Investigation of electrode material and applied current density in molten carbonate electrolysis

Abstract

Molten carbonate electrolysis is a technology that can convert CO₂ into valuable solid carbon and oxygen gas. Different materials of electrode pairs are considered in molten carbonate electrolysis, including a titanium cathode and a titanium anode (Ti–Ti), a titanium cathode and a nickel anode (Ti–Ni), a titanium cathode and an Alloy X anode (Ti–Ax), a nickel cathode and a titanium anode (Ni–Ti), and an Alloy X cathode and a titanium anode (Ax–Ti). The electrode pairs are assessed based on their electrolysis performance and characteristics of carbon, which are analyzed using scanning electron microscope (SEM), x-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). The results show that the Ti–Ni pair exhibits the most stable potential performance, the highest Faraday and voltage efficiency, the lowest specific electrical energy consumption, and the fastest carbon deposition rate. Moreover, regardless of the anode, the use of titanium as a cathode results in a spherical onion-like shape, while Alloy X as a cathode produces an amorphous structure with some tubular form.

Based on these results, the effect of the applied current density for the Ti–Ni electrode pair is investigated. The applied current density of 0.1 A cm⁻² shows the highest Faraday efficiency of 67.4% and voltage efficiency of 52.3%. It also has the lowest specific electrical energy consumption for a solid carbon product of 25.9 kW h kg⁻¹. The fastest carbon deposition rate is 0.027 g cm⁻² h⁻¹, which is obtained by applying a current density of 0.4 A cm⁻². The morphology of carbon produced with the applied current density between 0.1 and 0.4 A cm⁻² shows a dominant spherical-like onion shape with a larger diameter at a higher applied current density. Furthermore, some tubular forms are found as a result of the applied current densities of 0.2 A cm⁻² and 0.3 A cm⁻². Plate shapes are also observed, yet only as a result of the applied current density of 0.4 A cm⁻². This finding suggests that the applied current density affects the diameter of the carbon particles, but does not significantly change the morphology.