Prospects of fuel cell technologies

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 demonstrated but the most pressing issue is the formidable cost of infrastructures associated with the production, storage, transportation and distribution of high-purity H2 (99%). Increasing the operation temperature of PEMFCs to high temperatures of 200–300°C, or HT-PEMFCs, offers possibilities in the use of alternative fuels as hydrogen carriers with the prospect of minimization of hydrogen infrastructure cost for the employment of FCs in the transportation sector. For example, methanol is an interesting hydrogen source because it is liquid at ambient temperature and has a much higher energy density than hydrogen and low reforming temperature (240–260°C) due to the absence of C–C bonds [5]. However, integration of methanol steam reformers (MSRs) with FCs requires the FCs to be operated at temperatures of 200–300°C in order to take advantage of the efficient heat exchange between the two devices (FCs are exothermic while MSRs are endothermic). Operation at 200–300°C is also necessary for Pt-based electrocatalysts to have high tolerance towards poisoning species like CO because the reformed stream contains a high level of CO, at 500–8000 ppm [5]. FCs with integrated MSRs are also advantageous in applications of remote communication, unmanned aerial vehicles (UAV) and soldier-portable power where carrying bulk and heavy H2 bottles is considered to be unpractical. Phosphoric acid-doped polybenzimidazole (PA/PBI) is the state-of-the-art PEMs with operation temperature up to 160°C, but suffers from the leaching and thermal instability of PA [6]. HT-PEMFC can also accelerate the development of direct alcohol fuel cells (DAFCs), a subclass of PEMFCs. Progress in HT-PEMFCs is relatively slow but a recent report of the exceptional stability of hybrid PA/PBI membranes based on heteropolyacid-functionalized mesoporous silica filler at 200°C is encouraging in this aspect [7]. In the case of SOFCs, there is a global trend in decreasing the operation temperature of SOFCs to intermediate temperatures of 500–600°C or IT-SOFCs [8]. The primary benefit of IT-SOFCs is the decreased cost of cell fabrication, the increased selection of cell and stack materials, and the increased thermal stability. In the intermediate-temperature range, the direct use of alcohol-based fuels is also attractive for IT-SOFC-based devices. However, development of IT-SOFCs faces the challenges of the substantially reduced ionic conductivity of conventional electrolytes such as YSZ and high polarization losses of electrode reactions particularly at the cathodes for O2-reduction reactions. A nano-structured approach either by infiltration or by exsolution is an effective way to develop high-performance and stable electrodes for IT-SOFCs [9,10]. Nevertheless, for the hydrocarbon fuel (e.g. NG)-based SOFCs, reducing the operation temperature of SOFCs to below 700–800°C does not bring specific benefit if the internal or external reforming of hydrocarbons is part of the system. A key difference of FC technologies from other electrochemical energy devices is that FCs cannot be used without system integration. FCs can be fabricated on a micrometer and even a nano scale, but the system required for a workable FC would be orders of magnitudes larger and more complex. As shown in Fig. 2, commercial viability and successful application and market penetration of FC technologies require ingenious system integration. Thus, in the development of future FC technologies, system control and integration are as important as the FCs themselves. Compact and reliable system control and integration play an enabling role to accelerate the pace of the commercialization of FC technologies. In this aspect, challenges in FC technologies are not only scientific and technological, but also an engineering issue. Scheme of the application types and power ranges of main FC types as a function of operation temperatures. The arrows show the R&D trend in the development of HT-PEMFCs and IT-SOFCs. Among many challenges in FC technologies, one of the most critical is probably the electrolyte. A fundamental breakthrough in AEM or HT-PEM will undoubtedly open opportunities in the development of non-precious-metal catalysts and integrated FC/on-board fuel reformers for the practical employment of FC technologies in transportation, communication and portable devices in remote areas. The main benefit of the development of doped ceria and bismuth materials as alternative electrolytes for IT-SOFCs is the direct utilization of active and mixed ionic and electronic conducting cobaltite-based perovskite cathodes, but their chemical and structural stability and ionic conductivity need to be substantially improved under realistic FC-operation conditions. It is a hope that considerable R&D efforts, collaborations and investment, not only from government policy makers, research institutes and academies, but also from the industrial developers and private investors will continue in a sustainable manner for the development of truly commercial viable and competitive FC technologies for energy-convention and power-source applications in the future. We thank Dr Jin Zhang for assistance in drafting Figures 1 and 2. This work was supported by the Australian Research Council under the Discovery Project Scheme (DP150102025 and DP150102044) and the National Natural Science Foundation of China (51402100 and 21573066).