Figure 1A proton exchange membrane (PEM) water electrolysis cell is a device which produces hydrogen and oxygen gas by using DC electricity to electrochemically split water. The cell is named for the electrolyte, which is a solid conductive polymer. The most commonly used proton exchange membrane is based upon a perfluorosulfonic acid (PFSA) type material, although there are several lower cost alternative membranes under development. Figure 1 illustrates the PEM electrolysis reaction.

In the electrolysis cell, the water enters the anode and is split into hydrogen ions (protons), electrons, and oxygen gas. The protons are conducted through the membrane while the electrons pass through the electrical circuit.  The oxygen is carried from the cell with excess water flow. At the cathode, the protons and electrons recombine to form hydrogen gas. The flow of protons also results in water being dragged across the membrane to the cathode. The extent to which this water is removed from the hydrogen down stream of the cell determines the hydrogen purity. The electrolysis half-reactions are shown in the equation (Figure 2). 

A PEM cell contains an active area in which the presence of a catalyst permits the reactions to take place. The active area is surrounded by a seal area which is used as a manifold to enable the entrance and exit of reactants and products, to isolate these fluid streams from each other, to provide the overboard seal of pressure and fluids, and to provide the structure for holding internal cell pressure. The structure of the active area of PEM cells consists of the membrane, the anode and cathode electrodes, and the anode and cathode flow fields. Precious metal catalysts are typically used at both the anode and cathode. Due to the high operating potentials, carbon materials tend to corrode, and therefore are not generally used for things such as catalyst supports or flow field plates. PEM electrolysis cells use metallic components instead.

Figure 2The flow fields distribute the flow of reactants and permit the products to exit. In addition, the flow fields operate as current collectors, electrically connecting to the external circuit.

In practical implementations, individual electrolysis cells are assembled into stacks of cells. When these cells are stacked in a bipolar arrangement, the anode of one cell is adjacent to the cathode of the next cell. The anode of one cell is electrically connected to, but fluidically isolated from, the cathode of the next cell. In this way, the cells are electrically in series while the fluids follow a parallel circuit within the stack (Figure 3). The electrolysis stack is typically water-cooled by circulating excess reactant water through the cells.

Figure 3
Because electrolysis cells are assembled in stacks, they are very scalable and modular. For example, with Proton’s commercial hydrogen generators, the same basic cell is configured in 10-cell, 20-cell, and 34-cell stacks depending on the required output. Proton’s H-series system uses three 34-cell electrolysis stacks together, introducing modularity at a stack level. So, depending on the range of power and recharge rate required, the stack is highly scalable.

A unique capability of PEM electrolysis is the ability to generate hydrogen at elevated differential pressure. Designed with proper support of the proton exchange membrane, appropriate sealing, and allowances for material creep, generation pressures from 13 bar to 200 bar (200psi to 3,000psi) have been demonstrated. Pressurization of the hydrogen results in cell voltages that are slightly elevated, as predicted by the Nernst equation. This voltage increase corresponds to work under isothermal compression, representing the most efficient form of gas compression. The only appreciable loss is hydrogen diffusion back through the proton exchange membrane to the lower pressure side of the cell.

In addition to the electrochemical cell stack, other components of a PEM electrolysis system include a power supply/voltage regulator, a water pump and water supply system, water-gas separators for hydrogen and oxygen, a heat exchanger, controls, and instrumentation. Included with the controls are safety features that monitor the system operation to ensure safe operation.
 

Proton Energy Systems
Wallingford, CT
protonenergy.com