PEM electrolyzers

The best catalyst material for hydrogen evolution is platinum typical loadings are in the range of l-2mgcm. Suitable catalyst materials for the evolution of oxygen with acidic electrolytes are, in order of their catalytic activities, Ir/Ru Ir Rh Pt. It is also possible to use oxides and mixtures of the metals [49, 50]. 

The best performance was achieved with platinum for hydrogen evolution and iridium for oxygen evolution. The catalyst loading was between 0.5 and 3 mg cm . Cobalt clathrochelates were also tested for oxygen evolution, but their performance is much lower [51]. Typical voltages at 80-90 °C and 0.1 MPa are given in Table 8.7. 

As mentioned in Section 8.4, it is useful to improve the round-trip efficiency by applying the pressure for hydrogen and oxygen storage on to the liquid water. This 

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Figure 3. Picture of HyLYZER 2.0 Module PEM electrolyzer developed by Hydrogenics Corp.

Early research of ionomer membrane degradation was conducted in the context of PEM electrolyzers. The detection of fluoride and other chain fragments in the condensed effluent water indicates the decomposition of PFSA ionomer and has long been noticed. Baldwin15 reported the detection of fluoride in the effluent of PEM electrolyzer and believed that it is the result of membrane mechanical failure. Extensive research has been conducted to elucidate the reaction pathways for membrane decomposition. Many controversial results and mechanisms have been reported in the literature, demonstrating the complex nature and the current inadequate understanding of the membrane degradation mechanisms. 

TABLE 1. Summary of strengths and weaknesses of alkaline and PEM electrolyzers [3]... 

In Burlington, Vermont, at the Department of Public Works hydrogen fuel station, wind turbines and an H-series PEM electrolyzer from Proton Energy Systems are used to produce 12 kg/d of H2. Air Products and Chemicals participated in the design of this wind-to-hydrogen installation. 

A PEM electrolyzer is literally a PEM fuel cell operating in reverse mode. When water is introduced to the PEM electrolyzer cell, hydrogen ions (protons) are drawn into and through the membrane, where they recombine with electrons to form hydrogen molecules. Oxygen gas remains behind in the water. As this water is recirculated, oxygen accumulates in a separation tank and can then be removed from the system. Hydrogen gas is separately channeled from the cell stack and captured. 

The energy required in the theoretical efficiency limit of any water electrolysis process is 39.4 kWh per kilogram. PEM electrolyzers operating at low current density can approach this efficiency limit. However, the quantities of hy-... 

A second commercially available electrolyzer technology is the solid polymer electrolyte membrane (PEM). PEM electrolysis (PEME) is also referred to as solid polymer electrolyte (SPE) or polymer electrolyte membrane (also, PEM), but all represent a system that incorporates a solid proton-conducting membrane which is not electrically conductive. The membrane serves a dual purpose, as the gas separation device and ion (proton) conductor. High-purity deionized (DI) water is required in PEM-based electrolysis, and PEM electrolyzer manufacturer regularly recommend a minimum of 1 MQ-cm resistive water to extend stack life. 

PEM technology was originally developed as part of the Gemini space program.16 In a PEM electrolyzer, the electrolyte is contained in a thin, solid ion-conducting membrane rather than the aqueous solution in the alkaline electrolyzers. This allows the H+ ion (proton) or hydrated water molecule (HsO+) to transfer from the anode side of the membrane to the cathode side, and separates the hydrogen and oxygen... 

The PEM cell design chosen for tlie current work employs a significantly different geometry than the Westinghouse cell. The PEM electrolyzer consists of a membrane electrode assembly (MEA) inserted between two flow fields. Behind each flow field is a back plate, copper current collector and stainless steel end plates. The MEA consists of a Nafion proton-exchange-membrane with catalyst-coated gas diffusion electrodes bonded on either side. 

Solid electrolyte PEM (proton exchange membrane) electrolyzers can be used in systems to avoid use of caustics as an added safety factor and where no one is available to frequently monitor a fluid electrolyte system. PEM electrolyzers are much more expensive, and do not have the track record that alkaline electrolyzers have in use. Although they are reportedly almost trouble free during use, they do pose problems in terms of cost of replacement parts when they become inoperable. Failures in PEM electrolyzers are usually membrane blow-outs or catalyst degeneration. Both problems are costly to service with replacement parts. 

Cost was an important consideration for us in the development of our experimental solar hydrogen system, so a PEM electrolyzer was discounted in favor of a low pressure alkaline tank electrolyzer. High pressure and high temperature electrolyzers were also evaluated. These were discounted due to higher amounts of energy consumed and the need for more intensive monitoring of the system. 

The new advances in the development of novel technologies in PEM electrolyzers are based on the filtration-type configuration that has been conceived in the late 1970s. Before 1975, the PEM... 

For several decades, commercial alkaline electrolyzers have been available in a variety of series with outputs of up to approximately 750 Nm h hydrogen. Product development for PEM electrolysis only began around 25 years ago, and thus there are fewer commercial systems (<30 Nm h ) available compared to alkaline electrolysis. However, aU major manufacturers of PEM electrolyzers are currently developing and constructing 1 MW systems (see Table 11.4). High-temperature electrolysis is currently being pursued only sporadically by industry, which means that some demonstration systems exist, but no commercial products are yet available. 

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