Electrochemical Oxidation of Carbon at High Temperature: Principles and Applications

Abstract

Carbon is an important energy carrier. It is abundant in the world, existing richly in coal and biomass. Its energy is released mainly through oxidation. A heat energy of ∼393 kJ may be obtained from the complete oxidation of 1 mol of carbon. The most conventional method of carbon oxidation is combustion, which has the problems of pollutant emission and low efficiency. However, through electrochemical oxidation, the chemical energy of carbon can be converted into electricity with high efficiency and low pollution. Herein, we give a brief review of our work on a novel technology of generating electricity and carbon monoxide from electrochemical oxidation of carbon through a solid oxide fuel cell (SOFC) at high temperature. This so-called direct carbon SOFC (DC-SOFC) has an all-solid-state configuration without any purging gas or liquid medium. Electricity is generated by coupling of the electrochemical oxidation of CO at the anode and the reverse Boudouard reaction at the carbon fuel. Meanwhile, gaseous CO and CO2 are produced. We prepared electrolyte-supported and anode-supported SOFC single cells and stacks and operated them directly with Fe-loaded activated carbon, biochar derived from orchid leaf and corn cob, respectively, as the fuel. In particular, we investigated their potential applications in portable power supplies and electricity-gas cogeneration. Our experimental results show that the output performance of a DC-SOFC is comparable to that of a SOFC operated on hydrogen. A 3-cell stack of tubular anode-supported segmented-in-series DC-SOFC gives a peak areal power density of 465 mW cm–2 and a volumetric power density of 710 mW cm–3 at 850 °C. Furthermore, the composition of the gas emission can be controlled by tuning the operating electrical current and the catalysts applied for the Boudouard reaction so that CO gas and electricity cogeneration can be realized.

A steel slag–derived Boudouard reaction catalyst for improved performance of direct carbon solid oxide fuel cells

“…Novel technologies to utilize solid carbon cleanly and efficiently are urgently demanded for a sustainable energy future. Direct carbon fuel cells (DCFCs) are power generation devices that directly convert the chemical energy of solid carbon to electricity via electrochemical routines . Without the constriction of the Carnot cycle, DCFCs offer much high‐energy conversion efficiency (slightly more than 100% in theory or approximately 80% in practice), which is more than twice that obtained from conventional coal‐fired power plants .…”

“…Direct carbon fuel cells (DCFCs) are power generation devices that directly convert the chemical energy of solid carbon to electricity via electrochemical routines . Without the constriction of the Carnot cycle, DCFCs offer much high‐energy conversion efficiency (slightly more than 100% in theory or approximately 80% in practice), which is more than twice that obtained from conventional coal‐fired power plants . Furthermore, since the gas‐tight electrolyte of DCFCs just allows O 2 in the air to transport to the anode in ionic forms, the highly concentrated CO 2 product streams can be directly captured for industrial use or sequestration .…”

“…The hybrid DCFCs totally eliminate the CO 2 recirculation and management needed in molten carbonate DCFCs . Solid oxide electrolyte DCFCs, also called direct carbon solid oxide fuel cells (DC‐SOFCs), are the only type of DCFCs with all‐solid‐state structure . As one of the most promising types of fuel cells, SOFCs have a variety of electrode materials and configurations to suit different types of fuel .…”

“…As one of the most promising types of fuel cells, SOFCs have a variety of electrode materials and configurations to suit different types of fuel . Compared with the DCFCs with molten electrolytes, DC‐SOFCs are free from the troubles of corrosion of cell components and leakage of molten matter . However, the efficient mass transfer of solid carbon in the solid anode is the major challenge that DC‐SOFCs must face .…”

“…A series of investigations have essentially revealed that the anode processes of DC‐SOFCs are the coupling of the reverse Boudouard reaction (R1, Figure ) and the electrochemical oxidation of CO (R2, Figure ) . In fact, the anode processes are kinetically controlled ones . Even the operation of DC‐SOFCs is in a steady state, still no thermodynamic equilibrium could arrive .…”