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 .

The high ionic strength and complex speciation of the anolyte solution within the CuCl(aq)/HCl(aq) electrolytic cell have impeded predictions of the energy requirements for the cell's electrolytic reaction at 25 °C and 1 bar. After collecting experimental open circuit potential (OCP) data and comparing the values obtained with predictions from prospective thermodynamic models, an approach to predict thermodynamic values and the overall efficiency was formulated. The compositions of the experimental measurements ranged from 2-2.5 mol of CuCl(aq) with 8-9 mol of HCl(aq) per kilogram of water in anolyte solution and 8-9 mol of HCl(aq) per kilogram of water in catholyte solution. From the OCP data, it was found that

An electrochemical kinetics investigation of the CuCl(aq)/HCl(aq) electrolyzer identified methods to significantly reduce the platinum loadings required to achieve a high cell current density of 0.5 A/cm 2 at 0.7 V. As the CuCl(aq)/HCl(aq) electrolyzer is a key component of the Cu-Cl thermochemical cycle, the economic viability of the Cu-Cl thermochemical cycle was significantly improved by reducing the loading required to achieve 0.5 A/cm 2 . Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) with a rotating disc electrode were employed to investigate the kinetics of the aqueous Cu II /Cu I chloride complexes reaction on platinum and glassy carbon using a three-electrode cell. It was found that the standard exchange current density of the anodic Cu II /Cu I electrochemical reaction on platinum, 4-12 A/cm 2 , was significantly larger than the values reported for the HER cathodic reaction thus far. In addition, SEM was used to observe the effectiveness of different catalyst application techniques. Through SEM observations, and electrochemical data analysis, the amount of platinum used in a laboratory scale CuCl(aq)/HCl(aq) electrolyzer was reduced from 0.8 mg/cm 2 applied to both electrodes to 0.4 mg/cm 2 on the cathode and zero at the anode while still maintaining a current density of 0.5 A/cm 2 at 0.7 V of applied potential difference. As interest in an energy storage option capable of storing both thermal and electrical energy produced via solar resources increases, low temperature hybrid thermochemical cycles are becoming an attractive candidate to fill this role. In particular, the Cu-Cl hybrid thermochemical cycle's high efficiency and moderate temperature requirements have established itself as a promising option for inexpensive hydrogen generation through harnessing excess thermal and electrical energy from solar resources.1 In addition to utilizing excess energy, the Cu-Cl thermochemical cycle provides a means of efficiently producing hydrogen gas not reliant on fossil fuels.The Cu-Cl hybrid thermochemical cycle uses a number of intermediate compounds, heat, and a small amount of electrical energy, within a series of physical and chemical reactions to split water into hydrogen and oxygen.2-10 One of the most important components in the hybrid cycle is the CuCl(aq)/HCl(aq) electrolytic cell. 11 In the electrolytic step of this cycle, the general electrochemical reaction consists of an anode reaction in which aqueous Cu I chloride complex species are oxidized to aqueous Cu II chloride complex species, and a cathode reaction in which HCl(aq) is reduced to H 2 (aq) with transfer of H + (aq) through a cation conductive membrane. This process is usually simplified as follows:To increase the solubility of CuCl(s) in the anolyte, a high concentration of HCl(aq) is typically used. Similarly, the catholyte comprises of highly concentrated HCl(aq) to avoid copper deposition at the cathode. 9Previous research established an electrolytic cell design that employed membrane electrode assemb...

The Cu-Cl thermochemical cycle is a promising technology for hydrogen production due to moderate temperature requirements and compatibility with commercially demonstrated renewable heat sources, such as the solar power tower. CuCl/HCl electrolysis, the main hydrogen-producing step of the cycle, has been investigated using a newly developed electrolyzer system supplemented by a number of electrochemical characterization techniques. Nafion- based membranes and Pt/C catalysts were used to prepare membrane electrode assemblies for these tests. The anolyte contained 1 to 2 mol·L-1 CuCl dissolved in 6 mol·L-1 HCl(aq), and the catholyte was a 6 mol·L-1 HCl(aq) solution. The results of the study showed that the applied potential of 0.7 V can produce the current density around 0.5 A·cm-2 for a period of 24 hours at a temperature of 80 °C and ambient pressure. The hydrogen production efficiency was found to be at 91-99 % without any decline during the test. No copper deposition was observed after the test on any component of the electrolyzer.

A CuCl/HCl electrolytic cell was operated at different temperatures to quantify the effect of temperature on the overall performance. Polarization curves and EIS data were taken at 40, 60 and 80 °C to observe the changes in ohmic, charge-transfer and interfacial resistance. Our research showed a significant increase in the cell performance as the temperature was increased from 40 to 80 °C. While the polarization data were used to observe the overall increase in the current density in a specific range of applied potential, the corresponding EIS data showed a decrease in the ohmic and charge transfer resistance. It was also observed that pressure applied on the end plates during cell assembly has a significant effect on the ohmic resistance. An optimum pressure of 6.3 psi (bolt torque: 20 Nm) showed the best performance. This paper demonstrates how changes in the signature of EIS spectra with temperature reflect the cell performance.