Monolithic electrochemical systems are previously known in the art. A monolithic electrochemical system is an electrochemical system where the working electrode and the counter electrode are assembled in a single integrated body. The working electrode and the counter electrode are separated by means of an intermediate layer of a porous insulating material.
The working and counter electrodes are made of porous structures and an electrolyte is at least partially filled in the porous structure, which is a monolithic structure comprising a layer forming the working electrode, a layer forming the counter electrode and an insulating layer separating the counter electrode from the working electrode.
An early example of a monolithic photo-electrochemical system is disclosed in WO97/16838 which relates to a battery of photovoltaic cells consisting of a monolithic assembly of a plurality of serial-connected photovoltaic cells.
Traditionally photo-electrochemical systems includes a first substrate provided with a first electrode and a second substrate provided with a second electrode. The first and second substrates are positioned such that the electrodes are facing each other while leaving a small gap in between. In order to ensure that the gap is of a magnitude within a desired range, a spacer can be used to separate the substrates by a certain distance. The system is sealed at the edges of the first and second substrate and between adjacent electrically connected cells to prevent the electrolyte from making connections between the cells. In order to create photo-electrochemical systems having uniform properties over the entire active area of the system that is over the complete area of the electrodes, it is essential that the distance between the electrodes are kept within a narrow range, which makes production difficult. In order to avoid the electrolyte from making contacts between adjacent cells, the horizontal positioning of the two substrates must be made with very high precision which makes production difficult. A further drawback with this traditional, bilithic type of photo-electrochemical system is that electrolyte normally has to be introduced after assembly of the system. The opening of the passages where the electrolyte is introduced must be well-sealed after the introduction of the electrolyte to prevent the electrolyte from leaking and e.g. water to penetrate the cell. Separate openings are required for each cell resulting in a large number of openings for a system with many cells which makes production difficult. Introduction of electrolyte via narrow passages into the essentially closed space between the substrates may lead to creation of air pockets in the system or to uneven distribution of electrolyte, which both deteriorate the quality of the system.
Monolithic electrochemical systems has shown to provide for a very compact and simple design, where construction of the electrochemical system is made possible without the necessity of the first substrate being positioned at a specific distance from the second substrate. In this case, the electrochemical system can be constructed by a multilayer structure being applied to a substrate, after which the electrochemical system is closed. The light absorbing dye and the electrolyte are preferably introduced before closing of the electrochemical system. The structure can suitably be closed by means of a flexible sheet of at least one polymer layer, which is preferably applied to said structure in the presence of heat and sub-atmospheric pressure.
Experiments with monolithic photo-electrochemical systems have shown that it is difficult to manufacture arrays of monolithic photo-electrochemical cells on a substrate such that the cells are given identical properties or properties within a desired range, even though they are manufactured simultaneously on the same substrate. A single photo-electrochemical cell is characterised by its current-voltage-characteristics. The current-voltage-characteristics for a cell vary with the light intensity and the light spectrum. Important data that describe the current-voltage-characteristics of a photo-electrochemical cell are the short-circuit current (Isc), the open-circuit voltage (Uoc), and the maximum power point (Pmax) which is the highest energy output the cell can give under the specific light conditions that were used in the measurement. The term fill-factor (ff) is often used to describe the curve as ff=Pmax/(Uoc*Isc). In order to reduce differences in the current-voltage-characteristics of individual cells arranged on a common substrate, the requirements on production tend to become increasingly stricter requiring higher quality of the purity of the chemical components, the production environment, and the production processes. Such measures lead to a much more expensive production. An important example is the necessity of having a perfect control of the deposition of the working electrode, the insulating layer, and the counter electrode, in order to avoid the counter electrode from partially penetrating the insulating layer touching the working electrode and/or the intermediate conducting layer on the substrate, causing energy losses and thus differences of the current-voltage characteristics of the individual cells on a common substrate. This becomes even more critical since a thin insulating spacer layer is desirable to obtain the best cell performance since it simplifies the diffusion of the redox-couple containing electrolyte between the two working and the counter electrodes.
The porous counter electrode of a monolithic photo-electrochemical system should: 1) be a good electrical conductor to avoid energy losses during electron transport through the counter electrode; 2) be a good catalyst for the redox-couple of the electrolyte, and 3) have a good adhesion to the intermediate conducting layer on the substrate. Experiments with monolithic photo-electrochemical systems have shown that it is difficult to combine these three properties in one counter-electrode material. When trying to combine these three properties in one porous counter electrode material, at least one has always performed insufficiently resulting in energy losses and thus reduced efficiency of the monolithic photo-electrochemical system.
The electrodes in a monolithic photo-electrochemical system are conventionally formed via selective deposition of pastes in e.g. screen-printing processes whereupon the electrodes are sintered in order to burn off organic residues of the pastes and to create the electrical contact between the particles of the porous electrodes. This is in prior art made in one sintering process after that the electrode layers have been deposited. Experiments with monolithic photo-electrochemical systems have shown that the properties of the working electrode and the counter electrode are depending of the sintering temperature and that different layers have different optimum sintering temperatures.