Low-cost, low-carbon H2 is required to decarbonise fuels and chemicals. In this thesis, the costs and emissions inventories of H2 production methods were reviewed, assessed and a framework developed to determine the levelised cost of carbon mitigation and proportional decarbonisation fraction relative to the industry standard steam methane reforming. This analysis demonstrated CH4 pyrolysis to be a potentially cost-effective CO2 mitigation option for low-carbon H¬2 production by technology substitution, provided engineering challenges of carbon separation are surmountable. CH4 pyrolysis in molten salt bubble column reactors was identified as a promising, unexplored area of research with potential low-cost carbon separation advantages. In order to quantify specific reaction rates in molten salt pyrolysis systems, a novel analytical technique to characterise bubbles generated from single orifices in high temperature liquid systems based on pressure pulses was developed. The performance of simple alkali halide salts (NaCl, NaBr, KCl, KBr and NaBr:KBr (48.7:51.3 mol%)) as reactive media candidates in molten salt bubble columns for CH4 pyrolysis were subsequently evaluated to characterise the reaction rate dependence on temperature, CH4 partial pressure and surface area to volume ratios of bubbles generated in the melt. The carbon samples generated in each were all low-density (< 1g cm-3), highly porous and demonstrated relatively low-levels of structural order. The carbon separability and residual salt contamination were correlated to the carbon-salt adhesion energy. Suspensions of fluidised y-Al2O3 particles inside the molten salt bubble column (slurry reactor) were used as probing particles to elucidate mass transfer mechanisms at the surface of the bubble. It was found that the y-Al2O3 particles enhance the rate of reaction, potentially by acting as lower-energy nucleation sites for solubilised carbon precipitation. The carbon deposited on the surface of the particles was identified as the active site for the enhanced reaction rates observed.