Proton exchange membrane fuel cells are attractive power sources because they are highly efficient and do not pollute the environment. However, the use of Pt-based catalysts in present fuel cell technologies is not optimal: Pt is rare and expensive, and even the best commercial Pt cathodes have high overpotentials due to slow oxygen reduction kinetics. As a result, much effort has gone toward developing cheaper, more effective catalysts. Nanoparticles are attractive because they have different catalytic properties than analogous bulk systems, require less material, and have tunable reactivities based on their composition and size. It is important to perform detailed studies of nanoparticle catalysts since composition and size effects are poorly understood. Computational simulations of such materials can provide useful insights and potentially aid in the design of new catalysts. Here, I examine composition and size effects in nanoparticle catalysts using computational methods. Two bimetallic systems are investigated to explore composition effects: Pd-shell particles with several different core metals, and Pd/Cu random alloy particles. Depending on how the two metals are mixed (core-shell or random alloy), charge transfer and strain due to alloying are found to have different contributions to the catalytic activity. Size effects are studied for pure Pt particles, where corner and edge sites are found to play an important role. The binding geometries of molecular oxygen to corner and edge sites lead to peroxide formation instead of water on small Pt particles. Results form these calculations can provide useful information for designing novel catalysts in the future. By changing the composition and/or size of nanoparticles in the proper way, the interaction between the adsorbate and catalyst can be optimized, and better catalysts can be obtained. / text

Polymer electrolyte membrane fuel cells (PEFCs) have long been thought of as a promising clean alternative energy electrochemical device. They are lightweight, highly efficient, modular and scalable devices. A fuel such as H2 or methanol that can be readily produced from a variety of sources can be utilized in PEFCs to generate electricity with low or no emissions. Despite these advantages, fuel cell technologies have failed to reach mass commercialization mainly due to short operational lifetimes and the high cost of materials. In particular, the polymer membrane and the catalyst layer have been problematic in reducing the material cost. Currently, platinum is the dominant material used to catalyze fuel cell reactions. However platinum is very expensive and scarce. In order to pursue the mass commercialization of fuel cells, two methods have been proposed: 1) increasing the utilization of platinum to lower the loading required, and 2) replacing platinum completely with a non-precious material. The latter has been suggested to be the long term solution due to the increasing cost of platinum. This thesis explores the elimination of platinum through the use of nanostructured non-precious metal catalysts for polymer electrolyte fuel cells. Several catalysts have been synthesized without the use of platinum that are active for the oxygen reduction reaction (ORR) which occurs at the cathode. Three different synthetic techniques were utilized using different nitrogen precursors. Aside from the different nitrogen precursors, each set of experiments utilize a different approach to optimize the oxygen reduction performance. Different characterization techniques are used to learn more about the ORR on non-precious metal fuel cells. The first experiment utilizes ethylenediamine, a well-known nitrogen precursor for non-precious metal fuel cell catalysts. Ethylenediamine is deposited onto two different porous carbon black substrates to determine the effectiveness of different porosities in creating active sites for the ORR. Of the two carbon black species, Ketjenblack EC-600JD and Ketjenblack ED-300J, the former was found to be more porous and effective. This result was mainly attributed to the increased surface area of the carbon black which allowed for better dispersion and a greater active site density. In this experiment, the coating of ethylenediamine on carbon black was also refluxed for 3 hours prior to the pyrolysis. It was found that refluxed catalyst samples showed much improved performance than catalyst samples without this procedural modification. The next experiment utilized cyanamide as a nitrogen precursor. Cyanamide was chosen due to its ability to form larger amounts of pyridinic nitrogen on the surface of the catalyst after a high temperature pyrolysis stage. The catalysts were heat-treated at 1000oC and the performance was measured. NH3 was introduced during the pyrolysis, which could remove the excess coating from the carbon surface, and increase the surface area of the catalyst by unblocking the carbon pores. A third modification to the procedure was carried out, where the heat-treated sample was ball-milled, re-coated, and heat-treated again in ammonia conditions to increase the nitrogen functionalities and increase the active site density. The performance was slightly increased from the original heat-treated sample. However due to the decreased surface area, the limiting current density also decreased. It was believed that ball-milling the sample crushed the pores within the catalyst sample, thereby lowering the active surface area and thus the current density. Therefore, the last sample was prepared similarly to the procedure for the third sample, but without ball-milling. This sample had restored surface area and improved ORR performance over all the synthesized catalyst samples – these experiments allowed for important realizations regarding the nature of the Fe-cyanamide-KJ600 catalysts and allowed for a drastic improvement in onset and half-wave potentials from the first catalyst. The final experiment discussed in this thesis describes the work done with 1,2,4,5-tetracyanobenzene and tetracyanoethylene as phthalcyanine precursors for non-precious metal catalysts (NPMCs). Iron(II) acetate was mixed with these phthalocyanine precursors to form polymer sheets of iron phthalocyanine or its monomeric units. By the creation of these polymer sheets of iron phthalocyanine, it allowed for a uniform distribution of iron centres on the surface of the carbon after a heat-treatment step. This allowed for high active site density through the design of these sheet polymers and prevented agglomeration or blockage of active sites which is thought to be a common problem in the synthesis of many NPMCs. Both tetracyanobenzene and tetracyanoethylene as precursors were tested. The tetracyanobenzene catalyst was heat-treated at different temperature ranging from 700-1100oC and characterized through electrochemical tests for the ORR. As an overall conclusion to this work, several catalyst samples were made and different approaches were successfully employed to improve the ORR performance. Of the synthesis treatments utilized to improve performance, each specific catalyst had different parameters to tweak in order to improve ORR performance. With X-ray photoelectron spectroscopy (XPS) analysis, conclusions were also specific to the catalysts structure and synthesis procedure, however quaternary and pyrrolic nitrogen groups seemed to play an influential role to the ORR final performance. Although relative amount of pyridinic nitrogen was not seen to increase with increasing catalyst performance during the studies; it may still play an essential role in the reduction of oxygen on the catalyst surface. The author of this work has not ruled out that possibility. Several recommendations for future work were suggested to broaden the knowledge and understanding of nanostructure non-precious metal catalysts to design a high performing, durable, and low-cost alternative to platinum based catalysts.

Proton exchange membrane fuel cells (PEMFC) are attractive power sources as they offer high conversion efficiencies with low or no pollution. However, several challenges, especially the sluggish oxygen reduction reaction (ORR) and the high cost of Pt catalysts, impede their commercialization. With an aim to search for more active, less expensive, and more stable ORR catalysts than Pt, this dissertation focuses on the development of non-platinum or low-platinum Pd-based nanostructured electrocatalysts and a fundamental understanding of their structure-property-performance relationships. Carbon-supported Pd–Ni nanoalloy electrocatalysts with different Pd/Ni atomic ratios have been synthesized by a modified polyol reduction method, followed by heat treatment in a reducing atmosphere at 500–900 oC. The Pd–Ni sample with a Pd:Ni atomic ratio of 4:1 after heat treatment at 500 °C exhibits the highest electrochemical surface area and catalytic activity. The enhanced activity of Pd80Ni20 compared to that of Pd is attributed to Pd enrichment on the surface and the consequent lattice-strain effects. To improve the catalytic activity and long-term durability of the Pd–Ni catalysts, Pd–Pt–Ni nanoalloys have been synthesized by the same method and evaluated in PEMFC. The Pt-based mass activity of the Pd–Pt–Ni catalysts exceeds that of commercial Pt by a factor of 2, and its long-term durability is comparable to commercial Pt within the testing duration of 180 h. Both the favorable and detrimental effects of Pd and Ni dissolution on the performance of the membrane-electrode assembly (MEA) have been investigated by compositional analysis by transmission electron microscopy (TEM) of the MEAs before and after the fuel cell test. The MEAs of the Pd–Pt–Ni catalyst have then been characterized in-situ by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) to better understand the performance changes during cell operation. The surface state change from Pd-enrichment to Pt-enrichment and the consequent decrease in the charge transfer resistance during cell operation is believed to contribute to the activity enhancement. To further improve the MEA performance and durability, the as-synthesized Pd–Pt–Ni catalysts have been pre-leached in acid and Pd–Pt alloy catalysts have been synthesized to alleviate contamination from dissolved metal ions. Compared to the pristine Pd–Pt–Ni catalyst, the preleached catalyst shows improved performance and the Pd–Pt catalyst exhibits similar performance in the entire current density range. Finally, the catalytic activities for ORR obtained from the rotating disk electrode (RDE) and PEMFC single-cell measurements of all the catalysts are compared. The improvement in the activities of the Pd-Pt-based catalysts compared to that of Pt measured by the RDE experiments is much lower than that obtained in single cell test. In other words, RDE tests underestimate the value of the Pd-Pt-based electrocatalysts for real fuel cell applications. Also, based on the RDE data, the Pd–Pt–Cu catalyst exhibits the highest catalytic activity among all the Pd–Pt–M (M = Fe, Ni, Cu) catalysts studied. / text

Polymer electrolyte membrane fuel cells (PEFCs) have long been thought of as a promising clean alternative energy electrochemical device. They are lightweight, highly efficient, modular and scalable devices. A fuel such as H2 or methanol that can be readily produced from a variety of sources can be utilized in PEFCs to generate electricity with low or no emissions. Despite these advantages, fuel cell technologies have failed to reach mass commercialization mainly due to short operational lifetimes and the high cost of materials. In particular, the polymer membrane and the catalyst layer have been problematic in reducing the material cost. Currently, platinum is the dominant material used to catalyze fuel cell reactions. However platinum is very expensive and scarce. In order to pursue the mass commercialization of fuel cells, two methods have been proposed: 1) increasing the utilization of platinum to lower the loading required, and 2) replacing platinum completely with a non-precious material. The latter has been suggested to be the long term solution due to the increasing cost of platinum. This thesis explores the elimination of platinum through the use of nanostructured non-precious metal catalysts for polymer electrolyte fuel cells. Several catalysts have been synthesized without the use of platinum that are active for the oxygen reduction reaction (ORR) which occurs at the cathode. Three different synthetic techniques were utilized using different nitrogen precursors. Aside from the different nitrogen precursors, each set of experiments utilize a different approach to optimize the oxygen reduction performance. Different characterization techniques are used to learn more about the ORR on non-precious metal fuel cells. The first experiment utilizes ethylenediamine, a well-known nitrogen precursor for non-precious metal fuel cell catalysts. Ethylenediamine is deposited onto two different porous carbon black substrates to determine the effectiveness of different porosities in creating active sites for the ORR. Of the two carbon black species, Ketjenblack EC-600JD and Ketjenblack ED-300J, the former was found to be more porous and effective. This result was mainly attributed to the increased surface area of the carbon black which allowed for better dispersion and a greater active site density. In this experiment, the coating of ethylenediamine on carbon black was also refluxed for 3 hours prior to the pyrolysis. It was found that refluxed catalyst samples showed much improved performance than catalyst samples without this procedural modification. The next experiment utilized cyanamide as a nitrogen precursor. Cyanamide was chosen due to its ability to form larger amounts of pyridinic nitrogen on the surface of the catalyst after a high temperature pyrolysis stage. The catalysts were heat-treated at 1000oC and the performance was measured. NH3 was introduced during the pyrolysis, which could remove the excess coating from the carbon surface, and increase the surface area of the catalyst by unblocking the carbon pores. A third modification to the procedure was carried out, where the heat-treated sample was ball-milled, re-coated, and heat-treated again in ammonia conditions to increase the nitrogen functionalities and increase the active site density. The performance was slightly increased from the original heat-treated sample. However due to the decreased surface area, the limiting current density also decreased. It was believed that ball-milling the sample crushed the pores within the catalyst sample, thereby lowering the active surface area and thus the current density. Therefore, the last sample was prepared similarly to the procedure for the third sample, but without ball-milling. This sample had restored surface area and improved ORR performance over all the synthesized catalyst samples – these experiments allowed for important realizations regarding the nature of the Fe-cyanamide-KJ600 catalysts and allowed for a drastic improvement in onset and half-wave potentials from the first catalyst. The final experiment discussed in this thesis describes the work done with 1,2,4,5-tetracyanobenzene and tetracyanoethylene as phthalcyanine precursors for non-precious metal catalysts (NPMCs). Iron(II) acetate was mixed with these phthalocyanine precursors to form polymer sheets of iron phthalocyanine or its monomeric units. By the creation of these polymer sheets of iron phthalocyanine, it allowed for a uniform distribution of iron centres on the surface of the carbon after a heat-treatment step. This allowed for high active site density through the design of these sheet polymers and prevented agglomeration or blockage of active sites which is thought to be a common problem in the synthesis of many NPMCs. Both tetracyanobenzene and tetracyanoethylene as precursors were tested. The tetracyanobenzene catalyst was heat-treated at different temperature ranging from 700-1100oC and characterized through electrochemical tests for the ORR. As an overall conclusion to this work, several catalyst samples were made and different approaches were successfully employed to improve the ORR performance. Of the synthesis treatments utilized to improve performance, each specific catalyst had different parameters to tweak in order to improve ORR performance. With X-ray photoelectron spectroscopy (XPS) analysis, conclusions were also specific to the catalysts structure and synthesis procedure, however quaternary and pyrrolic nitrogen groups seemed to play an influential role to the ORR final performance. Although relative amount of pyridinic nitrogen was not seen to increase with increasing catalyst performance during the studies; it may still play an essential role in the reduction of oxygen on the catalyst surface. The author of this work has not ruled out that possibility. Several recommendations for future work were suggested to broaden the knowledge and understanding of nanostructure non-precious metal catalysts to design a high performing, durable, and low-cost alternative to platinum based catalysts.

>Magister Scientiae - MSc / Oxygen reduction reaction (ORR) is carried out in the cathode of the proton exchange membrane fuel cell (PEMFC) and it is known for its sluggish kinetics and the existence of two-pathway mechanism, related with the production of water and hydrogen peroxide. Nowadays, the design of novel cathode catalysts that are able to generate both high oxygen reduction currents and water as main product is a challenge since it causes an enhancement in the performance of PEMFC. Generally, these catalysts are composed of platinum nanoparticles, bearing in mind its high activity towards the ORR. However, the use of platinum means an increase in the total cost of PEMFCs due to its scarcity and high cost. This topic has been the motivation for a wide research in the field of PEMFCs during the last several years, being the main goal to design efficient and low cost catalysts for the cathode of PEMFCs. In this Master thesis project, platinum-palladium (Pt-Pd) catalysts supported on carbon black (CB), carbon nanofibers (CNF) and carbon xerogels (CX) were synthesised using methanol (MeOH), formaldehyde (FMY), n-propanol (nPrOH), ethanol (EtOH) and ascorbic acid (AA). The as-prepared materials were physically characterised by energy dispersive X-ray (EDS), X-ray diffraction (XRD) and transmission electronic microscopy (TEM), in order to determine its composition and morphological characteristics. The catalytic activity towards ORR was assessed by means of electrochemical techniques as rotating disc electrode (RDE) and cyclic voltammetry (CV).

This research exploits mild sonication in a thin layer electrochemical cell to enhance rates of reaction in systems under voltammetric perturbation. Sound waves propagate through a thin layer of condensed fluid to provide energy to the electrode solution interface in the form of pressure and temperature. The sonic energy provided in three dimensions can be exploited to enhance rates of heterogeneous electron transfer as the energy is harnessed at the two dimensional electrode interface. Enhanced rates of heterogeneous electron transfer are of interest both for fundamental reasons and for exploitation in electrochemical energy systems. The initial pilot studies were directed at demonstrating the impact of acoustic energy on heterogenous electron transfer. Redox couples with different electron transfer rates were evaluated. Whereas compounds with reversible electron transfer kinetics demonstrated little improvement, redox couples such as ferric ion (Fe3+) with slow electron transfer kinetics exhibited an increase in the standard heterogeneous electron transfer rate constant, k0 with an increase in acoustic energy. The reduction of oxygen is a complex four proton, four electron process that is of technological importance. Slow kinetics of the oxygen reduction is a primary loss of efficiency in electrochemical power sources. Much like the ferric ion, oxygen kinetic rats improve. Preliminary studies in the oxidation of methanol demonstrated a sonocatalyic effect in methanol electrolysis that is of particular interest for the development of liquid based fuel cells. Sonication can both clean and destroy surface materials. The cleaning power inherent in sonication improves electrocatalysis and removes deposits and oxides from the electrode surface.

Pt-based alloy surfaces are used to catalyze the electrochemical oxygen reduction reaction (ORR), where molecular oxygen is converted into water on fuel cell electrodes. In this work, we address challenges due to the cost of high Pt loadings in the cathode electrocatalyst, as well as those arising from catalyst durability. We aim to develop an increased understanding of the factors that determine ORR activity together with stability against surface segregation and dissolution of Pt-based alloys. We firstly focus on the problem of determining surface atomic distribution resulting from surface segregation phenomena. We use first-principles density functional theory (DFT) calculations on PtCo and Pt3Co overall compositions, as well as adsorption of water and atomic oxygen on PtCo(111) and Pt-skin structures. The bonding between water and surfaces of PtCo and Pt-skin monolayers are investigated in terms of orbital population. Also, on both surfaces, the surface reconstruction effect due to high oxygen coverage and water co-adsorption is investigated. Although the PtCo structures show good activity, a large dissolution of Co atoms tends to occur in acid medium. To tackle this problem, we examine core-shell structures which showed improved stability and activity compared to Pt(111), in particular, one consisting of a surface Pt-skin monolayer over an IrCo or Ir3Co core, with or without a Pd interlayer between the Pt surface and the Ir-Co core. DFT analysis of surface segregation, surface stability against dissolution, surface Pourbaix diagrams, and reaction mechanisms provide useful predictions on catalyst durability, onset potential for water oxidation, surface atomic distribution, coverage of oxygenated species, and activity. The roles of the Pd interlayer in the core-shell structures that influence higher ORR activity are clarified. Furthermore, the stability and activity enhancement of new shell-anchor-core structures of Pt/Fe-C/core, Pt/Co-C/core and Pt/Ni-C/core are demonstrated with core materials of Ir, Pd3Co, Ir3Co, IrCo and IrNi. Based on the analysis, Pt/Fe-C/Ir, Pt/Co-C/Ir, Pt/Ni-C/Ir, Pt/Co-C/Pd3Co, Pt/Fe-C/Pd3Co, Pt/Co- C/Ir3Co, Pt/Fe-C/Ir3Co, Pt/Co-C/IrCo, Pt/Co-C/IrNi, and Pt/Fe-C/IrNi structures show promise in terms of both improved durability and relatively high ORR activity.

Oxygen reduction reaction (ORR) has considerable potential for the pro-duction of electricity, issues with water splitting and many other applications in the energy sector. But in order to increase the efficiency of the reaction an electrocatalyst needs to be introduced. In today’s industrial devices precious and costly metals such as platinum (Pt) are used as catalysts. Other more abundant and cheaper alternatives, for example cobalt oxides, are therefore being investigated. In this thesis, pure cobalt (Co) thin films were synthesised to investigate if thin films can be used for the catalysis of ORR. This was successfully carried out by electrochemically modifying the thin films and grow catalytically ac-tive hexagonal cobalt oxide nanoparticles.   Multicomponent system CoCrFeNi is an emerging alloy system with high research interest for its high corrosion resistance suitable for harsh environments in which the applications for ORR are found. In this thesis, CoCrFexNi were synthesised as thin films. The corrosion resistance of the films was investigated in addition to their catalytic activity. The effect of Fe content on these properties was also studied. The presence of Fe was crucial for the electrochemical activation of films and catalytic activity towards ORR.  In summary, this thesis shows that cobalt based thin films can be used as catalysts combined with corrosion resistance for ORR applications.

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