Production of "green" hydrogen is a promising alternative to fossil fuels, especially for the metallurgical and chemical industries. Hydrogen is produced by electrolysis of water, using electricity from renewable sources. However, the process is hindered by bubbles that form on the electrodes and increase energy consumption.
Researchers from Cornell University (USA) and City University of Hong Kong have developed a three-dimensional mathematical model that describes the processes in an electrolyzer (a device for splitting water with electric current) at scales from nanometers to centimeters. The work was published on November 9 in the journal EES Catalysis. The study was led by Lenan Zhang (Cornell) and Weihong Li (Hong Kong), while Muhan Zhang and Kaiyu Li contributed equally as first authors.
Hydrogen and oxygen bubbles, appearing on the electrodes, cover active sites, prevent the liquid from reaching the surface and impede the movement of charged particles. This forces the system to expend additional energy. The problem intensifies as electrolyzers grow in size and current density increases.
To understand this, the scientists applied a new method for calculating the tiniest electrochemical interactions, which reduced the computational load by almost 1000 times without loss of accuracy. The result was a "digital twin" of the electrolyzer - a virtual device that reproduces real conditions and shows what is difficult to see in an experiment.
Simulation of a single bubble in an alkaline electrolyzer showed that the main source of losses is the blocking of the electrode's active area. A critical bubble radius was found: once a bubble exceeds this size, the additional voltage needed to sustain electrolysis grows exponentially (that is, faster and faster with each step). Under some simulated conditions, the addition to the voltage exceeded 50% of the value without bubbles. The critical radius depends on the electrode geometry and the wettability of its surface by the liquid, not on current density.
Hence the practical conclusion: the electrode surface should be designed so that bubbles detach before they reach the critical size. The researchers introduced a parameter - the ratio of the detachment radius to the critical radius. Simulations show that if this ratio is less than one, the additional voltage due to a single bubble will not exceed 3%.
So far the model describes a single bubble, but the authors are extending it to the interaction of many bubbles, which can merge and influence each other. The plans include combining the model with optimization and additive technologies (3D printing) to create electrodes with minimal losses, as well as turning it into accessible software for engineers.
It is important to understand: the work demonstrates a correlation between bubble size and energy losses in the model, but does not prove a causal relationship in real devices. This is a statistical association that by itself does not confirm that bubbles are what cause the increase in energy consumption. However, the model provides a quantitative basis for further experiments and the development of more efficient equipment.