The European Strategy plan for the Energy Technologies foresees that at least 65% of the electric energy should derive from renewable energy sources and CO2 emissions reduced by 50% within 2050. In this context, Polymer Electrolyte Fuel Cells (PEFCs) play an important role, above-all in terms of clean, safe and efficient electric power generation with low environment impact and, hence, independence from fossil fuels. As known, Direct H2-PEFCs (DH-PEFCs) convert the chemical energy of the reaction between H2 and air, respectively fed at anode and cathode sides, in electrical energy, generating water and heat. This occurs thanks to electrochemical reactions at the electrodes and the protons transportation from anode to cathode through the polymeric membrane, considered the core of the PEFC. In order to expand the application field, the research interest has been addressed towards development of High-Temperature PEFCs (HT-PEFCs). The increase of working temperature produces several important benefits, such as the improvement of the catalyst activity, the impurities tolerance and the simplification of the thermal and water management of the system. In drastic conditions, the most commonly used perfluoro-sulphonic membranes (in particular, Nafion®) are subjected to swelling and de-swelling processes that lower its particularly high proton conductivity and mechanical strength with a consequent performance failure. Different solutions have been considered to overcome these obstacles such as the development of membranes based on: i) short-side chain perfluorinated sulphonic acid (SSCA-PFSA) thermo-stable polyaromatic polymers reinforced membranes; iv) organic/inorganic polymer matrices containing hygroscopic and/or proton conductor fillers to develop composite membranes. The aim consists in the enhancing of the water retention and the mechanical properties of the membranes when hydrophilic inorganic oxides, such as SiO2, TiO2, ZrO2 are added and in the improvement of the proton conductivity when proton conductors, such as heteropolyacids (PWA, PMoA, SiWA, etc.) are used to modify the polymer matrix. In particular, inorganic sulphated or phosphated zirconia nanoparticles fillers with acidic functionalities can contribute to the proton conductivity of the membranes, while nano-sized Yttria- Stabilized-Zirconia (YSZ) fillers seem to provide hybrid membranes stable at high temperature with a reduced methanol crossover. It was demonstrated that nano-YSZ becomes a proton conductor below 120°C in water-saturated air and this low-temperature proton conductivity is greatly dependent on the grain size of nanostructured YSZ. YSZ has been investigated as a possible additive to mitigate the degradation by free radicals of the membranes and the interest in such YSZ based-materials as filler for Proton Exchange Membranes (PEMs) has been increased to work at intermediate temperatures <150°C. Hence, composite Nafion® membranes represent a possible solution to operate at temperatures higher than 100°C and low RH in DH-PEFCs and YSZ based-materials have got important properties to be investigated for the aim. For these reasons, in our previous works, a commercial Zirconia stabilised with an 8 mol.% of Yttria, usually used as conductor for Solide Oxide Fuel Cells (SOFCs) at extremely high cell temperature, was used as a filler for Nafion® composite membranes in a single HT-PEFC and also in self-made stack configuration. A 10 wt.% of this commercial filler was found to be the optimal content in terms of performance at high temperature. Starting from evidence that a protonic conduction at low-temperature in nano-YSZ was hypothesized in the actual work, a YSZ filler containing an 8 mol. % of yttrium oxide was synthesised and its ideal content was investigated. This filler was synthesised by basic hydrolysis and successive doping of zirconia with a suitable amount of a c-Y2O3, used as a phase inductor. Highly homogeneous and wide size composite Nafion® membranes with a variable content of this filler (3, 5, 10 wt. %) were cast by a widely standardised procedure, verifying the electrochemical behaviour under drastic conditions of T and RH. Successively, once individuated the total optimal YSZ content (5wt%.), with the aim of evaluating the influence of Y2O3 doping level in ZrO2 lattice, differently doped fillers using four Y2O3 loadings (from 4.8 up to 15 mol.%) were synthesised and the corresponding membranes have been cast and characterized. A deep chemical-physical characterisation in terms of XRD, Ion Exchange Capacity (IEC), water uptake and swelling at different temperatures (room T, 80 and 95°C), ex-situ oxidative stability tests (Fenton's tests) to evaluate the degradation, N2 adsorption-desorption tests (BET), Scanning-Electron-Microscopy (SEM), Dynamo-Mechanical Analysis (DMA) on fillers and membranes was carried out, together to an electrochemical characterisation based on proton conductivity measurements, polarisation curves, Accelerated Degradation Tests (ADTs) and H2-crossover tests in a single cell, at high T and reduced RH. The main results regarding filler were: about different ZrO2 doping level, XRD demonstrated the presence of a mixed t-, m-, c-crystalline structure in which the tetragonal and monoclinic phases decrease increasing the Yttria load. An average crystalline size of about 29-37 nm and nanometric agglomerates with size decreasing as a function of increasing Y2O3 amount were found. From BET analysis, the formation of a c-ZrO2 mesoporous structure was demonstrated, while SEM revealed a size reduction of the agglomerates increasing the doping level. The main results regarding composite membranes cast were: i) a fraction of Y3+ cations in syn- YSZ is able to replace the protons of Nafion® sulphonic groups, maintaining unaltered water retention and swelling of the membrane; DMA and chemical-physical characterisations, it was evidenced that the composite membranes have a similar Glass Transition Temperature (Tg) and a higher mechanical resistance for higher doping levels up to 90°C if compared to reference membrane. After this T, the membranes are equally mechanically stable; the membranes with higher doping levels of Y2O3 in the ZrO2 structure showed an increased IEC, probably attributable to a partial proton conduction. Regarding the electrochemical results: at100 %RH with 80<120°C, the proton conductivity (PC) of composite membranes is higher than pristine Nafion, meaning that the filler has intrinsic proton conduction properties (presence of proton charge carriers at this T); at 120°C, 75%RH, PC increases as a function of YSZ content, probably due to an enhancement of the path continuity within the polymer matrix when the filler loading is increased. In DH-PEFC, the composite membrane containing a 5 wt. % of syn- YSZ supplied the highest electrochemical performance with a cell potential of 0.617 V (@ 0.5 A cm-2) against a value of 0.534 V for pure Nafion and highest stability and degradation resistance in ADTs, exceeding 0.55 V (@0.4 A cm-2) supplying 48 cycles against to 28 cycles of the reference membrane. Moreover, from chemical oxidative stability measurements, the developed fillers with a higher yttria loadings (12 and 15 mol.%) showed to have an antioxidant effect, aspect confirmed from ADT results, carried out in drastic conditions for a PEFC (95°C and 50% RH). Hence, the composite membranes containing the highly yttria-doped fillers seem to be promising samples for the selected application, since they were able to work more than 110 cycles maintaining an optimal performance with very low H2-crossover values within the targets supplied by DoE as references.
Composite Nafion® Membranes with Differently Doped Nano-crystalline Yttria-Stabilised-Zirconia (YSZ) For Proton Exchange Membranes Fuel Cells (PEMFCs) Applications in Drastic Conditions
A Saccà
Primo
;A CarboneSecondo
;I Gatto;R Pedicini;S Maisano;A Stassi;Enza PassalacquaUltimo
2021
Abstract
The European Strategy plan for the Energy Technologies foresees that at least 65% of the electric energy should derive from renewable energy sources and CO2 emissions reduced by 50% within 2050. In this context, Polymer Electrolyte Fuel Cells (PEFCs) play an important role, above-all in terms of clean, safe and efficient electric power generation with low environment impact and, hence, independence from fossil fuels. As known, Direct H2-PEFCs (DH-PEFCs) convert the chemical energy of the reaction between H2 and air, respectively fed at anode and cathode sides, in electrical energy, generating water and heat. This occurs thanks to electrochemical reactions at the electrodes and the protons transportation from anode to cathode through the polymeric membrane, considered the core of the PEFC. In order to expand the application field, the research interest has been addressed towards development of High-Temperature PEFCs (HT-PEFCs). The increase of working temperature produces several important benefits, such as the improvement of the catalyst activity, the impurities tolerance and the simplification of the thermal and water management of the system. In drastic conditions, the most commonly used perfluoro-sulphonic membranes (in particular, Nafion®) are subjected to swelling and de-swelling processes that lower its particularly high proton conductivity and mechanical strength with a consequent performance failure. Different solutions have been considered to overcome these obstacles such as the development of membranes based on: i) short-side chain perfluorinated sulphonic acid (SSCA-PFSA) thermo-stable polyaromatic polymers reinforced membranes; iv) organic/inorganic polymer matrices containing hygroscopic and/or proton conductor fillers to develop composite membranes. The aim consists in the enhancing of the water retention and the mechanical properties of the membranes when hydrophilic inorganic oxides, such as SiO2, TiO2, ZrO2 are added and in the improvement of the proton conductivity when proton conductors, such as heteropolyacids (PWA, PMoA, SiWA, etc.) are used to modify the polymer matrix. In particular, inorganic sulphated or phosphated zirconia nanoparticles fillers with acidic functionalities can contribute to the proton conductivity of the membranes, while nano-sized Yttria- Stabilized-Zirconia (YSZ) fillers seem to provide hybrid membranes stable at high temperature with a reduced methanol crossover. It was demonstrated that nano-YSZ becomes a proton conductor below 120°C in water-saturated air and this low-temperature proton conductivity is greatly dependent on the grain size of nanostructured YSZ. YSZ has been investigated as a possible additive to mitigate the degradation by free radicals of the membranes and the interest in such YSZ based-materials as filler for Proton Exchange Membranes (PEMs) has been increased to work at intermediate temperatures <150°C. Hence, composite Nafion® membranes represent a possible solution to operate at temperatures higher than 100°C and low RH in DH-PEFCs and YSZ based-materials have got important properties to be investigated for the aim. For these reasons, in our previous works, a commercial Zirconia stabilised with an 8 mol.% of Yttria, usually used as conductor for Solide Oxide Fuel Cells (SOFCs) at extremely high cell temperature, was used as a filler for Nafion® composite membranes in a single HT-PEFC and also in self-made stack configuration. A 10 wt.% of this commercial filler was found to be the optimal content in terms of performance at high temperature. Starting from evidence that a protonic conduction at low-temperature in nano-YSZ was hypothesized in the actual work, a YSZ filler containing an 8 mol. % of yttrium oxide was synthesised and its ideal content was investigated. This filler was synthesised by basic hydrolysis and successive doping of zirconia with a suitable amount of a c-Y2O3, used as a phase inductor. Highly homogeneous and wide size composite Nafion® membranes with a variable content of this filler (3, 5, 10 wt. %) were cast by a widely standardised procedure, verifying the electrochemical behaviour under drastic conditions of T and RH. Successively, once individuated the total optimal YSZ content (5wt%.), with the aim of evaluating the influence of Y2O3 doping level in ZrO2 lattice, differently doped fillers using four Y2O3 loadings (from 4.8 up to 15 mol.%) were synthesised and the corresponding membranes have been cast and characterized. A deep chemical-physical characterisation in terms of XRD, Ion Exchange Capacity (IEC), water uptake and swelling at different temperatures (room T, 80 and 95°C), ex-situ oxidative stability tests (Fenton's tests) to evaluate the degradation, N2 adsorption-desorption tests (BET), Scanning-Electron-Microscopy (SEM), Dynamo-Mechanical Analysis (DMA) on fillers and membranes was carried out, together to an electrochemical characterisation based on proton conductivity measurements, polarisation curves, Accelerated Degradation Tests (ADTs) and H2-crossover tests in a single cell, at high T and reduced RH. The main results regarding filler were: about different ZrO2 doping level, XRD demonstrated the presence of a mixed t-, m-, c-crystalline structure in which the tetragonal and monoclinic phases decrease increasing the Yttria load. An average crystalline size of about 29-37 nm and nanometric agglomerates with size decreasing as a function of increasing Y2O3 amount were found. From BET analysis, the formation of a c-ZrO2 mesoporous structure was demonstrated, while SEM revealed a size reduction of the agglomerates increasing the doping level. The main results regarding composite membranes cast were: i) a fraction of Y3+ cations in syn- YSZ is able to replace the protons of Nafion® sulphonic groups, maintaining unaltered water retention and swelling of the membrane; DMA and chemical-physical characterisations, it was evidenced that the composite membranes have a similar Glass Transition Temperature (Tg) and a higher mechanical resistance for higher doping levels up to 90°C if compared to reference membrane. After this T, the membranes are equally mechanically stable; the membranes with higher doping levels of Y2O3 in the ZrO2 structure showed an increased IEC, probably attributable to a partial proton conduction. Regarding the electrochemical results: at100 %RH with 80<120°C, the proton conductivity (PC) of composite membranes is higher than pristine Nafion, meaning that the filler has intrinsic proton conduction properties (presence of proton charge carriers at this T); at 120°C, 75%RH, PC increases as a function of YSZ content, probably due to an enhancement of the path continuity within the polymer matrix when the filler loading is increased. In DH-PEFC, the composite membrane containing a 5 wt. % of syn- YSZ supplied the highest electrochemical performance with a cell potential of 0.617 V (@ 0.5 A cm-2) against a value of 0.534 V for pure Nafion and highest stability and degradation resistance in ADTs, exceeding 0.55 V (@0.4 A cm-2) supplying 48 cycles against to 28 cycles of the reference membrane. Moreover, from chemical oxidative stability measurements, the developed fillers with a higher yttria loadings (12 and 15 mol.%) showed to have an antioxidant effect, aspect confirmed from ADT results, carried out in drastic conditions for a PEFC (95°C and 50% RH). Hence, the composite membranes containing the highly yttria-doped fillers seem to be promising samples for the selected application, since they were able to work more than 110 cycles maintaining an optimal performance with very low H2-crossover values within the targets supplied by DoE as references.File | Dimensione | Formato | |
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