The generation of green hydrogen from water splitting by means of electrolysis is a promising strategy for integrating renewables into our fuels, where metal-organic frameworks (MOFs) have
been identified as materials of interest, offering excellent electrocatalytic activity towards hydrogen evolution reaction (HER). The aim of this study was to expand upon existing research and
investigate the impact of changing the organic linkers and metal composition on the structure of the MOF and its performance as HER catalyst. The methodology is presented for solvothermal
synthesis of the MOF followed by sample preparation for characterisation. In total, six materials were produced: three organic linkers (terephthalic acid, aminoterephthalic acid and dihydroxyterephthalic acid) were paired with two metal combinations (Ni-and-Fe, Ni-only). To investigate MOF structure, a range of characterisation methods were used, including SEM,
PXRD and ICP-MS. This was followed by electrochemical characterisation, involving custom two compartment electrolytic cell setup for HER experiments, including linear sweep voltammetry,
chronoamperometry and quantification of generated hydrogen. SEM and PXRD data shown that all investigated materials are crystalline, and in some cases highly crystalline, showing excellent coating of the nickel foam substrate. The electrochemical characterisation confirmed that all tested materials are excellent HER catalysts, with the Ni-ata MOF/NF achieving the faradic efficiency of up to 99.5%. With the use of crystallographic database, PXRD spectrum of the Ni-dhta MOF was matched to published structure, suggesting that it forms cyclic, hexagonal molecular structures that are interconnected at the sides, forming a honeycomb-like pattern. It was concluded that changing the linker is likely to change the MOF structure, which is not always true when changing the metal composition. The dihydroxyterephthalic acid was highlighted as the particularly interesting linker, for which change of metal composition resulted in virtually no change to the crystal structure. MOFs with this linker also shown high crystallinity and excellent surface coverage at macroscopic scale. The elemental analysis shown that there are significant differences between the metal ratio of MOF crystals and the bulk MOF powder, with the concentration of nickel increasing from Fe:Ni of 2:1 to approximately 1:1. Lastly, recommendations for future research were made, including improvements to the electrochemical characterisation and hydrogen quantification protocols, where particular care should be taken to ensure the electrochemical is gas-tight and no sample loss occurs during transfer.
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