Prospects of fuel cell technologies

Abstract

Fuel cells (FCs) are efficient and clean devices to electrochemically convert the chemical energy of fuels such as hydrogen, natural gas (NG), methanol, ethanol and hydrocarbons to electric energy with significantly high efficiency and much lower greenhouse-gas emission as compared to well-established internal combustion engine (ICE) technologies. Thus, FCs are regarded as the most promising energy-conversion strategies for the sustainable energy development. FC technologies can be categorized according to the nature of the electrolytes, including low-temperature proton-exchange membrane fuel cells (PEMFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs) to high-temperature molten-carbonate fuel cells (MCFCs) and solid-oxide fuel cells (SOFCs). Operating temperatures and ionic-transfer processes of FCs are determined by the nature of the electrolytes. As shown in Fig. 1, in general, increasing operation temperature will increase the efficiency and decrease the materials cost, while the system complexity and cell and stack fabrication costs increase. The high efficiency of MCFCs and SOFCs is also due to the high quality of heat generated, which can be used for hot-water supply or internal system use. In the case of AFC, PAFC and MCFC, the alkaline, phosphoric acid and molten-carbonate electrolytes are highly corrosive, which significantly limits their commercial viability and competitiveness. PEMFCs and SOFCs are considered to be the most promising and competitive in the future energy-conversion technology sectors [1]. Fuel cell types, showing the general trend in the relationship between the operation temperature, efficiency, system complexity, fabrication cost and materials cost of the FC technologies. PEMFCs use polymeric materials as the proton-exchange membrane (PEM). PEMFCs based on the most advanced perfluorosulfonic acid (PFSA)-based membranes such as Nafion show fast start-up, high efficiency, low operation temperature (80°C or lower), high power density, easy assembly and handling and are the most mature technologies for the development of FC-powered cars. Though there are significant activities in the non-precious-metal electrocatalysts, the majority of these catalysts only show reasonable performance under alkaline medium [2], targeting anion-exchange-membrane fuel cells (AEMFCs). However, the development of anion exchange membranes (AEMs) with high anion conductivity and chemical and structural stability is a grand challenge in AEMFCs [3]. Though Pt reserve is limited, Pt electrocatalysts used in PEMFCs can be in principle recycled at the end of their service. Thus, in the foreseeable future, Pt-based electrocatalysts are still the main players in PEMFCs. This in turn implies the importance of reducing the loading of precious Pt-metal catalysts with increased activity and stability under FC-operation conditions. The durability and activity of Pt-based electrocatalysts can also be significantly enhanced by the development of highly conductive and nano-structured oxides or carbides as non-carbon catalyst supports for PEMFCs. SOFCs are all-ceramic devices, have no corrosive components and do not require precious-metal electrocatalysts due to the high operation temperatures (800–1000°C). The high operation temperature also makes it possible to use directly a wide range of fuels including reformate H2, NG, hydrocarbons and alcohol-based liquid fuels – a distinct advantage as compared to other types of FCs. For this reason, SOFCs are particularly suitable for stationary and distributed power-supply applications, ranging from kW to MW. The most common and well-developed materials for the state-of-the-art SOFCs are Y2O3–ZrO2 (YSZ) electrolytes, Ni–YSZ cermet anodes and (La,Sr)MnO3-based perovskite cathodes. The key challenges in SOFC technologies are the thermal stability or thermal cycle capability, stack durability and system controllability, which are inter-related. In all solid-state devices like SOFCs, any thermal mismatch between components would lead to thermal instability, but good system control and management can significantly minimize the thermal stress and subsequently reduce the degradation and prevent system failure during the start-up and shut-down cycles and in the case of emergencies. The stack durability depends strongly on the activity and stability of electrode materials. However, a major challenge associated with a SOFC system over a five-year lifetime is the gradual degradation and deactivation of the electrodes by contaminants which can be either in the fuel and air streams or from the volatile species of cell components, such as metallic interconnects, sealants and manifolds. The recently discovered polarization-induced interface and its impact on microstructure and interface stability are emerging fundamental issues for the development of SOFCs [4]. Operation temperature is an important topic in FC research. PFSA PEM-based FC cars have been successfully demonstra