Alkali and Alkaline Earth Oxoacid Salts; Synthesis, Hydration, Stability, and Electrical Conductivity

116 Pages·2017·9.59 MB·English

Alkali and Alkaline Earth Oxoacid Salts; Synthesis,
Hydration, Stability, and Electrical Conductivity
Aleksander Andestad Elstad
Thesis for the degree of ’Master of Science’, Spring 2017

Summary
Proton-conductingelectrolytesaresoughafterforuseinvariousapplicationswithinthe
field of electrochemistry. Pure and high proton conductivity has been found in many
perovskite-type oxides like BaZrO (BZY) and BaCeO , with BaCeO -based materials
3 3 3
being among the best proton-conducting oxides. In the intermediate temperature range
of 400 to 800◦C, BZY has been established as one of the most promising materials,
exhibiting a protonic conductivity higher than 1×10−2Scm−1 over the whole temper-
ature range. However, it is difficult to process, and the resulting materials are usually
grainy and possess highly resistive grain-boundaries [1]. For low-temperature regions,
compounds like CsHSO and CsH PO show great potential with respect to protonic
4 2 4
conductivity, even displaying superprotonic transitions that immensely increase their
conductivity, however their stability is lacking with respect to temperature and solubil-
ityinwater[2].
With this project, the aim is to broaden the horizon and investigate compounds that
fall outside the common perovskite-definition. In this work, various solid acids (E.g.
KBaPO ,NaCaHSiO andBaH SiO ),inwhichthecationsarealkaliandalkalineearth
4 4 2 4
metals and the anionic groups are separated XO tetrahedra, are synthesized and subse-
4
quently characterized by X-Ray Diffraction (XRD), Thermogravimetric Analysis (TG),
as well as electrical characterization by Impedance Spectroscopy (IS). The work on
KBaPO culminatedinasubmittedpaper[3].
4
KBaPO hasbeenproposedtotransformintoagreatprotonicconductoruponhydration
4
at low temperatures. Effectively, hydration through steam at 80◦C is said to give the
compound a protonic conductivity of 1×10−2Scm−1 just below 100◦C [4]. This is
a remarkable result and, if it can be reproduced, it can become a viable rival to BZY.
For this reason, KBaPO was chosen as a topic for this work. Here, we synthesize
4
KBaPO through a high-temperature solid state reaction, and subsequently character-
4
ize the system with respect to thermal stability and its inherent electrical conductivity.
Through electrical measurements, we found that the conductivity of pure KBaPO was
4
very low, around 2×10−6Scm−1 at 600◦C, with an activation energy exceeding 1eV.
The compound is indifferent to the presence of humidity, and results indicate that the
chargecarrierinthecompoundisnotprotonic,butratheritistheorizedtobepotassium
ions, with potassium Frenkel defects being the predominating defect, however this has
not been explicitly confirmed. All in all, we propose a defect model for KBaPO with
4
Frenkeldefectsasthepredominatingdefects.
ThroughattemptsathydratingKBaPO inaccordancetothemethodproposedbyGood-
4
enough,wefoundthatitdoesnottransformintoahigh-conductivityphase,butratherde-
composes into potassium doped Ba (PO ) , and that the resulting system shows similar
3 4 2
i
properties,suchasthermalstability(Decomposingat300◦C)andprotonicconductivity
(1.6×10−6Scm−1 at 250◦C), to the system Ba K H (PO ) previously investigated
3-x x x 4 2
by Haile et al. [5], albeit with a significantly lower potassium content than the systems
theyhavecharacterized,possiblyindicatingthatasaturationofKinBa (PO ) hasbeen
3 4 2
reached.
By subsequently heating Ba K H (PO ) to high temperatures, the system is found
3-x x x 4 2
to expel potassium and form a two-phase system of Ba (PO ) and a secondary phase
3 4 2
of KBaPO , showing similarities to the system Ba K (PO ) previously investi-
4 3(1-x) 3x 4 2-x
gated by Iwahara et al. [6]. Through impedance spectroscopy of said system, we found
evidence that points toward the system being a protonic conductor, with a bulk conduc-
tivityslightlyhigherthan1×10−3Scm−1 at600◦C,andanactivationenergyofaround
0.67eV. This is one order of magnitude higher than the one previously reported by
Iwaharaetal.,andonlyoneorderofmagnitudelowerthanthatofBaZrO .
3
Parallelly,NaCaHSiO andrelatedcompoundsABHXO (A−−Li,NaorK. B−−Ca,Sror
4 4
Ba. X−−Si, Ge or Sn) were synthesized hydrothermally and subsequently characterized.
ElectricalcharacterizationofNaCaHSiO gavelowconductivities,althoughprotonic,of
4
1.8×10−8Scm−1 at 250◦C, with an activation energy of 0.9eV. Based on the results,
we propose a defect model in which interstitial hydroxide ions and interstitial protons
strsignificantdefectsinthecompound.
However, although NaCaHSiO could be successfully synthesized and subsequently
4
characterized, the other syntheses did not yield the desired results. In fact, the only
synthesis that yielded a pure product was that which gave Sr SiO , possibly providing
2 4
a hydrothermal approach to synthesizing a compound previously produced by a high-
temperaturesolidstatereaction.
Lastly, the compound BaH SiO was synthesized, according to a hydrothermal route,
2 4
and characterized with respect to thermal stability and electrical conductivity. It was
foundtoexhibitaconductivityof2.5×10−8Scm−1 at200◦Cwithanactivationenergy
of 0.88eV, comparable to that of NaCaHSiO . Due to BaH SiO showing similar re-
4 2 4
sponsetovariousatmospheresasNaCaHSiO ,adefectmodelcontaininghydroxideand
4
hydrogeninterstitialsisproposedforBaH SiO aswell.
2 4
Comparedtoearlierreports,adiscrepancywasfoundinthattheBaH SiO decomposes
2 4
priortotemperatureregionsinwhichdataonelectricalconductivityhasbeenpreviously
reported. Another,separateinvestigationintoBaH SiO isthereforerecommended.
2 4
ii
Acknowledgements
OverthelasttwoyearsIhavehadtheopportunitytoworkonaprojectthatwasabitout
oftheordinary. Ithasbeentwointerestingyears,withtheprojectcontinuouslyevolving
asexperimentsgaveunexpectedresults,andnewroadshavehadtobepaved. Intheend,
I arrived at a goal that was not the one I initially set out for. However, looking back, it
hasbeenquiteajourney,andIamleftwithalotofknowledgeandideasforthefuture.
I would like to express my gratitude to my supervisors, Truls Norby and Sabrina Sar-
tori, for their continuous support and for their eagerness to help me when I have had
questions,attimesresultinginalongemaildroppingintomyinboxinthemiddleofthe
night.
IwouldliketothankReidar,EinarandRagnarforhavingthetimeandpatiencetoassist
mewithinstruments,interpretations,defectchemistry,andeverythingelseIhaveneeded
helpwith. Additionally,IwouldliketothankallthepeopleinFASE.Therewasalways
someoneIcouldask.
Iwouldalsoliketogiveashout-outtoKevinNguyenforthemanydiscussionswehave
hadinordertounderstandwhatwehavebeendoing.
Lastly,Iamverygratefulformyfamilyfortheirloveandsupport,eventhoughtheyare
notentirelysureaboutwhatIhavebeenworkingonduringalltheseyears.
iii

Contents
Summary ii
Acknowledgements iii
ListofFigures xiii
ListofTables xvi
1 Introduction 1
1.1 FuelCells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 HighTemperature: MOFCsandSOFCs . . . . . . . . . . . . . 2
1.1.2 Low-andIntermediateTemperatures . . . . . . . . . . . . . . 3
1.2 GoalofProject . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 TheoreticalBackground 7
2.1 IonicConductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1 ConductionThroughDefectsinSolids . . . . . . . . . . . . . . 7
2.1.2 CalculationofConductivityandActivationEnergy . . . . . . . 8
2.2 DefectsandStructuralConsiderations . . . . . . . . . . . . . . . . . . 9
2.2.1 DefectChemistry . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2 DefectModels . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 ImpedanceSpectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.1 TheConceptofImpedance . . . . . . . . . . . . . . . . . . . . 18
2.3.2 CircuitElements . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.3 ImpedanceSweepsandModelling . . . . . . . . . . . . . . . . 21
3 LiteratureReview 25
3.1 IonicConductioninOxoacidSalts . . . . . . . . . . . . . . . . . . . . 25
3.2 KBaPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4
3.2.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2.2 AcceptorDopingKBaPO . . . . . . . . . . . . . . . . . . . . 26
4
3.3 Orthosilicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3.1 NaCaHSiO andRelatedCompounds . . . . . . . . . . . . . . 27
4
3.3.2 BaH SiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2 4
v
Contents
4 Experimental 29
4.1 SamplePreparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.1.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.1.2 PelletPreparation . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2.1 X-RayDiffraction-XRD . . . . . . . . . . . . . . . . . . . . 32
4.2.2 ScanningElectronMicroscopy-SEM . . . . . . . . . . . . . . 33
4.2.3 EnergyDispersiveX-RaySpectroscopy-EDS . . . . . . . . . 34
4.2.4 ThermogravimetricAnalysis-TGA . . . . . . . . . . . . . . . 34
4.3 ImpedanceSpectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.3.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.3.2 ElectricalMeasurements . . . . . . . . . . . . . . . . . . . . . 36
4.4 OtherExperimentalMethods . . . . . . . . . . . . . . . . . . . . . . . 37
4.4.1 HydrationofKBaPO . . . . . . . . . . . . . . . . . . . . . . 37
4
4.5 SourcesofError . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5 Results 43
5.1 KBaPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4
5.1.1 Synthesis&Characterization . . . . . . . . . . . . . . . . . . . 43
5.1.2 Hydration-KBaPO → Ba K H (PO ) . . . . . . . . . . . . 46
4 3-x x x 4 2
5.1.3 ImpedanceSpectroscopy . . . . . . . . . . . . . . . . . . . . . 51
5.2 NaCaHSiO andRelatedCompounds . . . . . . . . . . . . . . . . . . 61
4
5.2.1 Synthesis&Characterization . . . . . . . . . . . . . . . . . . . 61
5.2.2 ImpedanceSpectroscopy . . . . . . . . . . . . . . . . . . . . . 62
5.3 BaH SiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
2 4
5.3.1 Synthesis&Characterization . . . . . . . . . . . . . . . . . . . 65
5.3.2 ImpedanceSpectroscopy . . . . . . . . . . . . . . . . . . . . . 65
6 Discussion 67
6.1 KBaPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4
6.1.1 SynthesisandCharacterizationofKBaPO . . . . . . . . . . . 67
4
6.1.2 ConductivityofKBaPO . . . . . . . . . . . . . . . . . . . . . 69
4
6.1.3 HydrationandDecompositionofKBaPO . . . . . . . . . . . . 72
4
6.2 HydratedKBaPO -Ba K H (PO ) . . . . . . . . . . . . . . . . . . 75
4 3-x x x 4 2
6.2.1 ConductivityofK-containingBa (PO ) -phase . . . . . . . . . 75
3 4 2
6.2.2 Two-PhaseSystemofBa (PO ) andKBaPO . . . . . . . . . 77
3 4 2 4
6.2.3 Comparison of KBaPO , Ba K H (PO ) , Two-Phase System
4 3-x x x 4 2
ofBa (PO ) andKBaPO ,andPureBa (PO ) . . . . . . . . . 79
3 4 2 4 3 4 2
vi
Contents
6.3 SilicatesandRelatedCompounds(ABHXO ) . . . . . . . . . . . . . . 81
4
6.3.1 SynthesisofNaCaHSiO andOtherOrthosilicates . . . . . . . 81
4
6.3.2 ConductivitiesandDefectModelsofOrthosilicates . . . . . . . 82
7 ConclusionsandFurtherWork 87
7.1 KBaPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4
7.1.1 IonicConductivityofKBaPO . . . . . . . . . . . . . . . . . . 87
4
7.1.2 HydrationofKBaPO . . . . . . . . . . . . . . . . . . . . . . 87
4
7.2 SilicatesandRelatedCompounds . . . . . . . . . . . . . . . . . . . . 88
7.2.1 NaCaHSiO andABHXO . . . . . . . . . . . . . . . . . . . . 88
4 4
7.2.2 BaH SiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
2 4
7.3 FurtherWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
A Appendix 91
A.1 DerivationofRelativeUncertainties . . . . . . . . . . . . . . . . . . . 91
Bibliography 93
vii