Electrolytes protonic conductors

The Chemical Process Engineering Research Institute (CPERI) works on bench and pilot plant hydrogen production units, SOFCs, polymer electrolyte proton conductors, and high-temperature electrocatalytic processes. 

The majority of solid electrolyte sensors are based on proton conductors (Miura et al. 1989, Alberti and Casciola 2(X)1). Metal oxides that can potentially meet the requirements for application in solid electrolyte sensors are listed in Table 2.7. These proton condnctors typically do not have high porosity but rather can reach 96-99% of the theoretical density (Jacobs et al. 1993). Similar to oxygen sensors, solid-state electrochemical cells for hydrogen sensing are typically constructed by combining a membrane of solid electrolyte (proton conductor) with a pair of electrodes (electronic conductors) Most of the sensors that use solid electrolytes are operated potentiometrically. The voltage produced is from the concentration dependence of the chenucal potential, which at eqnihbrium is represented by the Nemst equation (Eq. 2.3). 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Nevertheless there are some reactions which never change. Thus NO reduction on noble metals, a very important catalytic reaction, is in the vast majority of cases electrophilic, regardless of the type of solid electrolyte used (YSZ or P"-A1203). And practically all oxidations are electrophobic under fuel lean conditions, regardless of the type of solid electrolyte used (YSZ, p"-Al203, proton conductors, even alkaline aqueous solutions). 

Significant advances have been made in this decade in electrochemical H2 separation, mostly through the use of solid polymer electrolytes. Since the overpotentials for H2 reduction and oxidation are extremely low at properly constructed gas diffusion electrodes, very high current densities are achievable at low total polarization. Sedlak [13] plated thin layer of Pt directly on Nafion proton conductors 0.1-0.2cm in thickness, and obtained nearly 1200 mA/cm2 at less than 0.3 V. The... 

Interest in new solid polymer electrolytes has driven some research groups to investigate other materials containing proton conducting moieties aside from sulfonic acid. Polymers and copolymers from monomers containing phosphonic-based proton conductors have been reported. Phosphonic and/or phosphinic acid containing polymers have not been well studied because of the rather limited synthetic procedures available for their preparation, compared with sulfonic acid derivatives. Miyatake and Hay... 

Figure 29. Conductivity of some intermediate-temperature proton conductors, compared to the conductivity of Nafion and the oxide ion conductivity of YSZ (yttria-stabilized zirconia), the standard electrolyte materials for low- and high-temperature fuel cells, proton exchange membrane fuel cells (PEMFCs), and solid oxide fuel cells (SOFCs).

Proton exchange membrane fuel cell (PEMFC) working at around 70 °C with a polymer membrane electrolyte, such as Nafion, which is a solid proton conductor (conducting by the H + cation). 

Solid-Electrolyte Hydrogen Sensor. Most of solid gas sensors so far developed need high temperature operation because of limited ionic conductivities when the electrolyte is near room temperature. If solid electrolytes with sufficiently large ionic conductivities are available, unique gas sensors operative near room temperature can be fabricated. An example is the following proton conductor hydrogen sensor proposed by our group (10, 11). 

It has been reported (4,5) that solid electrolyte sensors using stabilized zirconia can detect reducible gases in ambient atmosphere by making use of an anomalous EMF which is unusually larger than is expected from the Nernst equation. However, these sensors should be operated in a temperature range above ca. 300°C mainly because the ionic conductivity of stabilized zirconia is too small at lower temperatures. On the other hand, solid state proton conductors such as antimonic acid (6,1), zirconium phosphate (8), and dodecamolybdo-phosphoric acid (9) are known to exhibit relatively high protonic conductivities at room temperature. We recently found that the electrochemical cell using these proton conductors could detect... 

Fig. 4.26 The elements of the hydrogen fuel cell. Note-, (i) The student is reminded that a chemical species which loses electrons is oxidized one that gains electrons, reduced, (ii) The proton H is transported through the polymer electrolyte attached to water molecules, (H20)nH which causes a water management problem. Current research is aimed at developing proton conductors able to operate in the region of 200 °C when the proton migrates unattached to water molecules.

In the case of the high temperature SOFC discussed below the principles outlined above equally apply. The technical differences are that the cell runs typically on hydrocarbon fuels (e.g. natural or coal-gas) and that the electrolyte is an oxygen ion conductor rather than a proton conductor. The complex fuel molecules, in the presence of the water molecule and at the high operating... 

The PEFC was first developed for the Gemini space vehicle by General Electric, USA. In this fuel cell type, the electrolyte is an ion-exchange membrane, specifically, a fluorinated sulfonic acid polymer or other similar solid polymer. In general, the polymer consists of a polytetrafluoroethylene (Teflon) backbone with a perfluorinated side chain that is terminated with a sulfonic acid group, which is an outstanding proton conductor. Hydration of the membrane yields dissociation and solvation of the proton of the acid group, since the solvated protons are mobile within the polymer. Subsequently, the only liquid necessary for the operation of this fuel cell type is water [7,8],... 

Currently, the most widely applied electrolyte in PEFCs is Nation, manufactured by DuPont, Dow Chemical, Midland, MI, USA and other chemical companies. The Nation polymer electrolyte is a good proton conductor. Besides, it has very low electron conductivity, and is gas impermeable in order to provide the necessary spatial separation between the anode oxidation and the cathode reduction reactions. 

Many - gas sensors based on - solid electrolytes operate under potentiometric conditions [iii]. The sensors for oxygen use oxide -> conductors, such as ZrC>2 -based ceramic, those for halogens use halide conductors (e.g., KAg s), while -> hydrogen sensors use protonic conductors. There are sensors for C02, N02, NH3, S03) H2S, HCN, HF, etc. (see -> lambda probe). 

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. 

Iwahara, H. et al.. High temperature type proton conductor based on SrCeOj and its application to solid electrolyte fuel cells. Solid State Ionics, 9/10, 1021-1026 (1983). 

Concerning proton conductors, Govind and Zaho [100] stated that metal-based membranes could be outperformed by solid electrolyte membranes based on materials... 

Formally, ammonia synthesis is closely related to Fischer-Tropsch synthesis. Industrial operation involves the use of an iron catalyst promoted with calcium and potassium oxides. However, the reason we consider this process here is not directly in connection with alkali promotion of the catalyst. We are concerned with a remarkable achievement reported by Yiokari et al. [15], who use a ton-conducting electrolyte to achieve electrochemical promotion of a fully promoted ammonia synthesis catalyst operated at elevated pressure. Specifically, they make use of a fully promoted industrial catalyst that was interfaced with the proton conductor CaIno.iZro.903-a operated at 700K and 50 bar in a multipellet configuration. It was shown that under EP the catalytic rate could be increased by a factor of 13 when... 

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