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of process intensification by shifting the thermodynamic equilibrium of a reaction [77–79].
3.4.1 Proton Defects in Oxide Ceramics
In an environment containing hydrogen or water, protons are dissolved in the oxide lattice forming positively charged defects following Eq. (3.6) written in Kröger–Vink notation [78].
Table 3.1 Hydrogen‐selective membrane types [70, 71].
Source: Adapted from Kluiters [70] and Al‐Mufachi et al. [71].
| Dense polymer | Microporous ceramic | Dense metallic | Porous carbon | Dense ceramic | |
|---|---|---|---|---|---|
| Temperature range (°C) | <100 | 200–600 | 300–600 | 500–900 | 600–900 |
| H2 selectivity | Low | 5–139 | >1000 | 4–20 | >1000 |
| H2 flux (×10−3 mol m−2 s−1) DP = 100 kPa | Low | 60–300 | 60–300 | 10–200 | 6–80 |
| Stability issues | Swelling, compaction, mechanical strength | Stability in H2O | Phase transition | Brittle, oxidizing | Stability in CO2 |
| Poisoning issues | HCl, SO2, CO2 | H2S, HCl, CO | Strong adsorbing vapors, organics | H2S | |
| Materials | Polymers | Silica, alumina, zirconia, titania, zeolites | Pd alloy | Carbon | Proton conducting ceramics (mainly SrCeO3, BaCeO3) |
| Transport mechanism | Solution/diffusion | Molecular sieving | Solution/diffusion | Surface diffusion; molecular sieving | Solution/diffusion (proton conduction) |
| Development status | Commercial by air products, Linde, BOC, and Air Liquide | Prototype tubular silica membranes available up to 90 cm. Other materials only small samples (cm2) | Commercial by Johnson Matthey; prototype membrane tubes available up to 60 cm | Small membrane modules commercial, mostly small samples (cm2) available for testing | Small samples available for testing |
The equilibrium constant of proton defect formation reaction in oxide ceramic materials (KOH·) is depicted in Eq. (3.7), where
(3.7)
For large bandgap oxide materials (e.g. Ce, Ti, and Zr), the formation of proton defects at moderate temperatures takes places through the dissociative absorption of water [80]. Water dissociates into a hydroxide ion and a proton, the hydrogen ion then occupies an oxide ion vacancy, and the proton forms a covalent bond with a lattice oxygen. The formation of proton defects implies a significant weight gain; hence, the concentration of such defects can be measured by thermogravimetric analysis (TGA) as a function of temperature and water partial pressure.
3.4.2 Proton Transport Membrane Fundamentals
Understanding the mechanism of proton conduction is of utmost importance for the development of novel materials. It is generally accepted that proton diffusion in protonic conductors occur via the Grotthuss‐type mechanism assisted by water molecules [81–83]. Moreover, hydrogen separation is driven by the hydrogen partial pressure difference across the membrane.
Proton conductivity has been observed in different types of materials. Perovskite‐type oxide ceramics are known to be proton conductors since the early 1980s. In general, perovskite structure with a general formula A2+B4+O3 (type II–IV), where A is Ba and B is Zr, Tb, Ce, or Th, exhibits the best proton conductivities being higher than 10−2 S cm−1, the lowest activation energies for proton transport, and high negative hydration enthalpies [78]. In particular, ceramic materials such SrCeO3, BaCeO3, or SrZrO3 are the most widely studied high‐temperature proton‐conducting perovskite‐type materials. Zirconate‐based materials are more interesting than cerates regarding their application in CO2 environments because of their higher stability under reducing atmospheres; however, they present an important grain boundary resistance and a high sintering