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As an electrochemical device that converts the chemical energy of fuel directly into electrical energy, the fuel cell is a new alternative technology that uses fuel from renewable sources and generates power for sustainable development and energy security. Among various types of fuel cells, the direct carbon fuel cell (DCFC) has higher efficiency because carbon fuel has higher energy density than liquid or gas fuel. However, the current development of DCFCs is still limited by the sluggish activity of the cathode reaction. In this study, a new composite cathode made of Gd0.2Ce0.8O1.9 (GDC) and SrFe1−xTixO3–δ (SFT) is developed for a Nicotiana tabacum-derived carbon fuel-based DCFC. The structural, optical, and electrochemical properties of the materials are systematically evaluated. X-ray diffraction analysis results show a cubic structure of GDC and cubic perovskite phase of SFT in the sample, with crystallite sizes of 37 and 15 nm, respectively. Ultraviolet–visible spectroscopy reveals an indirect band gap which exhibits a red shift. Fourier transform infrared spectroscopy confirms the presence of Ce–O, Sr–Ti–O, and Fe–O functional groups in all the samples. Scanning electron microscopy analysis shows the morphology and particle size of the materials. The sample Gd0.2Ce0.8O2-δ–SrFe0.96Ti0.04O1.9 exhibits the highest electrical conductivity of 4.96 S cm−1 in an oxygen atmosphere at 600 °C and a higher power density of 40 mW cm−2 at 600 °C compared to other samples using Nicotiana tabacum carbon fuel. These findings indicate that the developed composite cathode is an efficient cathode for low-temperature DCFCs. [ABSTRACT FROM AUTHOR]

Developers of direct carbon fuel cell (DCFC) technology seek to convert carbon to a pure or highly concentrated carbon dioxide stream that could ultimately be transported and sequestered. Carbon black produced by natural gas cracking processes is very pure and is therefore a candidate for use in DCFCs. In this study the performance of As received and Treated carbon blacks are compared in a planar cell configuration. At a more viable operating temperature of 750 °C, the As received carbon was found to produce a peak power density of 51 mW/cm2 at a current density of 104 mA/cm2. The performance then increased to 105 mW/cm2 at a current density of 145 mA/cm2 at 850 °C. Acid treatment of the carbon was found to improve performance of the DCFC at 750 °C by 35% (69 mW/cm2 at 160 mA/cm2) and only by 5% at 850 °C (111 mW/cm2 at 201 mA/cm2). • A commercial carbon black was used to fuel a planar, direct carbon fuel cell (DCFC). • An acid treatment method applied to the carbon black increased its wettability. • A DCFC output higher power density with Treated than with As received carbon black. [ABSTRACT FROM AUTHOR]

As municipal solid waste (MSW) is a low-cost, available, and basically renewable source, energy recovery from MSW is a beneficial technique for reducing fossil fuel usage while also lowering MSW disposal challenges. The current study proposes a novel scheme consisting of plasma gasification melting (PGM) process and direct carbon fuel cell (DCFC) system for converting MSW into high-quality syngas and power. Plasma air and high-temperature steam are employed in the process as gasification agents. The DCFC runs on the pyrolysis char, which is a carbon-rich source and is extracted during the PGM process and sent to the DCFC. The process is simulated in Aspen Plus, and a computational model for estimating the DCFC's output voltage and power is developed in MATLAB. Effects of main operating parameters, including equivalence ratio, steam to biomass ratio, plasma energy ratio, char separation fraction, and fuel cell operating temperature on the system's performance indicators, namely syngas composition, syngas lower heating value, syngas yield, gasification temperature, overall efficiency, and DCFC power output were investigated. By increasing the char separation fraction, more pyrolysis char was extracted and forwarded to the DCFC, which leads to less amount of char entering the gasification section and higher power output through the DCFC. Keeping other parameters constant, at a fuel cell operating temperature of 923 K, the optimal value for char extraction fraction was 0.15, resulting in a 79.5% overall efficiency. • MSW to Syngas & Power, integrating Plasma Gasification & Direct Carbon Fuel Cell. • The Direct Carbon Fuel cell operates on pyrolysis char, a carbon-rich source. • Investigating key parameters for overall efficiency & high-quality syngas production. • Enhanced efficiency: Optimal char extraction fraction yields 79.5% overall efficiency. [ABSTRACT FROM AUTHOR]

There is much organic waste that comes from by-products of agriculture and product processing, solid waste from livestock, and municipal waste. Conventional methods that are widely used for the treatment and management of organic fractions of waste are landfilling, composting, anaerobic digestion, incineration, gasification, and pyrolysis. Among the above methods, pyrolysis is a relatively simple, robust, and scalable technology for transforming diverse organic waste feedstock into renewable energy products. Recently, the electrochemical conversion of biochar into electricity in direct carbon fuel cells (DCFC) has also been investigated and shown to be feasible and highly efficient. This paper focuses on the utilization of organic waste as a fuel and the investigation of their characteristics during electrochemical reactions in molten hydroxide direct carbon fuel cells (MH-DCFCs). Organic waste of different origins (the food-processing industry, urban and suburban areas, municipal solid organic waste, sewage sludge) with diversified characteristics was used as the main feedstock. The lowest power density was determined for sewage sludge (5.1 mW cm−2), and the best results were obtained for peanut shells (53.14 mW cm−2). This study concludes that higher elemental carbon, lower ash content and the presence of reactive surface oxygen functional groups in biochar obtained from organic waste might contribute to better cell performance. Moreover, the research establishes the potential of carbonized organic waste as a prospective alternative fuel source for power generation in an MH-DCFC. [ABSTRACT FROM AUTHOR]

To enhance the contact between the electrolyte (source of O2−) and the carbon fuel in solid oxide–direct carbon fuel cells (SO-DCFCs), molten metals and molten salts were used in the anode chamber. Oxygen ions can dissolve and be transported in the molten medium to the anode three-phase boundary to reach and oxidize the carbon particles. To improve the sluggish kinetics of the electrochemical oxidation of carbon, the same molten media can act as redox mediators. Moreover, using a liquid metal/salt anode, tolerant to fuel impurities, the negative effect of carbon contaminants on cell performance is mitigated. In this work, an overview of SO-DCFCs with liquid metals, liquid carbonates, and mixed liquid metals/liquid carbonates in the anode chamber is presented and their performance was compared to that of conventional SO-DCFCs. [ABSTRACT FROM AUTHOR]

Molten carbonate direct carbon fuel cells (MC-DCFCs) allow the efficient and clean use of coal. In this study, a novel anode structure is designed, and the performances of six coal-based fuels are investigated in MC-DCFC. The mechanisms of performance differences are investigated, as well as the effect of operating temperature on performance. The results reveal the fuel cell performance in the following order: meagre coal (109.8) ≈ bituminous coal (108.7) > bituminous coal char (98.1) > lignite coal (83.7) > lignite coal char (71.3) > meagre coal char (53.2) in mW cm−2. Coal performs better because of its high carbon content, high volatile content, rich oxygen-containing functional groups, larger specific surface area, stronger thermal reactivity, and other factors. The electrochemical reactivity of coal fuel increased with higher reaction temperatures and varied throughout the temperature ranges. This study implies that using coal fuel to commercialize MC-DCFC could be a realistic alternative. [Display omitted] • Performance of six coal fuels are investigated in an MC-DCFC with a new anode. • Maximum peak power density of 109.8 mW cm−2 is obtained with a meagre coal. • Mechanism of their performance differences is analyzed. • Their performance stability are discussed. • Effect of operating temperature on the performance is explored. [ABSTRACT FROM AUTHOR]

Hydrogen can be produced from reduction of water at the cathode of a solid oxide electrolysis cell (SOEC) and electricity and CO gas can be co-generated from the oxidation of carbon at the anode of a direct carbon solid oxide fuel cell (DC-SOFC). In the present work, a solid oxide cell (SOC) combining the cathode reaction of the SOEC and the anode reaction of the DC-SOFC is proposed, prepared, and investigated. It is demonstrated that hydrogen and CO can be separately produced from the cathode and anode through simultaneous reduction of water and oxidation of carbon in such a H 2 O-C SOC. Resistances from electrode polarizations are analyzed through measuring current-voltage characteristics, overpotentials, and AC impedances of the cell. The advantage of the H 2 O-C SOC over two separately operated SOEC and DC-SOFC is verified by the resistance analysis. An electrolyte-supported H 2 O-C SOC, with a cermet of silver and gadolinium-doped ceria (Ag-GDC) as the symmetrical electrode material and a steam concentration of 50 % at the cathode, gives an operating current density of 0.67 A cm-2 at the thermal neutral voltage (0.7 V). It stably operates at 0.2 A cm-2 under an applied voltage of only 0.16 V, producing pure hydrogen from the cathode and CO with a concentration of 88 % from the anode. The performance is significantly improved by using an anode-supported H 2 O-C SOC which gives an operating current density of 2.27 A cm-2 at the thermal neutral voltage. [ABSTRACT FROM AUTHOR]

There is much organic waste that comes from by-products of agriculture and product processing, solid waste from livestock, and municipal waste. Conventional methods that are widely used for the treatment and management of organic fractions of waste are landfilling, composting, anaerobic digestion, incineration, gasification, and pyrolysis. Among the above methods, pyrolysis is a relatively simple, robust, and scalable technology for transforming diverse organic waste feedstock into renewable energy products. Recently, the electrochemical conversion of biochar into electricity in direct carbon fuel cells (DCFC) has also been investigated and shown to be feasible and highly efficient. This paper focuses on the utilization of organic waste as a fuel and the investigation of their characteristics during electrochemical reactions in molten hydroxide direct carbon fuel cells (MH-DCFCs). Organic waste of different origins (the food-processing industry, urban and suburban areas, municipal solid organic waste, sewage sludge) with diversified characteristics was used as the main feedstock. The lowest power density was determined for sewage sludge (5.1 mW cm−2), and the best results were obtained for peanut shells (53.14 mW cm−2). This study concludes that higher elemental carbon, lower ash content and the presence of reactive surface oxygen functional groups in biochar obtained from organic waste might contribute to better cell performance. Moreover, the research establishes the potential of carbonized organic waste as a prospective alternative fuel source for power generation in an MH-DCFC.

To enhance the contact between the electrolyte (source of O2−) and the carbon fuel in solid oxide–direct carbon fuel cells (SO-DCFCs), molten metals and molten salts were used in the anode chamber. Oxygen ions can dissolve and be transported in the molten medium to the anode three-phase boundary to reach and oxidize the carbon particles. To improve the sluggish kinetics of the electrochemical oxidation of carbon, the same molten media can act as redox mediators. Moreover, using a liquid metal/salt anode, tolerant to fuel impurities, the negative effect of carbon contaminants on cell performance is mitigated. In this work, an overview of SO-DCFCs with liquid metals, liquid carbonates, and mixed liquid metals/liquid carbonates in the anode chamber is presented and their performance was compared to that of conventional SO-DCFCs.

Abstract Direct carbon fuel cell (DCFC) is a promising technology with high energy efficiency and abundant fuel. To date, a variety of DCFC configurations have been investigated, with molten hydroxide, molten carbonate or oxides being used as the electrolyte. Recently, there has been particular interest in DCFC with molten carbonate involved. The molten carbonate is either an electrolyte or a catalyst in different cell structures. In this review, we consider carbonate as the clue to discuss the function of carbonate in DCFCs, and start the paper by outlining the developments in terms of molten carbonate (MC)-based DCFC and its electrochemical oxidation processes. Thereafter, the composite electrolyte merging solid carbonate and mixed ionic–electronic conductors (MIEC) are discussed. Hybrid DCFC (HDCFCs ) combining molten carbonate and solid oxide fuel cell (SOFC) are also touched on. The primary function of carbonate (i.e., facilitating ion transfer and expanding the triple-phase boundaries) in these systems, is then discussed in detail. Finally, some issues are identified and a future outlook outlined, including a corrosion attack of cell components, reactions using inorganic salt from fuel ash, and wetting with carbon fuels.

Summary: The inefficient application of fossil energy is one of the fundamental factors leading to global warming. Bioenergy is a renewable energy and also a carbon‐neutral energy, with advantages of extensive sources, diverse products, and environmental friendliness. Direct carbon solid oxide fuel cell (DC‐SOFC) is a novel high‐efficient bioenergy technology, which is able to directly convert the chemical energy of biochar into power. A biochar derived from rice husk is applied to power an electrolyte‐supported DC‐SOFC. The maximum power density of the cell with pure rice husk biochar is 135 mW cm−2 at 850°C, which enhances to 214 mW cm−2 after using a Boudouard reaction catalyst. The microstructures, components and thermochemical parameters of the biochar are characterized and analyzed. The results show that the microstructure of the rice husk biochar is rough and porous. Na, K, Mg, Al, and other elements are found to present in the biochar. The physicochemical properties of the biochar and the enhanced electrical output of the DC‐SOFCs are demonstrated and discussed in detail. [ABSTRACT FROM AUTHOR]

Direct carbon fuel cell (DCFC) is a promising technology with high energy efficiency and abundant fuel. To date, a variety of DCFC configurations have been investigated, with molten hydroxide, molten carbonate or oxides being used as the electrolyte. Recently, there has been particular interest in DCFC with molten carbonate involved. The molten carbonate is either an electrolyte or a catalyst in different cell structures. In this review, we consider carbonate as the clue to discuss the function of carbonate in DCFCs, and start the paper by outlining the developments in terms of molten carbonate (MC)-based DCFC and its electrochemical oxidation processes. Thereafter, the composite electrolyte merging solid carbonate and mixed ionic–electronic conductors (MIEC) are discussed. Hybrid DCFC (HDCFCs) combining molten carbonate and solid oxide fuel cell (SOFC) are also touched on. The primary function of carbonate (i.e., facilitating ion transfer and expanding the triple-phase boundaries) in these systems, is then discussed in detail. Finally, some issues are identified and a future outlook outlined, including a corrosion attack of cell components, reactions using inorganic salt from fuel ash, and wetting with carbon fuels. [ABSTRACT FROM AUTHOR]

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. • Pretreated mesocarp fiber biochar as fuel for DCFC. • HCl pretreated biochar reduces ash from 2.5 to 0.1 wt% and increases O/C ratio. • Higher power density of DCFC with HCl treated biochar than activated carbon. • Porous fibrous biochar increases active sites for electrooxidation. [ABSTRACT FROM AUTHOR]

The design of a direct carbon fuel cell (DCFC) for continuous operation was successfully executed and is presented in this study. Petroleum coke, an abundant carbon source in Canada, was used as a fuel and pneumatically supplied to the anode side of the cell. The open circuit voltage (OCV) of the cell reached 1.1 V and generated a power density of 28 mW/cm2 at 700 °C and 85 mW/cm2 at 850 °C. After about 180 h of operation the cell's performance declined, due to loss of electrolyte. In the next design, a cell was fabricated with an electrolyte circulation loop. This cell generated a lower power density than the first cell assembly but its OCV drop during 180 h operation was significantly improved. These are the first molten carbonate DCFC results demonstrating continuous carbon supply and electrolyte recirculation with potential for ash and impurities removal. • The progression of a planar molten carbonate DCFC design is presented. • A new continuously-fed DCFC design is experimentally implemented. • A distribution plate improved even delivery of petroleum coke to the anode surface. • Electrolyte circulation improved cell performance. [ABSTRACT FROM AUTHOR]

Melon seed husk (MSH) biochar was used in a single cell direct carbon fuel cell (DCFC) as an alternative biofuel. The DCFCs belong to a generation of energy conversion devices that are characterised with higher efficiencies, lower emission of pollutants and MSH biochar as the fuel. Several analytical techniques (proximate, ultimate and thermo-chemical analysis) were employed to analyse the characteristics of the biomass fuel, their effects on the cell's performance, and the electrochemical reactions between the fuel and the electrolyte in the system. High carbon content and calorific values are some of the parameters responsible for good performances. The performance of a lab-scale DCFC made of ceramic tubes using molten carbonate electrolyte was investigated. Binary carbonates mixture (Na2CO3-K2CO3, 38-62 mol.%) was used as electrolyte and the waste MSH carbonised at 450°C as biofuel. A practical evaluation of the fuel used in the DCFC system was conducted, for varying temperature of 100-800³C. The maximum open circuit voltage (OCV) was 0.71 V. With an applied load resistance and active surface area of 5.73 cm² the maximum power density was 5.50 mWcm-2 and the current density was 29.67 mAcm-2 at 800°C. [ABSTRACT FROM AUTHOR]

• An alkali metal thermal electric converter is coupled with a direct carbon fuel cell. • The main irreversible losses in the coupling system are considered. • The maximum power output densities of two subsystems and whole system are calculated. • The coupling system is revealed to have a high efficiency. • The matching schemes of two subsystems and parametric choice criteria are provided. The aim of this study is to deeply search the performance of the coupling system comprised of a direct carbon fuel cell and an alkali metal thermal electric converter. The main contents of this study are to propose the model of the novel coupling system by considering the main irreversible losses, derive the efficiency and power output of the whole system, discuss the influences of the electric current of the fuel cell, the electrolyte thickness, and the ratio of the electrode areas of two subsystems on the systemic performances, and, evaluate and compare the performances of the whole system and the fuel cell. The main novelties are to provide parametric optimum choice criteria and give the reasonably matching schemes of two subsystems. The results obtained reveal the advantages of the present coupling system. [ABSTRACT FROM AUTHOR]

The present work deals with transient simulation of hydrodynamics and mass transfer in a direct carbon fuel cell (DCFC) using a hybrid Lattice Boltzmann and finite volume approach. A 2D mathematical model was develloped to describe the transport phenomena in the different componments of the direct carbon fuel cell (feed channels, anode, cathode and elyctrolyte) and the electrochemical reactions. The DCFC is assumed to operate at a fixed temperature. The numerical model was first validated against available experimental data. Then, a parametric study was carried out to investigate the effect of the specific surface area (SSA) of the carbon fuel on the DCFC performance. The increase of the SSA was found to reduce the anodic activation losses, minimize the voltage drop in the cell and ameliorate its output power density. [ABSTRACT FROM AUTHOR]

Abstract A novel system based on the indirect oxy-combustion of coal in a liquid Sb anode solid oxide fuel cell (SOFC) has been used to produce electricity for over 48 h. Pulverized anthracite was fed to the liquid-antimony-anode of the fuel cell, and a peak power density of 47 mW cm−2 was reached at 1023 K and 35 mW cm−2 at 973 K. The fuel cell was prepared using a porous stainless-steel tube as a support for an LSM cathode, antimony oxide (Sb 2 O 3)/yittria stabilized zirconia (YSZ, Y 0.08 Z 0.92 O 1.96) composite electrolyte (membrane), while liquid antimony acted as the anode. Liquid antimony/antimony oxide served as the intermediate medium for coal oxidation producing mainly carbon dioxide, which evolved as a separate gas stream. The fuel cell will facilitate carbon capture process, and simultaneously convert the chemical energy of coal directly to electricity. The experiment showed that while the fabricated electrolyte was porous, it became dense during the actual operation, preventing nitrogen leakage into the Sb/C side and producing reasonable open circuit voltage. Analysis of the experimental EIS data illustrates that the Ohmic resistance was the primary loss mechanism in the system. It further suggests approaches to improve the design. Continuous operation of this coal fueled oxy-combustion/fuel cell system achieved an overall efficiency of 28.2% despite of its tiny scale. Simple technologies can be employed to scale up this system at relatively low cost of fabrication and materials. [ABSTRACT FROM AUTHOR]

Summary The pomelo peel char (PC) was prepared and used as fuel for solid oxide electrolyte direct carbon fuel cells with nickel‐yttrium stabilized zirconia anode, thin‐film YSZ electrolyte, and La0.8Sr0.2MnO3 cathode. The power densities of fuel cells operating on PC and catalyst‐loaded PC (PCC) fuels achieved 309 and 518 mW cm−2 at 850°C, respectively, which are among the highest power densities reported in the literature on DCFCs. The PC exhibited superior gasification reactivity than coal char due to its unique reticulated foam carbon structure with a homogeneously distributed inherent catalyst. The stability tests at a current density of 50 mA cm−2 and 825°C indicate that the cell using PC fuel operated in a more stable manner than that using PCC, and the fuel availabilities for PC and PCC were 47.25% and 34.71%, respectively. The results suggest that PC is a promising solid carbonaceous fuel for solid oxide electrolyte direct carbon fuel cells based on its adequate gasification reactivity and high compatibility with the fuel cells. [ABSTRACT FROM AUTHOR]

Summary: The use of low‐rank coal in a clean and efficient manner is a major challenge facing the current coal technology. A high‐sulfur coal with 4.5 wt% sulfur is chosen to examine the compatibility of the pristine coal and the purified contrast with a solid oxide fuel cell (SOFC) with nickel cermet anodes. Desulfurization of the pristine coal is performed by molten caustic leaching method with a removal ratio of 80%. Analyses of the physicochemical properties of coal samples indicate that the purified coal has a more favorable structure and higher Boudouard reactivity, which is suitable as a fuel for fuel cells. The assessment of electrochemical performance reveals that the purification treatment not only makes the peak power density of SOFCs improve from 115 to 221 mW cm−2 at 900°C but also extends their durability from 1.7 to 11.2 hours under a current density of 50 mA cm−2 at 850°C with a fuel availability increasing from 6.25% to 40%. The postmortem analyses show that far less deposited carbon and nickel sulfide are observed on the anode surface. The fuel‐based investigation reveals that the purified coal is a promising fuel for direct carbon fuel cells. [ABSTRACT FROM AUTHOR]

Summary: Direct carbon solid oxide fuel cell (DC‐SOFC) is a promising technology for electricity generation from biomass with high conversion efficiency and low pollution. Biochar derived from wheat straw is utilized as the fuel of a DC‐SOFC, with cermet of silver and gadolinium‐doped ceria as the material of both cathode and anode and yttrium stabilized zirconia as electrolyte. The output performance of a DC‐SOFC operated on pure wheat straw is 197 mW cm−2 at 800°C and increases to 258 mW cm−2 when 5% of Ca, as a catalyst of the Boudouard reaction, is loaded on the wheat straw char. Higher power and fuel conversion utilization are achieved by using Ca as the Boudouard reaction catalyst. X‐ray diffraction, scanning electron microscopy, energy‐dispersive spectrometer, and programmed‐temperature gravimetric experiment are applied to characterize the leaf char. It turns out that the wheat straw char is with porous structure and composed of C, K, Mg, Cl, Fe, and Ca elements. The effects of the Ca catalyst on the Boudouard reaction, the performance of the DC‐SOFCs operated on the wheat straw char, and the economic advantages of the wheat straw char are demonstrated and analyzed in detail. [ABSTRACT FROM AUTHOR]

Molten carbonate direct carbon fuel cells (MC-DCFCs) are among the most promising devices for high-efficiency energy conversion and clean power generation from coal. Many studies have focused on the anode performance, while little attention was paid to the cathode. In the present study, we comprehensively investigate the cathode polarization performance, revealing the reactions taken place and the impact of several important operation parameters on reaction rate. The results show that the reduction of inputted gases is the primary reaction at moderate cathodic polarization of a gold electrode, which could benefit from incorporation of an optimal O2/CO2 molar ratio of 1/2, increase of the gas flux or reaction temperature to a certain extent. Interestingly, with a sufficient overpotential applied, an unexpected carbon deposition phenomenon on the cathode is observed. To give an insight into this reaction, several material characterization techniques and electrochemical tests are conducted to analyze the composition and formation conditions of the deposit, respectively. It shows that the carbon deposition is resulted from the reduction of carbonate ions in the electrolyte, which occurs when the cathode potential reaches a critical value of −1.5 V, corresponding to a current density of −32 mA cm−2. To avoid the contamination of the cathode surface by the carbon deposition when operating an MC-DCFC, a feasible strategy is using a sufficiently large cathode to keep the cathodic current density as well as overpotential below the critical value. [ABSTRACT FROM AUTHOR]

Summary: Among numerous modern high‐performance energy technologies that allow for conversion of chemical energy into electricity and heat, the most interesting are direct carbon fuel cells (DCFCs). They are the only ones among all types of fuel cells that allow for a direct conversion of chemical energy stored in the solid fuel (carbon) into electricity. Furthermore, they are characterized by high performance, which is not limited due to the Carnot's rule, low emissions of such substances as SO2, NOx, fly ashes and others, and a relatively simple design since there are no moving components. Nowadays, DCFCs have been developed all over the world. These cells differ first and foremost in the electrolyte they use. The type of electrolyte determines both configuration of the device and operating temperature. The paper discusses current state of knowledge concerning DCFC technology with alkaline (hydroxide) electrolyte and presents the results of research and development studies concerning such cells all over the world. Furthermore, main factors and parameters that impact on the operation of individual cells and potential challenges that have to be overcome in order to develop these technologies were characterized and discussed. The aim for this paper is to review the current state of knowledge concerning DCFC technology with alkaline (hydroxide) electrolyte and presents the results of research and development studies conducted by the key technology players related to those fuel cells all over the world. Furthermore, main factors and parameters that impact on the operation of individual cells, key areas for further improvements and potential challenges that have to be overcome in order to develop this technology have also been characterized and discussed. [ABSTRACT FROM AUTHOR]

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The objective of this study is the preliminary investigation of the feasibility of using a low-temperature molten hydroxide direct carbon fuel cell as an additional energy source for steel production by electric arc furnace. For this purpose, four carbonaceous materials related to the steel industry (electrographite, coke, torrefied biochar and hydrochar) were selected and characterized to predict their electrical behavior before their actual introduction as fuels. Special attention was paid to both the morphological effect (bulk/pellet or powder) and the chemical composition of the fuels on the electrical performance of the cell. Electrical measurements showed the positive influence of powder morphology, with coke powder having the highest peak power density value (49.6 mW/cm2). Electrographite was found to be useable only as a powder (18.7 mW/cm2), as the high chemical stability of the bulk morphology, provided by the smooth surface and the pitch used as a binder, acted as inhibitors of the carbon oxidation reaction. Although biochar appeared superior to hydrochar when inserted as powder (23.5 vs. 18.2 mW/cm2), the latter showed promising results also inserted as pellet. the latter also showed promising results when inserted as a pellet. Specifically, once inserted within the molten hydrochar, the binder used to produce the hydrochar is removed changing the morphology from pellet to sandy/powdery, negating the penalizing effect of the lower surface to volume of bulk morphology (15.8 vs. 18.2 mW/cm2) and offering the advantage of avoiding the milling process and related fine particulate production from an industrial point of view. [Display omitted] • Biochar and steelmaking related carbonaceous materials are used as fuel for a DCFC. • The promising synergy between DCFC and the steel industry has been highlighted. • Biochar and hydrochar showed promising performance in both pellet and powder form. • Powder morphology was highly recommended for steelmaking related materials. • Potential power generation by DCFC could offset some of EAF electricity demand. [ABSTRACT FROM AUTHOR]

The molten hydroxide electrolyte Direct Carbon Fuel Cell (MH-DCFC) is a promising type of DCFC due to its advantages, such as high ionic conductivity, higher electrochemical activity of carbon (higher anodic oxidation rate and lower overpotentials) and high efficiency of carbon oxidation due to lower operating temperature (the dominant product of carbon oxidation is CO2 vs. CO). Accordingly, the MH-DCFC can be operated at lower temperatures (roughly 673–873 K), and thus cheaper materials can be used to manufacture the cell. Nonetheless, MH-DCFCs are still under development due to several fundamental and technological challenges such as corrosion problems. Selection of materials and development of a structure that ensures adequate availability and durability of the cell is crucial for the optimization of the MH-DCFC performance and the further development of that technology. This article presents the operating characteristics of the MH-DCFC made of different construction materials, such as carbon steel, stainless steel, and nickel and its alloys. Nickel and its alloys have proven to be the best materials for the construction of individual elements of the fuel cell. Inconel alloy 600 was a good catalytic material for cathodes with good corrosion resistance.

Abstract Direct carbon fuel cells (DCFC) that employ solid oxide electrolytes and liquid antimony (Sb) anodes are efficient electrochemical cells for converting various types of solid carbon fuels directly into power. Though the liquid Sb anode exhibits decent performance, electrolyte corrosion by molten Sb 2 O 3 during fuel cell operation has long been an issue. The present study investigates the behavior of scandia stabilized zirconia (ScSZ) electrolytes fabricated through different approaches in liquid antimony anode DCFCs. As for conventional sintered ScSZ electrolyte, we observed severe electrolyte corrosion by molten Sb 2 O 3 , which agrees with previous reports. In contrast, corrosion or thinning by the oxide was not detected in ScSZ electrolyte prepared with atmospheric plasma spray (APS) technology. Both electrochemical testing and microscopic characterization results suggest that plasma spray is a promising method to prepare robust electrolytes for liquid antimony anode based DCFCs. Highlights • Metal supported SOFC prepared by atmospheric plasma spray (APS) technology. • Direct conversion of solid carbon fuel into power in liquid Sb anode. • Electrolyte's resistivity against Sb 2 O 3 corrosion confirmed in the 120-h long term test. [ABSTRACT FROM AUTHOR]

Abstract Direct carbon solid oxide fuel cell has garnered increasingly attention due to its high conversion efficiency. However, when a liquid metal is used as the anode, the electrolyte is subjected to hot corrosion by the liquid metal and its oxide. In the present study, both chemical and electrochemical corrosions of Gd 2 O 3 -doped CeO 2 electrolyte by liquid Sb and Sb 2 O 3 are conducted at 750 °C for up to 100 h to simulate the situations occurring in the direct carbon solid oxide fuel cell. The tested specimens are characterized by a scanning electron microscope and an electron probe micro-analyzer. In the case of chemical corrosion, both liquid Sb and Sb 2 O 3 penetrates into grain boundaries of the electrolyte, resulting spallation of grains from the bulk. The depth of liquid Sb penetration is deeper than that of liquid Sb 2 O 3. Electrochemical corrosion of the electrolyte by liquid Sb occurs in the same way as the chemical corrosion at an increased rate promoted by applied current. It is also found that the electrochemical corrosion of the electrolyte by liquid Sb improves the cell performance with the sacrifice of electrolyte. Highlights • The corrosion of GDC electrolyte was studied in two experimental modes. • Both chemical and electrochemical corrosion of GDC electrolyte were happened. • Liquid Sb or Sb 2 O 3 penetrate into GDC grain boundaries, and then grains detached. [ABSTRACT FROM AUTHOR]

Abstract An integrated natural gas decarbonisation and dual fuel cells power cycle (NGDDFC) was evaluated. The hydrogen and solid carbon produced in the natural gas decarbonisation process are fed to hydrogen and carbon fuel cells respectively. A fraction of the produced hydrogen is combusted to supply the heat required for natural gas decarbonisation. A pure carbon dioxide stream is produced from the carbon fuel cell, suitable for sequestration. The power cycle shows an efficiency of about 61% with near 100% carbon capture, 15 to 20% points higher than natural gas combined cycle (NGCC). Highlights • An advanced natural gas dual fuel cell system is proposed. • Natural gas decarbonisation followed by a hydrogen fuel cell and a carbon fuel cell. • Anode exhaust from carbon fuel cell is a pure CO 2 , ready for sequestration. • A fraction of hydrogen is burnt to supply heat for natural gas cracking. • The power cycle has an efficiency of 61% (HHV of natural gas) with 100% carbon capture. [ABSTRACT FROM AUTHOR]

Activated carbons (AC) pretreated with HNO 3 and NaOH are investigated as the fuel of direct carbon fuel cells. HNO 3 and NaOH treatments both increase the oxygen content and decrease the graphitization degree of AC. The amount of hydroxyl groups on the surface of AC increases after the treatments with HNO 3 and NaOH, and then decreases during the subsequent heating process in an inert atmosphere. On the contrary, the carbonyl and quinone groups on the surface of the activated carbon remain stable during the heating process. The activated carbon treated with HNO 3 shows the highest reactivity towards oxidation and reverse Boudouard reactions due to its lowest graphitization degree and highest oxygen content. The single cell with a 380 μm-thick yttria stabilized zirconia layer as the electrolyte and the activated carbon treated with HNO 3 as the fuel exhibits the lowest polarization and the highest maximum power density of 128 mW cm −2 at 800 °C. [ABSTRACT FROM AUTHOR]

• Woody and non-woody biomass for direct carbon fuel cells (DCFC). • Structural properties of biochar fuels affect DCFC performance. • H/C and the O/C atomic ratios are important indicators for biochar quality. • Characteristics of biochar as potential DCFC fuels are highlighted. This review explores two areas: firstly, the production of biochars from an array of different woody and non-woody waste biomass through pyrolysis and its physical-chemical characterization. Secondly, the influence of the structural properties of biochars for application in direct carbon fuel cells (DCFCs) is also explored. The need for developing low cost and environmentally favourable carbon fuel material puts lignocellulosic biomass-derived char (biochar) into the forefront. This char fuel is produced from biomass feedstock by the process of slow pyrolysis which is regarded as the most common route of production. The degree and magnitude of decomposition of each of the biomass components is dependent on process variables such as feedstock type, reaction temperature and heating rate. The generated biochar is then utilized as a fuel in a DCFC. This review establishes an understanding of the most significant properties of the biochar fuels which include proximate analysis (information of moisture, ash, volatile and fixed carbon content), and ultimate analysis (C, H, N, S and O composition), heating values and surface area. The H/C and the O/C atomic ratios are considered as important pyrolysis indicators required for a better quality fuel. [ABSTRACT FROM AUTHOR]

To explore the correlation between the performance of direct carbon fuel cells and the characteristics of semi-coke as the fuel, a strategy for adjusting the characteristics of semi-coke is employed by changing the coal pyrolysis conditions. It is found that the ratios O/C and H/C of semi-coke are decreased with an increase in the final pyrolysis temperature, a slowing of the heating rate and an extension of the pyrolysis residence time. In addition, CO 2 activation can increase the specific surface area of semi-coke. The performance of a direct carbon fuel cell increases with an increase in O/C and H/C in semi-coke. Meanwhile, the combustion characteristic index of semi-coke has a linear correlation with the maximum output power of a direct carbon fuel cell, so the combustion characteristic index can be used as a reference indicator for a direct carbon fuel cell to select solid carbon fuel. [ABSTRACT FROM AUTHOR]

For the first time, a combination of coal grain analysis (CGA) and mineral liberation analysis (MLA) has been applied to coal chars prepared at different pyrolysis temperatures. Information collected from these powerful techniques is used here to assess electrochemical activity of coal chars in a solid anode of the direct carbon fuel cell. Mineral distribution and consideration of particle size gives very useful information in terms of showing differences between chars of differing origin, although major differences in mineral distribution between pyrolysis HHT are not observed here. As a result of this analysis, it is proposed that electrochemical performance of coal chars at solid anodes in molten carbonate appear to be dependent on several, interlinked factors. The physical properties of the char including its porosity, resistivity, surface area and crystalline structure have previously been demonstrated to affect electrochemical oxidation of carbons, and these factors are also observed to be important here. Further, however, the prevalence, size and type of mineral matter in the coal char and its contact with char particles also is suggested to play an equally important role. The mechanism of this interaction may include alteration of previously observed sensitivities, such as mineral enhanced gasification of the coal char leading to carbon morphology changes. Additional influences of mineral matter are suggested here to include modification of surface polarity and subsequent contact between carbon and molten carbonate electrolyte (effectively enhancing surface area), or the ability of mineral components to act as a shuttle for active species. [ABSTRACT FROM AUTHOR]

The combined impact of carbon type (anthracite coal, bituminous coal and pine charcoal) and in situ , catalyst-aided, carbon gasification process on the electrochemical performance of a Direct Carbon Fuel Cell (DCFC) is explored. The effect of operation temperature (700–800 °C) and catalyst (Co/CeO 2 ) infusion to carbon feedstock under CO 2 atmosphere at the anode chamber is systematically investigated in a cell of the type: Carbon + CO 2 |Cu-CeO 2 /YSZ/Ag|Air. All fuel samples were characterized, in terms of chemical composition, crystallite structure (XRD), pore structure (BET), surface morphology (SEM), particle size distribution (PSD) and thermogravimetric analysis (TGA), in order to obtain a close relationship between the carbon characteristics and the DCFC performance. The results reveal that in the absence of catalyst, the optimum performance is obtained for the charcoal sample (P max  ~ 12 mW/cm 2 ), due to its high volatile matter, oxygen content, porosity and carbon disorder as well as its low amount of impurities. Catalyst infusion to carbon feedstock results in a considerable increase in the achieved cell power density up to 225%, which is more pronounced for the less reactive coals and low temperatures. The enhanced performance obtained by infusing Co/CeO 2 catalyst into carbon is ascribed to the positive effect of catalyst on the in situ carbon gasification, through the reverse Boudouard reaction (C + CO 2  → 2CO), and the subsequent faster diffusion and electro-oxidation of formed CO at the anodic three-phase boundary. [ABSTRACT FROM AUTHOR]

This review presents three directions in solid oxide fuel cell (SOFC) technology development involving solid-state carbon in some stage of the fuel-to-electricity conversion process: direct carbon (DC-SOFC), integrated gasification (IG-SOFC) and deposited carbon (rechargeable SOFC). Recent achievements of science and technology were studied in order to identify the most widely developed concepts. In addition, the review contains a statistical approach to published patents and articles, naming the people and institutions active in the field. Simultaneous development of all three technologies could bring synergies and contributed to a major breakthrough in the efficiency of coal-fired power plants. [ABSTRACT FROM AUTHOR]

In this study, a numerical model of activation and ohmic polarization is modified, taking into account the correlation function between surface properties and inner resistance. To investigate the correlation function, the surface properties of coal are changed by acid treatment, and the correlations between the inner resistance measured by half-cell tests and the surface characteristics are analyzed. A comparison between the model and experimental results demonstrates that the absolute average deviations for each fuel are less than 10%. The numerical results show that the sensitivities of the coal surface properties affecting polarization losses change depending on the operating temperature. The surface oxygen concentrations affect the activation polarization and the sensitivity decreased with increasing temperature. The surface ash of coal is an additional index to be considered along with ohmic polarization and it has the greatest effect on the surface properties at 973 K. [ABSTRACT FROM AUTHOR]

An integrated power cycle comprising natural gas decarbonization in conjunction with a hydrogen fuel cell and a carbon fuel cell is proposed, termed a natural gas decarbonization dual fuel cell system (NGDDFC). In this process hydrogen and solid carbon produced in a natural gas decarbonization step are fed to hydrogen and carbon fuel cells, respectively. A pure carbon dioxide stream is produced from the carbon fuel cell, suitable for sequestration. Process modeling of this cycle carried out using UniSim R430, for the non-CO 2 capture case, showed a cycle power efficiency of about 61%, which is 15 percentage points higher than the current natural gas combined cycle (NGCC) case. With 82% CO 2 capture, the NGDDFC had a net efficiency about 20 percentage points higher than the published NGCC option, albeit with a slightly lower capture percentage (82% vs. 90%). [ABSTRACT FROM AUTHOR]

Solid fuel particles will become increasingly important in the future. Present energy conversion systems for solid fuels are too inefficient. New energy conversion systems for solid fuels with higher energy conversion efficiencies are possible. Fuel cell technology is a key-technology in these new conversion systems. The direct carbon fuel cell (DCFC) operates on carbon particles obtained from a variety of solid fuel feedstocks. The DCFC is the only fuel cell designed to directly oxidize carbon particles in a special anode chamber. The particles are generally graphite structure with high purity. The electrolyte used is the high temperature solid oxide, molten carbonate or hydroxide electrolyte. Since a pure stream of CO2 is produced the stream can easily be sequestered and disposed. Pure carbon dioxide produced as a by-product would also have a market in many industries. A well defined technology roadmap identifying key research and development (R&D) issues is necessary to provide a framework for the development of these systems and to prevent entrenchment in inherently inefficient technologies. This review paper describes the direct carbon fuel cell and its system, how it works, the developmental status, the characteristics of the carbon particles needed, and the research and development issues for the technology.