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.     

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.     

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.     

Performance assessment of concentrated solar power plants based on carbon and hydrogen fuel cells

In spite of the recent success on the implementation of Concentrating Solar Power (CSP), still this technology needs a substantial enhancement to achieve competitiveness. This paper provides thorough insight after previous analyses on an alternative concept for higher efficiency CSP systems based on the replacement of the power block by an electrochemical conversion system. Concentrating solar energy is herewith used to decompose methane into hydrogen and carbon, which are used in hydrogen and carbon fuel cells for electricity generation. This approach envisages modular, efficient and flexible generation plants. Dispatchability can be achieved by storing the solid carbon. Solar-to-electricity efficiency was calculated assuming thermodynamic equilibrium composition and experimental data available from literature, and compared with those of conventional power generation systems and commercial CSP plants. It is concluded that this new-generation CSP concept is potentially able to produce power more efficiently than the current state-of-the art solar thermal power plants.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Pretreated mesocarp fibre biochars as carbon fuel for direct carbon fuel cells

This work focuses on the effect of acid and alkali pretreatment of palm mesocarp fibre (PMF) on its fuel performance in a direct carbon fuel cell (DCFC). PMF is pretreated with acid and alkali in the range of 0.1 M–4 M and followed by pyrolysis to produce biochar fuel. Performance is evaluated in the DCFC at 750 °C, 800 °C, and 850 °C. This work reveals that 2.0 M HCl treated PMF biochar gives the lowest ash value (0.1 wt%) and the highest O/C ratio among all tested biochars. The acid pretreatment contributes to enhanced electrochemical reactivity of the PMF biochar, which gives a peak power density output of 11.8 mW cm^?2 at 850 °C in the DCFC. This obtained peak power density is higher than the power density of untreated biochar, recorded at a value of 0.70 mW cm^?2. The results indicate that reduced ash, the existence of oxygen functional groups, and porous fibrous structure have increased the electro-oxidation active sites of the pretreated biochar fuel in DCFC.     

Highly efficient CO2 bubble removal on carbon nanotube supported nanocatalysts for direct methanol fuel cell

In this paper, we investigate the CO₂ microbubble removal on carbon nanotube (CNT)-supported Pt catalysts in direct methanol fuel cells (DMFCs). The experiments involve the incorporation of near-catalyst-layer bubble visualization and simultaneous electrochemical measurements in a DMFC anodic half cell system, in which CH₃OH electro-oxidation generate carbon dioxide (CO₂) microbubbles. We observe rapid removal of smaller CO₂ bubble sizes and less bubble accumulation on a Pt-coated CNT/CC (Pt/CNT/CC, CC means carbon cloth) electrode. The improved half cell performances of the high CO₂ microbubble removal efficiency on the CNT-modified electrode (Pt/CNT/CC) were 34% and 32% higher than on Pt/CC and Pt/CP electrodes, respectively.     

Direct methane cracking using a mixed conducting ceramic membrane for production of hydrogen and carbon

The direct cracking of methane can be used to produce CO_x and NO_x-free hydrogen for proton exchange membrane fuel cells. Recent studies have been focused on enhancing the hydrogen production using the direct thermocatalytic decomposition of methane as an attractive alternative to the conventional steam reforming process. We present the results of a systematic study of methane direct decomposition using a mixed conducting oxide, Y-doped BaCeO₃, membrane. A dense disk-shaped BaCe_0.85Y_0.15O₃ membrane was successfully prepared and covered with Pd film, as the catalyst for the methane decomposition. For the methane thermocatalytic decomposition, the methane gas was employed as reactant on the membrane side with a pressure of 10² kPa and rate of 70 ml/min at the reaction temperatures of 600, 700, and 800 °C. The hydrogen was selectively transported through the mixed conducting oxide membrane to the outer side. In addition, the carbon, which is a by-product after methane decomposition, showed the morphologies of sphere-shaped nanoparticles and the transparent sheets.     

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).     

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.