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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]. is a positively charge (2+) oxygen vacancy, represents an oxygen atom with a neutral charge placed on its original place in the crystal lattice, and is a proton defect (+1 charged).

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

(3.6)

      The equilibrium constant of proton defect formation reaction in oxide ceramic materials (K_OH·) is depicted in Eq. (3.7), where represents the proton defect concentration, is the oxygen vacancy concentration, represents the concentration of oxygen atoms with a neutral charge placed on its original place in the crystal lattice, and p_H2O is the water vapor pressure.

(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

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