Cairong Jiang

Demonstration of high power, direct conversion of waste-derived carbon in a hybrid direct carbon fuel cell

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 CO2 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. 40000 hours minimum for system. In the present study, we demonstrate excellent performance from a hybrid direct carbon fuel cell based upon an yttrium-stabilised 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−2 using a lanthanum doped strontium manganite (LSM) cathode and 878 mW cm−2 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.

Highly efficient, coking-resistant SOFCs for energy conversion using biogas fuels

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% CH4, 36% CO2, 20% H2O, 4% H2 and 4% CO used here was derived from an anode recirculation method. A BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (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−1 biogas, the cell with a BCZYYb-Ni-YSZ anode, generated 1.69 A cm−2 at 0.8 V with an optimal amount of 0.6 wt% BCZYYb, whereas only 0.65 A cm−2 was produced with a non-infiltrated Ni-YSZ in the same conditions. At 750 °C, a maximum power density of 1.43 W cm−2 was achieved on a cell with a BCZYYb-Ni-YSZ anode, a 3 μm dense YSZ film electrolyte, a Gd0.1Ce0.9O2 (GDC) buffer layer and a La0.6Sr0.4Co0.2Fe0.8O3–Gd0.1Ce0.9O2 (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−2. 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.

Role of coal characteristics in the electrochemical behaviour of hybrid direct carbon fuel cells

There is a growing interest in hybrid direct carbon fuel cells (HDCFCs) now considered as one of the most efficient options for the generation of clean energy from mineral coals. In this work, two different hard coals (bituminous and anthracite) have been modified via carbonisation and oxidation and their electrochemical behaviour has been compared in an electrolyte supported HDCFC. A new insight into the HDCFC reaction mechanism is presented, providing an exhaustive analysis taking into account not only the evolution of the properties of the coals upon treatment but also other relevant parameters such as the effect of the cell preparation step or the interaction of the coal with the other cell components. The results show that the carbon content, the carbonaceous structure and the reactivity of the coals are key characteristics for optimal electrochemical behaviour. The plasticity of bituminous coals, an important parameter overlooked in previous works, can help to extend the area for electrochemical reactions beyond the current collector/anode interface. The reaction mechanism proposed shows that additional gas phase electrochemical reactions are an important contribution during the early stages of the electrochemical testing and that the direct electrochemical oxidation of solid carbon is the dominant reaction at longer times.