrode. This work aims to understand the relationship between process conditions, mass transfer, and reactor performance by developing an easy-to-use analytical model. We find that the film thickness and the volume ratio of CO2/electrolyte fed to the reactor significantly affect the current density and the faradaic efficiency. Additionally, we find industrially relevant performance when operating the reactor at an elevated pressure beyond 5 bar. We compare our predictions with numerical simulations based on the unit cell approach, showing good agreement for a large window of operating parameters, illustrating when the easy-to-use predictive expressions for the current density and faradaic efficiency can be applied.

Isabell Bagemihl, Chaitanya Bhatraju, J. Ruud van Ommen and Volkert van Steijn

Earlier Publications