Solid oxide fuel cell technology for sustainable

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e2 2

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Solid oxide fuel cell technology for sustainable development in China: An over-view Yuzheng Lu a,*, Yixiao Cai b,c,**, Loembe Souamy d, Xiang Song a, Lei Zhang a, Jun Wang e a

School of Electronic Engineering, Nanjing Xiaozhuang University, Nanjing, 211171, PR China College of Environmental Science and Engineering, Donghua University, 2999 Ren'min North Road, Shanghai, 201620, PR China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, PR China d Jiangsu Provincial Key Laboratory for Novel Software Technology, Nanjing University, 210023, PR China e Jiangsu Provincial Key Laboratory of Solar Energy Science and Technology, School of Energy and Environment, Southeast University, Nanjing, 210018, PR China b

article info

abstract

Article history:

Fuel cell technology motivates a variety of benefits, which are barely offered by other

Received 21 January 2018

energy technologies. The fuel cell can be obtained through natural resourcesdbiomass,

Received in revised form

coal and water, which are abundant in China. More importantly, most of these are sus-

26 April 2018

tainable and realize ecological circulation. Being blessed with a source of renewable en-

Accepted 1 May 2018

ergy, fuel cell technology is favorably promoted in China. Simultaneously, fuel cell

Available online xxx

technology offers China great opportunities to meet the energy consumption demand for its sustainable development. In this proposed method, useful results of leading research in

Keywords:

solid oxide fuel cell relevant research in China are reviewed and the hybrid system based

Renewable energy

fuel cell technology is particularly detailed. Additionally, the effects of some important

Sustainable development

renewable energy parameters, future challenges and constructive recommendations for

Solid oxide fuel cells

China's energy technology are suggested.

China

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction World power crises have increased in the demand of energy [1]. Over the past two decades, Chinese population has exceeded 1.3 billion and the economy has grown by 8%. It was stated by the National Energy Administration of China that since 2009, China has become the world's largest consumer of energy products and derivatives. The total energy

consumption in China from 1978 to 2016 is shown in Fig. 1. The total energy consumption of standard coal grew from 540 million tons in 1978 to 4.36 billion in 2016 [2]. It is worth noting that although China is the largest consumer of energy products in the world, China's per capita energy consumption is far less than the world average. Presently, China is facing its worst energy crisis due to the growing population and industrial needs [3]. In China, most coal resource reserves exist in the north, and the total coal

* Corresponding author. ** Corresponding author. College of Environmental Science and Engineering, Donghua University, 2999 Ren'min North Road, Shanghai, 201620, PR China. E-mail addresses: [email protected] (Y. Lu), [email protected] (Y. Cai). https://doi.org/10.1016/j.ijhydene.2018.05.008 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lu Y, et al., Solid oxide fuel cell technology for sustainable development in China: An over-view, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.008

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Fig. 1 e The Total Energy Consumption from 1978 to 2015. The data from National Bureau of Statistics of the People's Republic of China.

resources are over 1 trillion tons, which accounts for 90% of the total fossil resources. Recoverable coal is detected at more than 110 billion tons, which accounts for 11.6% of the world's recoverable coal. Also, there are about 104 billion tons of oil resources and 47 trillion m3 of natural gas resources in China. In general, China is rich in coal resources and short on oil and gas resources, which indicates an unreasonable structure of energy consumption. For instance, when compared to other countries, China's coal consumption accounts for 67.7% of the world's coal consumption in 2004 and 66% in 2013. However, it only accounts for 27.2% in 2004 and 30% in the world in 2013 as shown in Fig. 2(A) and (B). Oil and gas are also in short supply, only 18.4% and 5.8% as of 2013, respectively. Additionally, the world consumption of oil and gas is shown in Fig. 2(C) and (D), 33% and 24% in 2013, respectively. In short, the proportion of coal consumption as a structure is too large in most developed districts of China, which may be direct causes of the

environmental degradation, such as the high level of particulate matter 2.5 and haze. Environmental conditions are likely to worsen unless local administrative agencies take effective action [4]. Deng et al. [5] predicts that the total demand of standard coal will be approximately 5.55 billion tons in 2020 and will approach 7.89 billion by 2030. To reduce the excessive consumption of fossil fuels and achieve sustainable energy development, intense research is conducted and encouraged by local governments to develop renewable energy techniques and advance the development of a clean, renewable energy. Major power enterprises in China continuously increase investments in clean energy research and development; e.g., hydropower, wind power, photovoltaic, and fuel cell energy with the aim of efficient power improvement and carbon dioxide emission reduction. In China, there are hundreds of universities and research and development (R&D) organizations dedicated to energy research. Moreover, the government has established various plans to promote the development of new energy. Significantly, in 2013, the Chinese government issued 9 key tasks within the 12th Five-Year Plan of Energy Development of which more than 2 key tasks are in relation to new energy. For example, speeding up the development of renewable energy sources by introducing wind power, solar energy, etc.; to develop distributable energy and energy-saving or new energy vehicles [6]. The Ministry of Science and Technology of China is actively supporting the development of new energy. Meanwhile, the National Natural Science Foundation of China stimulates the research on new energy and the National Development and Reform Commission has issued a batch of preferential policies for the development of new energy. There are 900 billion kWh that will be replaced by new energy in the Qinghai province during the year of 2015 with an emphasis on solar energy. This is equivalent to reducing 111 thousand tons coal consumptions, 275 thousand tons of carbon dioxide emissions and 2650

Fig. 2 e Energy Consumption Structure of the World and China. A) Energy Consumption Structure of China in 2004, B) Energy Consumption Structure of China in 2013, C) Energy Consumption Structure of the World in 2004 and D) Energy Consumption Structure of the World in 2013. The data from National Bureau of Statistics of the People's Republic of China. Please cite this article in press as: Lu Y, et al., Solid oxide fuel cell technology for sustainable development in China: An over-view, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.008


tons of sulfur dioxide emissions per year [7]. Multiple R&D organizations and research institutions have started the exploration of cheap, clean, and alternative energy resources including solar power, hydro power, wind power, geothermal, tidal, biomass using fuel cell (FC) technology, and more. In particular, the research on FC technology has attracted more attention because it offers unique benefits like highconversion efficiency, lower noise production and lower harmful emissions. Among the following research projects; low temperature FC, high temperature FC, fuel flexible FC, direct carbon FC, bio FC and FC hybrid system technologies are of peak fascination. FC technology has a variety of applications ranging from stationary power plants to portable energy consumption. Different types of fuels are used, such as air, hydrogen gas, biogas, natural gas etc. [9]. FC technology and FC based energy sources have successfully launched worldwide. India has spent billions of dollars on FC and hydrogen energy research [8]. In the USA, FC system/stacking have been developed and integrated with hydrocarbon fuels like gasoline and diesel. The sufficiency of fuels found in nature and the applications of FC technology makes it a promising candidate for stimulating a sustainable development of China. Thus, the use of FC technology must be extensively investigated as it offers cheaper alternatives to conventional and expensive power sources. In this sense, a comprehensive overview of FC technology and its applications are illustrated to advance FC technology through R&D principally in China. This summary is eminently desired because of its reference value and utility for the general public. By starting from the basics on different types of fuel cells, the advantages and usefulness of diverse FC technologies are discussed. Integrated FC hybrid systems, FC based polygeneration systems (a process that produces more than one product simultaneously from a single fuel source) and the potentials of different renewable energy resources in China are also presented. Furthermore, we discuss the status and involvement of different research institutes who are developing FC technology in China. To conclude, challenges to implement FC technology for a sustainable development of China is described and several recommendations are advised. China is blessed with several renewable energy resources in abundance like wind, solar, hydro, biomass, tidal, geothermal, bio-fuels, etc. These resources must be utilized in order to achieve the goal of non-fossil energy consumption accounting for 15% of the total energy consumption in 2020 and up to 20% in 2030 [10], as stated in “the development of renewable energy is an important part of the 13th Five-Year Plan” by the National Energy Administration of China. Recently, the Ministry of Science and Technology of China, the Ministry of Finance of China, the Ministry of Industry and Information Technology of China, and the National Development and Reform Commission of China are in joint consensus that it is crucial to develop new energy strategies, especially new energy vehicles. For national research grants, the 973 Plan and 863 Plan include three major projects: the first is the ‘Basic Research on Carbon-based Solid Fuel Cell System’ held by Professor Han at China University of Mining and Technology, started in January 2012 and concluded in August 2018. The second project is called ‘Key Technology of Fuel Cell and Distributed

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Generation System’ hosted by Dalian Institute of Chemical Physics, Chinese Academy of Science, and the final project is ‘2-kW Flat Type Medium Temperature Solid Oxide Fuel Cell System’ hosted by the Shanghai Institute of Ceramics, Chinese Academy of Science [11e13]. Meanwhile, the Ministry of Education of China declared the importance to enhance talent cultivation and develop and establish key laboratories for new energy research. Moreover, there are some associational entities in China. These associations strive to develop renewable energy technology, but each association's target is different. For instance, The China Energy Association concentrates on creating new energy, energy conservation, emissions reduction, and new energy vehicles [14]. The Chinese Renewable Energy Industries Association is committed to promoting renewable energy progress and advanced technology to actively promote the commercialization of China's renewable energy industry [15]. Also, the China Hydrogen Energy and Fuel Cell Association focuses on aspects like industrialization of hydrogen, fuel cell development, and international cooperation [16]. A standard fuel cell can transform chemical energy into electrical energy with high efficiency. FCs can convert some renewable fuel (hydrogen gas, natural gas, biogas, etc.) into electricity. Fuel cells are renewable, because they are abundantly available in the universe and thus, can be renewed and replenished without any end [17,18]. However, the main defect is the high cost of a fuel cell system. A normal commercialized FC usually has a 3-layer anode-electrolytecathode structure which has remained the standard since its invention. In 1839, the inventor, Grove [19], proposed that if electricity can be used to split water then it may be possible to generate electricity using hydrogen and oxygen in a reverse process. In 1889, the term, fuel cell was proposed. Years later in 1932, a fuel cell was fabricated that employed nickel electrodes and alloys as electrolytic material and used oxygen and hydrogen as fuel. A complete fuel cell system that was capable of producing 5 kW of electricity was launched in 1959 [20e23]. Generally, fuel cells are categorized by the type of electrolyte material - of which there a multiple possibilities for fuel cell construction. In this sense, fuel cells can be classified as; proton exchange membrane fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, solid oxide fuel cell, alkaline fuel cell, and direct methanol fuel cell [20e27]. A proton exchange membrane fuel cell (PEMFC) allows protons to pass through. The electrolyte is made of a proton exchange membrane that conducts charged ions. The working temperature of these cells range from 60 to 80 C and the power density is quite high [23]. However, pure hydrogen is strictly required during the operation, thus limiting further development. In the case of phosphoric acid fuel cell, concentrated phosphoric acid is used as the electrolyte material. Electrodes are often made from platinum or related alloys that serve as a catalyst. The working temperature of the cell ranges from 150 to 220 C [20e23]. For the molten carbonate fuel cell, a liquid of lithium, sodium or potassium carbonates are used as the electrolyte material. The operating temperature is around 65 C, which is favorable because the carbonate electrolyte achieves good conductivity [20e23].

Please cite this article in press as: Lu Y, et al., Solid oxide fuel cell technology for sustainable development in China: An over-view, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.008

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For an alkaline fuel cell (AFC), the electrolyte consists of liquid alkaline potassium hydroxide. The high performance of an AFC is induced by a rapid reaction happening in the cathode. This type of cell has been successfully used for producing electricity and water in space applications. The operating temperature ranges from 150 to 200 C [20e23]. The use of an electrolyte in a direct methanol fuel cell (DMFC) is similar to the PEMFC. However, for direct methanol fuel cell, the working temperature ranges from 50 to 100 C, which is lower than most other types [20e23]. Fuel cells can be adapted for multiple systems and there are three main applicable areas, transportation, stationary power and portable power, as shown in Fig. 3. Fuel cells have huge potential for the above applications. The list of benefits is excessive; the important ones have been summarized below [20,21]. Effectiveness: The efficiency of fuel cells are high because of the minimized impact by the Carnot cycle which is a theoretical thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. It provide an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work, or conversely, the efficiency of refrigeration system in creating a temperature difference by the application of work to the system. It is not an actual thermodynamic cycle but is a theoretical construct [22,23]. Simplicity: Fuel cells are reliable devices with a simple configuration. The quick starting-up process and controllable settings can regulate the power grid rapidly. Some of them have demonstrated a flexible multi-fuel that can direct alcohol and biogas. Furthermore, fuel cells can be stacked in a modular form to match different power requirements [22,23]. Environmentally friendly and minimized noise: Fuel cells do not emit any harmful gas but instead produce water and heat as a byproduct which can be further used for other purposes [22,23]. In the solid oxide fuel cell (SOFC), solid oxide material is used as an electrolyte. Typical materials for the electrolyte in

SOFCs include zirconia along with a small amount (8 mol%) of ytrria doping, samarium doped ceria, or gadolinium doped ceria. SOFCs are suitable for generating electric power on a relatively large scale. Such systems are reliable and fuel adaptable with a low emission of harmful and unfriendly gases. SOFC stack systems are also suitable for local power generation systems in rural areas without access to public grids. Furthermore, they have numerous advantages like high-efficiency, low noise, long-term stability and low maintenance costs [25]. The operating temperature is in a range of 600e1000 C and is the primary aspect why the SOFC is exceedingly suitable for polygeneration [26]. However, compatibility weaknesses prevent the development of SOFCs as evidenced by the long start-up and cooling-down times. Because of this, several groups and researchers have been searching for alternative strategies to lower the operating temperature. They claim that SOFCs may represent the next generation for energy production if successful and sustainable solutions are established [28e31].

The development of fuel cell technology in China China is facing dual threats of energy deficiency and environmental degradation. According to the report, ‘The oil and gas industry development in domestic and overseas’ published in 2014, oil import is up to 308 million tons and 59.5% is imported from overseas. Therefore, it is essential to develop new energy technologies in China. Although solar energy, wind, and hydro energy are plentiful in China, they have not been transferred into large-scale productions for their own shortcomings. For example, the conversion efficiency for a solar cell is still too low. Currently, the conversion efficiency of a commercialized crystalline silicon solar cell is approximately 16%e18% and the highest reported record is only 22% [32]. Whereas, wind energy applications have noise problems and high maintenance costs making it difficult to move forward. Simply, energy conversion must be efficient, clean, and safe. Among new energy sources, fuel cell is a fair candidate [33]. Fuel cell technology with a high conversion efficiency, nonmechanical moving function, low noise and high flexibility

Fig. 3 e Fuel cell and its applications. Please cite this article in press as: Lu Y, et al., Solid oxide fuel cell technology for sustainable development in China: An over-view, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.008


has gained great attention in China. Compared with most developed countries, the development of fuel cell technology in China started rather late. In general, the development timeline for fuel cells in China can be summarized in Fig. 4. Research on fuel cells in China was launched by Dalian Institute of Chemical Physics in the mid-1950s [34]. However, the main research work started in 1970 for primarily aerospace applications. PEMFC and SOFC have achieved great progress in the national program the Eighth Five-Year Plan. Currently, China is initiating the Thirteenth Five-Year Plan.

Study consequences The main reason for developing fuel cells in China is to provide electric power for countless applications. In stationary power applications, fuel cells can be used at a fixed location like homes, grocery stores, industrial buildings and other backup power units where a diesel generator or lead-acid batteries could not be used due to the pollution they generate. Fuel cells can also be applied in transportation systems. Moreover, there were various international events in China, which required the participation of fuel cell technologies [35]. The use of fuel cell technologies will help to decrease the dependence on power production through coal or oil. Thus, hydrogen and fuel cells will play a major role in the future energy market if national policies give high priority to efficient hydrogen technologies [33].

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Potential of a fuel cell based hybrid system in China Power generation and related environmental impacts have become important issues in China. Today, most electrical power is provided by conventional power generation technologies that largely rely on fossil fuel combustion. However, these will induce two serious threats; global warming and air pollution. To avoid this situation, energy systems must be updated and a new power generation system should be developed immediately. FC technology can also be integrated with several renewable energy resources to work in hybrid systems for multiple applications. As discussed earlier, China is rich in renewable energy resources such as solar energy, wind and biomass. Out of all the fuel cell types, the SOFC has gained great attention for its high energy conversion efficiency and fuel flexibility. For instance, when solid carbon is used as the fuel, SOFC technology can achieve a theoretical electrical efficiency close to 100% [36]. Different kinds of fuels such as gases (e.g., hydrogen, syngas, and bio-gas), liquids (e.g. methanol, ethanol and glycerol) and solids (carbon and lignin) can be used in SOFCs. SOFCs are also easy to actualize cogeneration, where power and heat are generated at the same time during operation. Therefore, a multi-functional SOFC can be used as both a hybrid and a polygeneration system. In the following sections, several FC hybrid systems with different renewable energy resources available in China are demonstrated.

Fig. 4 e Fuel cell development timeline in China. Please cite this article in press as: Lu Y, et al., Solid oxide fuel cell technology for sustainable development in China: An over-view, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.008

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a) SOFC-Biomass Hybrid System

theoretical bases and optimization criterion for the design and operation of practical syngas SOFC based hybrid systems.

In general, China is largely an agricultural country. The total biomass resource is about 700 million tons according to statistic reports [37]. The biomass energy resource is usually classified into four main categories: straw, firewood, various kinds of organic wastes, and agricultural residues, as shown in Fig. 5. These wastes can be converted into useful biogas using a bio-gasifier. Biogas is the first energy source utilized by mankind and represents a renewable fuel source. The combination of SOFCs with biomass gasification can greatly increase the value of biogas [38,39]. An SOFC-biomass hybrid system can be used for several applications such as electrical portable devices, stationary units, combined heat and power system, and so on [40]. The block diagram for an SOFCbiomass hybrid system is shown in Fig. 5. There are several important researches that have been conducted in China. Zhang et al. [41] made a comprehensive review on integration strategies for solid fuel cells. They demonstrated basic hybrid integration strategies and advanced integration cycles for improved power generation. Their research work includes several SOFC applications, such as an SOFC combined heat and power system, a Brayton cycle, a thermophotovoltaic power generation, a hydrocarbon fuel air turbo rocket (ATR), a biomass gasification, a reversible SOFC and Photovoltaic (PV) etc. Zhu et al. [42] used biomass gas as the fuels for the electromotive force of SOFCs. The study showed that when using biomass gas as a fuel, the depression of the electromotive force for a SOFC using doped-ceria as the electrolyte is relatively smaller when compared with the use of yttria stabilized zirconia. Li et al. [43] investigated a cermet-supported tubular solid oxide fuel cell and found that it can be operated with biomassbased syngas through supercritical water. Zhang et al. [44] established a novel model for the solid oxide fuel cell-gas turbine hybrid system with fuel reforming, where the residual fuel from the fuel cell is further burned in a combustor. The SOFC and combustor act as the high temperature reservoirs of the gas turbine (GT). The results may provide some

b) SOFC-Wind Hybrid System The total wind power reserves are around 32.26 1011W in China. Among them, there are approximately 2.53 1011W of wind energy that can be harnessed [45]. Wind is a suitable renewable energy source that can produce electric power 24/7 as compared to solar energy that cannot be harnessed during the night. China is now the world leader in wind power with a total installed capacity, which is also strongly supported by relevant policies [46]. Wind energy can be converted from the environment into electricity using a wind turbine/mill. A wind turbine first converts the kinetic energy from wind into mechanical energy by rotating the shaft of the electric generator to generate electricity. The electric power from wind energy is usually unstable because wind speed is influenced by meteorological conditions [47]. In other words, wind power fluctuates because of wind speed. When the capacity of a wind turbine is too large, it will influence the stability of the grid and may even induce potential safety hazards. Compared to other new energiesdincluding solar energy, biomass, geothermal energy, etc.dthe cost of wind power is relatively low [48]. Hence, it is feasible to use electric power from wind to produce hydrogen and oxygen by the electrolysis of water. The produced hydrogen can be used as a fuel and oxygen as an oxidant in a solid oxide fuel cell system, respectively. The water produced from the SOFC can be recycled to produce hydrogen and oxygen in an electrolyzer. In this way, the power from the FC is quite stable and can be used in a variety of applications such as transportation, power stations, portable devices, etc. Lo and Wu et al. [46] have evaluated the performance of wind farms in China. They explored the relationships between the operational performance of a wind farm and several key factors, e.g., resource area, regional location, and scale. The research results showed that policies and approvals may affect the performance of wind farms, leading to an expansive difference on the performance of wind farms in different

Fig. 5 e Solid oxide fuel cell- Biomass hybrid system. Please cite this article in press as: Lu Y, et al., Solid oxide fuel cell technology for sustainable development in China: An over-view, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.008


regions of China. Zhang et al. [49] presented a coordinated control strategy for SOFCs and superconducting magnetic energy storage systems to match the intermittent wind power generation and compensate for the rapid load changes. The proposed method had better anti-interference performance in various operating situations comparing with the conventional control methods. c) SOFC-Solar Hybrid System China is rich in solar resources with an average daily radiation of 4 kW h/m2. More than two thirds of the country's domain can receive a radiation of more than 5000 MJ/m in a year and more than 2200 h of sunshine [50]. The SOFC-solar hybrid system is one type of clean, inexhaustible and widely scattered energy. The renewable energy usage took up around 8% of the total energy consumption in China in 2011. The Chinese government has an agenda to increase the proportion of renewable energy up to 15% in 2020, with solar energy playing a critical role [10]. However, solar energy has its disadvantages such as having low density, and being unstable and discontinuous. The effective method is integrating it to other new energies, especially an SOFC system. A simple integration method is shown in Fig. 6. The output of the PV cells and fuel cells are direct current power. It is easy to realize the integration of the PV and FC subsystem in the engineering application. However, this method is inadvisable in the industry because the costs of these subsystems are expensive. There is a viable way to use hydrogen energy, an ideal energy for the future [51]. The technology of solar hydrogen production mainly includes solar direct thermal decomposition of water, thermochemical cycles, photo-electrochemical cells, and semiconductor photo-catalytic decomposition [52]. The hydrogen produced from water through solar energy can be used as fuel for the FC system, as shown in Fig. 7. The water is produced during the operation of fuel cells, which can be recycled to produce hydrogen and oxygen in the electrolyzer. Lu et al. investigated

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the methods of integrating solar energy and fuel cells as a hybrid system [53]. The hybrid system has numerous advantages, such as a new device application, low cost and high efficiency on different scientific research that can convert solar energy, and chemical energy to fuel electric energy simultaneously within the same device. In this case, the upmost consideration of the application design is needed; Lu et al. also built a novel mode for the hybrid system of solar energy and solid oxide electrolyte cells [54]. It is found inevitably that an increase in the temperature and a decrease in operating voltage can reduce the energy loss in the hybrid system. d) SOFC-Coal Hybrid System and Polygeneration System Due to its high operating temperature, the SOFC is a highly efficient energy conversion device that can be easily coupled with a thermal recovery system to satisfy a possible contemporary request of heat and electricity. Moreover it can utilize a wide spectrum of fuels such as natural gas, syngas, methanol, kerosene, etc. Hydrogen with a higher calorific value has a great ability to meet the future energy requirements. One of the greatest advantages of hydrogen is the ability to be produced from an extensive source, especially coal. Hydrogen can be produced from coal by a coal gasification process in which a wide variety of products such as diesel, natural gas, ethanol, hydrogen and industrial gas can be produced. Some of them can be directly used as fuel for SOFCs [55]. An energy efficient and low emission SOFC with a combined heat and power system is a promising electric and thermal energy generation for future commercial buildings [56]. The efficiency of a SOFC can reach up to 85% when it has combined heat and power systems, which is higher than a thermal power unit. Polygeneration can be a promising technology to meet different demands regarding energy sustainability and reducing global warming [57]. Dr. Liu and Li [58] presented a conceptual system based on a SOFC and gas turbine combination and investigated how

Fig. 6 e Solid oxide fuel cell and PV cell. Please cite this article in press as: Lu Y, et al., Solid oxide fuel cell technology for sustainable development in China: An over-view, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.008

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Fig. 7 e Integration methods of Solid oxide fuel cell and solar energy.

those parameters affected the heat to power ratio. Their simulation results showed the advantages of such a system by comparing it with other conventional engines and found that it can achieve a low heat to power ratio with a low stack running temperature. Zhe et al. [59] analyzed the low temperature solid oxide fuel cell's (LTSOFC) hybrid system. Based on a steady-state mathematical model of LTSOFCs, results showed that a hybrid system can achieve high efficiency.

Involvement of different research sectors in China a) The Shanghai Institute of Ceramics, Chinese Academy of Sciences focused on SOFCs. They have made remarkable achievements and recently found a novel method to enhance the conductance of the transitional metal oxide electrodes. Their research mainly focuses on SOFCs, especially on the plate type and MW level [60]. Yang's group succeeded in synthesizing the orthorhombic-perovskite structure, BaCe0.8-xZr0.2InxO3-d (x ¼ 0, 0.1, 0.2, 0.3, 0.4), materials. A maximum power density of 151 mW/cm2 at 700 C was obtained in the SOFC mode. Then, high current density of 729 mA/cm2 was obtained at the same temperature with an applied voltage of 1.5 V in solid oxide electrolysis cell mode [61]. Qi et al. are tremendous for the work and applications they can achieve and are consistently promising candidates for intermediate-temperature SOFC interconnect known as TiC/hastelloy composites. The research results indicate that the decrease of the graphite particle size has no effect on an effective route to optimize the oxidation resistance of composites [62,63]. For a long-term stability of metal-supported SOFC, a

NieCe0.8Sm0.2O2-d infiltrated 430 L anode with a La0.6Sr0.4Fe0.9Sc0.1O3-d(LSFS) infiltrated Scandia-stabilized zirconia (SSZ) cathode has proven to be stronger in recent tests. After a 1500 h durability test at 650 C and 900 mA/cm2, the results show a degradation rate of 1.3% k/h [64]. b) The Division of Fuel Cell and Battery, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences has interesting research in power fuel cell, DMFCs, medium temperature SOFCs, and an advanced secondary battery. They are divided into four teams. The fuel cell system and engineering research team directed by Professor Shao, the team of medium temperature SOFC research directed by Professor Cheng, the team of DMFC research directed by Professor Sun, and the team of advanced secondary battery research directed by Professor Chen. These teams have made remarkable achievements on fuel cells in China. i. The fuel cell system and engineering research team focuses on key materials and components, FC systems, renewable FC systems, new conceptions of fuel cells, alkaline anion exchange membrane FCs, and fuel cell electric piles. They had built a 75 kW reforming of a natural gas fuel cell and regenerative fuel cell system. ii. Dr. Liu and co-workers [65] prepared a YSZ (Y2O3-stabilized ZrO2) interlayer that consisted of nanoparticles smaller than 10 nm, and was introduced by spinning and coating hydrolyzed YSZ sol solution on the electrolyte and sintering at 800 C. The interlayer constructive meeting slightly increased the ohmic resistance but significantly decreased the polarization resistance.



iii. Professor Jiang's group [66] synthesized a novel cathode (PB0.92CoCu) for material of PrBa0.92CoCuO6-d intermediate-temperature SOFCs. Their attempts were to demonstrate that PB0.92CoCu is a promising cathode material of intermediate-temperature SOFCs. (LSM-YSZ) iv. The (La0.8Sr0.2)0.9MnO3-d-Y0.15Zr0.85O1.93 nanocomposite layer was fabricated by Liu's group for anode supported cells. They have shown that the LSMYSZ nanocomposite layer provides considerable high oxygen reduction reaction (ORR) activity through use of its own electronic and ionic conducting network. Unfortunately, the electrochemical performance seems to be constrained by its limited ionic and electronic conductivity [67]. v. Professor Wang and his group investigated sealing performance and chemical compatibility of SrOe La2O3eAl2O3eSiO2 glass sealants with bare and MneCo spinel coated SUS430 ferritic alloys. The results show that the sealing performance can be improved by applying the MneCo spinel coating on the SUS430 alloy, especially for the bonded couple with high-SrO glass [68,69]. c) The Union Research Center of Fuel Cell at the China University of Mining &Technology (Beijing) is led by Professor Han who is also the chief scientist of Chinese Academy of Sciences. The team, in conjunction with Shanghai Jiaotong University, built a coal gasification fuel cell at the union research center in 2005. A significant amount of scientific research projects, including the national 863 Project, and other key projects of the Ministry of Education have been undertaken by the team, especially in 2011, when they assumed the head research position for the national 863 Project, which namely researches systems of carbon-based SOFCs. The team focuses on enhancing the project's stability and reducing its cost. Their work paves the way for the industrialization of SOFCs [70,71]. The cathode of SOFCs suffer from various contaminations such as Si and Cr arising from the interconnect and sealing materials, and the presence of humidity and CO2, which are inherent in ambient air. All of these result in serious issues of longterm performance degradation. The impacts of certain poisoning and corrosion on SOFC cathodes are presented, and the latest results of durability research on the corrosion resistant properties of cathodes under CO2, humidity, Cr and Si-containing conditions are reviewed by Han's group [72]. The mechanism of CO2 corrosion on high performance Ba0.9Co0.7Fe0.2Nb0.1 O3-d (BCFN) cathodes has been studied by Han and her co-workers. The results show that the BCFN electrode contaminated by CO2 can be recovered by purging the air a second time, which could be a very promising cathode material for SOFCs [73]. The BCFN materials can operate at intermediate temperature SOFCs using the La0.8Sr0.2Ga0.83Mg0.17O3-d (LSGM) electrolyte. Peak power densities of 550, 770 and 980 mW/cm2 have been achieved at 700, 750 and 800 C, respectively [74]. Han and her team's research indicate that BCFN is a great potential cathode material for intermediate-temperature SOFCs [74,75]. Recently, they have also succeeded in fabricating Gd0.1Ce0.9O1.95-Li2CO3eNa2CO3 (GDC-LNC) ceramic carbonate dual-phase membranes for the electrochemical

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separation of CO2 by a novel phase inversion combined with a carbonate solidification method. The GDC-LNC dual-phase membranes exhibit stable CO2 permeation, thus suggesting a promising prospect towards future carbon storage and separation applications [76]. d) Ningbo Institute of Industrial Technology, Chinese Academy of Science gives further attention to the SOFC and high temperature electrolysis. The research team, which is composed of nearly 100 researchers led by Professor Wang, built the SOFC R&D platform including nano-powder and battery production, manufactured electric piles, and SOFC systems. They also developed anode-supported, electrolyte-supported and electric pile module technology [77]. The NiOþ8YSZ/8YSZ/LSMþ8YSZ anode-supported single cell was fabricated by Xue and added fish oil to increase porosity. The power density can reach 0.592 W/cm2 and 0.416 W/cm2 at 850 C and 750 C, respectively [78]. The degradation rate of the fabricated cell at 750 C discharges below a constant current of 10 A for 215 h, which is 0.68%/ kh. Furthermore, they fabricated the NiOþ8YSZ/8YSZ/ LSMþ8YSZ anode-supported single cells with a large size of 30 cm by 30 cm. The maximum power output voltage was 0.8 V for one cell, 1.6 V for two cells and a stack can reach 32.6 W, 34.7 W and 67.3 W at the temperature of 750 C, respectively [79]. More importantly, a company called SOFCMAN Energy (www.sofcman.com) was founded by Professor Wang's SOFC group. SOFCMAN works from the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences. The group began to focus on SOFC research in 2006 and has set up a R&D team for materials, chemicals, mechanical design, thermal and electrical engineering, etc., for SOFCs. After years of intense research, SOFCMAN has had outstanding achievements in the development of SOFC power systems. Its productions include powders, cells, stacks, systems, as well as a variety of SOFC testing equipment and services. SOFCMAN is currently developing a 200 kW SOFC system with a single hot box configuration. Such a system will consist of 192 stack modules. These stack modules are using cells the size of 14 cm by 14 cm and totals 25 cells. SOFCMAN has dedicated itself to making clean, reliable and affordable energy for the world. Professor Guan is also from Ningbo Institute of Industrial Technology, Chinese Academy of Science. The probing temperature inside a SOFC stack lies at the heart of the development of high-performance and stable SOFC systems as reported by Guan's group. Their recent work is based on the direct measurements of temperature in three types of SOFC systems: a 5-cell short stack, a 30-cell stack module, and a stack series consisting of two 30-cell stack modules. During a steady-state operation, the temperature of the 5-cell stack was stable while the temperature was increased in the 30-cell stack [80]. When two Pt voltage probes are embedded into the cathode/electrolyte interface, results show that the voltage and maximum output power density measured by the probes on both sides of the La0.7Sr0.3Cr0.5Fe0.5O3-d- Gd0.1Ce0.9O2d (LSCF-GDC) and LSM-YSZ composite cathodes are 7% and 4%, respectively. The possible applications of the system indicated have been investigated and found the corresponding


10


cell during instantaneous current voltage testing [81e83]. Professor Xu's group and Professor Guan's group have developed a wide variety of SOFCs by use of materials, device fabrication and stacked systems [84,85]. e) The medium temperature solid oxide fuel cell research team was created by Professor Sun at the Harbin Institute of Technology. They made a breakthrough with core technology in cell preparation of SOFCs with large sizes. In their research, the power and power density of a single cell is the most advanced in the world [86e88]. Recently, La0.8Sr0.2MnO3/Zr0.92Y0.08O2 (LSM-YSZ) composite nanotubes of different diameters were co-synthesized by a pore-wetting technique for cathode materials in solid oxide fuel cells. A fast firing method was introduced to improve the contact between the LSM-YSZ composite cathode and the YSZ electrolyte to retain the original nanotube structure. At 700, 750, and 800 C, the area specific resistances (ASRs) were 0.55, 0.40, and 0.26 Ucm2, respectively. The low ASR was primarily due to the small grain size, homogeneous particle distribution, and fine pore structure of the material [89]. f) SOFC Research Team at the Department of Materials Science ＆Engineering, University of Science and Technology of China is administrated by Professor Xia. The group's research focuses on key materials of SOFCs [90]. g) Changchun Institute of Applied Chemistry, Chinese Academy of Science has studied solid polymer electrolytes for many years. The team, directed by Professor Wei, participates in international cooperation. Aimed at the forefront of FC research, they have succeeded in producing high efficiency anode anti-platinum catalysts of direct formic acid fuel cells. Its power density is as high as 550 mW/cm2 [91]. Recently, the group was successful at synthesizing direct formic acid fuel cells via a traditional sol-gel process and evaluated it as a cathode for intermediate-temperature SOFCs. The results indicated that introducing a microdose of Mg into the Ni-site of the La2NiO4-d compound is an efficient way to obtain an excellent cathode material for intermediate-temperature SOFCs [92]. Ni1-xCux/Sm-doped ceria materials have been prepared for the anode of SOFCs. Under dry CH4 situation, the cell exhibits a maximum power density of 379 mW/cm2 at 600 C. It also shows great long-term stability [93]. h) Advanced Energy Materials Research Laboratory at the School of Physics and Engineering, Sun Yat-sen University, focuses on FC material research. The current director of the laboratory, Professor Shen, has greatly advanced fuel cell research. The aim of the lab is to conduct basic research on new materials and develop novel theories and approaches to solve problems in advanced energy materials with the ultimate aim of bringing such materials to the market. Since the establishment of the lab, progress has been made in fostering international collaboration, securing national funded projects and building up an active research team [94]. i) Professor Li, along with a group from Huazhong University of Science and Technology, have independently developed a 5 kW SOFC power system. Its power generation efficiency

reached 46.5% and a comprehensive utilization of combined heat and power that can reach 79.7% which reaches an internationally advanced level [95]. Their research results reveal that Sr and Co segregations toward the surface have great contributions to the chemical instability of La1xCo1-yFeyO3-d (LSCF) during annealing [96]. A new composite cathode material of La2NiO4þd-coated PrBa0.5Sr0.5Co1.5Fe0.5O5þd (PBSCF) is prepared for the intermediate temperature solid oxide fuel cells. The polarization resistance (R-p) value of the PBSCF-LN cathode is significantly lower than that of the PBSCF cathode [97]. Professor Li and associates reported a viable approach to substantially enhance the sulfur poisoning resistance of a Nigadolinia-doped ceria (Ni-GDC) anode through impregnation of proton conducting perovskite BaCe0.9Yb0.1O3-d(BCYb) to overcome the hydrogen sulfide. The preliminary results of the cell with the BCYb þ Ni-GDC anode in methane fuel, containing 5000 ppm of H2S show the promising potential of the BCYb infiltration approach through the development of highly active and stable Ni-GDC based anodes that are fed with hydrocarbon fuels containing a high concentration of sulfur compounds [98]. An intermediate-temperature SOFC with (La0.8Sr0.2)0.95MnO3-d-Y0.16Zr0.84O2 Pd0.95Mn0.05O-infiltrated composite cathodes was presented by Li's group. The maximum power density increases from 328 to 734 mW/cm2 with the increase of testing temperatures from 600 to 800 C, which is nearly 2.6 times higher than that of cells with the conventional LSM-YSZ cathode [99]. They also found that its improved electrochemical performance and redox cycling resistance are attributed to its stabilized microstructure consisting of nano-scale Ni particles distributed on the surface of the pre-sintered YSZ scaffold [100]. Dr. Guo has worked with the same organization. His research is on nanostructured ZrO2, including nanocrystalline ceramics, polycrystalline and epitaxial thin films, and heterostructures. Lower total conductivity has been achieved through use of nanocrystalline ceramics and polycrystalline thin films [101]. Dr. Guo and researchers also reported a number of new material and technology developments at the recent 3rd Jiangsu-Europe International Conference on New Energy and Hydrogen & Fuel Cells Forum (http://www.nanocofc.com/ news/359.html), a joint China-Europe advanced fuel cell forum in Wuhan, held 11th to the 13th of December, 2017 [102,103]. j) Professor Geng from Northeastern University conducted excellent research of SOFC interconnect applications. MnCu (Mn:Cu ¼ 1:1, atomic ratio) metallic coatings have been deposited for planar SOFC interconnect applications [104]. They find that the sputtering low-Cr alloy nanocrystalline coating exhibits a promising perspective for intermediate-temperature SOFC interconnects applications [105]. k) Professor Shao at Nanjing Tech University successfully developed low-cost, durable electro catalysts for ORRs at intermediate temperatures, which is critical for broad commercialization of SOFCs. This resolutiondissolution of electro catalytically active nanoparticles



l)

m)

n)

o)

on an electrode surface may be applicable to the development of other high performance cobalt-free cathodes for fuel cells and other electrochemical systems [106]. A single-perovskite (SP)/double-perovskite (DP) composite with the nominal composition of SrCo0.7Fe0.2W0.1O3-d was also designed [107]. They anticipate that the synthesis of this perovskite-type composite could pave the way towards discovery of more perovskite-based composite catalysts for SOFCs. La1-xSrxMnO3-d, which is a pure electronic conductor, is the traditional cathode material of SOFC technology. Although it has many advantages, such as high catalytic activity, stability, and chemical compatibility, the activity of the cathode is decreased greatly with a notable decrease in operation temperature. It is necessary to find new cathode materials for intermediate or LTSOFCs [108e111]. Hence, Cobalt based perovskite is vital for LTSOFCs, including La1-x SrxCoO3-d, La1-XSrxCo1-yFeyO3d, Ba1-xSrxCoyFe1-yO3-d, and so on [112]. YSZ/Sm-doped CeO2 (SDC) bilayer electrolyte film was successfully fabricated on a NiO/YSZ anode substrate using a stepwise sintering processing by a screenprinting technique. It was developed by Professor Huang and co-workers at Harbin Institute of Technology. Open-circuit voltages of 1.06 V is achieved at 750 C for single cells. After two thermal cycles, the maximum power density of the single cell exhibited a significant decrease. The destruction of the YSZ/SDC bilayer electrolyte structure was not observed during the thermal cycles [113]. Professor Wei's group, also from Harbin Institute of Technology, studied an oxygen electrode of Sm0.5Sr0.5CoO3eCe0.8Sm0.2O1.9 composite. It exhibited a better stability under a low anodic polarization current. The results reveal that the polarization current is a key operating parameter for the oxygen electrode under the SOFC mode [114]. Professor Wang [115] from Qingdao University has shown a novel liquid-phase synthesis strategy which was demonstrated for the preparation of the Nbcontaining ceramic oxide SrCo0.9Nb0.1O3-d. BaCe0.4Zr0.4Y0.2O3-d works as the electrolyte and the SrCo0.9Nb0.1O3-d is used as the cathode. The cell shows a peak power density of 348 mW/cm2 at 700 C. Professor Xu [116] at China University of Mining & Technology made a newly solid oxide fuel cell with symmetric electrode in which the electrodes are designed by a La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-dGd0.1Ce0.9O2d and the electrolyte is developed by Zirconia. Through different various techniques, the experimental results show that the maximum power density are 348 and 324 mW/cm2 at 800 and 850 C, respectively. Professor Niu [117] at Jilin University reported a nickelfree double perovskite anode material, Sr2TiFe0.5Mo0.5O6-d. The cell exhibits excellent electrochemical performance using H2 as fuel. The anode shows the electrical conductivity value of 22.3 S/cm in H2 and the maximum power densities of 547 mW/cm2 with an electrolyte-supported cell at 800 C.

11

Development of LTSOFC by nanocomposites in China In order to reach good performance levels and improve the electrical efficiency of the solid oxide fuel cells, electrolyte materials with high ionic conductivity are needed. Traditional solid oxide fuel cells have been achieved at an intermediate to high temperature (600e1000 C) range [118]. Fortunately, there is a worldwide trend of research to decrease the operating temperatures of the device. The reduced temperatures would pave a way for the solid oxide fuel cell commercialization. As known, the electrolyte pays a significantly important role in the cell's performance. There are two major solutions for decreasing the temperature of the system. One is to reduce the thickness of the electrolyte. The other is developing new highionic conductive and stable electrolyte materials. In the last two decades, Zhu and his co-workers at the Royal Institute of Technology have advanced SOFC research by decreasing operation temperatures of materials including the electrolyte, cathode and anode materials. It may be pointed out that ceria-carbonate composites and applied for low temperatures, (300e600 C) were first introduced by Zhu [119,120]. Many similar composite electrolytes have been discovered and explored for LTSOFC applications [121e132]. In order for international cooperation, Dr. Zhu and his colleagues have established the nanocomposites for advanced fuel cells (NANOCOFC)d through the EC-China Multifunctional Nanocomposites for Advanced Fuel Cells network (www.nanocofc.com). Following this line, a trend of LTSOFC R&D and several reviews of this field can be found in literature [125e139]. Another unique approach is to use the composite technology to develop hybrid ion conductors combining protons and oxygen ions to make them function together, thus to enhance the solid oxide fuel cell performance. Wang et al. [140] proposed the swing mode if O2 conduction is due to a bulk vacancy mechanism. This mode gives another simple explanation to the dual Hþ and O2 transport but is limited as it does not cover other ceria-based composites, where both Hþ and O2 make comparable contributions to conductivities, and in some cases, O2 can play a dominate role [141]. On the other hand, highly mobile CO2 3 anions from the carbonate phase can also promote oxygen ion mobility [142]. By using the two-phase material nanocomposite approach, the two-phase structure can directly be made by integrating two proton and oxygen conductive oxide materials. Thus Hþ and O2 conduction paths can be constructed with dual Hþ/O2 conductivity. This has been discovered in the composite proton conductor BaCe0.9Y0.1O3-d and oxygen ion conductor samarium-doped ceria [143]. It should be noted that dual Hþ and O2 or hybrid ions of Hþ, O2, and CO2 3 functions are some of the most essential and unique characteristics for LTSOFCs that differ from traditional fuel cells [144,145]. We can conclude here that improvements in the electrode activity, or FC performance by values, range from several times to more than three orders of magnitudes, as well as remarkable characteristics. The nanocomposite approach provides a platform for the current research work on further improving the electrochemical performance of LTSOFC technology.


12


SOFC research has been significantly expanded and extended by the efforts of other groups in China that can be reviewed in the following subsections. The purpose of this minor review is to focus on recent relevant developments within China.

fuel cell environment. They have synthesized nanocrystalline, Ce0.8Sm0.2O1.9 by using a combined Ethylenediaminetetraacetic acid (EDTA)-citrate complexing solgel process for low LTSOFCs based on composite electrolytes [155e160].

a) Professors Li, State Key Laboratory for Chemical Engineering, School of Chemical Engineering, Tianjin University, focuses on the low or intermediate temperature SOFCs by using nanocomposites [146]. They have analyzed high performance composite ionic conducting electrolytes for intermediate-temperature fuel cells and found evidence for ternary ionic conduction [147]. They have also researched a carbon in molten carbonate anode model for a direct carbon fuel cell and intermediate-temperature fuel cells with a doped ceria carbonate composite electrolyte. It is found that the surface properties of SDC and the electrolyte's thickness have considerable influence on the fuel cell's performance. When the co-precipitated SDC is used as the electrolyte component and a CO2/O2 gas mixture is adopted as the cathode oxidant gas, a fuel cell with an excellent performance is obtained, which has a peak power output of 1704 mW/cm2 at a density of 3000 mA/cm2 at 650 C [148]. Professor Li's group made a record by introducing triple ions of Hþ/O2/CO2 3 transportation in a fuel cell. b) Another group under Professor Wang in Tianjin University developed hydrocarbon fueled SOFCs and direct carbon fuel cells [149]. In parallel, direct carbon fuel cells, i.e. using solid C as the anode to develop a corresponding fuel cell conversion of C to direct electricity is relevant to SOFC-Coal fundamental research. This work can be traced back to the early 2000s where Wang's group from Tianjin University developed direct carbon fuel cells [150,151]. Later, Professor Li's group in Tianjin University carried out further research and development in direct carbon fuel cells [152,153]. c) Similarly, Dr. Fan and Professor Wang have made advancements on LTSOFCs. Potential low-temperature applications and hybrid-ionic conducting properties of ceriacarbonate composite electrolytes for SOFCs have been studied. Results show that the hybrid-ionic conduction improves the total ionic conductivity and fuel cell performance. These results highlight potential low-temperature applications of ceria-carbonate composite electrolytes for SOFCs [154]. d) Mao from the Institute of Nuclear and New Energy Technology at Tsinghua University made great achievements in the direction of LTSOFCs. He is the vice chairman of the International Association for Hydrogen Energy and chief scientist of hydrogen energy of the 973 Project for the Ministry of Science and Technology of China. They have worked on the development of SOFC materials for intermediate-to-low temperature (ILT) operation. They have successfully developed new electrolyte materials for intermediate-to-low SOFCs, including Ce0.8Sm0.2O1.9 (SDC), and SDC-carbonate composites. BaCe0.8Sm0.2O2.9, Compared with the state-of-the-art YSZ, these materials exhibit much higher ionic conductivity at ILT range. Especially so, SDC-carbonate composites show an ionic conductivity of 102 to 1 S cm1 between 400 and 600 C in a

The calcium cobaltite cathodes are post-treated by dipping in hydrogen peroxide [161]. The experiment results show that the mesopores are created on the surface of the cathode particles and the pore channels of the cathode are cleaned up after leaching with 10 wt% H2O2, resulting in a remarkable decrease of the area-specific resistance. e) Ji et al. [162,163] presented an optimal control-based inverse method that may be required for distribution of the electrodes for the electroosmotic micromixers with an external driven flow from the inlet. Results are provided to demonstrate the effectiveness of the proposed method; the step-shaped distribution of the external electric potential imposed on the sidewalls is obtained, measures are taken, and the electrodes with an interlaced arrangement are inversely derived according to the obtained external electric potential. f) Dr. Pei reported a novel electrolyte material of introducing detonation nano-diamonds into samarium-doped ceria. The power density of this new device can reach 762 mW/ cm2 at 800 C. The results show that it remains at high power density in the intermediate temperature [164]. g) Professor Cui [165] synthesized porous YFe0.5Co0.5O3 (YFC) thin sheets as cathodes for SOFCs in which the electrolyte is BaZr0.1Ce0.7Y0.2O3 (BZCY). The cell can operate at intermediate-temperature and its power density is up to 303 mW/cm2 at 750 C. The results indicate that YFC is a promising cathode candidate for an intermediatetemperature SOFC. h) Professor Wang [166], at Xi'an Jiaotong University, recently fabricated a dense La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) electrolyte at a low sintering temperature. The cell power density is up to 592 mW/cm2 and open circuit voltage is up to 0.956 at 750 C. i) Professor Ding [167] fabricated a solid oxide fuel cell with both anode and cathode for symmetry. The electrodes (anode and cathode) were prepared with La0.6Ca0.4Fe0.8 Ni0.2O3-d (LCFN)-Sm0.2Ce0.8O1.9 (SDC). The results from Xray diffraction and a scanning electron microscope indicated that FeeNi bimetallic nano-particles were exsolved in situ from LCFN perovskite and distributed on the surface of the LCFN backbone after H2 reduction at high temperatures. These results suggest that the LCFN-SDC composite electrodes are promising bi-electrode materials for high performance and cost-effective SOFCs according to the research. j) Meng's group, from the Department of Materials Science＆ Engineering, University of Science and Technology of China, focused on the development and exploitation of SOFCs. They have completed the 863 Project of the key material and preparation technology for intermediate temperature SOFC and direct gas SOFC power generation with a capacity of 5 kW. They then completed the 973 Project of research on new materials for SOFCs during the



tenth Five-year Plan [168,169]. Professor Xia, also from the same institution, is interested in developing materials for energy conversion, pollutant removal, and membrane separation. He is also interested in a fundamental understanding of the effect of structure, defect, and microstructure on transport and electrical properties of bulks, surfaces and interfaces. His current research activities are development of SOFCs using hydrocarbons as the fuel and fabrication ceramic membranes, thin films, and coatings for solid-state ionic devices. They hope that their work could provide a method for the energy conversion device with high efficiency and be environment friendly [170e173]. k) A symmetrical cell composed of Ce0.9Gd0.1O2-d electrolyte has become the focus of Chen at Southnorth University [174]. Currently, the applications are undergoing thorough research and development. Thus, the acquired electrode will have a design of a typical element, Ni0.8Co0.15Al0.05LiO2 (NCAL)-foam Ni composite. The maximum power of the densities are 93.6 and 159.7 mW/cm2 at 500 and 550 C, respectively. The polarization resistances of the cathode are 0.393 and 0.729 U/cm2. The results reveal that Li2CO3 and the cation-disordered NiO-like phase are formed on the surface of the layered NCAL structure due to a prolonged exposure to air and a large content of oxygen vacancies. It must be stated that such NCAL symmetrical fuel cell configurations were introduced by Dr. Zhu at Hubei University, who successfully mixed conducting membrane fuel cells by achieving 800e1000 mW/cm2 at 550 C [175e178]. These all new structure OSFC will be reviewed bellow in another line of LTSOFC R&D for EFFCs.

Electrolyte free or single layer solid oxide fuel cells in China Towards LTSOFC R&D, we can see all activities and developments have been done either; (1) using thin film technologies to develop a thin YSZ electrolyte layer to reduce the electrolyte's low-conductivity caused resistance so to reduce operational temperatures, or; (2) by developing new electrolyte materials. The electrolyte plays a central role in determining the LTSOFC progresses and achievements, which becomes a barrier to limit the SOFC commercial progress. If we can eventually avoid, or remove, the electrolyte limitations, certainly a radical new situation will emerge. A breakthrough researchdelectrolyte-free fuel cell (EFFC) device has been presented. This novel technology stems from the electrolyte and electrode materials forming a new multifunctional nanocomposite based on the NANOCOFC. Dr. Zhu is the first researcher who discovered the electrolyte free solid oxide fuel cell (or single component fuel cell) and introduced this work in China in 2012. His co-workers and him have made significant contributions to the field [179e185]. Electrolyte free fuel cell (EFFC) was first published in 2011 [186e190] and highlighted by Nature Nanotechnology [187]. Before discovering this novel dive, Zhu's group paid greater attention to the nanocomposites for low or intermediatetemperature SOFCs [188,189]. They also gave the first review and results summary on ceria carbonate composite electrolytes [132,190].

13

Another structure, EFFC with a Schottky barrier [191], is a new emerging energy conversion technology. The EFFC consists of a single-component of nanocomposite material which works as a one-layer fuel cell device, contrary to the traditional three-layer anode-electrolyte-cathode structure, in which an electrolyte layer plays a critical role [192]. Today, they are using ceria-based multi-functional nanocomposites as materials for LTSOFCs. Multifunctional materials have been developed by integrating semi- and ion conductors, which have resulted in an emerging insight knowledge concerned with their R&D on single-component EFFCs [185]. They have also found that some natural mineral materials and industrial-grade mineral materials can work as electrolyte materials using some doped semi-conductor materials, such as Ni0.8Co0.2Al0.05LiO2-d. These findings may pave a way for the industrialization of SOFCs [193]. Based on perovskite solar cell principle, the results succeeded in creating a new novel fuel cell with a nanocomposite functional layer that achieved a stable power output of 1080 mW/ cm2 at 550 C [194]. More importantly, they have made a breakthrough in a new generation SOFC science and technology called semiconductor-ionic fuel cells which combine ionic and semiconducting properties. The first theoretical approach (which is also suitable for a ceria-carbonate composite) may stem from the 2008 semiconductor-ionic heterostructure composites [178] to semiconductor energy band designs [193,195]. In connection with the heterojunction mechanism, the single-layer can simultaneously transport Hþ and O2 effectively through electrodes without an electrolyte separator. This can give rise to major performance enhancements of the device. The fuel conversion processes of EFFCs are thus similar to FCs, i.e. the cathode ORR on an air electrode and the HOR on a hydrogen electrode [194], as shown in Fig. 8. In the O2 conducting case, the ORR and HOR can be expressed as: Air electrode 0:5O2 þ 2e /O2

(1)

Hydrogen electrode H2 þ O2 /2H2 O þ 2e

(2)

þ

In the H conducting case, the ORR and HOR can be expressed as: Air electrode 2Hþ þ 2e þ 0:5O2 /H2 O

(3)

Hydrogen electrode H2 /2Hþ þ 2e

(4)

In both case, the overall reaction is: H2 þ 0:5O2 /H2 O

(5)

A more fundamental mechanism for the underlying process can be found in the references. [194]. Because the conducting band is higher in the N-semiconductor LaSrCaTiO3 (LSCT), and even higher in the semiconductor-ionic LaSrCoFeO3-SCDC (SmeCa co-doped


14


Fig. 8 e A typical semiconductor band design of (a) perovskite solar cell, (b) fuel-to-electricity conversion device inspired by the PSC structure. CeO2) membrane than in the electrode, electrons cannot jump over the anode to the LSCT and not to the LSCF-SCDC, either. Therefore, such an energy band alignment can effectively prevent the electronic cross-over and avoid a short-circuiting problem. A new method used through an internal electron-hole redox cycle results in the ionic electrons avoidance of ceria electronic conduction problems thus developing an excellent electrolyte [196]. An enhancement in the ionic conductivity of a heterostructure between the semiconductor SrTiO3 and the ionic conductor YSZ (yttrium stabilized zirconia) can be expected to have a profound effect in oxygen ion conductors and SOFCs [195]. Meng's group from State Key Laboratory of Rare Earth Resource Utilization at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences has made headway with EFFCs. They have reported the manipulated concentration ratios of ionic to electronic conductors in an electrolytefree Ce0.8Sm0.2O2-d-Li0.15Ni0.45Zn0.4 by adjusting the relative weight between its two inside compositions. These findings are relevant for the technological improvement of this new species of electrolyte-free fuel cells and represent an important step towards commercialization of the single-component fuel cell [197,198]. Zhao's group at the School of Environmental and Chemical Engineering, Yanshan University paid close attention to the synthesis of new nano (porous) electrode material and its application including fuel cells, supercapacitors, photo catalysis, solar cells, etc. Recently, the group successfully synthesized hierarchically porous composite metal oxide, LiNiCuZnoxide, through a sol-gel method with a bio-artemia cyst shell as a hard template. The as-synthesized material was used as symmetrical electrodes, anode and cathode, for the SDCLiNaCO3 electrolyte-based LTSOFCs, achieving a maximum power density of 132 mW/cm2 at 550 C [199,200]. Tan's group, from the Department of Environment Engineering, Nanjing Institute of Technology, is focusing on the mechanism of electric conduction of EFFCs. They are also interested in the gas phase analysis. The group has researched electrochemical behaviors of Y-doped La0.7Sr0.3CrO3Ld anodes in sulfur-containing SOFCs. The results by electrochemical impedance spectroscopy (EIS) indicate a charge transfer loss in polarization resistant dominates in the total resistance,

especially lower than 650 C. They also analyzed catalytic activities and sulfur tolerance for Mn-substituted La0.75Sr0.25CrO3±d in gas containing H2S. The results demonstrate that Mn-substituted La0.75Sr0.25CrO3±d with a moderate Mnsubstitution content has the highest activity for H2S, which is attributed to considerable reducibility of Mn4þ determined by H2-TPR [201,202]. Li's group in Tianjin University made a development on the EFFC based on a single layer fuel cell (SLFC). They successfully fabricated a new kind of single layer fuel cell (SLFC) based on a composite material of Ce0.8Sm0.2O2-d(SDC)-Na2CO3 and Sr2Fe1.5Mo0.5O6-d(SFM). The device made of 30 wt% SFM and 70 wt% SDC-Na2CO3 exhibits the highest open circuit voltage of 1.05 V and output of 360 mW/cm2 at 750 C. Likewise, a discussion of the influence factors of the OCV of the cell, the reason why the SLFC can make a considerably decent OCV and output compared with the traditional solid oxide fuel cell, has been explained in detail [203]. The novel semiconductor perovskite LaSrCaTiO3 was applied for all functional components of the SOFC device [204] suggests a revolutionary concept to develop advanced perovskite LTSOFCs [204]. Another novel semiconductor-ionic, La0.6Sr0.4Co0.2Fe0.8O2-d/SCDC nanocomposite, has been developed as a membrane, which is sandwiched between two layers of Ni0.8Co0.15Al0.05Li-oxide to construct semiconductorion membrane fuel cells (SIMFCs). Such a device presented an open circuit voltage (OCV) above 1.0 V and a maximum power density of 814 mW/cm2 at 550 C [205e207]. Dr. Wang, Dong and Xia with Zhu's group in Hubei University obtained achievements in electrolyte free- or singlelayer SOFCs. They report that industrial-grade rare-earth and perovskite oxide for high performance electrolyte layerfree fuel cell. With an optimum ratio of 60 wt% LCP to 40 wt % LSCF reached the highest OCV at 1.01 V and a maximum power density of 745 mW/cm2 at 575 C, displaying good performance stability. The findings show that industrialgrade rare-earth oxide is a good candidate for innovative low temperature solid oxide fuel cell (LTSOFC) technology [193]. Lu reported further progress in EFFC [208] and reviewed the recent development achievement requirements for the devices and patents in LTSOFCs from nanotechnology perspectives. He reports of advancements including fabrication



methods, material compositions, characterization techniques and cell performances in China [209]. Ji's group from Jilin University, in cooperation with Zhu's group in Hubei University, also carried out extensive research and development on EFFCs with a number of publications involving semiconductor designs and redox-stable perovskite oxide materials to enhance the EFFC stability [148,210]. EFFC or SLFC were recently developed with the scientific understanding of semiconductor-ionic fuel cells [183,211e214]. Meng and Ji et al. from Jilin University also conducted research on EFFCs or semiconductor-ionic fuel cells within multiple publications [210]. There is also an interesting R&D inclination emerging of using natural mineral materials, which are very often a natural type semiconducting property. For example, Wu's group co-operated at China University of Geosciences (Wuhan) and Zhu's group at Hubei University and found that the natural hematite can function as an electrolyte material for SOFCs. Its maximum power density is up to 467 mW/cm2 at 600 C. A composite of the industrial grade rare-earth precursor for agriculture and the Li0.3Ni0.9Cu0.07Sr0.03O2-dfor EFFCs were reported by Zhu's group. The report reveals that this material possesses a comparable ionic and electronic conductivities [180]. On the other hand, the commercial YSZ nanocomposite with a natural CuFe-oxide mineral exhibits an enhanced ionic conductivity in the low temperature range [181]. The electrolytes made from natural resources of industrial grade mixed rare-earth carbonate and composite materials were able to achieve excellent fuel cell performances [184]. A commercial lithium battery, LiMn-oxide, was reported as a positive electrode material for fuel cell applications. The cell performance has significantly improved from 10 mW/cm2 to 210 mW/cm2 below 600 C. The single-layer fuel cell was also constructed using the LiMn-oxide/SDC-metal oxide composite and achieved even better performances than those for conventional fuel cells [215]. The results demonstrate that natural materials for next-generation SOFCs may pave way for industrialization [185,216]. There are numerous teams making headline in fuel cell researches in China that listing one by one is impossible, due to limited space, but most presented their work at the Wuhan Advanced Fuel Cell Forum and represented works were selected for the International Journal of Hydrogen Energy publications. Several distinguished achievements in this new field of research and development on EFFC, SLF or latest semiconductor-ionic fuel cells should be highlighted: Semiconductor-ionic fuel cells have achieved state-of-theart performance, with 1000 mW/cm2 at 550 C to be higher than that of early reported SLFC and EFFC at around 600 mW/cm2 at 550 C; The advanced state-of-the-art semiconductor-ionic materials are based on layer structured oxides and perovskite semiconductors; it should be established that pure semiconductor perovskite can be used as the electrolyte function for advanced fuel cells [176e178,182,191,192,194]. SLFC and EFFC scientific understanding and principles are upgraded to semiconductor energy band designs and junction designs. Typical energy band designs and

15

junctions can be found in bulk Pn junction [179], Schottky junction [191] and semiconductor energy band designs [194,195]. Despite being efficient, environmentally friendly and highly suitable for several applications, FC technology still faces several hurdles towards effective utilization and commercialization, such as high costs of fuel cell components, and flexibility of fuel storage place. Our group is working to overcome these barriers by focusing on the following objectives: Develop functional nanocomposite materials for new fuel cells: Electrolyte-Free Fuel Cells; Develop fuel cell based hybrid and polygeneration system; Develop prototypes of energy conversion devices; Develop electrolytic hydrogen production at low or medium temperatures; Find investments from industries for commercialization of the technology.

Challenges and recommendations for China To protect the environment, ensure energy sustainability and stimulate innovation, the Chinese government invested several programs in renewable energy including FC, solar energy, etc. Nowadays R&D investment on FC development still largely relies on public funding and governmental administration. There is still a large gap in terms of R&D investment between China and developed countries. For the development of sustainable FC technology, certain factors must be considered. The energy crisis can be overcome when we increase the use of renewable energy sources instead of non-renewable sources. Moreover, the government should encourage a private sector to invest in renewable energy technologies. Therefore, according to the current situations in China, there are some suggestions about the fuel cell systems [33].

Policy orientation New policies must be established to improve the pertinence of industrial policies. The administrative agencies should give further attention to integrate the connections between research institutions and enterprises. It is crucial to build core competitiveness by developing key technologies. More importantly, the fuel cell systems should be modified according to different situations of regions and schedules, and only then can proper development strategies be formulated.

Public sector involvement Public sector involvement is one of the fundamentals to start and sustain development in any field. There are several incentives for the government to launch FC technology in China. The government must also reduce taxation on R&D equipment and create reasonable financial management for developing SOFC technology. Nonetheless, there should be more emphasis on the ultimate commercialization of the


16


technology. In this sense, government funded R&D projects and facilitation in setting up the markets will help and boost the FC technology in China.

Private sector participation New energy development in China can be quickened with the active participation of private partnerships. The Chinese government should encourage the private sectors to participate in the investment of FCs since nowadays private sectors are at a promising scale in China. The government should formulate relevant incentive guidelines to stimulate their funds towards the development of FCs.

International cooperation International cooperation should be strengthened. Advanced technology should be introduced to shorten the differences between developed countries. Only in this way can fuel cell products in China be competitive in the world.

Resources distribution To avoid excess development in China, the government should focus more on resource distributions such as funds, materials, industrialization and so on.

Combining to other new energies Solar energy, wind and other clean energies are abundant in China. FCs may obtain faster development through a combination with other clean energies.

Public awareness The public must be aware of the potential benefits from these emerging technologies. This can be achieved through public media, education and so on. Fuel cell technology is the most promising clean energy in the 21st century, and could be the best choice for replacing traditional fossil energy. Many developed countries, such as the United States, Japan and the European Union, are also continually increasing their investment in R&D of fuel cells. Fuel cells will become one of the development trends of world energy. For China's future energy strategies, although other renewable energies were developed earlier, the fuel cell systems have their own superiority. The fuel sources of fuel cell systems are extensive and can be obtained by conversion of solar, wind and other clean energies. In addition, the fuel cell systems can be used in a wide range of applications. With growing needs of fuel cell systems within the international environment, the prospect of China's fuel cells will be consistently promoted.

Conclusions China is abundant in renewable energy resources that can potentially fulfill its energy needs. However, the current Chinese government is still trying to conquer its energy crisis by means of conventional energy technologies. An effective and

sustainable long-term solution must be adopted in terms of renewable energy technologies. It is with regret that these resources have not been harvested, due to social, economic, and bureaucratic barriers. To strengthen the renewable energy technologies in China, the public and private sectors should greatly devote investments in renewable energy technologies for creating a sustainable future. Fuel cell technology is one of the most promising renewable energy technologies due to its compatibilities with several renewable energy resources and being combustion-free. In addition, the diversity of FC technology makes it a suitable candidate to justify our future energy demands and sustainable development of China. The fuel used in FC technology is highly effective compared to conventional fuels. In the future, we will expect that the cost of FCs will be largely reduced by the development of improved fuel storage techniques. Therefore, FC's hybrid systems will enact a brighter perspective in China.

Acknowledgement This work was supported by the Scientific Research project of Nanjing Xiao Zhuang University (Grant No. 2017NXY39, 4178011), the Jiangsu Natural Science Foundation (Grant No. BK20151574), the Swedish Research Council (Grant No. 6212011-4983), the Jiangsu Provincial Key Laboratory of Solar Energy Science and Technology.

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