Electrolysis is a method for production of a chemical reaction that is electrically driven by passage of an electric current, typically a direct current (DC), through an electrolyte between an anode electrode and a cathode electrode. An electrochemical cell is used for electrochemical reactions and comprises anode and cathode electrodes immersed in an electrolyte with the current passed between the electrodes from an external power source. The rate of production is proportional to the current flow in the absence of parasitic reactions. For example, in a liquid alkaline water electrolysis cell, a direct current (DC) is passed between two electrodes in an aqueous electrolyte to split water (the reactant) into the component product gases: hydrogen and oxygen where the product gases evolve at the surfaces of the respective electrodes.
The achievement of a preselected level of production involves a trade-off between increasing the operating current density and increasing the number of the cells. Due to the physical nature of the electrolytic processes, the higher the current density, the higher the energy consumption per unit of production, and so the trade-off facing the cell designer is whether to bear the increase in capital cost of more cells or to pay higher operating costs through reduced energy efficiency. Increasing current density will also lead to more stressful operating conditions such as higher electrolyte temperature that will impose additional design requirements and added costs. In the final analysis, the trade-off is determined on a case-by-case basis by the external variables primarily driven by the cost of electricity.
In the conventional bi-polar electrolyser a voltage is applied between the end electrode of a stack of electrode plates. One side of a plate acts as an anode and produces oxygen and the other side acts as a cathode producing hydrogen in the case of electrolysis. The key implications to this in terms of current flow is that the current flow is through the stack perpendicular to the plane of the electrode (the plane of the electrode defined by the gas evolving surfaces of the electrodes) and importantly that the current flow is contained within the cell stack. Current flows in the electrode from all edges of the electrode towards the centre of the electrode plate.
In the conventional mono-polar cell design presently in wide commercial use today, one cell or one array of (parallel) cells is contained within one functional electrolyser, or cell compartment, or individual tank. Therefore each cell is made up of an assembly of electrode pairs in a separate tank where each assembly of electrode pairs connected in parallel acts as a single electrode pair. The connection to the cell is through a limited area contact using an interconnecting bus bar such as that disclosed in Canadian Patent Number 302,737 issued to A. T. Stuart (1930). The current is taken from a portion of a cathode in one cell to the anode of an adjacent cell using point-to-point electrical connections using the above-mentioned bus bar assembly between the cell compartments. The current is usually taken off one electrode at several points and the connection made to the next electrode at several points by means of bolting, welding or similar types of connections and each connection must be able to pass significant current densities. Current flows from the point of connection over the area of the electrode. Current in the electrode flows only in the plane of the electrode. Current between cells occurs outside the nominal cell stack as each cell is in a separate tank. A drawback to such connections is that they are prone to oxidation and other types of degradation resulting in significant potential drops between cells which reduce the efficiency of the electrolyser.
Most filter press type electrolysers insulate the anodic and cathodic parts of the cell using a variety of materials which may include metals, plastics, rubbers, ceramics and various fibre based structures. In many cases, O-ring grooves are machined into frames or frames are moulded to allow O-rings to be inserted. Typically at least two different materials form the assembly necessary to enclose the electrodes in the cell and create channels for electrolyte circulation, reactant feed and product removal. One of the materials is, for example, a hard engineering plastic and the other a material soft enough to allow sealing to be achieved. In large bipolar filter press systems, cell stacks could be many tens of meters in length. Such systems require hard and rigid materials with compatible coefficients of thermal expansion and minimal temperature/pressure related creep.
It would be very advantageous to provide an electrochemical system which eliminates the need for external contacts connecting adjacent electrodes, which avoids the drawbacks to conventional monopolar and bipolar systems but incorporates the advantages of each into a system, and which reduces the number of components making up the system.
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