Jensen, Annemette Hindhede

Brændselsceller ved 200-400°C er af interesse da de kan kombinere fordele fra lav- og højtemperaturteknologier. En forøgelse af temperaturen i forhold til hvad der bruges for polymer elektrolyt membran (PEM) brændselsceller forbedrer kinetikken af katalysatorerne, og dermed åbnes der mulighed for anvendelse af ikke-ædelmetal katalysatorer. Dog er driftstemperaturen lav nok til at også relativt billige materialer kan anvendes som konstruktionsmaterialer. I dette arbejde er et formålsbygget set-up og brændselscellehardware konstrueret til demonstration af brændselsceller til mellemtemperaturområdet. Til elektroderne blev karbonklæde og karbonpapir testet som gasdiffusionslag med forskellige katalytiske lag. Af de to havde karbonpapir med en platinmængde på 7 mg cm−2 en bedre virkning. Karbon er ustabilt under driftsforholdene i en brændselscellekatode og andre materialer må findes. Det blev forsøgt at anvende rustfrit stål (316L), hvilket imidlertid korroderede og stålet blev derfor belagt med tantal. Tantalbelægningerne blev fundet til at være korrosionsmodstandsdygtige og resulterede endvidere i ekstrem lav kontaktmodstand på 1,3 mΩ cm2 . I litteraturen rapporteres celler med cæsium dihydrogen fosfat (CsH2PO4) elektrolyt som de mest lovende for mellemtemperaturområdet. CsH2PO4 gennemgår faseovergang ved 230°C. Med en stigning i ledningsevne fra 8,5 x 10−6 ved 223°C til 1,8 x 10−2 S cm−1 ved 233°C kaldes denne overgang for superprotonisk faseovergang. Denne elektrolyt, ligesom andre elektrolytter til dette temperaturområde, lider af dårlige mekaniske egenskaber og stabile brændselsceller er kun demonstreret med tykke elektrolytter. Derudover, for at bibeholde høj ledningsevne af elektrolytten, er en høj befugtning af fødegasserne nødvendig. Kompositter med CsH2PO4 blev i dette arbejde lavet for at forbedre egenskaberne af materialet. Kompositter i formation med mekanisk stærke materialer inkusiv ZrO2, TiO2 og NdPO4·0.5H2O mindskede permeabiliteten af elektrolytten, som ydermere resulterede i forbedret stabilitet af brændselscellen. Open circuit voltage (OCV) for sådanne celler var høje, over 0,9 V, og stabile op til 250°C. Kompositdannelse med ZrO2 resulterede desuden i forøget ledningsevne ved højere temperaturer sandsynligvis på grund af fysisk stabilisering af den superprotoniske fase. Ved 250°C var cellen stabil i mere end 60 timer med et partial tryk af vand på kun 0,12 atm, og blev opereret op til 275°C hvor brændselscellen med en ren CsH2PO4 elektrolyt ikke længere var funktionsdygtig. Kompositdannelsen mellem CsH2PO4 og NdPO4·0,5H2O gav sandsynligvis anledning til dannelsen af en ny fase, CsH5(PO4)2, som har en høj ledningsevne fra omkring 150°C. Mekanismen bag stigningen i ledningsevnen for CsH2PO4/NdPO4·0,5H2O kompositten blev dog ikke fuldt afklaret. Ved at bruge sådanne elektrolytter kunne brændselscellen opereres ved 285°C hvor den højeste ydelse blev målt. Strøm- og effekttætheder blev måltviii til henholdsvis 117 mA cm−2 og 27,7 mW cm−2 . Kompositdannelse med melamin cyanurat resulterede i øget ledningsevne i hele det målte temperaturinterval i.e. fra 120 til 260°C. En ledningsevne så høj som 0,18 S cm−1 blev målt for en 90CsH2PO4/10melamin cyanurat komposit ved 250°C. Ydermere blev det observeret at disse kompositter havde gode mekaniske egenskaber. Indenfor PROCON projektet blev en screening udført for at søge efter nye elektrolytter. Fra denne screening blev niobium og bismut fosfat fundet til at have høje ledningsevner (&gt;10−2 S cm−1 ) med fornuftig stabilitet, og det blev derfor forsøgt at fabrikere elektrokemiske celler med disse. De ublandede fosfater havde dårlig mekanisk stabilitet og derfor blev polybenzimidazole (PBI) tilført. Ved at tilføre PBI i stor mængde blev der målt stabile OCVer, disse forblev stabile i 10 og 70 timer for henholdsvis niobium og bismuth fosfat. Ved højere temperaturer faldt OCV, som ved 200°C var under 0,9 V. Wolframkarbid (WC) blev evalueret som en ikke-ædelmetal katalysator til hydrogen udvikling og oxidation reaktionerne. Karbider blev fremstillet på forskellig vis for at opnå højere overfladeareal i forhold til det meget lave overfladeareal som kommercielt karbid har, dette havde så lavt et overfladeareal at det ikke kunne kvantificeres. Ved at fremstille karbid fra WO3 (WC-mWO3) som blev fremstillet fra en mesoporøs silica skabelon ved karburisering med methan ved 900°C i tre timer blev et overfladeareal på 6 m2 g −1 målt. Ved at introducere et ekstra synteseskridt ved først at omdanne WO3 til W2N som derefter blev omdannet til WC (WC-mW2N) blev et højere overfladeareal på 18 m2 g −1 målt. Brugen af methan i forhold til ethan som karburiseringsmiddel blev undersøgt ved karburisering af commercielt WO3 under de samme betingelser. Fra karburisering med methan kunne der ikke måles noget overfladeareal, mens karburiseringen med ethan (WC-ethane) resulterede i et overfladeareal på 12 m2 g −1 . En ekstra wolframkarbid (WC-05-VN) med et overfladeareal på 31 m2 g −1 blev undersøgt for sammenligning. Hydrogen evolution aktiviteter for karbiderne blev målt i fosforsyre ved 185°C og -100 mV. Det blev fundet at bortset fra WC-mW2N steg aktiviteten med stigende overfladeareal, og afvigelsen for WC-mW2N kan skyldes et amorft karbon overfladelag. Aktiviteterne blev fundet til 1,5, 2,07, 10,7 og 18,73 A g−1 for henholdsvis WC-mWO3, WC-mW2N, WCethane og WC-05-VN. Karbiderne blev desuden undersøgt som katalysatorer til anoden i brændselscellen. Den bedste ydelse blev fundet ved den højest målte temperatur (270°C) hvor effekttæthederne blev fundet til henholdsvis 2,7, 3,1, 7,4 og 8,2 for WC-mW2N, WC-mWO3, WC-05-VN og WC-ethane. Til disse målinger blev CsH2PO4 brugt som elektrolyt og WC loadingen var 10 mg cm−2.The intermediate temperature region for fuel cells (200-400°C) is of interest as it may combine advantages from low and high temperature technologies. Increasing the temperature above what is used in polymer electrolyte membrane (PEM) fuel cells enhances the catalyst kinetics, and therefore it might become possible to use non-noble metal catalysts. On the other hand, the temperature is low enough for a wide range of materials to be used as construction materials. In this work a set-up was built and fuel cell hardware was made for demonstration of fuel cells for the intermediate temperature range. For the electrodes, carbon cloth and carbon paper were tested as gas diffusion layers with different catalytic compositions, and of the two, carbon paper with a platinum loading of 7 mg cm−2 had the better performance. However, carbon is unstable at the conditions in the fuel cell cathode and other materials must be sought for. It was attempted to use stainless steel (316L), this however corroded and therefore a protective tantalum coating was applied. The tantalum coatings were found to be corrosion resistant and furthermore provided extremely low interfacial contact resistances of only 1.3 mΩ cm2 . From a literature review it was found that the most promising results for this temperature range have been performed using cesium dihydrogen phosphate (CsH2PO4) electrolytes. CsH2PO4 undergoes a phase transition at around 230°C, with a rise in conductivity from 8.5 x 10−6 at 223°C to 1.8 x 10−2 S cm−1 at 233°C this is called superprotonic. This electrolyte as well as other electrolytes for this temperature range, however, suffers from poor mechanical properties, and stable fuel cell performance had only been achieved by use of thick electrolytes. Furthermore to maintain high conductivity of the electrolyte, a high level of humidification was necessary. Composites with CsH2PO4 were made to improve the properties of the electrolyte material. Composites in formation with mechanically strong materials including ZrO2, TiO2 and NdPO4·0.5H2O improved the densification of the electrolyte, which further resulted in improved stability of the fuel cell. Open circuit voltages (OCVs) using such fuel cells were found to be high, above 0.9 V, and stable up to 250°C. Composite formation with ZrO2 furthermore resulted in increased conductivity at higher temperatures probably due to the physical stabilization of the high conducting phase. At 250°C the cell was stable for more than 60 hours with a partial pressure of water of only 0.12 atm, and it was operational up to 275°C, where the fuel cell using pure CsH2PO4 no longer performed. When CsH2PO4 was used in composite with NdPO4·0.5H2O there were indications of a new phase formed, CsH5(PO4)2, which has been reported to have high conductivity from 150°C. The mechanism behind an increase in conductivity for the CsH2PO4/NdPO4·0.5H2O ofvi several orders of magnitude was not fully clarified. Using an 29CsH2PO4/71NdPO4·H2O electrolyte enabled fuel cell performance measurements up to 285°C, where the highest performance was recorded. At this temperature current and power densities were found to be 117 mA cm−2 and 27.7 mW cm−2 , respectively. Composite formation with melamine cyanurate resulted in increased conductivity in the entire temperature interval measured i.e. from 120°C to 260°C. A conductivity as high as 0.18 S cm−1 was measured for a 90CsH2PO4/10melamine cyanurate composite at 250°C. Good mechanical properties were furthermore observed for the composites. Within the research project a screening was made in order to search for new electrolytes. From this screening niobium and bismuth phosphates were found to have high conductivities (&gt;10−2 S cm−1 ) with reasonable stability, and it was therefore attempted to fabricate electrochemical cells from these. The pure phosphates were however suffering from poor mechanical stability and therefore polybenzimidazole (PBI) was added. By adding high amounts of PBI stable OCVs were achieved, these remained stable for around 10 and 70 hours for niobium and bismuth phosphates, respectively. At high temperatures, however, the OCVs were found to drop, at 200°C the OCVs were below 0.9 V. Tungsten carbide was evaluated as a non-noble catalyst for the hydrogen evolution and oxidation reactions. Tungsten carbides were prepared in different ways in order to achieve higher surface areas compared to the very low surface area of the commercial carbide which was too low to be quantified. By preparing the carbide from WO3 (WC-mWO3) which had been prepared by use of a mesoporous silica template by carburization with methane at 900°C for 3 hours, a surface area of 6 m2 g −1 was measured. By introducing an extra synthesis step by first converting the WO3 into W2N which was then converted into WC (WC-mW2N) a higher surface area of 18 m2 g −1 was measured. The use of methane versus ethane as carburizing agents were investigated, by carburizing commercial WO3 with both agents under the same conditions. From carburization with methane no surface area could be quantified, while the carburization with ethane resulted in a carbide (WC-ethane) with a surface area of 12 m2 g −1 . An additional tungsten carbide (WC-05-VN) with a BET area of 31 m2 g −1 was used for comparison. Hydrogen evolution activities for the carbides were measured in phosphoric acid at 185°C and -100 mV. It was found that apart from WC-mW2N, the activities were increasing with surface area, this deviation may be due to an amorphous carbon surface layer. Activities were found as 1.5, 2.07, 10.7 and 18.73 A g−1 for WC-mWO3, WC-mW2N, WC-ethane and WC-05-VN, respectively. The carbides were furthermore investigated as fuel cell anode catalysts. The best performances were achieved at the highest temperature measured i.e. 270°C where power densities of 2.7, 3.1, 7.4 and 8.2 mW cm−2 for WC-mW2N, WC-mWO3, WC-05-VN and WC-ethane, respectively, using CsH2PO4 electrolytes and WC loadings of 10 mg cm−2 

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Preparation and Characterization of Components for Intermediate Temperature Fuel Cells And Electrolyzers

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Preparation and Characterization of Components for Intermediate Temperature Fuel Cells And Electrolyzers

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