The ever increasing demand for a clean and renewable energy source has stimulated research for alternatives for the use of fossil fuels, which contribute significantly to global warming. The SO2 oxidation reaction was studied for production of hydrogen as a clean and renewable energy carrier. This reaction occurs at a lower standard electrode potential (0.158 V vs. SHE) than normal water electrolysis (1.23 V vs. SHE). This is a theoretical indication that the SO2 oxidation reaction has possible potential when compared to normal water electrolysis, since hydrogen production may occur at lower potentials and therefore lower cost. Rh was compared with Pt for the SO2 oxidation reaction since little research has been done on this catalyst and many studies exist in which Pt was used as catalyst. The oxygen reduction reaction and ethanol oxidation reaction were also included in this study to create a foundation for the catalysts studied, since the SO2 oxidation reaction is complicated by different adsorbed species that can form according to various mechanisms. The electrochemical techniques employed in this study to characterize the catalysts included cyclic voltammetry from which onset potentials and limiting current densities were determined, as well as from which some qualitative analysis was done. Linear polarization experiments were used during rotating disk electrode studies from which Levich and Koutecky-Levich analyses were done and the number of electrons transferred calculated and compared between the two catalysts. From the Koutecky-Levich analysis the kinetic current density was also obtained for use in Tafel analysis for further comparison between catalysts. It was found that Rh showed good behaviour for the oxygen reduction reaction when compared to Pt with similar onset potentials and limiting current densities. From Levich analysis it was concluded that both catalysts achieved diffusion limitation at high overpotentials. However, from the calculated number of electrons transferred it was evident that a difference in mechanism existed between catalysts and that the mechanism for both changed in the potential range studied, which is confirmed by the Tafel slopes. For the ethanol oxidation reaction it was shown that Rh exhibited very low catalytic activity in comparison with Pt. However, it was concluded from cyclic voltammetry and rotating disk electrode studies that more adsorbed species were present on the surface of Rh than on Pt. These results confirmed the possibility of using Rh as a co-catalyst together with Pt since it was shown from rotating disk electrode studies that low adsorption of ethanol and its oxidation products caused species to be transported away from the surface of the electrode during rotation. For the SO2 oxidation reaction it was found that Rh exhibited very poor catalytic activity together with being very susceptible to poisoning by adsorbed species. Pt showed very good behaviour, which corresponded well with what had been observed in literature. Levich analysis revealed that Pt did not exhibit diffusion limitation and Koutecky-Levich analysis revealed that a 2 electron reaction occurred on Pt, which corresponds with the SO2 oxidation reaction during which 2 electrons are transferred. It was, therefore, shown that Rh could exhibit good behaviour and act as a suitable catalyst in certain circumstances. However, for the SO2 oxidation reaction, which was the main focus of this study it was shown that Rh is not a suitable catalyst, either alone or as co-catalyst. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

The ever increasing demand for a clean and renewable energy source has stimulated research for alternatives for the use of fossil fuels, which contribute significantly to global warming. The SO2 oxidation reaction was studied for production of hydrogen as a clean and renewable energy carrier. This reaction occurs at a lower standard electrode potential (0.158 V vs. SHE) than normal water electrolysis (1.23 V vs. SHE). This is a theoretical indication that the SO2 oxidation reaction has possible potential when compared to normal water electrolysis, since hydrogen production may occur at lower potentials and therefore lower cost. Rh was compared with Pt for the SO2 oxidation reaction since little research has been done on this catalyst and many studies exist in which Pt was used as catalyst. The oxygen reduction reaction and ethanol oxidation reaction were also included in this study to create a foundation for the catalysts studied, since the SO2 oxidation reaction is complicated by different adsorbed species that can form according to various mechanisms. The electrochemical techniques employed in this study to characterize the catalysts included cyclic voltammetry from which onset potentials and limiting current densities were determined, as well as from which some qualitative analysis was done. Linear polarization experiments were used during rotating disk electrode studies from which Levich and Koutecky-Levich analyses were done and the number of electrons transferred calculated and compared between the two catalysts. From the Koutecky-Levich analysis the kinetic current density was also obtained for use in Tafel analysis for further comparison between catalysts. It was found that Rh showed good behaviour for the oxygen reduction reaction when compared to Pt with similar onset potentials and limiting current densities. From Levich analysis it was concluded that both catalysts achieved diffusion limitation at high overpotentials. However, from the calculated number of electrons transferred it was evident that a difference in mechanism existed between catalysts and that the mechanism for both changed in the potential range studied, which is confirmed by the Tafel slopes. For the ethanol oxidation reaction it was shown that Rh exhibited very low catalytic activity in comparison with Pt. However, it was concluded from cyclic voltammetry and rotating disk electrode studies that more adsorbed species were present on the surface of Rh than on Pt. These results confirmed the possibility of using Rh as a co-catalyst together with Pt since it was shown from rotating disk electrode studies that low adsorption of ethanol and its oxidation products caused species to be transported away from the surface of the electrode during rotation. For the SO2 oxidation reaction it was found that Rh exhibited very poor catalytic activity together with being very susceptible to poisoning by adsorbed species. Pt showed very good behaviour, which corresponded well with what had been observed in literature. Levich analysis revealed that Pt did not exhibit diffusion limitation and Koutecky-Levich analysis revealed that a 2 electron reaction occurred on Pt, which corresponds with the SO2 oxidation reaction during which 2 electrons are transferred. It was, therefore, shown that Rh could exhibit good behaviour and act as a suitable catalyst in certain circumstances. However, for the SO2 oxidation reaction, which was the main focus of this study it was shown that Rh is not a suitable catalyst, either alone or as co-catalyst. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

The pressure on clean and sustainable energy supplies is increasing. In this regard energy conversion by electrochemical processes plays a major role, for both fuel cell reactions and electrolysis reactions. The sulphur dioxide oxidation reaction (SOR) is a common reaction found in the Hybrid Sulphur Cycle (HyS) and the HyS is a way to produce large-scale hydrogen (H2). The problem with the use of the HyS and fuel cells is the cost involved as large amounts of Pt are required for effective operation. The aim of the study was to determine whether there was an alternative catalyst which was more efficient and cost-effective than Pt. The oxygen reduction reaction (ORR), the ethanol oxidation reaction (EOR) and SOR were studied by means of different electrochemical techniques (cyclovoltammetry (CV), linear polarization (LP) and rotating disk electrode (RDE)) on polycrystalline platinum (Pt) and palladium (Pd). The SRR and EOR are common reactions occurring at the cathode and anode, respectively, in fuel cells and these reactions have been investigated extensively. The reason for studying the reactions was as a preparation for the SOR. This study compared polycrystalline Pt and Pd for the different reactions, with the main focus on the SOR as Pd is considerably cheaper than Pt, and for the SOR polycrystalline Pd has by no means been investigated intensively. Polycrystalline Pt and Pd were compared by different electrochemical techniques and analyses. The Koutecky-Levich and Levich analyses were used to (i) calculate the number of e- involved in the relevant reaction, (ii) to determine whether the reaction was mass transfer controlled at high overpotentials and (iii) whether the reaction mechanism changed with potential. Next the kinetic current density ( k) was calculated from Koutecky-Levich analyses, which was further used for Tafel slope analyses. If it was not possible to carry out the analyses, the activation energy (Ea) was used to determine the electrocatalytic activity of the catalyst. The electrocatalytic activity was also determined by comparing onset potentials (Es), peak potentials (Ep) and limited/maximum current density ( b/ p) of each catalyst. This study was only a preliminary study for the SOR and therefore, further studies are certainly required. It seemed Pd shows better electrocatalytic activity than Pt for the SRR in an alkaline electrolyte because of similar Es, but Pd produced a higher cathodic current density. Pt showed a lower Es than Pd for the SRR in an acid electrolyte, but Pd delivered a higher cathodic current density. This, therefore, means that the SRR in an acid electrolyte is kinetically more favourable on Pd than on Pt. For the EOR better electrocatalytic activity was obtained with Pd than with Pt in an alkaline electrolyte due to higher current densities at lower potentials and Pd showed lower Ea values than Pt in the potential range normally used for fuel cells. Pd was inactive for EOR in an acid electrolyte, while a reaction occurred on Pt. A possible reason for this observation may be due to the H2 absorbing strongly on Pd thus blocking the active positions on the electrode surfaces, preventing further reaction. Pd showed higher electrocatalytic activity for the SOR due to lower Es and higher current densities at low potentials. From the RDE studies it was established that the SRR in an alkaline electrolyte on polycrystalline Pt and Pd was mass transfer controlled at low potentials (high overpotentials), but the SRR in an acid electrolyte was only mass transfer controlled on Pt. The SOR was not mass transfer controlled on polycrystalline Pt and Pd at high potentials (high overpotentials). These assumptions were confirmed by Levich analysis. Using Koutecky-Levich analysis, it was determined that the reaction mechanism on polycrystalline Pt and Pd changed with potential for SRR in an alkaline electrolyte and the SOR. For the SRR in an acid electrolyte the reaction mechanism remained constant with changes in potential on polycrystalline Pd, but the reaction mechanism on polycrystalline Pt changed with potential. These assumptions were confirmed by the number of e-, calculated using Koutecky-Levich analyses. Levich and Koutecky-Levich analyses were not performed for EOR as an increase in rotation speed did not produce an increase in current density. Tafel slope analyses were conducted by making use of overpotentials and k, where possible. As in the case of ethanol, it was not possible to execute Koutecky-Levich analyses and, therefore, it was not possible to perform Tafel slope analyses using k. Tafel slope analyses for the EOR was therefore performed with normal current densities at 0 rotations per minute (rpm). The reaction mechanisms on Pt and Pd for the SRR in alkaline and acidic electrolytes differed due to different Tafel slopes. Pt and Pd displayed similar Tafel slopes for the EOR in alkaline electrolyte, thus suggesting that the reaction mechanisms on Pt and Pd were the same. For the SOR it seemed that the reaction mechanism on Pt and Pd were similar because of similar Tafel slopes. This was only a preliminary and comparative study for polycrystalline Pt and Pd, and the reaction mechanism was not further studied by means of spectroscopic techniques. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

The pressure on clean and sustainable energy supplies is increasing. In this regard energy conversion by electrochemical processes plays a major role, for both fuel cell reactions and electrolysis reactions. The sulphur dioxide oxidation reaction (SOR) is a common reaction found in the Hybrid Sulphur Cycle (HyS) and the HyS is a way to produce large-scale hydrogen (H2). The problem with the use of the HyS and fuel cells is the cost involved as large amounts of Pt are required for effective operation. The aim of the study was to determine whether there was an alternative catalyst which was more efficient and cost-effective than Pt. The oxygen reduction reaction (ORR), the ethanol oxidation reaction (EOR) and SOR were studied by means of different electrochemical techniques (cyclovoltammetry (CV), linear polarization (LP) and rotating disk electrode (RDE)) on polycrystalline platinum (Pt) and palladium (Pd). The SRR and EOR are common reactions occurring at the cathode and anode, respectively, in fuel cells and these reactions have been investigated extensively. The reason for studying the reactions was as a preparation for the SOR. This study compared polycrystalline Pt and Pd for the different reactions, with the main focus on the SOR as Pd is considerably cheaper than Pt, and for the SOR polycrystalline Pd has by no means been investigated intensively. Polycrystalline Pt and Pd were compared by different electrochemical techniques and analyses. The Koutecky-Levich and Levich analyses were used to (i) calculate the number of e- involved in the relevant reaction, (ii) to determine whether the reaction was mass transfer controlled at high overpotentials and (iii) whether the reaction mechanism changed with potential. Next the kinetic current density ( k) was calculated from Koutecky-Levich analyses, which was further used for Tafel slope analyses. If it was not possible to carry out the analyses, the activation energy (Ea) was used to determine the electrocatalytic activity of the catalyst. The electrocatalytic activity was also determined by comparing onset potentials (Es), peak potentials (Ep) and limited/maximum current density ( b/ p) of each catalyst. This study was only a preliminary study for the SOR and therefore, further studies are certainly required. It seemed Pd shows better electrocatalytic activity than Pt for the SRR in an alkaline electrolyte because of similar Es, but Pd produced a higher cathodic current density. Pt showed a lower Es than Pd for the SRR in an acid electrolyte, but Pd delivered a higher cathodic current density. This, therefore, means that the SRR in an acid electrolyte is kinetically more favourable on Pd than on Pt. For the EOR better electrocatalytic activity was obtained with Pd than with Pt in an alkaline electrolyte due to higher current densities at lower potentials and Pd showed lower Ea values than Pt in the potential range normally used for fuel cells. Pd was inactive for EOR in an acid electrolyte, while a reaction occurred on Pt. A possible reason for this observation may be due to the H2 absorbing strongly on Pd thus blocking the active positions on the electrode surfaces, preventing further reaction. Pd showed higher electrocatalytic activity for the SOR due to lower Es and higher current densities at low potentials. From the RDE studies it was established that the SRR in an alkaline electrolyte on polycrystalline Pt and Pd was mass transfer controlled at low potentials (high overpotentials), but the SRR in an acid electrolyte was only mass transfer controlled on Pt. The SOR was not mass transfer controlled on polycrystalline Pt and Pd at high potentials (high overpotentials). These assumptions were confirmed by Levich analysis. Using Koutecky-Levich analysis, it was determined that the reaction mechanism on polycrystalline Pt and Pd changed with potential for SRR in an alkaline electrolyte and the SOR. For the SRR in an acid electrolyte the reaction mechanism remained constant with changes in potential on polycrystalline Pd, but the reaction mechanism on polycrystalline Pt changed with potential. These assumptions were confirmed by the number of e-, calculated using Koutecky-Levich analyses. Levich and Koutecky-Levich analyses were not performed for EOR as an increase in rotation speed did not produce an increase in current density. Tafel slope analyses were conducted by making use of overpotentials and k, where possible. As in the case of ethanol, it was not possible to execute Koutecky-Levich analyses and, therefore, it was not possible to perform Tafel slope analyses using k. Tafel slope analyses for the EOR was therefore performed with normal current densities at 0 rotations per minute (rpm). The reaction mechanisms on Pt and Pd for the SRR in alkaline and acidic electrolytes differed due to different Tafel slopes. Pt and Pd displayed similar Tafel slopes for the EOR in alkaline electrolyte, thus suggesting that the reaction mechanisms on Pt and Pd were the same. For the SOR it seemed that the reaction mechanism on Pt and Pd were similar because of similar Tafel slopes. This was only a preliminary and comparative study for polycrystalline Pt and Pd, and the reaction mechanism was not further studied by means of spectroscopic techniques. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014