The potentiometric method is one of the most exact techniques of electrochemistry. Its use is widespread in both routine measurements, such as pH determination, and in applications which demand the highest accuracy. Perhaps its severest limitation is the error due to liquid junction potentials. Although this has been the subject of much study, and although various means of combating it have been proposed, it has defied all attempts at a satisfactory solution. Indeed, so serious is the position that it has become the accepted practice in the more accurate fields to abandon altogether the use of potentiometric techniques in favour of others, usually less convenient and otherwise less accurate, when the presence of liquid junctions cannot be avoided. Intro. p. 1.

As the world is moving towards a more sustainable energy perspective, construction equipment sees the requirement to change its current way of operation with fossil fuels to reduce its environmental impact. In order to pursue the electrification of construction equipment a dense power source is essential, where hydrogen powered fuel cells have the potential to be a sufficient energy source. This thesis work is carried out in order to find the least CO2 emissive pathway for hydrogen to various construction sites. This is done by collecting state of the art data for production, processing and storage technologies. With the assembled data an optimization model was developed using mixed integer linear programming. The technologies found that showed promising adaptability for construction equipment in the state of art regarding production were steam methane reforming (SMR), proton exchange membrane electrolyser (PEMEC) and alkaline electrolyser. They showed promising characteristics due to their high level of maturity and possibility for reducing the environmental impact compared to the current operation. To investigate the hydrogen pathway and its possibilities, four scenarios were created for four types of construction sites. The scenarios have different settings for distance, grid connection and share of renewables, where the operations have various energy profiles that is to be satisfied. The optimal hydrogen pathway to reduce the CO2 emissions according to the model, were either PEMEC on-site or gaseous delivery of SMR CCS produced hydrogen. The share of renewables in the energy mix showed to be an important factor to determine which of the hydrogen pathways that were chosen for the different scenarios. Moreover, in the long run PEMEC was considered to be a more sustainable solution due to SMR using natural gas as feedstock. It was therefore concluded that for a high share of renewables PEMEC was the optimal solution, where for a low share of renewables SMR CCS produced hydrogen was optimal as the energy mix would result in a more emissive operation when using PEMEC.

Thesis (M. Sc. (Chemistry)) -- University of Limpopo, 2017. / In order to meet the great demand of energy supply globally, there must be a transition from dependency on fossil fuel as a primary energy source to renewable source. This can be attained by use of hydrogen gas as an energy carrier. In the context of hydrogen fuel cell economy, an effective hydrogen generation is of crucial significant. Hydrogen gas can be produced from different methods such as steam reforming of fossil fuels which emit greenhouse gases during production and from readily available and renewable resources in the process of water electrolysis. Hydrogen generated from water splitting using solar energy (photocatalysis) or electric energy (electrocatalysis) has attracted most researchers recently due to clean hydrogen (without emission of greenhouse gases) attained during hydrogen production. In comparison with photocatalytic water splitting directly using solar energy, which is ideal but the relevant technologies are not yet commercialized, electrolysis of water using catalyst is more practical at the current stage. The platinum group noble metals (PGMs) are the most effecting electrocatalysts for hydrogen evolution reactions (HER) but their scarcity and high cost limit their application. In this study, we presented the noble metal free organic-inorganic hybrid composites and their HER electrocatalysis performances were investigated. Polyaniline-metal organic framework (PANI/MOF) composite was prepared by chemical oxidation of aniline monomer in the presence of MOF content for hydrogen production. The properties of PANI, MOF and PANI/MOF composite were characterised for their structure and properties by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), Raman, transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), atomic absorption spectroscopy (AAS), square wave (SWV) and cyclic voltammetry (CV). There was a clear interaction of MOF on the backbone of the PANI matrix through electrostatic interaction as investigated by both Raman and FTIR. The MOF exhibited irregular crystals with further wrapping of MOF by PANI matrix as evidenced by both SEM and TEM analyses. The PANI composite exhibited some nanorods and microporous structure. x The determined energy band gap of the composite was in good agreement with previously reported catalysts for hydrogen evolution reaction (HER). The thermal stability of PANI increased upon addition of MOF. Experiments probing the electrochemical, HER and photophysical properties revealed that the composite was very stable and robust with significant improvement in properties. The resulting composite is a promising low-cost and environmentally friendly hydrogen production material. In this work we also reported about novel poly (3-aminobenzoic acid)-metal organic framework referred as PABA/MOF composite. Spectroscopic characterisations (UV-vis and FTIR) with support of XRD and TGA revealed a successful interaction between PABA and MOF. Morphological characterisation established that PABA is wrapping MOF and the amorphous nature of the materials were not affected. The catalytic effect of PABA and PABA/MOF composites on HER was studied using exchange current density and charge transfer coefficient determined by the Tafel slope method. A drastic increase in catalytic H2 evolution was observed in PABA and composite. Moreover, they merely require overpotentials as low as ~-0.405 V to attain current densities of ~0.8 and 1.5 Am-2 and show good longterm stability. We further demonstrated in the work the electrocatalytic hydrogen evolution reaction of MOF decorated with PABA. These novel MOF/PABA composites with different PABA loading were synthesised via in situ solvothermal synthesis of MOF in the presence of PABA. It was deduced that PABA with different loading amount have an influence on the morphologies, optical properties and thermal stabilities of MOF. Interestingly, the MOF/PABA composites exhibited the great significant on the HER performance and this is potentially useful in HER application for hydrogen fuel cell. / Sasol Inzalo foundation and National Research Foundation of South Africa

Philosophiae Doctor - PhD / Today fossil fuels such as oil, coal and natural gas are providing for our ever growing energy needs. As the world’s fossil fuel reserves fast become depleted, it is vital that alternative and cleaner fuels are found. Renewable energy sources are the way of the future energy needs. A solution to the looming energy crisis can be found in the energy carrier hydrogen. Hydrogen can be produced by a number of production technologies. One hydrogen production method explored in this study is electrolysis of water.

Hartree-Fock, Moller-Plesset, and density functional theory calculations have been carried out using 6-31+G(d), 6-31+G(d,p) and 6-31++G(d,p) basis sets to study the properties of low-barrier or short-strong hydrogen bonds (SSHB) and their potential role in enzyme-catalyzed reactions that involve proton abstraction from a weak carbon-acid by a weak base. Formic acid/formate anion, enol/enolate and other complexes have been chosen to simulate a SSHB system. These complexes have been calculated to form very short, very short hydrogen bonds with a very low barrier for proton transfer from the donor to the acceptor. Two important environmental factors including small amount of solvent molecules that could possibly exist at the active site of an enzyme and the polarity around the active site were simulated to study their energetic and geometrical influences to a SSHB. It was found that microsolvation that improves the matching of pK as of the hydrogen bond donor and acceptor involved in the SSHB will always increase the interaction of the hydrogen bond; microsolvation that disrupts the matching of pKas, on the other hand, will lead to a weaker SSHB. Polarity surrounding the SSHB, simulated by SCRF-SCIPCM model, can significantly reduce the strength and stability of a SSHB. The residual strength of a SSHB is about 10--11 kcal/mol that is still significantly stable compared with a traditional weak hydrogen bond that is only about 3--5 kcal/mol in any cases. These results indicate that SSHB can exist under polar environment. Possible reaction intermediates and transition states for the reaction catalyzed by ketosteroid isomerase were simulated to study the stabilizing effect of a SSHB on intermediates and transition states. It was found that at least one SSHB is formed in each of the simulated intermediate-catalyst complexes, strongly supporting the LBHB mechanism proposed by Cleland and Kreevoy. Computational results on the activation energy for catalyzed and uncatalyzed model reactions shows that strong hydrogen bonding between catalyst and the substrate at the transition state can significantly reduce the activation energy. This implies that LBHBs are possibly playing a crucial role in enzyme catalysis by supplying significant stabilizing energy to the reaction transition state.

No description available.

No description available.

No description available.

No description available.

The friction force of an Annco iron-on-Annco iron sliding system was measured in laboratory air, nitrogen, and hydrogen. The coefficient of friction for each environment was calculated and the amplitude of the "stick-slip" behavior from each environment was observed. It was found that the coefficient of friction obtained in the hydrogen environment was significantly smaller than the values obtained in laboratory air and nitrogen. Also, the amplitude of the "stick-slip" behavior observed in a hydrogen environment was less than that obtained in laboratory air and nitrogen. These results were attributed to a decrease in the fracture strength of the Annco iron along the interface between the pin and the disk which was caused by the presence of gaseous hydrogen. The surface energy and decohesion models for hydrogen embrittlement, both mechanisms which explain brittle behavior, were considered valid in this test. The localized plasticity model of hydrogen embrittlement was considered invalid in this test. Copper, a noble metal not susceptible to hydrogen embrittlement, was also used in friction tests. Tests conducted with copper-on-copper sliding systems showed no statistical difference between the coefficient of friction and the amplitude of the "stick-slip" behavior as the gaseous environment was varied between laboratory air, nitrogen, and hydrogen. Finally, recommendations for further study and testing were presented. The recommendations included changing the gaseous hydrogen pressure to both high and vacuum levels, raising the temperature of the hydrogen gas, and using anyone of the great number of iron-based alloys susceptible to hydrogen embrittlement. / Master of Science