<p>This thesis work is the continuation and final part of a joint project between the Department of Materials Technology, NTNU and Norsk Hydro Research Center in Porsgrunn, looking at the possibility of using fuel cells for production of hydrogen chloride and electric power. The experimental work encompass an evaluation of three hydrogen - chlorine fuel cell design concepts, development and implementation of a mathematical fuel cell model and a kinetic study of the chlorine reduction reaction. </p><p>The evaluated fuel cell designs consisted of a) a conventional PEM fuel cell applying a Nafion membrane, b) a composite system applying an aqueous HCl electrolyte and Nafion membrane and c) a phosphoric acid doped PBI membrane fuel cell operating at intermediate temperatures of 150 - 175 ◦C. From the evaluation it was found that the chlorine reduction kinetics are much faster than the corresponding oxygen reduction reaction, leading to low activation losses on the fuel cell cathode. However, the nature of the reactant, chlorine, and the product, HCl, places strict demands on the corrosion resistance of the construction materials and drastically increases the difficulties related to water management in the cells. Due to these effects, none of the investigated systems were able to demonstrate stable operation under the conditions used in this study. The PBI cell showed best potential and seems to be the system in which the humidification and corrosion difficulties easiest can be remedied. The first design criteria for such a system should be the minimisation of the existence of liquid water, ideally a hydrogen - chlorine fuel cell system should operate in totally water free environment and consist of a high temperature proton conductor. </p><p>A two dimensional, isothermal mathematical model of a hydrogen - chlorine single fuel cell with an aqueous HCl electrolyte is presented. The model focuses on the electrode reactions in the chlorine cathode and also includes the mass and momentum balances for the electrolyte and cathode gas diffusion layer. There is good agreement between the model predictions and experimental results. Distributions of physical parameters such as reactant and product concentrations, solution and solid phase potentials and local current densities and overpotentials as a function of cell voltage are presented. Effects of varying the initial electrolyte concentration and operating pressure are analysed. It was found that an electrolyte inlet concentration of 6 mol dm−3 gave the best cell performance and that an increase of operating pressure gave a steady increase of the fuel cell performance.</p><p>The rate and mechanism of the electroreduction of chlorine on electrochemically oxidised Pt and Ru electrodes has been investigated relative to the state of oxide formation. Current/potential curves for the reduction process in 1 mol dm−3 HCl solution saturated with Cl2 have been obtained for electrode surfaces in various states of preoxidation with the use of the rotating disc electrode technique (RDE). In the case of chlorine reduction on platinum, the results indicate that adsorption of chlorine molecules with a subsequent rate determining electrochemical adsorption step is the dominant mechanism. The exchange current density seems to decrease linearly with the logarithm of the amount of surface oxide. Chlorine reduction on ruthenium is best described by a Heyrovsky-Volmer mechanism with the first charge transfer reaction as the rate determining step. The Krishtalik mechanism incorporating adsorbed O•Cl+ intermediates is also able to describe the reaction successfully. The reaction order is constant for all oxide coverages while the exchange current density apparently moves through a maximum at intermediate oxide coverages (∼100 mC cm−2). The results show that the electrocatalysis of the cathodic reduction of chlorine is very sensitive to the state of the oxidation of the electrode surface.</p><p>The rate and mechanism of the electroreduction of chlorine on electrooxidised ruthenium has further been investigated with focus on the effect of solution pH. Current/potential curves for the reduction process in solutions with constant chloride concentration of 1.0 mol dm−3 and varying H+ concentration have been obtained with the use of the rotating disk electrode technique (RDE). It was found that the chlorine reduction rate is highly inhibited in solutions with high H+ concentrations and that it can be satisfactorily described by the Erenburgh mechanism, previously suggested for the chlorine evolution on RuO2 and ruthenium titanium oxides (RTO). The expression of the kinetic current as a function of chlorine and H+ concentration was obtained by solving the elementary rate equations of the kinetic mechanism. The kinetic constants obtained from the correlation of the kinetic current expression to the experimental data were used to simulate the dependence of the surface coverages and elementary reaction rates on overpotential.</p>

<p>This thesis presents work performed on five subjects of the polymer fuel cell:</p><p>• A polymer fuel cell test-facility was optimised. A fuel cell housing was designed and built, enabling measurements of ocal fuel cell temperatures. The fuel cell voltage was measured between the gas diffusion backings using thin platinum wires. This assures that the true fuel cell voltage is measured. </p><p>• The Nafion® content and content of Acetylene Black in the fuel cell electrode was optimised in a 3₂ – factorial experiment. The amount of Nafion varied between 15,25 and 35 wt%, and Acetylene Black varied at 0, 5 and 10 wt%. The data was analysed with the Bootstrapping method, and reproducibility was assedded. The optimal amount for the Acetylene Black content.</p><p>• A novel fuel cell membrane was tested in the polymer fuel cell. The membrane was a proton irradiated and directly sulfonated poly(vinyl fluride) (PVF.SA) membrane. The performance was lightly better than for a Nafion® 117 membrane tested under the same conditions.</p><p>• A fuel cell model based on irreversible thermodynamics was presented. The model was a one-dimensional model solving the heat and water transport perpendicular to the membrane surface. A potential and temperature profile was calculated, based on literature data. Temperatures inside the membrane (Nafion® 117) were estimated to be 5°C higher at 1 A/cm² in the gas channels. </p><p>• The local temperatures were measured inside the polymer fuel cell. At 1 A/cm² the temperature difference between gas channel and membrane was measured to 6°C. The thermal conductivities in the membrane, backing and catalytic layer were estimated from temperature measurements. The thermal conductivity of the gas diffusion backing with electrode was 0.19 ± 0.05 Wmk and the thermal conductivity of the Nafion® 115 membrane was wstimated to 0.1 ± 0.1 W/mK. The heat-transfer coefficient of the electrodes was calculated to 1000 ± 300 W/m²K. </p>

This thesis presents work performed on five subjects of the polymer fuel cell: • A polymer fuel cell test-facility was optimised. A fuel cell housing was designed and built, enabling measurements of ocal fuel cell temperatures. The fuel cell voltage was measured between the gas diffusion backings using thin platinum wires. This assures that the true fuel cell voltage is measured. • The Nafion® content and content of Acetylene Black in the fuel cell electrode was optimised in a 32 – factorial experiment. The amount of Nafion varied between 15,25 and 35 wt%, and Acetylene Black varied at 0, 5 and 10 wt%. The data was analysed with the Bootstrapping method, and reproducibility was assedded. The optimal amount for the Acetylene Black content. • A novel fuel cell membrane was tested in the polymer fuel cell. The membrane was a proton irradiated and directly sulfonated poly(vinyl fluride) (PVF.SA) membrane. The performance was lightly better than for a Nafion® 117 membrane tested under the same conditions. • A fuel cell model based on irreversible thermodynamics was presented. The model was a one-dimensional model solving the heat and water transport perpendicular to the membrane surface. A potential and temperature profile was calculated, based on literature data. Temperatures inside the membrane (Nafion® 117) were estimated to be 5°C higher at 1 A/cm2 in the gas channels. • The local temperatures were measured inside the polymer fuel cell. At 1 A/cm2 the temperature difference between gas channel and membrane was measured to 6°C. The thermal conductivities in the membrane, backing and catalytic layer were estimated from temperature measurements. The thermal conductivity of the gas diffusion backing with electrode was 0.19 ± 0.05 Wmk and the thermal conductivity of the Nafion® 115 membrane was wstimated to 0.1 ± 0.1 W/mK. The heat-transfer coefficient of the electrodes was calculated to 1000 ± 300 W/m2K.

This thesis work is the continuation and final part of a joint project between the Department of Materials Technology, NTNU and Norsk Hydro Research Center in Porsgrunn, looking at the possibility of using fuel cells for production of hydrogen chloride and electric power. The experimental work encompass an evaluation of three hydrogen - chlorine fuel cell design concepts, development and implementation of a mathematical fuel cell model and a kinetic study of the chlorine reduction reaction. The evaluated fuel cell designs consisted of a) a conventional PEM fuel cell applying a Nafion membrane, b) a composite system applying an aqueous HCl electrolyte and Nafion membrane and c) a phosphoric acid doped PBI membrane fuel cell operating at intermediate temperatures of 150 - 175 ◦C. From the evaluation it was found that the chlorine reduction kinetics are much faster than the corresponding oxygen reduction reaction, leading to low activation losses on the fuel cell cathode. However, the nature of the reactant, chlorine, and the product, HCl, places strict demands on the corrosion resistance of the construction materials and drastically increases the difficulties related to water management in the cells. Due to these effects, none of the investigated systems were able to demonstrate stable operation under the conditions used in this study. The PBI cell showed best potential and seems to be the system in which the humidification and corrosion difficulties easiest can be remedied. The first design criteria for such a system should be the minimisation of the existence of liquid water, ideally a hydrogen - chlorine fuel cell system should operate in totally water free environment and consist of a high temperature proton conductor. A two dimensional, isothermal mathematical model of a hydrogen - chlorine single fuel cell with an aqueous HCl electrolyte is presented. The model focuses on the electrode reactions in the chlorine cathode and also includes the mass and momentum balances for the electrolyte and cathode gas diffusion layer. There is good agreement between the model predictions and experimental results. Distributions of physical parameters such as reactant and product concentrations, solution and solid phase potentials and local current densities and overpotentials as a function of cell voltage are presented. Effects of varying the initial electrolyte concentration and operating pressure are analysed. It was found that an electrolyte inlet concentration of 6 mol dm−3 gave the best cell performance and that an increase of operating pressure gave a steady increase of the fuel cell performance. The rate and mechanism of the electroreduction of chlorine on electrochemically oxidised Pt and Ru electrodes has been investigated relative to the state of oxide formation. Current/potential curves for the reduction process in 1 mol dm−3 HCl solution saturated with Cl2 have been obtained for electrode surfaces in various states of preoxidation with the use of the rotating disc electrode technique (RDE). In the case of chlorine reduction on platinum, the results indicate that adsorption of chlorine molecules with a subsequent rate determining electrochemical adsorption step is the dominant mechanism. The exchange current density seems to decrease linearly with the logarithm of the amount of surface oxide. Chlorine reduction on ruthenium is best described by a Heyrovsky-Volmer mechanism with the first charge transfer reaction as the rate determining step. The Krishtalik mechanism incorporating adsorbed O•Cl+ intermediates is also able to describe the reaction successfully. The reaction order is constant for all oxide coverages while the exchange current density apparently moves through a maximum at intermediate oxide coverages (∼100 mC cm−2). The results show that the electrocatalysis of the cathodic reduction of chlorine is very sensitive to the state of the oxidation of the electrode surface. The rate and mechanism of the electroreduction of chlorine on electrooxidised ruthenium has further been investigated with focus on the effect of solution pH. Current/potential curves for the reduction process in solutions with constant chloride concentration of 1.0 mol dm−3 and varying H+ concentration have been obtained with the use of the rotating disk electrode technique (RDE). It was found that the chlorine reduction rate is highly inhibited in solutions with high H+ concentrations and that it can be satisfactorily described by the Erenburgh mechanism, previously suggested for the chlorine evolution on RuO2 and ruthenium titanium oxides (RTO). The expression of the kinetic current as a function of chlorine and H+ concentration was obtained by solving the elementary rate equations of the kinetic mechanism. The kinetic constants obtained from the correlation of the kinetic current expression to the experimental data were used to simulate the dependence of the surface coverages and elementary reaction rates on overpotential.

No description available.

No description available.

<p>The main objectives of the thesis work were: (1), to study the oxidation of methanol and formic acid on platinum electrodes by employing conventional and advanced electrochemical methods, and (2), to develop membrane electrode assemblies based on polybenzimidazole membranes that can be used in fuel cells up to 200 °C.</p><p>D.c. voltammetry and a.c. voltammetry studies of methanol and formic acid on polycrystalline platinum in sulphuric acid electrolyte were performed to determine the mechanism and kinetics of the oxidation reactions.</p><p>A combined potential step and fast cyclic voltammetry experiment was employed to investigate the time dependence primarily of methanol oxidation on platinum. Charge measurements clearly demonstrated the existence of a parallel path at low potentials and short times without formation of adsorbed CO. Furthermore, experimental results showed that only the serial path, via adsorbed CO, exists during continuous cycling, with the first step being diffusion controlled dissociative adsorption of methanol directly from the bulk electrolyte. The saturation charge of adsorbed CO derived from methanol was found to be significantly lower than CO derived from formic acid or dissolved CO. This was attributed to the site requirements of the dehydrogenation steps, and possibly different compositions of linear, bridged or multiply bonded CO. The coverage of adsorbed CO from formic acid decreased significantly at potentials just outside of the hydrogen region (0.35 V vs. RHE), while it did not start to decrease significantly until about 0.6 V vs. RHE for methanol. Adsorbed CO from dissolved CO rapidly oxidized at potentials above about 0.75 V due to formation of platinum oxide.</p><p>Data from a.c. voltammograms from 0.5 Hz up to 30 kHz were assembled into electrochemical impedance spectra (EIS) and analyzed using equivalent circuits. The main advantages of collecting EIS spectra from a.c. voltammetry experiments are the ability to directly correlate the impedance spectra with features in the corresponding d.c. voltammograms, and the ability to investigate conditions with partially covered surfaces that are inaccessible in steady-state measurements.</p><p>A variety of spectral types were observed, and for methanol these showed only a single adsorption relaxation aside from the double-layer/charge-transfer relaxation, though some structure in the phase of the latter relaxation hints at another process. The charge-transfer resistance showed Tafel behaviour for potentials in the rising part of the oxidation peak consistent with a one-electron process in the rate-determining step. The rate limiting step was proposed to be the electrochemical reaction between adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites. Only a single adsorption relaxation in methanol oxidation was observed, implying that only one single coverage is required to describe the state of the surface and the kinetics. It was assumed that this single coverage is that of OH, and all the surface not covered with OH is covered with CO so that the coverage of CO is not an independent variable. Inductive behaviour and negative relaxation times in the methanol oxidation were attributed to nucleation and growth behaviour. Linear voltammetry reversal and sweep-hold experiments also indicated nucleation-growth-collision behaviour in distinct potential regions, both in the forward and reverse potential scan for methanol oxidation on platinum.</p><p>In both methanol oxidation and formic acid oxidation, a negative differential resistance (NDR) was observed in the potential regions that possess a negative d.c. polarization slope, and was attributed to the formation of surface oxide which inhibited the oxidation of methanol or formic acid.</p><p>EIS spectra for formic acid clearly showed the presence of an additional low frequency relaxation at potentials where we expect adsorbed dissociated water or platinum oxide to be present, implying that more than one single coverage is required to describe the state of the surface and the kinetics. Two potential regions with hidden negative differential resistance (HNDR) behaviour were identified in the positive-going sweep, one prior to platinum oxide formation, assumed to involve adsorbed dissociated water, and one just negative of the main oxidation peak, assumed to involve platinum oxide. Oscillatory behaviour was found in the formic acid oxidation on platinum by adding a large external resistance to the working electrode circuit, which means that there is no longer true potentiostatic control at the interface. By revealing the system time constants, impedance measurements can be used to assist in explaining the origin of the oscillations. In the case of formic acid, these measurements showed that the oscillations do not arise from the chemical mechanism alone, but that the potential plays an essential role.</p><p>Preparation and optimization of gas-diffusion electrodes for high temperature polymer electrolyte fuel cells based on phosphoric acid doped polybenzimidazole (PBI) membranes was performed. This fuel cell allows for operating temperatures up to 200 °C with increased tolerance towards catalytic poisons, typical carbon monoxide. In this work we employed pure hydrogen and oxygen as the fuel cell feeds, and determined the optimum morphology of the support layer, and subsequently optimized the catalytic layer with respect to platinum content in the Pt/C catalyst and PBI loading. A smooth and compact support layer with small crevices and large islands was found to be beneficial with our spraying technique in respect to adhesion to the carbon backing and to the catalyst layer. We found that a high platinum content catalyst gave a significantly thinner catalyst layer (decreased porosity) on both anode and cathode with superior performance. The PBI loading was found to be crucial for the performance of the electrodes, and a relatively high loading gave the best performing electrodes.</p>

The main objectives of the thesis work were: (1), to study the oxidation of methanol and formic acid on platinum electrodes by employing conventional and advanced electrochemical methods, and (2), to develop membrane electrode assemblies based on polybenzimidazole membranes that can be used in fuel cells up to 200 °C. D.c. voltammetry and a.c. voltammetry studies of methanol and formic acid on polycrystalline platinum in sulphuric acid electrolyte were performed to determine the mechanism and kinetics of the oxidation reactions. A combined potential step and fast cyclic voltammetry experiment was employed to investigate the time dependence primarily of methanol oxidation on platinum. Charge measurements clearly demonstrated the existence of a parallel path at low potentials and short times without formation of adsorbed CO. Furthermore, experimental results showed that only the serial path, via adsorbed CO, exists during continuous cycling, with the first step being diffusion controlled dissociative adsorption of methanol directly from the bulk electrolyte. The saturation charge of adsorbed CO derived from methanol was found to be significantly lower than CO derived from formic acid or dissolved CO. This was attributed to the site requirements of the dehydrogenation steps, and possibly different compositions of linear, bridged or multiply bonded CO. The coverage of adsorbed CO from formic acid decreased significantly at potentials just outside of the hydrogen region (0.35 V vs. RHE), while it did not start to decrease significantly until about 0.6 V vs. RHE for methanol. Adsorbed CO from dissolved CO rapidly oxidized at potentials above about 0.75 V due to formation of platinum oxide. Data from a.c. voltammograms from 0.5 Hz up to 30 kHz were assembled into electrochemical impedance spectra (EIS) and analyzed using equivalent circuits. The main advantages of collecting EIS spectra from a.c. voltammetry experiments are the ability to directly correlate the impedance spectra with features in the corresponding d.c. voltammograms, and the ability to investigate conditions with partially covered surfaces that are inaccessible in steady-state measurements. A variety of spectral types were observed, and for methanol these showed only a single adsorption relaxation aside from the double-layer/charge-transfer relaxation, though some structure in the phase of the latter relaxation hints at another process. The charge-transfer resistance showed Tafel behaviour for potentials in the rising part of the oxidation peak consistent with a one-electron process in the rate-determining step. The rate limiting step was proposed to be the electrochemical reaction between adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites. Only a single adsorption relaxation in methanol oxidation was observed, implying that only one single coverage is required to describe the state of the surface and the kinetics. It was assumed that this single coverage is that of OH, and all the surface not covered with OH is covered with CO so that the coverage of CO is not an independent variable. Inductive behaviour and negative relaxation times in the methanol oxidation were attributed to nucleation and growth behaviour. Linear voltammetry reversal and sweep-hold experiments also indicated nucleation-growth-collision behaviour in distinct potential regions, both in the forward and reverse potential scan for methanol oxidation on platinum. In both methanol oxidation and formic acid oxidation, a negative differential resistance (NDR) was observed in the potential regions that possess a negative d.c. polarization slope, and was attributed to the formation of surface oxide which inhibited the oxidation of methanol or formic acid. EIS spectra for formic acid clearly showed the presence of an additional low frequency relaxation at potentials where we expect adsorbed dissociated water or platinum oxide to be present, implying that more than one single coverage is required to describe the state of the surface and the kinetics. Two potential regions with hidden negative differential resistance (HNDR) behaviour were identified in the positive-going sweep, one prior to platinum oxide formation, assumed to involve adsorbed dissociated water, and one just negative of the main oxidation peak, assumed to involve platinum oxide. Oscillatory behaviour was found in the formic acid oxidation on platinum by adding a large external resistance to the working electrode circuit, which means that there is no longer true potentiostatic control at the interface. By revealing the system time constants, impedance measurements can be used to assist in explaining the origin of the oscillations. In the case of formic acid, these measurements showed that the oscillations do not arise from the chemical mechanism alone, but that the potential plays an essential role. Preparation and optimization of gas-diffusion electrodes for high temperature polymer electrolyte fuel cells based on phosphoric acid doped polybenzimidazole (PBI) membranes was performed. This fuel cell allows for operating temperatures up to 200 °C with increased tolerance towards catalytic poisons, typical carbon monoxide. In this work we employed pure hydrogen and oxygen as the fuel cell feeds, and determined the optimum morphology of the support layer, and subsequently optimized the catalytic layer with respect to platinum content in the Pt/C catalyst and PBI loading. A smooth and compact support layer with small crevices and large islands was found to be beneficial with our spraying technique in respect to adhesion to the carbon backing and to the catalyst layer. We found that a high platinum content catalyst gave a significantly thinner catalyst layer (decreased porosity) on both anode and cathode with superior performance. The PBI loading was found to be crucial for the performance of the electrodes, and a relatively high loading gave the best performing electrodes.