Bipolar membrane electrode

The bipolar membranes are used in a more or less conventional ED stack together with conventional unipolar membranes. Such a stack has many acid—alkah producing membranes between a single pair of end electrodes. The advantages of the process compared to direct electrolysis seem to be that because only end electrodes are required, the cost of the electrodes used in direct electrolysis is avoided, and the energy consumption at such electrodes is also avoided. 

Much more simply, the same result can be attained with bipolar membranes, membranes consisting of an anion- and cation-permeable (an anion- and cation-exchange) membrane laminated together. At such a membrane, when mounted between electrodes so that the cation-exchange layer faces the anode, water is split into and OH ions so that the acidic and alkaline solutions required for regeneration as above are produced at the respective surfaces of the bipolar membrane. When such membranes are suitably integrated into the sequence of membranes in the electrodialysis unit above, gas evolution at the electrodes is not needed the acid-base pair is produced with about half the power. 

The costs of a PEMFC stack are composed of the costs of the membrane, electrode, bipolar plates, platinum catalysts, peripheral materials and the costs of assembly. For the fuel-cell vehicle, the costs of the electric drive (converter, electric motor, inverter, hydrogen and air pressurisation, control electronics, cooling systems, etc.) and the hydrogen storage system have to be added. Current costs of PEM fuel-cell stacks are around 2000/kW, largely dominated by the costs of the bipolar plates and... 

Cost targets exist for all parts of the fuel cell for bipolar plates, from 10/kW (2004) to 3/kW in 2015 for electrocatalysts, from 40/kW (2005) to 3/kW in 2015 and for membrane electrode assemblies (MEA), from 50/kW (2005) to 5/kW in 2015 (Freedom Car, 2005 these cost targets are somewhat different from those mentioned by the IEA (2005)). Since 2004, the number of fuel-cell cars has been growing and at the time of writing they numbered approximately 1000 worldwide there are also around 100 fuel-cell buses in use worldwide in several demonstration projects. But these cars are produced as individual (hand-built) models and are extremely expensive, with production costs per vehicle currently estimated at around one million large-scale production is not expected before 2015, see Section 13.1. 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

Bipolar membranes consist of an anionic and a cationic membrane laminated together [13]. When placed between two electrodes, as shown in Figure 10.19, the interface between the anionic and cationic membranes becomes depleted of ions. The only way a current can then be carried is by the water splitting reaction, which liberates hydrogen ions that migrate to the cathode and hydroxyl ions that... 

The key element in electrodialysis with bipolar membranes is the bipolar membrane. Its function is illustrated in Figure 5.11(a), which shows a bipolar membrane consisting of an anion- and a cation-exchange layer arranged in parallel between two electrodes. 

An elementary PEMFC comprises several elements and components the membrane-electrode assembly (MEA), the flow-field plate (bipolar plate, which also ensures electric contact with the next cell), gaskets to ensure tightness to reactants and end plates (Figure 9.4). 

Fig. 1. Schematic presentation of a PEFC cross-section. The cell (left) consists of a membrane catalyzed on both sides (referred to as a membrane/electrode (M E) assembly ), gas-diffusion backing layers and current collectors with flow fields for gas distribution. The latter become bipolar plates in a fuel cell stack. The M E assembly described schematically here (right) shows catalyst layers made of Pt/C catalyst intermixed with ionomer and bonded to the membrane (large circles in the scheme correspond to 10 nm dia. carbon particles and small circles to 2 nm dia. platinum particles).

Fig. 8 S implified cross section through a single element of a bipolar membrane cell. The electrodes are supported by corrugated bands, which are in turn supported by contact plates. Compression of the single cells together gives the series connections for the stack [14, p. 442],...

Ayers and Farley employed an electrolytic cell in which a Pd membrane worked as a bipolar electrode. CO2 is cathodically reduced at the front side of the Pd membrane electrode with hydrogen atoms cathodically produced and dissolved at the back side. Hydrogen atoms permeate through the Pd membrane to CO2 reduction side. Both cathodic reactions can be regulated independently by two electrochemical systems. They reported that methanol was formed at low electrolytic current densities. 

Cells usually have a bipolar configuration. The electrocatalysts are bonded to each side of the membrane (15), and the resulting SPE is a structurally stable membrane-electrode assembly as shown in Figure 1. A multi-layer package of expanded metal screens which presses up against the electrode on one side serves as the current collector and fluid distributor. 

The current efficiency of acid/base generation and the purity of the acid and base made with bipolar membranes drops off as concentrations increase, because Donnan exclusion diminishes with increasing solution concentrations. Further, the production rate is limited by the rate of diffusion of water into the bipolar membrane. Nevertheless, there are substantial advantages to the process. Since there are no gases evolved at the bipolar membranes, the energy associated with gas evolution is saved, and the power consumption is about half that of electrolytic cells. Compared to the electrodes used in conventional electrolytic cells, the bipolar membranes are inexpensive. Where dilute (e.g., 1 N) acids or bases are needed, bipolar membranes offer the prospect of low cost and minimum unwanted by-products. 

Design and demonstrate a reformate-capable fuel cell stack, utilizing CO-tolerant membrane electrode assemblies (MEAs) and low cost bipolar collector plates. 

If an electrical potential difference is established between the electrodes all charged components will be removed from an aqueous interphase between the two ion-exchange layers. If only water is left in the solution between the membranes further transport of electrical charges can only be accomplished by protons and hydroxyl ions which are available in very low concentrations in completely de-ionized water. Protons and hydroxyl ions removed from the interphase arc replenished because of the water dissociation equilibrium. A bipolar membrane thus consists of a cation- and anion-exchange layer laminated together. 

Membrane-electrodes composite 7 Anodic support collector 8 Bipolar collector 9 Anodic end plate 10 Anode side bus bar... 

The technological development of electrolyzers started with a mono polar cell consisting of a cathode part and an anode part separated by a diaphragm, hi multi-cell systems, bipolar plates are used carrying the cathode material for one cell and on its backside the anode material for the neighbor cell. The functions of the bipolar plate are the continuous supply of the membrane electrode with H2 on one side and with O2 or air on the other side and the regulation of the water balance by providing moisture for the membrane on the H2 side and remove the product water on the O2 side. 

The PEM fuel cell is principally buUt up by the bipolar plates and the gas diffusion layers with the membrane-electrode assembly (MEA). Located on both... 

The peripheral equipment needed for direct methanol fuel cells is largely analogous to that of polymer electrolyte membrane fuel cells. The mechanical basis of fuel cells and stacks on the whole consists of bipolar plates between which the sandwiched membrane-electrode assemblies are arranged. For the venting of heat, cooling plates with a circulating heat transfer agent are set up in a particular order between individual fuel cells in the stack. 

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