Direct carbon fuel cells offer highly efficient means of converting carbon from waste, biomass or coal to electricity producing an exhaust stream that is well-suited to CO 2 sequestration and, hence could underpin a new, clean carbon economy. If this technology is to contribute significantly to improving our impending global energy crisis, three aspects must first be addressed: competitive performance with extant fuel cell technologies, development of practical systems to handle available carbon resources and demonstration of sufficient durability, i.e. 40 000 hours minimum for system. In the present study, we demonstrate excellent performance from a hybrid direct carbon fuel cell based upon an yttriumstabilised zirconia electrolyte to use solid carbons as fuels directly. Good stability of the zirconia is observed during and after fuel cell testing and in corrosion tests under reducing conditions; however, significant intergrain erosion is observed under oxidising conditions. The carbon fuel chosen is a waste product, Medium Density Fibreboard, which is widely available and difficult to recycle. Cells exhibit excellent electrochemical performance at 750 C, with a maximum power density of 390 mW cm À2 using a lanthanum doped strontium manganite (LSM) cathode and 878 mW cm À2 using a lanthanum doped strontium cobalt (LSC) cathode under flowing air. This is comparable with current commercial Solid Oxide Fuel Cell and significantly in excess of commercial Molten Carbonate Fuel Cell (MCFC) performance. This hybrid direct carbon fuel cell therefore offers the clean utilisation of coal, waste and renewable carbon sources and hence merits development as a realistic alternative technology.

A direct carbon fuel cell (DCFC) can produce electricity with both superior electrical efficiency and fuel utilisation compared to all other types of fuel cells. Although the first DCFC prototype was proposed in 1896, there was, until the 1970s, little sustained effort to investigate further, because of technology development issues. Interest in DCFCs has recently been reinvigorated as a possible method of replacing conventional coal-fired power plants to meet the demands for lower CO emissions, and indeed for efficient utilisation of waste derived chars. In this article, recent developments in direct carbon conversion are reviewed, with the principal emphasis on the materials involved. The development of electrolytes, anodes and cathodes as well as fuel sources is examined. The activity and chemical stability of the anode materials are a critical concern addressed in the development of new materials. Redox media of molten carbonate or molten metal facilitating the transportation of ions offer promising possibilities for carbon oxidation. The suitability of different carbon fuels in various DCFC systems, in terms of crystal structure, surface properties, impurities and particle size, is also discussed. We explore the influence of a variety of parameters on the electrochemical performance of DCFCs, with regard to their open circuit voltage, power output and lifetime. The challenges faced in developing DCFCs are summarised, and potential prospects of the system are outlined.

No abstract

No abstract

Previous experimental results have shown that a system with molten carbonate and solid oxide electrolyte is feasible for Direct Carbon Fuel Cell (DCFC). A study is presented to investigate cell performance with a range of solid carbon (i.e. coals, biochars, graphite) and operation mode in this hybrid electrolyte system. The results show that less crystalline coal with high fixed carbon, low sulfur, medium volatile material and moisture is best suited to this system. Using high rank of fuel such as anthracite coal, good cell performance can be obtained only by elevating temperature and with adequate pretreatment to remove impurities. Discussion of cell operation indicates that cell degradation and operation failure were due to coal agglomeration, ash buildup, and limited fuel supply in potentiostatic mode. Instead, galvanostatic operation gave stable cell performance over 60 hours. This result allows better understanding of anode reaction mechanism on the hybrid electrolyte system. Thus, long-term operation is promised when suitable solid fuel and optimized operation parameters are applied. Direct Carbon Fuel Cell (DCFC) is a promising technology to use solid carbon for energy production. This technology allows direct conversion of chemical energy to electricity, thus giving a high thermodynamic efficiency, i.e. 100%, when carbon is oxidized to carbon dioxide. The concept of DCFC is simple. Solid carbon is fed to a fuel cell and electrochemically oxidized at the anode to produce electricity.1-3 Pure product, carbon dioxide (CO 2 ), is obtained in complete oxidation and is easy for carbon sequestration. Furthermore, application of DCFC benefits from sufficient fuel supply with abundant coal and biomass in the world. These advantages as well as established infrastructure of transportation, storage, and processing for solid carbon make a DCFC feasible and very attractive.One of design challenges to develop a DCFC is choice of fuel. 4Despite large reserves of solid carbon available, different sources of carbon have varying activity and indeed affect DCFC performance. In the past, many attempts have been made to evaluate effect of solid carbon on DCFC for electric power generation. Cooper et al. 5 reported cell polarization on nine particulate carbon derived from fuel oil, coal, biochar, petroleum coke, etc. The highest discharge rate, 100-125 mA cm −2 at 0.8 V and 800• C, was obtained with biochar-derived carbon. They found that properties of carbon fuel which control discharging rates are (i) crystallographic disorder, (ii) electrical conductivity, (iii) active surface sites, and (iv) sulfur and ash impurities. Less graphitized and high disordered carbon such as chars is more reactive to oxidation; however, this property counterbalances electrical conductivity since chars are poor conductors. By contrast, neither particle size, surface area, nor morphology was found to have much impact on the electrochemical discharge rate. Moreover, Zhu et al 6 investigated factors that determine performance of carbon fuel in the D...

Solid oxide fuel cells (SOFCs) afford an opportunity for the direct electrochemical conversion of biogas with high efficiency; however, direct utilisation of biogas in nickel-based SOFCs is a challenge as it is subject to carbon deposition. A biogas composition representative of a real operating system of 36% CH 4 , 36% CO 2 , 20% H 2 O, 4% H 2 and 4% CO used here was derived from an anode recirculation method. A BaZr 0.1 Ce 0.7 Y 0.1 Yb 0.1 O 3Àd (BCZYYb) infiltrated Ni-YSZ anode was investigated for biogas conversion. The infiltration of BCZYYb significantly promoted the electrochemical reactions and the cells exhibited high power output at the operational temperatures of 850, 800 and 750 C. At 800 C, supplied with a 20 ml min À1 biogas, the cell with a BCZYYb-Ni-YSZ anode, generated 1.69 A cm À2 at 0.8 V with an optimal amount of 0.6 wt% BCZYYb, whereas only 0.65 A cm À2 was produced with a non-infiltrated Ni-YSZ in the same conditions. At 750 C, a maximum power density of 1.43 W cm À2 was achieved on a cell with a BCZYYb-Ni-YSZ anode, a 3 mm dense YSZ film electrolyte, a Gd 0.1 Ce 0.9 O 2 (GDC) buffer layer and a La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 -Gd 0.1 Ce 0.9 O 2 (LSCF-GDC) composite cathode. The cell remained stable, while operating at 0.8 V for 50 hours with a current density of 1.25 A cm À2 . A well-designed cell structure and selected components made it possible to obtain excellent performance at good fuel utilisation. The analysis of gases in open-circuit conditions or under various current loads suggested that the prevalent reaction was reforming of methane without coking. This study demonstrates that the BCZYYb-Ni-YSZ is a promising electrode for carbon-containing fuel.Depending on the reaction conditions, the equilibrium for the water-gas shi can be pushed in either direction. In fuel cell systems, the dominant reactions will be dependent on the gas 133 4463808; Tel: +44 (0) 133 4463817 † Electronic supplementary information (ESI) available. See

Hybrid direct carbon fuel cells (HDCFCs) are now considered as one of the most efficient options for the generation of clean energy from mineral coals. This work sheds light on the reaction mechanism of the HDCFC operated with raw and pre-treated mineral coals which have been thoroughly characterised and tested.

No abstract