Materials for solid alkaline fuel cells

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Universidade de Aveiro Ano 2018

Departamento de Engenharia de Materiais e Cerâmica

Nuno André

Carvalho Sousa

Materiais para Pilhas de Combustível

Alcalinas Sólidas

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Universidade de Aveiro Ano 2018

Departamento de Engenharia de Materiais e Cerâmica

Nuno André

Carvalho Sousa

Materiais para Pilhas de Combustível

Alcalinas Sólidas

Materials for Solid Alkaline Fuel Cells

Tese apresentada à Universidade de Aveiro para cumprimento dos requisitos necessários à obtenção do grau de Doutor em Ciência e Engenharia de Materiais, realizada sob a orientação científica do Doutor Filipe Miguel

Henriques Lebre Ramos Figueiredo, Investigador Principal da Universidade de Aveiro e do Doutor Armando Jorge Domingues Silvestre Professor Catedrático da Universidade de Aveiro

Trabalho realizado com o apoio financeiro do projeto UniRCell POCI-01-0145-FEDER-016422 (Ref. SAICTPAC/0032/2015).

Trabalho realizado com o apoio da FCT sob a forma de bolsa individual de doutoramento com a referência SFRH/BD/89670/2012.

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o júri

presidente Prof. Doutor Vítor José Babau Torres

professor catedrático da Universidade de Aveiro

Prof. Doutora Maria de Fátima Grilo da Costa Montemor professora catedrática do Instituto Superior Técnico da Universidade de Lisboa

Prof. Doutor Fernando Manuel Bico Marques professor catedrático da Universidade de Aveiro

Prof. Doutor Francisco Miguel Portela da Gama professor associado com agregação da Universidade do Minho

Prof. Doutor Duncan Paul Fagg

equiparado a investigador principal da Universidade de Aveiro

Prof. Doutor Filipe Miguel Henriques Lebre Ramos Figueiredo equiparado a investigador principal da Universidade de Aveiro

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agradecimentos

Em primeiro lugar queria agradecer à FCT pelo financiamento deste ciclo. À Universidade de Aveiro e ao CICECO, pela cedência das instalações para a realização deste trabalho.

Aos meus orientadores Doutor Filipe e Professor Armando, pelos ensinamentos e esclarecimentos.

Aos amigos que acumulei ao longo destes anos, em Vila Real, a minha cidade natal, em Braga durante os anos de universitário e durante esta mais recente experiência da minha vida, em Aveiro.

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palavras-chave Célula de Combustível, Membrana, Nanocompósito, Celulose Bacteriana, Condutividade Aniónica, Cátodo, Perovesquite, Fase Ruddlesden-Popper, Electrocatálise, Estabilidade em Condições Alcalinas.

resumo O principal objetivo deste trabalho é o desenvolvimento de uma nova geração de materiais sustentáveis para células de combustível alcalinas de eletrólito solido (SAFC) e tecnologias de hidrogénio. A tecnologia SAFC oferece vantagens a dois níveis: i) o eletrólito polimérico solido é menos sensível a envenenamento por CO2,

e especialmente ii) a cinética das reações de elétrodo é melhorada no meio básico criado na célula, que pode não requerer o uso de catalisadores à base da escassa platina. Estas vantagens implicam desafios. O transporte de OH-_{é inerentemente}

mais lento que o de H+_{, e, portanto, polímeros altamente condutores são}

necessários. O meio alcalino ameaça a integridade tanto da membrana polimérica como do catalisador, então materiais quimicamente estáveis são necessários. Esta tese explora o potencial da celulose bacteriana (BC) como suporte de polieletrólitos catiónicos/quaternários, formando membranas nanocompósitas com excelentes propriedades mecânicas e estáveis em ambientes alcalinos. A comparação de membranas de BC pura e com tratamento ácido ou básico não revela uma deterioração evidente da membrana com tratamento alcalino em termos de degradação térmica, condutividade iónica, propriedades viscoelásticas e até desempenho da célula de combustível. Este último é severamente limitado pela baixa condutividade da BC (<1 ms.cm-1_{a 98% de humidade relativa (RH)),}

produzindo menos de 1 mW.cm-2_{. Sintetizaram-se membranas nanocompósitas de}

BC com três polieletrólitos diferentes funcionalizados com grupos NH4+:

Poli[2-(acrililoxi)etil]trimetilamonio (PAETA), poli(3-acrilamidopropil)-

trimetilamonio (PAPTA) e poli(vinilbenzil)trimetilamonio (PVBTA). Membranas à base de PAETA não são estáveis em condições alcalinas, mas nanocompósitos de PAPTA e PVBTA mantêm as excelentes características viscoelásticas da BC, com módulos de armazenamento superiores a 1 GPa e permanecendo estáveis até cerca de 200 °C. O comportamento de absorção de água correlaciona-se com a condutividade iónica, que aumenta com o aumento de RH e de temperatura, atingindo a 94 °C e 98% RH 72.8 mS.cm-1_{para BC:PAPTA e 12.4 mS.cm}-1_para

BC:PVBTA. Testes de célula de combustível de BC:PAPTA atingiram 10 mW.cm-2

a 55 mA.cm-2_{, um desempenho determinado por polarização dos elétrodos. O}

estudo dos eletrólitos é complementado pelo estudo de uma serie de perovesquites e fases Ruddlesden-Popper para aplicação enquanto catalisadores para a redução de oxigénio (ORR) e de peroxido de hidrogénio (HPRR) em meio alcalino. Testes de estabilidade em ambiente alcalino (pH>14) indicam que Sr, Ni, Cu e Co nas perovesquites e nas fases Ruddlesden-Popper tendem a dissolver, em

conformidade com diagramas de Pourbaix calculados. A superfície dos elétrodos sem catiões metálicos, que por sua vez se acumulam no eletrólito, é provável que resulte em efeitos difíceis de prever nas propriedades electrocatalíticas da amostra. De acordo com os diagramas de Pourbaix, materiais à base de Mn devem ser capazes de resistir a ambientes alcalinos. A perovesquite La0.7Sr0.3MnO3 apresenta

a melhor atividade electrocatalítica, e emerge de entre as composições testadas como a única alterativa estável para a catálise de ORR em meio fortemente alcalino. Uma tentativa é feita no sentido de correlacionar composição, estabilidade química e comportamento eletroquímico dos materiais, baseado em modelos

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keywords Fuel Cell, Membrane, Nanocomposite, Bacterial Cellulose, Anionic Conductivity, Cathode, Perovskite, Ruddlesden-Popper Phase, Electrocatalysis, Stability in Alkaline Conditions.

abstract The development of a new generation of sustainable materials for solid alkaline fuel cells (SAFC) and hydrogen technologies is the major driver of this work. SAFC technology offers advantages at two levels: i) the base solid polymer electrolyte is less sensitive to CO2 poisoning, and especially ii) the kinetics of the electrode

reactions is improved in the basic environment created inside the cell, which may avoid the use of scarce platinum catalysts. These advantages entail challenges. The OH-_{transport is inherently slower than of H}+_{, and thus highly conductive}

polymers are necessary. The strong alkaline medium threatens the integrity of both the polymer membrane and the catalyst, so chemically stable materials are

necessary. This thesis explores the potential of bacterial cellulose (BC) as support to cationic/quaternary polyelectrolytes, forming nanocomposite membranes with excellent mechanical properties and stable in alkaline environments. Comparison of pristine BC membranes and submitted to acidic or basic treatments reveals no noticeable degradation of the alkaline-treated membrane in terms of thermal degradation, ionic conductivity, visco-elastic properties and even fuel cell

performance. The latter is severely limited by the low conductivity of BC (<1 mS.cm -1_{under 98% relative humidity (RH)), delivering less than 1 mW.cm}-2_.

Nanocomposite membranes of BC with three different polyelectrolytes functionalized with NH4+ groups: Poly[2-(acryloyloxy)ethyl]trimethylammonium

(PAETA), Poly(3-acrylamidopropyl)trimethylammonium (PAPTA), and

Poly(Vinylbenzyl)trimethylammonium (PVBTA) were synthesized. PAETA-based membranes are not stable in alkaline conditions, but both PAPTA and PVBTA nanocomposites maintain the excellent visco-elastic characteristics of BC, with storage modules in excess of 1 GPa, remaining stable up to about 200 °C. The water adsorption behaviour correlates with the ionic conductivity, both increasing with increasing RH and temperature, reaching, at 94°C and 98% RH a maxima of 72.8 mS.cm-1_{for BC:PAPTA and 12.4 mS.cm}-1_{for BC:PVBTA. Fuel cell tests of}

BC:PAPTA delivered 10 mW.cm-2_{at 55 mA.cm}-2_{, a performance determined by}

electrode polarization. The electrolyte study is complemented by the study of a range of transition metal perovskite and Ruddlesden-Popper phases for application as catalysts for the oxygen reduction (ORR) and hydrogen peroxide reduction (HPRR) in alkaline media. Stability tests in alkaline environments (pH>14) indicate that Sr, Ni, Cu and Co in perovskite and Ruddlesden-Popper phases tend to dissolve, in agreement with calculated Pourbaix diagrams. An electrode surface depleted of metallic cations, which accumulate on the electrolyte, is likely to have unpredictable effects on the electrocatalytic properties of the system. According to the Pourbaix diagrams, Mn-based materials should be able to withstand the alkaline environments. The perovskite La0.7Sr0.3MnO3 displays the best electrocatalytic

activity and emerges from the tested compositions as the only stable alternative to catalyse the ORR in strong alkaline medium. An attempt is made to correlate the composition, chemical stability and electrochemical behaviour of these materials based on known molecular-orbital models. This, however, must be taken with

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List of Figures

Figure 1. Schematics of a SAFC running on hydrogen. ... 5

Figure 2. Conduction mechanism of OH-_{through polymeric AEMs. Adapted from}12_{... 7}

Figure 3. QA removal in the presence of hydroxide by nucleophilic substitution (top) and Hofmann elimination (bottom). The hydroxide attacks at α-carbon and β-proton, respectively.22_{... 9}

Figure 4. Cationic functional groups for AEMs (R – alkyl groups).3,7,86_{... 15}

Figure 5. AETACl, APTACl and VBTACl structure. ... 16

Figure 6. a - Cellulose structure and b - BC supramolecular organization.106_{... 19}

Figure 7. Scanning electron micrograph of: a - the surface of a freeze-dried BC pellicle and b - a fractured edge of BC sheet.109_{... 20}

Figure 8. Synthetic route to produce QABC. Adapted from123_{. ... 21}

Figure 9. Adsorption profiles for hemoglobin onto BC and QABC as function of pH.123_{... 22}

Figure 10. Design of a three-layer electrode for a bipolar stack design. ... 23

Figure 11. a - polarization losses and b - current efficiency in alkaline and acidic media.133_{... 25}

Figure 12. Modified form of Pourbaix diagram: pH dependence of equilibrium potential for reaction (8) proceeding as an inner sphere reaction (ΔGads = −30 kJ.mol-1 is the adsorption free energy of O2,ad−).134 ... 25

Figure 13. Kinetic currents (jk) at 0.85 V for ORR different metal single crystals in 0.1 mol.L-1 NaOH solution as functions of the calculated metal d-band center (εd - εF; relative to the Fermi level).158 ... 33

Figure 14. jk at 0.85 V for O2 reduction on: a - carbon-supported metal nanoparticles and b - Pt monolayers supported on the different single-crystal surfaces.158_{... 34}

Figure 15. ABO3 perovskite structure with La in the A-site and a metal (M) in the B-site. ... 36

Figure 16. Trends in catalytic activity towards oxygen reduction with: a - substitution of the B‐site,169,194,197_b - composition of the A‐site170,198,199_{and c - substitution of the A‐site}200_{as compiled in Risch’s review}201_{... 37}

Figure 17. a - Potentials at 25 μA.cm-2_{of the perovskites oxides depicting an M-shaped relationship with} d-electron number; b - O22-/OH- exchange, where the shape of the eg electron pointing directly towards the surface O atom plays an important role during O22-/OH- exchange; c - Potentials at 25 μA.cm-2 of the perovskite oxides as function of the eg-orbital filling. Adapted from 169. ... 38

Figure 18. ORR mechanism on perovskite oxide catalysts via four steps occurring at the surface: 1, hydroxide displacement; 2, peroxide formation; 3, oxide formation; 4, hydroxide regeneration.169_{... 39}

Figure 19. Crystal structure of La2MO4 Ruddlesden-Popper consisting of LaMO3 (Perovskite) and LaO (rock salt) layers alternating in the c direction.205_{... 40}

Figure 20. Publications per year for the different search terms. Source: Web of knowledge search on 29.11.2017. ... 42

Figure 21. Publications per year for the “polymer ”+”alkaline fuel cell” and “solid alkaline fuel cell” search terms. Source: Web of knowledge search on 29.11.2017. ... 42

Figure 22. SWOT analysis of SAFCs. ... 43

Figure 23. Preparation PAA:MBA (B* step was expected to result in the formation of PAETAOH:MBA). . 48

Figure 24. Preparation of PAPTAOH:MBA. ... 48

Figure 25. Preparation of PVBTAOH:MBA. ... 49

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Figure 28. Temperature profile for the sintering process. ... 53 Figure 29. Ceramic electrode prepared for the electrochemical tests. ... 55 Figure 30. Photograph of a – catalysed GDL, b – membrane with a piece of GDL on the back side. ... 57 Figure 31. Photograph of single fuel cell testing system and of the interconnector plate with an interdigitated flow field design. ... 58 Figure 32. FTIR-ATR spectra of pristine BC, alkaline-treated BC and acid-treated BC with identification of the BC chain peaks. ... 61 Figure 33. Chemical structure of BC. Adapted from 225_{. ... 62}

Figure 34. 13_{C CP-MAS NMR spectra for pristine, alkaline and acid-treated BC samples. (acid-treated BC}

spectrum reproduced from 120_{) ... 63}

Figure 35. XRD diffractograms of pristine, alkaline and acid-treated120_{BC samples. ... 63}

Figure 36. a- SEM micrographs of alkaline-treated BC (in 1 M NaOH for 24 h) and b - ESEM image of BC (treated in 3.75 M NaOH for 30mins) recorded by Xi Chen.224_{... 64}

Figure 37. SEM micrographs of pristine, alkaline and acid-treated120_{BC samples. ... 65}

Figure 38. TGA and derivative (dashed lines) curves for pristine, alkaline- and acid-treated120_{BC samples,}

obtained under N2 atmosphere. ... 66

Figure 39. Storage modulus and tan δ obtained by DMA in air atmosphere for pristine, alkaline and acid-treated120_{BC samples. ... 68}

Figure 40. Arrhenius plots of the in-plane σ of pristine, alkaline and acid-treated BC samples measured under variable RH (60, 80 and 98%). Lines are for visual guidance. Relative humidity increases from the bottom to the top. ... 69 Figure 41. Evolution of the σ in 3 distinct conditions: 60 °C/60% RH (blue), 80 °C/60% RH (orange) and 80 °C/98% RH (grey). ... 70 Figure 42. Arrhenius plot of the through-plane (full symbols - ●) and in-plane (contours - ○) conductivities of alkaline and acid-treated BC at different RH: ○ – 60%, ∆ - 80% and □ – 98%. ... 71 Figure 43. Polarization and power curves of alkaline and acid-treated BC MEAs collected at room

temperature under humidified hydrogen/air gradient. ... 72 Figure 44. Tafel plots for alkaline and acid-treated BC membranes. ... 74 Figure 45. Photo of: a - Dry BC membrane; b - Dry BC:PAPTAOH:MBA (1:3:0.2) membrane, c -

PAPTAOH:MBA (1:0.2) powder and d – PVBTAOH:MBA (1:0.2) powder. ... 75 Figure 46. Synthesis of the acidic PAA:MBA proton conductor resulting from the transformation of the PAETACl electrolyte in alkaline medium... 76 Figure 47. FTIR-ATR spectra of BC, PAETACl:MBA and BC:PAETACl:MBA with identification of the PAETACl:MBA chain peaks (PAETACl:MBA is also presented). ... 77 Figure 48. FTIR-ATR spectra of PANa:MBA and BC:PANa:MBA with identification of the

PAETACl:MBA chain peaks. ... 78 Figure 49. EDS pattern of BC:PAETACl:MBA (1:5:0.4) and BC:PANa:MBA (1:5:0.4) (before and after the immersion in the 1 M NaOH). ... 79 Figure 50. Liquid 13_{C NMR spectra of the 1 M NaOH solution after the immersion of PAETCl:MBA (1:0.2).}

... 79 Figure 51. 13_{C CP-MAS NMR spectra of BC, PAETACl:MBA and two representative nanocomposites}

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Figure 55. FTIR spectra of the 4 nanocomposites before (BC:PAPTACl:MBA) and after

(BC:PAPTAOH:MBA) the exchange step with identification of the PAETACl:MBA C=O str. peak. ... 83 Figure 56. EDS pattern of BC:PAPTACl:MBA (1:2:0.05), BC:PAPTAOH:MBA (1:2:0.05) and

BC:PAPTAOH:MBA (1:5:0.4). ... 84 Figure 57. 13_{C CP-MAS NMR spectra of BC, PAPTACl:MBA and the BC:PAPTACl:MBA nanocomposites.}

... 85 Figure 58. IEC and the relative intensity of the C=O str. peak in the FTIR-ATR (normalized by the OH str.) as function of the measured fraction of PAPTAOH+MBA. ... 86 Figure 59. VBTACl structure. ... 86 Figure 60. FTIR-ATR spectra of BC, PVBTACl:MBA and BC:PVBTACl:MBA with identification of the PVBTACl:MBA chain peaks. ... 87 Figure 61. FTIR spectra of the (1:3:0.4) nanocomposites before (BC:PVBTACl:MBA) and after

(BC:PVBTAOH:MBA) the exchange step with identification of the trimethylammonium δC-H peak. ... 87 Figure 62. 13_{C CP-MAS NMR spectra of BC, PVBTACl:MBA and the BC:PVBTACl:MBA nanocomposites.}

... 88 Figure 63. EDS pattern of BC:PVBTACl:MBA (1:3:0.4), BC:PVBTAOH:MBA (1:3:0.4). ... 89 Figure 64. Benzenic group containing polymer alkaline degradation by SN2. Adapted from 249. ... 89

Figure 65. a - IEC and b - water uptake capacity plotted as function of the measured fraction of

PAPTAOH+MBA. ... 90 Figure 66. XRD pattern of a representative sample of each nanocomposite system in comparison with the alkaline-treated or acid-treated BC. ... 91 Figure 67. XRD pattern of PAPTAOH:MBA. ... 92 Figure 68. SEM micrographs of the nanocomposite membranes of the BC:PAETACl:MBA and

BC:PANa:MBA (after the alkaline hydrolysis) systems. ... 93 Figure 69. SEM micrographs of the nanocomposite membranes of the BC:PAPTA:MBA system

(BC:PAPTAOH:MBA is after the exchange step). ... 94 Figure 70. SEM micrographs of the nanocomposite membranes of the BC:PVBTA:MBA system

(BC:PVBTAOH:MBA is after the exchange step). ... 94 Figure 71. BC:PVBTAOH:MBA (1:3:0.2) SEM micrograph. ... 95 Figure 72. Thermograms and the corresponding derivative, obtained under N2 atmosphere, of PAA:MBA

(1:0.2) (the acid-treated BC is shown as a term of comparison). ... 96 Figure 73. Thermograms and the corresponding derivative, obtained under N2 atmosphere, of the

BC:PAA:MBA nanocomposites system. ... 97 Figure 74. Thermograms and the corresponding derivative, obtained under N2 atmosphere, of

PAPTACl:MBA (1:0.2) and the corresponding alkaline form: PAPTAOH:MBA (1:0.2). ... 98 Figure 75. Thermograms and the corresponding derivative, obtained under N2 atmosphere, of the

BC:PAPTACl:MBA and BC:PAPTAOH:MBA nanocomposites. ... 99 Figure 76. Thermograms and the corresponding derivative, obtained under N2 atmosphere, of

PVBTAOH:MBA (1:0.2). ... 100 Figure 77. Thermograms and the corresponding derivative, obtained under N2 atmosphere, of the

BC:PVBTAOH:MBA nanocomposites system. ... 101 Figure 78. Storage modulus and tan δ obtained by DMA for the different nanocomposites. ... 104 Figure 79. IP EIS spectra for several representative samples showing the evolution of the shape of the impedance spectra with increasing RH (○ – 20%, □ – 40%, ◊ - 60%, ∆ - 80% and ● – 98%) at 40 ºC for: a - BC:PAPTAOH:MBA (1:3:0.2), b - BC:PAPTAOH:MBA (1:5:0.2), c - BC:PVBTAOH:MBA (1:3:0.2) and d

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Figure 80. Arrhenius plots of two BC:PAA:MBA representative samples as a function of RH. Lines are for visual guidance. RH increases in the arrow direction (20, 40, 60, 80 and 98%). ... 106 Figure 81. Arrhenius plots as a function of RH and evolution of conductivity as a function of RH, at different temperatures, for two BC:PAPTAOH:MBA representative samples. The straight lines are linear fits to the Arrhenius model, whereas the dashed curves are fits to the VTF equation. ... 107 Figure 82. Graphical representation of top: log σ vs. log RH (□ 20 °C, × 40 °C, ◊ 60 °C and ∆ 80 °C), bot: ln(σ2_/(pH

2O)g) vs. the reciprocal temperature (○ 20% RH, □ 40% RH, × 60% RH, ◊ 80% RH and ∆ 98% RH)

, for two BC:PAPTAOH:MBA representative samples. ... 110 Figure 83. Arrhenius plots of two BC:PVBTAOH:MBA representative samples as a function of RH. RH increases in the arrow direction (20, 40, 60, 80 and 98%, for the (1:3:0.2) nanocomposite the σ was measured from 40% RH up). The straight lines are linear fits to the Arrhenius model, whereas the dashed curves are fits to the VTF equation. ... 111 Figure 84. Graphical representation of top: log σ vs. log RH (□ 20 °C, × 40 °C, ◊ 60 °C and ∆ 80 °C), bot: ln(σ2_/(pH

2O)g) vs. the reciprocal temperature (○ 20% RH, □ 40% RH, × 60% RH, ◊ 80% RH and ∆ 98% RH)

, for two BC:PVBTAOH:MBA representative samples... 112 Figure 85. Evolution of the anionic conductivity of the BC:PAPTAOH:MBA samples at various

temperatures and RH as function of: a - the [OH-_{] and b – of the λ. ... 113}

Figure 86. Evolution of the anionic conductivity of the BC:PVBTAOH:MBA samples at various

temperatures and RH as function of: a - the [OH-_{](the WU evolution is indicated by the dashed arrow) and b}

– of the λ. ... 114 Figure 87. Polarization and power curves of a - of BC:PAPTAOH:MBA (1:5:0.2) and b -

BC:PVBTAOH:MBA (1:3:0.2) collected at room temperature under humidified hydrogen/air gradient. .... 115 Figure 88. Polarization and power curves of BC:PAA:MBA (1:5:0.2) collected at room temperature under humidified hydrogen/air gradient. ... 116 Figure 89. Tafel plots of a - of BC:PAPTAOH:MBA (1:5:0.2) and b - BC:PVBTAOH:MBA (1:3:0.2) ... 116 Figure 90. Tafel plot for of the BC:PAA:MBA (1:5:0.2) membrane. ... 117 Figure 91. σAC and σDC for all the membranes tested as MEA (the σAC was measured at 40 °C/98% for the

alkaline and acid-treated membrane and at 30 °C/98% for the nanocomposites, using the IP configuration in all cases). ... 118 Figure 92. XRD patterns collected at the surface of the fresh electrodes. ... 121 Figure 93. SEM micrographs taken at the edge of a fracture revealing the surface and the cross-section of the ceramic electrodes after the electrochemical measurements. ... 123 Figure 94. EDS in 3 distinct zones of the La0.84Sr0.16CoO3 sample. ... 124

Figure 95. Calculated Pourbaix diagrams for a – LaCoO3 (50%La:50%Co), b – LaFeO3 (50%La:50%Fe), and

c – LAMnO3 (50%La:50%Mn) showing the relevant stability fields. Shadows indicate existence of solids

(solid phases are signalled in grey while ions are in red). The dashed lines correspond to the water oxidation ( top, 2H2O → O2 + 4H+ + 4e-) and reduction (bottom, 2 H2O + 2 e- → H2 + 2 OH-).260 ... 125

Figure 96. CV scans of the four tested perovskite electrodes at 25 °C. Scan rate: 25 mV.s-1_{: a - LaCoO} 3, b -

La0.84Sr0.16CoO3, c -La0.8Sr0.2Fe0.8Co0.2O3 andd -La0.7Sr0.3MnO3. ... 127

Figure 97. Adaptation of the Volcano plot for the ORR proposed by Suntivich169_{to the selected}

perovskite-oxides. ... 128 Figure 98. Volcano plot for the HPRR of the tested perovskite-oxides. ... 129 Figure 99. Effect of the potential scan rate on the CVs obtained with the La0.7Sr0.3MnO3 electrode in 0.4 M

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Figure 103. TEM images illustrating La2NiO4 formation at the interface of La2O3 and NiO. ... 133

Figure 104. Phase evolution for the annealing at different temperatures of: a - non-activated sample and b - mechanically activated sample. (◘ La2NiO4, La2O3,  NiO) ... 133

Figure 105. Powder XRD patterns of (left) nickelates (including identified secondary phases La2O3, 

NiO  CeO2, (La,Sr)NiO3), (right) cuprates. ... 134

Figure 106. Lattice parameters for the a - nickelates and b - cuprates A2BO4 compositional series. ... 136

Figure 107. SEM micrographs of surface of the ceramic electrodes after exposure to 2 M NaOH and cathodic bias, and of the cross-section obtained after a fresh fracture aiming to show the sample non-exposed to the alkaline solution. ... 138 Figure 108. Calculated Pourbaix diagrams for: a – LaNiO3 (50%La:50%Ni-O-H), b – LaCuO3

(50%La:50%Cu-O-H) and c – (LaSr)CuO3 (40%La:10%Sr:50%Cu-O-H) systems showing the relevant

stability fields. Shadows indicate existence of solids. The dashed lines correspond to the water oxidation (top, 2H2O → O2 + 4H+ + 4e-) and reduction (bottom, 2 H2O + 2 e- → H2 + 2 OH-).260 ... 140

Figure 109. Electrical σ as vs temperature for a - nickelates (+ La2NiO4,  La1.9Sr0.1NiO4,  La1.9Ce0.1NiO4,

 La1.9Pr0.1NiO4,  La1.8Pr0.2NiO4), and b - cuprates ( La2CuO4,  La1.9Sr0.1CuO4,  La1.9Pr0.1CuO4). ... 142

Figure 110. CVs recorded at a scan rate of 50 mV.s-1_{in three different 2 M NaOH electrolyte media:}

deaerated; O2-saturated; and after adding 0.05 M H2O2 at 25 °C. ... 143

Figure 111. Voltammograms of La2NiO4 recorded in 0.05 M H2O2 + 2 M NaOH solution using scan rates in

the 5 – 1000 mV.s-1_{range at temperature of 25 °C. ... 145}

Figure 112. CVs of nickelates and cuprates electrodes recorded at a scan rate of 25 mV.s-1_{in three different 2}

M NaOH electrolyte media: deaerated; O2-saturated; and after adding 0.05 M H2O2 at 25 °C. ... 146

Figure 113. jmax and concentration of solubilized ions vs. electronic σ. ... 147

Figure 114. LSVs of La2CuO4 displaying a – the effect of potential scan rate (in 0.05 M H2O2 + 2 M NaOH)

and b – of the H2O2 concentration (in 0.05, 0.5 and 1 M in 2 M NaOH at scan rate of 25 mV.s-1). ... 148

Figure 115. La2CuO4 electrode CA curves: a - for different applied potentials (0.05 M H2O2 + 2 M NaOH), b

- CA for different H2O2 concentrations (at -0.6 V); CP curves of La2CuO4 electrode in a – for different

applied j (0.05 M H2O2 + 2 M NaOH), and b - for different H2O2 concentrations (at applied current density of

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List of Tables

Table 1. Preparation route, IEC and anionic conductivity at given temperature and relative humidity (RH) for

different AEMs. ... 11

Table 2. Structure, IEC and 𝜎 of the VBTACl based materials tested by Tsai et al.64_{... 18}

Table 3. Comparison of some of the properties of VC and BC 106_{... 20}

Table 4. Literature references on for AFCs electrodes. ... 30

Table 5. List of the metal oxides samples tested. ... 52

Table 6. Lattice parameters estimated from the XRD patterns. ... 64

Table 7. Decomposition temperature, weight loss at maximum temperature decomposition, water loss temperature, water lost percentage and content of ashes of pristine and treated BC membranes. ... 66

Table 8. IP and TP ex-situ σAC and σDC estimated from the polarization curves. ... 73

Table 9. List of the prepared membranes including composition, IEC, σ and WU capacity. ... 76

Table 10. Lattice parameters estimated from the XRD patterns. ... 92

Table 11. Decomposition temperature, weigh loss at maximum temperature decomposition, water loss temperature, water loss and ashes of the different cross-linked polyelectrolytes and nanocomposite membranes. ... 102

Table 12. Fitting parameters of the BC:PAPTAOH:MBA samples conductivity data to the Arrhenius or the VTF equation. ... 108

Table 13. Fitting parameters of the BC:PVBTAOH:MBA samples conductivity data to the Arrhenius or the VTF equation. ... 111

Table 14. IP ex-situ σAC and σDC estimated from the polarization curves. ... 117

Table 15. Lattice parameters and unit cell volume (indexed on R3c space group) estimated from the XRD patterns before and after the electrochemical measurements. ... 122

Table 16. Element stoichiometry estimated by EDS. ... 124

Table 17. Electrical σ at room temperature. ... 126

Table 18. Lattice parameters and unit cell volume estimated from the XRD patterns collected before and after immersion in 2 M NaOH. ... 135

Table 19. Sintering temperature and density of the samples. ... 137

Table 20 Elemental analysis of the 2 M NaOH solutions used for the stability tests of the various oxides. Values are given in µg of element per L of solution. ... 139

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Abbreviations and Acronyms

AB Acetylene black

AEM Anion exchange membrane

AETACl 2-(Acryloyloxy)ethyl trimethylammonium chloride AFC Alkaline fuel cell

AL Active layer

APTACl (3-Acrylamidopropyl)trimethylammonium chloride ATRP Atom transfer radical polymerization

BC Bacterial cellulose

MBA N,N-methylene-Bis-acrylamide

BM Backing material

CA Chronoamperometry

CHPTAC 3-Chloro-2-hydroxypropyl trimethyl ammonium chloride CNT Carbon nanotubes

CP Chronopotentiometry

CP-MAS Cross polarization-magic angle spinning CV Cyclic voltammetry

DABCO 1,4-Diazabicyclo[2.2.2]-octane DMA Dynamic mechanical analysis

DMF N,N-dimethylformamide

EDA Ethylenediamide

EDS Energy-dispersive X-ray spectroscopy EIS Electrochemical impedance spectroscopy

FC Fuel Cell

FTIR-ATR Fourier transform infrared-attenuated total reflectance

GA Glutaraldehyde

GC Glassy carbon

GDL Gas diffusion layer

HOR Hydrogen oxidation reaction HPRR Hydrogen peroxide reduction reaction

HTCC N-[(2-hydroxy-3-trimethylammonium)propyl] chitosan chloride

ICP-MS Inductively coupled plasma-mass spectrometry IEC Ionic exchange capacity

IP In-plane

KPS Potassium persulfate

LEIS Low-energy ion scattering spectroscopy MEA Membrane electrode assembly

NMR Nuclear magnetic resonance OCV Open-circuit voltage

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PAA Poly(acrylic acid)

PAETACl Poly[2-(acryloyloxy)ethyl]trimethylammonium chloride PAETAOH Poly[2-(acryloyloxy)ethyl]trimethylammonium hydroxide PANa Poly(sodium acrylate)

PAPTACl Poly(3-acrylamidopropyl)trimethylammonium chloride PAPTAOH Poly(3-acrylamidopropyl)trimethylammonium hydroxide PBC Prussian blue nanotubes dispersed on carbon composite PBI Polybenzimidazole

PE Polyethylene

PECH Poly(epichlorohydrin)

PETFE Poly(ethylene-co-tetrafluoroethylene PEGDA Polyethylene glycol diacrylate PEM Proton exchange membrane PEMFC Proton exchange membrane fuel cell PEN Poly(ether nitrile)

PHEMA Poly(2-hydroxyethyl methacrylate) PLD Pulsed laser deposition

PPO Poly(2,6-dimethyl-1,4- phenylene oxide)

PS Polystyrene

PAESF Poly(arylene ether sulfone) PSSA Poly(4-styrene sulfonic acid)

PSU Polysulfone

PTFE Polytetrafluoroethylene PVA Poly(vinyl alcohol) PVBC Poly(vinylbenzyl chloride)

PVBTACl Poly(vinylbenzyl)trimethylammonium chloride PVBTAOH Poly(vinylbenzyl)trimethylammonium hydroxide PVIm Poly(1-vinylimidazole)

QA Quaternary ammonium

QABC Quaternary ammonium bacterial cellulose QAPSU Quaternary ammonia polysulfone

RAFT Reversible addition-fragmentation chain transfer RH Relative humidity

ROMP Ring-opening metathesis polymerization SADP Selected area diffraction patterns SAFC Solid alkaline fuel cell

SCE Saturated calomel electrode SEM Scanning electron microscopy

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VBTACl (Vinylbenzyl)trimethylammonium chloride VC Vegetable cellulose

WU Water uptake

xQAPSU Self-cross-linked quaternary ammonia polysulfone XRD X-ray diffraction

Symbols

A area of the cross-section

C* _{concentration of the limiting electroactive species}

D diffusion coefficient

E potential generated

Ea activation energy

eg antibonding orbital

Eo _{theoretical cell potential/electromotive force}

f frequency

fm morphology factor in anionic conductivity

g solvating water molecules

G Gibbs free energy

h bar height

Had hydrogen atom adsorbed to the catalyst surface

I electrical current

ik kinetic current

j electrical current density j0 exchange current density Ksolv _{solvation constant}

l bar width

Lbar length of ceramic bar

Ldisc thickness of the ceramic pellets

Lmen thickness of the membrane

MNaOH molar concentration of NaOH solution n number of electrons transferred [OH-_] _{alkaline load}

P power

R ohmic resistance

VNaOH volume of NaOH solution added to reach the equivalence point

w weight

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Greek Symbols

α/β transfer coefficients in Butler -Volmer equation

γ-ray gamma ray

δ thickness of the diffusion layer tan δ loss tangent

ΔGads adsorption free energy

ΔHads heat of adsorption

ΔHsolv solvation enthalpy

ΔSsolv solvation entropy

εd metal d-band centre energies

η overpotential

λ hydration level

λr radiation wavelength

 electrical conductivity

c intrinsic anionic conductivity

*-band antibonding band

ϕc volume fraction of anionic conductive phase

 angular frequency

∇COH- concentration gradient

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8.2.3 Microstructure of the nanocomposites ... 93 8.2.4 Thermogravimetry ... 95 8.2.4.1 BC:PAETACl:MBA ... 95 8.2.4.2 BC:PAPTACl:MBA ... 98 8.2.4.3 BC:PVBTACl:MBA ... 100 8.2.5 Visco-elastic behaviour ... 103 8.2.6 Ionic conductivity ... 105 8.2.6.1 Impedance spectroscopy ... 105 8.2.6.2 Influence of extrinsic factors: temperature and RH ... 106 8.2.6.3 Influence of intrinsic factors: structure, IEC and WU ... 113 8.2.7 MEA testing ... 115

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9.1.2 Electrical Conductivity and Electrochemical characterization ... 126

9.2 La2MO4 Ruddlesden-Popper phases ... 132

9.2.1 Synthesis of AB2O4 assisted by high energy milling ... 132

9.2.2 Structure, microstructure and stability ... 134 9.2.3 Electrical conductivity and Electrochemical characterization ... 142

IV. CONCLUSIONS AND OUTLOOK ... 151 V. REFERENCES ... 155

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I. INTRODUCTION

This chapter starts with a brief identification of the motivation and objectives of this work, in order to introduce the reader to fuel cells and in particular to solid alkaline fuel cells.

Since we will be dealing with two different fuel cell components, the literature on solid alkaline fuel cells electrolytes and electrodes is reviewed separately. The structure adopted in this introductory chapter, where the matters concerning the electrolyte and electrode materials are discussed in two distinct sections will be followed throughout this Thesis.

The electrolyte section briefly presents the ion transport in anion exchange membranes, the application requirements of such membranes in this kind of fuel cell and gives an extensive overview of the state-of-the-art membrane materials, emphasising biopolymers and polyelectrolytes.

The catalysts section starts by describing the key features of solid alkaline fuel cell electrodes and their link to the relevant electrochemical reactions. The description of the application requirements precedes the literature review of electrocatalyst materials, covering separately metals and simple metal oxides on one side, and perovskite and Ruddlesden-Popper oxides on the other side.

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1 Motivation and objectives – Fuel Cells and the energy paradigm

With the increasing energy demands, generating electricity from renewable sources in replacement of

pollutant fossil fuels has been an important way to reduce carbon dioxide (CO2) emissions, with more

and more countries installing wind and solar power plants to help meeting targets to cut CO2 emissions.

This is a major concern of Europe's ambitious political agendas, as demonstrated by the Energy Union strategy, unveiled in February 2015, that aims at ensuring that Europe has access to secure, affordable and sustainable energy until 2030.1

Nevertheless, renewable energy sources also present drawbacks, among which the variability is particularly concerning. The wind tends to blow intermittently, and solar power is only available during the daytime. Hence, renewable power plants must be over-engineered to take account of this lower capacity factor, or they need to be supported by additional systems. In many cases backup gas turbines are used to provide a spinning reserve, which means that CO2 is still being emitted.2

Ideally, the excess energy generated during periods when renewable sources exceed the needs could be stored, and later used when sufficient electricity is not available. But storing this energy is a difficult task, with batteries and similar technologies performing well over short timescales, but for longer periods an alternative approach is still necessary. Energy storage in the form of hydrogen is one possibility. The excess of electricity can be used by an electrolyser to split water into its elemental constituents, oxygen and hydrogen, which can then be used in Fuel Cells (FCs) to produce electricity.2 FCs are electrochemical devices that convert the chemical energy of a fuel, such as hydrogen, directly into electricity, heat and water by means of controlled electrochemical reactions.3 FC technologies are thus the core of the virtuous cycle translated by reaction (1) using hydrogen as the energy vector of a truly sustainable paradigm.

𝐻2(𝑔) + 1

2𝑂2(𝑔) ↔ 𝐻2𝑂(𝑙) + 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 + ℎ𝑒𝑎𝑡 1

The merit for the finding of the FC principle is shared by Christian Friedrich,4 that in 1839 found that a current could be measured between two platinum plates immersed in a sulphuric acid electrolyte while H2 and O2 were separately fed to the electrodes, and William Grove, that made the same observation

while experimenting on the reversibility of water electrolysis, publishing in 1843 an article describing the gaseous voltaic cell, which is often considered the first FC stack.5

Recognising their role in the decarbonisation of energy generation, the EU has been supporting research on FCs and hydrogen technologies since the 4th Framework Programme (1994-1998) with ever-increasing intensity. In 2008, this led to the establishment of the Fuel Cells and Hydrogen Joint

Undertaking, a partnership between industry and research entities.1 There are several examples of

projects that reflect the importance of integrated hydrogen energy systems. Located on France, the Renewable Hydrogen Mission for Integration into the Electric Grid, is a system that combines solar power with electrolysers, hydrogen storage and FCs. Other examples include a wind-hydrogen hybrid power plant in Prenzlau, Germany, that during periods of excess wind generates hydrogen through a series of electrolysers. The stored hydrogen is used either as fuel by FC vehicles or, after being mixed with biogas, is fed into cogeneration plants to produce electricity and heat.2

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Although the FC technology has matured substantially over the past decades, in part due to the investment promoted by the above-mentioned actions, it still presents some technological barriers, such as insufficient durability, cell life time and components cost. For instance, the most mature and wide spread FC technology, the proton exchange membrane fuel cell (PEMFC), requires the use of expensive polymer membrane electrolytes and platinum electrodes, which are drawbacks that limit their future mass exploitation.6–8

Alkaline fuel cells (AFCs), have the potential to be substantially less expensive due to the improved electrode kinetics in basic environments, allowing the use of non-precious metal catalysts.8 It is the oldest FC technology and normally uses concentrated potassium hydroxide as electrolyte. It appeared almost 90 years after the discovery of the FC principle, in 1932, when Francis Bacon took the principle developed by Grove and used Ni gauzes as electrode and a KOH solution as electrolyte, instead of the expensive Pt electrode and the acid electrolyte.9

The most serious drawback of AFC is related to the KOH electrolyte. This component tends to be particularly sensitive to carbon dioxide poisoning, leading to the formation of hydrogencarbonate (HCO3-) and carbonate (CO32-) anions and, consequently, the precipitation of KHCO3 and K2CO3 that

reduce the catalyst activity and the electrolyte conductivity (σ), and limit fuel mass transfer in the electrode, all contributing to a rapid and drastic decrease in FC performance. Replacement of the liquid electrolyte by a solid polymer anion exchange membrane (AEM) with covalently tethered cationic groups, eliminates the carbonate precipitation problems in the so-called solid alkaline fuel cells (SAFCs).10

SAFCs have the same working principle of AFCs, as described in Figure 1. The oxygen is reduced on the cathode, producing hydroxide ions which are transferred through the electrolyte to the anode, where the oxidation of hydrogen into water takes place.3,7 The theoretical H2/O2 cell potential (Eo) under

reference conditions of temperature and pressure, the difference between the O2 and the H2 electrodes

potentials, is 1.23 V. However, real open circuit electromotive force is close to 0.9 V due to the cathode activation potential, humidification of the fuel stream (essential to avoid anode dehydration) and finally by the use of air instead of oxygen.11 Under load the electric current is generated by the system, leading to a voltage drop due to the internal ohmic resistance of the cell, and to local over-potentials at the electrodes and concentration and mass transfer limitations.11

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Figure 1. Schematics of a SAFC running on hydrogen.

The paradigm of hydrogen and FCs is the driving force for this PhD Thesis in Materials Science and Engineering, focused on studying cost-effective and eco-friendly SAFC materials. The possibility to replace totally or partially synthetic polymers by biopolymers, using at the same time the improved electrode kinetics offered by the alkaline environment, to replace the expensive precious metal catalysts (usually Pt) is the focus of this work. The major scientific objectives of this Thesis are:

- to establish efficient and reliable routes to prepare nanocomposite membranes, based on bacterial cellulose (BC) bearing cationic groups, to promote enhanced hydroxyl conductivity (higher than 1 mS.cm-1) while ensuring high chemical stability in alkaline media and adequate mechanical performance;

- to identify the underlying relationships between composition, microstructure, conductivity and catalytic activity for oxygen and hydrogen peroxide reduction of multicomponent oxides of the perovskite and Ruddlesden-Popper systems: La1-xDxM1-yNyO3+δ and La2-xDxMO4+δ, with M, N

and D being elements selected from Ce, Pr, Sr, Co, Cu, Ni, Fe and Mn;

- to identify and test suitable combinations of the developed BC-based membranes and the oxide electrocatalysts in a single FC.

Global fuel cell reaction

O

₂

+ 2

H

₂

→ 2

H

₂

O

cathode

anode

electrolyte

4 OH-4e

-O

₂

+ 2

H

₂

O +

4 e

-

→ 4

_O

_H

-4

O

H

-

_{+ 2}

_H

2 → 4

H

2 O

+

4 e

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-2 Anion exchange membranes

2.1 Ionic transport through AEMs and the role of water

The conduction of hydroxide ions in AEMs has been discussed invoking similar mechanisms to those explaining protons conduction , namely the Grotthuss mechanism (or structural diffusion), molecular diffusion/migration, and convection (Figure 2).12

Figure 2. Conduction mechanism of OH-_{through polymeric AEMs. Adapted from}12_.

The Grotthuss mechanism occurs due to surface site hopping of protons along the membrane ionic conductive side chains, or by structural diffusion in absorbed water molecules. Molecular diffusion and migration occur in the presence of a concentration and/or electrical potential gradient acting on charged particles (∇COH- and ∇φ respectively). Finally, the convection is the result of a pressure gradient and/or

species being carried by a net ionic concentration subject to an electrostatic potential gradient. Transport can be the product of any combination of these 3 mechanisms.12

It has been demonstrated that the conductivity of hydroxyl anions is lower than that of protons, being one of the weaknesses of the alkaline systems. In water, both charged defects are believed to be transported by a structural diffusion mechanism, in addition to the vehicular process (relatively slow transport mechanism that occurs close to the pore walls). However, this mechanism has different diffusion coefficients for the transport of hydroxide or proton ions: 5.310-9 m2.s-1 and 9.310-9 m2.s-1 respectively.12 Therefore, in conditions where there is sufficient hydration and the transport inside the membrane follows the same trends of pure water, hydroxide conductivity in AEMs tend to be around half of the protonic conductivity achieved with proton exchange membranes (PEMs).13–15

The diffusion coefficient is in largely affected by the hydrodynamic radius, the ion own radius and its hydration shell. This is because ions do not move through the water in an isolated manner but with a noticeable hydration shell. In the specific case of hydronium (H3O+) and hydroxide (OH–) ions, the

diffusion coefficients are very high because, in addition to hydrodynamic vehicular diffusion, their proton exchange with the protons of the surrounding water via hydrogen bond rearrangement in a

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structural diffusion process. The implication is that structural diffusion requires the presence of an excess of water.16

One factor that may help explaining the differences in conductivities between proton and hydroxide ions is the key role that coordination/solvation plays in structural diffusion. While the protons are hypo-coordinated by three water molecules the hydroxide ion is hyper-hypo-coordinated by approximately 4.5 water molecules. This results in hydration enthalpies of -520 kJ.mol-1 for OH- and -1150 kJ.mol-1 for H+.13,17 The enthalpy of OH- surpasses that of water diffusion at low water contents, which may be rationalized by an increasing contribution of the conductivity activation enthalpy due to dissociation. Therefore, the conduction process is energetically more demanding when less water is available.12 In summary, in conditions where enough water is available the higher diffusion coefficient of protons can explain the differences in the conductivities of hydroxide ions and protons. At low water contents the degree of dissociation is the most significant difference between these ionic species. As expected, the current hydroxide ion conducting membranes do not offer the same level of conductivity as state-of-the-art proton conductors such as Nafion®. Judging from the difference in ionic mobility, AEMs should present conductivities only half of the typical values for similar PEM. However, the differences observed are usually higher, typically less than 0.01 S.cm-1_{for HO}-_{in AEMs and of the order of 0.1}

S.cm-1 for H+ in PEMs.13 The requirements for AEMs, and the causes that in many cases lead to the membranes failing to meet these requirements, are addressed in the following sub-section.

2.2 Requirements and challenges

Most of the current AEMs (including the commercial ones) have low chemical stability at both high temperature (above 60 °C) and pH. A common effect of the low stability is that, while carbonate precipitates are not expected to be formed in SAFCs, carbon dioxide can still react with the hydroxide anions to form carbonate and bicarbonate anions. Hence, carbonation is still an issue in these systems.3,7

To meet the requirements for FC applications, AEMs should, at least, possess the following properties:3,7

- hydroxide conductivity higher than 10-2 S.cm-1, preferentially higher than 10-1 S.cm-1; - high chemical and thermal stability under FC operating conditions (e.g., 80 °C);

- high mechanical strength and low swelling degree when used in the form of flexible membranes with a thickness as low as possible (< 100 µm).

2.2.1 Degradation of the quaternary ammonium groups in alkaline conditions

The promotion of anion exchange in AEM requires a material with positively charged groups (cationic group). If these positive charges are neutralized, the membrane loses the capacity to perform its function. This is a critical problem with AEMs in which the highly nucleophilic hydroxide ion promotes the degradation of the functional cationic group and/or the polymer backbone.3,7,18–22 This possible degradation when exposed to strong alkaline environments is the reason why the chemical stability is

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the main reason why AEMs rapidly lose performance above 60 °C, with the operation of the SAFC being severely limited unless this degradation can be reduced to acceptable levels.

Attempts to solve this problem have addressed the use of different cationic functional groups. When testing groups such as ammonium, phosphonium or sulfonium it was found that the quaternary ammonium (QA) group was among the most stable, sulfonium the least stable and phosphonium groups displaying intermediate behaviour.20,22 Alternative cations exhibit QA-comparable stability, but consist of more exotic structures based on phosphonium with extremely bulky substituents or transition metal complexes, with large ligands that require complex and expensive synthesis methods.23 Since the advantage of low-cost platinum-free SAFC would be counteracted by expensive cations, QA groups are usually chosen as they are comparatively simple to synthesize and easily attached to a polymeric backbone.23

The main degradation pathways of QA compounds in the presence of a strong base such as a hydroxide, are nucleophilic substitution and β-elimination (also known as Hofmann elimination). The schematic representations seen in Figure 3 show that in the substitution reaction, the hydroxide attacks an α-carbon, while β-elimination requires the presence of a proton in a β-position, which is abstracted by hydroxide.3,7

Figure 3. QA removal in the presence of hydroxide by nucleophilic substitution (top) and Hofmann elimination (bottom). The hydroxide attacks at α-carbon and β-proton, respectively.22

While substitution and elimination are the most critical degradation pathways, other reactions can occur as well, depending on the molecular structure. In the absence of β-protons, Stevens rearrangement is reported to occur. For aromatic compounds, there are examples of neutralization through Sommelet-Hauser rearrangement or ortho-attacks.24,25 The reactivity of the QA is therefore significantly affected by the chemical nature of the backbone to which it is attached, the degree of functionalization as measured by the ionic exchange capacity (IEC), the water content and the heterogeneous nature of the membrane.

To stabilize the QA in an alkaline environment, the degradation reactions have to be suppressed or inhibited. This might be achieved through specific molecule structures that are resistant against the various attack pathways of the hydroxide anion.

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nevertheless, a problem, due to difficulty in synthesizing increasingly bulky ammonium groups, since the ammonium ion is relatively small and the space around it is limited. Additionally, the nucleophilic nature of ammonium groups decreases for large substituents, making it difficult to attach bulky ammonium groups to a polymer backbone or to polymerize monomers already containing the bulky groups. For these reasons steric shielding of the positive charge is more suited for cations other than nitrogen.22,26,27 The inhibition of β-elimination through the removal of all β-protons (as in benzyltrimethylammonium) removes any possibility for this reaction pathway to increase the alkaline stability, since it is one of the fastest degradation reactions.22

2.2.2 CO2 contamination

Other of previously mentioned challenges when considering the stability of AEMs is to avoid or to reduce carbonation, which, though not as worrying as in the case of the KOH electrolytes, can also substantially impact the performance of the solid AEM.28 When CO2 dissolves in water, it forms

carbonic acid, which dissociates into H+, HCO3-, and CO32-. The HCO3- and CO32- displace at least some

of the OH- counterions in the AEM. This leads to a conductivity decrease for these membranes, mostly due to the larger size of the carbonate ions compared to the hydroxide one. Carbonate ions also cause the decrease of the pH in the electrodes and electrolyte, and it is unclear the effect this could have on the anode and cathode kinetics.28–30 The formation of carbonic acid and carbonate is expressed in the following equations31_:

𝐶𝑂₂(𝑔) + 𝐻₂𝑂 (𝑙) ↔ 𝐻₂𝐶𝑂₃(𝑎𝑞) 2

𝐻₂𝐶𝑂₃(𝑎𝑞) + 2𝑂𝐻−(𝑎𝑞) ↔ 𝐶𝑂₃−2_{(𝑎𝑞) + 𝐻}

2𝑂(𝑙) 3

Despite the reduction in conductivity operated by the substitution of hydroxide by carbonate or bicarbonate anions, an FC operating in the carbonate cycle could result in improved stability of the electrolyte and in a significant improvement of the device operational lifetime. The carbonate anion is a weaker nucleophile compared to hydroxide and should have a lower tendency to attack the cationic sites of the AEM through the nucleophilic attack or other degradation mechanisms described above.30,31 When comparing the effect of two different cathode streams, CO2 and N2, Vega and co-workers31

verified an improvement in the FC performance in the kinetic region current when using CO2,

suggesting that the FC was operating through the carbonate cycle. Further studies are still required to find carbonate selective catalysts under fully hydrated cell conditions.31

2.3 Materials and synthesis of anion exchange membranes

The AEMs can be prepared by polymerization reactions of monomers bearing cationic groups, by functionalization of polymers with groups containing cationic moieties, or by grafting a membrane with cationic groups, as summarized by the examples given in Table 1. The table lists the composition, preparation route, IEC and anionic conductivity of each AEM, when presented by the authors in the various references listed.

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Table 1. Preparation route, IEC and anionic conductivity at given temperature and relative humidity (RH) for different AEMs.

Membrane Preparation route IEC

(mmol.g−1) σ (mS.cm −1₎ T(°C) / RH Refs P o ly mer fun ct io na liza tio n

Poly(2,6-dimethyl-1,4-phenylene oxide) containing pendant QA groups Chloromethylation 0.70 4.3 30 / 100 32

Poly(phthalazinone ether ketone) with QA groups Chloromethylation and cross-linking 2.63 30 / 100 33

Poly(arylene ether sulfone)s bearing aromatic side-chain QA groups Bromination, cross-linking and quaternisation 1.33 – 1.68 22.2 - 29.4 25 / 100 34

Polystyrene (PS)-poly(ethylene-ran-butylene) Chloromethylation, quaternisation and alkalization 0.3 5.12 30 / 100 35

Poly(epichlorohydrin) Thermal cross-linking with 1,4-diazabicyclo[2.2.2]-octane (DABCO);

Chloride substitution on the –CH2Cl function of the epichlorhydrin units 1.3 2.5 20 / 100 36

Poly(2,6-dimethyl-1,4- phenyleneoxide) (PPO); Partially fluorinated polybenzimidazole (PBI);

Sulfonated polyethersulfone

Bromination Cross-linking with DABCO or

N,N,N′,N′-Tetramethylethylenediamine; Morpholinium-functionalized blend 3.3 6.1 (Cl

-₎ _{30 / 90} 37

Polypropylene-ethylenediamine Amination of chlorinated polypropylene 4.2–5.1 4–10 25 / 100 38

N-[(2-hydroxy-3-trimethylammonium)propyl] chitosan chloride (HTCC) Chitosan quaternisation with glycidyltrimethylammonium chloride and

cross-linking with glutaraldehyde; 4.8–7.5 25 / 100

39

Polyvinyl alcohol (PVA) –HTCC Quaternisation of PVA with (2,3-epoxypropyl) trimethylammonium

chloride and cross-linking with Glutaraldehyde (GA) 0.85–0.25 3–7 30 / 100

40

Di-o-butyrylchitosan Prepared by reacting chitosan with butyric acid anhydride in the presence

of perchloric acid 10

-7 _{25 / 100} 41

Deacetylated-chitosan Prepared by film casting, deacetylation with acetic acid 10-6 _{25 / 100} 42

Deacetylated-chitosan Prepared by film casting, deacetylation with acetic acid and NaOH 1 25 / 100 43

Quaternized-chitosan Glycidyltrimethylammonium chloride as quaternizing reagent 0.86 10 25 / 100 44

Cellulose p-toluenesulfonate bearing QA moieties Reaction with DABCO; Cross-linking with diiodobutane 3.5 5.4 25 / 100 45

Phosphorylated-chitosan Prepared from the reaction of orthophosphoric acid and urea on the

surface of chitosan membranes in N,N-dimethylformamide 1.2 25 / 100

46

Quaternary ammonia polysulfone (QAPSU) Chloromethylation; Conversion of the chloromethyl group into an

ammonium group 1.18 20 25 / 100

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12

PAES tetraphenylolethane glycidyl ether-trimethylammonium Chloromethylation with chloromethyl methyl ether and amination with

trimethylamine 1.31 12.8 30 / 100

50

PSU Tetramethylethylenediammonium Chloromethylation with chloromethyl ether, quaternisation and

alkalization 31 25 / 100

51

Poly(phthalazinon ether sulfone ketone) Trimethylammonium Chloromethylation and quaternisation 1.69 140 25 / 100 52

PPO Trimethylammonium Blend of brominated and chloroacetylated PPO; Quaternary-amination in

trimethylamine 2 22–32 25 / 100

53

Poly(ether imide) with trimethylammonium groups Chloromethylation, quaternisation and alkalization 0.92 2.3 25 / 100 54

Self-cross-linked QAPSU (xQAPSU) Chloromethylation, quaternisation and alkalization; Self cross-linking

using the tertiary amino groups 1.34 15 20 / 100

55

Poly(ether nitrile) (PEN) with imidazolium and morpholinium

PEN synthesized by the polycondensation of MBAphenol A and 2,6-difluorobenzonitrile, imidazolium and morpholinium integrated by

chloromethylation

1.76 32 30 / 100 56

Polyethylene-b-poly(vinylbenzyl trimethylammonium) Prepared by anionic polymerization and postpolymerization

functionalization reactions (bromination and quaternization) 1.21 5 (Br

-₎ _{50 / 95} 57 M o no mer P o ly meriza tio n

Poly(vinylbenzyl chloride) (PVBC) with QA groups Amination and Cross-linking with

N,N,N’,N’-tetramethylhexane-1,6-diamine 1.14 9.2 30 / 100

58

Poly[ 9,9׳-MBA (4-hydroxy-3, 5-dimethylphenyl) fluorine]-tetramethyl

MBAphenol fluorene-containing poly(sulfone) Co-polymerization; Bromination; Amination of the bromomethyl groups 2.18 88.5 30 / 100

59

Tetraalkylammonium-functionalized norbornene-dicyclopentadiene Ring-opening metathesis polymerization (ROMP) 1.4 18 20 / 100 60

Tetraalkylammonium-cyclooctene ROMP; Conversion of bromide into hydroxide 68.7 22 / 100 14

Poly(methyl methacrylate-co-butyl-acrylate-co-vinylbenzyl chloride) Co-polymerization of methyl methacrylate, butyl acrylate and

vinylbenzyl chloride (VBC); quaternisation with trimethylamine 1.25 85.3 25 / 80

61

Cardo polyetherketone-poly(VBC-co-divinylbenzene) In-situ bulk polymerization; Cross-linked with Tetraethylenepentamine 2.63 31 30 / 100 62

Poly(4,4’-dichlorodiphenyl sulfone), 4,4-isopropylidenediphenol and di-, tri-,

and tetramethylhydroquinones Polycondensation; Quaternisation of benzyl bromide groups 2.6 9.7 (Br

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Ra dia tio n -ind uced g ra ft ing

Poly(ethylene-co-tetrafluoroethylene) (ETFE)/PVB with QA groups E-beam irradiation; VBC immersion followed by Cl-_substitution _1.42 ₁₀ _{20 / 100} 67

ETFE with QA groups E-beam irradiation; Quaternisation 2.25 68

Poly(hexafluoropropylene-co-tetrafluoroethylene)-PVB QA groups Grafting of VBC with γ-irradiation; amination with trimethylamine and

ion-exchange with aqueous potassium hydroxide 1.3-2 20 25 / -

69

Quaternary ammonia ETFE-PVB γ-ray irradiation of ETFE 0.74 30 30 / 100 70

Poly(4-vinylpiridine) Grafting and plasma-polymerization; haloalkylation 4.3 25 / 100 71

P hy sica l met ho ds

PPO polymer matrix with quaternary phosphonium hydroxide functional group Reinforcement method: van der Waals interaction Chloromethylation

and quaternisation with tris(2,4,6-trimethoxyphenyl)phosphine 1.25 55 20 / 100

72

Polyethylene (PE)/VBC Pore Filling; Polymerization on porous PE; Cross-linked with

divinylbenzene 1.76 57 30 / 100

73

PVA-poly(acrylic acid) (PAA) PVA-PAA Blending prepared by sol-gel; N,N′-methylene double

acrylamide as cross-linker; immersion in KOH 19 25 / 100

74

Porous PSU impregnated with (3-Acrylamidopropyl)trimethylammonium

chloride (APTACl) UV-irradiation promoted polymerization of APTA 100 25 / 100

75

Tetra-pyrrolidinium-based block poly(arylene ether sulfone) Pyrrolidinium groups introduced into the polymer scaffold using 4, 4′-_{oxyMBA (2,6-MBA(pyrrolidinyl-1-methyl) phenol)} 2.2 28 30 / 100 76

PVA and polybenzimidazole blend PVA-PBI membranes were synthesized using a casting method 45 25 / 100 77

Incorporation of a PVA matrix with PVBC and poly(1-vinylimidazole)(PVIm)

PVBC and PVIm act as bifunctional macromolecular cross-linking agents to provide both imidazolium cations and to form cross-linked

network in the membranes

1.86 21.9 30 /100 78

Chitosan-KOH composite KOH was incorporated into chitosan membranes as ionic functionality

(glutaraldehyde was used as cross-linker) 10 25 / 100

79,80

Quaternized chitosan - PS Semi-interpenetrating polymer network; polymerization of styrene

monomers in a quaternized chitosan emulsion + acetic acid 1.39 10 30 / 100

81

Bacterial cellulose/TiO2/3-chloro-2-hydroxypropyl trimethyl ammonium

chloride (CHPTAC)/PVA composite

Bacterial cellulose/TiO2 membrane used as substrate, CHPTAC

converted to hydroxide form by NaOH 0.99 10 25 / 100

(40)

Polymerizing a monomer containing either the final cationic functional group or its precursor, is a flexible way to design a material where the degree of functionalization can be controlled precisely in the polymerization step, by a well-defined initial ratio of monomers, with or without functional groups.15,32 This synthetic method requires polymeric materials functionalized with cationic groups, either ionomers or polyelectrolytes (depending on if the polymeric material has less or more than 15% of constitutional units with ionic/ionisable groups, respectively). While polyelectrolytes seem the obvious choice, the table shows that many membranes employ ionomers instead, since polyelectrolytes tend to dissolve in water due to their high concentration of ionic groups.

On the other hand, ionomer-based AEMs with sufficiently high IEC to attain good levels of OH -conductivity is also challenging. High IEC also tend to increase the water uptake (WU), eventually leading to an excessive swelling and decline of the mechanical strength, or even dissolution in hot water.55 Controlling the swelling is thus essential and may be achieved through designing structures similar to Nafion®, with separated hydrophilic channels for easy ionic transport, and a hydrophobic polymer backbone providing mechanical stability.75,83–85 Swelling may also be controlled by cross-linking of polymeric chains, or casting the AEM onto a supporting mesh, or a combination of these approaches. In a good example of such strategies Pan et al. were able to design a self-cross-linked quaternary ammonia polysulfone (xQAPSU) membrane with exceptional anti-swelling behaviour (<3%) and a claimed conductivity similar to that of Nafion®.55 This happens due to the tertiary amino group acting as a short-range cross-linker with polymer backbones in close proximity. Using the self-aggregation, the authors focused on designing efficient ionic channels for OH- conduction, namely by adding additional hydrophobic side-chains to drive the microscopic phase separation and the aggregation of ionic channels.

A starting polymer can easily be functionalized with haloalkyl groups by haloalkylation (typically chloromethylation) or halogenation (e.g. bromination) reactions, where a cationic functional group such as QA can be formed through a quaternisation reaction with a tertiary amine precursor. It is a simple, reliable and flexible method since the polymer backbone and cationic functional group can be chosen independently, giving origin to membranes with distinctive chemical structures. However, the control of the degree of functionalization may be difficult, with the functional sites in the polymer matrix limited to those that are most reactive for the initial haloalkylation/halogenation step, contrarily to what happens in the polymerization process.15 Radiation grafting of a non-functionalized membrane with haloalkyl groups that can be subsequently quaternised (as in polymer functionalization) has also been used to prepare AEMs. The membrane can be exposed to gamma rays or electron irradiation to create polymer radicals where an haloalkyl-containing unsaturated monomer (e.g. vinylbenzyl chloride (VBC)) can be linked. The polymer is then functionalized through the haloalkyl group of the grafted monomer. However, radiation may compromise the stability of the materials, particularly concerning in fluoroaklylene polymers. Controlling the degree of grafting is also an issue in this technique.15

Physical methods such as polymer blending, pore filling and tuning of Van der Waals interactions are also used to strengthen AEMs. In polymer blending, a reinforcing material (usually a hydrophobic and

Materials for solid alkaline fuel cells

Nuno André

Carvalho Sousa

Materiais para Pilhas de Combustível

Alcalinas Sólidas

Nuno André

Carvalho Sousa

Materiais para Pilhas de Combustível

Alcalinas Sólidas

Materials for Solid Alkaline Fuel Cells

List of Figures

List of Tables

Abbreviations and Acronyms

Symbols

Greek Symbols

Table of contents

I. INTRODUCTION

1 Motivation and objectives – Fuel Cells and the energy paradigm

Global fuel cell reaction

O

2

+ 2

H

2

→ 2

H

2

O

cathode

anode

electrolyte

4

OH-4e

-O

2

+ 2

H

2

O +

4

e

-

→ 4

O

H

-4

O

H

-

+ 2

H

2

→ 4

H

2

O

+

4

e

-2 Anion exchange membranes

2.1 Ionic transport through AEMs and the role of water

2.2 Requirements and challenges

2.3 Materials and synthesis of anion exchange membranes

₂

₂

₂

₂

₂

_O

_H

_{+ 2}

_H