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The Cu-Cl thermochemical cycle is among the most attractive technologies proposed for hydrogen production due to moderate temperature requirements and high efficiency. In this study, the key step of the cycle, H 2 gas evolution via oxidation of CuCl͑s͒ dissolved in high concentrated HCl͑aq͒, was experimentally investigated. The electrolysis parameters and system performance were studied by linear sweep voltammetry and electrochemical impedance spectroscopy at ambient temperature. Promising performance of the electrolyzer was obtained when pure water was used as catholyte. A thermodynamic model previously developed for speciation of the CuCl-CuCl 2 -HCl aqueous solutions was used to speculate on the effects of reagent concentration, flow rate, and temperature on electrolysis kinetics. The experimental decomposition potential necessary to initiate the hydrogen evolution reaction was more than 3 times lower than the potential necessary for water electrolysis at the same conditions. Close correspondence of the hydrogen production rate to Faraday's law of electrolysis indicated the current efficiency of about 98%, while the voltage efficiency was estimated at 80% at 0.5 V and 0.1 A/cm 2 .

This contribution provides an overview of available experimental, thermodynamic, and molecular data on Au aqueous speciation, solubility, and partitioning in major types of geological fluids in the Earth's crust, from low-temperature aqueous solution to supercritical hydrothermal-magmatic fluids, vapours, and silicate melts. Critical revisions of these data allow generation of a set of thermodynamic properties of the AuOH, AuCl−2, AuHS, and Au(HS)−2 complexes dominant in aqueous hydrothermal solutions; however, other complexes involving different sulphur forms, chloride, and alkali metals may operate in high-temperature sulphur-rich fluids, vapours, and melts. The large affinity of Au for reduced sulphur is responsible for Au enrichment in S-rich vapours and sulphide melts, which are important gold sources for hydrothermal deposits. Thermodynamic, speciation, and partitioning data, and their comparison with Au and S contents in natural fluid inclusions from magmatic-hydrothermal gold deposits, provide new constraints on the major physical-chemical parameters (temperature, pressure, salinity, acidity, redox) and ubiquitous fluid components (sulphur, carbon dioxide, arsenic) affecting Au concentration, transport, precipitation, and fractionation from other metals in the crust. The availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations