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.     

Surface modification of carbon fuels for direct carbon fuel cells

The direct carbon fuel cell (DCFC) is a promising power-generation device that has much higher efficiency (80%) and less emissions than conventional coal-fired power plants. Two commercial carbons (activated carbon and carbon black) pre-treated with HNO₃, HCl or air plasma are tested in a DCFC. The correlation between the surface properties and electrochemical performance of the carbon fuels is explored. The HNO₃-treated carbon fuels have the highest electrochemical reactivity in the DCFC due to the largest degree of surface oxygen functional groups. The overall effect on changing the electrochemical reactivity of carbon fuels is in the order HNO₃ > air plasma ≈ HCl. Product gas analysis indicates that complete oxidation of carbon to CO₂ can be achieved at 600–700 °C.     

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.     

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.     

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.     

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.     

Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells

The microstructure of anode has a significant influence on the whole electrochemical performance of solid oxide direct carbon fuel cells (SO-DCFCs). The tubular SO-DCFCs based on cathode supported solid oxide fuel cells was fabricated by dip-coating and co-sintering techniques. As the anode porosity mainly came from the pore former (graphite) in the dip-coating process, different contents of graphite were added into the anode slurries. When the graphite was 10.1% wt., the SO-DCFCs showed the best performance and stability. The peak power density reached 242 mW cm⁻² at 850 °C, with carbon black (located 5% Fe) as the fuel and air as the oxidant.     

Evaluation of raw coals as fuels for direct carbon fuel cells

As a promising high-temperature fuel cell, the direct carbon fuel cell (DCFC) has a much higher efficiency and lower emissions compared with conventional coal-fired power plants. In the present DCFC system, four Australian coals from Central Queensland are successfully tested at 600-800 °C. The electrochemical performances of these coals are highly dependent on their intrinsic properties, such as chemical composition, surface area, concentrations of oxygen-containing surface functional groups and the nature of mineral matter in their ashes. Impurities such as Al₂O₃ and SiO₂ lead to an inhibitive effect during the anodic reaction in the DCFC, while CaO, MgO and Fe₂O₃ exhibit a catalytic effect on the electrochemical oxidation of carbon.     

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.     

Electrochemical behavior of surface treated metal bipolar plates used in passive direct methanol fuel cell

Corrosion resistance performance of SS316L treated by passivation solution was investigated in a simulated environment of the passive direct methanol fuel cell (DMFC). Electrochemical impedance spectroscopic (EIS) test showed that polarization resistance of untreated and treated SS316L were 1191 Ω cm² and 9335 Ω cm², respectively. The above result agreed with the Tafel slope analysis of potentiodynamic polarization curves. Comparing the untreated and treated SS316L in the simulated environment of DMFC anode working conditions, it was observed that the corrosion current density of treated SS316L as estimated by 4000 s potentiostatic test reduced from 38.7 μA cm⁻² to 0.297 μA cm⁻², meanwhile, the current densities of untreated and treated SS316L in cathode working conditions were 3.87 μA cm⁻² and 0.223 μA cm⁻², respectively. It indicated that the treated SS316L should be suitable in both anode and cathode environment of passive DMFCs. The treated SS316L bipolar plates have been assembled in a passive single fuel cell. A peak power density of 1.18 mW cm⁻² was achieved with 1 M methanol at ambient temperature.     

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.     

Effect of coal based pyrolysis gases on the performance of solid oxide direct carbon fuel cells

Solid oxide direct carbon fuel cells (SO-DCFCs) offer a promising way to convert common carbon materials such as coal and biomass to electricity efficiently. It is inevitable that these carbon fuels decompose during SO-DCFCs operation and release pyrolysis gases, which may affect the cell performance and change the anode reaction pathway. In this study, three coals and an activated carbon are used to investigate the role of pyrolysis gases in SO-DCFCs. The cell polarization curve and durability performance are measured for all raw fuels and a pyrolysis pretreated coal at temperatures ranging from 1023 to 1123 K. Mass spectrometry is employed to analyze pyrolysis gases under open circuit operation. The results show that all raw fuels have good electrochemical performance at 1123 K. The cell power density is greatly improved by the presence of pyrolysis gases (mainly H₂ and CO) in the anode chamber. And the cell durability performance is largely dependent on both the amounts of active pyrolysis gases and the carbon gasification reactivity. It is demonstrated that the dominant electrochemical reactions are the electrooxidation of gaseous fuels, namely pyrolysis gases in the initial stage of discharge, then CO produced by carbon gasification. The direct electrooxidation of carbon is less significant than that of gaseous fuels.     

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.     

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.     

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.     

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.     

Cell performance of polymer electrolyte fuel cell with urchin-like carbon supports

Urchin-like structured carbon comprising carbon nanotubes grown on Fe catalyst-seeded mesoporous carbon have shown promising results as catalyst supports for use in direct methanol fuel cells (DMFCs) and proton exchange membrane fuel cells (PEMFCs). The Fe catalyst is prepared on the mesoporous carbon by immersion process followed by a high temperature reduction. The growth of carbon nanotubes then progress, for a predetermined time, through the thermal decomposition of acetylene at 800 °C. The resulting structure, comprising intimately connected mesoporous carbon and carbon nanotubes, is shown to offer performance advantages as a catalytic support for DMFCs and PEMFCs. When the hot-pressing pressure is fixed 20 kg cm⁻² to fabricate a membrane electrode assembly (MEA) with urchin-like carbon supports, the CNT growth time is found to be 60 min for a highest maximum power density in both DMFCs and PEMFCs. The maximum power densities are 43 and 79% higher than those with purely mesoporous carbon in DMFCs and PEMFCs, respectively. In a direct comparison with commercial E-TEK catalyst, the urchin-like catalyst shows higher maximum power densities, in DMFC and PEMFC, by approximately 17 and 31%, respectively.     

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).     

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.