Dissertations

2022

Nicolaas Engelbrecht, “The development of autothermal microchannel reactor technology for hydrogen-based gas processing “, PhD thesis, Promoter: Prof. R.C. Everson, Co-promoter: Prof. D.G. Bessarabov, Faculty of Engineering, North West University, 2021.

Key challenges associated with the production, transport, storage and continuous use of hydrogen (H2), generated from renewable energy, is the natural intermittency of some renewable resources such as solar photovoltaic and wind, and the physical properties of H2 that complicates its handling in the industry, i.e. its low volumetric energy density and high flammability. The work presented in this thesis demonstrates the use of process intensifying microchannel reactor technology for the thermo-catalytic processing of renewable H2 via attractive energy carriers: (i) the decomposition of ammonia (NH3) to form H2, as well as (ii) the synthesis of methane (CH4) using renewable H2 as feedstock (CO2 methanation).

Furthermore, these processes require heat management strategies for effective autothermal operation. The NH3 decomposition reaction is endothermic and a coupled exothermic process (NH3 oxidation) is demonstrated to provide the heating requirements for H2 release. Conversely, the CO2 methanation process is exothermic, and cooling is required to reach equilibrium favouring reaction temperatures that promote the conversion of H2 and CO2 towards CH4. Extensive experimental investigations were carried out into these thermally coupled processes, and reported on for NH3 decomposition and for CO2 methanation. Then an evaluation of a compact methanation demonstrator unit incorporating the microchannel-based reactor was carried out, followed by computational fluid dynamic (CFD) modelling to evaluate associated heat and mass transport properties in the microchannel reactor.

The scale of the microchannel reactors investigated here was such that NH3 up to a flow rate of 6 NL min-1 was processible towards H2 at a high conversion rate (99.8%), and corresponding to an equivalent H2 fuel cell power of 0.71 kWe, while total methanation flow rates of up to 7 NL min-1 were used to demonstrate CO2 methanation (90.5% CO2 conversion). Respective thermal efficiencies of 75.9% and 76.6% were obtained at the recommended steady-state operating points, which were close to thermodynamic equilibrium. These thermal efficiencies are deemed remarkable, considering the compact- and R&D-scale reactor technologies investigated herein. Overall, the work described in this thesis contributes to the development of micro-engineered reactors that are multifunctional catalytic conversion units and heat exchangers, and which support modular technologies for the processing of renewable H2.

Publications in the thesis:

        • “Microchannel reactor heat-exchangers: A review of design strategies for the effective thermal coupling of gas phase reactions” in Chemical Engineering and Processing – Process Intensification (2020) Elsevier, doi.org/10.1016/j.cep.2020.108164.
        • “A highly efficient autothermal microchannel reactor for ammonia decomposition: Analysis of hydrogen production in transient and steady-state regimes” in Journal of Power Sources (2018) Elsevier, doi.org/10.1016/j.jpowsour.2018.03.043.
        • “Thermal management and methanation performance of a microchannel-based Sabatier reactor/heat exchanger utilising renewable hydrogen” in Fuel Processing Technology (2020) Elsevier, doi.org/10.1016/j.fuproc.2020.106508.

 

Phillimon Mokanne Modisha, “Evaluation of perhydrodibenzyltoluene dehydrogenation parameters and durability using noble metal catalysts for hydrogen production” Doctoral thesis, Promoter: Dr Dmitri Bessarabov, Faculty of Engineering, North-West University, 2021.

Hydrogen storage using liquid organic hydrogen carrier (LOHC) technology offers numerous advantages over conventional hydrogen storage systems. LOHCs are organic compounds that can store and release hydrogen through catalytic hydrogenation and dehydrogenation reactions. Globally, there are a few key-industry players involved in this technology. In South Africa, there are identified opportunities for the future application of LOHC technology. Probably the most widely used LOHC material for commercial applications is dibenzyltoluene. However, dibenzyltoluene (technical grade) (H0-DBT) consists of various structural isomers that are difficult to analyse by conventional gas chromatography. Products of reaction mixtures of H0-DBT contain numerous isomeric compounds (>20). This results in overlapping of peaks in conventional gas chromatography. Further challenging topics, associated with the use of H0-DBT as a LOHC, include the following: development and optimization of suitable catalytic materials for the dehydrogenation reaction, durability of the H0-DBT molecule, and the unknown number of hydrogenation and dehydrogenation cycles that a H0-DBT molecule can withstand. Investigations into these and other areas in this field are of a great importance. Specific topics addressed in this work included the following: determination of a suitable chromatographic method and optimum conditions for the separation, identification and quantitative analysis of isomeric reaction mixtures of H0-DBT-based LOHC; evaluation and characterization of monometallic catalysts (Pt/Al2O3 and Pd/Al2O3) and their bimetallic counterparts (Pt–Pd/Al2O3) for the dehydrogenation of perhydrodibenzyltoluene (H18-DBT); optimization of the catalyst metal loading and reaction temperature for the reaction, towards achieving a reasonably high degree of dehydrogenation (dod), coupled with high-purity hydrogen; and determination of the durability of H0-DBT-based LOHCs.

Publications emanated from this thesis:

        • Modisha, P., Jordaan, J. H., Bösmann, A., Wasserscheid, P., & Bessarabov, D. (2018). Analysis of reaction mixtures of perhydro-dibenzyltoluene using two-dimensional gas chromatography and single quadrupole gas chromatography. International journal of hydrogen energy, 43(11), 5620–5636. https://doi.org/10.1016/j.ijhydene.2018.02.005
        • Modisha, P., Ouma, C. N., Garidzirai, R., Wasserscheid, P., & Bessarabov, D. (2019). The prospect of hydrogen storage using liquid organic hydrogen carriers. Energy & fuels, 33(4), 2778–2796. https://doi.org/10.1021/acs.energyfuels.9b00296
        • Modisha, P., Gqogqa, P., Garidzirai, R., Ouma, C. N., & Bessarabov, D. (2019). Evaluation of catalyst activity for release of hydrogen from liquid organic hydrogen carriers. International Journal of Hydrogen Energy, 44(39), 21926–21935. https://doi.org/10.1016/j.ijhydene.2019.06.212
        • Modisha, P., & Bessarabov, D. (2020). Stress tolerance assessment of dibenzyltoluene-based liquid organic hydrogen carriers. Sustainable Energy & Fuels, 4(9), 4662–4670. https://doi.org/10.1039/D0SE00625D

Alina Kozhukhova, “Development of catalyst support structures for hydrogen applications”, PhD dissertation, Promoters: Prof. Dmitri Bessarabov and Dr. Stephanus Petrus du Preez, Faculty of Engineering, North-West University, 2022.

Anodized aluminium oxide (AAO) has attracted the attention of scientists and engineers as a source material for the preparation of catalyst support nanostructures.  AAO represents a hexagonally distributed pore structure with parallel arranged pore channels.  The oxide layer formed during the anodization of aluminium (Al) has increased hardness, thermal conductivity, and corrosion resistance compared to Al’s native oxide layer.  However, widespread utilization of AAO as a support material is perplexed by the high cost of pure Al (>99%) and difficulties related to the anodization process, such as the requirements for low-temperature (approximately 0 °C) and extensive anodization times.  Therefore, numerous studies have focused on the use of an Al substrate of lower purity (<99%) and the development of a suitable anodization method.  Despite this, a research gap exists in the preparation of ordered (or at least semi-ordered) AAO structures, and their application as a catalyst support material in hydrogen-based applications.  A comprehensive study is therefore proposed, to develop an acceptable anodization method for low-purity Al, and then to evaluate and demonstrate its application as a catalyst support for high-temperature catalytic hydrogen combustion (CHC).

The anodization process for a low-purity Al substrate (Al6082) was investigated.  The effects of voltage (30–60 V), the type and concentration of the electrolyte (oxalic and phosphoric acid solutions), temperature (10–40 °C), and duration of the anodization process (1–4 h) on the AAO pore arrangement were investigated.  The morphological characteristics (pore diameter, interpore distance, pore density, porosity, and thickness of the oxide layer) were determined to assess the prepared AAO layers.  Because the Al alloy consisted of alloying additives (classified as impurities here), the effects thereof on the anodization process were considered.

The wet impregnation method, using hexachloroplatinic acid (H2PtCl6), was applied to prepare a Pt/AAO catalyst.  Characterization of the catalyst (i.e., Pt particle size, surface characterization, and Pt loading) was performed using advanced techniques such as scanning electron microscopy, focused ion beam (FIB) and transmission electron microscopy (TEM), and inductively coupled plasma optical emission spectroscopy.  The catalytic activity of the Pt/AAO catalysts was evaluated upon its exposure to hydrogen.  Results showed high activity (near-complete hydrogen conversion, spontaneous initiation of the reaction at room temperature) and durability (530 h of the reaction with no catalyst deactivation) of the Pt/AAO catalyst towards the CHC reaction.  High activity of the Pt/AAO catalyst was achieved due to the structural features of the AAO layer that allowed high catalyst dispersion on the AAO support.  Furthermore, it was the prepared AAO layer closely adhered to the metallic core, which promoted high thermal conductivity of the Al/AAO system.  This, in turn, promoted a uniform temperature distribution throughout the catalyst surface, which prevented localized Pt aggregation.  High thermal conductivity of catalysts plays a key role when the catalyst is intended to be used for safety (e.g. passive autocatalytic recombiner, PAR) and combustion/heating applications (e.g. cooking, spatial heating) as it prevents the formation of hotspots.

The new Pt/AAO catalyst was then tested and evaluated for the passive autocatalytic recombination of hydrogen.  Five Pt/AAO catalysts were installed in an in-house-developed recombiner section testing station and then tested for combustion of 0.5–4 vol% hydrogen fuel.  The thermal distribution throughout the catalyst surface was investigated using an infrared (IR) camera.  The Pt/AAO catalyst showed high thermal conductivity; a temperature gradient throughout the catalyst surface of 23 °C was observed during the experiment.  In addition, the CHC reaction was initiated at room temperature and low hydrogen concentrations (<1 vol%), suggesting high catalytic activity of the Pt/AAO catalyst.  The activation energy for CHC on the Pt/AAO catalyst was determined to be 19.2 kJ/mol.  To confirm that Pt aggregation/catalyst deactivation was avoided, the Pt-containing AAO layer was cross-sectioned by FIB and characterized using TEM before and after prolonged CHC.

A Pt/CeO2, ZrO2, Y2O3 mixed oxide (CZY)/AAO catalyst was prepared and evaluated for CHC applications.  The CZY support was prepared by a co-precipitation process with aqueous ammonia solution, using aqueous solutions of Ce(NO3)3, ZrO(NO3)2, and Y(NO3)3.  The Pt/CZY/AAO catalyst was then prepared by spray-deposition Pt/CZY intermediate on an AAO layer adhered to aluminium core.  Evaluation was carried out in 1–8 vol% hydrogen/air mixtures.  The thermal distribution throughout the catalyst surface was investigated using an IR camera.  Thermal imaging revealed a maximum temperature gradient of 36 °C throughout the catalyst surface (3 × 1 cm) for all studied hydrogen concentrations (1–8 vol%).  Further, to assess the catalyst durability, the Pt/CZY/AAO catalyst was subjected to prolonged CHC.  The catalyst maintained a continuous combustion temperature of 239.0±10.0 °C for 53 days at a hydrogen flow rate of 138 NmL/min.  Finally, a Pt/CZY/AAO catalytic plate (14.0 × 4.5 cm) was prepared to investigate the thermal distribution.  A maximum temperature gradient of 5.4 °C was obtained throughout the catalyst surface.

Publications emanated from the thesis:

          • Kozhukhova, A.E., du Preez, S.P., Bessarabov, D.G., Preparation of anodized aluminium oxide at high temperatures using low purity aluminium (Al6082). Surface and Coating Technology, 2019. 378: 124970.
          • Kozhukhova, A.E., du Preez, S.P., Bessarabov, D.G., The effects of pore widening and calcination on anodized aluminium oxide prepared from Al6082. Surface and Coating Technology, 2020. 383: 125234.
          • Kozhukhova, A.E., du Preez, S.P., Shuro, I., Bessarabov, D.G., Development of a low purity aluminium alloy (Al6082) anodization process and its application as a platinum-based catalyst in catalytic hydrogen combustion. Surface and Coating Technology, 2020. 404: 126483.
          • Kozhukhova, A.E., du Preez, S.P., Malakhov, A.A., Bessarabov, D.G., A thermally conductive Pt/AAO catalyst for hydrogen passive autocatalytic recombination. Catalysts, 2021. 11: 491.
          • Kozhukhova, A.E., du Preez, S.P., Bessarabov, D.G., Catalytic hydrogen combustion for domestic and safety applications: A critical review of catalyst materials and technologies. Energies, 2021. 14: 4897.
          • Kozhukhova, A.E., du Preez, S.P., Bessarabov, D.G., Preparation of Pt/Ce-Zr-Y mixed oxide/anodized aluminium oxide catalysts for hydrogen passive autocatalytic recombination. International Journal of Hydrogen Energy, 2022. (accepted)

Alexander Malakhov, “Mitigation strategies for hydrogen safety in the confined environment: CFD modelling and validation“, PhD dissertation, Promoter: Dr. M.H. du Toit, Co-promoters: Prof. D.G. Bessarabov, Dr. A.V. Avdeenkov, Faculty of Engineering, North West University, 2022.

Over the last twenty years, the importance of hydrogen as a prospective energy source has significantly increased. Utilizing hydrogen offers advantages in reducing carbon emissions generated by transportation systems. The current hydrogen economy growth is owed to the rapid development and employment of hydrogen fuel cell technology. Moreover, in efforts to achieve zero-carbon emission in the mining industry, they are looking for more perspective technologies in terms of clean and environmentally friendly energy carriers. Hydrogen fuel cell technology can make zero-carbon emissions a reality. However, due to specific combustion properties, the usage of hydrogen energy is accompanied by a risk of unintended leakage, dispersion, ignition, and deflagration inside a confined space. In this regard, mitigation strategies for hydrogen safety need to be investigated. The emergency hydrogen removal systems must be deployed to guarantee that the H2 concentration is diluted below the minimum flammability limit, even under severe accidents.

One of the widely applied hydrogen safety technologies is utilizing a ventilation system. Forced ventilation can promote the removal of hydrogen from a confined space by means of mixing and diluting a flammable gas below the ignition limit. Several accident scenarios in underground private garages, semi-closed rooms, and other confined spaces were investigated in past. However, none of these studies have investigated the efficiency of FV to mitigate horizontally directed hydrogen releases in the full-scale tunnel. This highlights the importance of developing and validating CFD models that can be applied to various applications and scenarios. Specific to the release of hydrogen in an underground tunnel may be caused by the pipe/vessel/PDR damage and the release orifice can be of various sizes. It is therefore important to conduct a parametrical study of horizontally directed hydrogen release based on different inlet conditions.

Another important approach that goes toward mitigation strategies for hydrogen safety is a passive autocatalytic recombiner (PAR). The spontaneous exothermic chemical reaction between hydrogen and oxygen leads to the heating of the catalyst surface and then activates natural convection inside the PAR. Eventually, by means of autocatalytic recombination reaction, the concentration of hydrogen reduces to lower than the flammable limit. In efforts to research and define the characteristics of PAR, numerous experimental programs have been conducted in small- and full-scale recombiner facilities in the past. However, the only plate-type catalyst has been studied in observed literature. In order to measure the temperature profile, only thermocouples have been applied along the catalyst plate length. It is therefore identified, the cylindrical-type catalyst requires attention. Moreover, advanced technologies should be considered to measure the temperature of cylindrical-type catalysts. While most studies have been focused on the plate-type PAR catalyst, the developed one- and two-dimension numerical models have enough capability to predict the catalyst thermal profile, and gas composition along a single recombiner channel for low inlet hydrogen concentrations rather well. Nevertheless, in order to simulate much complex PAR catalyst geometry (i.e. cylindrical-type), a three-dimension model is required. Much less attention has been given to developing a CFD model that uses a conjugated approach with accounted both gas-phase (homogeneous) and surface (heterogeneous) reactions.

Related to this, the CFD simulations method can be very helpful in terms of hydrogen safety research. Hydrogen release and subsequent dispersion, as well as a multi-step reaction of hydrogen autocatalytic recombination, can be investigated using commercially available CFD software.

This research is based on experimental evaluation and computational fluid dynamics (CFD) modelling validation. Full-scale tests to investigate unintended hydrogen release were conducted at the HySA Mining Platform Test Facility for Green Mining. The cylindrical-type PAR catalyst section has been tested in an in-house developed recombiner section testing station. Hydrogen concentrations were measured by means of high-precision H2 sensors, while the temperature of PAR catalyst has been evaluated utilizing a high-resolution infrared camera. The three-dimensions CFD models have been developed in the STAR-CCM+ software package to simulate each experiment in detail.

Publication emanated from the thesis:

          • Malakhov, A.A., Avdeenkov, A.V., Du Toit, M.H. and Bessarabov, D.G., (2020). CFD simulation and experimental study of a hydrogen leak in a semi-closed space with the purpose of risk mitigation. International Journal of Hydrogen Energy, 45(15), pp.9231–9240. (doi.org/10.1016/j.ijhydene.2020.01.035)
          • Malakhov, A.A., du Toit, M.H., Du Preez, S.P., Avdeenkov, A.V. and Bessarabov, D.G., (2020). Temperature profile mapping over a catalytic unit of a hydrogen passive autocatalytic recombiner: an experimental and computational fluid dynamics study. Energy & Fuels, 34(9), pp.11637–11649. (doi.org/10.1021/acs.energyfuels.0c01582)
          • Kozhukhova, A. E., du Preez, S. P., Malakhov, A. A., Bessarabov, D. G. (2021). A Thermally Conductive Pt/AAO Catalyst for Hydrogen Passive Autocatalytic Recombination. Catalysts, 11(4), 491. (doi.org/10.3390/catal11040491)
          • Malakhov, A.A., Avdeenkov, A.V., Du Toit, M.H. and Bessarabov, D.G., (2021). Study of the efficiency of a PAR Pt catalyst and CFD verification of detailed mechanism of H2/O2 oxidation. (has been accepted for publication 08/10/2021 in 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19))
          • Avdeenkov, A. V., Sergeev, V. V., Stepanov, A. V., Malakhov, A. A., Koshmanov, D. Y., Soloviev, S. L., Bessarabov, D. G. (2018). Math hydrogen catalytic recombiner: Engineering model for dynamic full-scale calculations. International Journal of Hydrogen Energy, 43(52), 23523–23537. (doi.org/10.1016/j.ijhydene.2018.10.212)

Rudaviro Garidzirai, “Dehydrogenation of perhydrodibenzytoluene for hydrogen production: the effect of Mg and Zn dopants on the catalytic activity of Pt/Al2O3 “, Master dissertation, Promoters: Prof. D.G. Bessarabov, Co-promoters: Dr P.M. Modisha, Faculty of Engineering, North West University, 2022.

Liquid organic hydrogen carriers (LOHC) are hydrocarbon molecules that have the capacity to store and release hydrogen through catalytic hydrogenation and dehydrogenation reactions. The dibenzyltoluene/perhydrodibenzyltoluene (H0-DBT/H18-DBT) system, has been identified as the most promising LOHC molecules for hydrogen storage. However, conventional Pt/Al2O3 catalyst used for dehydrogenation of H18-DBT still need to be further improved to obtain high throughput hydrogen production with low deactivation. Modification of Pt/Al2O3 is another way of improving the catalytic performance (productivity, degree of dehydrogenation, selectivity and conversion). Therefore, in this contribution the effect of Mg and Zn dopants on the catalytic performance of Pt/Al2O3 is investigated for the dehydrogenation of H18-DBT. Firstly, the ү-Al2O3 supports were modified with Mg(NO3).6H2O and Zn(NO3).6H2O precursors to produce Mg-Al2O3 and Zn-Al2O3 aiming at 3.8 wt % metal loading. Thereafter, the unmodified and modified ү-Al2O3 supports were impregnated using H2PtCl6.xH2O to produce Pt/Al2O3, Pt/Mg-Al2O3 and Pt/Zn-Al2O3 aiming at 0.5 wt % Pt loading. The catalysts were characterised using inductively coupled plasma-optical emission spectrometry (ICP-OES), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), hydrogen temperature programmed reduction (H2-TPR), ammonia temperature programmed desorption (NH3-TPD), CO pulse chemisorption, Branuer-Emmett-Teller (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Moreover, the catalysts’ performance for dehydrogenation of H18-DBT was evaluated using a batch and fixed bed reactor. The samples resulting from dehydrogenation reaction were analysed using gas chromatography and refractive index techniques.

The initial four dehydrogenation runs (cycles) using a batch reactor at temperatures from 260–300 °C indicated that an increase in temperature increases the performance of all catalysts. Notably, the performance of the catalysts is in the following order: Pt/Mg-Al2O3 > Pt/Zn-Al2O3 > Pt/Al2O3 which suggests that Mg is a suitable and promising dopant. However, all catalysts showed deactivation during the initial four dehydrogenation runs. Furthermore, initial catalysts stability was investigated using a fixed bed reactor at weight hourly space velocity (WHSV) of 0.61 h–1, 22 h time-on-stream and 300 °C. Pt/Mg-Al2O3 catalyst showed improved performance when compared to Pt/Al2O3 and Pt/Zn-Al2O3, and it also showed lower deactivation. The calculated turnover frequencies (TOF) for dehydrogenation of H18-DBT at 300 °C using Pt/Al2O3, Pt/Mg-Al2O3 and Pt/Zn-Al2O3 are: 202, 586 and 269 min-1, respectively. This indicate that Pt/Mg-Al2O3 catalyst has the highest amount of active metal sites available for dehydrogenation reaction. This was also confirmed by a high frequency factor (2.3 x 1011) min–1 which suggests high rate of molecular collisions. The dehydrogenation of H18-DBT followed first order reaction kinetics and the obtained activation energy of Pt/Al2O3, Pt/Mg-Al2O3 and Pt/Zn-Al2O3 are 101, 151 and 131 kJ/mol, respectively. In this case, high amount of active sites corresponds to high value of frequency factor and this is connected to high activation energy. Therefore, lower activation energy for dehydrogenation of H18-DBT will not always produce high reaction rate. Mg lowered the support acidity (as proven by NH3-TPD) and the less acidic catalyst is found to be more selective towards the product H0-DBT. Therefore, Mg weakens the adsorption of H0-DBT on Pt surface and promotes H0-DBT desorption. However, the most active catalyst (Pt/Mg-Al2O3) produced high amounts of by-products. This is because more aromatic molecules produced are susceptible to the acidic sites of the support which provides the C–C cracking function.

Publication emanated from the thesis:

2021

Henning Petrus Cornelius Buitendach, “The effect of a ripple wave power supply on PEM electrolysis efficiency“, Master dissertation, Promoters: Prof. R. Gouws, Co-promoters: P.C. Minnaar, C.A. Martinson and Prof. D.G. Bessarabov, Faculty of Engineering, North West University, 2021.

The two matured water electrolysis technologies commercially available today are Alkaline water electrolysis and Proton exchange membrane water electrolysis (PEMWE). Although alkaline water electrolysis is in a more mature stage of development, PEMWE holds a few advantages over this technology. These advantages include minimum mass transport limitations, the compact cell design which results in rapid response times, the simplicity of the cell design, the high current density capabilities, the ability to discharge hydrogen gas at high pressures and the high purity hydrogen gas produced. However, PEMWE is still a relatively new technology compared to Alkaline water electrolysis and consequently has the highest need for research and development to reduce the cost and improve the efficiency of this technology.

Past research and development in this field focused mainly on material science of the electrolyser components and the operating conditions such as temperature and pressure to improve the efficiency and reduce the cost of this technology. Water electrolysers are usually supplied with a steady DC current, and it is common to use the electrical characteristics (i.e. voltage, current density, and impedance) as a measure of the electrolyser’s performance. Unfortunately, very little research is available on the effect that specific electrical characteristics of the power supplied to the PEM electrolyser (i.e. ripple current) have on the performance and efficiency of the cell. In the little literature that is available, the main focus was on the efficiency of the converters used and the effect that different converter topologies have on the efficiency of a PEM electrolyser.

Much more literature is however available on the effect of electrical characteristics, such as a fluctuating current or voltage, on the cell efficiency of alkaline water electrolysers. These studies indicated that electrical characteristics such as the ripple factor, frequency, and waveform of the applied electrical power affect the hydrogen production rate as well as the power consumption rate of the electrolyser. Although some respectable information was obtained from the literature, all the studies made use of alkaline water electrolysis systems and the effects of a ripple current on PEMWE are still unknown.

This study aims to determine how the efficiency of a PEM electrolyser is affected by a ripple current. The primary objective was determining how the ripple current frequency, ripple factor, and waveform affects the hydrogen production, power consumption, efficiency and durability of a PEM electrolyser. Electrochemical impedance spectroscopy (EIS) was used to characterise the PEM electrolyser after which the impedance data were used to develop an equivalent electrical circuit (EEC) model. The EEC model was then used to simulate the effects of different ripple currents on the voltage response and power consumption of the cell. An active laboratory sized PEM electrolysis system was used to further investigate the impact of varying ripple currents on the efficiency of the system and to verify and validate the simulation results.

Publication emanated from the thesis: “Effect of a ripple current on the efficiency of a PEM electrolyser” in Results in Engineering (2021) Elsevier, doi.org/10.1016/j.rineng.2021.100216.

Ashleigh Townsend, “Optimising hydrogen based energy storage systems using super capacitors”, Master dissertation, Promoters: Prof. R. Gouws, Co-promoters: C. Martinson and Prof. D.G. Bessarabov, Faculty of Engineering, North West University, 2021.

One of the greatest challenges when it comes to the efficient energy use in unmanned aerial vehicles is that of the energy storage systems. As drones require minimal weight to have optimal mobility the capacity of the energy source is reduced to decrease the weight. This reduction decreases the flight time of a drone. Batteries overcome the weight issue but have a low power density and perform poorly at peak power demands. A larger power source is found in hydrogen fuel cells that possess a very high energy density, but this comes with an increase in weight and they too have a low power density. A solution to this problem is found in the hybrid combination of these two power sources. However, as they both have low power densities, they still perform poorly at peak power requirements. Super-capacitors are presented as a solution to this problem as they have high power densities and respond significantly better to peak power demands. Previously, super-capacitors have been combined with fuel cells, and in some cases batteries, to improve the functionality of hydrogen vehicles, with small improvements observed and suggestions made to incorporate all three power sources to supply the required load.

The research presented in this dissertation requires the analysis of a hybrid system designed with drone application in mind. Due to the low energy density of super-capacitors many are required to deliver a sufficient voltage level and an increase in the quantity leads to an increase in weight – in some cases – preposterously. This research thus presents the solution of combining the super-capacitors with an appropriate DC-DC converter to assess the effect thereof on the super-capacitors’ capacity as well as the operation of the hybrid system as a whole. The proposed DC-DC converter was verified through simulation and validated alongside the entire system through laboratory implementation. A load profile obtained from an existing hydrogen fuel cell drone was used to assess the experimental operation of the system. Six tests were conducted using the different power sources and the load profile. The fuel cell system and super-capacitor bank were all tested individually for the first two tests; the next two tests consisted of combining the super-capacitor bank with the fuel cell and a switching module – these tests consisted of two rounds varying the order of connection; two additional tests were conducted with the inclusion of the DC-DC converter.

From the results obtained through the experimental tests the fuel cell – super-capacitor combination utilizing a DC-DC converter was seen to provide the longest duration of 365 s, the highest energy and power density of 0.70 Wh/kg and 73.5 W/kg, respectively, and an overall efficiency of 96.25%. In obtaining these results the objectives and requirements of this research were met. The findings of this dissertation are not restricted to drone applications as they include outcomes pertaining to super-capacitors and their combination with a DC-DC converter.

Publication emanated from the thesis: “A comprehensive review of energy sources for unmanned aerial vehicles, their shortfalls and opportunities for improvements” in Heliyon 6.11 (2020) Elsevier, doi.org/10.1016/j.heliyon.2020.e05285.

Kyatsinge Cédric Musavuli, “Catalytic microchannel reactor development for the removal of carbon monoxide from hydrogen-rich gas streams”, Master dissertation, Promoters: Nicolaas Engelbrecht, Raymond Everson and Dmitri Bessarabov, Faculty of Engineering, North-West University, 2020.

Currently, hydrogen (H2) fuel cells are among the fastest emerging clean power generation technologies worldwide. The emergence of H2 energy and fuel cells for power generation originates from the renewable energy sector, and the storage of renewable energy in the form of a chemical energy carrier to provide power grid balancing. At the moment, H2 production is still dominated by fossil fuel processing, leading to carbon-based impurities such as carbon monoxide (CO) in the H2 streams––which is known to deactivate fuel cell anode catalysts. Selective methanation and preferential oxidation of CO are two common methods of catalytic CO removal from H2-rich gas streams.

In this work, three of the most popular catalysts used for CO abatement (Ni-Pt/Al2O3, Au/Al2O3, and Ru-Cs/Al2O3) were tested for their ability to remove ca. 1.4 vol.% CO from a synthetic H2-rich gas stream. Stainless steel microchannel reactors, containing the three respective washcoated catalysts, were used during the experimental work. Due to micro-scale dimensions of the channels, limit gas diffusional effects were expected, with the channels providing close contact between the bulk gas phase and the catalytic layer. Experiments were conducted isothermally at reaction temperatures of 80–400°C (depending on the CO abatement reaction applied), and space velocities of 32.6–130.4 NL.gcat-1.h-1. The Ru-Cs/Al2O3 catalyst was found to be the most suitable catalyst. CO concentrations lower than 100ppm were obtained via CO preferential oxidation at reaction temperatures of 120–180°C, with a peak CO conversions of more than 99.7% at 120–140°C and space velocities of 65.2–97.8 NL.gcat-1.h-1. This corresponds to CO levels as low as 42 ppm in the product gas. The conversion of H2 did not exceed 6.5%.

Additionally, the inability to characterise the dynamic region and the transport phenomena within the microchannels led to a theoretical study of the preferential CO oxidation process. A full three-dimensional model (using COMSOL Multiphysics® V4.4 software) showed that CO oxidation was the dominant reaction in a temperature range of 80–160°C. At temperatures above 160°C, the effects of the RWGS reaction was more pronounced, leading to a noticeable decrease in the CO conversion. Kinetic approximations were used to validate the reactor model to the full set of experimental data, and the model fit was noticed to yield accurate approximations of CO conversion in the simulated microchannel reactor over the range of reaction parameters.

It has been demonstrated that microchannel reactor technology is suitable for CO removal from H2-rich gas streams by preferential oxidation, at relatively low reaction temperatures (below 200°C) and high gas throughput compared to the reactor’s physical size. H2 originating from fossil or bio-based processes can be successfully treated to near complete CO purification standards, before fuel cell technology is applied for power generation purposes.

2020

Leandri Vermaak, “High temperature electrochemical hydrogen membrane separation using a PGM-based catalyst”, Master dissertation, Promoters: Hein Neomagus and Dmitri Bessarabov, Faculty of Engineering, North-West University, 2019.

Hydrogen, as energy carrier, is expected to play an indispensable role in the future energy prospects. Subsequently, significant advancements in research and development of hydrogen-based technologies and infrastructure are required. Among several technologies under consideration for hydrogen infrastructure, the use of an electrochemical hydrogen separator is very probable, since both hydrogen purification/separation and hydrogen compression (for storage purposes) are integrated and addressed. Some advances have already been made in this field, including the development of high-temperature membranes doped with phosphoric acid, to overcome the limitations associated with conventional low-temperature membranes. However, limited studies had been performed on these membranes and their operations under various conditions and feed compositions are largely unexplored.

The first part of this dissertation, addresses the aforementioned issue through experimental investigation of the performance of a high-temperature TPS-based membrane under various feed compositions (containing CH4, CO2 and NH3, balance hydrogen) over a temperature range of 100-160°C. The performance parameters used included polarisation curves, electrochemical impedance spectroscopy, hydrogen purity, hydrogen separation selectivity, hydrogen flux/permeability, and general efficiencies (current, voltage and power). The second part of the work focussed on the poisoning effect of CO on Pt. An integrated approach of high-temperature operation and Pt-Ru as bimetallic catalyst was implemented and tested with a 2% CO (balance hydrogen) inlet. The performance of Pt-Ru/C and Pt/C was compared under the same operating conditions. The electrochemical active surface area was then determined to evaluate the CO poisoning of the two catalysts. In generaral, Pt-Ru showed better CO tolerance over the entire temperature range (80-160°C). Also, temperature played a crucial role in the mitigation of CO poisoning in the case of Pt-Ru.

2019

Carel Minnaar, “Comparative study of power converters for coupling a renewable source to an electrolyser“, Master dissertation, Promoters: Prof Andre Grobler, Prof D.G. Bessarabov and Dr Gerhard Human, Faculty of Engineering, North West University, 2019.

Renewable energy sources are receiving much attention due to their clean and sustainable properties. The problem with these renewable energy sources is that they do not provide stable and constant power. Instead, they are only able to provide intermittent power when certain conditions are met. Therefore, to maximise the energy utilization from these renewable energy sources, the energy must be stored in such a way that no harmful by-products or gasses are formed. Hydrogen gas, produced from water using electrolysis, can be used to store solar and wind energy. Photovoltaic panels are conventionally used to power an electrolyser using a power converter. This configuration is both simple to implement and very environmentally friendly. Transitioning to hydrogen as an energy source will also ensure a more secure source of energy as there is less risk of depletion and price volatility that can result in economic pressures. An LLC resonant converter is a kind of DC/DC power converter that is able to achieve zero voltage switching across a wide load range by utilizing the transformer leakage and magnetizing inductances in series with a capacitor, hence the term LLC. Therefore, these power converters are associated with very high efficiencies up to 97%. Presented here is the design, simulation, test and evaluation of an LLC resonant converter for coupling a photovoltaic array to a polymer electrolyte membrane water electrolyser (PEMWE). The resonant converter will be compared to a hard switched converter. A microprocessor is used to control the power converter to ensure that the solar panels are operating at their maximum power point which will result in optimal hydrogen production and maximum overall system efficiency.

Retha Peach, “Development of novel PBI-blended membranes for SO2 electrolysis”, PhD Thesis, Promoter: Prof HM Krieg, Co-promoters: Dr AJ Krüger, Dr DG Bessarabov and Dr JA Kerres, Faculty of Agricultural and Natural Sciences, North-West University, 2019.

Novel PBI-blended membranes were synthesized and evaluated in terms of membrane composition (polymer, cross-linking and ratio), H2SO4 stability (ex situ) and SO2 electrolyser performance (in situ) to identify suitable proton exchange membranes (PEMs) for future SO2 electrolyser applications both below and above 100 °C. For this purpose, partially fluorinated and non-fluorinated acidic and basic polymer components were blended to obtain 4-component PBI-based blend membranes with ionic and covalent crosslink combinations. After a harsh sulphuric acid treatment the chemical and thermal suitability of blend membranes for SO2 electrolysis operations were monitored using % weight changes, ion-exchange capacities (IECs) and thermal gravimetric analysis (TGA). For SO2 electrolysis evaluation, a sufficiently stable and conductive PBI-blend membrane (1Ai) was selected and found to surpass the performance obtainable with a benchmark Nafion®115 at 80 °C. At 120 °C, a decrease in cell voltage of nearly 50% was obtained while reaching a maximum current density (1 A/cm2) double that achieved when operating at 80 °C. It was concluded that the combined fluorinated nature of acidic (SFS) and basic (F6PBI, BrPAE-1) polymer components contributed to the development of a compatible PBI-blended membrane (1Ai) with improved H2SO4 stability and sufficient conductivity for SO2 electrolyser application, both at 80 and 120 °C.

2018

Neels le Roux, “Complexation of palladium(II) with monovalent anionic ligands – a spectrophotometric investigation”, Doctoral thesis, Promoters: Promoters: Cobus Kriek, Department of Chemistry, North-West University, 2018.

The hydrometallurgical processing of platinum group metals (PGMs) requires exact knowledge of the speciation (identity and distribution of specific metallic species/complexes in solution) of the metal complexes involved. The stability constants (also known as formation constants) convert into Gibbs free energies that, when different, result in a shift of the stability regions of different complexes that are illustrated by a Pourbaix diagram.

Information on the stability constants of palladium complexes with monovalent ligands (i.e. thiocyanate, chloride, bromide and mixed palladium(II) chloro-hydroxo and palladium(II) bromo-hydroxo complexes) in solution is not readily available and quite limited. The majority of the methodsemployed involved the preparation of individual solutions with varying concentrations of the base metal, ligand or both. These techniques yielded only a few useable data points. To improve on this experimental shortcoming, an improved automated titration method was developed and employed to investigate the spectrophotometry of these complexes at 25 C and an ionic strength of 1.0 M. This new experimental method was developed by combining two specialized analytical apparatus that made it possible to automatically vary the ligand concentration over various intervals ensuring a very accurate and consistent set of absorbance data. The ideal titration conditions were determined by employing the Hyperquad Simulation and Speciation (HySS) software, while the stability constants for all of the palladium systems were calculated with the software program HypSpec. HypSpec software is specifically developed for the determination of stability constants from spectrophotometric data.

The stability constants, log β n , determined in this study for the palladium(II) thiocyanate complexes [Pd(SCN) n (H 2 O) 4–n ] 2–n (n = 0 – 4), which are generally accepted to be square planar, are: log β 1 = 8.14, log β 2 = 15.46, log β 3 = 21.94, and log β 4 = 27.42. However, a five-coordinated species, i.e. [Pd(SCN 5 ] 3– , with a formation constant of log β 5 = 31.94, would seem to exist (proposed to be square pyramidal) and form part of the system. These published results represent the first complete set of communicated stability constants.

The experimental procedure was customized to determine the stability constants for the palladium(II)-chloride and -bromide complexes. For [PdCl n (H 2 O) 4–n ] 2–n (n = 0 – 4), the log β values are: log β 1 = 4.49, log β 2 = 7.80, log β 3 = 10.18 and log β 4 = 11.54, while the log β values for [PdBr n (H 2 O) 4–n ] 2–n (n = 0 – 4), are: log β 1 = 5.04, log β 2 = 9.12, log β 3 = 12.38 and log β 4 = 14.55. For the chloride system our results represent a refinement of already published data, whereas our results for the bromide system represent only the second full set of published stability constants that are deemed to be much more accurate and reliable. Similarly, the experimental procedure was again modified and applied to the mixed palladium(II) chloro-hydroxo and palladium(II) bromo-hydroxo complexes. The stability constants for the mixed chloro-hydroxo complexes [PdCl 4n (OH) n ] 2 (n = 0 – 4), are:

log β 1 = 18.36, log β 2 = 23.21, log β 3 = 26.91 and log β 4 = 29.68. The stepwise stability constants of the palladium(II) bromo-hydroxo system, [PdBr 4n (OH) n ] 2 (n = 0 – 4), are as follows: log β 1 = 18.73, log β 2 = 22.25, log β 3 = 25.58 and log β 4 = 28.47. For the chloro-hydroxo system our results represent a refinement of already published data, whereas our results for the bromo-hydroxo system represent the first set of stability constants.

The newly developed experimental technique, linking and automating a double burette auto-titrator with a UV-vis spectrophotometer equipped with a flow-through cuvette, was applied to five different palladium(II) systems with great success. This technique can be applied to similar metal-ligand systems for the accurate determination of stepwise stability constants.

Faan du Preez, “Hydrogen generation by the reaction of mechanochemically activated aluminium and water”, Master dissertation, Promoters: Hein Neomagus and Dmitri Bessarabov, Faculty of Engineering, North-West University, 2018.

This dissertation presents a method to generate on-demand and pure hydrogen from neutral pH water using a hydrolysing material, i.e. mechanochemically activated aluminium (Al), under standard ambient conditions. The individual and combined effects of the considered activation compounds, i.e. bismuth (Bi), indium (In), and tin (Sn), on Al during mechanochemical processing were evaluated. Of importance in this study were i) composite hydrolysis reactivity towards water, ii) the effects of activation compounds on Al particle behaviour during mechanochemical activation, i.e. cold-welding, strain hardening, fracturing, and iii) the distribution of activation compounds in Al particles. Several activation compound combinations were considered for investigation, i.e. Bi-In-Sn, Bi-In, Sn- In and Bi-Sn. SEM and EDS analyses were applied to determine particle morphology and surface/subsurface chemical compositions of Al particles pre- and post mechanochemical activation procedures. Scanning electron microscopy (SEM) energy dispersive x-ray spectrometer (EDS) results presented in this study suggests that the considered activation compounds could be distributed relatively homogeneously throughout Al particles by mechanochemical activation. Such a distribution promoted micro-galvanic activity between anodic Al and cathodic Bi, In, and Sn. X-ray diffraction indicated various intermetallic phase formation between Al-activation compound and activation compound-activation compound. These phases formed as a result of mechanochemical activation and in some cases affected the structural failure and/or reactivity of Al particles. Numerous high hydrogen yielding (&gt;95%) composites were prepared. Furthermore, a preliminary method to recover activation compounds from hydrolysed Al using common acids was proposed.

Carel Minnaar, “Comparative study of power converters for coupling a renewable source to an electrolyser”, Master dissertation, Promoters: Andre Grobler, Dmitri Bessarabov and Gerhard Human, Faculty of Engineering, North-West University, 2018.

Renewable energy sources are receiving much attention due to their clean and sustainable properties. The problem with these renewable energy sources is that they do not provide stable and constant power. Instead, they are only able to provide intermittent power when certain conditions are met. Therefore, to maximise the energy utilization from these renewable energy sources, the energy must be stored in such a way that no harmful by-products or gasses are formed. Hydrogen gas, produced from water using electrolysis, can be used to store solar and wind energy. Photovoltaic panels are conventionally used to power an electrolyser using a power converter. This configuration is both simple to implement and very environmentally friendly. Transitioning to hydrogen as an energy source will also ensure a more secure source of energy as there is less risk of depletion and price volatility that can result in economic pressures. An LLC resonant converter is a kind of DC/DC power converter that is able to achieve zero voltage switching across a wide load range by utilizing the transformer leakage and magnetizing inductances in series with a capacitor, hence the term LLC. Therefore, these power converters are associated with very high efficiencies up to 97%. Presented here is the design, simulation, test and evaluation of an LLC resonant converter for coupling a photovoltaic array to a polymer electrolyte membrane water electrolyser (PEMWE). The resonant converter will be compared to a hard switched converter. A microprocessor is used to control the power converter to ensure that the solar panels are operating at their maximum power point which will result in optimal hydrogen production and maximum overall system efficiency.

2017

Bianca Friend, “Modelling and experimental characterization of an ionic polymer metal composite actuator”, Master dissertation, Promoters: Andre Grobler, George van Schoor and Dmitri Bessarabov, Faculty of Engineering, North-West University, 2017.

This study is about modelling an ionic polymer metal composite (IPMC) actuator and the experimental characterization thereof. In this study a brief background on IPMCs are given to the reader and then the research that has been done in various fields that are of importance to this study was discussed. From this the equipment required to develop an experimental setup was determined. The experimental setup was designed and mainly consist of a load cell, a laser displacement sensor, a data acquisition system, and a clamp for the IPMC.

A grey box model was used that consist of an electrical equivalent circuit and an electromechanical model. The model was implemented in Simulink and was verified by using parameters and results from literature. The parameter estimation that was done in Simulink was also verified with those values. The model was developed for a Nafion N117 sample plated with a Platinum loading of 10 mgPt/cm2. The model could sufficiently predict the absorbed current and the blocked force.

The behaviour of 7 different samples were investigated. Samples varied in terms of the Platinum loading and the membrane thickness. The response of each sample was investigated for different input voltages. The influence of input voltage, input frequency, humidity and temperature was also investigated. It was seen that the amplitude of the input voltage, the relative humidity and the temperature effect the response of the IPMC greatly. The experimental data from the sample the model was developed for was used to validate the model. The model could suffienctly predict the blocked forces and the displacement for a step input.

Isabella Ndlovu, “An evaluation of a microchannel reactor for the production of hydrogen from formic acid”, Master dissertation, Promoters: Raymond Everson, Steven Chiuta, Hein Neomagus, Henrietta Langmi, Jianwei Ren and Dmitri Bessarabov, Faculty of Engineering, North-West University, 2017.

This dissertation evaluates the performance of a microchannel reactor for the decomposition of vaporised formic acid as a promising technology for the production of hydrogen for proton exchange membrane fuel cell applications. Accordingly, a combined experimental and modelling approach was used to evaluate the microchannel reactor coated with a gold supported on alumina (1.15 wt. % Au/Al2O3) catalyst. For the experimental evaluation, two sets of experiments were carried out where pure formic acid (99.99 %) and dilute formic acid (50 vol. %) were taken as the feed to the reactor respectively. The first set of the experimental evaluation involved measuring key performance parameters such as, formic acid conversion, formic acid residual concentration, selectivity to hydrogen and hydrogen yield at different temperatures of 250 – 350°C and formic acid (99.99 %) vapour flowrates of 12 – 48ml/min. Overall, the reactor performed well in decomposing pure formic acid (99.99 %), achieving conversions (98 to 99 %) close to equilibrium at 350 oC and all studied vapour formic acid flowrates of 12- 48 ml/min. The highest hydrogen production rate (0.04mol.h-1) was measured at the highest formic acid flowrate of 48 ml/min when the reactor was operated at 350 oC. At all studied temperatures however, both dehydrogenation (HCOOH → H2 +CO2) and dehydration (HCOOH → H2O+CO) reactions occurred and the dehydrogenation reaction was found to be dominant. The dehydration reaction was mostly favoured at high temperatures and carbon dioxide concentrations ranged between 4 – 15 % while the corresponding selectivity towards H2 production ranged between 0.7 and 0.88.  Effort was made to improve the H2 yields in the second set of the experiments through decomposing a mixture of formic acid and water (50/50 vol. %) thereby promoting the occurrence of the forward water gas shift reaction. Under these conditions, carbon monoxide concentrations decreased to a range of 2 – 7 % while selectivity towards hydrogen production increased to a range of 0.84 – 0.94. Overall, the reactor was found stable at a continuous period of 144 hours after running for approximately 1 200 hours. A computational fluid dynamic model was developed for concentrated formic acid (99.99 %) experiments aimed at describing reaction-coupled transport phenomena relating to velocity, mass and temperature profiles within the microchannel reactor. Kinetic rate expressions that best described the experimental results were successfully estimated using a model-based parameter optimisation and refinement on Comsol Multiphysics™ 4.3b. Validation of the model against the experimental results showed that the developed model was an acceptable fit to the experimental conversions and hydrogen yields especially at temperatures higher than 250 oC. Overall, this dissertation highlights the first steps in the development and use of microchannel reactors in promoting formic acid as a future hydrogen storage medium for portable and distributed fuel cell applications.

2016

Nicolaas Engelbrecht, “Carbon dioxide methanation in a catalytic microchannel reactor”, Master dissertation, Promoters: Raymond Everson, Steven Chiuta, Hein Neomagus and Dmitri Bessarabov, Faculty of Engineering, North-West University, 2016.

The work reported in this dissertation demonstrated the practicality of a catalytic microchannel reactor for CO2 methanation implemented via the Sabatier reaction for potential power-to-gas applications. A combined experimental and computational fluid dynamic (CFD) modelling approach was used to evaluate the microchannel reactor washcoated with an 8.5 wt.% Ru/Al2O3 catalyst. For the experiments, a stoichiometric feed ratio (1:4) of CO2 and H2 was used. The reactor was evaluated for CO2 methanation at different reaction temperatures (250‒400°C), pressures (atmospheric, 5 bar and 10 bar), and gas hourly space velocities (32.6–97.8 NL.gcat-1.h-1). The highest CO2 conversion of 96.8% was achieved for the lowest space velocity (32.6 NL.gcat-1.h-1) and conditions corresponding to 375°C and 10 bar. The CH4 production was however maximised operating the reactor at conditions corresponding to high space velocity (97.8 NL.gcat-1.h-1), high temperature (400°C) and at 5 bar. At this operating point the reactor showed 83.4% CO2 conversion, 83.5% CH4 yield and high CH4 productivity (16.9 NL.gcat-1.h-1). The microchannel reactor demonstrated good long-term performance and no observable catalyst deactivation even after start-stop and continuous cycles, thereby proving its ability to handle dynamic operation required for power-to-gas applications. A CFD model was developed and used to interpret the experimental reactor performance, as well as provide fundamental insight into the reaction-coupled transport phenomena within the reactor. Most importantly, global kinetic rate expressions were developed using model-based parameter estimation. Results from the CFD model corresponded with good agreement to the experimental reactor performance in terms of CO2 conversion and CH4 yield over a wide range of operating parameters. The model also provided velocity and concentration distributions to better understand the transport principles established within the reactor. Overall, the results presented in this dissertation pinpointed the important aspects of realising CO2 methanation at the micro-scale and could provide a platform for future studies using microchannel reactors for power-to-gas applications.

Wikus Kirsten, “Characterisation pf proton exchange membranes using high-pressure gas membrane rupture test”, Master dissertation, Promotors: Hein Neomagus and Dmitri Bessarabov, Faculty of Engineering, North-West University, 2016.

A biaxial tensile testing method was proposed to characterise the viscoelastic properties that affect the mechanical durability of proton exchange membranes. It served as a good representation of the operational environment found within electrochemical hydrogen energy systems, replicating stresses induced on the constrained membranes. The highpressure membrane rupture test was used to determine the Young’s modulus of Nafion® membranes at three temperatures (20 ℃, 50 ℃ and 80 ℃) and four relative humidity levels (35 %, 50 %, 70 % and 90 %). The results showed that the Young’s modulus decreases with increased temperature and RH with the change in temperature having a significantly larger effect. Nafion® 1110 membrane samples were found to have a higher rupture pressure at sub-zero temperatures than at the studied temperature larger than 0 ℃. It was also shown that the properties of the membrane remain constant for the two temperatures.  Nafion® 1110 membranes were subjected to ion exchange with cations (Na+, Mg2+ and Fe3+). An increase in the Young’s modulus was observed with the presence of foreign cations as a result of reduced moisture uptake. Reinforced membranes were ruptured at 90 % RH and 50 ℃ with the rupture pressures compared to Nafion® membranes with similar thicknesses at the same environmental conditions. The rupture pressure of the reinforced membranes showed a nearly 100 % increase in strength compared to that of the Nafion® membranes. It is therefore clear that the e-PTFE layer of the reinforced membranes strongly improves the mechanical strength of the specimen. Unhydrolyzed perfluorinated membranes were partially hydrolysed for up to 46 hours to investigate the effect of the equivalent weight of the membrane specimen on the mechanical strength. These tests showed that the equivalent weight of the specimens decreased as the hydrolysis time increased, which in turn resulted in an increase of the rupture pressure of the specimen at 50 % RH and 50 ℃.

Gerhardus Human, “Power management and sizing optimisation of renewable energy hydrogen systems”, PhD Thesis, Promotors: George van Schoor and Kenny Uren, Faculty of Engineering, North-West University, 2016.

An integrated sizing and control optimisation strategy for renewable energy hydrogen production systems is developed. The aim was to provide insight into the design of these systems which comprise complex non-linear components and intermittent renewable energy sources, in this case wind and sun. A multi-objective strength Pareto evolutionary algorithm to optimise a simulated system model that he developed and configured for three different geographic sites is implemented. A reduced fuzzy rule base was derived for each site, resulting in reduced complexity and allowing ease of rule interpretation. The unique contributions are the insight into the relationship between system design and performance parameters, and also the process followed to generate Pareto optimal solutions.

Angelique Janse van Rensburg, “Multi-scale model of a valve-regulated lead-acid battery with electromotive force characterization to investigate irreversible sulphation”, PhD Thesis, Promotors: George van Schoor and Pieter Andries van Vuuren, Faculty of Engineering, North-West University, 2016.

Operating modes that cause irreversible sulphation (IS) in a lead-acid battery were investigated using a multi-scale electrochemical model. Experimental data from a valve-regulated lead-acid battery with an immobile electrolyte were used in a novel concentration-based method for electromotive force (EMF) characterization. The EMF curve was used for model calibration during parametric analysis of the multi-scale model. Variance-based sensitivity analysis confirmed that the parameters for electrode kinetics have the most significant effect on the simulated voltage. After verification and experimental validation of the multi-scale model, it was used to simulate partial state-of-charge (SOC) operation. It was found that the available active surface area suffers irreversible decreases due to minor errors in SOC indication. Additionally, the internal resistance during the initial voltage drop increases from one discharge to the next. It was concluded that IS cannot be prevented satisfactorily using SOC information because SOC is not indicative of a specific damage mechanism.

Tshiamo Segakweng, “Hydrogen physisorptive storage in metal-organic frameworks (MOFs)”, MSc, Dissertation, Study leaders: Philip Crousea, Jianwei Renb, Henrietta Langmi, University of Pretoria, bHySA Infrastructure CSIR.

The overall aim of this study was to evaluate the potential offered by MOFs for hydrogen storage. Three types of MOFs were studied namely: Zn-based MOF (MOF-5); Zr-based MOF (UiO-66); and Cr-based MOF (MIL-101). The study investigated the optimisation of the three different MOFs for hydrogen storage by gaining an understanding of their optimal synthesis conditions. It was shown that a simple variation in the synthesis of the MOFs was able to have a significant effect on the hydrogen storage properties of the MOFs, and enable a fast and reproducible synthesis method. The synthesised MOFs were characterised and their hydrogen adsorption capacity measured. Hydrogen uptake was found to be related to the quality of the MOF crystals and also directly related to the surface area, pore volume and/or pore size of the particular MOF.

2015

Steven Chiuta, “Experimental and modelling evaluation of an ammonia-fuelled microchannel reactor for hydrogen generation”, PhD., dissertation, Study Leaders: Raymond Everson; Hein Neomagus; Dmitri Bessarabov, Faculty of Natural Science, North-West University.

Ammonia decomposition was assessed as a fuel processing technology for producing hydrogen on-demand for fuel cell applications. An experimental study of an ammonia-fuelled microchanel reactor was undertaken, and the performance data subsequently used to develop and validate a mathematical model for the effective design of microchannel reactors for ammonia decomposition. The work described in the thesis was motivated by the need for a convenient and efficient method of providing hydrogen for a proton-exchange membrane fuel cell (PEMFC) in portable and distributed power applications.

Andries Krüger, “Evaluation of process parameters and membranes for SO2 electrolysis”, PhD., dissertation, Study Leader: Henning Kriega and Dmitri Bessarabovb, Faculty of Natural Science, Chemical Resource Beneficiationa, Faculty of Engineeringb, North-West University.

The use of the SO2 electrolyser was evaluated for the production of both hydrogen gas and sulfuric acid.  Various operating parameters were investigated for the SO2 electrolyser which included cell temperature, catalyst loading, membrane thickness and MEA manufacturing procedure.  Using the optimised operating conditions the performance of the SO2 electrolyser was evaluated using electrochemical impedance spectroscopy (EIS) to separate the kinetic and membrane resistance while the mass transport limitations could be quantified.  The impact of H2S presence in the SO2 feed was also investigated using cyclic voltammetry and CO stripping to determine the electrochemical surface area (ECSA).  The results obtained showed that the SO2 electrolyser could possibly be applied in a mining environment.  To further increase the electrolyser efficiency various PBI based membranes were also evaluated for application in the SO2 electrolyser environment.

2014

Marcelle Potgieter, “A comparative study between Pt and Rh for the electro-oxidation of aqueous SO2 and other model electrochemical reactions”, M.Sc., Thesis, Study Leaders: Cobus Kriek; Vijay Ramani, Faculty of Natural Science, North-West University.

Adri Calitz, “A comparative study between Pt and Pd for the electro-oxidation of aqueous SO2 and other model electrochemical reactions”, M.Sc., Thesis, Study Leaders: Cobus Kriek; Vijay Ramani, Faculty of Natural Science, North-West University.

Francois Stander, “Evaluation of an advanced fixed bed reactor for sulphur trioxide conversion to sulphur dioxide using supported platinum catalysts”, PhD., Dissertation, Study Leaders: Ray Everson; Hein Neomagus, Faculty of Engineering, North-West University.

2013

Bongibethu Hlabano-Moyo, “Seperation of SO2/O2 using membrane techynology”, M.Eng., Thesis, Study Leaders: Percy van der Gryp; Dmitri Bessarabov; Henning Krieg, Faculty of Engineering, North-West University

Anzel Falch, “Synthesis, characterisation and potential employment of Pt-modified TiO2 photocatalysts towards laser induced H2 production”, M.Sc., Thesis, Study Leaders: Cobus Kriek; Vijay Ramani, Faculty of Natural Science, North-West University.

Christiaan Martinson, “Characterisation of a proton exchange membrane electrolyser using the current interrupt method”, M.Eng., Thesis, Study Leaders: Kenny Uren; George van Schoor; Dmitri Bessarabov, Faculty of Engineering, North-West University.

Jan-Hendrik van der Merwe, “Characterisation of proton exchange membrane electrolyser using electrochemical impedance spectroscopy”, M.Eng., Thesis, Study Leaders: Kenny Uren; George van Schoor; Dmitri Bessarabov, Faculty of Engineering, North-West University.

Rudolph Petrus Louw, “Design optimisation and costing analysis of a renewable hydrogen system”, M.Eng., Thesis, Study Leader: Willie Venter, Faculty of Engineering, North-West University.

Martinus (Gerhard) de Klerk, “Development of a simulation model for a small scale renewable energy system”, M.Eng., Thesis, Study Leader: Willie Venter, Faculty of Engineering, North-West University.

Richard Sutherland, “Performance of different proton exchange membrane water electrolyser components”, M.Eng., Thesis, Study Leaders: Henning Krieg; Percy vd Gryp; Dmitri Bessarabov, Faculty of Engineering, North-West University.

Sammy Rabie, “SO2 and O2 separation by using ionic liquid absorption”, M.Eng., Thesis, Study Leader: Marco le Roux, Faculty of Engineering, North-West University.

 2012

Boitumelo Mogwase, “An electrochemical study of the oxidation of platinum employing ozone as oxidant and chloride as complexing agent”, M.Sc., Thesis, Study Leader: Cobus Kriek, Faculty of Natural Science, North-West University.

Andries Kruger, “SO2 electrolyser development for hydrogen production with the hybrid sulfur process”, M.Sc., Thesis, Study Leader: Henning Krieg, Faculty of Natural Science, North-West University.

Hugo Opperman, “A theoretical and experimental approach to SO2 permeation through Nafion and a novel sFS-PBI membrane”, M.Sc., Thesis, Study Leader: Henning Krieg, Faculty of Natural Science, North-West University.

Hannes Schoeman, “H2SO4 stability of PBI blend membranes for SO2 electrolysis”, M.Sc., Thesis, Study Leader: Henning Krieg, Faculty of Natural Science, North-West University.

Boitshoko Modingwane, “Investigation of Pt supported on carbon, ZrO2, Ta2O5 and Nb2O5 as electrolycatalysts for the electro-oxidation of SO2“, M.Sc., Thesis, Study Leader: Cobus Kriek, Faculty of Natural Science, North-West University.

HME Dreyer, “A comparison of catalyst application techniques for membrane electrode SO2 depolarised electrolysers”, M.Eng., Thesis, Study Leader: Johan Markgraaf, Faculty of Engineering, North-West University.

JH Hodgman, “The feasibility and application of multi-layer vacuum insulation for cryogenic hydrogen storage”, M.Eng., Thesis, Study Leader: Johan Markgraaf, Faculty of Engineering, North-West University.

Herman Retief, “A review of hydrogen storage for vehicular application and the determination of the effect of extraction boil-off”, M.Eng., Thesis, Study Leader: Johan Markgraaf, Faculty of Engineering, North-West University.

Morne Coetzee, “Upscaling of a sulfur dioxide depolarized electrolyser”, M.Eng., Thesis, Study Leader: Johan Markgraaf, Faculty of Engineering, North-West University.

Marco van Rhyn, “Recuperation of H2SO4 in the hybrid sulfur process using prevaporation”, M.Eng., Thesis, Study Leaders: Marco le Roux; Percy vd Gryp, Faculty of Engineering, North-West University.

Dian Kemp, “Technical evaluation of the copper chloride water splitting cycle”, M.Eng., Thesis, Study Leader: Ennis Blom, Faculty of Engineering, North-West University.

Marizanne Gouws, “Evaluation of the reduction of CO2 emissions from a coal-to-liquids utilities plant by incorporating PBMR energy”, M.Eng., Thesis, Study Leader: Ennis Blom, Faculty of Engineering, North-West University.

Liberty Mapamba, “Simulation of the copper-chloride thermochemical cycle”, M.Eng., Thesis, Study Leaders: Percy vd Gryp; Mike Dry, Faculty of Engineering, North-West University.

2011

Bothwell Nyoni, “Simulation of the sulphur iodine thermochemical cycle.”, M.Eng. (Chemical Enbgineering) Thesis, Study Leaders: Percy van der Gryp; Mike Dry, Faculty of Engineering, North-West University.

The demand for energy is increasing throughout the world, and fossil fuel resources are diminishing. At the same time, the use of fossil fuels is slowly being reduced because it pollutes the environment. Research into alternative energy sources becomes necessary and important. An alternative fuel should not only replace fossil fuels but also address the environmental challenges posed by the use of fossil fuels. Hydrogen is an environmentally friendly substance considering that its product of combustion is water. Hydrogen is perceived to be a major contender to replace fossil fuels. Although hydrogen is not an energy source, it is an energy storage medium and a carrier which can be converted into electrical energy by an electrochemical process such as in fuel cell technology. Current hydrogen production methods, such as steam reforming, derive hydrogen from fossil fuels. As such, these methods still have a negative impact on the environment. Hydrogen can also be produced using thermochemical cycles which avoid the use of fossil fuels. The production of hydrogen through thermochemical cycles is expected to compete with the existing hydrogen production technologies. The sulphur iodine (SI) thermochemical cycle has been identified as a high-efficiency approach to produce hydrogen using either nuclear or solar power. A sound foundation is required to enable future construction and operation of thermochemical cycles. The foundation should consist of laboratory to pilot scale evaluation of the process. The activities involved are experimental verification of reactions, process modelling, conceptual design and pilot plant runs. Based on experimental and pilot plant data presented from previous research, this study presents the simulation of the sulphur iodine thermochemical cycle as applied to the South African context. A conceptual design is presented for the sulphur iodine thermochemical cycle with the aid of a process simulator. The low heating value (LHV) energy efficiency is 18% and an energy efficiency of 24% was achieved. The estimated hydrogen production cost was evaluated at $18/kg.