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disadvantage of both families of materials, solid solutions of doped BaCeO₃ and BaZrO₃ have been developed by different research groups [84–87].

      The reader is referred to other literature sources to dwell into other examples of proton conducting materials such as rare earth oxides, rare earth ortho‐niobates and tantalates, rare earth tungstates, phosphates, and pyrochlores [75, 78, 88, 89].

Because of the low electronic conductivity, the abovementioned materials do not present a sufficiently high hydrogen permeation flux for their integration in practical applications. Composite membranes composed by two phases, an electronic and a protonic phase, have been developed in the past years to overcome this problem, obtaining promising permeation values [90–93].

      3.4.3 Application Concepts of Proton Conducting Membranes

      The applications of CCS using proton conducting membrane technologies has emerged as a hot topic to provide an industrial solution for the mitigation of the greenhouse effect. H₂‐related membranes can operate at intermediate and high temperatures (400–900 °C), and the processes in which they have been integrated to can be divided into (i) CO₂ reduction into valuable chemicals such as methane or methanol (catalytic membrane reactors [CMR]), (ii) conversion of chemicals into electrical energy (fuel cells), and (iii) generation of H₂ as a fuel.

SMR has proven to be one of the most energy‐efficient way to produce hydrogen from methane from an industrial point of view. SMR converts methane and water into syngas in an endothermic reaction (Eq. (3.8)); this step can be subsequently combined with water gas shift reaction (WGSR) to produce extra H₂ together with CO₂ (see Eq. (3.9)).

(3.8)

(3.9)

      Using hydrogen‐selective membranes, a pure H₂ stream is obtained, resulting in the shift of the thermodynamic equilibrium and hence in process intensification. The majority of the reported membranes are metal‐based membranes, i.e. Pd or Pd/Ag alloys [71]. However, these membranes do not achieve full CH₄ conversion nor H₂ permeation [94–96].

      WGSR at temperatures ranging from 700 to 900 °C have been performed using MPEC‐based membranes, SrCe_0.9Eu_0.1O_3−δ‐ and SrCe_0.7Zr_0.2Eu_0.1O_3−δ‐supported tubular membranes, yielding interesting results [97, 98]. By using the SrCe_0.7Zr_0.2Eu_0.1O_3−δ‐membrane, an increase of 77% in the CH₄ conversion as compared with thermodynamic conversion was obtained at 900 °C (H₂O/CO ratio = 1/1).

      The selective conversion of natural gas to higher hydrocarbons and aromatics remains an important industrial challenge. Non‐oxidative coupling of methane to produce olefins and aromatics (see Eq. (3.10)) are reactions limited thermodynamically. The selective extraction of H₂ will allow the shift of the thermodynamic equilibrium, i.e. the conversion toward the product side, giving rise to a significant improvement in the reaction yield. However, the H₂ extraction accelerates coking and catalyst deactivation.

(3.10)

Methane dehydroaromatization (MDA) reaction was performed by Li and coworkers [99] using a 2 μm dense membrane made of SrCe_0.95Yb_0.05O_3−δ and 4 wt% Mo/HZSM‐5 catalyst. A modest increase of the CH₄ conversion as compared with the conventional reactor was obtained at 720 °C. However, a slightly higher catalyst deactivation was also observed.

      Caro and coworkers studied the MDA reaction by using a U‐shape La_5.5W_0.6Mo_0.4O_11.25−δ [100] membrane and 6 wt% Mo/HZSM‐5 as a catalyst at 700 °C. Higher aromatics yield than that without membrane was obtained during the first five hours on the stream because of the important H₂ extraction, reaching 40–60% of the H₂ produced in the reaction. In this case, a catalyst deactivation was also observed, giving rise to aromatic yield lower than that without H₂ extraction after 10 hours on the stream.

3.5 Membranes for Electrochemical Applications

      As previously mentioned, hydrogen has been identified as a potential alternative fuel source and a key energy carrier for the near future energy supply with a low CO₂ footprint. Hydrogen can be used directly to produce energy (fuel cells) or be easily transformed into other forms of energy for different end applications. Nevertheless, pure hydrogen is not abundant in the atmosphere. Its production is generally accomplished by steam reforming, coal gasification, or the partial oxidation of some heavy hydrocarbons. On the other hand, CO₂‐neutral processes are also studied and are based on reforming, pyrolyzing, and fermentation of biomass, but efficiencies are not good enough for decreasing time‐to‐market. H₂ produced from water is an efficient, green, and commercial technology, where the common techniques include solar thermochemical or photocatalytic water splitting and electrical water electrolysis. One advantage of such water electrolysis systems is the possibility to perform CO₂ electrolysis or H₂O/CO₂ co‐electrolysis to produce syngas [101] (H₂ and CO). The opportunity to conduct H₂O and CO₂ co‐electrolysis is a very interesting way for the production of clean synthetic fuels: power to gas and power to fuel technologies. Therefore, these technologies have been pointed out as promising alternatives to store energy from renewable intermittent energy sources.

      3.5.1 Electrolysis and Co‐electrolysis Processes

      The equivalent energy required for the water and carbon dioxide split reaction (ΔH) is determined from the free energy and the entropy as follows:

      (3.11)

where the electrical energy needed for the electrolysis will be defined by the free energy term (ΔG), whereas the thermal energy is defined by the entropy term (ΔS). As can be seen in Figure 3.6, the enthalpy of the water splitting reaction drops sharply at 100 °C because of the phase transition from liquid water to steam. In addition, the electrical energy needed for the electroreduction of water and carbon monoxide decreases as the temperature is increased. Furthermore, as temperature increases, the enthalpy of the CO₂ electrolysis decreases. Therefore, by increasing temperature and operating at higher temperatures, the system will work more efficiently. Reaction kinetics are also enhanced at higher temperatures, which indicates higher H₂ or/and CO production at the same operation voltage.

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