Enhancing triple-phase boundary at fuel electrode of direct carbon fuel cell using a fuel-filled ceria-coated porous anode

A new type of high-temperature fuel cell using solid carbon as a fuel, which is called a direct carbon fuel cell (DCFC), recently attracts scientific and industrial attention due to its excellent electrochemical efficiency, less production of CO₂, and no need of CO₂ separation. However, the state-of-the-art technology on the DCFC still stays in an idea developing stage, mainly because of fuel-related difficulties: a discontinuous fuel supply and a very limited formation of triple phase boundary. In this study, we focused on how to enhance the formation of triple phase boundary at the fuel electrode: using a porous Ni anode filled with carbon particles to enhance the fuel-electrode physical contact and making the porous anode wettable by ceria coating the anode. We demonstrated for the first time that the two ideas are quite successful, leading to 700% increase in a maximal power density and 500% increase in a maximal current density with respect to the standard case.     

Evaluation of carbon materials for use in a direct carbon fuel cell

The direct carbon fuel cell (DCFC) employs a process by which carbon is converted to electricity, without the need for combustion or gasification. The operation of the DCFC is investigated with a variety of solid carbons from several sources including some derived from coal. The highly organized carbon form, graphite, is used as the benchmark because of its availability and stability. Another carbon form, which is produced at West Virginia University (WVU), uses different mixtures of solvent extracted carbon ore (SECO) and petroleum coke. The SECO is derived from coal and both this and the petroleum coke are low in ash, sulfur, and volatiles. Compared to graphite, the SECO is a less-ordered form of carbon. In addition, GrafTech, Inc. (Cleveland, OH) supplied a well-fabricated baked carbon rod derived from petroleum coke and conventional coal–tar binder. The open-circuit voltage of the SECO rod reaches a maximum of 1.044 V while the baked and graphite rods only reach 0.972 V and 0.788 V, respectively. With this particular cell design, typical power densities were in the range of 0.02–0.08 W cm⁻², while current densities were between 30 and 230 mA cm⁻². It was found that the graphite rod provided stable operation and remained intact during multi-hour test runs. However, the baked (i.e., non-graphitized) rods failed after a few hours due to selective attack and reaction of the binder component.     

直接碳燃料电池性能研究

直接碳燃料电池(DCFC)勿需碳和氧气气化、重整,而直接通过电化学反应产生电能,效率可达80%,燃料的理论利用率可达100%,是一种高效、清洁的燃料电池.文章所介绍的组装DCFC单体电池,以石墨作阳极,不锈钢作阴极,加湿氧气作氧化剂,采用熔融氢氧化物作电解质,并掺入一定量的催化剂,该电池工作温度为500～700℃.对不同工作温度、不同电解质和不同氧气流量下DCFC的输出性能进行了试验研究.结果表明:随着工作温度的升高,电池输出性能有很大提高;KOH比NaOH的导电性好,电池运行更稳定,更有利于电池的输出;氧气流量为70mL/min,温度为650℃时,该电池的输出性能最佳,最大电流密度、功率密度分别为118mA/cm2和0.054 W/cm2,开路电压达到0.76 V.     

Characteristics of a fluidized bed electrode for a direct carbon fuel cell anode

The characteristics of a fluidized bed electrode applied as a direct carbon fuel cell anode, which has an inner diameter of 35 mm and height of 520 mm and employed bamboo-based activated carbon (BB-AC) as a feedstock, are vigorously studied under various experimental conditions. The optimal performance of the fluidized bed electrode direct carbon fuel cell (FEBDCFC) anode with the BB-AC as a fuel is obtained under the following conditions with a limiting current density of 95.9 mA cm⁻²: reaction temperature, 923 K; N₂ flow rate, 385 ml min⁻¹; O₂/CO₂ flow rate, 10/20 ml min⁻¹; nickel particle content, 30 g; and a cylindrically curved nickel plate as a current collector. Under the same optimal conditions, the limiting current density of the FEBDCFC anode with oak wood-based activated carbon and activated carbon fiber as the fuel is determined to be 94.5 and 88.4 mA cm⁻², which is lower than that determined for BB-AC as the fuel. Comparatively, the limiting current density for graphite, which is utilized as the carbon fuel for this fuel cell system, could not be unequivocally determined because no plateau of the limiting current density against the overpotential is observed.     

Analysis of the carbon anode in direct carbon conversion fuel cells

The total electrochemical efficiency of a direct carbon fuel cell with molten carbonate electrolyte is dominated by the product of coulombic efficiency (electron yield (n) per carbon atom, divided by 4) and voltaic efficiency (ratio of cell voltage to theoretical voltage). The voltaic efficiency is acceptably high (70–80%) for many atomically-disordered carbon materials. High coulombic efficiency is more difficult to achieve but ranges from below 50% at low current densities in porous material to 100% in certain monolithic and particulate carbon anodes at high current densities where substantially pure CO₂ is the product gas. We find evidence for two competing anode reactions associated with distinct low- and high polarization segments, respectively: (1) a charge-transfer controlled, linear–polarization reaction occurring predominately within pores, proportional to specific area, and tending toward low efficiency by co-production of CO and CO₂; and (2) a flow-dependent reaction occurring on the exterior surface of the anode, requiring > 100 mV polarization and tending to produce CO₂. Based on this interpretation, high electrochemical efficiency of a carbon fuel cell is expected with anodes made of atomically disordered ("turbostratic") carbon that have negligible porosity, or with anodes of disordered carbon for which interior pores are intentionally blocked with an impervious solid material, such as an inert salt or readily carbonized pitch.     

A performance study of hybrid direct carbon fuel cells: Impact of anode microstructure

Direct carbon fuel cells (DCFCs) have recently attracted great interest because they could provide a considerably more efficient means of power generation in comparison with conventional coal-fired power plants. Among various types of DCFCs under development, a hybrid system offers the combined advantages of solid oxide and molten carbonate electrolytes; however, there is a significant technical challenge in terms of power capability. Here, we report an experimental study demonstrating how anode microstructure influences the power-generating characteristics of hybrid DCFCs. The anode microstructure (pore volume and surface area) is modified by using poly(methyl methacrylate) (PMMA) pore-formers. Polarization studies indicate that cell performance is strongly dependent on the anode surface area rather than on the pore volume. The incorporation of PMMA-derived pores into the anode leads to improved power capability at typical operating temperatures, which is attributed to an enlarged active zone for electrochemical CO oxidation.     

The effect of Nafion content in a graphitized carbon nanofiber-based anode for the direct methanol fuel cell

The performance and stability of a direct methanol fuel cell (DMFC) with membrane electrode assemblies (MEA) using different Nafion^® contents (30, 50 and 70 wt% or MEA30, MEA50 and MEA70, respectively) and graphitized carbon nanofiber (GNF) supported PtRu catalyst at the anode was investigated by a constant current measurement of 9 days (230 h) in a DMFC and characterization with various techniques before and after this measurement. Of the pristine MEAs, MEA50 reached the highest power and current densities. During the 9-day measurement at a constant current, the performance of MEA30 decreased the most (−124 μV h⁻¹), while the MEA50 was almost stable (−11 μV h⁻¹) and performance of MEA70 improved (+115 μV h⁻¹). After the measurement, the MEA50 remained the best MEA in terms of performance. The optimum anode Nafion content for commercial Vulcan carbon black supported PtRu catalysts is between 20 and 40 wt%, so the GNF-supported catalyst requires more Nafion to reach its peak power. This difference is explained by the tubular geometry of the catalyst support, which requires more Nafion to form a penetrating proton conductive network than the spherical Vulcan. Mass transfer limitations are mitigated by the porous 3D structure of the GNF catalyst layer and possible changes in the compact Nafion filled catalyst layers during constant current production.     

Recent progress in direct carbon solid oxide fuel cell: Advanced anode catalysts,diversified carbon fuels,and heat management

Direct carbon solid oxide fuel cells (DC-SOFCs) are promising energy-conversion devices that can directly convert the chemical energy of carbon into electricity with high efficiency and low pollution. The efficient and durable operation of DC-SOFCs is maintained by the effective coupling of electrochemical oxidation in the anode and the reverse Boudouard reaction in the carbon fuel. In this paper, we review recent advances in material design, fuel development, and heat management for improving the electrochemical output and conversion efficiency of DC-SOFCs. First, developments in state-of-the-art anode materials for the electrochemical oxidation of carbon are briefly reviewed. Second, various kinds of carbon fuels and catalysts for carbon gasification are outlined. Third, we introduce the present status of heat management in DC-SOFC systems. Finally, some conclusions involving challenges in DC-SOFCs and perspectives on them are systematically summarized.     

Investigation of the anode reactions in solid oxide electrolyte based carbon fuel cells

The anode reactions of solid oxide electrolyte based carbon fuel cells (SO-CFCs) are explored by comparing the electrochemical behaviors of SO-CFCs under varying anode carrier gas flow rates (FAr) and at different contact modes. The electrochemical performance of four raw carbon fuels, including a graphitic carbon (GC), two coals (lignite CF and anthracite YQ) and an activated carbon (AC), and their chars is tested to investigate the influence of carbon fuel properties on the cell performance. The results show that CO electro-oxidation and C-CO₂ gasification were main anode reactions. The direct carbon electro-oxidation is insignificant under high F_Ar. Polarization performance of the chars under high F_Ar was similar with that of 5–10% CO. It is also concluded that the cell performance is greatly dependent on the carbon fuel gasification reactivity with CO₂. Thermal pretreated AC displays the best durability performance for its stable and moderate CO₂ gasification rate. Additionally, the coal ash does not affect the cell performance significantly.     

Nickle-cobalt composite catalyst-modified activated carbon anode for direct glucose alkaline fuel cell

Glucose is the most abundant monosaccharide in nature and has great potential as high-density hydrogen carrier for fuel cells. However, the practical application of direct glucose alkaline fuel cell (DGAFC) is hampered by lack of cost-effective anode catalyst. In this study, nickel-cobalt composite catalysts were prepared by NaBH₄ reduction method and electrochemical, morphological and chemical properties of catalysts were characterized by LSV, EIS, SEM, TEM, XRD and XPS techniques. The nickel-cobalt composite catalyst and modified activated carbon anode was evaluated in one-chamber DGAFC. Our results demonstrated that the DGAFC performance was greatly improved with the addition of Ni-Co composite catalyst in the anode. Fuel cell achieved the peak power density of 23.97 W m^?2 under the condition of 1 M glucose, 3 M KOH and ambient temperature. The enhancement of the anode performance could be attributed to the synergic effect of two reversible redox systems, Ni(II)/Ni(III) and Co(II)/Co(III), which improved interfacial charge-transfer kinetics. Our study may facilitate the development of cost-effective renewable energy devices.     

A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells

A direct carbon fuel cell based on a conventional anode-supported tubular solid oxide fuel cell, which consisted of a NiO-YSZ anode support tube, a NiO-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode, has been successfully achieved. It used the carbon black as fuel and oxygen as the oxidant, and a preliminary examination of the DCFC has been carried out. The cell generated an acceptable performance with the maximum power densities of 104, 75, and 47 mW cm⁻² at 850, 800, and 750 °C, respectively. These results demonstrate the feasibility for carbon directly converting to electricity in tubular solid oxide fuel cells.     

Effect of anode diffusion layer fabricated with mesoporous carbon on the performance of direct dimethyl ether fuel cell

In this study, we present the novel membrane electrode assembly (MEA) for direct dimethyl ether fuel cell (DDFC). The anode gas diffusion layer (AGDL) of the MEA is fabricated with mesoporous carbon to facilitate the anode mass transport and enhance the performance of DDFC. The major differences of mesoporous carbon AGDL (MAGDL) and XC-72 AGDL (XAGDL) are the BET surface, the pore volume, and the pore size distribution. The MAGDL provides many more passageways for mass transport than XAGDL. The MAGDL possesses hydrophilic small and hydrophobic middle pores, which benefit the liquid and gas transport simultaneously. The maximum power density of DDFC increases by 20% when using MAGDL instead of XAGDL at 60 °C. The electrochemical measurements indicate that the promotion of the anode two-phase mass transport is the main reason for the significant improvement of DDFC performance.     

Effect of contact type between anode and carbonaceous fuels on direct carbon fuel cell reaction characteristics

The contact between the anode and the carbonaceous fuel has a strong effect on the direct carbon fuel cell (DCFC) reaction characteristics. These effects are experimentally investigated by measuring the electrochemical behavior of a detached anode, an anode in physical contact with the fuel and an anode with carbon deposited on the surface in a DCFC. The results show that for the detached type DCFC, the reaction characteristics are closely related to the anode gas. In an Ar atmosphere, the main anode reactions are the electrochemical reaction to produce O₂ and the carbon gasification with the formed O₂. In a CO₂ atmosphere, the main anode reactions are the carbon gasification with CO₂ and the electrochemical oxidization of the formed CO. For the physical contact type DCFC, the anode reaction mechanisms are the same as for the detached type DCFC with no electrochemical oxidization of carbon at the physical contact interface between the carbonaceous fuel and the anode. Thus, the increased contact does not result in better performance. The carbon-deposited type DCFC has better performance with a significant activation polarization due to the electrochemical oxidization of the deposited carbon.     

Mechanism for carbon direct electrochemical reactions in a solid oxide electrolyte direct carbon fuel cell

The carbon direct electrochemical reactions in a solid oxide electrolyte direct carbon fuel cell (DCFC) are investigated experimentally with CH₄-deposited carbon at the anode as fuel. The surface morphology of the anode cross-sections is characterized using a scanning electron microscope (SEM), the elemental distribution using an energy dispersive spectrometer (EDS) and an X-ray photoelectron spectroscopy (XPS), and the deposited carbon microstructures using a Raman spectrometer. The results indicate that all the carbon deposited on the yttrium-stabilized zirconium (YSZ) particle surfaces, the Ni particle surfaces, as well as the three-phase boundary, can participate in the electrochemical reactions during the fuel cell discharging. The direct electrochemical reactions for carbon require the two conditions that the O²⁻ in the ionic conductor contact with a carbon reactive site and that the released electrons are conducted to the external circuit. The electrochemical reactions for the deposited carbon are most difficult on the Ni particle surfaces, easier on the YSZ particle surfaces and easiest at the three-phase boundary. Not all the carbon deposited in the anode participates in the direct electrochemical reactions. The deposited carbon and the O²⁻ in the YSZ react to form the double-bonded adsorbed carbonyl group CO.     

Graphene supported platinum nanoparticles as anode electrocatalyst for direct borohydride fuel cell

The graphene supported Pt nanoparticles are prepared by ethylene glycol reduction method. The obtained Pt/graphene (Pt/G) nanocomposites are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). TEM images show that the spherical Pt nanoparticles with sizes of 3.1 nm disperse uniformly on the surface of the graphene, which is consistent with the XRD date of 2.97 nm. The Pt/G nanocomposites show electrochemically active surface area (ECSA) of 62.7 m²/g. It has been found by electrochemical measurements (i.e., cyclic voltammetry, chronoamperometry) that the Pt/G nanocomposites exhibit good electrocatalytic activity and stability toward borohydride oxidation. Besides, the Pt/G nanocomposites are used as anode electrocatalyst in a direct borohydride fuel cell at 298 K, and the maximum power density is 42 mW/cm², which is apparently higher than Vulcan XC-72R supported Pt (Pt/C) nanoparticles (34 mW/cm²).     

Modeling and simulation of a single direct carbon fuel cell

A mathematical model was developed to simulate the performance of a direct carbon fuel cell. The model takes account of the electrochemical reaction dynamics, mass-transfer and the electrode processes. An improved packed bed anode was adopted. Polarization losses for the cell components were examined supposing graphite as the fuel and molten carbonate as the electrolyte. The results indicated that the anode activation polarization was the major potential loss in 923–1023 K. The effects of temperature, anode dimension, and carbon particle size on the cell performance were investigated. The model predicted that the power density can be as high as 200–500 W m⁻², with carbon particle size in the range 1.0 × 10⁻⁷ to 1.0 × 10⁻⁴ m and in 923–1023 K and that the overall efficiency of the cell is higher than 55% for low current density and is 45–50% for high current density.     

Enhancement of electrooxidation activity of activated carbon for direct carbon fuel cell

Direct carbon fuel cells are promising power sources using solid carbon directly as fuel. Their performances significantly depend on the electrooxidation activity of carbon fuel. Electrooxidation of activated carbon particulates in molten Li₂CO₃–K₂CO₃ was investigated by potentiodynamic and potentiostatic method. Results indicated that the electrooxidation performance of activated carbon was significantly enhanced by pre-soaking with Li₂CO₃–K₂CO₃ and by treatment with HF, HNO₃ and NaOH, respectively. The onset potential negatively shifted by around 100 mV and the current density increased by around 50 mA cm⁻² after pre-soaking. The non-oxidant acids (HF) treatments are more effective than oxidant acid (HNO₃) and base (NaOH) treatments. HF treated activated carbon exhibited the highest activity among all the samples. The enhancement in electrooxidation performance can be closely correlated with the increase in surface area and porosity caused by acid and base treatments.     

Electrochemical performance of different carbon fuels on a hybrid direct carbon fuel cell

In this work, three processed carbon fuels including activated carbon, carbon black and graphite have been employed to investigate influence of the chemical and physical properties of carbon on the HDCFC performance in different anode atmospheres at 650–800 °C. The results reveal that the electrochemical activity is strongly dependent on crystalline structure, thermal stability and textural properties of carbon fuels. The activated carbon samples demonstrate a better performance with a peak power density of 326 mW cm^?2 in CO₂ at 750 °C, compared to 147 and 59 mW cm^?2 with carbon black and graphite samples, respectively. Compared to the ohmic resistance, the polarization resistance plays a more dominated role in the cell performance. When replacing N₂ by CO₂ purge gas, the power density is the strongly temperature dependent due to the Boudouard reaction.     

Electricity generation from corn cob char though a direct carbon solid oxide fuel cell

Solid oxide fuel cell (SOFC) is a potential technology for utilizing biomass to generate electricity with high conversion efficiency and low pollution. Investigations on biomass integrated gasification SOFC system show that gasifier is one of the high cost factors which impede the practical application of such systems. Direct carbon solid oxide fuel cell (DC-SOFC) may provide a cost effective option for electricity generation from biomass because it can operate directly using biochar as the fuel so that the gasification process can be avoided. In this paper, the feasibility of using corn cob char as the fuel of a DC-SOFC to generate electricity is investigated. Electrolyte-supported SOFCs, with yttrium stabilized zirconia (YSZ) as the electrolyte, cermet of silver and gadolinium-doped ceria (GDC) as the anode and the cathode, are prepared and tested with fixed bed corn cob char as fuel and static ambient air as oxidant. The maximum power output of a DC-SOFC operated on pure corn cob char is 204 mW cm⁻² at 800 °C and it achieves 270 mW cm⁻² when Fe of 5% mass fraction, as a catalyst of the Boudouard reaction, is loaded on the corn cob char. The discharging time of the cell with 0.5 g corn cob char operated at a constant current of 0.1 A lasts 17 h, representing a fuel conversion of 38%. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and Raman spectroscopy have been applied to characterize the char-based fuels.     

Lignite as a fuel for direct carbon fuel cell system

Lignite, also known as brown coal, and char derived from lignite by pyrolysis were investigated as fuels for direct carbon solid oxide fuel cells (DC-SOFC). Experiments were carried out with 16 cm² active area, electrolyte supported solid oxide fuel cell (SOFC), using pulverized solid fuel directly fed to DC-SOFC anode compartment in a batch mode, fixed bed configuration. The maximum power density of 143 mW/cm² was observed with a char derived from lignite, much higher than 93 mW/cm² when operating on a lignite fuel. The cell was operating under electric load until fuel supply was almost completely exhausted. Reloading fixed lignite bed during a thermal cycle resulted in a similar initial cell performance, pointing to feasibility of fuel cell operation in a continuous fuel supply mode. The additional series of experiments were carried out in SOFC cell, in the absence of solid fuels, with (a) simulated CO/CO₂ gas mixtures in a wide range of compositions and (b) humidified hydrogen as a reference fuel composition for all cases considered. The solid oxide fuel cell, operated with 92%CO + 8%CO₂ gas mixture, generated the maximum power density of 342 mW/cm². The fuel cell performance has increased in the following order: lignite (DC-SOFC) < char derived from lignite (DC-SOFC) < CO + CO₂ gas mixture (SOFC) < humidified hydrogen (SOFC).