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.     

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.     

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).     

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.     

Thermodynamic analysis of gasification-driven direct carbon fuel cells

The gasification-driven direct carbon fuel cell (GD-DCFC) system is compared with systems using separate gasification steps prior to work extraction, under autothermal or indirect constraints. Using simple system exergy analysis, the maximum work output of the indirect gasification scheme is 4–7% lower than the unconstrained direct approach, while the work output of the autothermal gasification approach is 12–13% lower than the unconstrained case. A more detailed calculation for the DCFC and indirect gasification plants, using common solid fuel compositions, gives conversion efficiencies in the range of 51–58% at an operating voltage of 0.7 V selected for both systems in this study. In contrast, the conversion efficiency of the autothermal gasification approach is estimated to be 33–35% at 0.7 V. DCFC efficiencies can be increased to over 60% by an increase in operating voltage and/or inclusion of a bottoming cycle. The thermodynamic model also indicates that steam gasification yields similar work output and thermal efficiency as for CO₂ gasification. Open circuit potential measurements agree with equilibrium calculations both for the C–O and C–H–O gasification systems, confirming the governing mechanism and feasibility of the GD-DCFC. Current–voltage measurements on an un-optimized system demonstrate power densities of 220 mW cm⁻² at 0.68 V during operation at 1178 K.     

Novel mesoporous carbon ceramics composites as electrodes for direct methanol fuel cell

In this work, a new family of materials for electrodes of direct methanol fuel cell (DMFC) is presented. Mesoporous carbon ceramics (MCCs) are obtained by the addition of commercial graphite into the synthesis gel of SBA-15 mesoporous silica with SiO₂/C weight ratios of 1/1 and 1/3. X-ray diffraction confirms both the formation of organized silica and the presence of graphite, and nitrogen physisorption measurements show that the presence of a graphitic phase does not interfere in the silica pore diameter although it diminishes the surface area. The MCCs modified with Pt or PtRu are tested as DMFC electrodes and compared with the commercial support Vulcan XC-72R. When used as cathode, the system using MCC-SBA-15 with SiO₂/C weight ratios of 1/1 presents a negligible performance, while the MCC-SBA-15 with SiO₂/C weight ratios of 1/3 is 2.9 times less active than the commercial support. On the other side, when used as anode, the MCC-SBA-15 with SiO₂/C weight ratios of 1/3 displays performances comparable to Vulcan XC-72R.     

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.     

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.     

Integration of direct carbon fuel cells with concentrated solar power

In this paper, we will report on a study on the thermodynamic feasibility of a concept that realizes the cracking of methane with a concentrated solar power (CSP) reactor and electricity production with a direct carbon fuel cell (DCFC) and its possible contribution to a clean energy supply for Europe in the long-term future. The natural gas (methane) is decomposed in an endothermic reaction into hydrogen and carbon. The separated carbon is fed to a direct carbon fuel cell (DCFC) and converted with high efficiency to electric power. A model of the proposed concept is carried out in the flow sheet program Cycle-Tempo and the results of the simulations and the corresponding analysis are presented in this paper. Finally the location factors influencing the implementation of this concept in the north of Africa are evaluated.     

Highly efficient utilization of industrial barium slag for carbon gasification in direct carbon solid oxide fuel cells

Direct carbon solid oxide fuel cells (DC-SOFCs) are recognized as an efficient energy conversion device. With regard to their operation mechanism, the reverse Boudouard reaction rate is the crucial factor influencing cell performance. In this work, a new-type catalyst derived from industrial barium slag (BS) was first developed to enhance the reverse Boudouard reaction and DC-SOFC performance. The chemical composition and micro-morphologies of BS and barium slag-derived catalyst (BSC) were characterized in detail. The superiorities of BS and BSC were reflected in the enhanced DC-SOFC performance and high fuel utilization. The single cell fueled by BSC-loaded carbon yielded the best output of 249 mW cm⁻² at 850 °C. This result was comparable to the 266 mW cm⁻² output of a hydrogen-fueled SOFC due to the superior catalytic activity of metallic catalysts toward carbon gasification. The advantage of the BSC was also observed in the durable operation of the corresponding DC-SOFCs, which lasted for 36.2 h at 50 mA with the fuel utilization of 29.0%. This work provides a new channel for green and efficient utilization of BS and other industrial residues, and a novel option to the development of energy conversion technology.     

Use of ash-free “Hyper-coal” as a fuel for a direct carbon fuel cell with solid oxide electrolyte

“Hyper-coal”, produced by the Kobe Steel Company, was investigated by analytical and physico-chemical methods to consider its potential usability as a fuel for a direct carbon fuel cell with solid oxide electrolyte (DC-SOFC). The performed tests showed that DC-SOFCs fed with this processed fossil coal were characterized by stable operation with reasonable current and power densities. The performance of the fuel cells can be improved by using iron as a catalyst for the anodic reaction and by the choice of appropriate working conditions.     

Optimization of a direct carbon fuel cell for operation below 700 °C

Direct carbon fuel cells (DCFCs) are the most efficient technology to convert solid carbon energy to electricity and thus could have a major impact on reducing fuel consumption and CO₂ emissions. The development of DCFCs to commercialisation stage is largely prohibited by their poor power densities due to the high resistive loss from anode. Here, we report a high-performance Sm_0.2Ce_0.8O_1.9 electrolyte-supported hybrid DCFC with Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ cathode and optimised anode configuration. The catalytic oxidation of carbon is improved, which results in an area specific resistance of only 0.41 Ω cm² at 650 °C at the anode. The hybrid DCFC achieves a peak power density of 113.1 mW cm⁻² at 650 °C operating on activated carbon. The stability of the fuel cell has also been improved due to the optimised current collection.     

Nitrogen-doped carbon xerogel as high active oxygen reduction catalyst for direct methanol alkaline fuel cell

A novel catalyst based on nitrogen-doped carbon xerogel for oxygen reduction reaction (ORR) was prepared via a sol–gel process, following by the subsequent pyrolysis under ammonia atmosphere. The catalytic activity in alkaline media was optimized by tuning the metal (cobalt) ratio to the gel precursor. Sample with the optimum activity was characterized by transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis and electrochemical measurements. Results show that the catalyst possesses an amorphous microstructure with nitrogen doped on the surface. The nitrogen-doped carbon xerogel displays comparable ORR activity and superior methanol tolerance than Pt/C in alkaline medium, demonstrating its promising application in direct methanol alkaline fuel cells as non-precious cathode catalyst.     

Planar polyphthalocyanine cobalt absorbed on carbon black as stable electrocatalysts for direct methanol fuel cell

In this work, a novel catalyst is prepared by dispersing planar polyphthalocyanine cobalt (PPcCo) synthesized by polymerizing cobalt (II)-4, 4′,4″,4?-phthalocyanine tetracarboxylic acid (TcPcCo) using a high surface area carbon powder (Vulcan XC 72), and then heat-treated in argon (Ar) atmosphere. The polymer and PPcCo/C catalysts are characterized systematically by a variety of methods, such as ultraviolet-visible (UV-vis) spectrophotometer, Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM). Results show that the PPcCo obtained is stable below 600 °C. The active site of PPcCo/C is CoN₄ in phthalocyanine ring, and the PPcCo is dispersed homogeneously on the surface of XC 72. Electrocatalytic properties and electrochemical stability of the catalysts in 0.5 mol L⁻¹ H₂SO₄ are evaluated by RDE measurements. The initial potential for O₂ reduction in O₂-saturated H₂SO₄ is 0.81 V and it catalyzed O₂ reduction mainly through a four-electron process. Almost no performance degradation is observed over continuous cyclic voltammetry (CV) at 10,000 cycles (4 days). Polarization curves obtained by linear sweep voltammetry (LSV) at 200 cycles also show no change. PPcCo/C catalysts display significant electrocatalytic performance for O₂ reduction, tolerance towards methanol, and long-term stability.     

A novel Chinese parasol leaf biochar fuelled direct carbon solid oxide fuel cell for high performance electricity generation

A direct carbon solid oxide fuel cell is a new technology for clean and efficient utilization of carbon resources to generate electricity, with the advantages of high power generation efficiency and wide available fuel flexibility. Biomass, in virtue of large specific surface area, numerous oxygen-containing functional groups which can promote the electrooxidation of carbon, and low ash content which can increase the cell stability, reveals promising feasibility as a fuel for direct carbon fuel cells. Here we report a high-performance direct carbon fuel cell utilizing Chinese parasol leaf biochar as fuel, among which Ag–Gd_0.1Ce_0.9O_2-δ and Al₂O₃ doped yttria-stabilized zirconia are employed as symmetrical electrodes and electrolyte materials, respectively. The cell with pure leaf biochar fuel gives a maximum power density of 249 mW cm^?2 and an open circuit voltage (OCV) of 1.008 V at 850 °C while an improved performance of 272 mW cm^?2 and OCV of 1.01 V are achieved for the cell fuelled by Fe catalyst-loaded leaf biochar. The above results demonstrate that Chinese parasol leaf biochar can be applied as a potential fuel for high performance direct carbon solid oxide fuel cells.     

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.     

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.     

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.     

Preparation of PVdF/PSSA composite membranes using supercritical carbon dioxide for direct methanol fuel cells

Composite membranes were prepared using supercritical carbon dioxide (scCO₂) impregnation and polymerization procedures and were optimized as electrolytes by controlling the amount of divinylbenzene (DVB) for polymerization. These poly(vinylidene fluoride)/polystyrene sulfonic acid (PVdF/PSSA) membranes were characterized by various methods. The cross-sectional superficial morphology and structure of the PVdF/PSSA membranes were analyzed by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), FT-IR and small-angel X-ray scattering (SAXS). The ion exchange capacity (IEC), ion conductivity, methanol permeability and cell performance of PVdF/PSSA membranes were measured and compared with Nafion 115. As the concentration of added DVB increased, the ion conductivity and methanol permeability of the PVdF/PSSA membranes decreased. The PVdF/PSSA membrane containing 7.5 wt% DVB achieved 94% of the current density with Nafion 115.     

Integration of direct carbon and hydrogen fuel cells for highly efficient power generation from hydrocarbon fuels

In view of impending depletion of hydrocarbon fuel resources and their negative environmental impact, it is imperative to significantly increase the energy conversion efficiency of hydrocarbon-based power generation systems. The combination of a hydrocarbon decomposition reactor with a direct carbon and hydrogen fuel cells (FC) as a means for a significant increase in chemical-to-electrical energy conversion efficiency is discussed in this paper. The data on development and operation of a thermocatalytic hydrocarbon decomposition reactor and its coupling with a proton exchange membrane FC are presented. The analysis of the integrated power generating system including a hydrocarbon decomposition reactor, direct carbon and hydrogen FC using natural gas and propane as fuels is conducted. It was estimated that overall chemical-to-electrical energy conversion efficiency of the integrated system varied in the range of 49.4–82.5%, depending on the type of fuel and FC used, and CO₂ emission per kW_elh produced is less than half of that from conventional power generation sources.