1,720,992 research outputs found
Ti3C2Tx as an Advanced Support Material for Polymer Electrolyte Fuel Cell Catalysts to Facilitate the Oxygen Reduction Reaction
Get PDFPolymer electrolyte fuel cell's (PEFC's) have the potential to offer a leading energy conversion technology. These fuel cells make use of hydrogen and oxygen and by means of a chemical reaction, electricity, heat and liquid water are produced. In 2015 the Department of Energy (DoE) of the United States declared a 5000-hour lifetime target for transport applicable fuel cells. With current technological limitation, the achieved lifespan is, however, restricted to only 1700-hours. An assessment to find a more active but primarily more durable support for the oxygen reduction reaction (ORR) in a PEFC than the currently employed carbonaceous support was therefore undertaken. A MXene, Ti3AlC2 was selected for assessment based on its theoretically suitable electrical and thermal conductivities, as well as its possession of among the strongest resistance to oxidation of the many different MAX phases. The synthesis of high (>50 m2 g -1 ), specific surface area, delaminated Ti3C2Tx flakes was attempted first with mild in-situ HF conditions. While this method could both etch and delaminate flakes in a single stage, because the flake size remained large and unchanged, the specific surface area was not seen to increase to the outlined requirements. To synthesize Ti3C2Tx flakes with a high specific surface area, HF etching was therefore employed. In this report, 0.5 g of 400-mesh Ti3AlC2 flakes synthesized by hot pressing were etched in 10 ml of 48 wt % HF for 24 hours at 30 °C. After micronizing for 10 minutes and probe sonicating in solution for a further 40 minutes, high specific surface area (86 m2 g -1 ), delaminated Ti3C2Tx flakes were attained. Using metal organic chemical deposition, well dispersed 2- 5 nm platinum particles were successfully deposited onto the support material. Initial electrochemical performance evaluations indicated a lack of conductivity which restricted electron transport and therefore limited catalyst activity. This was determined to be the result of more defective flakes and by correlation, an increase in interfaces leading to increased resistance. With the incorporation of carbon to the catalyst material to synthesize a hybrid electrode, a positive result confirming ORR activity was attained. While the electrochemical surface area (ECAS) was less than half of that of Pt/C (80 vs 28 m2 g -1 ), it confirmed where the synthesis constraints lie. In review of the durability results, it was found that trapped intermediates between high specific surface area MXene sheets not only restricts access to catalytic sites but are further protonated and reduced to form hydrogen peroxide which causes irreversible damage the PEFC's catalyst membrane
CuAg bimetallic nanoparticles for the electrochemical reduction of carbon dioxide
Get PDFThe electrochemical reduction of carbon dioxide is a surface reaction, involving the conversion of carbon dioxide and water to hydrocarbons and oxygenates in an electrolytic environment. This reaction grants an opportunity for the rerouting of carbon dioxide from expulsion to the atmosphere towards the production of chemical products. Due to the stable C-O bond in carbon dioxide, this reaction requires a catalyst and an external energy source to activate it. The use of renewable energy as an energy source would ensure that the electrochemical reduction process is carbon neutral. Cu has been identified as a promising catalyst for the electrochemical reduction of carbon dioxide, as it is more active and produces higher amounts of hydrocarbons and oxygenates relative to other transition metals,. However, Cu is unselective towards a specific product, and it highly active for the undesirable hydrogen evolution reaction (Kuhl et al., 2014). On the other hand, under electrochemical conditions, Ag yields mainly CO, which has been shown to compete with the hydrogen evolution reaction (Hori, Murata & Takahashi, 1989). This study focuses on the synthesis of different ratios of CuAg bimetallic nanoparticles, and their electrocatalytic performance evaluation for the electrochemical reduction of carbon dioxide. Bimetallic nanoparticles were synthesised via a wet chemical method using two synthesis routes. One synthesis was performed in the presence of hexadecylamine (HDA), surfactant, while the other was performed in its absence. The electrocatalytic performance evaluation was conducted using two reactors, a batch reactor with a gas diffusion electrode, and a rotating disc electrode reactor. It was found that catalysts synthesised in the absence of HDA had a phase-separated atomic arrangement, forming islands of Cu and Ag. On the other hand, synthesis conducted in the presence of HDA culminated in a CuAg solid solution. The two synthesis routes resulted in catalysts that had distinct product distributions. Catalysts prepared in the absence of HDA predominantly formed formate, with catalysts that had a higher Cu content forming methanol and CO. The yield of formate for catalysts synthesised under the absence of HDA did not decline at higher potentials relative to Cu catalysts which suffered from hydrogen production. On the other hand, bimetallic catalysts synthesised in the presence of HDA demonstrated behaviour similar to monometallic catalysts. Catalysts with a higher Cu content predominantly produced formate, while catalysts with a high Ag content produced a CO rich stream. This study indicates a profound dependency of the catalyst activity and product distribution on the CuAg bimetallic ratio and atomic arrangement. This study adds knowledge on the synthesis of CuAg bimetallic nanoparticles, and the design of catalysts for the electrochemical reduction of carbon dioxide
Pulsatile Electropolishing of Nitinol Stents
Get PDFAlloys that oxidize easily such as those containing titanium or chromium present a challenge to electropolishing because the polarization that dissolves the metal species produces positive ions, these oxidize and form stable surface layers of metallic oxides that prevent further dissolution. This is usually overcome with the use of acid solutions that dissolve the metallic oxide. This thesis aims to shift the primary control of the electropolishing e_ect from electrolyte variables to a combination of potential variation and hydrodynamic interference. Traditionally this is achieved with one continuous mass removal process that operates after a steady state of dissolution is established, generally requiring hydro_uoric or phosphoric acid to achieve titanium dioxide breakdown. The resulting concentration gradient is heavily a_ected by electrolyte variables such as viscosity and electrical resistance, while the electrical polarization is constrained by the metallic oxide reaction rate which creates a complex net of interdependent variables that can be di_cult to tune. A rapidly changing electric _eld was applied to modulate the alloying element dissolution rates. In tandem with the electropolishing development, stages prior to the electropolishing step were selectively removed to simplify the process. Utilizing a three electrode system and an external potentiostat controller to permit greater _exibility, a variety of alternating current pulsatile waveforms were investigated and the resulting e_ect on surface topology was observed using SEM and AFM microscopes. Di_erential pulse voltammogram yielded a feedback parameter on surface composition, and various pulse parameters were adjusted to optimize for surface smoothness, and identify the primary control variable. An electropolishing method is presented which achieves a :50% reduction in the Sa surface roughness value to an area average of 45 nm on a laser cut tubular stent geometry. It is shown that this method can be adapted to eliminate the need for chemical etching or mechanical polishing prior to electropolishing. The resulting polished surface displays corrosion resistance equivalent or better than other electropolished Nitinol surfaces from literature with a breakdown potential >1V vs SCE, and a similarly high repassivation potential. Balancing the charge in the anodic and cathodic pulses was the key to minimizing the resulting surface roughness, and eliminating micropits. Nitinol is a nearly binary alloy of NiTi and a charge transfer ratio of 1 yielded the smoothest surfaces at current densities around :1 A/cm2. The initial surface condition was found to be irrelevant to electropolishing control with respect to oxide composition, provided enough mass was removed to fully dissolve the initial layers of mixed composition
Towards identifying platinum anchor sites on carbon via a model electrochemical system
Get PDFThe interaction between Pt and its carbon support was investigated by a model electrochemical system. This entailed aggressively oxidising a two-dimensional carbon substrate, i.e. highly orientated pyrolytic graphite (HOPG) and mirror finish graphite (MFG) quartz crystal, to incorporate oxygen terminated groups into the graphitic matrix. This study focusses on potential cycling to determine the mobility of Pt across these carbon surfaces and the effect of the Pt anchoring to carbon on the electrocatalyst durability. This work incorporates both a conventional three electrode electrochemical setup and the use of the electrochemical quartz crystal nano-balance (EQCN). The objectives of this study were to better understand the Pt mobility across the carbon substrate surface and to gain insight into the solid-liquid interface of Pt dissolution due to potential cycling. Initial results on HOPG as discussed in chapter 2, indicated minimal Pt dissolution of between 13% and 15% of total electrochemical active surface area loss. These results, however, did not provide adequate evidence to conclusively determine the extent of Pt mobility on the carbon surface and the effect of oxygen terminated groups in hindering Pt dissolution. In order to gain a more thorough understanding of the Pt dissolution processes, the use of the EQCN technique was utilised. Firstly, it was shown that the mirror finished graphite quartz crystals used in the EQCN technique, are qualitatively comparable to the electrochemical measurements recorded with the HOPG samples. Secondly, potential cycling under the same conditions as HOPG produced similar electrochemical results. The frequency response curves from the EQCN yielded the most promising results. This study showed, qualitatively, that the surface of Pt is non-monotonic, and that the surface charge changes with increased potential cycling. Pt/MFG-A had consistent frequency responses over the entire potential range during Pt dissolution, thus, with the above understanding of surface charge, it is concluded that acid treated carbon substrates show a stronger affinity for Pt anchoring
Improving on quality control methods used in commercial MEA manufacturing
Get PDFWith the current rate of global warming, temperatures are continuously increasing and could have severe impacts on natural and human systems. The transport sector has the largest potential for short term reduction of emissions and the fuel cell industry could be a viable replacement for fossil fuel consumption. The hydrogen economy, alongside the fuel cell industry, can target decarbonisation of long-haul and heavy-duty transport by pursuing a green hydrogen mobility ecosystem. One of the key barriers hindering fuel cell commercialisation is the lack of standard methods for quality control in membrane electrode assembly (MEA) manufacturing, specifically the lack of in-line, non-destructive and roll-to-roll quality control methods. There are currently 5 typical methods for assessing the quality of an MEA, which are gravimetric and thickness analysis, visual and optical microscopy inspection and single cell characterisation. The development of efficient, contactless and non-destructive techniques for in-line quality control testing is a critical enabler in alleviating manufacturing costs by detecting and flagging manufacturing defects thus ensuring a desired quality standard is met at each step in the process. Nonconforming products are detected early and removed at different stages in the process, reducing manufacturing costs that would have otherwise been wasted on defective products. (Yuan, et al., 2021; Phillips, Ulsh , Neyerlin, Porter, & Bender, 2018) In this work, three techniques have been proposed and developed for improving quality control. They are automated visual inspection, infrared thermography (IRT) and x-ray fluorescence (XRF). A rig was successfully designed and built for the automated visual inspection and IRT and methods were successfully developed for the use of the 3 techniques in MEA fabrication. The results from the thickness and gravimetric analysis yielded values that were on par with the experimental targets and commercial standards. A linear relationship was also demonstrated, showing the link between thickness and gravimetric loading. If the relative humidity is well controlled, one of the two quality control methods could be omitted during manufacturing to decrease manufacturing time. To overcome the downfall associated with relative humidity for both these test methods, a newer technology, XRF spectrometry, was investigated which determines the PGM loading and is independent of changes in the relative humidity. The PGM loading can then be used to calculate the approximate thickness of the electrode layers if needed and is less labour intensive and time consuming. It was discovered that the XRF needs to be specifically programmed for each matrix that it is to be used on – where the matrix includes the substrate thickness, type of substrate, type of catalyst in the ink, ionomer content and sample orientation (cathode or anode facing XRF eye). It is assumed that the matrix expands to other parameter changes as well, such as type of ionomer, ink solids content and presence of backers or covers film. However, these parameters were not tested within this study due to time and resource constraints. To assess the aesthetic and physical quality of the CLs produced, visual and optical microscopy inspection were done. Visual inspection involves human observation of the layer under light, therefore, methods were developed in this study to improve the test procedure and omit human error by digitising the test procedure. The results yielded from the visual and optical microscopy inspection were compared and were found to be almost identical. For the purpose of commercial manufacturing to identify light translucency above 10%, it would be less labour intensive and time consuming to use the automated visual inspection method developed. The downfall of either method arises when coating the anode layer (in this study) or when coating the second electrode (in general) due to themasking effect of first layer on the second, which prevents inconsistencies in the second layer being identified. The other downfall of either method is the inability to determine if a defect is a light spot or if it is a pinhole through the MEA. This is where IRT comes in as it was successful in identify pinholes. SEM and single cell performance characterisation were used to test whether the methods developed for automated visual inspection, XRF analysis and/or IR thermography had a negative or destructive effect on the MEAs – the results proved that there was no negative effect from any of the 3 methods. All three techniques were successfully developed for local use in laboratory and small-medium scale manufacturing. The automated visual inspection technique proved to be a key quality control technique and further work into its next generation and incorporation into a high-volume manufacturing line is needed as this could not be investigated due to time and budget constraints. The study was successful in developing a method for the use of XRF to determine PGM loadings and in investigating key parameters associated with its calibration procedures. The value of XRF over gravimetric analysis was shown and the XRF was successfully incorporated into a simulated high- volume line, however, further work is needed for its next generation in a high-volume line. A method for the use of IRT for defect detection was also successfully developed and an attempt to create a defect catalogue was started, however, cataloguing the appearance of defects in IRT still requires more work
CdSe based nanowires for the photocatalytic production of hydrogen gas
Get PDFPhotocatalytic production of hydrogen was investigated towards achieving a decarbonized supply of hydrogen gas for clean energy conversion technologies such as the proton exchange membrane fuel cell (PEMFC). This study uses a template-directed electrodeposition technique to synthesize multi-segmented CdSe based nanowires for use as a photocatalyst device for hydrogen production. CdSe, Ni, Au and Pt nanowires were successfully synthesized with dimensions ranging from 100 nm to 350 nm in diameter and up to 10 µm long. The CdSe stoichiometry was not easily controlled despite following literature protocols and requires a more systematic investigation. The electrodeposition of Ni nanowires was found to be most effective with very few problems encountered. Improvements in the morphology of Au and Pt nanowires were made by using a constant current as opposed to constant potential electrodeposition techniques. Multi-segmented nanowire devices were prepared with nanowires left embedded in a porous anodized aluminium oxide (AAO) template. Polymer PEDOT: PSS and noble metal Pt was used as an anode and cathode electrocatalyst materials respectively. A prototype photocatalytic testing system was set-up using a 1600 W xenon arc lamp as a light source, an in-house made photoreactor as the device holder, and a mass spectrometer for online gas detection measuring ionic currents of evolved species. The set-up was able to successfully detect hydrogen evolved during the tests but does require further development if more complete photocatalytic testing is to be conducted in future. Photocatalytic hydrogen production from the irradiated devices was inconclusive, but hydrogen detection from devices was observed in an 80 % MeOH solution with no irradiation. Through these tests it was learned that photocatalytic activity needs to be differentiated from regular catalytic activity. This is particularly the case if testing is conducted in organic media and if the photocatalytic phenomena is to be properly isolated and understood correctl
Design, construction and commissioning of an automated optical fibre catalyst coating process for use in photocatalytic reactor systems
Get PDFClimate change is one of the greatest challenges facing humanity. Fossil fuels are the primary source of energy on Earth. Since the global economic growth is closely linked to the global energy demand, fossil fuel usage remains the largest contributor to the steadily increasing atmospheric carbon dioxide concentration (CO2). CO2 mitigation through carbon capture and conversion are of great interest. Capturing CO2 from point source emitters is possible by absorption in a basic, sodium hydroxide (NaOH) containing solution, which is then converted into sodium bicarbonate (NaHCO3). Conversion of CO2 is thermodynamically demanding as it will require a large amount of energy, which renders currently used technologies infeasible. A promising alternative is the conversion of captured NaHCO3 into useful hydrocarbons at moderate operating conditions using solar energy, by a process called photocatalysis. Photocatalysis is the acceleration of a photo-induced reaction in the presence of a catalyst. Photocatalytic reactors have not yet been commercialised due to suboptimal catalyst and reactor designs. The typically low catalyst activity has to be countered by efficiently loading a large amount of catalyst in the reactor. This results in a problem regarding the photon transfer limitations to the catalytically active site, which limits illumination of the catalyst in the reactor. This can be overcome by using optical fibre to guide photons, which are coated with the photocatalyst. However, it is estimated that a reactor containing ca. 1 g of catalyst will require ca. 1.8 km of identically coated optical fibre. The aim of the project is to design, construct and commission an automated controllable process to increase the production volume of catalyst coated optical fibre using either a solgel suspension or a slurry containing P25 (TiO2). A multi-step optical fibre coating process was developed to achieve the desired coated optical fibre as a product. It consists of 6 major units that process raw (polymer-coated) optical fibre into catalyst coated optical fibre. The steps include the 4 essential steps required for optical fibre preparation by-hand, these steps are stripping, washing, coating and heat treatment. This automated optical fibre catalyst coating process (AOFCCP) can make the coating of optical fibres time-efficient and controllable. The latter can be achieved by controlling the effect various process parameters affecting the coating thickness and homogeneity of the coating, such as pH, heat treatment, catalyst slurry concentration as well as pulling speed. The AOFCCP produced coating thicknesses ranging from 0.47 µm - 0.59 µm and 0.37 µm - 0.46 µm for the P25 slurry and sol-gel coating methods respectively. The pH of the P25 slurry was found to have a negligible effect on both the coating thickness and surface morphology, therefore is no longer regarded as a process variable in the AOFCCP. The thickness of the coating increased with an increase in P25 slurry concentration with a maximum achievable coating thickness of 0.87 µm using a slurry concentration of 20 wt.-%. The temperature of heat treatment which was tested showed different relationships between the coating methods. For the sol-gel coating method, the increase in temperature resulted in a decrease in coating thickness possibly due to the decrease in porosity whereas for the P25 slurry method the increase in temperature showed an increase in coating thickness possibly due to the higher evaporation rates. An increase in the pulling speed in the AOFCCP resulted in an increase in coating thickness on the optical fibre independent of the coating method; coating thicknesses ranging from 0.41 µm - 0.71 µm and 0.23 µm - 2.14 µm were obtained using the P25 slurry and sol-gel coating methods, respectively, by varying the pulling speed. The critical cracking thickness is defined as the thickness of the film, produced by the sol-gel method, at which coating deformations become observable which was found to be 0.37 µm at 600 °C, and 0.77 µm at a pulling speed of 2.30 mm.s -1 . The results obtained from the commissioning experiments showed that the AOFCCP can produce coated optical fibre with controllable thickness. The controllability was discovered to be in the adjustment of the process variables investigated which showed a significant effect on the coating thickness, except for pH. Based on the statistical analysis that was performed, it was confirmed that the results obtained from the system were repeatable and that the coating was uniform for all process variables that were investigated except for sol-gel coating at high speeds of 2.88 mm.s -1 – 3.46 mm.s -1 . The system was able to produce fibre with coating thickness's between 0.4 – 1.1 µm. It is recommended that a combination of the process variables be used in order to achieve better controllability in the process and to achieve thicker coating layers. Furthermore, the operating ranges of the process variables should be increased in order to determine the extent of the relationship between the process variable and the coating thickness and surface morphology
Preparation and characterisation of inorganic nanostructured support materials for polymer electrolyte fuel cells
Get PDFPolymer electrolyte fuel cells (PEFCs) have been identified as a safe, clean and reliable alternative energy conversion technology to conventional, fossil fuel based, ones. However, the hindrance to worldwide commercialisation of this technology lies in the poor durability and high costs associated with the current carbon supported platinum (Pt/C) catalysts. Carbon support corrosion and Pt dissolution/aggregation on the catalyst layer within the fuel cell have been confirmed as the major contributors to the degradation of the Pt/C (Shao, et al., 2007). Attention needs to be paid to the improvement of catalyst components to produce an electrocatalyst with better degradation resistance and low Pt loading in order to overcome these two major commercialisation barriers. The physico-chemical and electronic interaction between the Pt catalyst and the support material play a crucial role in the catalytic activity and stability of the electrocatalysts (Wang, et al., 2011). A comprehensive understanding of the effects of catalyst support material and morphology on the mechanism and kinetics of the oxygen reduction reaction (ORR) needs to be developed. This study investigated alternative, novel catalyst support materials and structures for the catalyst layer as opposed to carbon for PEFC applications. This material consisted of TiB2 electrospun nanofibers, powder and crushed electrospun nanofibers. Methods used to reliably and accurately deposit Pt onto these materials were identified, developed and analysed. These methods include platinum deposited onto TiB2 powder, electrospun crushed nanofibers and nanofiber mats via DC magnetron sputter deposition and thermally induced chemical deposition (TICD). The synthesised catalysts were physically characterised using X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Inductively Coupled Plasma Optical emission spectrometry (ICP-OES). Platinum effectively deposited on the TiB2 support structures via these deposition techniques within two standard deviations of the desired Pt loadings
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