Eletrocatalisadores Pt/C, Pd/C, PtAu/C, PtIr/C, PdAu/C, PdIr/C, PtAuIr/C e PdAuIr/C foram preparados via redução por borohidreto de sódio em diferentes proporções atômicas, com 20% em massa de metal e suportados em carbono Vulcan XC72 de alta área superficial. Os materiais foram caracterizados pelas técnicas de espectroscopia de energia dispersiva de raios-X (EDX), análise de difração de raios-X (DRX) e microscopia eletrônica de transmissão (MET). A oxidação eletroquímica do etanol foi estudada por voltametria cíclica (VC) e cronoamperometria, utilizando a técnica do eletrodo de camada fina porosa e o estudo da oxidação eletroquímica de etanol \"in situ\" utilizando espectroscopia de infravermelho com transformada de Fourier (FTIR). Posteriormente os materiais foram testados em células à combustível alcalinas alimentadas diretamente com etanol. Os resultados demonstraram que houve formação de ligas, com tamanho médio de nanopartículas entre 4,0 - 10 nm. De acordo com os experimentos eletroquímicos os eletrocatalisadores ternários apresentaram maior atividade eletrocatalítica e os estudos em FTIR indicaram que o produto principal da oxidação eletroquímica de etanol em meio alcalino para todos eletrocatalisadores sintetizados foi o acetato, sugerindo que a oxidação ocorre de forma incompleta pelo mecanismo indireto. Os testes em célula mostraram os melhores resultados para PdAuIr/C (50:40:10) com o qual obteve-se potencial de circuito aberto de aproximadamente 0,78 V e densidade de potência máxima de aproximadamente 15 mW cm-2, cerca de 333% superior a Pd/C. / Pt/C, Pd/C, PtAu/C, PtIr/C, PdAu/C, PdIr/C, PtAuIr/C and PdAuIr/C electrocatalysts, were prepared by the sodium borohydride reduction process in different atomic proportions, with 20 wt.% of metal loading and supported on Vulcan XC72 carbon with high surface area. The materials were characterized by X-ray dispersive energy spectroscopy (EDX), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The ethanol electrochemical oxidation was studied by cyclic voltammetry (CV), chronoamperometry and in situ using Fourier Transform Infrared Spectroscopy (FTIR) using the thin porous coating technique, Afterwards the materials were tested on direct ethanol alkaline fuel cells. The results suggest alloys formation, with the mean nanoparticles sizes are from 4 - 10 nm. According to the electrochemical experiments the ternary electrocatalysts presented higher electrocatalytic activity, while that the FTIR studies indicated that the main product of the electrochemical oxidation of ethanol in alkaline medium for all synthesized electrocatalysts was acetate, also suggesting that the oxidation occurs incompletely by the indirect mechanism. The fuel cell experiments showed the best results for PdAuIr/C (50:40:10), where these materials had an open circuit potential of approximately 0,78 V and maximum power density of about 15 mW cm-2, about 333% higher than Pd/C.

Eletrocatalisadores Pt/C, Pd/C, PtAu/C, PtIr/C, PdAu/C, PdIr/C, PtAuIr/C e PdAuIr/C foram preparados via redução por borohidreto de sódio em diferentes proporções atômicas, com 20% em massa de metal e suportados em carbono Vulcan XC72 de alta área superficial. Os materiais foram caracterizados pelas técnicas de espectroscopia de energia dispersiva de raios-X (EDX), análise de difração de raios-X (DRX) e microscopia eletrônica de transmissão (MET). A oxidação eletroquímica do etanol foi estudada por voltametria cíclica (VC) e cronoamperometria, utilizando a técnica do eletrodo de camada fina porosa e o estudo da oxidação eletroquímica de etanol \"in situ\" utilizando espectroscopia de infravermelho com transformada de Fourier (FTIR). Posteriormente os materiais foram testados em células à combustível alcalinas alimentadas diretamente com etanol. Os resultados demonstraram que houve formação de ligas, com tamanho médio de nanopartículas entre 4,0 - 10 nm. De acordo com os experimentos eletroquímicos os eletrocatalisadores ternários apresentaram maior atividade eletrocatalítica e os estudos em FTIR indicaram que o produto principal da oxidação eletroquímica de etanol em meio alcalino para todos eletrocatalisadores sintetizados foi o acetato, sugerindo que a oxidação ocorre de forma incompleta pelo mecanismo indireto. Os testes em célula mostraram os melhores resultados para PdAuIr/C (50:40:10) com o qual obteve-se potencial de circuito aberto de aproximadamente 0,78 V e densidade de potência máxima de aproximadamente 15 mW cm-2, cerca de 333% superior a Pd/C. / Pt/C, Pd/C, PtAu/C, PtIr/C, PdAu/C, PdIr/C, PtAuIr/C and PdAuIr/C electrocatalysts, were prepared by the sodium borohydride reduction process in different atomic proportions, with 20 wt.% of metal loading and supported on Vulcan XC72 carbon with high surface area. The materials were characterized by X-ray dispersive energy spectroscopy (EDX), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The ethanol electrochemical oxidation was studied by cyclic voltammetry (CV), chronoamperometry and in situ using Fourier Transform Infrared Spectroscopy (FTIR) using the thin porous coating technique, Afterwards the materials were tested on direct ethanol alkaline fuel cells. The results suggest alloys formation, with the mean nanoparticles sizes are from 4 - 10 nm. According to the electrochemical experiments the ternary electrocatalysts presented higher electrocatalytic activity, while that the FTIR studies indicated that the main product of the electrochemical oxidation of ethanol in alkaline medium for all synthesized electrocatalysts was acetate, also suggesting that the oxidation occurs incompletely by the indirect mechanism. The fuel cell experiments showed the best results for PdAuIr/C (50:40:10), where these materials had an open circuit potential of approximately 0,78 V and maximum power density of about 15 mW cm-2, about 333% higher than Pd/C.

The lack of catalytic efficiency towards the complete ethanol oxidation reaction (EOR) has hindered the development of direct ethanol fuel cells (DEFCs). Ir-based catalysts have recently been shown promise in the complete EOR. However, the reaction mechanism of the complete EOR remains unclear, which impedes the development of better Ir-based catalysts. Herein, we performed extensive density functional theory (DFT) calculations to develop a comprehensive reaction network of EOR on Ir(100). The EOR process consists of four dehydrogenation steps of ethanol leading to the generation of CH2CO species followed by two competitive reaction pathways, i.e., a C-O bond cleavage to poisoning species (e.g., CHC) and the surface diffusion of CH2CO leading to CO2. Furthermore, our studies show that for all CHxCOy (x = 1, 2, or 3 and y = 0 or 1) species, only when the C and O atoms (or two C atoms) bind to two different surface Ir atoms can the C-C/C-O bond cleavage occur. This work highlights the essential roles of adsorption structure and diffusion of CH2CO for the complete EOR and serves as a benchmark for the future investigation of the electronic and solvent effects.Pt-Ir-based alloy electrocatalysts have shown encouraging catalytic performance on the EOR in direct ethanol fuel cells. Nevertheless, designing a suitably qualified EOR electrocatalyst remains challenging because of several convoluted factors (e.g., C1 species poisoning, acetate acid formation, and C-C bond splitting). To understand the relationship between the EOR performance and the type of catalysts, we model three kinds of (100)-exposed Pt-Ir layered catalysts and perform density functional theory (DFT) calculations to explore 58 elementary reactions of the EOR over three catalyst surfaces. According to the calculated activation energies and reaction energies, we mapped out the reaction mechanisms for EOR on different catalysts, indicating corresponding rate-limiting steps (RLSs) of the complete EOR. We demonstrated that the C-O coupling/decoupling ability of the catalyst surface plays a leading role in the overall EOR performance because a perfect complete EOR not only has to avoid some C-O coupling reactions (e.g., CH¬3CO+OH→CH3COOH) but also needs to promote some C-O coupling reactions (e.g., CO+O→CO2). We further illustrated that Pt and Ir exhibit excellent C-O coupling and decoupling abilities, respectively, implying that modifying the compositions and structures of Pt-Ir catalysts is a promising way to achieve the complete EOR. Furthermore, the Ir@Pt(100) surface (Ir monolayer over Pt(100) surface) with a Pt-doped active site possesses the most significant potential on EOR, which could impede the acetate acid formation and facilitate the CO2 formation simultaneously. This work highlights the role of tuning the C-O coupling/decoupling ability of electrocatalyst in EOR activity, providing a new strategy for designing and selecting the EOR electrocatalyst. The solvent effect has always been a non-negligible factor for aqueous reactions. In computational chemistry, researchers have been looking for a compromise between computational efficiency and the rationality of solvent models to mimic the solvent environment. In this work, I investigated the ethanol dehydrogenation and C-C bond cleavages of EOR over Ir(100)using both implicit and explicit solvation models. The implicit model exhibited little impact on the adsorbates without the hydroxyl group, whereas the explicit model can reasonably describe the system’s hydrogen bonding and van der Waals interaction. This solvent effect study showed how different solvent models affected the elementary reactions geometrically and energetically.

In this study the focus was on the electrochemical techniques and aspects behind the establishment of the better catalyst (platinum or gold) for the sulphur dioxide oxidation reaction (SDOR). One of the primary issues regarding the SDOR is the catalyst material, thus the comparative investigation of the performance of platinum and gold in the SDOR, as found in this study. Ultimately, the SDOR could lead to an effective way of producing hydrogen gas, which is an excellent energy carrier. The electrochemical application of the oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR) is an integral part of the catalytic process of water electrolysis, and by using fuel cell technology, it becomes even more relevant to this study and can therefore be used as a control, guide and introduction to the techniques required for electrochemical investigation of catalyst effectiveness. Subsequently, the EOR as well as the ORR was used as introduction into the different electrochemical quantification and qualification techniques used in the electrochemical analyses of the SDOR. Considering the ORR, gold showed no viable activity in acidic medium, contrarily in alkaline medium, it showed good competition to platinum. Gold also lacked activity towards the EOR in acidic medium compared to platinum, with platinum the best catalyst in both acidic and alkaline media. Ultimately, platinum was established to be the material with better activity for the ORR with gold a good competitor in alkaline medium, and platinum the better catalyst for the EOR in both acidic and alkaline media. With the main focus of this study being the SDOR, gold proved to be the best catalyst in salt and gaseous forms of SO2 administration compared to platinum when the onset potential, maximum current density, Tafel slope and number of electrons transferred are taken into consideration. The onset potential was determined as 0.52 V vs. NHE for both platinum and gold using SO2 gas and 0.54 V and 0.5 V for gold and platinum respectively, using Na2SO3 salt. The maximum current density using gaseous SO2 for platinum at 0 RPM was 400 mA/cm2 with a Tafel slope of 891 mV/decade whereas gold had a maximum current density of 300 mA/cm2 and a Tafel slope of 378 mV/decade. Using Na2SO3 salt, the maximum current density of gold was 25 mA/cm2 with a Tafel slope of 59 mV/decade whereas platinum only achieved 18 mA/cm2 with a Tafel slope of 172 mV/decade. Concerning the number of electrons transferred, gold achieves a transfer of 2 while platinum only 1 for both SO2 gas and Na2SO3 salt. Taking all these summarised determinations into account, gold was established to be a very competitive catalyst material for the SDOR, compared to platinum. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2015

In this study the focus was on the electrochemical techniques and aspects behind the establishment of the better catalyst (platinum or gold) for the sulphur dioxide oxidation reaction (SDOR). One of the primary issues regarding the SDOR is the catalyst material, thus the comparative investigation of the performance of platinum and gold in the SDOR, as found in this study. Ultimately, the SDOR could lead to an effective way of producing hydrogen gas, which is an excellent energy carrier. The electrochemical application of the oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR) is an integral part of the catalytic process of water electrolysis, and by using fuel cell technology, it becomes even more relevant to this study and can therefore be used as a control, guide and introduction to the techniques required for electrochemical investigation of catalyst effectiveness. Subsequently, the EOR as well as the ORR was used as introduction into the different electrochemical quantification and qualification techniques used in the electrochemical analyses of the SDOR. Considering the ORR, gold showed no viable activity in acidic medium, contrarily in alkaline medium, it showed good competition to platinum. Gold also lacked activity towards the EOR in acidic medium compared to platinum, with platinum the best catalyst in both acidic and alkaline media. Ultimately, platinum was established to be the material with better activity for the ORR with gold a good competitor in alkaline medium, and platinum the better catalyst for the EOR in both acidic and alkaline media. With the main focus of this study being the SDOR, gold proved to be the best catalyst in salt and gaseous forms of SO2 administration compared to platinum when the onset potential, maximum current density, Tafel slope and number of electrons transferred are taken into consideration. The onset potential was determined as 0.52 V vs. NHE for both platinum and gold using SO2 gas and 0.54 V and 0.5 V for gold and platinum respectively, using Na2SO3 salt. The maximum current density using gaseous SO2 for platinum at 0 RPM was 400 mA/cm2 with a Tafel slope of 891 mV/decade whereas gold had a maximum current density of 300 mA/cm2 and a Tafel slope of 378 mV/decade. Using Na2SO3 salt, the maximum current density of gold was 25 mA/cm2 with a Tafel slope of 59 mV/decade whereas platinum only achieved 18 mA/cm2 with a Tafel slope of 172 mV/decade. Concerning the number of electrons transferred, gold achieves a transfer of 2 while platinum only 1 for both SO2 gas and Na2SO3 salt. Taking all these summarised determinations into account, gold was established to be a very competitive catalyst material for the SDOR, compared to platinum. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2015

L'étude du mécanisme de la réaction d'oxydation de l'éthanol (EOR) a été réalisée sur des électrocatalyseurs bi- et tri-métalliques à base de Pt, Rh et SnO2 sur support carboné à l'aide de méthodes électrochimiques couplées (DEMS, in situ FTIR). Deux importantes problématiques de l'EOR ont été abordées: la déshydrogénation de la molécule d'éthanol et la cassure de sa liaison C-C.L'investigation de certains paramètres expérimentaux, comme l'épaisseur de la couche d'électrocatalyseur, a permis de démontrer q'une couche active épaisse conduit à une meilleure électrooxydation plus complète de l'éthanol en CO2, mais également que l'empoisonnement de l'électrocatalyseur par de très forts adsorbats advient dans l'épaisseur de couche active.Les performances de chaque électrocatalyseur ont été comparées entre elles et ont mis en évidence une meilleure sélectivité de l'EOR sur Pt-Rh-SnO2/C, ainsi que l'engendrement de courants plus élevés à bas potentiel à température ambiante. La tendance est amplifiée à température plus élevée (T = 60 °C). / The study of the ethanol oxidation reaction (EOR) mechanism was performed on carbon supported bi- and tri-metallic Pt-, Rh-, SnO2-based electrocatalysts via electrochemical coupled techniques (DEMS, in situ FTIR). Two of the most important issues related to the EOR have been broached: the dehydrogenation of the ethanol molecule and its C-C bond breaking.The investigation of some experimental parameters, such as the thickness of the electrocatalyst layer, enabled demonstrating the better complete ethanol electrooxidation into CO2 for large electrocatalysts layers, combined to the enhanced poisoning effect inside the catalyst layer by very strong adsorbates.The performances of each electrocatalyst were compared and evidenced an improved selectivity of the EOR on Pt-Rh-SnO2/C, as well as the generation of higher currents at low potential at room temperature. The tendency was amplified at elevated temperatures (T = 60 °C).

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