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 temperature is needed to produce dense samples. In order to overcome the 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)).
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, SrCe0.9Eu0.1O3−δ‐ and SrCe0.7Zr0.2Eu0.1O3−δ‐supported tubular membranes, yielding interesting results [97, 98]. By using the SrCe0.7Zr0.2Eu0.1O3−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 SrCe0.95Yb0.05O3−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 La5.5W0.6Mo0.4O11.25−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
