CIGS for hydrogen production

The original article can be read here

Producing hydrogen gas from solar energy is an effort to make society more sustainable by providing a way to store energy from the sun and possibility for a renewable fuel. For example here at Ångström laboratory, we have used electrolysis with our CIGS-solar cells with more than 10 % efficiency from sunlight to hydrogen (read more here). Two methods for this kind of hydrogen production, which is often separated in the literature, is photoelectrochemical cells (PEC-cells) and photovoltaic electrolyzers (PV-electrolyzers) as above. In this study, the processes governing these methods are analyzed, and through this it is demonstrated that they are closely related and rather should be seen as variations of the same basic theme: a photo driven catalysis. The analysis is performed by successively transforming a PEC-cell to a PV-electrolyzer.

Producing hydrogen with sunlight is done by splitting water molecules to hydrogen and oxygen using electrons excited by the photons in the light. This can be described by four fundamental processes: charge carrier generation, charge carrier separation, charge carrier transport, and charge carrier transfer for catalysis. The effectiveness of these four will determine the total efficiency of the device. The potential that is needed to split water at the end is 1.229 V, but due to losses in the processes a higher potential will be needed in reality. For a good generation of charge carriers, the material where it takes place must therefore have a bandgap that is large enough to accomplish the required potential. The bandgap will then represent the energy that can be transferred from the electrons before the losses. The electron and hole must then be separated from each other, either by diffusion or an electric field in the material, and transported to the site where they act in the reaction. At the reaction, the charge carriers are finally transferred to the reactants at the electrolyte surface where they will act to split the water. Involved in this are two half-reactions: the oxidation of oxygen atoms in the water molecules, where oxygen gas is released, and the reduction of hydrogen atoms, where hydrogen gas is released. A catalyst could be used to decrease the overpotential for the reactions.

In the classical type of monolithic PEC-cells, all processes above takes place in the same material immersed into a water-based electrolyte. This has the advantage of being a simple design, but it creates high demands on the material since this has to be effective for all four processes. The device could therefore be divided into several materials, where the demand on each part is lower. This could for example be done by having electrodes of several layers where the catalysis for hydrogen and oxygen takes place at different sides of the only electrode and the light absorption happens between these. Another step towards the PV-electrolyzer is separating the anode from the cathode so that the hydrogen production and oxygen production takes place at different electrodes connected to each other with a wire, where only one of them has light absorption. With the wire, the distance for the electron transport increases, but the flexibility in the choice of materials and geometry is a big advantage which leads to very efficient devices. Another step is to separate also the other catalysis from the light absorption so that there are three electrodes connected to each other; one for light absorption, one for hydrogen production and one for oxygen production. If the photoelectrode is now placed outside the electrolyte we have come to the PV-electrolyzer since this is equivalent to a solar cell for light absorption and an electrolyzer for the reactions.

There is a lot of research on the two main types above, PEC-cells and PV-electrolyzers, as well as on the transitions between these devices. In spite of the obvious similarities they are however classified as separate research fields. By classifying PEC-cells and PV-electrolyzers as variations of the same theme, the research could focus on the challenges and goals they have in common, such as materials and processes, instead of defining differences between the two types. This way, the accumulated knowledge can be applied to all forms of photo driven catalysis.