Impact of an electrode-diaphragm gap on diffusive hydrogen crossover in alkaline water electrolysis

Hydrogen crossover limits the load range of alkaline water electrolyzers, hindering their integration with renewable energy. This study examines the impact of the electrode-diaphragm gap on crossover, focusing on diffusive transport. Both finite-gap and zero-gap designs employing the state-of-the-art Zirfon UTP Perl 500 and UTP 220 diaphragms were investigated at room temperature and with a 12 wt% KOH electrolyte. Experimental results reveal a relatively high crossover for a zero-gap configuration, which corresponds to supersaturation levels at the diaphragm-electrolyte interface of 8–80, with significant fluctuations over time and between experiments due to an imperfect zero-gap design. In contrast, a finite-gap (500 μm) has a significantly smaller crossover, corresponding to supersaturation levels of 2–4. Introducing a cathode gap strongly decreases crossover, unlike an anode gap. Our results suggest that adding a small cathode-gap can significantly decrease gas impurity, potentially increase the operating range of alkaline electrolyzers, while maintaining good efficiency.</p

Alkaline water electrolysis: with or without iron in the electrolyte?

The presence of iron in the electrolyte has a significant impact on the performance of the electrodes in alkaline water electrolysis. For nickel-based anodes, the presence of iron is needed to achieve and maintain low overpotentials in the oxygen evolution reaction (OER). In contrast, in hydrogen evolution, the presence of iron can lead to deactivation of noble metal-based cathodes, which are more active than non-noble metal cathodes. Since the catholyte and anolyte can mix through the porous separator, stack developers need to decide on the optimal iron content in their system. It seems most promising to focus further development on the ‘iron-rich’ system, also considering the costs of construction materials and water purification.</p

Ohmic resistance in zero gap alkaline electrolysis with a Zirfon diaphragm

Alkaline water electrolyzers are traditionally operated at low current densities, due to high internal ohmic resistance. Modern diaphragms with low internal resistance such as the Zirfon diaphragm combined with a zero gap configuration potentially open the way to operation at higher current densities. Data for the Zirfon diaphragm show that the resistance is only 0.1–0.15 Ω cm2 in 30% KOH at 80 °C, in line with estimations based on the porosity. Nevertheless, an analysis of data on zero gap alkaline electrolyzers with Zirfon reveals that the area resistances are significantly higher, ranging from 0.23 to 0.76 Ω cm2. A numerical simulation of the secondary current distribution in the zero gap configuration shows that an uneven current distribution, imperfect zero gap and the presence of bubbles can probably only partly explain the increased resistance. Therefore, other factors such as the presence of nanobubbles could play a role

Optimal operating parameters for advanced alkaline water electrolysis

Advanced zero-gap alkaline electrolyzers can be operated at a significantly higher current density than traditional alkaline electrolyzers. We have investigated how their performance is influenced by diaphragm thickness, temperature and pressure. For this a semi-empirical current-voltage model has been developed based on experimental data of a 20 Nm3/h electrolyzer. The model was extrapolated to thinner diaphragm thicknesses and higher temperatures showing that a nominal current density of 1.8 A cm−2 is possible with a 0.1 mm diaphragm at 100 °C. However, these operating parameters also lead to increased gas crossover, which limits the ability to operate at low loads. A gas crossover model has been developed, which shows that crossover is mainly driven by diffusive transport of hydrogen, caused by a high local supersaturation at the diaphragm surface. To enable a low minimum load of 10% the operating pressure should be kept below 8 bara

Current efficiency and mass transfer effects in electrochemical oxidation of C1 and C2 carboxylic acids on boron doped diamond electrodes

The oxidation of acetic, glycolic, glyoxylic, oxalic and formic acid has been studied on boron doped diamond electrodes. Our voltammetry study on rotating disk electrodes highlights clear differences between the carboxylic acids (formic, oxalic, glyoxylic and glycolic acid) that can be oxidized via a direct electron transfer (DET) and acetic acid which can presumably solely be oxidized by •OH radicals formed in the region where water oxidation takes place. In glycolic and glyoxylic oxidation oxalic acid is the main intermediate formed. Surprisingly, glyoxylic acid could not be detected as an intermediate in the glycolic acid oxidation. Chronoamperometric experiments confirm that all compounds except acetic acid react further to CO2, which could be deduced from the electron balances. It was shown that formic, oxalic, glyoxylic and glycolic acid can be selectively oxidized at 2.3 V vs. Ag/AgCl with high current efficiencies, below or close to mass transfer limiting rates. At higher potentials (2.4 V and 2.5 V) simultaneous water electrolysis results in lower current efficiencies. At these potentials the conversion rates can exceed the limiting rates, which might be attributed to effects related to water oxidation (i.e. O2 evolution and •OH radical formation)

Separating kinetics and mass transfer in formic acid and formate oxidation on boron doped diamond electrodes

This paper describes the electrochemical oxidation of formic acid (pH 2) and formate (pH 6 and pH 10) on boron doped diamond electrodes and separates kinetic and mass transfer information using RDE and flow cell experiments. The voltammetry experiments show that the oxidation of formic acid and formate takes place before water oxidation, which indicates that the oxidation takes place via a direct electron transfer (DET) mechanism. The current densities measured in the linear sweep voltammograms were in line with the expected limiting current densities confirming mass transfer limitations. During chrono-amperometric experiments current densities were significantly lower, indicating a decrease in the kinetic rates. This is probably related to a surface modification at the electrode. This modification is reversible as bringing the electrode to low potentials restores the activity. The stable currents observed in the chronoamperometry experiments in the region before water oxidation indicate that there is probably a second DET mechanism, which has a much higher overpotential and a different pH dependency than the first DET mechanism. Koutecky-Levich plots showed clear mixed kinetic and mass transfer control and were used to deduce the Tafel slopes of this second mechanism. The observed high Tafel slopes of 240–300 mV/dec are in line with values reported for other reactions on BDD. Experiments in a parallel plate electrolyser show that the current efficiency is approximately 100% at 2.3 V for all pHs. At higher potentials current efficiency decreases due to the water oxidation. Interestingly at these potentials the formic acid oxidation exceeds the limiting current density, possibly related to the diffusion of ·OH radicals or other oxidation mediators to the bulk solution or the formation of oxygen bubbles on the electrode

Bipolar membrane electrodialysis for the alkalinization of ethanolamine salts

Bipolar membrane electrodialysis for the production of organic bases, in contrast to organic acids, has received little attention in the scientific literature. In the present work we have investigated and compared different membrane configurations for the alkalinization of monoethanolamine salts into the organic base monoethanolamine. A current utilization of only 36% has been obtained for a two compartment configuration with bipolar and anion-exchange membranes. Proton tunneling through the anion-exchange membrane has been identified as the main reason for this relatively low current utilization. Minimizing proton tunneling by employing proton blocking anion-exchange membranes and using a three compartment configuration, the current utilization could be increased to 80%. This bipolar membrane configuration acts as a concentration step as well. A MEA concentration of 32 wt% in the base compartment could be achieved. A disadvantage of the three compartment configuration is the relatively high unit cell potential drop of 3.1 V at a current density of 1000 A m−2 using a compartment thickness of 0.75 mm