Effects of electrodes, current density, and dynamic operation on solid carbon production in molten carbonate electrolysis

Abstract

The urgency to reduce CO2 emissions has accelerated research into electrochemical conversion of CO2 into valuable products, such as solid carbon, carbon monoxide gas, and hydrocarbons. Investigations into each of these target products are ongoing, with the goals of achieving high production efficiency, high purity, and low production cost. The CO2 electrolysis for solid carbon production involves a cathode and an anode that are immersed in molten salt electrolytes. When a potential difference is applied, the reduction reaction at the cathode produces solid carbon, while oxidation at the anode generates oxygen gas.

The carbon product from CO2 electrolysis in molten salt, commonly referred to as electrolytic carbon, exhibits diverse morphologies including graphite, carbon nanotubes (CNTs), spherical-like onion carbon, and amorphous carbon. Metal elements or compounds originating from the cathode and the anode are commonly detected through scanning electron microscopy (SEM) and X-ray diffraction (XRD). Some of these metals are separated from the carbon particles, while others remain on the surface or are incorporated within the carbon particles. Electrolytic carbon has numerous potential applications, such as a lithium-ion battery anode, an adsorbent, and bucky paper. Despite its promising properties, commercialization remains challenging because production costs are significantly higher compared with carbon obtained through conventional methods.

In this doctoral dissertation, experimental studies were conducted to investigate the effects of electrode pair selection, applied current density, and dynamic operating conditions using various waveforms during electrolysis on the properties of the solid carbon product. Additionally, the influence of dynamic operation on electrical power consumption during electrolysis was examined. Titanium, nickel, and nickel-based superalloy (Alloy X) were tested as cathode and anode materials in a molten lithium carbonate. Among the electrode pairs evaluated, a titanium cathode with a nickel anode (Ti–Ni) demonstrated the most stable voltage performance, achieving the highest Faraday efficiency of 62% and voltage efficiency of 48%. The SEM analysis of carbon produced using a Ti–Ni pair revealed predominant spherical-like onion structures, whereas tubular and amorphous carbon morphologies were observed when an Alloy X cathode and a titanium anode (Ax–Ti) were employed.

Subsequently, using the Ti–Ni electrode pair, current densities ranging from 0.1 to 0.4 Acm−2 were applied. The highest Faraday efficiency of 67% and voltage efficiency of 52% were achieved at 0.1 Acm−2. Notably, the trend of Faraday efficiency did not correlate with the magnitude of applied current density. The SEM analysis indicated that the applied current density had no significant effect on the morphology of the electrolytic carbon, although it slightly influenced the diameter of the spherical onion-like shape of the carbon structure.

Furthermore, dynamic operations using triangular, square, sine, ramp-up, and ramp-down waveforms were applied during electrolysis. The SEM analysis revealed that waveform variation had a minimum impact on the carbon morphology, with most products exhibiting spherical-like onion structures, and a small fraction of tubular and hollow sphere structures. An analysis of electrical power consumption during electrolysis under dynamic operation indicated higher electrical power consumption compared with constant DC, due to additional losses introduced by the alternating current component. Ripple loss under dynamic operation increased with amplitude, and among the tested waveforms, the square waveform produced the highest ripple loss.