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of selective membranes devoted for hydrogen separation
Visot Mao
To cite this version:
Nouveaux matériaux à conduction mixte
protonique-électronique : Développement
de membranes sélectives destinées à la
séparation de l’hydrogène
Délivré par l’Université de Montpellier
Préparée au sein de l’école doctorale 459
Sciences Chimiques Balard
Et de l’unité de recherche UMR 5253 Equipe AIME
Agrégats, Interfaces, et Matériaux pour l’ÉnergieSpécialité : Chimie et Physicochimie des matériaux
Présentée par Visot MAO
Soutenue le 05 décembre 2016 devant le jury composé de
M. Gilles CABOCHE, Pr, Université de Bourgogne Rapporteur
M. Julian DAILLY, Dr, European Institute for Energy Research Examinateur
Mme Deborah JONES, DR, Université de Montpellier Président du Jury
M. Mathieu MARRONY, Dr, European Institute for Energy Research Examinateur
M. Fabrice MAUVY, Pr, Université de Bordeaux Rapporteur Statut Jury
M. Jacques ROZIÈRE, Pr émérite, Université de Montpellier Invité
M. Gilles TAILLADES, Pr, Université de Montpellier Directeur de Thèse
This dissertation is a part of the requirements to obtain the degree of doctor of philosophy (Ph.D.) conferred by the University of Montpellier. The research work is accepted by the Institut Charles Gerhardt Montpellier (ICGM) and hosted by the laboratory of Aggregates, Interfaces, and Materials of Energy (AIME) UMR 5253. The research work began on 1st October 2013 until the thesis defense which will take place on 5th December 2016. The research work has been jointly financed by the French Government Scholarship (Bourses du Gouvernement Français – BGF) during the first and second years and by an EIFER-EDF doctoral contract for the final third year.
I would like to address my acknowledgement to Professor Gilles Caboche (Laboratoire Interdisciplinaire Carnot de Bourgogne, University of Bourgogne) and Professor Fabrice Mauvy (Institut de Chimie de la Matière Condensée de Bordeaux, University of Bordeaux) for having accepted as the reviewers of my thesis.
I would like to show my deep acknowledgement to Dr. Deborah Jacqueline Jones, Head of the Department of Solid Chemistry and Divided Matter and equally to Emeritus Professor Jacques Rozière for giving me the permissions to work at the AIME laboratory from my Master’s thesis until my PhD thesis. Most importantly, they both give me the inspirations for the pursuit of science through their remarkable and outstanding scientific works in the fields of fuel cells and hydrogen energy.
I would like to express my sincere thanks and my deepest respect to my thesis advisor Professor Gilles Taillades for accepting me as a PhD student and for giving me sound scientific guidances from which I have learned so much and have improved my competences. More than an advisor, he is also an amazing collaborator who would give technical assistance in carrying out experiments any time necessary. I would say I am so lucky to have him as my supervisor and I can certainly say that without his involvements, I would not be able to successfully complete my PhD thesis within this 3-year period.
research work on track and bring the results to a fruitful level. I would like also to thank all the colleagues and staffs at the AIME laboratory for sharing memorable time and exciting experience with me during these unforgettable years.
Last but not least, I would like to express my profound thanks to my parents, family, and friends who have been always besides me during my ups and downs and who have always provided endless supports and cares.
Montpellier, October 2016
La formulation de matériaux à conduction mixte protonique-électronique (MIEC-H+) performants constituerait une avancée majeure pour le développent d’applications liées au vecteur hydrogène. En particulier cette classe de matériaux constitue une alternative prometteuse aux membranes métalliques ou poreuses pour les dispositifs dédiés de la séparation de l’hydrogène. L’objectif de ce travail de thèse a ainsi été de développer, de caractériser des matériaux présentant des conductivités suffisantes pour l’application visée, de mettre en forme des membranes et d’évaluer leurs performances.
La première approche a consisté développer des matériaux monophasiques par substitution d’oxydes conducteurs protoniques par un élément multivalent, Ba(Ce0,5Zr0,5)0,9-xPrxY0,1O3-δ).
Parallèlement, nos travaux ont porté sur des composites céramique-céramique constitués d’une phase à conduction protonique et d’une phase à conduction électronique dans des conditions réductrices, xBaCe0,9Y0,1O3-δ-(1-x)Ce0,9Y0,1O2-δ et xBaZr0,9Y0,1O3-δ
-(1-x)Ce0,9Y0,1O2-δ.
Les résultats les plus prometteurs en terme de conductivité (> 100 mS.cm-1 @ 600°C) ont été obtenus avec le composite de composition 20BaZr0,9Y0,1O3-δ-80Ce0,9Y0,1O2-δ qui a présenté
une perméabilité à l’hydrogène (0,13 ml.cm-1.min-1) du même ordre de grandeur que les meilleures reportées dans la littérature.
The formulation of high-performance mixed protonic-electronic conductors (MPEC) presents a major advancement for the development of hydrogen-linked application. In particular, this class of materials constitute a promising alternative to the metallic or porous membranes for devices devoted for separation of hydrogen. Thus, objective of this thesis work is to develop, characterize materials presenting sufficient conductivities for the targeted applications, to fabricate the membrane and to evaluate their performances.
The first approach consisted of developing single-phase materials by substitution of proton-conducting oxides by a multivalent element, Ba(Ce0.5Zr0.5)0.9-xPrxY0.1O3-δ. In parallel, our
works focused on the ceramic-ceramic composites which were consisted of a proton-conducting phase and an electron-proton-conducting phase in reducing conditions, xBaCe0.9Y0.1O3-δ
-(1-x)Ce0.9Y0.1O2-δ et xBaZr0.9Y0.1O3-δ-(1-x)CeY0.1O2-δ.
The most promising results in terms of conductivity (> 100 mS.cm-1 @ 600°C) was obtained
with the composite of composition 20BaZr0.9Y0.1O3-δ-80CeY0.1O2-δ which presented hydrogen
permeability (0.13 ml.cm-1.min-1) in the same order of magnitude as the best values reported
in the literature.
Table I.1 Energy content of different fuels [Dutta] ... 8
Table II.1 Shifted syngas compositions given in mol% from steam methane reforming and coal gasification [Voldsund et al.] ... 19
Table II.2 Adsorbent materials [Wiessner] ... 20
Table II.3 Boiling point of gas constituents at atmospheric pressure ... 23
Table II.4 Technical targets for hydrogen dense ceramic separation membranes ... 24
Table II.5 Most important rubbery and glassy polymers used in industrial membranes gas separation ... 29
Table II.6 Hydrogen permeabilities and selectivities of the selected polymer membranes .... 30
Table II.7 Advantages and disadvantages of polymer membranes ... 30
Table II.8 Hydrogen fluxes and selectivities obtained with supported Pd membranes [Hughes] ... 32
Table II.9 Improvement in hydrogen permeability of various binary Pd alloy at 350°C, 20.7 bar [Knapton] ... 32
Table II.10 Advantages and disadvantages of palladium-based membranes ... 33
Table II.11 Advantages and disadvantages of carbon membranes ... 35
Table II.12 Selected hydrogen separation microporous ceramic membranes and their performances ... 36
Table II.13 Advantages and disadvantages of microporous ceramic membranes ... 36
Table II.14 Hydrogen permeation from galvanic hydrogen pumping cells ... 39
Table II.15 Advantages and disadvantages of proton conducting membranes ... 39
Table II.16 Hydrogen permeation performance of single-phase dense ceramic membranes . 44 Table II.17 Hydrogen permeation performance of dual-phase dense ceramic membranes .... 44
Table II.18 Different types of separation membranes and their important characteristics [Kluiters] ... 45
Table III.1 Properties and applications of perovskites ... 54
Table III.2 Summary of the Kröger-Vink notation ... 57
Table III.3 Results of structure and phase analysis of Y-doped BaZrO3. Data obtained from X-ray diffraction [Kreuer et al., 2001]. ... 76
Table III.4 Phase transitions of BaCeO3 from 200°C to 950°C [Knight, 1994] ... 81
Table III.7 Overview of proton conductors reported in literature ... 95 Table III.8 Overview of dual-phase mixed protonic-electronic conductors reported in
literature ... 107
Table IV.1 Comparison of lattice parameters (!) and unit cell volume (!") of BaCe0.9Y0.1O3-δ elaborated in this study with those of Y-doped BaCeO3 compounds reported in literature.
Note: GNP = Glycine-Nitrate Process, SSR = Solid-State Reaction, and SG = Sol-Gel ... 139
Table IV.2 Comparison of the total conductivity at 600°C in reducing atmospheres of
BaCe0.9Y0.1O3-δ elaborated in this study with that of other Y-doped BaCeO3 compounds
reported in literature ... 141
Table IV.3 Comparison of lattice parameters (!) and unit cell volume (!") of BaZr0.9Y0.1O3-δ
elaborated in this study with those of Y-doped BaZrO3 compounds reported in literature.
Note: GNP = Glycine-Nitrate Process, SSR = Solid-State Reaction, and SG = Sol-Gel. ... 143
Table IV.4 Comparison of the total conductivity at 600°C in humidified reducing
atmospheres of BaZr0.9Y0.1O3-δ elaborated in this study with that of other Y-doped BaZrO3
compounds reported in literature. ... 146
Table IV.5 Comparison of lattice parameters (!) and unit cell volume (!") of
BaCe0.8Zr0.1Y0.1O3-δ elaborated in this study with those of other Y-doped BaCe1-xZrxO3
compounds reported in literature. Note: GNP = Glycine-Nitrate Process, SSR = Solid-State Reaction, and SG = Sol-Gel. ... 149
Table IV.6 Comparison of the total conductivity at 600°C in reducing atmospheres of
BaCe0.8Zr0.1Y0.1O3-δ elaborated in this study with that of other Y-doped BaCe0.9-xZrxO3-δ
compounds reported in literature. ... 152
Table IV.7 Compositions and corresponding designations for the studied oxides. ... 153 Table IV.8 Refinement of unit cell parameters for Ba(Ce0.5Zr0.5)0.9-xPrxY0.1O3-δ compounds
calcined at 900°C for 10h. ... 156
Table IV.9 Refinement of unit cell parameters for Ba(Ce0.5Zr0.5)0.9-xPrxY0.1O3-δ compounds
sintered at 1200°C for 10h. ... 158
Table IV.10 BE values of the Pr3d core levels for Pr6O11, Pr10, Pr20, Pr30, Pr40, Pr50, and
interval. ... 162
Table IV.12 Compositions and designations of the composite materials BCY and YDC are
introduced for comparison. ... 170
Table IV.13 Unit cell lattice parameters for single phases and composites sintered at 1400°C.
... 172
Table IV.14 Compositions and designations of the composite materials. BZY and YDC are
introduced for comparison. ... 179
Table V.1 Values of ARSohm and ASRpol for symmetrical membrane
Ni-BCZY/BCZY-ZnO/Ni-BCZY measured in dry 5%H2/Ar at 600°c and 700°C... 206
Table V.2 Values of ASRohm and ASRpol for the symmetrical membrane
Ni-BCZY/BCZY-ZnO/Ni-BCZY at 600°C in 5% and 20% H2 balance Ar at 600°C. ... 207
Table V.3 Hydrogen flux for Ni-BCZY/BCZY-ZnO/Ni-BCZY membrane measured at 600°C
and 700°C under dry 5% and 20%H2/Ar. ... 208
Table V.4 Summary of hydrogen flux and permeability measured at 600°C under dry
5%H2/Ar obtained with reference and composite membranes. ... 218
Table V.5 Hydrogen permeation rates through ceramic-metallic and ceramic-ceramic
Figure I.1 World production of fossil energy from 1800 to 2010 [Höok et al.]... 3 Figure I.2 Consumption of primary energy in the world in 2013 [IEA] ... 4 Figure I.3 CO2 emission from fossil fuels in Gt per year from 1971 to 2009 [Höok et al.] .... 5
Figure I.4 Evolution of fossil fuels (coal, oil, natural gas) consumption since 1965 with
projection until 2030 [Shafiee et al.] ... 6
Figure I.5 Growth in the shipments of fuel cells (portable, stationary and transport
applications) from 2009 to 2015 [FuelCell] ... 7
Figure II.1 Shares of hydrogen production methods worldwide [Voldsund et al.] ... 15
Figure II.2 Schematic description of a steam reformer plant for production of high-purity H2
[Rydén et al.] ... 20 Figure II.3 Schematic drawing of a PSA adsorption bed consisting of alumina, carbon and
zeolite as first, second, and third layer respectively [Baksh et al.] ... 21
Figure II.4 Schematic of a six-bed PSA unit [Weissner] ... 22 Figure II.5 Schematic presentation of mechanisms for permeation of gases through
membranes ... 25
Figure II.6 Important types of membrane technology for hydrogen separation ... 28 Figure II.7 A model proposed by Brandt for the transport of small penetrant molecules in
polymers [Pandley et al.]. ... 29
Figure II.8 Schematic representation of hydrogen separation from a gas mixture using a
Pd-based membrane [Al-Mufachi et al.] ... 31
Figure II.9 Schematic representation of hydrogen separation from hydrocarbon mixture [Pandley et al.] ... 34 Figure II.10 Schematic representation of a thin microporous membrane layer deposited on a
macroporous support. ... 36
Figure II.11 Schematic representation of dense protonic membrane pumping hydrogen under
applied external voltage ... 38
Figure II.12 Schematic representation of hydrogen permeation through a dense mixed
protonic-electronic membrane under the gradient of hydrogen partial pressure ... 40
Figure III.1 The perovskite structure with the larger A-site ion shown in orange, the B-site in
blue and the oxygen ion in red [Bentzer]. ... 54
Figure III.4 Schematic representation of point defects in non-stoichiometric compounds [Kasap] ... 58
Figure III.5 Schematic showing the creation of oxygen vacancies in the ABO3 perovskite
upon substitution by a trivalent acceptor. (Reproduction with permission from Prof. Gilles Taillades). ... 59
Figure III.6 Incorporation of water into the lattice structure of the perovskite oxide.
(Reproduction with permission from Prof. Gilles Taillades). ... 60
Figure III.7 Quantum MD simulation showing Grotthus mechanism for the proton transfer [Kreuer, 2003] ... 63 Figure III.8 Isotope effects on the conductivity of BaCe0.95Dy0.05O3-α in Ar-H2O (pH2O =
0.023 atm) and Ar-D2O (pD2O = 0.023 atm). ... 66
Figure III.9 Dependence of conductivity on oxygen partial pressure [Iwahara, 2009] ... 67 Figure III.10 (a) transport numbers of different charge carriers and (b) corresponding
conductivities as a function of inverse temperature in O2+H2O atmosphere. ... 71
Figure III.11 transport numbers of different charge carriers and (b) corresponding
conductivities as a function of inverse temperature in H2+H2O atmosphere. ... 71
Figure III.12 SrZrO3 orthorhombic unit cell [Cavalante et al.] ... 73
Figure III.13 Evolution of lattice parameter of SrZrO3 as a function of temperature [Howard
et al.] ... 73 Figure III.14 Conductivities of SrZr0.95M0.05O3-α in H2 atmosphere [Yajima et al.] ... 74
Figure III.15 Cubic crystal structure of BaZrO3 [Yamanaka et al.] ... 74
Figure III.16 XRD patterns of anode-supported BZI membrane fired at 1450°C, before (a)
and after exposure to pure CO2 at 700°C for 3 h (b) and to boiling water for 3 h (c). [Bi et al.] ... 75
Figure III.17 Mobility of protonic defects in BaZrO3 with (a) different kinds of acceptor
dopants and (b) with different concentration of Y as acceptor dopant. [Kreuer, 2001] ... 76
Figure III.18 Analysis of BaZr0.95Y0.05O3 using orientation imaging microscope (OIM). (a)
grain boundary map and (b) grain orientation map [Iguchi et al.]. ... 77
Figure III.19 SEM images of the fractured section of (a) unmodified BZY and (b) 4 mol.%
ZnO added BZY, both sintered at 1300°C [Babilo et al.] ... 78
Figure III.20 Proton conductivities of various oxides as calculated on basis of proton
Figure III.22 Orthorhombic crystal structure of BaCeO3 [Yamanaka et al.] ... 80
Figure III.23 Structural phase transition of BaCeO3 as a function of temperature [Knight,
1994] ... 81 Figure III.24 Plots of transport number of protons and oxygen ions in BaCe0.9M0.1O3-δ
against ionic radii of dopant M under fuel cell condition [Iwahara et al., 1994]. ... 82
Figure III.25 Electrical properties of BaCeO3-based specimens as a function of temperature
in wet 5%H2-95%Ar [Fu et al.] ... 83
Figure III.26 Conductivity at 700°C for BaCe1-xYxO3-δ as a function of Y-doping under dry
and wet conditions [Chiodelli et al.] ... 83
Figure III.27 Powder X-ray diffraction patterns of BCY10 before and after heating in 100%
CO2 at 850°C [Zakowsky et al.] ... 84
Figure III.28 X-ray diffraction patterns of BaCe0.9-xZrxY0.1O3-α before and after exposing to
CO2 atmoshpere at 900°C for 2h [Katahira et al., 2000] ... 86
Figure III.29 Conductivities of BaCe0.9-xZrxY0.1O3-α in wet H2 (pH2O = 1.7# × 1$3 Pa)
[Katahira et al., 2000] ... 86 Figure III.30 Valence states of lanthanides in oxides [Han et al.] ... 88 Figure III.31 Schematic diagrams showing two types of B-site ordering in complex
perovskite oxides: (a) 1 :1 ordering giving a cubic unit cell and (b) 1 :2 ordering producing a trigonal unit cell [Nowick et al., 1999] ... 91
Figure III.32 A comparison of the Arrhenius conductivity plots for Sr2(Sc1.05Nb0.95)O6-δ,
Sr2(Sc1.1Nb0.9)O6-δ, Ba3(Ca1.18Nb1.182)O9-δ with those of 5%Yb-doped SrCeO3 and
5%Nd-doped BaCeO3 following water vapor treatment [Liang et al., 1994] ... 92
Figure III.33 Backscattered electron images of (a) cermet anode in a symmetrical cell
assemblies after reduction and (b) cermet membrane sintered at 1400°C for 10h [Essoumhi et
al., Kim et al., 2014]. ... 97 Figure III.34 The total electrical conductivity of BCY and various Ni-BCY cermets in a wet
4%H2/balance He [Kim et al., 2014] ... 98
Figure III.35 Ionic and electronic conductivities of Ni-BCY composite membrane [Zhang et al., 2003] ... 98
Figure III.36 Time dependence of hydrogen flux through Ni-BCZY (0.4 % & % 0.8)
membranes in a feed gas containing (a) 20%CO2 and (b) 30%CO2 balance 40%H2/He at
silver droplets in the pores [Ruiz-Trejo et al.]. ... 101
Figure III.38 Arrhenius plots for (a) bulk conductivity and (b) grain-boundary conductivity if
Ag/BCZYZ composite in air [Ruiz-Trejo et al]. ... 101
Figure III.39 OCV measurement as a function of temperature for a moist 10%H2,
Ag/BCZYZn-Ag/Ag, air cell [Ruiz-Trejo et al]. ... 102
Figure III.40 STEM image of a selected area of STN95/BCZY27 sample (a) and
corresponding XEDS map of the same area showing Sr (red) and Ba/Ce (green) [Fish]. .... 103
Figure III.41 SEM images of 30BCY-70CYO surface (a) and cross-section (b), sintered at
1600°C in air for 10h [Liu et al.] ... 104
Figure III.42 Temperature dependence of total conductivity measured in 4%H2/Ar of
BCY-CYO composite with different ratios sintered at 1550°C [Liu et al.] ... 105
Figure III.43 XRD patterns of 30BCY-70CYO sintered at 1550°C in air after exposure to
different atmospheres for 10h: (A) sintered sample, after (B) 4%H2/Ar treatment, (C) H2O
treatment and (D) 100% CO2 treatment [Liu et al.]. ... 106
Figure IV.1 Preparation of fine powders ... 122 Figure IV.2 Typical steps in solid state reaction method ... 123 Figure IV.3 X-ray diffraction pattern of BaCeO3 perovskites prepared by solid state reaction
at different calcination temperatures [Cai et al.]. ... 124
Figure IV.4 Field emission scanning electron microscope (FESEM) image of BaCeO3
perovskite powders prepared by solid state reactions [Cai et al.] ... 124
Figure IV.5 Typical steps in sol-gel method. ... 125 Figure IV.6 Flowchart for solution combustion synthesis. ... 127 Figure IV.7 Particle size of as-synthesized and calcined BaZr0.7Ce0.1Y0.2O2.9 as a function of
G/N ratio [Chien et al., 2010] ... 129
Figure IV.8 SEM images of (a) as-synthesized and (b) calcined BaCe0.8Y0.2O3-δ powders at
900°C for 4h [Jadhav et al.] ... 130
Figure IV.9 SEM micrograph of BaCe0.8Gd0.2O3 powder calcined at 1050°C [Boskovic et al.]
... 131
Figure IV.10 Flowchart for the preparation of BZCYYb by EDTA-assisted GNP method [Zhou et al.]. ... 132 Figure IV.11 XRD pattern of BZCYYb powders calcined at various temperatures ranging
Figure IV.13 Cross-sectional SEM images of (a) BZY pellet sintered at 1700°C for 10h and
(b) BZY-ZnO pellet sintered at 1450°C for 10h [Peng et al.] ... 135
Figure IV.14 Effect of transitional metal oxide additives as sintering aids on densification of
BZY [Babilo et al.]. ... 135
Figure IV.15 Before and after sintering photos of BCZY pellets sintered at 1450°C for 12h.
A) Legend showing the metal oxide additives (5mol.%), B) Corresponding pellets before sintering and C) Same pellets after sintering. Note: Con = Blank BCZY control sample
[Nikodemski et al.]. ... 136 Figure IV.16 Relative density achieved after solid state reactive sintering vs. ionic radius of
the transition metal cation additive used. All samples were sintered at 1450°C for 12h
[Nikodemski et al.]. ... 136 Figure IV.17 Schematic representation of preparation procedure for BCY10 powders. ... 138 Figure IV.18 X-Ray Diffraction patterns of BCY10 after calcination at 900°C for 10h. ... 139 Figure IV.19 (a) photo of a sample holder for impedance measurements, (b) 3D-exploded
view and (c) labelled 2D drawing. ... 140
Figure IV.20 Total conductivity as a function of inverse temperature of BCY measured in
wet 5%H2/Ar ... 141
Figure IV.21 X-ray diffraction patterns of powder BZY10 after calcination. ... 142 Figure IV.22 SEM image of a fracture cross-section of BZY sintered at 1500°C for 10h. . 144 Figure IV.23 Mean grain size of sintered BZY10 pressed at 200, 500, 750 and 998 MPa [Duval] ... 144 Figure IV.24 Total conductivity as a function of inverse temperature of BZY measured in wet
5%H2/Ar. ... 145
Figure IV.25 Flowchart for the preparation of BCZY powders by EDTA-assisted GNP
method. ... 147
Figure IV.26 XRD patterns of calcined BCZY powders at 1000°C (blue line), at 1200°C (red
line) and of sintered BCZY pellet at 1350°C (green line). ... 148
Figure IV.27 Relative density as a function of temperature for BCZY with and without ZnO [Pers]. ... 149 Figure IV.28 SEM pictures of a fracture cross section of BCZY prepared by EDTA-GNP
Figure IV.29 Comparison of the total conductivity as a function of inverse temperature for
BCY, BCZY and BZY measured in wet 5%H2/Ar. ... 151
Figure IV.30 Thermal cycle for calcination process. ... 154
Figure IV.31 Support-sample assembly configuration for sintering. ... 154
Figure IV.32 Thermal cycle for sintering process. ... 154
Figure IV.33 XRD patterns of BaCe0.4Zr0.4Pr0.1Y0.1O3-δ (Pr10), BaCe0.35Zr0.35Pr0.2Y0.1O3-δ (Pr20), BaCe0.3Zr0.3Pr0.3Y0.1O3-δ (Pr30), BaCe0.25Zr0.25Pr0.4Y0.1O3-δ (Pr40), BaCe0.2Zr0.2Pr0.5Y0.1O3-δ (Pr50), and BaCe0.15Zr0.15Pr0.6Y0.1O3-δ (Pr60) powders calcined at 900°C for 10h in air. ... 155
Figure IV.34 Unit cell parameters and volume as a function of Pr content. ... 156
Figure IV.35 XRD patterns of BaCe0.4Zr0.4Pr0.1Y0.1O3-δ (Pr10), BaCe0.35Zr0.35Pr0.2Y0.1O3-δ (Pr20), BaCe0.3Zr0.3Pr0.3Y0.1O3-δ (Pr30), BaCe0.25Zr0.25Pr0.4Y0.1O3-δ (Pr40), BaCe0.2Zr0.2Pr0.5Y0.1O3-δ (Pr50), and BaCe0.15Zr0.15Pr0.6Y0.1O3-δ (Pr60) pellets sintered at 1200°C for 10h in air. ... 157
Figure IV.36 XRD patterns of the main peaks for Pr10, Pr20, Pr30, Pr40, Pr50, and Pr60 158 Figure IV.37 Unit cell lattice parameters and volume as a function of Pr content. ... 159
Figure IV.38 SEM micrographs of sintered specimens (Pr10, Pr20, Pr30, Pr40, Pr50, and Pr60) ... 160
Figure IV.39 (a) XPS spectra for the Pr3d core levels of Pr6O11 commercial powders (b) Peak fitting for the Pr3d core levels of Pr6O11 and (c) Peak fitting for the Pr3d core levels of BaCe0.3Zr0.3Pr0.3Y0.1O3-δ (Pr30) as-prepared powders in air ... 161
Figure IV.40 Mean Pr valence as a function of Pr content. Mean Pr valence of Pr6O11 is given as a reference. ... 162
Figure IV.41 X-ray diffraction patterns of Pr10, Pr20, Pr30, Pr40, Pr50, and Pr60 before and after exposure to humidified atmospheric air (~ 3 vol.% H2O and 400 ppmv of CO2) at 600°C for 480h. ... 164
Figure IV.42 Comparison of the total conductivity as a function of inverse temperature for Ba(Ce0.5Zr0.5)0.9-xPrxY0.1O3-δ (x = 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) under dry and wet air. ... 165
Figure IV.43 Total conducivity of Pr10, Pr20, Pr30, Pr40, Pr50 and Pr60 sintered pellets in dry air. ... 166
Figure IV.47 XRD patterns of the calcined composite powders 60BCY40YDC,
40BCY60YDC, and 20BCY80YDC are shown along with that of the parent materials BCY and YDC for phase identification. ... 171
Figure IV.48 XRD patterns of the BCY-YDC composite electrolytes sintered at 1400°C for
10h. ... 173
Figure IV.49 SEM images and EDS maps of (a) Ba, (b) Ce, and (c) Y distributions for
60BCY40YDC, (d) Ba, (e), Ce, and (f) Y distributions for 40BCY60YDC and (g) Ba, (h) Ce and (i) Y distributions for 20BCY80YDC. ... 174
Figure IV.50 Electrical conductivity measured in dry 5%H2/Ar as a function of temperature
for BCY-YDC composite electrolytes with different compositions sintered at 1400°C for 10h. ... 175
Figure IV.51 OCV measurement of Pt/20BCY80YDC/Pt in 5%H2-air as a function of
temperature. Theoretical values calculated in 5%H2-air and 100%H2-air are added for
comparison. ... 176
Figure IV.52 XRD patterns of the BCY-YDC composite electrolytes before and after
conductivity test in dry 5%H2/Ar for 120h. ... 177
Figure IV.53 SEM micrographs of the fracture cross-section for (a) before and (a’) after
conductivity test for 60BCY40YDC, (b) before and (b’) after conductivity test for 40BCY60YDC, and (c) before and (c’) after conductivity test for 20BCY80YDC. The
conductivity test was performed in 5%H2/Ar for 120h for each specimens. ... 177
Figure IV.54 Flowchart for the preparation of BZY-YDC composites ... 179 Figure IV.55 XRD patterns of BZY, YDC, and the BZY-YDC composite electrolytes. For
BZY and YDC, the samples are powders calcined at 1000°C and 900°C respectively for 10h whereas for 60BZY40YDC, 40BZY60YDC, and 20BZY60YDC, the specimens are sintered at 1500°C for 10h ... 180
Figure IV.56 SEM micrographs of the fracture cross-section of 20BZY80YDC pellet sintered
at (a) 1400°C and (b) 1500°C for 10h and EDS maps for (c) Ba, (d) Zr, (e) Ce, and (f) Y of the pellet sintered at 1500°C ... 181
Figure IV.57 Electrical conductivity measured in dry 5%H2/Ar as a function of temperature
Figure V.1 Schematic representation of a PCFC single cell. ... 192 Figure V.2 Explicative schematics of a PCFC integrated with a dense mixed
protonic-electronic layer ... 193
Figure V.3 Photo of the Pr30 layer deposited on Ni-BCZY substrate taken after heat
treatment at 1550°C for 9h. The dimension of the cell is 3x3 cm2. ... 194
Figure V.4 SEM image of thin film BaCe0.3Zr0.3Pr0.3Y0.1O3-δ deposited on a cermet substrate
Ni-BCZY ... 194
Figure V.5 EDX analysis of the BaCe0.3Zr0.3Pr0.3Y0.1O3-δ layer ... 195
Figure V.6 Schematic representation of the cell configuration showing material compositions,
elaboration processes, and component dimensions. ... 196
Figure V.7 I-V characteristics and power density curves obtained under dry hydrogen at
600°C. ... 196
Figure V.8 Photos of a PCFC cells taken (a) before the test and (b) after the test. ... 197 Figure V.9 Schematic diagram of an experimental setup for the measurement of the hydrogen
permeation flux. A mixed protonic-electronic membrane was illustrated as an example. .... 198
Figure V.10 Schematic diagram of alumina manifold/gasket/membrane assembly where leaks
from feed side to sweep side occur. ... 199
Figure V.11 Volume of Ar leaks as a function of fraction of hydrogen in the feed gas
composition during a hydrogen permeation test measured at 600°C for the 20BZY80YDC composite membrane. ... 201
Figure V.12 flux of (a) leaked hydrogen and (b) permeated hydrogen after leak correction, as
a function of hydrogen fraction in the feed gas stream. ... 201
Figure V.13 Hydrogen permeation flux across a palladium membrane for hydrogen partial
pressure of 5% and 10%. The result from the PhD works of Pons was included for comparison
[Pons], indicated by the legend, 5%H2 Limoges. ... 202
Figure V.14 Preparation procedure for NiO-BCZY cermet powders. ... 203 Figure V.15 SEM image of cross section of a symmetrical membranes
Ni-BCZY/BCZY-ZnO/Ni-BCZY, (a) showing whole layer of BCZY membrane with parts of the Ni-BCZY electrodes on both sides and (b) showing an enlarged interface between porous Ni-BCZY electrode and dense BCZY membrane. The sample was reduced in 5%H2 at 700°C for 2h
Figure V.17 Complex impedance spectra of symmetrcal membrane
Ni-BCZY/BCZY-ZnO/Ni-BCZY at 600°C and 700°C under 5%H2/Ar. ... 206
Figure V.18 Complex impedance spectra of symmetrical membrane
Ni-BCZY/BCZY-ZnO/Ni-BCZY at 600°C under 5% and 20%H2/Ar. ... 207
Figure V.19 Comparison of H2 flux for SrCe0.95Yb0.05O3-α thin films and 1 mm dense disk
(950K) [Hamakawa et al.] ... 209
Figure V.20 SEM micrograph of polished surface (a) in secondary mode and (b) in
backscattered mode and (c) fracture cross-section of the 20BCY80YDC membrane sintered at 1400°C for 10h. ... 210
Figure V.21 Hydrogen permeation flux as a function of H2 concentration in feed stream
through 20BCY80YDC membrane at 600°C. ... 211
Figure V.22 Thermal cycle for the adhesion of Pt layer on the membrane surfaces ... 212 Figure V.23 SEM image of the surface microstructure of the Pt layer in (a) Secondary mode
and (b) back-scattered mode... 212
Figure V.24 SEM image of the fracture cross-section of the Pt layer ... 213 Figure V.25 H2 permeation flux as a function of H2 concentration in the feed gas measured
for the 20BCY80YDC membrane with no catalytic layer and with Pt layer deposited on both sides of the membrane. ... 214
Figure V.26 SEM images of (a) polished surface and (b) fractured cross-section of the
20BZY80YDC membrane sintered at 1500°C in air for 10h. ... 215
Figure V.27 SEM image of the surface of Pt layer (a) in secondary mode and (b) in
back-scattered mode. ... 215
Figure V.28 H2 permeation flux as a function of H2 concentration in feed stream, measured at
different temperatures. ... 216
Figure V.29 (a) H2 permeation flux measured in ml.cm-2.min-1 and (b) H2 permeability
Reference ... 46 Chapter III : Protonic and Mixed Protonic-Electronic Conductors: Theory and
Materials ... 51 III.1. Fundamentals ... 53 III.1.1. Structure of perovskite oxides ... 53 III.1.2. Defect chemistry ... 57 III.1.3. Proton incorporation ... 59 a. Protonic defect formation by hydration ... 60 b. Protonic defect formation under H2 atmosphere ... 61
III.1.4. Proton transport and conductivity ... 61 a. Proton transfer mechanism ... 61 b. Mobility and conductivity ... 63 c. Isotope effect ... 65 III.1.5. Mixed conductivity in conducting perovskites ... 66 III.2. Mixed conducting materials ... 72 III.2.1. Single-phase materials ... 72 a. Accepter-doped cerates and zirconates ... 72 i. AZrO3 compounds ... 72
ii. ACeO3 compounds ... 79
iii. BaCeO3-BaZrO3 solid solution ... 84
IV.2. Reference materials ... 137 IV.2.1. BaCe0.9Y0.1O3-δ (BCY) ... 137
a. Synthesis ... 137 b. Structural characterization ... 138 c. Electrical conductivity ... 140 IV.2.2. BaZr0.9Y0.1O3-δ (BZY) ... 142
a. Synthesis ... 142 b. Structural characterization ... 142 c. Microstructure ... 143 d. Electrical conductivity ... 145 IV.2.3. BaCe0.8Zr0.1Y0.1O3-δ (BCZY) ... 146
a. Synthesis ... 146 b. Structural characterization ... 148 c. Microstructure ... 150 d. Electrical conductivity ... 151 IV.3. Single-phase mixed protonic-electronic conductors: Ba(Ce0.5Zr0.5)0.9-xPrxY0.1O3-δ system ... 152
IV.3.1. Experimental ... 153 IV.3.2. Result and discussion ... 155 a. Phase formation and structure determination ... 155 b. Sintering and microstructure ... 157 c. Mixed valence state determination ... 160 d. Chemical stability ... 163 e. Total conductivity ... 164 IV.4. Dual-phase mixed protonic-electronic conductors:
xBaCe0.9Y0.1O3-δ-(1-x)Ce0.9Y0.1O2-δ system ... 168
IV.4.1. Experimental ... 169 IV.4.2. Results and discussion ... 171 a. Phase formation ... 171 b. Sintering and microstructure ... 172 c. Electrical conductivity ... 174 d. Chemical stability ... 176 IV.5. Dual-phase mixed protonic-electronic conductors:
xBaZr0.9Y0.1O3-δ-(1-x)Ce0.9Y0.1O2-δ system ... 177
b. Microstructure ... 180 c. Electrical conductivity ... 181 IV.6. Conclusion ... 184 Reference ... 185 Chapter V : Characterizations of electrochemical device ... 189 V.1. H2 filtration membrane ... 191
I.1 Energy context
Traditionally, biomass in the form of wood, charcoal, leaves, agricultural residue and animal/human waste was a dominant source of energy for cooking, drying, space heating and lighting [Karekeri et al.]. However, the industrial revolution that began in England in the 18th century and then rapidly spread to Western Europe and North America made a significant change in the forms of energy that would be used. This began in 1810 with a massive quantity of coal being utilized as a sole substitute for traditional biomass in order to provide the energy services required. Then, the arrival of petroleum, a refined form of crude oil, and natural gas, normally found in oil fields, in the first half of the 20th century tremendously fueled the industrialization even greater. Figure I.1 shows the amount of fossil fuels (coal, oil, and natural gas) being produced between 1800 and 2000 within which a skyrocketed increase in the production of fossil fuels is observed since 1950 [Höok et al.].
Today, fossil fuels are unarguably the most important and dominant sources of energy needed for socioeconomic developments of the rich countries and those in process of development, particularly China and India. A report made by IEA (International Energy Agency) shows that fossil fuels represent more than 80% of the world’s primary energy supply in 2013 in which coal accounts for 28.9%, oil takes up 31.1% and natural gas provides 21.4% (Figure I.2)
[IEA].
Figure I.2 Consumption of primary energy in the world in 2013 [IEA]
Although fossil fuels are heavily relied on for energy supply, they present two serious drawbacks with regards to their utilizations. Firstly, the reserves are shown to be limited and will be rapidly depleted based on the actual rate of energy consumption. Furthermore, oil is the most consumed form of fossil fuel, and in terms of geographical distributions, it is very much localized in only certain areas, which gives rise to geopolitical consequences. In this regards, oil price is famously considered as a manipulative tool used by the oil-producing countries, mostly the OPEC nations in the Middle East regions. This organization could exert geopolitical pressures on the consuming nations by increasing the oil price and thus causing an immediate threat to the national security of the consuming nations. For this reason, an effort to achieve an energy independence, that is to be essentially free from fossil fuels, has attracted a lot of attentions from the non-oil-producing countries. Secondly, the uses of fossil fuels inevitably generate the emission of CO2, a major greenhouse gas, which is claimed to be
responsible for global warming and climate change according to the scientific community (Figure I.3) [Höok et al.]. Besides CO2, burning fossils also emit other toxic, polluting and
hazardous gases such as CO, H2S, NOX, SOX, and particulate matter which can cause serious
Coals are generally combusted in traditional thermal power plants that emit huge amount of CO2 more than any other forms of fossil fuels. However, since CO2 is produced on-site, it can
be easily captured and stored. Oil is used in almost all internal-combustion-engine vehicles and
CO2 emitted is very difficult to be trapped since it is released from the exhaust pipe of the
vehicles. Although natural gas is considered the cleanest form of fossil fuels since it produces about half of the CO2 emission compared to coal, it presents a huge risks and damages to the
surrounding ecosystem in case of leaking. About 85% of natural gas is made up of methane, another greenhouse gas, and it is even worse than CO2 in terms of the impacts of greenhouse
effect. Consequently, its leakage into the air can cause severe environmental damages.
Figure I.3 CO2 emission from fossil fuels in Gt per year from 1971 to 2009 [Höok et al.]
With regards to the scarcity of the fossil fuel resources and the necessity to reduce the CO2
fundamental drawback of these natural energy sources concerns their relatively low energy density such that a large amount of space is required to produce the equivalent amount of energy that would be provided by much densely packed fossil fuels. Another issue regarding the reliability of these sources of energy is inherited from their intermittent nature. As a result, the utilization of these forms energy is a huge technical challenge, and it is obviously far from being the major sources of energy.
As projected, the demand for fossil fuels is continue to increase for many decades to come regardless of the availability of other energy sources (Figure I.4) [Shafiee et al.]. In this regards, a realistic strategy that can answer the ever-growing energy needs during this so-called “energy transition period” is to use fossil fuels in a more efficient and environmentally friendly way by prolonging the depletion rates of fossil fuel resources as long as possible and at the same time, significantly reducing the CO2 emissions.
Figure I.4 Evolution of fossil fuels (coal, oil, natural gas) consumption since 1965 with projection until 2030 [Shafiee et al.]
uses has been growing at an exponential rates. Three main sectors for which fuel cells are developed include portable, stationary and transport applications (Figure I.5) [FuelCell]. Among these, stationary applications occupy the biggest share in the growth.
Figure I.5 Growth in the shipments of fuel cells (portable, stationary and transport applications) from 2009 to 2015 [FuelCell]
Fundamentally, a fuel cell is an electrochemical device that produces electricity, heat, and water when fed with hydrogen and oxygen. Unlike in a combustion process where hydrogen and oxygen from air are physically mixed together and reacted chemically in a single chamber where heat and water vapor are produced, in a fuel cell, hydrogen and oxygen is physically separated in two different compartments in which electrochemical processes take place when hydrogen and oxygen are consumed at each compartment.
From an environmental point of view, hydrogen is the cleanest fuel known since transformation of hydrogen into useful energy whether via thermal or electrochemical route would only result in formation of water [Majlan et al.]. In this case, sufficient quantity and high purity of hydrogen is required. Unlike fossil fuels, hydrogen does not exist solely on its own in nature. It must be, therefore, produced from compounds in which it is chemically combined with other elements. Moreover, hydrogen has an interesting property in terms of its energy contents. As shown from the Table I.1, its energy density is much higher compared to other fuels available in various forms [Dutta].
Table I.1 Energy content of different fuels [Dutta]
Fuel Energy content [MJ/kg]
Hydrogen 120
Liquefied natural gas (LNG) 54.4
Propane 49.6 Aviation gasoline 46.8 Automative gasoline 46.4 Automative diesel 45.6 Ethanol 29.6 Methanol 19.7 Coke 27 Wood (dry) 16.2 Bagasse 9.6
Nowadays, about 75% of hydrogen fuel production is derived from fossil fuels, hydrocarbon compounds [Dutta]. However, through this process, hydrogen is always mixed with other gases that are generally undesirable. As a result, purification of hydrogen is necessary and is primarily carried out using commercial gas separation processes such as pressure swing adsorption (PSA) or cryogenic distillation [Adhikari et al.]. However, these technologies are somewhat limited in terms of selectivity, fabrication costs or energy needed for operation. As a consequence, among alternative technologies related to separation and purification of hydrogen, membrane technology is nowadays increasingly considered as a good candidate for substituting the conventional systems. These technologies for hydrogen separation includes:
1. Dense polymer membranes 2. Porous carbon membranes 3. Dense metal membranes
4. Microporous ceramic membranes 5. Dense ceramic membranes
I.2 Objectives of research
The overall objectives of this PhD research is to effectively gain a good understanding of the phenomena and mechanisms of the simultaneous conductions of proton and electron in the solids by carrying out the physico-chemical characterization techniques. This allow for a selection of a proper reference materials from which novel materials with desired properties can be developed as selective membranes for the hydrogen separation application.
In this regards, the methodology of the research will be to:
Ø Formulate, develop, and characterize mixed proton/electron-conducting materials from a structural and an electrical standpoints and in term of chemical stability. Ø Carry out shape processing of materials for hydrogen separation application according
to two configurations:
1. A standalone configuration in order to evaluate the hydrogen flux rates and hydrogen permeability.
2. A symmetrical configuration electrode / MPEC / electrode in order to study the influence of the electrocatalyst on the permeation rates.
Ø Performance testing of hydrogen separation in the temperature range of 300 – 600°C.
I.3 Layout of the research work
The dissertation is organized into 6 chapters as following:
Chapter I introduces the motivations for the search by laying the ground for which
Chapter II presents overview of various hydrogen production and purification
technologies separation membranes with a particular interest in separation membranes.
Chapter III discusses the theory of mixed conduction and the existing materials that
exhibit this investigated electrical property.
Chapter IV provides the results from the synthesis, elaboration, characterization, and
chemical stability study of the single-phase and dual-phase materials.
Chapter V deal with hydrogen permeation tests of the selected membranes in
comparison to the reference materials by evaluating the permeation rates and permeability.
References
[Adhikari et al.] S. Adhikari and S. Fernando. Hydrogen membrane separation techniques. Industrial
& Engineering Chemistry Research 45 (2006) 875-881.
[Dutta] S. Dutta. A review on production, storage of hydrogen and its utilization as an energy source.
Journal of Industrial and Engineering Chemistry 20 (2014) 1148-1156.
[EIA] EIA, 2015. Key World Energy Statistics.
https://www.iea.org/publications/freepublications/publication/KeyWorld_Statistics_2015.pdf
[FuelCell] The Fuel Cell and Hydrogen Annual Review, 2015.
[Höok et al.] Höok M. and Tang X. Depletion of fossil fuels and anthropogenic climate change—A
review. Energy Policy 52 (2013) 797-809.
[Karekeri et al.] Karekeri S., Lata K., Teixeira S. and Coelho S.T. (2006). Traditional Biomass
Energy: Improving Its Use and Moving to Modern Energy Use. Assmann D., Laumanns U. and Uh D. (Eds.), Renewable Energy: A Global Review of Technologies Policies, and Markets. (pp. 230). Earthscan.
[Majlan et al.] Majlan E.H., Daud W.R.W., Iyuke S.E., Mohamad A.B., Kadhum A.A.H., Mohammad
A.W., Takriff M.S., Bahaman N. Hydrogen purification using compact pressure using adsorption system for fuel cell. International Journal of Hydrogen Energy 34 (2009) 2771-2777.
[Shafiee et el.] Shafiee S. and Topal E. When will fossil fuel reserves be diminished? Energy Policy 37
(2009) 181-189.
Chapter II :
This chapter presents a brief description of hydrogen production, separation, and purification technologies. Classical purification technologies are compared to membrane technologies. Each types of separation membranes are presented. In particular, the main focus is made on the dense ceramic membranes in which proton-conducting and mixed protonic-electronic materials offer the most efficient solution for hydrogen separation.
II.1. General Overview on Hydrogen Production Technologies
Hydrogen is the future fuel and energy carrier; it is carbon-free and hence environmentally friendly. Hydrogen is considered a clean and efficient energy carrier since its combustion only produces water as by-product. There are various ways to produce hydrogen based on the feedstocks and the types of the process implemented. In this context, hydrogen can be generated cleanly or in mixture with carbonaceous compounds as by-products. There are several production routes from which hydrogen can be generated. These can be separated into two categories: (1) carbon-free production via water electrolysis and (2) fossil fuel-derived production. The later includes steam reforming of natural gas, coal gasification, and oil-based partial oxidation. [Andreassen, Hauserman, Oertel et al., Foster et al.]. On a global scale, current hydrogen production is largely fossil-fuel based. The shares of various production methods of hydrogen are shown in Figure II. 1 [Voldsund et al.]. Conversion of fossil fuels is responsible for 96% while water electrolysis accounts for the remaining 4%. Steam reforming of natural gas holds the largest share, the second largest part of oil-based partial oxidation and the rest is derived from coal gasification.
II.1.1 Water electrolysis
The water electrolysis is one of the most important industrial processes commercialized for pure hydrogen production. The process has reached mature status in terms of technological progress but is still being intensively under research and development in order to improve the hydrogen production capacity. Water electrolysis is expected to become even more important in the future. Three major technologies currently under consideration for electrolytic hydrogen production includes alkaline, polymer membrane, and ceramic oxide electrolyzers. Water electrolysis is an electrochemical process that requires energy input in the form of DC electricity and involves splitting of water molecule into hydrogen and oxygen.
'
*$(+) , -/2569 : '
*(6) ,
;*$
*(6)
(II.1)
However, presently very less percent of hydrogen is produced through water electrolysis while the rest of it is still derived from fossil fuels.
II.1.2 Fossil fuel-derived production
Fossil fuels comprising of natural gas, coal and petroleum are the main feedstocks for hydrogen production. Steam reforming of natural, coal gasification or oil-based partial oxidation is a thermochemical process that generates a mixture of CO and H2, commonly
known as syngas. Today, syngas is produced in large scale for the synthesis of chemicals like ammonia and methanol and for the generation of electricity in thermal power plants.
a. Steam reforming of natural gas
Natural gas contains essentially methane (CH4), which is the simplest form of hydrocarbons.
natural gas, steam and methane react to yield hydrogen and carbon monoxide in an equilibrium-limited, high endothermic reaction:
<'
=, '
*$ > <$ , 3'
*?H
@#A#*BC#DE F0G#kJImol (II.2)
The reaction is normally carried out at temperature of 500-900°C, pressure at 20-35 atm and steam-to-carbon (S/C) ratios of 2.5 to 3 [Ritter et al., Subramani et al.]. High conversion is favored by high temperature, low pressure and high S/C ratio [Subramani et al.].b. Gasification
The solid fuels like coal or biomass can be converted to syngas by gasification. In this process, the fuel is reacted with oxygen and steam at high temperature and pressure, resulting in a mixture of hydrogen, carbon monoxide and possibly carbon dioxide. Some of the reactions taking place are:
< , '*$ > <$ , '*
?H
@#A#*BC#D E 131#kJImol(II.3)
< , $* > <$*
?H
@#A#*BC#D E K#3L4#kJImol(II.4)
< ,;*$* > <$#
?H
@#A#*BC#D E K111#kJImol(II.5)
< , <$* > F#<$?H
@#A#*BC#D E 17F#kJImol(II.6)
c. Partial oxidation
Partial oxidation (POX) of hydrocarbon is a method for producing hydrogen from heavy hydrocarbon such as diesel fuel and residual oil. The reaction occurs when a fuel is partially oxidized burnt with a sub-stoichiometric amount of air in a reformer. Partial oxidation is an exothermic process and can be performed with or without a catalyst. The general reaction equation is:
<M'N,M*##$* > O<$ ,N*'*
(II.7)
Large-scale uncatalysed reactors typically operate at temperature in the range of 1150-1500°C and pressure between 20 and 80 bar. For catalytic POX process, operating temperature and pressure are reduced. Temperatures are between 780°C and 900°C, and pressures are between 25 and 35 atm. Depending on the desired products from the POX process, either atmospheric air or pure oxygen is supplied.
d. Water-gas shift
After reforming, gasification or partial oxidation, the produced CO is typically reacted with steam to produce CO2 and additional hydrogen in a process called Water-Gas Shift (WGS)
reaction. This reaction is particularly important for hydrogen production with CO2 capture due
to the conversion of CO to CO2, and because hydrogen yield is maximized. The reaction is
equilibrium-limited and slightly exothermic:
<$ ,#'*$ > <$*, '*
?H
@#A#*BC#D E # K41#PQIOR+(II.8)
In practical situation, the products of the reaction normally contain H2, CO2, CO,Table II.1 Shifted syngas compositions given in mol% from steam methane reforming and coal gasification [Voldsund et al.]
Gas composition Steam methane reforming + WGS Coal gasification + WGS
H2 70-80% 54-57% CO2 15-25% 38-44% CO 0.5-3% 0.6-1.7% N2 Trace 1.0-4.8 Ar 0 0.47-0.9% CH4 3-6% % 1.33S H2S 0 % 0.0GS
II.2 Overview on Hydrogen Separation Technologies
II.2.1 Current Hydrogen Separation Technology
Producing hydrogen via steam reforming of natural gas or gasification of coal/biomass results in a gas mixture with a variety of contaminants including CO, CO2, CH4, H2S, H2O,…etc
[Meinema et al.]. A variety of methods to separate and purify hydrogen exist. This involves
the removal of the contaminants from the mixed gas stream and produce a purified of hydrogen gas stream.
Large-scale, industrial hydrogen production has been done by purifying hydrogen-rich gas mixture via PSA technology (Pressure Swing Adsorption) or cryogenic distillation. [Meinema
et al., Ma, Doong et al.]. The prime separation technology is the PSA which has several
Figure II.2 Schematic description of a steam reformer plant for production of high-purity H2 [Rydén
et al.]
a. Pressure Swing Adsorption (PSA)
Pressure Swing Adsorption (PSA) has been used in hydrogen plants since 1980s [Ritter et
al.], and the process has seen tremendous growth during the last decades. The PSA units are
designed and developed for the recovery and purification of hydrogen from different hydrogen-rich gas streams such as synthesis gas from steam reforming or gasification processes with possibility of obtaining hydrogen purity exceeding 99.9999 mol.% [Sircar et
al.].
PSA technology operates based on adsorption which is a physical process where molecules fix themselves onto the surface of adsorbents (usually solid materials). Molecules have different affinities for various types of adsorbent, and this phenomena can be used to purify gases.
In a PSA unit, five types of adsorbent materials are mainly used (Table II.2) [Wiessner].
Table II.2 Adsorbent materials [Wiessner]
Type Gas component to be adsorbed
Alumina H2O, NH3
Activated carbon CO2, CH4, C2, N2
Molecular-sieving zeolite CH4, CO, N2
Molecular-sieving carbon O2
For the purpose of hydrogen purification multiple types of adsorbents can be selected so that all impurities are adsorbed. However, the correct selection and combination of these materials is a complicated matter. Figure II.3 shows a typical PSA adsorption bed consisting of three layers of absorbent materials which will selectively adsorb different gas components based on their adsorbing functionalities [Baksh et al.].
Figure II.3 Schematic drawing of a PSA adsorption bed consisting of alumina, carbon and zeolite as first, second, and third layer respectively [Baksh et al.]
The separation process is based on differences in binding forces to the absorbent material. Highly volatile components with low polarity such as hydrogen are practically non-adsorbable as opposed to molecules like N2, CO, CO2, hydrocarbons and water vapor. Consequently,
these impurities can be adsorbed from a hydrogen-containing stream and high purity hydrogen is recovered.
Adsorption of impurities is carried out at high pressure being determined by the pressure of the feed gas. The feed gas flows through the adsorber vessels in an upward direction, where impurities such as water, heavy hydrocarbons, light hydrocarbons, CO2, CO, N2 are
with very limited adsorption. Highly pure hydrogen thus exits the adsorber vessels at the top. After a defined time when the adsorbent is saturated, the adsorption phase of the vessel stops and regeneration starts. Another adsorber vessel takes over the task of adsorption to ensure continuous hydrogen supply. The regeneration phase consists of basically five steps in chronological order as follow:
- Pressure equalization: lowering the pressure by depressurization - Provide purge: providing pure hydrogen to purge
- Dump: desorption of impurities to leave the adsorber
- Purging: final desorption of impurities by highly pure hydrogen into the tail gas system - Repressurization: increasing pressure to reach the required pressure level again
Figure II.4 shows a typical six-bed PSA unit for the separation of hydrogen [Weissner]. The
production capacities range from a few hundred Nm3/h to more than 400 000 Nm3/h.
Figure II.4 Schematic of a six-bed PSA unit [Weissner]
b. Cryogenic distillation
operates by cooling the gas and condensing some or all of the constituents of the gas stream. The process is applicable to hydrogen separation because gaseous impurities in a crude hydrogen gas stream such as hydrocarbon, water vapor, carbon monoxide, carbon dioxide, nitrogen,…etc condense at much higher temperature than hydrogen. Table II.3 shows the boiling point of principle gas constituent found in a typical syngas or gasified stream.
Table II.3 Boiling point of gas constituents at atmospheric pressure
Gas constituent Boiling point (°C)
H2 - 253 N2 - 195.8 CO - 192 Ar - 185.9 O2 - 183 CH4 - 161.5 CO2 - 78.5 H2S - 60 Propylene - 47.7 Propene - 43 H2O 100
For hydrogen separation, the gas mixture is compressed and cryogenically cooled to < -150°C. Contaminant gases are condensed at different temperature level while hydrogen remains in gaseous phase. The concentrated hydrogen and liquefied components are separately discharged from a condenser kept at cryogenic temperature. The purity level of hydrogen are normally in the range of 90-98% in commercial processes, and the recovery rate of hydrogen in terms of a quantitative product to feed ratio is typically 95%.
II.2.2 Hydrogen Membrane Separation Technologies
The purity of hydrogen produced from these current separation technologies is mediocre and they are energetically inefficient and costly processes [Meinema et al.]. The separation and purification by means of membrane technology offers better solutions and thus is a potential alternative to the PSA [Adhikari et al.]. It can produce high-purity or at least moderately pure hydrogen at much lower costs. In order to develop a high-performance membrane that meet the industrial standards, general requirements on the properties of the membrane must be verified.
· High permeability and permeation flux rates · Excellent selectivity for targeted species
· Sufficient mechanical, chemical, and thermal stability under applied conditions · Minimal or no parasitic power requirements
· Long and reliable service life
· Robust performance under harsh operating environments · Cost-effective production
No membrane separation technology can simultaneously meet all of the above requirements. In particular, there is often a trade-off between the permeability and selectivity: as selectivity increases, permeability decreases and vice versa [Robeson]. However, dense ceramic membranes are attracting attentions in the field of hydrogen separation and purification due to key advantages over the other types of membrane technology. Table II.4 shows the specific target values set for dense ceramic hydrogen separation membranes by the U.S. Department of Energy.
Table II.4 Technical targets for hydrogen dense ceramic separation membranes
Characteristic Units 2003 Status 2007 Targets 2010 Targets 2015 Targets
Flux rate ml.cm-2.min-1 30.5 50.8 101.7 152.5
Cost US $/m2 1940 1620 1080 <1080
Durability Year(s) <1 1 3 >5
Operating temp. °C 300-600 400-700 300-600 250-500
Parasitic power kWh/1000 m3 H
2 generated 113 113 105 <100
ΔP operating capability MPa 0.69 1.38 <2.76 2.76-6.89
Hydrogen recovery % of total gas 60 70 80 90
b. Transport Mechanism
There are a variety of membrane types being investigated and developed which can be grouped into three categories based on the hydrogen transport mechanism across the membrane: molecular, atomic, and ionic (proton or mixed proton/electron).
i. Molecular transport
It is a mechanism by which chemical species are transported across the membrane in a molecular form. Depending on the architecturing of material microstructure, the mechanism can be further divided into three processes such as molecular sieving, surface diffusion and solution diffusion. Figure II.5 illustrates the molecular transport mechanism across a porous and dense membrane.
Figure II.5 Schematic presentation of mechanisms for permeation of gases through membranes
Membranes operate on a molecular-sieve effect are essentially microfilters. In order to function as a molecular sieve, the membrane must has pore size between the diameter of the smaller and larger gas molecules. Then only the smaller molecule can permeate and a very high separation would be achieved. In case of hydrogen separation, the pore of the membrane
Membranes Porous
Dense
Molecular sieving or surface diffusion
are sized at F.8L#!#to enable the very small hydrogen molecule to move through the membrane while larger gas molecules are left behind.
Surface diffusion has been studied as another means of enhancing separation factor. Gas molecules are adsorbed on the pore walls of the membrane and migrate along the surface. Surface diffusion increases the permeability of the components adsorbing more strongly to the membrane pores. Consequently, transport of non-adsorbing components is reduced and selectivity is increased. This positive contribution of surface diffusion only works for certain temperature and pore diameters.
The solution-diffusion mechanism is considered to consist of three steps: (1) absorption or adsorption at the upstream, (2) activated diffusion (solubility) through the membrane and (3) desorption or evaporation on the other side of the membrane at the downstream. This solution-diffusion mechanism is driven by a difference in the thermodynamic activities existing at the upstream and downstream of the membrane. The activity difference causes a concentration difference in the direction of decreasing activity.
ii. Atomic transport
iii. Ionic transport
Membranes based on ionic transport mechanism are dense materials having the ability to conduct protons or mixed protons/electrons independently such that hydrogen is dissociated on one side and re-constituted on the other. Based on the conduction mechanism, the ionic transport membrane can be divided into two types:
1. Proton conducting membranes: this type of membrane materials exhibit predominantly protonic conductivity in the operating temperature range and thus require external electrical current in order to assure hydrogen permeation.
2. Mixed proton/electron conducting membranes: this type of membrane is capable of conducting both protons and electrons simultaneously inside the materials to provide adequate hydrogen flux. Moreover, based on the material composition, these mixed conducting membranes can be further separated into two types: (1) Purely mixed conducting membrane and (2) mixed conducting composite membrane.
a. Different Types of Membrane Materials
Figure II.6 Important types of membrane technology for hydrogen separation
i. Polymer Membranes
It is a dense-type membrane which transports the targeted species based on the solution diffusion mechanism through the bulk of the membrane material. Figure II. 7 gives a schematic representation of a model proposed by Brandt for transport of small penetrant molecules through the polymer matrix [Pandley et al.]. The model shows a bundle of parallel polymer chains and inclusion of gas molecules. In order to move into the polymer matrix the gas molecule pushes the polymer chain and jumps into a new position.
Atomic transport Molecular transport Polymer Glassy Rubbery Ionic transport Carbon Molecular sieving Surface diffusion
Microporous ceramic Silica, Alumina, Zironia, Zeolite
Dense ceramic
Protonic
Mixed protonic/electronic Metal Palladium or Palladium alloys
H2 separation
Figure II.7 A model proposed by Brandt for the transport of small penetrant molecules in polymers [Pandley et al.].
The polymer membrane is used industrially for hydrogen separation from gaseous mixtures that consist of N2, CO, or hydrocarbons [Freeman et al.]. The membranes can be further
divided into two types (Table II.5):
1. Glassy-type membrane : prepared at temperature below the glass transition temperature
2. Rubbery-type membrane : prepared at temperature above the glass transition temperature
Table II.5 Most important rubbery and glassy polymers used in industrial membranes gas separation
Rubbery Glassy
Poly(dimethylsiloxane) Cellulose acetate
Ethylene oxide/propylene oxide Polyperfluorodioxides
Polycarbonates Polyimides
Poly(phenylene oxide) Polysulfone
The membrane can operate in the temperature range of 90-100°C. The glassy membranes show low flux but high selectivity whereas the rubbery membranes exhibit higher flux but lower selectivity. Table II. 6 gives hydrogen permeabilities (at 300K and 2.07 bar feed gas pressure) for selected polymer membranes and selectivities over nitrogen (N2), methane
