Exceptional hydrogen permeation of all-ceramic composite robust membranes based on BaCe 0.65 Zr 0.20 Y 0.15 O 3 (cid:2) d and Y-or Gd-doped ceria †

Mixed proton and electron conductor ceramic composites were examined as hydrogen separation membranes at moderate temperatures (higher than 500 1 C). In particular, dense ceramic composites of BaCe 0.65 Zr 0.20 Y 0.15 O 3 (cid:2) d (BCZ20Y15) and Ce 0.85 M 0.15 O 2 (cid:2) d (M = Y and Gd, hereafter referred to as YDC15 and GDC15), as protonic and electronic conducting phases respectively, were successfully prepared and tested as hydrogen separation membranes. The mixture of these oxides improved both chemical and mechanical stability and increased the electronic conductivity in dual-phase ceramic membranes. The synthetic method and sintering conditions were optimized to obtain dense and crack free symmetric membranes. The addition of ZnO as a sintering aid allowed achieving robust and dense composites with homogeneous grain distribution. The chemical compatibility between the precursors and the influence of membrane composition on electrical properties and H 2 permeability performances were thoroughly investigated. The highest permeation flux was attained for the 50:50 volume ratio BCZ20Y15–GDC15 membrane when the feed and the sweep sides of the membrane were hydrated, reaching values of 0.27 mL min (cid:2) 1 cm (cid:2) 2 at 755 1 C on a 0.65 mm thick membrane sample, currently one of the highest H 2 fluxes obtained for bulk mixed proto-nic–electronic membranes. Increasing the temperature to 1040 1 C, increased the hydrogen flux up to 2.40 mL min (cid:2) 1 cm (cid:2) 2 when only the sweep side was hydrated. The H 2 separation process is attributed to two cooperative mechanisms, i.e. proton transport through the membrane and H 2 production via the water splitting reaction coupled with oxygen ion transport. Moreover, these composite systems demonstrated a very good chemical stability under a CO 2 -rich atmosphere such as catalytic reactors for hydrogen generation.


Introduction
Nowadays, industrial production of hydrogen is mainly conducted by steam reforming of methane (SMR).][3] This journal is © The Royal Society of Chemistry 2015 In this field, mixed proton and electron conductor (MPEC) oxides have recently attracted much interest as materials for dense separation membranes.These systems have the enormous advantage of being 100% selective towards H 2 avoiding subsequent purification procedures (and additional costs).Moreover, they can be directly integrated into the reforming or gasification plants. 4Under the non-galvanic conditions (no external power supply), both protons and electrons diffuse across the membrane using a H 2 partial pressure gradient as driving force.The H 2 flux is proportional to the ambipolar protonic-electronic conductivity. 5Therefore, MPEC oxides suitable for H 2 separation must be excellent protonic and electronic conductors.6][7][8] Several perovskite-type materials exhibit reasonable proton conductivity in a hydrogen atmosphere at temperatures higher than 500 1C. 9,10Barium cerate ceramics show the highest proton conductivity, particularly when doped with 15% or 20% of Y, reaching values around 10 À2 O À1 cm À1 at 600 1C. 9,11][19] The electronic conductivity of these materials could be enhanced through doping the B-site on the ABO 3 perovskite ideal structure ( xii A 2+ iv B 4+ O 3 ) with metals of variable oxidation states.However, this strategy could be detrimental to other key properties such as the protonic transport. 5On the other hand, the development of a dualphase dense composite membrane by adding a secondary phase as an electronic conductor to the proton conducting perovskite could overcome these drawbacks.Indeed, composites have the advantage of providing a wide range of materials with different functional properties.1][22][23][24][25][26] Another approach is to develop dual-ceramic composite membranes based on all-ceramic (cer-cer) composites, formed by a phase being predominantly an electronic conductor (of electrons or electron holes under H 2 separation conditions, that is n-type or p-type semiconducting ceramics) and another phase as a proton conductor.The all-ceramic membranes offer significant advantages in terms of mechanical and chemical stability over competing membrane concepts such as cer-met.3][34] The addition of doped ceria to barium cerate or cerate-zirconate above the percolation limit ensures the electronic transport across the H 2 separation membrane under operational conditions (i.e.high temperatures and reducing atmospheres).Moreover, doped ceria enhances the stability against CO 2 and H 2 O of the cerate material in the cer-cer composite because it shifts the equilibrium of the degradation reaction towards the reactant side, thereby preserving the desired perovskite composition. 29,35n the present work, symmetric dense cer-cer membranes based on BaCe 0.65 Zr 0.20 Y 0.15 O 3Àd (BCZ20Y15) and Ce 0.85 M 0.15 O 2Àd (M = Y and Gd, hereafter referred to as YDC15 and GDC15) were explored as hydrogen separation membranes at temperatures higher than 500 1C.Aiming at obtaining a good compromise between protonic and electronic conductivity with suitable chemical stability, compositions containing 50% and 60% of volume fraction of the BCZ20Y15 phase were investigated (hereafter referred to as BCZ20Y15-YDC15 50 : 50, BCZ20Y15-YDC15 60 : 40, BCZ20Y15-GDC15 50 : 50 and BCZ20Y15-GDC15 60 : 40).After checking the chemical and mechanical compatibility between BCZ20Y15 and GDC15 and YDC15 phases, the influence of the different composites on electrical properties and H 2 permeability performances was thoroughly studied.Finally, the stability of these materials under a CO 2 -containing atmosphere was evaluated.
BCZ20Y15-GDC15 and BCZ20Y15-YDC15 pellets (E20 mm or 15 mm diameters and a thickness of E1 mm) and bars were prepared by direct uniaxial pressing.The sintering treatments were performed at 1400 1C, 1450 1C and 1500 1C for 5 hours in air with a constant heating rate of 2 1C min À1 .Moreover, the

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Energy & Environmental Science addition of 1 wt% ZnO (Sigma Aldrich, ACS reagent) as the sintering aid was evaluated.

Structural and microstructural characterization
The crystal structure and phase purity of ceramic powders and sintered composites were characterized by using a Philips PW 1830 diffractometer with Bragg-Brentano geometry, employing a Cu anode X-ray tube operated at 40 kV and 30 mA.Powder X-ray diffraction patterns were recorded at room temperature using a step scan procedure (0.021/2y step, 0.5 s time per step) in the 2y range of 20-1001.Rietveld refinements on the X-ray powder diffraction profiles have been performed using the software MAUD by using the full-profile fitting method. 37owder morphology as well as external and fractured surfaces of the sintered samples were observed by field emission SEM (FESEM) using both a SIGMA Zeiss instrument (Carl Zeiss SMT Ltd, UK), equipped with a field emission gun, operating under high vacuum conditions at an accelerating voltage varying from 0.2 to 30 kV and a Fei-Esem FEI Quanta 200 FEG instrument, equipped with a field emission gun operating under high vacuum conditions.
Sample density behaviour was studied by measuring the geometrical parameters and the weight of the specimens after sintering and by SEM investigations.Theoretical densities were calculated from crystal lattice parameters measured by XRD.Relative densities were calculated according to theoretical densities derived from the XRD data and experimental densities measured by geometrical parameters.

Electrochemical tests
Electrochemical tests were conducted on samples (composites and single-phase precursors) prepared by uniaxial pressing and sintering at 1450 1C for 5 hours with ZnO as the sintering aid.A near full dense material is a requirement to employ the studied composites as dense membranes.The relative density experimentally achieved was superior or equal to 93% for all the samples tested.From the reported data, the obtained densities were observed to be high enough to define near full dense materials.
The conductivity of sintered pellets (bars with dimensions E11 Â 2 Â 2 mm) was measured by a.c.impedance spectroscopy (EIS) over the 1-1 Â 10 6 Hz frequency range (10 points per decade) using an Autolab PGSTAT100 potentiostat/galvanostat.Conductivity measurements were performed in the 500-900 1C temperature range ensuring equilibrium conditions at each point.Impedance data were acquired using a two-electrode cell configuration under symmetrical cell conditions, with platinum electrodes sputtered onto the surface of the specimens, and were analysed by using the Zview2 modelling software (Scribner Associates, Inc.).Impedance spectra were collected in dry air, and dry and wet 5% hydrogen in argon atmospheres.Wet 5% hydrogen in argon was obtained by saturation at 25 1C.
The mixed conduction behaviour of membranes was evaluated by electromotive force (EMF) measurements (or open circuit voltage, OCV) in the cell fuel mode. 38Different dense specimens (20 mm in diameter and 1 mm in thickness) were prepared with porous Pt electrodes (Gwent Electronic Material, UK) pasted onto both side of the disks.The samples were mounted on a Probo-Statt (NorECs) test-rig with a gas-tight ceramic paste seal (Aremco Ceramabond 552, Valley cottage, NY).A voltage was generated by fluxing the two electrode surfaces with different gases: one surface was exposed to water-saturated 5% H 2 balanced Ar and the other to water-saturated synthetic air.Humidification of gases (H 2 O 3%) was accomplished by saturation at 25 1C.The theoretical EMF, E N , across the membranes is given by eqn (1) where E 0 is the standard EMF of the cell which is derived from the standard free energy E 0 = DG 0 /2F, 39 R is the universal gas constant, T is the temperature, F is the Faraday's constant and p H 2 , p O 2 , p H 2 O are the partial pressures of hydrogen, oxygen, and water, respectively.

Hydrogen permeation
Permeation measurements were carried out on 15 mm diameter disc shaped dense samples.H 2 permeation for the four composites was measured from 760 1C to 550 1C.In addition, BCZ20Y15-GDC15 50 : 50 was also measured from 1040 1C to 750 1C.The thickness of the membranes ranges between 600 and 700 mm: 610 mm for BCZ20Y15-YDC15 50 : 50, 651 mm for BCZ20Y15-YDC15 60 : 40, 650 mm for BCZ20Y15-GDC15 50 : 50 and 660 mm for BCZ20Y15-GDC15 60 : 40.The corresponding thickness for the BCZ20Y15-GDC15 50 : 50 membrane measured up to 1040 1C was 700 mm.Both disk sides were polished by using abrasive discs Presi P120 and P320 and then coated by screen printing with a 20 mm porous layer of a Pt ink (Mateck, Germany) in order to improve the surface catalytic activity of the membranes.Permeation measurements were performed on a double chamber quartz reactor as reported elsewhere. 40Hydrogen was separated from a mixture of H 2 -He (dry or saturated in water at 25 1C) using argon as sweep gas (permeate side).The flow rates used were 100 mL min À1 for feed and 150 mL min À1 for sweep under all the conditions and they were controlled using mass flow controllers (MFCs).Feed and sweep humidification was accomplished by saturation at 25 1C using Milli-Q water.The H 2 content in the permeate side was analyzed using a micro-GC Varian CP-4900 equipped with Molsieve5A and PoraPlot-Q glass capillary modules.The permeation fluxes in mL min À1 cm À2 were calculated by dividing the permeation rates by the effective surface area of the membrane.Sealing was done using silver and gold rings (depending on the maximum temperature in the measurement) and appropriate sealing was confirmed by measuring the He concentration in the permeate stream.An acceptable sealing was achieved when the helium concentration was lower than 5% of the H 2 permeated.Data reported in the present study were recorded at a steady state after thirty minutes of stabilization.

CO 2 stability tests
In order to assess the chemical stability of composites, thermogravimetric analyses were performed on BCZ20Y15-GDC15 and BCZ20Y15-YDC15 sintered powders in N 2 (100 mL min À1 )/CO 2 (20 mL min À1 ) flow over the temperature range of 30-1300 1C, by means of a simultaneous SDT Q600 TA Instruments Analyser.Thermal treatments were completed at heating rates of 10 1C min À1 .Analyses were performed on sintered powders instead of densified pellets, with the aim of emphasizing the reactivity with CO 2 due to the higher surface area.To this end, sintered composites were ground in an agate mortar and then sieved through a 75 mm mesh, in order to obtain final powders with finer grain size and homogeneous distribution.
Furthermore, H 2 permeation measurements were also performed using 15% CO 2 in Ar as sweep gas under C3 conditions for 24 hours in order to demonstrate the stability of the compounds under permeation operation.

Results and discussion
Phase composition, sintering and microstructure BCZ20Y15-YDC15 and BCZ20Y15-GDC15 composite membranes in both 50 : 50 and 60 : 40 volume ratios were prepared by mixing BCZ20Y15 powders from Marion Technologies 36 and YDC15 or GDC15 powders that were prepared by a conventional SSR method.XRD analyses of ''as received'' BCZ20Y15 powders and of YDC15 and GDC15 powders after calcination at 1200 1C were performed.YDC15 and GDC15 diffractograms were indexed to the reflections of a cubic crystal system in the Fm% 3m space group (no.225).No additional reflections were detected, which indicates that single phases of Y-and Gd-doped cerium oxides were both successfully synthesized.On the other hand, the BCZ20Y15 perovskite solid solution profile can be fitted using a model based on an orthorhombic crystal system, Pnma space group (no.62).Some traces of BaCO 3 (Pmcn space group, no.62) and CeO 2 (Fm% 3m space group, no.225) phases are revealed in the profile of the ''as received'' BCZ20Y15 commercial powders (see also Fig. S1, ESI †).
Hence, the effect of the temperature and sintering aid on the density of BCZ20Y15-YDC15 and BCZ20Y15-GDC15 50 : 50 membranes was investigated.With increasing temperature, high densities were reached but a reduction in mechanical strength was observed: all pellets sintered at 1500 1C showed large cracks in both the surface and the bulk.In many cases, samples sintered at 1500 1C even broke during this thermal treatment.Conversely, the addition of ZnO as the sintering aid is tremendously effective in enhancing the densification process and preserving good mechanical properties.Indeed, mean relative densities of BCZ20Y15-MDC15 composite membranes after sintering at 1450 1C with 1 wt% of ZnO (sintered disks of + E 15 mm), are higher than 96% for all composition specimens (Table S1, ESI †).
2][43] Different oxides such as ZnO, NiO or CuO are employed as sintering aids to prepare dense ionic conducting materials.In most cases, the mechanism implies the formation of low melting phases. 16The influence of ZnO on BaCeO 3 -BaZrO 3 based materials has been investigated by different research groups. 16,41,42,44,45Some authors concluded that Zn incorporates into the lattice 16 whereas others assumed that the enhancement of the sintering behaviour occurs through the formation of BaOÁZnO eutectic in the intergranular region. 41,44In this work, the addition of 1 wt% of ZnO allowed the preparation of high-density composites and therefore suitable for application as hydrogen separation membranes.Moreover, the use of a sintering aid allowed its preparation at temperatures at least 100 1C lower than in similar materials obtained by the SSR method. 27,29This minimum amount of ZnO reduces fabrication costs but also (i) limits barium oxide evaporation and (ii) prevents abnormal and discontinuous grain growth that are detrimental to both electrical and mechanical properties of this kind of material. 18,41EM investigations of sintered pellets confirmed the macroscopic observations.Fig. 1 shows the SEM micrographs of crosssections of BCZ20Y15-YDC15 (a-c) and BCZ20Y15-GDC15 50 : 50 (d-f) composite membranes sintered at 1450 1C, 1500 1C and 1450 1C with ZnO as the sintering aid, respectively.As observed in micrographs of Fig. 1(a-d) and (b-e), densities obtained after sintering at 1450 1C and 1500 1C were not satisfactory.In fact, the structures of these samples are porous, with evident and numerous apertures.On the contrary, as shown in micrographs Fig. 1(c) and (f), the addition of ZnO as the sintering aid favoured the grain coarsening, allowing a complete densification of the composite materials with homogeneous distribution of the grain No ZnO accumulated at the grain boundaries was detected within the limits of SEM-EDS.In some specimens, isolated residual grains of ZnO were observed (Fig. S3, ESI †).One phase corresponds to YDC15 or GDC15 and the other phase to BCZ20Y15, thus indicating that BCZ20Y15-YDC15 and BCZ20Y15-GDC15 sintered membranes are the mixture of BCZ20Y15 and doped-ceria in a cer-cer composite system with no reactivity between ceramic phases.Therefore, these results evidence a good chemical compatibility among the BCZ20Y15 proton conductor and YDC15 or GDC15 electronic conductor phases.BaCO 3 and CeO 2 traces that were identified on the ''as received'' BCZ20Y15 powders are not present on the sintered sample.Moreover, no ZnO reflections were detected.Table 1 summarizes the space group and lattice parameters for all the phases.As in powders, YDC15 and GDC15 peaks of sintered pellets were indexed to the reflections of a cubic crystal system, Fm% 3m space group (no.225).The pattern of BCZ20Y15 sintered sample matches with an orthorhombic crystal system, Pnma space group (no.62).
Very slight reduction of the crystal lattice parameters of BCZ20Y15 and YDC15 or GDC15 phases was detected for the BCZ20Y15-YDC15 and BCZ20Y15-GDC15 composite membranes with respect to the single sintered precursor materials.This phenomenon could be due to a possible structural rearrangement in the composite systems.XRD analyses show no evidence of any significant cell volume change due to ZnO addition, as depicted from the similar volume contraction detected also for composite membranes sintered at 1450 1C without a sintering aid (see samples referred to as * data of Table 1).

Electrochemical characterization
Total conductivity of cer-cer composites was studied as a function of temperature (500-900 1C) in different atmospheres by means of EIS.Measurements were performed on both 50 : membranes and sintered single-phase BCZ20Y15, YDC15 and GDC15 (prepared under analogous conditions, i.e. sintering at 1450 1C with 1 wt% of ZnO as the sintering aid).Data are plotted in the Arrhenius arrangement in (a)-(c) graphs of Fig. 3.
The total conductivity values of the samples in dry air and dry and wet hydrogen (5% H 2 balanced Ar) at 700 1C are reported in Table 2. Total conductivity values for 50 : 50 composites under reducing atmospheres are in the same order of magnitude as YDC15 and GDC15, indicating a good percolation threshold of the electronic phase.The conductivity of composites increases up to about one order of magnitude under reducing conditions with respect to dry air and this fact is ascribed to the increase of the electronic conductivity.Total conductivities of composites are a result of the nearly linear combination of the conductivity of doped ceria oxides and BCZ20Y15 and thus, depend on the specific volume ratio and percolation of each phase.The significant increase of the total conductivity under dry H 2 as compared with wet H 2 of YDC15, GDC15 and composite samples is related to the predominant n-type electronic conductivity (electrons) under more reducing conditions (dry H 2 is a more reducing atmosphere than wet H 2 ).In addition, the change in the mobile oxygen-vacancy concentration of doped ceria was expected to be minimal in a reducing atmosphere. 34On the contrary, BCZ20Y15 shows higher conductivities in wet H 2 as expected from the predominant protonic transport in this material (hydration effect s H 2 O+H 2 4 s H 2 ).Note that the Arrhenius plots under dry H 2 conditions of composites display different slopes as a function of the temperature: a higher slope at T o 600 1C compared with the one at T 4 600 1C.This change is proportional to the activation energy, which is apparently lower due to a predominant electronic character under dry reducing conditions at these temperatures if compared with oxygen ion transport (with higher activation energy). 46he good percolation of the BCZ20Y15 phase is also evidenced in Fig. 3(c), where the composite presents a conductivity magnitude closer to one exhibited by the BCZ20Y15 specimen.These data are in agreement with the OCV measurements (See also Fig. S4, ESI †), which show the lowering of the open circuit voltage in 50 : cer-cer composites in comparison with both the theoretical and the BCZ20Y15 values.This current leakage indicates that BCZ20Y15-YDC15 and BCZ20Y15-GDC15 membranes displayed an enhanced electronic conduction behaviour with respect to BCZ20Y15.Therefore, it is expected that the hydrogen permeation process should not be limited by the electronic conductivity in the composite membranes.
Regarding total conductivity in dry air, the higher values measured for BCZ20Y15 are related to the p-type (electron holes) conductivity presented by this material under oxidizing conditions. 47On the contrary, the values obtained for GDC15,

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YDC15 and the composites are lower under oxidizing conditions due to the important contribution of the n-type electronic conductivity under reducing atmospheres.On the other hand, the GDC15 sample presents higher conductivity as compared to YDC15.These results are in good agreement with literature data, which report that Gd-doped ceria shows higher total and oxygen ionic conductivity than Y-doped ceria. 32,34drogen permeation Fig. 4 plots the H 2 flux for BCZ20Y15-YDC15 50 : 50, BCZ20Y15-YDC15 60 : 40, BCZ20Y15-GDC15 50 : 50 and BCZ20Y15-GDC15 60 : 40 membranes measured under C3 conditions (both sides of the membrane humidified) feeding 50% H 2 in He, as a function of (a) the reciprocal temperature and expressed in mL min À1 cm À2 , and (b) as a function of the temperature and expressed in mL min À1 cm À1 , the latter in order to disregard the effect of the thickness.The hydrogen permeation process for the measured membranes is limited by the bulk diffusion, which is described by the Wagner Equation, 48,49 due to (i) the magnitude of the thickness and (ii) surface catalytic activity that is promoted by the coating of a catalytic Pt porous layer on both sides of   fluxes within the studied temperature range.However, BCZ20Y15-GDC15 50 : 50 membrane allowed the significant improvement of the H 2 flow rate, reaching values up to 0.27 mL min À1 cm À2 at 755 1C.This value is higher than that reported for the 50% volume-LWO/LSC composite measured under analogous permeation conditions (0.15 mL min À1 cm À2 at 700 1C for a 370 mm membrane) 50 , currently one of the highest H 2 fluxes obtained for bulk mixed protonic-electronic membranes.The BCZ20Y15-GDC15 50 : 50 sample was also measured without a Pt catalytic layer and the H 2 fluxes were significantly lower than those obtained for the sample coated with Pt (see Fig. S5, ESI †).Furthermore, the difference between the H 2 fluxes is more pronounced at lower temperatures where surface processes are slower, since the activation energy of surface exchange processes is higher than that exhibited by bulk proton diffusion. 51These results are ascribed to the modest surface catalytic activity of BCZ20Y15-MDC15 compounds which can be substantially enhanced through the use of catalytic layers.H 2 flux under C3 conditions could be produced via two different processes: (1) proton transport through the membrane from the feed side to the sweep side (from higher pH 2 to lower pH 2 ) due to the protonic conductivity of the BCZ20Y15 phase and (2) H 2 produced in the sweep side via water splitting because of the oxygen ion transport from the sweep side to the feed side (from higher pO 2 to lower pO 2 ).From the results plotted in Fig. 4(a), activation energies were calculated (Table 3).Activation energies range from 1.00 to 1.20 eV above 650 1C, which can be related to the prevailing oxygen ion transport through the membrane, 46,52 principally through the fluorite phase.However, at lower temperature (below 650 1C) the activation energies decreased as a consequence of the higher contribution of proton transport.Consequently, the high H 2 flux obtained with the BCZ20Y15-GDC15 50 : 50 membrane at T 4 650 1C could be ascribed to the larger H 2 production via water splitting due to the higher oxygen ionic conductivity that the GDC phase possesses as compared with YDC.For example, as it can be seen in Table 3, from the EIS measurements at 700 1C under dry air GDC15 total conductivity was 2.9 higher than YDC15.
A deep study was performed by using the BCZ20Y15-GDC15 50 : 50 membrane due to two different reasons: (1) this membrane exhibited the highest H 2 flux obtained among the developed composites and (2) the other three compounds presented lower redox stability and they broke when the conditions in the permeation measurements (H 2 concentration and hydration configuration) were changed.
In order to characterize the nature of predominant transport mechanisms in the BCZ20Y15-GDC15 50 : 50 membrane, H 2 measurements were performed under different hydration conditions: (C1) both sides dry, (C2) only the feed side humidified, (C3) both sides of the membrane humidified and (C4) the sweep side humidified.Fig. 5(a) shows the H 2 flux obtained as a function of the temperature for this composite under the different hydration conditions feeding 50% H 2 in He.H 2 flux was very low when only the feed side was humidified (C2), due to the lack of protonic charge carriers.However, an important H 2 flux was obtained when both sides of the membrane were hydrated (C3), due to the contribution of the H 2 permeation through the membrane in addition to the production of H 2 via  water splitting.When only the sweep side was humidified (C4), a further increase in the H 2 flux was obtained due to the increase in the oxygen ion transport through the membrane related to the higher pO 2 gradient between both membrane sides.This behaviour was observed before in other protonic materials with a prevailing oxygen ion transport at high temperatures. 52,53Fig. 5(b) plots the H 2 variation produced by the step-change from C3 to C4 conditions at 750 1C.In this process, H 2 flux sharply increases thus evidencing the main H 2 production via water splitting 54 under the abovementioned conditions.The pH 2 effect on the H 2 flux is also reported in Fig. 5(c), and the H 2 flux rises with increasing temperature and pH 2 as it is postulated by the Wagner equation. 49esults obtained under C1 conditions are not shown because the sample broke under these conditions probably due to mechanical failure after expansion of ceria under reducing conditions and the constraints related to the gold sealing at high temperatures.Finally, permeation measurements were performed at higher temperature, from 750 to 1040 1C, by using a BCZ20Y15-GDC15 50 : 50 membrane with a thickness of 700 mm and a gold gasket for the sealing (Fig. 6).At 1040 1C, H 2 flux reached values up to 2.40 mL min À1 cm À2 and 1.75 mL min À1 cm À2 under C4 and C3 conditions, respectively.Note that H 2 fluxes are lower than the values reached with the membrane measured at lower temperature (not justified by the different thickness of the samples), which can be related to the different maximum sealing/testing temperature.This difference in the maximum temperature can cause variations in the redox properties of the material and consequently in the electron and oxygen vacancy concentration.In fact, under reducing atmospheres, the oxygen deficiency (d) for Ce 0.9 Gd 0.1 O 1.95Àd is lower than the predicted by the mass action law and this deviation can be ascribed to defect interaction when the material presents a high concentration of oxygen deficiency. 55efect interaction can provoke the reduction of the oxygen ion transport through the membrane with the subsequent decrease of the H 2 flux obtained by water splitting.The obtained values are among the highest values reported currently for bulk mixed ionic (protonic)-electronic conductors; however more investigations are needed in order to quantify the proton and oxygen ion transport contribution in the H 2 flux obtained.Table 4 summarizes H 2 fluxes measured by different research groups on different bulk membranes.It is noteworthy that the BCZ20Y15-GDC15 50 : 50 sample shows one of the highest H 2 permeation flux measured for a bulk mixed protonic-electronic membrane, only surpassed by some cer-met systems (at higher pH 2 and temperature) where hydrogen could be transported also through the metallic phase. 5,20The use of doped ceria as the electronic conductor phase instead of a metal (most commonly Ni in this type of cer-met membrane) avoids problems during the processing such as non-uniformity in microstructure, metallic phase agglomeration or exudation. 20The H 2 flux for BCZ20Y15-YDC15 50 : 50 is two times smaller than the equivalent composite with GDC15, but this value (i.e.0.12 mL min À1 cm À2 at 735 1C) is still in line with the best fluxes for dense ceramic membranes.
A better understanding of the permeation phenomenon should allow optimizing the membrane design and composition for future development.The production of H 2 by water splitting should be considered as an advantage because it provides an extra H 2 flux to the separated H 2 and it could be tailored for more efficient industrial processes for H 2 production (such as in catalytic membrane reactors).Furthermore, this material could be used as an O 2 separation membrane  View Article Online under reducing atmospheres.Note that several attempts to measure the permeation of all-GDC15 membranes were done under the identical testing conditions but all membranes broke quickly under those conditions.After permeation measurements, the BCZ20Y15-GDC15 50 : 50 membrane was examined by SEM investigations and XRD analyses.Fig. 7 shows two SEM micrographs of fractured cross-sections of the sample after the tests conducted under C3 and C4 conditions up to 1040 1C and feeding 50% H 2 in He.SEM investigations on the bulk suggest that no apparent alteration occurred during permeation measurements: indeed, samples are characterized by well-defined grain-boundary having similar grain coarsening and dimension of as-sintered fresh samples.Moreover, no secondary phases or precipitates were detected in reference to the detection limit of SEM-EDS at the grain interior or at the grain boundaries.These results are in good agreement with XRD data (Fig. S6, ESI †) which show only BCZ20Y15 and GDC15 peaks without any secondary phases.On the other hand, some porous structures very close to the surface, underneath the catalytic Pt porous layer were observed from SEM investigations (Fig. S7, ESI †).From EDS analysis, these regions were identified as BCZ20Y15 with barium deficiency, probably due to the evaporation of BaO.This phenomenon seems to be limited to these layers of about 100-200 nm of thickness.
It has been demonstrated that this reaction is critical for systems with high cerium content in the structure: in fact, for x higher than 0.2 essentially no reaction with CO 2 was observed. 17,58he incorporation of ceria, doped or un-doped, is expected to improve the thermodynamic stability of the composite material in CO 2 or H 2 O atmospheres: indeed, the CeO 2 phase shifts the equilibrium towards the reactant side, thus stabilizing the perovskite structure. 29,35n order to assess the potential use of these composite systems under operational conditions, their chemical stability was evaluated under CO 2 -rich atmospheres by means of thermogravimetric analyses.Fig. 8(a) shows TG curves recorded for BCZ20Y15-YDC15 and BCZ20Y15-GDC15 sintered powders (sintered at 1450 1C for 5 hours with ZnO) under N 2 (100 mL min À1 )/CO 2 (20 mL min À1 ) continuous flow up to 1300 1C.For comparison, TG curves recorded for BaCe 0.85 Y 0.15 O 3Àd (BCY15) and BaCe 0.65 -Zr 0.20 Y 0.15 O 3Àd (BCZ20Y15) powders exposed to the same treatment are shown in the graph as reference materials.Unlike BCY15 and BCZ20Y15 samples, very smaller weight uptakes (o0.2 wt%) were observed at T 4 600 1C for BCZ20Y15-YDC15 and BCZ20Y15-GDC15 cer-cer membranes, thus indicating that the formation of barium carbonate was almost negligible (BaCO 3 r 1.8 mol%).It should be noted that these tests were performed on sintered powders in order to magnify the degradation effects of CO 2 over the composites.In fact, no carbonation process could be ascribed from the TG curves registered under the same conditions for the cer-cer membranes in the form of sintered pellets.
These data are in agreement with XRD analyses of the sintered powders checked by TG: indeed, no phase changes are evident and only a few weak peaks ascribable to two BaCO 3 phases (R% 3m space group no.166 and Pmcn no.62 respectively) are present in a practically negligible quantity (see also Fig. S8, ESI †).
H 2 permeation measurements for a BCZ20Y15-GDC15 50 : 50 membrane were also performed at 750 1C for 24 h using 15% CO 2 in Ar as sweep gas, 50% H 2 in He as the feed gas, and C3 conditions.After a stabilization time under Ar, sweep gas was switched to 15% CO 2 in an Ar atmosphere.H 2 flux decreases quickly when CO 2 is introduced and this drop can be related to the CO 2 /H 2 competitive adsorption on the membrane surface. 59hen, the H 2 flux increases for at least 24 hours as can be observed in Fig. 8(b).This increase can be related to a higher hydration of the membrane as it was previously reported for Nd 5.5 W 0.5 Re 0.5 O 11.25Àd . 59H 2 flux was recovered and surpassed when the sweep gas was switched to Ar, indicating the reversibility of the surface adsorption and the integrity of the membrane under CO 2 .These results attest a very good chemical stability of these cer-cer composite membranes against CO 2 that is mandatory for their application under operational H 2 separation conditions.

Conclusions
BCZ20Y15-YDC15 and BCZ20Y15-GDC15 cer-cer composite systems were investigated as H 2 separation membranes at temperatures higher than 500 1C.In order to evaluate the influence of the protonic and electronic conductor phase on H 2 permeation, symmetric dense BCZ20Y15-YDC15 and BCZ20Y15-GDC15 composite membranes both in 50 : 50 and 60 : 40 volume ratios were prepared.Chemical compatibility among precursors was confirmed by means of XRD analyses.Dense and crack free samples with homogeneous grain distribution and free-pore structures were obtained with reproducible densities by sintering treatments at 1450 1C with 1 wt% of ZnO as the sintering aid.This preparation procedure avoids high sintering temperatures, which are both expensive from the economic point of view and detrimental to the mechanical and electrical properties of composite membranes.

Paper
Energy & Environmental Science Permeation measurements were performed under different hydration degree conditions and H 2 pressure gradients.BCZ20Y15-YDC15 50 : 50, BCZ20Y15-YDC15 60 : 40 and BCZ20Y15-GDC15 60 : 40 composite membranes exhibited very similar hydrogen fluxes under all studied conditions.BCZ20Y15-GDC15 50 : 50 membrane showed the highest H 2 flux among all compositions studied reaching a value of 0.27 mL min À1 cm À2 at 755 1C when both sides of the membrane were hydrated.Moreover, by increasing the temperature, the flux noteworthily increased reaching at 1040 1C values up to 1.75 mL min À1 cm À2 and 2.40 mL min À1 cm À2 when both sides of the membrane (700 mm thick) or only the sweep side were hydrated respectively.These values are currently among the highest H 2 fluxes recorded for bulk mixed protonic-electronic membranes.Moreover, these composite systems demonstrated a very good chemical stability under a CO 2 -rich atmosphere, as in real hydrogen separation processes.
Further investigations are underway in order to have a better understanding of the H 2 permeation process in these materials, i.e. to determine the proton and oxygen ion transport contributions.Moreover, a thorough study on chemical and mechanical stability under a syngas atmosphere is necessary to validate the use of this kind of membrane under real operational conditions.The research aims to develop the optimal membrane materials and designs.

Fig. 1
Fig. 1 SEM micrographs of the fracture cross-sections of BCZ20Y15-YDC15 (a-c) and BCZ20Y15-GDC15 (d-f) 50 : 50 membranes after sintering in air at 1450 1C, 1500 1C and 1450 1C with ZnO as the sintering aid.In the inset, the relative densities are indicated.

Fig. 4
Fig. 4 H 2 flux (mL min À1 cm À2 ) as a function of reciprocal temperature (a) and H 2 flux (mL min À1 cm À1 ) as a function of temperature (b) for the different composites measured under C3 conditions feeding 50% H 2 in He.

Fig. 5
Fig. 5 H 2 flux as a function of temperature for the BCZ20Y15-GDC15 50 : 50 composite (650 mm thickness) measured in three configurations (C2, C3 and C4) feeding 50% H 2 in He (a).H 2 flux variation when hydration configuration changes from C3 to C4 at 755 1C feeding 50% H 2 in He (b) and H 2 flux as a function of temperature under C3 conditions feeding different pH 2 (c).

Fig. 6
Fig. 6 H 2 flux as a function of temperature for the BCZ20Y15-GDC15 50 : 50 composite (700 mm thickness) measured under C3 and C4 conditions and feeding 50% H 2 in He.

Fig. 7
Fig. 7 Secondary electron (a) and back-scattered electron micrographs (b) of the fracture cross-sections of BCZ20Y15-GDC15 50 : 50 dense membranes after permeation tests under C3 and C4 conditions up to 1040 1C and feeding 50% H 2 in He.

Table 3
Activation energies obtained from the H 2 permeation measurements under C3 conditions and feeding 50% H 2 -50% He