DIRECT CARBON FUEL CELL: FUNDAMENTALS AND RECENT DEVELOPMENTS

by dianxue-cao

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 Direct carbon fuel cell: Fundamentals and recent developments
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• Journal of Power Sources 167 (2007) 250–257 Direct carbon fuel cell: Fundamentals a uil ineeri ruary y 200 Abstract The direc direc power gener n the emissions th arbon available an chem Recent deve also © 2007 Else Keywords: D logy 1. Introdu The dire cell techno century [1, DCFC resemble those of the high temperature fuel cells, such as the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC). A DCFC consists of three key components: the anode, the cathode and the electrolyte. Differing from the MCFC and hydrogen g hol, etc., D introduced CO2 at hig The overal C + O2 = DCFC, unique attr dynamic ad and SOFC efficiency because th ∗ Correspon E-mail ad 1.6 J sta J m −39 duc phases, therefore their chemical potentials (activities) are fixed and independent of extent of conversion of the fuel or position within the cell. This may allow a full conversion of the carbon fuel in a single pass with the theoretical voltage of DCFC 0378-7753/$ doi:10.1016/j SOFC, instead of operating on a gaseous fuel, e.g. enerated by reforming of natural gas, coal gas, alco- CFC uses solid carbon as fuel. Solid carbon is directly into the anode compartment and electro-oxidized to h temperature generating electrical power (Fig. 1). l cell reaction is given by Eq. (1). CO2, E◦ = 1.02 V (1) the only fuel cell type using solid fuel, has several active features. First, the DCFC offers great thermo- vantages over other fuel cell types, such as MCFC [3–9]. Its theoretical electrochemical conversion based on Eq. (1) slightly exceeds 100%. This is e entropy change for the cell reaction is positive ding author. Tel.: +86 451 82589036; fax: +86 451 82589036. dress: caodianxue@hrbeu.edu.cn (D. Cao). remaining nearly constant at ∼1.02 V during the operation (minimal Nernst loss). Consequently, the fuel utilization efficiency could reach 100%, giving a practical typical coal to electricity efficiency of around 80% (direct electrical generation alone without cogeneration). This value is higher than MCFC or SOFC running on hydrogen or natural gas (nominal efficiency of 45–60%, see Table 1) [10]. So DCFC is potentially one of the most efficient electrochemical power generation systems. Second, DCFC releases lower emissions than coal-firing power plants. DCFC may cut carbon emissions from coal by 50% and reduce off-gas volume by 10 times compared to con- ventional coal-burning power plants [11,12]. This is because, in contrast to combustion in a boiler, the oxidation of carbon in a DCFC occurs electrochemically at the anode compartment with- out the direct mixing with air, and thus the CO2 produced is not mixed with other gases. The majority of the ingredients in the off-gas are carbon dioxide, which can be sequestered or injected into an oilfield to enhance oil recovery and at the same time used – see front matter © 2007 Elsevier B.V. All rights reserved. .jpowsour.2007.02.034 Dianxue Cao ∗, Yong Sun, G College of Material Science and Chemical Engineering, Harbin Eng Received 15 December 2006; received in revised form 15 Feb Available online 25 Februar t carbon fuel cell is a special type of high temperature fuel cell that ator for power plants, it has a higher achievable efficiency (80%) tha an conventional coal-burning power plants. More importantly, its solid c d abundant. In this review, some fundamental study results of electro lopments in direct carbon fuel cell configurations and performance are vier B.V. All rights reserved. irect carbon fuel cell; Carbon electrooxidation; Clean coal to electricity techno ction ct carbon fuel cell (DCFC) is an almost forgotten fuel logy that actually has a long history date to mid-19 2]. The configuration and theoretical principles of the (�S = larger 395.4 k (�H = the pro nd recent developments ing Wang ng University, Harbin 150001, PR China 2007; accepted 15 February 2007 7 tly uses solid carbon as anode and fuel. As an electrical molten carbonate and solid oxide fuel cells, and has less -rich fuels (e.g. coal, biomass, organic garbage) are readily ical oxidation of carbon in molten salts are summarized. discussed. K−1 mol at 600 ◦C), which results in a slightly ndard Gibbs free energy change (�G = − ol−1 at 600 ◦C) than the standard enthalpy change 4.0 kJ mol−1 at 600 ◦C). The reactant carbon and t carbon dioxide exist as pure substance in separate
• D. Cao et al. / Journal of Power Sources 167 (2007) 250–257 251 Fig. 1. Schematic of a direct carbon fuel cell configuration. to store car This can fu also release ronment br are signific on coal, fo trical powe CO2 emiss sion (ranke 67% of its electricity [15]. Thirdly, different re (rice hulls, garbage. C and accoun 80% of the mer Soviet remain und ticles whic requires les for MCFC kilograms o in the Unite unit volum ing, in thi as hydroge (9.0 kWh L Fourthly reformers o a coal mine saving ener coal shippi possibility converting raw coal directly to electrical power without com- bustion, gasification (reforming) and the thermal efficiency limitations of heat engines. The first literature-recorded DCFC may be traced back to the mid of 19 century. Bacquerelle in 1885 and Jablochkoff in 1877 built electrochemical devices using electrode-grade car- bon as anode, Pt/Fe as cathode, and fused KNO3 as electrolyte [16,17]. Such devices produced electrical power, but were unsta- ble due to electrolyte degradation. In 1896, Dr. William Jacques demonstrated a large assembly of cells consisting of 100 single cells with rods of baked coal as anode, iron pots as cathode, and molten sodium hydroxide as electrolyte [2]. By blowing air to n pot ◦ nace cal p e co ver t gene d to ce t uiva gard as e al re e ina the vers drive logy he 1 Pa , M ossi city t of for d by l cel DCF d th the on t CFC the d [1 t rev elop Table 1 Efficiency of Fuel C CH4 H2 Note: Efficien μ][voltage effi bon dioxide permanently beneath the earth’s surface. rther reduce the release of CO2 into air. The DCFC s no particulates (fly ash). These benefits to the envi- ought about by using a DCFC to produce electricity ant and important for those regions heavily relying r example, China, in which, around 80% of the elec- r is from burning coal, which releases 70% of its total ion (ranked 2nd in the world), 90% of its SO2 emis- d 1st in the world), 70% of its total particles, and total NOx [13,14]. Coal-fired plants produce 55% of in the U.S., as well as a large amount of pollutants a solid carbon fuel can be easily produced from many sources, including coal, petroleum coke, biomass nut shells, corn husks, grass, woods) and even organic oal is the earth’s most abundant fossil resource, ts for 60% of the world’s fossil fuel resources, and coal belongs to the United States, Canada, the for- Union and China. The vast energy reserves of coal erused. The pyrolysis production of tiny carbon par- h can be used in DCFC, consumes less energy and s capital than the production of hydrogen-rich fuels or SOFC by steam-reforming processes. Billions of f carbon blacks were produced annually by pyrolysis d States [6]. Carbon releases a very high energy per e on oxidation with oxygen (20.0 kWh L−1) exceed- s regard, many widely used fuel cell fuels, such n (2.4 kWh L−1), methane (4.0 kWh L−1), gasoline −1), and diesel (9.8 kWh L−1) [4,5]. , DCFC system is mechanically simple because no r heat engines required. It can be built on the site of , thus eliminating coal transportation, consequently gy and reducing environmental pollution caused by ng and handling. Therefore, the DCFC provides a for the realization of the 150-year-old dream of the iro in a fur electri could b tions o power believe and sin tion eq was re stack w chemic and th well as cal con steam- techno until t (Menlo (NTEL cally p electri opmen before impose the fue oping clarifie firmed of carb of a D nent to Howar presen the dev fuel cells Theoretical limit = �G◦(T )/�H◦std Utilization efficiency (μ) V 1.003 1.0 0 0.895 0.80 0 0.70 0.80 0 cy of a fuel cell is defined as: (electrical energy out)/(heat of combustion (HHV) of ciency εV] = [�G(T)/�H◦][μ][V/V◦] = [μ][nFV]/�H◦ (where �G(T) = −nFV◦ = �H containing the electrolyte and heating to 400–500 C , a current density of as high as 100 mA cm−2 and an ower of 1.5 kW were achieved from the system. This nsidered the first DCFC, but there are many specula- he actual performance and debates about the electrical ration mechanism. For instance, the cell reaction was be C + 2NaOH + O2 = Na2CO3, E◦298 = 1.42 V [17], he electrolyte was consumed by an irreversible reac- lent to CO2 + 2NaOH = Na2CO3 + H2O, this device ed as not a fuel cell but rather a battery. The cell ven suspected to generate electricity not by electro- action, but by a thermoelectric effect. These doubts bility of reproducing Jacques’s results by others, as diminishment of incentive for seeking electrochemi- ion of coal caused by the improved efficiency of the n generator at the early of 20 century, put DCFC to rest for nearly more than two-third of a century 970s, when a series of studies at SRI International rk, CA, a National Energy Technology Laboratory organtown, WV) contractor) verified that it is practi- ble to completely electro-oxidize carbon to generate [18–20]. In recent years, with the significant devel- fuel cell technology and the urgent needs than ever a clean and efficient coal to electricity technology the crude oil crisis and environmental deterioration, l research community regained the interest of devel- C. Studies on the DCFC in the last few years have e earlier misunderstanding of the DCFC, and have electrochemical foundation of the direct conversion o electricity, and have demonstrated the feasibility at least on a laboratory scale. Early research perti- DCFC has been reviewed by Baur and Tobler [21], 6], and Liebhafsky and Cairns [17]. The aim of this iew is to provide an overview on the recent progress in ment of DCFC and to point out the most important (i)/V(i = 0) = εV Actual efficiency = (�G/H◦std)(μ)(εV) .80 0.80 .80 0.57 .80 0.45 fuels input) = [theoretical efficiency �G/�H][utilization fraction −T�S).
• 252 D. Cao et al. / Journal of Power Sources 167 (2007) 250–257 technological challenges to be solved for DCFC becoming a viable power source. 2. Electro 2.1. Basic Electroc ture becaus performed bonates, an experiment molten car (1) The pr than ar as to 1 off-gas that the same a firmed from g based o curren temper the dep and fo at high ilar stu the CO [23,28 anodic inant s well-k C + CO2 = These o carbon to C mation of C current den sound foun (2) Carbon ductivi less, th poor c more r face de Carbon ohmic reactio is mor graphi surface al. fou discha 2.2. Mechanism The study of the mechanism is difficult due to the lack of ques to detect the reaction intermediates in molten salts temperature (usually higher than 600 ◦C). Supported by ndirect evidence, a mechanism for the anodic oxidation on in molten cryolite/alumina electrolyte (acidic melts) rocess) has been proposed and summarized by Haupin et 3,29,30] 2F4]2− → 2O2− +2Al2OF4 O2− formation (3) O2− → CRSO2− First adsorption (4) − → → + O 2− → 2− → me uoalu reac ed o o fo oms n ion e spe dsor erpo he “ n un O per mol nism n for iate Coo empe the s )–(9 − → Pictor gen io n [3] chemical oxidation of carbon findings hemical oxidation of carbon requires high tempera- e of its sluggish kinetics and is therefore generally in molten salt electrolytes (e.g. cryolites, molten car- d molten hydroxides) [3,12,22–25]. Some important al findings regarding anodic oxidation of carbon in bonate electrolytes are summarized below: edominant product is CO2 at polarizations greater ound 0.1 V at temperatures above 700 ◦C. As early 935, Tamaru et al. already found by analyzing the composition that CO2 is dominant and concluded overall electrochemical oxidation of carbon was the s its complete combustion [26]. Hauser later con- this result through the analysis of the gas evolved raphite anode [27]. He found the current efficiency n four electron process was more than 99% at applied t densities between 20 and 120 mA cm−2 over the ature range of 650–800 ◦C. Weaver et al. measured endence of off-gas composition on current density und that more than 90% of the anode gas was CO2 current density [18,19]. Vutetakis et al. in a sim- dy reported that the anodic product was CO2 and /CO2 ratio increases as current density decreases ]. These results overturned the assumption that the oxidation of carbon would produce CO as the dom- pecies at temperature above 750 ◦C according to the nown Boudouard reaction equilibrium (Eq. (2)). 2CO (2) bservations proved that complete electrooxidation of O2 (a four-electron process) is feasible, and the for- O (a two-electron process) could be avoided at high sity (polarized condition), and therefore built the dation for the DCFC. properties, such as, crystallization, electrical con- ty, surface area and particle size, affect, more or e reactivity of the carbon reaction. It seems that the rystallized, highly lattice disordered carbons are the eactive probably due to the existence of more sur- fects, such as, edges, steps, which act as active sites. s with good electrical conductivity would lower the polarization and benefit the carbon electrochemical n [3]. Weaver et al. reported that devolatilized coal e reactive than spectroscopic carbon and pyrolytic te [18]. They attributed the high reactivity to large area and poor crystallization. However, Cooper et nd that surface area has no strong effects on carbon rge rate. techni at high some i of carb (Hall p al. as [ 2[Al2O CRS + CRSO2 CRSO− CRSO CRSO2 CRSO2 The plex fl on the adsorb steps t bon at oxyge surfac This a able ov step. T form a edge C Coo bon in mecha gen io dissoc ature, high t tiates Eqs. (4 2CO32 Fig. 2. first oxy formatio CRSO− + e− Fast discharge (5) CRSO + e− Fast discharge (6) 2− → CRSO22− Slow adsorption (rate-determining step) (7) CRSO2− + e− Fast discharge (8) CO2(g) + e− Fast discharge and evolution (9) lt is the source of O2−. The dissociation of a com- minate ion generates a free oxide ion, which adsorbs tive carbon surface sites (like edges or steps). The xygen ion undergoes discharge in two, single-electron rm a C O C (“C2O”) bridge between reactive car- on the exposed carbon surface (Fig. 2A). The second adsorbs right next to the C2O site to extend the cies to a C O C O C (“C3O2”) bridge (Fig. 2B). ption is kinetically hindered and requires consider- tential, and thereby constitutes the rate-determining C3O2” is discharged in two, one-electron steps to stable group, and readily releases CO2 by cutting of bonds. et al. suggested that the anodic oxidation of car- ten carbonates (basic melts) might follow a similar to the Hall process with the exception of the oxy- mation step [3,5]. Since molten carbonates readily into CO2 and O2− at the DCFC operation temper- per et al. proposed that carbonates decompose at a rature to form oxygen ions (Eq. (10)), which ini- ubsequent carbon oxidation reactions as shown in ). 2CO2 + 2O2− (10) ial description of the carbon electrochemical oxidation. (A) the n adsorption and (B) the second oxygen ion adsorption and CO2 .
• D. Cao et al. / Journal of Power Sources 167 (2007) 250–257 253 The mechanism for anodic oxidation of carbon in molten hydroxides is unknown at present. Whether the Cooper mech- anism works or not for molten hydroxide electrolytes needs experimental verification. This is because in molten hydroxides other oxygen containing ions than O2− exist, such as O22−, O2− and OH−, which may take part in the anode oxidation of carbon [4,5]. The electrooxidation of carbon contacting a solid electrolyte, like Y2O3–ZrO2 (YSZ), might follow the similar process to molten carbonates due to presence of O2−, however this is only this author’s hypothesis. 3. DCFC with a molten carbonate electrolyte Mixed molten carbonates (e.g. Li2CO3–K2CO3) have been widely used in the MCFC as electrolytes. They are also attractive for DCFC because of their high conductivity, good stability in the presence of CO2 (the product of carbon electrooxidation), and suitable melting temperature [3,24]. In molten carbonate electrolytes, the anode and cathode reactions might be expressed by Eqs. (11) and (12), respectively. The cell voltage is given by Eq. (13). CO2 is formed at the anode side and consumed at the cathode side, therefore, its partial pressure has an influence on the cell voltage. C + 2CO3 O2 + 2CO2 Ecell = E◦ +(R Cooper (LLNL, Li entation de K2CO3 me carbon par Fig. 3. Schem anode [11]. Fig. 4. P as the curre fine nickel anode and zirconia fel ments in a lithiated by active struc 5–45◦ angl elec e. S e siz . Cu chie ial) ( calc l bla pro hic d e site anc to d the kel s lity. de p LLN l/car [33– veloped a DCFC model based on the LLNL cell design in o provide a theoretical base for DCFC system [36]. The tion results indicated the system having a net electrical ncy of 78%. FC with a molten hydroxide electrolyte first DCFC successfully demonstrated by William s used molten sodium hydroxide as electrolyte. However, hen for a long time, molten hydroxides have been rejected DCFC electrolyte because they react with CO2 produced rbon oxidation to form carbonates. In recent years, hers in Scientific Application & Research Associates 2− → 3CO2 + 4e− (11) + 4e− → 2CO32− (12) − (RT/4F ) ln [CO2]3anode T/4F ) ln([O2][CO2]2cathode) (13) et al. at Lawrence Livemore National Laboratory vermore, CA) constructed a DCFC with a tilted ori- sign (Fig. 3) [6,11,12,31,32]. A 32% Li2CO3–68% lt was used as the electrolyte. The anode is a paste of ticles (
• 254 D. Cao et al. / Journal of Power Sources 167 (2007) 250–257 (SARS, Cypress, CA) revived the investigation of DCFC using molten hydroxide as electrolyte [37]. Comparing with molten carbonates acting as el higher acti means a hi [4]. So wi be operate The low op materials fo costs. Besi low tempe Boudouard The above carbonate f and Tremi carbon ele chemical p (Eq. (15)). a fast chem (Eq. (17)), 2OH− + C C + 6OH− 6OH− = 3 C + 3O2− The rate of O2− and the water c (14) and (1 Taking adv developed humidified the electrol but also in DCFC take like a semi graphite ro molten sod container, fied air is via a gas d 650 ◦C. Se which, nick alytic activ material, ai The open c age power with over 180 mW cm greater tha can be sign electrode m has no sepa carbon, a m chem ano ed a s this s th ratur nic m l tes r to v lyte ment ith s ele ce i cell y pure NaOH. The reduced cell performance was mainly the reduced cathode performance. Therefore, it seems car- content does not significantly affect the cell deterioration ithin 35 mol% carbonates. Performance of the SARA direct carbon fuel cell with different anode ]. , molten hydroxides have a number of advantages in ectrolytes, such as a higher ionic conductivity and a vity of the carbon electrochemical oxidation, which gher carbon oxidation rate and a lower overpotential th molten hydroxides as the electrolyte DCFC can d at a lower temperature, typically around 600 ◦C. eration temperature allows the use of less expensive r DCFC fabrication and thus brings down the DCFC des, the dominant product of carbon oxidation at rature (i.e.
• D. Cao et al. / Journal of Power Sources 167 (2007) 250–257 255 Saddawi et al. at West Virginia University developed a method to produce solid cylindrical carbon rods for SARA’s DCFC [42]. The fuel rods were made with varying amounts of petroleum coke, coal tar binder pitch, and either one or two coal-derived fuels. The chemical composition, density, and electrical resistivity of the resulting carbon rods were analyzed. SARA test results indicated that coal-derived rods perform significantly better than their graphite counterparts due to increased electrochemical activity [9]. Notably, the mecha- nisms for the electrooxidation of carbon (anode reaction) and the electro-reduction of oxygen (cathode reaction) in molten sodium hydroxide are not well understood at present. The over- all electrode reaction may be given by Eqs. (18) and (19) for anode and Eq. (20) for cathode [4]. C + 6OH− → CO32− + 3H2O + 4e− (18) C + 2CO32− → 3CO2 + 4e− (19) O2 + 2H2O + 4e− → 4OH− (20) 5. DCFC with YSZ-based solid electrolyte Balachov et al. at SRI International patented an unique DCFC design which combined advances in SOFC and MCFC tech- nology (Fig. 7A) [43,44]. The key component of their DCFC is a U-tube consisting of, from inner to outer of the tube, a Fig. 7. Schem SOFC and M anode [43,44] Fig. 8. Pe metal mesh thanum Str YSZ), and immersed Li2CO3 + K better oper tact betwe enhance m been tested and mixed SRI has ach 950◦ (Fig. achieved by A similar s was used as at oxid ng 0 ZrO de a W c ed at Akron has recently started the investigation of SOFC olid carbon as fuel [49]. The focus is on the anode cat- he preliminary results indicated that with coke as fuel, en circuit voltage can reach to around 0.8 V at temper- round 700 ◦C, and a current density of 50 mA cm−2 at can be obtained at a cell temperature of 950 ◦C. The for DCFC using solid electrolyte include poor contact n the carbon anode and the electrolyte and the high oper- emperature, which may lead to formation of CO due to uard reaction [50]. Duskin and Gu¨r at Clean Coal Energy Stanford, CA) recently envisions a DCFC combining and fluidized-bed technologies (Fig. 9) [51,52], but they have an operating system at present. The configuration s continuous carbon feeding and good contact between fuel and solid electrolyte reducing mass transport ion. atic of the SRI direct carbon fuel cell combining advances in the CFC technology. (A) cell configuration and (B) flowing liquid . [45]. Tao direct By usi thick ( as ano of 10 m obtain sity of with s alyst. T the op ature a 0.8 V issues betwee ation t Boudo (CCE, SOFC do not enable carbon limitat rformance of the SRI direct carbon fuel cell liquid anode [43]. cathode current collector, a cathode layer (e.g. Lan- ontium Managanate, LSM), an electrolyte layer (e.g. a metal mesh anode current collector. The U-tube is into a liquid anode comprising a mixture of molten 2CO3 + Na2CO3 and carbon particles. The DCFC is ated in a flow mode (stirring) to facilitate the con- en carbon particles and anode current collector to ass transport (Fig. 7B) [43]. A variety of fuels have , including coal, tar, coke, acetylene black, plastic waste. Using conventional coal without pretreatment, ieved power densities greater than 100 mW cm−2 at 8) [43], which is comparable to the power densities commercial MCFC plants operating on natural gas. tudy was recently reported by Pointon et al. The cell a high energy density battery for military application CellTech Power LLC (Westborough, MA) tested ation of coal in a SOFC-like structure [46–48]. .6 mm thick La0.84Sr0.16MnO3 as cathode, 0.12 mm 2)(HfO2)0.02(Y2O3)0.08 as electrolyte, carbon black nd Pt as anode current collector, a power output m−2 at 0.248 V and 50 mW cm−2 at 0.507 V were 800 and 1002 ◦C, respectively. Chuang at Univer-
• 256 D. Cao et al. / Journal of Power Sources 167 (2007) 250–257 ing SO 6. Conclu Studies conversion ical step in configurati scale. Effic The payoff that the DC convention reduce the tenth. Ther coal and ot efficient an Since D stantial eff challenges, The mecha ous molten the carbon investigate at the mol science, e. ture and th will provid coal and fo pensive an biomass an rities, like s of these im trolyte, ano materials s for the dev determine Ash accum lifetime. T Weaver et y ash the . Th elec on. A t int of cher n be s ha hode nent cata manc sca FCs de, a nit s , ado Fig. 9. Schematic of CCE envisioned direct carbon fuel cell combin sions and outlook in the last few years have demonstrated that direct of solid carbon to electricity in a single, electrochem- a fuel cell is feasible. Several DCFCs with different ons have been successfully tested on the laboratory iencies higher than 80% were shown to be achievable. for DCFC development is extraordinary considering FC might operate at about twice the efficiency of al steam power plant and more significantly might emissions of a coal-firing power plant to about one- efore, the DCFC provides a technical option for using her solid carbon-rich fuels in a manner which is more d cleaner. CFC technology is still at the beginning stage, sub- orts need to be undertaken to address many serious both in the fundamental and the engineering aspects. nism of electrochemical oxidation of carbon in vari- salt electrolytes and the dynamic behavior within coal fl change system ery of attenti withou release Resear that ca DCFC the cat compo electro perfor The the DC electro large u SOFC -electrolyte double layer region should be further d in order to better understand the anodic process ecular level [22]. Understanding of the underlying g. the relationship between the carbon nanostruc- e electrochemical reactivity is required. Such studies e useful information for extraction of carbon from r developing anode catalysts if needed. Many inex- d readily available carbon fuels, such as coal, coke, d organic garbage (e.g. waste plastics) contain impu- ulfur, hydrogen, nitrogen and minerals. The influence purities on the carbon electrooxidation rate, elec- de current collector and other fuel cell component hould be examined. Such research is very important elopment of a practical DCFC, and the results will to what extent carbon fuels have to be pretreated. ulation could be a key factor determining the DCFC he only literature on the ash effect was reported by al. [20]. They claimed that addition of 10 wt% of trodes to s with a soli impossible fuel withou performanc cleaned and of electricit produced b temperatur integration account all less feasibl Acknowled We gra sity and H RC2006QN FC and fluidized-bed technologies [52]. to molten carbonate electrolyte did not measurably carbon polarization curve measured in a half cell e separation of ash from the electrolyte and recov- trolyte from exhausted melt also deserve research DCFC allowing continuous feeding of solid carbon errupting the cell operation and without explosive volatile components demands a smart cell design. s at LLNL have recently proposed a self-feeding cell refueled pneumatically with cleaned coal. Current ve adopted MCFC or SOFC components, such as, catalyst and/or electrolyte. The suitability of these s to the DCFC requires further confirmation. New lysts and molten salts systems might improve DCFC e. le-up of DCFC will face several real challenges. All tested so far are essentially mono-polar with a small nd their scale-up will lead to a high IR drop and a ize. Fuel cells using gaseous fuel, e.g. MCFC and pt compact bipolar stack designs with large elec- cale-up for good economics. However, for a DCFC d carbon feedstock, the bipolar configuration seems . Raw coal might not be directly used as the DCFC t cleaning because its impurities will decrease cell e and shorten the cell life. So the DCFC requires processed coal. Cleaning coal will increase the cost y generated by DCFC power plants. Notably, the heat y a DCFC may not be enough to maintain the cell e. A possible solution to this problem might be the of a DCFC with a MCFC or a SOFC. Taking into these remaining issues, the DCFC system might be e than other type of fuel cells, e.g. MCFC and SOFC. gements tefully acknowledge Harbin Engineering Univer- arbin Bureau of Science and Technology (Grant 001021) for supporting this work.
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Direct carbon fuel cell: Fundamentals and recent developments Introduction Electrochemical oxidation of carbon Basic findings Mechanism DCFC with a molten carbonate electrolyte DCFC with a molten hydroxide electrolyte DCFC with YSZ-based solid electrolyte Conclusions and outlook Acknowledgements References