Modified Nafion membranes for hydrogen production in Cu-Cl thermochemical cycle

Abstract

The decomposition of water by a thermochemical cycle is a promising green method of producing hydrogen on a large scale. A thermochemical cycle is a combination of thermal and chemical reactions that ultimately leads to splitting water into hydrogen and oxygen. The Cu-Cl cycle, which is one of the most promising hybrid cycles, has many potential advantages. First, all the copper chloride compounds used in the cycle are recyclable. Second, the copper chlorine cycle has relatively low operating temperatures (500-600 °C) compared to other thermochemical cycles. Third, it can have high energy efficiency by utilizing low-grade waste heat. One step in the Cu- Cl cycle employs an electrochemical process where Cu(I) is oxidized to Cu(II) at the anode and hydrogen is generated at the cathode. The electrochemical cell used is similar to a water electrolysis cell and employs a membrane electrode assembly (MEA) that utilizes a proton exchange membrane (PEM). Nafion, the most widely used PEM, is permeable to cations and neutral species. Because of this, Cu can permeate the membrane and enter the cathode where it is readily oxidized to metallic copper and deposited on Pt catalytic sites. This drastically reduces cathode efficiency as well as the whole cell. In this research, we have modified Nafion membranes, with different thicknesses, by in situ polymerization with pyrrole and aniline compounds. Ex situ permeation experiments were performed along with impedance spectroscopy to investigate the rate of the copper crossover and the proton conductivity of the composite membranes, respectively. The electrochemical behavior of the composite membranes was investigated using a full copper chlorine electrolytic cell. The efficiency of both copper conversion and hydrogen production were used as indicators of the membrane performance.