Water electrolyzers are generally classified according to temperature: high temperature and low temperature; and low temperature electrolyzers are in turn classified according to the pH of the operating medium: alkaline electrolyzers (AE) and proton exchange membrane (PEM) electrolyzers. At the commercial level, PEM and EA electrolyzers are already available, the latter being the most widely used by the industry due to its production capabilities. In fact, commercial EAs are capable of producing up to 750 Nm³/h of Hydrogen, while commercial PEM electrolyzers can produce up to 30 Nm³/h of H2.
The implementation of both electrolyzers on a large commercial and industrial scale requires scientific and technological improvements to increase the power densities of the stacks, to simplify the complexity of the system, to improve the long-term stability of its components due, and to decrease the high cost associated with the cost of catalysts (usually of critical or scarce materials) and membranes.
Scientific and technological challenges of conventional electrolyzers
Alkaline electrolyzers are currently more attractive for industrial applications due to their higher performance, greater durability and better price-performance ratio. However, one of the main problems of alkaline electrolyzers is their large size, but recent research has been able to reduce their total volume by assuming a "zero-gap" configuration (when the space between membrane and electrodes is zero).
However, this configuration is not free of challenges as it has conductivity problems and troubles associated with the accumulation of bubbles of Hydrogen and Oxygen formed on the electrode surface. As a result of these technical drawbacks, the power density of alkaline electrolyzers is affected.
The efficiency of current conventional electrolyzers, and therefore the H2 production rate, is also conditioned by how fast the oxygen is formed, which is a complex, slow reaction and the cause of catalyst degradation in the long term.
In addition, one of the most relevant challenges in both EA and PEM electrolyzers is the diffusion of gases through the membranes. Such gas diffusion, if uncontrolled, can result in explosive mixtures of H2 and O2. And although membrane design has advanced significantly in recent years by increasing their ionic conductivity and decreasing their gas permeability, existing membranes are not completely impermeable to gases and, in particular, to hydrogen permeation. This is a challenge for the safety of the system, and for this reason, the generated pressures of H2 and O2 must be very carefully controlled to prevent them from permeating between the anode and cathode compartments.
Keeping the gases separated in both compartments is not trivial. When an electrolizer is connected to renewable energy sources like wind or solar, the input power is variable and, therefore, the generation rate, and so are the pressures of H2 and O2. Also, when current densities are low, H2 production is also slow and very similar to the gas permeation rate through the membrane. For example, an electrolizer connected to solar panels with an efficiency of 10% sun-fuel energy transformation and under solar exposure of 100 mW cm-2 would operate at a current density of 10 mA cm-2, which is considered a good benchmark. However, at these current densities, hydrogen permeation can be potentially dangerous since the potential explosion limit for hydrogen-oxygen mixtures is only 4%.
Innovative solutions to conventional problems: Decoupled electrolyzers.
A fast and cheap technology that allows us to solve these problems is uncoupled cells. In decoupled electrolyzers, the evolution of H2 occurs simultaneously with the oxidation of an intermediate mediator, which will subsequently be reduced by the formation of oxygen (Figure 1). Therefore, in decoupled electrolyzers the formation of H2 and O2 are separated in time and space, thus avoiding gas mixing when the device is connected to renewable energy sources.
Furthermore, since H2 and O2 production does not occur simultaneously, there is also no need for additional, usually costly, H2 purification steps.