Carbon anode in direct carbon fuel cell

Direct carbon fuel cell (DCFC) is a kind of high temperature fuel cell using carbon materials directly as anode. Electrochemical reactivity and surface property of carbon were taken into account in this paper. Four representative carbon samples were selected. The most suitable ratio of the ternary eutectic mixture Li₂CO₃–K₂CO₃–Al₂O₃ was determined at 1.05:1.2:1(mass ration). Conceptual analysis for electrochemical reactivity of carbon anode shows the importance of (1) reactive characteristics including lattice disorder, edge-carbon ratio and the number of short alkyl side chain of carbon material, which builds the prime foundation of the anodic half-cell reaction; (2) surface wetting ability, which assures the efficient contact of anode surface with electrolyte. It indicates that anode reaction rate and DCFC output can be notably improved if carbon are pre-dispersed into electrolyte before acting as anode, due to the straightway shift from cathode to anode for CO₃²⁻ provided by electrolyte soaked in carbon material.     

Investigation of direct carbon conversion at the surface of a YSZ electrolyte in a SOFC

Fuel cells offer a promising way to produce electricity efficiently. In this work, a direct carbon fuel cell (DCFC) based on a solid oxide fuel cell (SOFC) has been investigated, in which solid carbon has been used as fuel in form of a pellet. The DCFC is an interesting technology because it offers the possibility to use, as fuel source, available and abundant raw materials with only minor pretreatment. Moreover, the thermodynamic efficiency slightly exceeds 100% in a wide temperature range due to the positive near-zero value of reaction entropy change. As pure carbon dioxide is produced at the anode, it can be easily captured and sequestered. Direct carbon conversion is competed by the Boudouard reaction, which produces carbon monoxide at high operating temperatures. This reaction is endothermic and leads to a fuel loss. The present paper relates to the contribution of both reactions by a long-term run over about 12 h with a non-porous anode layer.     

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.     

Strategies and new developments in the field of molten carbonates and high-temperature fuel cells in the carbon cycle

This paper introduces the special issue of this Journal dedicated to the “International Workshop on Molten Carbonates and Related Topics”, IWMC2011. Molten carbonates are mostly known as electrolytes in molten carbonate fuel cells (MCFCs). Many advances have been reached in the last decade on the development of such electrochemical generators. MCFCs can be considered as a mature technology close to the real market. However, new materials and approaches are necessary to increase their performance and lower their costs. Lately, molten carbonates are also used for CO₂ capture with ambitious industrial projects and a growing interest in many countries, in particular in Europe. Furthermore, molten carbonates are also seriously considered in direct carbon fuel cells (DCFCs), directly as electrolytes or as media to feed solid oxide fuel cells with solid carbon fuel. Noteworthy is the astonishing research activity on composite fuel cells combining molten carbonates with solid oxide fuel cells. All these promising topics and others are briefly described emphasising their novelty.     

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.     

Insight into graphite oxidation in a NiO-based hybrid direct carbon fuel cell

A direct carbon fuel cell is an electricity generation device using solid carbon as a fuel directly with no reforming process. In this study, three-carbon fuels, graphitic carbon (GC), carbon black (CB), and biomass carbon (BC) are tested as the fuel to investigate the influence of carbon fuel properties on the cell performance in HDCFC with a traditional nickel oxide as the anode. Either an electrolyte-supported cell with a thin nickel oxide anode or an anode-supported cell with a thick nickel oxide anode is used to evaluate the electrochemical reactivity of carbon samples. These three-carbon fuels are characterised on the crystal structure, particle size, composition, and surface property. It is found that GC shows excellent cell performance on thin nickel oxide anode. However, it displays relatively slow electrochemical reactivity on the thick anode due to its great extent of carbon oxidation. BC shows good initial cell performance but fast degradation of the cell performance, as much more hydrogen is released at the beginning of the cell test. The anode reactions of HDCFCs are explored by the in-situ gas analysis in open circuits and under current load conditions. It is observed that GC produces the highest amount of CO among these three fuels, suggesting that carbon oxidation is the dominant electrochemical process in HDCFCs after a certain time when most of the hydrogen is released from the pyrolysis process.     

The synergistic catalyst-carbonates effect on the direct bituminous coal fuel cell performance

The current work explores the feasibility to improve the performance of a Direct Carbon Fuel Cell (DCFC): CO₂ + bituminous coal|Co-CeO₂/YSZ/Ag|Air by infusing a gasification catalyst (Co/CeO₂) and/or Li-K carbonates mixture into the carbon fuel. The different fuel feedstock mixtures were characterized by various methods, involving chemical composition and proximate analysis, particle size distribution (PSD), X-ray diffraction (XRD), N₂ adsorption-desorption (BET method), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM), to gain insight into the effect of catalyst and/or carbonates addition to fuel mixture physicochemical characteristics. An increase of the power output up to ca. 20 and 80% is achieved for carbon/catalyst and carbon/catalyst/carbonates mixtures, respectively, in comparison to bare carbon at 700 °C, demonstrating the pronounced effect of catalyst as well as its potential synergy with carbonates. It was also shown that the achieved maximum power density is directly associated with the CO formation rate, implying the importance of in situ formed CO on the electrochemical performance. The obtained findings are further discussed based also on the corresponding AC impedance spectroscopy studies, which revealed the beneficial effect of fuel feedstock additives (catalyst and/or carbonates) on ohmic and electrode polarization resistances. The present results clearly revealed the feasibility to improve the DCFC performance by concurrently infusing a gasification catalyst and carbonates mixture into fuel feedstock.     

Development of tubular hybrid direct carbon fuel cell

The hybrid direct carbon fuel cell is a direct carbon fuel cell concept that combines a solid oxide fuel cell with a molten carbonate fuel cell electrode. This offers efficient conversion of coal or biomass derived carbons to electricity. In this study we aim to improve the electrical performance of this cell by using gadolinia doped ceria (GDC) as either a protection layer over a YSZ electrolyte or as the electrolyte itself. In our study, the electrical performance of several tubular cell geometries were investigated using impedance spectroscopy both with and without gas flows of carbon dioxide or nitrogen. Integrity of microstructure including possible layer delamination effects were investigated by SEM. Promising values of power and resistance were observed using a GDC material as electrolyte at intermediate temperature reducing the operation temperature compared to YSZ, doubling the power of each cell.     

Biomass to power conversion in a direct carbon fuel cell

Direct carbon fuel cells (DCFC) offer clear advantages over conventional power generation systems including higher conversion efficiency, low emissions and production of a near pure CO₂ exit stream which can be easily captured for storage. When operated on biomass-derived fuels and combined with carbon capture and storage they have the potential to be a carbon negative technology. Currently most studies relating to DCFC's focus on the use of synthetic high purity fuels. Although of significant academic interest, the high energy requirements for the production of such fuels and high cost would negate the advantages offered by DCFCs over conventional combustion technologies that can produce power from lower-grade fuels. A number of industrial processes (such as pyrolysis or gasification) can produce high carbon containing and low cost chars from biomass sources. This paper describes the operation of a novel solid state direct carbon fuel cell operated on two such commercially available bio-mass derived chars, an agricultural waste derived bio-char used for soil enrichment and coconut char used for the processing of ceramics. Chemical analysis (ICP, XRF), X-ray diffraction and thermo-gravimetric analysis have been used to characterise the fuels. Testing on small button cells showed that it is possible to operate fuel cells directly on low grade unprocessed chars. Although initial power densities were low, significant improvements to cell materials and designs can lead to practical devices. Overall the stability of the fuel cell materials in contact with bio-chars appeared to be good with no phase decomposition of any material observed.     

Catalysis and oxidation of carbon in a hybrid direct carbon fuel cell

The hybrid direct carbon fuel cell (HDCFC), combining molten carbonate fuel cell and solid oxide fuel cell technology, is capable of converting solid carbon directly into electrical energy without intermediate reforming. Here, we report the investigation of the HDCFC with yttria stabilized zirconia (YSZ) electrolyte, NiO-YSZ anode and lanthanum strontium manganite (LSM) cathode using the eutectic mixture of 62 mol% Li₂CO₃ and 38 mol% K₂CO₃. An open circuit voltage (OCV) of 0.71 V at 800 °C is recorded without the carbonate which increases to 1.15-1.23 V in the presence of the carbonate at the same temperature. In addition, the cell's OCV is enhanced not only by the thermal history but also by the carbonate, which is in excess of 1.57 V after the high temperature treatment. Electrochemical performance analysis indicates a suitable amount of the carbonate enhanced the carbon oxidation. With 1 mm robust thick electrolyte and commercial carbon, the cell (1.13 cm² active area) generates the peak density of 50 mW cm⁻² at 800 °C. There are significant losses from electrolyte resistance, which would be overcome by the application of a thinner electrolyte.     

A comprehensive review of direct carbon fuel cell technology

Fuel cells are under development for a range of applications for transport, stationary and portable power appliances. Fuel cell technology has advanced to the stage where commercial field trials for both transport and stationary applications are in progress. The electric efficiency typically varies between 40 and 60% for gaseous or liquid fuels. About 30–40% of the energy of the fuel is available as heat, the quality of which varies based on the operating temperature of the fuel cell. The utilisation of this heat component to further boost system efficiency is dictated by the application and end-use requirements. Fuel cells utilise either a gaseous or liquid fuel with most using hydrogen or synthetic gas produced by a variety of different means (reforming of natural gas or liquefied petroleum gas, reforming of liquid fuels such as diesel and kerosene, coal or biomass gasification, or hydrogen produced via water splitting/electrolysis). Direct Carbon Fuel Cells (DCFC) utilise solid carbon as the fuel and have historically attracted less investment than other types of gas or liquid fed fuel cells. However, volatility in gas and oil commodity prices and the increasing concern about the environmental impact of burning heavy fossil fuels for power generation has led to DCFCs gaining more attention within the global research community. A DCFC converts the chemical energy in solid carbon directly into electricity through its direct electrochemical oxidation. The fuel utilisation can be almost 100% as the fuel feed and product gases are distinct phases and thus can be easily separated. This is not the case with other fuel cell types for which the fuel utilisation within the cell is typically limited to below 85%. The theoretical efficiency is also high, around 100%. The combination of these two factors, lead to the projected electric efficiency of DCFC approaching 80% - approximately twice the efficiency of current generation coal fired power plants, thus leading to a 50% reduction in greenhouse gas emissions. The amount of CO₂ for storage/sequestration is also halved. Moreover, the exit gas is an almost pure CO₂ stream, requiring little or no gas separation before compression for sequestration. Therefore, the energy and cost penalties to capture the CO₂ will also be significantly less than for other technologies. Furthermore, a variety of abundant fuels such as coal, coke, tar, biomass and organic waste can be used. Despite these advantages, the technology is at an early stage of development requiring solutions to many complex challenges related to materials degradation, fuel delivery, reaction kinetics, stack fabrication and system design, before it can be considered for commercialisation. This paper, following a brief introduction to other fuel cells, reviews in detail the current status of the direct carbon fuel cell technology, recent progress, technical challenges and discusses the future of the technology.     

Mixed ionic electronic conducting perovskite anode for direct carbon fuel cells

The conversion of carbonaceous materials to electricity in a Direct Carbon Fuel Cell (DCFC) offers the most efficient process with theoretical electric efficiency close to 100%. One of the key issues for fuel cells is the continuous availability of the fuel at the triple phase boundaries between fuel, electrode and electrolyte. While this can be easily achieved with the use of a porous fuel electrode (anode) in the case of gaseous fuels, there are serious challenges for the delivery of solid fuels to the triple junctions. In this paper, a novel concept of using mixed ionic electronic conductors (MIEC) as anode materials for DCFCs has been discussed. The lanthanum strontium cobalt ferrite, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) was chosen as the first generation anode material due to its well known high mixed ionic and electronic conductivities in air. This material has been investigated in detail with respect to its conductivity, phase and microstructural stability in DCFC operating environments. When used both as the anode and cathode in a DCFC, power densities in excess of 50 mW/cm² were obtained at 804 °C in electrolyte supported small button cells with solid carbon as the fuel. The concept of using the same anode and cathode material has also been evaluated in electrolyte supported thick wall tubular cells where power densities around 25 mW/cm² were obtained with carbon fuel at 820 °C in the presence of helium as the purging gas. The concept of using a mixed ionic electronic conducting anode for a solid fuel, to extend the reaction zone for carbon oxidation from anode/electrolyte interface to anode/solid fuel interface, has been demonstrated.     

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.     

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.     

Experimental investigation of direct carbon fuel cell fueled by almond shell biochar: Part I. Physico-chemical characterization of the biochar fuel and cell performance examination

A wide variety of abundant low cost carbonaceous fuels such as coal-derived coke, petroleum coke (refinery wastes) and biochar from biomass carbonization can be utilized to generate electricity in a Direct Carbon Fuel Cell (DCFC) system. In this paper, a promising Intermediate–Temperature DCFC based on ceria-carbonate composite electrolyte is successfully tested at 600–750 °C using as fuel a biochar issued from almond shell carbonization. This bio-resource fuel gives the best open circuit cell potential (1.07 V) at 700 °C. Measurements of the cell performance indicate a peak power density of 127 mW cm⁻² for the almond shell biochar fuel, which is compared favorably with a peak power density of 100 mW cm⁻² for activated carbon fuel used in previous work. Obtained results show that the electrochemical performance of the almond shell biochar as fuel is highly dependent on their intrinsic physico-chemical properties, such as chemical composition, surface area, the nature of mineral matter and content of oxygen- functional groups not only on the edges but also in the interior of the graphite structure of the almond shell biochar where the degree of coverage is higher. The cell performance is showed maintained due to the replenishment of oxygen-containing reactive sites which continuously appear at the reactive edge of the carbon crystal, as the edge continuously advances.     

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.     

A direct carbon fuel cell with (molten carbonate)/(doped ceria) composite electrolyte

A composite electrolyte containing a Li/Na carbonate eutectic and a doped ceria phase is employed in a direct carbon fuel cell (DCFC). A four-layer pellet cell, viz. cathode current collector (silver powder), cathode (lithiated NiO/electrolyte), electrolyte and anode current collector layers (silver powder), is fabricated by a co-pressing and sintering technique. Activated carbon powder is mixed with the composite electrolyte and is retained in the anode cavity above the anode current collector. The performance of the single cell with variation of cathode gas and temperature is examined. With a suitable CO₂/O₂ ratio of the cathode gas, an operating temperature of 700 °C, a power output of 100 mW cm⁻² at a current density of 200 mA cm⁻² is obtained. A mechanism of O²⁻ and CO₃²⁻ binary ionic conduction and the anode electrochemical process is discussed.     

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.     

Performance improvement of direct carbon fuel cell by introducing catalytic gasification process

In this paper, the effects of catalytic gasification on the solid oxide electrolyte DCFC (direct carbon fuel cell) performance are experimentally investigated and analyzed using K, Ca, Ni as catalyst in carbon black and controlling the temperatures of cell and carbon black at 750 °C and 700-1000 °C, respectively. The average power densities are 976, 1473 and 1543 W m⁻² respectively for 900, 950 and 1000 °C pure carbon black gasification. Catalytic gasification improves the DCFC performance significantly. For the same performance of pure carbon black, the gasification temperatures decrease about 200, 130 and 150 °C with K, Ca and Ni additives, respectively. The catalytic effects for carbon black gasification with CO₂ are: K > Ni > Ca. For typical identical temperature DCFC operating at 750 °C, the power densities of 0.7 V discharging are 1477, 1034 and 1123 W m⁻² for the carbon black with K, Ca and Ni additives, respectively. It is possible to reduce the operation temperature of DCFC to the medium temperature range of solid oxide electrolyte (600-800 °C) by introducing catalytic gasification process.     

Durability of different carbon nanomaterial supports with PtRu catalyst in a direct methanol fuel cell

PtRu catalysts with similar particle size and composition were deposited on three different carbon supports: Vulcan, graphitized carbon nanofibers (GNF) and few-walled carbon nanotubes (FWCNT) and their performance for methanol oxidation was studied in an electrochemical cell and in a single cell DMFC. The electrochemical results indicate that with PtRu/GNF and PtRu/FWCNT higher current densities are obtained and oxidation intermediates deactivate the surface less compared to the same catalyst on Vulcan support. Conversely, PtRu/Vulcan provided the highest open circuit voltage OCV and current densities in DMFC experiments due to a well-optimized electrode layer structure. Because stability is a key requirement for fuel cell commercialization, 6-day-long fuel cell stability tests were carried out, showing that PtRu/Vulcan degraded significantly. This was due to the collapse of the secondary structure of the electrode layer revealed by post characterization of the membrane electrode assembly (MEA) with SEM and TEM. PtRu/GNF exhibited slightly poorer initial performance but better stability because the structure of the anode layer was maintained. PtRu/FWCNT showed the worst initial performance and long-term stability. The good stability of non-optimized PtRu/GNF MEAs shows the potential of these novel nanocarbon supported catalysts as stable fuel cell components after proper MEA optimization.