Electrochemical Oxidation Of Carbon Research Articles

(Keynote) Insight into Carbon Corrosion of Different Carbon Supports for Pt-Based Electrocatalysts for Polymer Electrolyte Fuel Cells from Interfacial Perspective

A mechanistic understanding of carbon corrosion in polymer electrolyte fuel cells (PEFCs) is required to design catalyst layers that are durable. Electrochemical oxidation of carbon occurs during uncontrolled startup and shutdown events of PEFCs leading to multitude of degradation events, including loss of electrochemical surface area (ECSA), pore structure collapse, Pt detachment, loss in ionomer and others. To understand carbon corrosion physics and resulting degradation mechanisms a detailed electrochemical characterization is needed to probe interfaces between Pt-ionomer, carbon-ionomer and to measure transport properties and the ECSA in the catalyst layer. In this presentation we will provide an overview of carbon corrosion understanding and our approaches to quantify the sequence of events. In these studies, PEFCs were subjected to the Department of Energy carbon corrosion accelerated stress test (AST) protocol. Various electrochemical techniques were used to describe interfaces and morphology, as well as transport properties for the cells at different stages of the AST. ECSA measurements, polarization curves, CO-displacement/stripping to determine sulfonic acid group coverage in dry and wet conditions, electrochemical impedance spectroscopy, oxygen transport resistance measurements and other techniques were used. In addition, physical characterization methods at the beginning of life (BOL) and end of life (EOL) were used, such as micro x-ray fluorescence mapping, focused ion beam scanning electron microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy and Raman spectroscopy. A combination of electrochemical and physicochemical methods revealed that amorphous carbon oxidizes first, after which Pt detach resulting in the loss of the ECSA, followed by severe structure collapse, resulting in a poor oxygen transport resistance. We extend the study to carbon supports that are more graphitized to alleviate some of the issues observed with amorphous carbon support.

Molten Metals and Molten Carbonates in Solid Oxide Direct Carbon Fuel Cell Anode Chamber: Liquid Metal Anode and Hybrid Direct Carbon Fuel Cells

To enhance the contact between the electrolyte (source of O2−) and the carbon fuel in solid oxide–direct carbon fuel cells (SO-DCFCs), molten metals and molten salts were used in the anode chamber. Oxygen ions can dissolve and be transported in the molten medium to the anode three-phase boundary to reach and oxidize the carbon particles. To improve the sluggish kinetics of the electrochemical oxidation of carbon, the same molten media can act as redox mediators. Moreover, using a liquid metal/salt anode, tolerant to fuel impurities, the negative effect of carbon contaminants on cell performance is mitigated. In this work, an overview of SO-DCFCs with liquid metals, liquid carbonates, and mixed liquid metals/liquid carbonates in the anode chamber is presented and their performance was compared to that of conventional SO-DCFCs.

Analysis of Electrode Degradation By High-Potential Environment-Using Electrochemical Impedance Spectroscopy in Polymer Electrolyte Membrane Fuel Cells

A Polymer electrolyte membrane fuel cell (PEMFC) is one of the most important applications for decarbonization of energy using hydrogen as an energy source and is considered a promising future power source due to its high efficiency, low operating temperature, fast startup, and low noxious emissions. In particular, improvement in durability of PEMFC is one of the major issues to spur the commercialization of the PEMFC system. Therefore, to improve the durability of PEMFC systems, diagnostic strategies are required to identify faulty conditions of PEMFC systems such as fuel starvation, dehydration, flooding, load cycling, and startup-shutdown (SU/SD). The problematic form of the membrane electrode assembly (MEA) that causes a decrease in the performance of PEMFCs in steady-state and dynamic operating conditions includes Pt dissolution, ionomer degradation, carbon corrosion [1] , and mechanical problems of the membrane in some circumstances. Among them, the undesired corrosion reaction, namely, the electrochemical carbon oxidation reaction, results in a drastic decrease in electrochemical double layer capacitance (EDLC), generation of oxygen functional groups in carbon supports, and severe electrochemical surface area (ECSA) fading [2, 3], followed by deteriorated PEMFC performance. Typically, the high potential inducing carbon corrosion reactions would occur during SU/SD and fuel starvation conditions. In detail, as for fuel starvation, a temporary shortage of H2 supply to one or several cells in a PEMFC stack causes the cell voltage reversal, accompanied by a potential strike over 1.0 V vs. RHE (Figure 1) with significant electrode degradation [4 , 5]. Also, the generation of hydrogen/air boundary at the anode due to the gas crossover through the membrane results in a high potential (~1.4 V vs. RHE), accelerating the decomposition of carbon support [6]. Therefore, in this work, to investigate the effects of high potential inducing electrode degradation on PEMFC performance, we conducted accelerated stress tests (AST), and in particular, we studied the correlation between electrode degradation behaviors as well as structural properties of porous carbon such as ionic resistance, EDLC with PEMFC performance in the systemic point of view. Specially, we could see the contrasting degradation behaviors and various performance decay rates using electrochemical impedance spectroscopy (EIS) analysis. EIS can be used to isolate the contribution of many processes to performance loss, allowing investigation of the effect of carbon corrosion-induced catalyst layer changes on fuel cell performance [ 7]. The measured EIS data is evaluated through the parametric fitting of the transmission line model (TLM) to the EIS spectrum. In addition, relaxation time distribution (DRT) analysis for direct analysis of internal resistance factors by reinforcing the TLM-based impedance analysis results was applied as a diagnostic method to evaluate electrode degradation. During the ASTs, electrochemical measurements (i-V, CV, LSV, and EIS) were performed to estimate the degree of PEMFC degradation, we also conducted ex-situ surface analysis of MEA using SEM, TEM, and XPS to elucidate the specific degradation components.

Edge and defect sites in porous activated coke enable highly efficient carbon-assisted water electrolysis for energy-saving hydrogen production

Developing energy-efficient water electrolysis (WE) process is critical to the mass production of H2. As the theoretical thermodynamics potential for carbon oxidation into CO2 is 0.21 V, the introduction of carbon is possible to reduce the power consumption for WE to 1/6. Herein, we adopt three kinds of microporous activated coke to study the electrochemical carbon oxidation reaction (ECOR) properties. Experimental and computational results demonstrate that carbon atoms located at edges and defects could be active sites in ECOR, whereas CO groups generated after electro-oxidized inhibit the process of ECOR. Additionally, a new Fe(II)/Fe(III) redox-coupled hydrogen evolution reaction system is designed for WE. The assembled electrolyzer only requires a voltage of 1.2 V to achieve a current density of 97 mA cm−2 without any metal catalyst at the anode and the corresponding electricity consumption is 2.87 kWh Nm−3 (H2). These findings provide a promising strategy for low-cost, efficient H2 production.

A completely sealed high temperature carbon-air battery with carbon dioxide absorber

A completely sealed high temperature carbon-air battery is proposed and prepared using a tubular oxygen-ion-conducting yttrium stabilized zirconia electrolyte with anode inside and cathode outside. CaO and carbon powders, with a molar ratio of 0.5, 0.75, and 1.0, are respectively loaded in the anode chamber, in two ways: One is mixing CaO and C together (mixing mode) and the other is loading them one by one (separating mode). The electrochemical output performance and discharging time of the batteries increase with decreasing CaO/C ratio. Thermodynamic calculation results show that the pressure of the anode chamber of a battery increases with more carbon oxidized. CO 2 produced through the electrochemical oxidation of carbon can be absorbed by CaO to produce CaCO 3 , resulting in relieve of pressure. However, insufficient CO 2 for maintaining the operation of battery is also caused by CaO. This evidence may be mitigated by loading C and CaO separately to avoid immediate reaction between CO 2 and CaO. A carbon-air battery with 0.25 g carbon powder separately loaded with CaO in a CaO/C ratio of 0.5 discharges at 200 mA for 6.9 h, giving a practical specific energy density of 1300 Wh kg −1 , considering the mass of both C and CaO. • Carbon-air battery with C as anode active material and CaO as CO 2 absorber. • Battery based on YSZ electrolyte with the anode chamber completely sealed. • CaO relieves anode pressure but causes insufficient CO 2 for maintaining operation. • Loading CaO and C separately can mitigate the negative effect of CaO. • Battery with CaO and C separately loaded in a molar ratio of 0.5 gives 1300 Wh kg −1 .

Tracking oxygen atoms in electrochemical CO oxidation - Part II: Lattice oxygen reactivity in oxides of Pt and Ir

Electrochemical oxidation of carbon monoxide (CO oxidation) is often used as a model reaction to investigate the surface of metallic electrocatalysts, most notably in CO stripping experiments. In this report, we use chip-based electrochemistry mass spectrometry with 18O isotope-labeled oxides Pt and Ir to investigate the involvement of lattice oxygen in the electrochemical oxidation of water (the oxygen evolution reaction, OER), adventitious carbon, and CO. For Pt, we find that the labeled oxygen from Pt18Ox is incorporated into the CO2 resulting from CO oxidation at the potential at which oxygen at the electrode surface is reduced to hydroxyl (*OH), confirming that *OH is the reactive species in the Langmuir-Hinshelwood electrochemical oxidation of CO. For Ir we find that lattice oxygen in Ir18O2 is similarly involved in electrochemical CO oxidation, but only if it is first activated by a reductive sweep. The labeled CO2 signal is transient, indicating that activated lattice oxygen provides the “ignition sites” for the Langmuir-Hinshelwood electrochemical oxidation of CO on Ir. We also confirm the previously reported result that electrochemically prepared, amorphous, Ir18Ox incorporates much more lattice oxygen in O2 evolved during OER than does rutile Ir18O2, but we also quantify the amount and show that in all cases the labeled O2 is a very small portion of the total O2 evolved, and that more lattice oxygen is released in CO2 when oxidizing CO and adventitious carbon than is released in O2 when oxidizing water. Through these results, we demonstrate that EC-MS in concert with isotope labeling and CO as a probe molecule can provide insight into lattice oxygen reactivity, extending the utility of CO oxidation to the study of noble metal oxides used in e.g. PEM electrolyzer anodes.

Honeycombed Porous, Size-Matching Architecture for High-Performance Hybrid Direct Carbon Fuel Cell Anode.

Direct carbon fuel cells (DCFCs) demonstrate both superior electrical efficiency and fuel utilization compared to all other types of fuel cells, and it will be the most promising carbon utilization technology if the sluggish anode reaction kinetics that derives from the use of solid fuel can be addressed. Herein, the electrode morphology and fuel particle size are comprehensively considered to fabricate an efficient DCFC anode skeleton. A honeycombed and size-matching anode architecture with dual-scale porous structure is developed by water droplet templating, which demonstrates an efficient strategy to address the challenge of poor carbon reactivity and improve the electrochemical performance of DCFCs. Single cell with this designed anode framework demonstrates excellent performance, and the maximum power density is as high as 765 mW cm-2 at 800 °C when using the matching carbon fuel. The size-matching between carbon fuel and anode framework shows a remarkable effect on the improvement of mass-transfer processes at the anodes. The significant contribution of the difficult electrochemical oxidation of carbon to the output performance is also demonstrated. These results represent a promising structural design strategy in developing high-performing fuel cells.

Direct Carbon Fuel Cell-Cleaner and Efficient Future Power Generation Technology

Increasing greenhouse effect due to the burning of fossil fuels has stirred the attention of researchers towards cleaner and efficient technologies. Direct carbon fuel cell (DCFC) is one such emerging technology that could generate electricity from solid carbon like coal and biogas in a more efficient and environmental-friendly way. The mechanism involves electrochemical oxidation of carbon to produce energy and highly pure carbon dioxide. Due to higher purity, the produced carbon dioxide can be captured easily to avoid its release in the environment. The carbon dioxide is produced in a gaseous state while the fuel used is in a solid state. Due to different phases, all of the fuel can be recovered from the cell and can be reused, ensuring complete (100%) fuel utilization with no fuel losses. Moreover, DCFC operates at a temperature lower than conventional fuel cells. The electric efficiency of a DCFC is around 80% which is nearly double the efficiency of coal thermal plant. In addition, DCFC produces pure carbon dioxide as compared to the thermal power plant which reduces the cost of CO2 separation and dumping. In different types of DCFCs, molten carbon fuel cell is considered to be superior due to its low operating temperature and high efficiency. This paper provides a comprehensive review of the direct carbon fuel cell technology and recent advances in this field. The paper is focused on the fundamentals of fuel cell, history, operating principle, its types, applications, future challenges, and development.

Carbon xerogels electrochemical oxidation and correlation with their physico-chemical properties

The electrochemical oxidation of carbon represents a relevant limitation for the application of carbon-based materials at the electrodes of several electrochemical devices, like fuel cells, metal-air batteries, water electrolyzers or supercapacitors. Understanding the key influencing parameters is thus of paramount importance to prevent and avoid the degradation of carbon materials. In the present investigation, carbon xerogels (CXGs) as a model of synthetic amorphous carbon, were studied on the basis of different surface chemistry and structure. Electro-oxidation experiments consisted of potential holding (1.2 V vs. RHE) on electrodes prepared with CXGs. The results indicate that the presence of oxygen groups (-C-O) and a higher ordering degree hinder the carbon oxidation reaction. A sub-structural parameter considering both the amount of oxygen-free carbon atoms (C-C + C*-C-O) from X-ray photoelectron spectroscopy, and the disordering degree from Raman spectra (in terms of ID/IG) is here proposed as a new factor to evaluate the tendency of an amorphous carbon material, like xerogels, to be electrochemically oxidized.

Parallel implementation of a three-dimensional cellular automaton model of the electrochemical oxidation of carbon “Ketjenblack EC-600JD”

The paper presents a three-dimensional cellular automaton model of electrochemical oxidation of the carbon. The sample of the electro-conductive carbon black “Ketjenblack EC-600JD” consisting of granules of carbon is simulated. The electrochemical oxidation of the carbon granules occurs through a few successive stages. Parallel implementation of the three-dimensional cellular automaton model of carbon corrosion is developed. The efficiency and speedup of the parallel code are analyzed. The portions of surface carbon atoms and atoms with different degree of oxidation are computed by the parallel code. Based on the obtained values of atom portions the electrochemical capacity is calculated. The results of computer simulation are compared with the experimental data.

Electrochemical Oxidation of Carbon at High Temperature: Principles and Applications

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...

A self-regenerable soot sensor with a proton-conductive thin electrolyte and a nanostructured platinum sensing electrode.

In recent years, exhaust sensors have become increasingly attractive for use in energy and environmental technologies. Important issues regarding practical applications of these sensors, especially for soot measurements, include the further development of ion-conductive electrolytes and active electrode catalysts for meeting performance and durability requirements. Herein, we design a proton conductor with a high breakdown voltage and a sensing electrode with high sensitivity to electrochemical carbon oxidation, enabling continuous soot monitoring with self-regeneration of the sensor. A Si0.97Al0.03HxP2O7-δ layer with an excellent balance between proton conductivity and voltage endurance was grown on the surface of a Si0.97Al0.03O2-δ substrate by reacting it with liquid H3PO4 at 600 °C. Specific reactivity of the electrochemically formed active oxygen toward soot was accomplished by adding a Pt-impregnated Sn0.9In0.1HxP2O7-δ catalyst into a Pt sensing electrode. To make the best use of these optimized materials, a unipolar electrochemical device was fabricated by configuring the sensing and counter electrodes on the same surface of the electrolyte layer. The resulting amperometric mode sensor successfully produced a current signal that corresponded to the quantity of soot.

Observable Electrochemical Oxidation of Carbon Promoted by Platinum Nanoparticles.

The radical degradation of Pt-based catalysts toward oxygen reduction reaction (ORR), predominantly caused by the oxidation of carbon supports, heavily blocks the commercialization of polymer electrolyte membrane fuel cells (PEMFCs). As reported, the electrochemical oxidation of carbon could be accelerated by Pt catalysts; however, hitherto no direct evidence is present for the promotion of Pt catalysts. Herein, a unique ultrathin carbon layer (approximately 2.9 nm in thickness) covered Pt catalyst (Pt/C-GC) is designed and synthesized by a chemical vapor deposition (CVD) method. This magnifies the catalysis effect of Pt to carbon oxidation due to the greatly increased contact sites between the metal-support, making it easy to investigate the carbon oxidation process by observing the thinning of the carbon layer on Pt nanoparticles from TEM observations. Undoubtedly, this finding can better guide the structural design of the durable metal catalysts for PEMFCs and other applications.

Introducing Anode Fluidization into a Tubular Liquid Antimony Anode Direct Carbon Fuel Cell

Liquid antimony is considered to be a promising anode for direct carbon fuel cell. The pilot system built in this research is made up with fuel supplement unit, cooling sub-system and power generating core block with tubular SOFC (solid oxide fuel cell) planted into liquid antimony bath. The metal bath serves as anode of direct carbon fuel cell and shall be fluidized during fuel cell operation. The fluidize wind acts as a carrier as well as a stirrer, for it carries pulverized coal directly into liquid antimony bath to feed the fuel cell before mixing the coal up with antimony anode. Well-stirred fluidized anode promotes the fuel cell performance, enhancing heat and mass transfer in the anode region, reducing the risk of thermal shock of tubular SOFC as well as accelerating the electrochemical oxidation of carbon fuel at the same time. The system exhibits stable performance during operation, and well-designed setup of liquid metal bath makes it much easier for stack assembly and system scale up, on the other hand, continuous fuel supply procedure employed in this system will also inspire direct carbon fuel cell operation in its future application. Figure 1

Enhancing the efficiency of direct carbon fuel cells by bubbling Ar gas in carbon/carbonate slurry

This study aims to enhance the direct carbon fuel cell (DCFC) efficiency by bubbling Ar gas in a carbon/carbonate slurry. The current discharge curve and CO and CO2 production rates were measured during discharge at 20 mA cm−2. When carbon content in carbonates (WC/carbonate) was 1.0 wt%, Ar bubbling decreased the CO/CO2 production ratio from 0.16 to 0.051, and increased the CO2 production rate, which approached the theoretical value for complete electrochemical oxidation of carbon. Moreover, bubbling increased the cell voltage by about 0.1 V at steady state. At WC/carbonate = 3.0 wt%, the CO/CO2 production ratio without bubbling increased significantly to 8.5, indicating that partial electrochemical oxidation became dominant. The C/O2− ratio is key for explaining the electrochemical oxidation of the carbon. When the C/O2− ratio was low, such as at WC/carbonate = 1.0 wt%, complete electrochemical oxidation became dominant, and bubbling enhanced the complete electrochemical oxidation. When the C/O2− ratio was high, such as at WC/carbonate = 3.0 wt%, partial electrochemical oxidation became dominant, and bubbling did not inhibit it. For WC/carbonate = 1.0 wt%, bubbling increased both coulombic and voltage efficiencies, resulting in an increase in total efficiency from 52% to 64%.

Electrochemical durability of heat-treated carbon nanospheres as catalyst supports for proton exchange membrane fuel cells.

Carbon nanospheres is wildly used to support noble metal nanocatalysts in proton exchange membrane (PEM) fuel cells, however they show a low resistance to electrochemical corrosion. In this study, the N-doped treatment of carbon nanospheres (Vulcan XC-72) is carried out in ammonia gas. The effect of heating treatment (up to 1000 degrees C) on resistances to electrochemical oxidation of the N-doped carbon nanospheres (HNC) is investigated. The resistance to electrochemical oxidation of carbon supports and stability of the catalysts are investigated with potentiostatic oxidation and accelerated durability test by simulating PEM fuel cell environment. The HNC exhibit a higher resistance to electrochemical oxidation than traditional Vulcan XC-72. The results show that the N-doped carbon nanospheres have a great potential application in PEM fuel cells.

Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: From Model Experiments to Real-Life Operation in Membrane Electrode Assemblies

The electrochemical oxidation of carbon is a pivotal problem for low-temperature electrochemical generators, among which are proton-exchange membrane fuel cells (PEMFCs), and (non)aqueous-electrolyte Li–air batteries. In this contribution, the structure-sensitivity of the electrochemical corrosion of high-surface area carbon (HSAC) used to support catalytic materials in PEMFC electrodes is investigated in model (liquid electrolyte, 96 h potentiostatic holds at different electrode potentials ranging from 0.40 to 1.40 V at T = 330 K) and real PEMFC operating conditions (solid polymer electrolyte, 12,860 h of operation at constant current). Characterizations from Raman spectroscopy demonstrate that the disordered domains of HSAC supports (amorphous carbon and defective graphite crystallites) are preferentially oxidized at voltages related to the PEMFC cathode (0.40 1.00 V, witnessed during start-up and shut-down of PEMFC systems, accelerate this pheno...

Analysis of the carbon anode in carbon conversion cells

Abstract We examine further the electrochemical oxidation of carbon in molten carbonate, based on analysis of published research. Ascending and descending branches of voltage hysteresis found in current sweeps of atomically-ordered graphite and of disordered carbon (coal char) are separated by about 0.20–0.25 V and by 0.10–0.15 V for ordered and disordered forms, respectively, over a wide band of current density, 0.03–0.10 A/cm2. The higher voltage of the descending branch is in rough agreement with prediction of the Y. Li model for the carbon/carbonate electrode in the same current range, for ordered graphite (La = 70–100 nm) and for disordered structures (La = 3–5 nm), respectively. We suggest that the amplitude of the hysteresis represents the difference between the overvoltage requirements for 2- and 4 electron net transfer processes, respectively. The 2 e− reaction (C + CO32− = CO + CO2 + 2e−) dominates the low current segment (LCS) of our previous analysis, and the more hindered 4e− transfer reaction (C + 2CO32− = 3CO2 + 4e−) dominates the high current segment (HCS). The voltage increase separating LCS from HCS is effected by accumulation of CO2 within small, melt-filled pores to form highly supersaturated solutions of CO2, which enhance anode voltage by a concentration overpotential of 0.10–0.25 V. Overpotential increases with reaction extent until (1) overall polarization inhibits the interior reaction and shifts CO2 production to the more accessible exterior surface, or, (2) at a critical concentration (dependent on surface tension and pore diameter) bubbles nucleate and block current flow in the pores. Further support for this picture comes from the often-reported deviation of the gas composition from the CO/CO2 ratio of the Boudouard equilibrium at atmospheric pressure, as open circuit conditions are approached in an electrochemical cell. Our interpretation accounts for the mole fraction of CO2 at open circuit being greater than predicted from the Boudouard equilibrium.

The Influence of Reverse Boudouard Reaction on Direct Carbon Fuel Cell Anodic Reaction

The Direct Carbon Fuel Cell (DCFC) based on the concept of direct transfer of chemical energy from solid carbon and oxygen into electric power has recently been gaining increasing attention. The DCFC has the advantage of inexpensive fuel and high energy efficiency. It has been demonstrated that more than 80% energy efficiency can be achieved in a molten-carbonate based DCFC, with a fuel utilization of almost 100%[1].However, the chemical mechanism of carbon oxidation in the molten-carbonate based DCFC is not well understood. It is not clear which factors control the extent of reverse Boudouard reaction, leading to CO rather than CO2production, and thereby decreasing the utilization. An important question is whether or not in the carbonate medium other reactions, electrochemically driven, occur than the well-known solid/gas reaction, Eq. 1.C + CO2→ 2CO (1)In previous work in this Institute, Chen et al.[2]investigated the wetting behavior of graphite in molten carbonate, and found that the wetting of a graphite rod at open circuit in molten carbonate is driven not only by capillary forces, but also by the electric fields associated with corrosion circuits set up between the “dry” and “wetted” parts of the graphite surface. The combined capillary and electrochemical forces control the gradual wetting of the internal and external surface, by micropore-filling and external film formation, respectively. Further exploring the wetting process, the work reported here, in parallel with a similar project reported in Ref. 3 aimed to evaluate the influence of the reverse Boudouard reaction on DCFC anodic reaction under different temperatures and gas atmospheres.The principal finding is that the reverse Boudouard reaction, or “carbon corrosion reaction”, in a molten carbonate medium greatly increases the wettability of graphite in molten carbonate. Upon “dry” exposure to a CO2atmosphere (enabling only the reverse Boudouard reaction [1]), the graphite surface corrodes into a stratified structure, which leads to an increase in surface area (Figures 1a and 1b), and thereby enhances the driving force for rapid wetting during subsequent exposure to the melt. During the wetting experiment, gas bubbles were observed which appear to be generated at the solid/liquid interface of the wetting film. Moreover, after the film formation was complete, a thin film of carbon was found to have formed on the graphite surface under the carbonate film. This newly formed carbon is shown in Figure 1d.To explain these observations, we propose a reaction mechanism which includes, at open circuit, a main anodic reaction of 4-e-carbon oxidation and two cathodic reactions that generate gas bubble and new carbon, respectively, as follows. . We believe that the reverse Boudouard reaction plays an important role in this process. Main anodic reaction: C + 2CO3 2- → 3CO2+4e- (2)First cathodic reaction:CO2 + 2e- → CO+O2- (3)Second cathodic reaction: CO → C + O2- + 2e2- (4)To confirm this mechanisms, further experiments including the measurement of polarization during wetting are in process.ReferencesJ. F. Cooper, J. R. Selman, “Electrochemical Oxidation of Carbon for Electric Power Generation: A Review”, Electrochemical Society Transactions, 19, 14 (2009)C. C. Chen, T. Maruyama, P. H. Hsieh, and J. R. Selman, “Wetting Behavior of Carbon in Molten Carbonate”, J. Electrochem. Soc., 159, 10 (2012)Feng Peng, Yue Li, S. Parulekar, and J. R.Selman , “Direct Carbon Fuel Cells – Wetting Behavior of Carbon in Molten Carbonate”, #31981, 225th Mtg ECS (May 2014)

Performance evaluation of a tubular direct carbon fuel cell operating in a packed bed of carbon

The DCFC (direct carbon fuel cell) technology, based on the direct electrochemical oxidation of carbon, has the potential to double the electric efficiency and half the CO2 emissions compared to conventional coal fired power plants. In order to assess the scalability of the technology in terms of fabrication and fuel feed system, and to elucidate the possible causes of the cell degradation, a tubular DCFC has been fabricated and operated in a pulverised carbon packed bed at around 800 °C. The cell was operated for a total period of 11 days with many thermal cycles. The electrochemical impedance spectroscopy was used to elucidate the possible causes of the cell degradation. Post-mortem analysis of the cell with SEM (scanning electron microscopy) and XRD (X-ray diffraction) confirmed structural stability of both air and fuel electrodes. A peak power density of 30 mW cm−2 was obtained by direct contact of carbon to the fuel electrode with high purity He as the purge gas. The cell, at the end of operation was still found to produce 60% of the power relative to the power at the beginning of operation, and this study demonstrates the feasibility of continuous operation of the tubular fuel cell in a packed bed of carbon.