Feasibility of an oxygen-getter with nickel electrodes in alkaline electrolysers

Alkaline electrolysis is the long-established technology for water splitting to produce hydrogen and has been industrially used since the nineteenth century. The most common materials used for the electrodes are nickel and derivatives of nickel (e.g. Raney nickel). Nickel represents a cost-effective electrode material due to its low cost (compared to platinum group metals), good electrical conductivity and exhibits good resistance to corrosive solutions. The steady degradation of the nickel electrodes over time is known as a result of oxide layer formation on the electrode surface. Reducing oxide layer growth on the electrode surface will increase the efficiency and lifetime of the electrolyser. Titanium has a higher affinity to oxygen than nickel so has been introduced to the electrolyser as a sacrificial metal to reduce oxide layer formation on the nickel. Two identical electrolysers were tested with one difference: Cell B had titanium chips present in the electrolyte solution, whilst Cell A did not have titanium present. SEM results show a reduction of 16 % in the thickness of the Cell B oxide layer on nickel compared to the Cell A nickel, which is supported by the large increase in oxide layer build-up on the titanium in Cell B. EDX on the same samples showed on average a 59 % decrease in oxygen on the Cell B nickel compared to Cell A. XPS surface analysis of the same samples showed a 17 % decrease in the oxygen on Cell B nickel. These results support the hypothesis that adding titanium to an alkaline electrolyser system with nickel electrodes can reduce the oxide layer formation on the nickel

Structure-property relationships in metal-organic frameworks for hydrogen storage

Experimental hydrogen isotherms on several metal-organic frameworks (IRMOF-1, IRMOF-3, IRMOF-9, ZIF-7, ZIF-8, ZIF-9, ZIF-11, ZIF-12, ZIF-CoNIm, MIL-101 (Cr), NH2-MIL-101 (Cr), NH2-MIL-101 (Al), UiO-66, UiO-67 and HKUST-1) synthesized in-house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analyzed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF-7 and ZIF-9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of "breathing", a phase transition in which the pore space increases due to adsorption. This work presents novel correlations using the modelled total hydrogen capacities of several MOFs. These capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume capacities of on-board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand.</p

Structure-property relationships in metal-organic frameworks for hydrogen storage

Experimental hydrogen isotherms on several metal-organic frameworks (IRMOF-1, IRMOF-3, IRMOF-9, ZIF-7, ZIF-8, ZIF-9, ZIF-11, ZIF-12, ZIF-CoNIm, MIL-101 (Cr), NH2-MIL-101 (Cr), NH2-MIL-101 (Al), UiO-66, UiO-67 and HKUST-1) synthesized in-house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analysed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF-7 and ZIF-9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of “breathing”, a phase transition in which the pore space increases due to adsorption. This work presents novel and more useful correlations using the modelled total hydrogen capacities of several MOFs. These total hydrogen capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume of on-board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand

PIM-MOF Composites for Use in Hybrid Hydrogen Storage Tanks

It is well understood that fossil fuels (coal, oil and natural gas) are non-renewable and their combustion is a major driver of anthropogenic climate change, and therefore replacement energy sources and energy vectors must be found. Hydrogen has long been touted as an alternative energy vector, due to its very high gravimetric energy density, and that its full combustion releases only water as a side product. However, hydrogen is a very sparse gas, and as a result its volumetric energy density is very low, making hydrogen storage a difficult technical challenge. The current industrial standard for hydrogen storage in vehicles is compression, in which hydrogen gas is compressed to 70 MPa. This technique has a high energy penalty, and safety concerns due to the high pressure. The tank must also be made out of low weight, high strength carbon fibre composite, which incurs a large economic cost. One alternative solution is adsorption, in which high surface area microporous use the physical bonding between hydrogen molecules and the solid surface area to achieve high hydrogen storage densities at reasonable pressures and temperatures.One potential class of materials for use in such a class are polymers of intrinsic microporosity (PIMs), which are polymeric materials featuring spiro-centres in their molecular chains that cause kinking and thus inefficient packing of the material, which leads to an inherent microporosity. These materials are attractive for a hybrid adsorption-compression tank due to their flexibility and processability. The central material being investigated in this study, PIM-1, is soluble in polar-aprotic solvents such as chloroform and tetrahydrofuran, and forms robust, flexible films upon solvent casting [1]. However, the BET surface area of these films is relatively low (~ 600 m2 g-1) [1], and therefore hydrogen storage capacity must be added for this material to achieve the U.S. Department of Energy targets for hydrogen storage systems [2]. This project aims to achieve this by incorporating crystals of MOF-5, a well understood metal-organic framework (MOF) that has been receiving industrial attention due to its high BET surface area and hydrogen storage capacity [3]. This work aims to synthesise and characterise PIM-1 and MOF-5 independendetly, before combining into a composite-type film material. Characterisation work on these materials is focussed on adsorption isotherms between 0 – 20 MPa at 77 K, using both nitrogen and hydrogen as sorbents. This work is supported through the use of helium pycnometry and thermogravimetric analysis. This characterisation data is compared between the materials, and the relationship between the characteristic properties of the composite and the ratio of materials in its composition is discussed. References:[1] P. M. Budd, E. S. Elabas, B. S. Ghanem, S. Makhseed, N. B. McKeown, K. J. Msayib, C. E. Tattershall &amp; D. Wang, Adv. Mater. 16 (2004) 456-459[2] United States Department of Energy (2009) http://energy.gov/sites/prod/files/2015/01/f19/fcto_myrdd_table_onboard_h2_storage_systems_doe_targets_ldv.pdf [Accessed 13/04/2015][3] M. Veenstra, J. Yang, C. Xu, M. Gaab, L. Arnold, U. Müller, D. Siegel &amp; Y. Ming (2014) http://www.hydrogen.energy.gov/pdfs/review14/st010_veenstra_2014_o.pdf [Accessed 13/04/2015

Fuel Gas Storage:The Challenge of Hydrogen

Among today’s societal challenges, arguably some of the most important are the safe, sustainable and affordable supply of clean water, food and energy. Energy is certainly a crucial challenge as demand is likely to increase greatly, due to the economic development of poorer nations and an increase in world population. This will put enormous pressure on the future exploration and production of energy sources. Perhaps an even more important aspect are current and predicted environmental problems associated with fossil fuels, the most notorious being climate change, which demands that we decarbonise our economies or risk catastrophic consequences. A solution for this problem would be to use a clean, sustainable energy system; one which converts, transports and uses energy safely, with no harmful emissions to the atmosphere and at an affordable cost.This clean and sustainable energy system would almost certainly require a great share in conversion of energy from renewable energy sources, and probably a clean energy vector, along with electricity, to decarbonise the transport sector and to balance supply and demand in the electric grid. Hydrogen is the one of the best alternatives for a clean and sustainable energy vector as it presents obvious advantages, among those the fact that it has the highest energy per unit mass of any chemical fuel, can be efficiently used in a fuel cell with no emissions, and can be produced, stored, distributed and used through a variety of different sustainable pathways.One of the biggest problems with hydrogen energy is its storage. Hydrogen is a very low density gas and methods of increasing its density, usually compression at 35 or 70 MPa or liquefaction at 20 K, carry high materials and energy penalties. Some proposed alternatives include chemical storage, which consists of reacting hydrogen with another element and storing it as a hydride or using highly porous materials to enhance the density of hydrogen on its surface. Adsorptive storage of hydrogen can be an attractive solution to the storage problem, as it can store equal amounts of hydrogen in the same volume at much milder conditions of pressure and temperature. Experiments on different adsorbent materials, including the metal-organic framework MIL-101 and activated carbons AX-21 and TE7, were done to identify and possibly tailor optimal materials for hydrogen storage. The results were analysed taking into account a number of different requirements for a hydrogen storage system, including capacity of the material, gravimetric and volumetric density, optimal operating conditions for storage, thermal management of the system and optimum kinetics and diffusion in adsorbent systems. These results were analysed in a systems approach context and used as input into the design of an improved adsorbent storage system

Mechanical characterisation of polymer of intrinsic microporosity PIM-1 for hydrogen storage applications

Polymers of intrinsic microporosity (PIMs) are currently attracting interest due to their unusual combination of high surface areas and capability to be processed into free-standing films. However, there has been little published work with regards to their physical and mechanical properties. In this paper, detailed characterisation of PIM-1 was performed by considering its chemical, gas adsorption and mechanical properties. The polymer was cast into films, and characterised in terms of their hydrogen adsorption at −196 °C up to much higher pressures (17 MPa) than previously reported (2 MPa), demonstrating the maximum excess adsorbed capacity of the material and its uptake behaviour in higher pressure regimes. The measured tensile strength of the polymer film was 31 MPa with a Young’s modulus of 1.26 GPa, whereas the average storage modulus exceeded 960 MPa. The failure strain of the material was 4.4%. It was found that the film is thermally stable at low temperatures, down to −150 °C, and decomposition of the material occurs at 350 °C. These results suggest that PIM-1 has sufficient elasticity to withstand the elastic deformations occurring within state-of-the-art high-pressure hydrogen storage tanks and sufficient thermal stability to be applied at the range of temperatures necessary for gas storage applications