In water electrolysers, gas crossover caused by elevated dissolved gas concentrations poses a major challenge, reducing product purity, safety, and operational stability. However, most existing electrolyser models assume that all generated products leave the electrode in the gaseous phase, neglecting the dynamics of dissolved gas transport. Experimental observations from the literature reveal that bubbles can continue to grow even after detachment, suggesting significant dissolved gas supersaturation. In this work, we develop a computational multiphase flow model that couples the transport of dissolved gas, gas fraction, and volumetric interfacial area to quantify bubble growth within electrogenerated plumes. The model is validated using experimental data extracted from literature videos, where individual bubbles are tracked to determine their size and position over time. The absolute average relative error in predicting bubble diameters is below 7%, demonstrating the model’s accuracy. Results show that at a gas-evolution efficiency of 40%, detached bubbles can grow up to 1.4 times their initial diameter, corresponding to a threefold increase in volume. This confirms that the observed post-detachment bubble growth can be quantitatively explained by the uptake of dissolved gas within the plume. By incorporating this mechanism, the model enables improved prediction of dissolved gas distributions, supporting more reliable design and operation of industrial electrolysers.

Modern alkaline water electrolysers for hydrogen production often use a zero-gap configuration in which electrodes are pressed directly against the separator. Counterintuitively, the inclusion of a small electrode-diaphragm gap was previously shown to reduce the cell potential significantly. This work aims to understand, quantify, and model this effect, and take first steps towards optimisation of the gap width. We present experimental measurements and simulations of the cell potential and kinetic and ohmic losses for expanded metal electrodes. We find that the configuration with our smallest used gap, created using a 60µm spacer, yields the lowest cell potential, while the zero-gap configuration incurs additional voltage losses of approximately 80mV at a current density of 10⁴A/m². This can be explained by bubbles and gas films in between the electrode and diaphragm, which block part of the diaphragm and electrode area. We introduce an analytical model that predicts the vertical gas fraction and current density distribution in the electrode-diaphragm gap, which is in good agreement with experimental and simulation results. For a maximum gas fraction below 0.7, the model can explain why there is no optimal gap width based solely on the gap resistance. Instead, the gap allows gas to escape, which mitigates the additional zero-gap overpotential. Our findings confirm, explain, and quantify that intentionally adding a small gap can be an effective way to improve the performance of alkaline electrolysers with perforated plate-like electrodes.

As current densities in alkaline water electrolysers increase, the resistive losses become increasingly important due to the locally high gas fraction around the electrodes, even in zero-gap configurations. Nonetheless, quantitative measurement of the distribution of these high gas fractions is difficult. Consequently, a numerical approach is useful to assess the impact of bubbles on electrolysis. However, models that couple current density and gas fraction distributions in a non-trivial geometry are currently lacking. We show that typically used models in the literature predict unrealistically high gas fractions in electrode-resolved simulations. To improve this, we added to the mixture model equations a solid pressure model similar to that used in simulations of dense granular flows. With the addition of this model, two-dimensional simulations of a lab-scale electrolysis cell accurately reproduce previously reported experimental results. This allows, for the first time, to predict local overpotentials influenced by the bubble distribution, opening the way towards computational optimisation of the electrode geometry.

The high mass transfer to or from gas-evolving electrodes is an attractive feature of electrochemical reactors, which can be partly attributed to the large convective flows that arise due to the buoyancy of bubbles. We derive exact analytical expressions for mass transfer coefficients for the case of constant gas flux boundary conditions. For the mass transport both Dirichlet and Neumann boundary conditions are considered. We deploy a recently derived self-similar solution of laminar two-phase flows, with density, hydrodynamic diffusivity, and viscosity dependent on the local gas fraction. Combining this with the Lévêque approximation, new mass transfer coefficients are obtained analytically. These new results are relevant for various electrochemical processes with gas evolution as well as boiling. The new formulation shows the mass transfer coefficient to scale with the vertical coordinate z proportional to z^−1/5 for short electrodes and low current densities and z^−4/15 for long ones and high current densities. The former limit also applies when buoyancy is due to temperature or concentration differences in the case that density differences are small. We provide a general overview considering all possible gas and mass boundary conditions combinations and a comparison with the Boussinesq approximation of small density differences.