Fuel cells that operate at 300° to 500°C

doi:10.1126/science.abc9136

GSTDTAP > 气候变化

DOI	10.1126/science.abc9136
	Fuel cells that operate at 300° to 500°C
	Meng Ni; Zongping Shao
	2020-07-10
发表期刊	Science
出版年	2020
英文摘要	Reducing the operating temperature of ceramic fuel cells (CFCs) from 800° to 1000°C to the 300°-to-500°C range would improve efficiency, sealing, durability, and cost while still maintaining favorable electrode reaction kinetics as compared with those of low-temperature fuel cells such as polymer electrolyte fuel cells. Developing stable electrolytes with a low ionic resistance and negligible electronic conductivity, however, is challenging. In principle, reducing the electrolyte thickness can reduce the resistance. Fabricating an ultrathin electrolyte requires advanced techniques that inevitably make mass production difficult and costly. Developing new high-conductivity electrolyte materials is another way to address this problem. On page 184 of this issue, Wu et al. ([ 1 ][1]) report a fuel cell with a distinct, high proton conductivity electrolyte. Conventional CFCs have an oxygen-ion conducting electrolyte (OCFC) that usually operates at above 800°C to allow for fast oxygen-ion transport. Because the proton has a lower barrier for diffusion, protonic CFCs (PCFCs) are generally accepted to be more promising than conventional OCFCs for operation at reduced temperatures. In addition, because the fuel-diluting H2O is produced in the cathode chamber of PCFCs, the expectation is for higher fuel utilization and higher theoretical performance. The authors' electrolyte consists of a Na x CoO2/CeO2 composite, which shows a conductivity of 0.1 to 0.3 S cm−1 at 370° to 520°C. The authors believe that the proton diffusion, which is different from the wellknown Grotthuss and vehicle mechanisms ([ 2 ][2], [ 3 ][3]), was confined to the NaxCoO2/CeO2 heterostructure interface (see the figure). This was driven by an induced local electric field (LEF) within the interface region because of the unbalanced charge distribution between the Ce–O and Co–O layers. The authors demonstrated a cell with 400-µm electrolyte thickness that achieved peak power densities of 1000 and 830 mW cm−2 at furnace temperatures of 520° and 490°C. These values are substantially higher than the benchmark PCFC with a well-known thin-film BaCe0.7Zr0.1Y0.1Yb0.1O3-δ electrolyte (455 mW cm−2 at 500°C) ([ 4 ][4]) and the OCFC (500 mW cm−2 at 550°C) ([ 5 ][5]). ![Figure][6] Three different proton diffusion mechanisms Proton (H+) transport through electrolytes is attractive for lower-temperature applications. The Grotthuss mechanism allows either an “excess” H+ or a defect to diffuse through the hydrogen bond network of water molecules through the formation and cleavage of bonds. The vehicle mechanism allows H+ to migrate bound to another element. An example is the formation of OH− and an oxygen vacancy (Vo) that occurs in perovskite structured oxides. Wu et al. propose transport by a local electric field (LEF)–promoted diffusion mechanism in which H+ migrates along the oxide interface. This requires an unbalanced charge distribution generated from oxygen and sodium vacancies (VNa). GRAPHIC: KELLIE HOLOSKI/ SCIENCE Protonic conductors with high conductivity are also in demand for hydrogen production, electrochemical synthesis of ammonia, dehydrogenation or hydrogenation, hydrogen separation, and other applications. In the reversed operation, the PCFCs can function as protonic ceramic electrolyzer cells (PCECs) for hydrogen production through steam electrolysis ([ 6 ][7]). Compared with low-temperature (below 100°C) polymer-based electrolyzer cells that require a noble metal catalyst such as platinum and high electrical energy input, a nonprecious catalyst can be used for PCECs, with the required electrical energy input partially replaced by thermal energy. Thus, the reversible operation of PCFCs makes them a good candidate for storing excess renewable solar or wind power and effective for utilization of industrial waste heat. Ammonia synthesis is a key industrial chemical process because ammonia is widely used in fertilizer and many other industries. Unlike the dominating Haber-Bosch process at a high pressure (∼30 MPa) and with a low conversion (15%), protonic conductor–based electrochemical cells can synthesize ammonia at atmospheric pressure with a high hydrogen-to-ammonia conversion of 78% ([ 7 ][8]). Successful implementation of this technology requires protonic conductors with high conductivity and suitable catalysts with high selectivity toward ammonia synthesis. The fuel cell properties reported by Wu et al. are inspiring because they offer a different strategy for the development of proton-conducting electrolyte materials. It is notable how the LEF can accelerate the hopping of the proton but not the electron. If such a mechanism is reliable, it may also be applied for the development of electrolytes with high oxygen-ion conductivity. Much more work is still needed to realize the practical application of the authors' device. The composite electrolytes were developed by means of dry pressing without high-temperature sintering, which may make the electrolytes porous. Direct oxidation of fuel by oxygen gas through the pores will reduce the overall fuel efficiency ([ 8 ][9]). The metastability of Na x CoO2 under a hydrogen atmosphere is another concern. The reduction of this compound will lead to the formation of metallic cobalt and NaO. The NaO could further react with water to form NaOH. This compound itself has a low melting point of 318°C, and molten NaOH can conduct OH− at the fuel cell's operating temperature of 370° to 520°C. The electrode that the authors developed is rich in cobalt and nickel oxides, and both of these compounds are widely used as electrodes in conventional alkaline fuel cells. Additional comprehensive studies will help illuminate the processes involved in the fuel cell operation. An evaluation of the long-term stability of the cell is important for determining whether there is a viable pathway to commercialization. Different cathode and anode materials are needed for improving highly efficient PCFCs that operate in the 300° to 500°C range. For example, sulfur deposits and coke can build up on anode materials, so developing materials that are resistant to these processes will improve performance and durability. Water produced at the cathode side is especially harmful because it may impede the oxygen reduction over the cathode of PCFCs. Solving these sorts of problems should lead to the successful application of protonic conductors with high conductivities in PCFCs and other relevant electrochemical systems. 1. [↵][10]1. Y. Wu et al ., Science 369, 184 (2020). [OpenUrl][11][Abstract/FREE Full Text][12] 2. [↵][13]1. H. Wang et al ., J. Ind. Eng. Chem. 60, 297 (2018). [OpenUrl][14] 3. [↵][15]1. C. Zhou et al ., J. Mater. Chem. A 7, 13265 (2019). [OpenUrl][16] 4. [↵][17]1. C. Duan et al ., Science 349, 1321 (2015). [OpenUrl][18][Abstract/FREE Full Text][19] 5. [↵][20]1. T. Suzuki et al ., Science 325, 852 (2009). [OpenUrl][21][Abstract/FREE Full Text][22] 6. [↵][23]1. L. Lei et al ., Adv. Funct. Mater. 29, 1903805 (2019). [OpenUrl][24] 7. [↵][25]1. G. Marnellos, 2. M. Stoukides , Science 282, 98 (1998). [OpenUrl][26][Abstract/FREE Full Text][27] 8. [↵][28]1. H. Xu et al ., J. Power Sources 440, 227102 (2019). [OpenUrl][29] Acknowledgments: M.N. is thankful for the grants (project nos. PolyU 152214/17E and PolyU 152064/18E) from the Research Grant Council, University Grants Committee, Hong Kong SAR. 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/283379
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Meng Ni,Zongping Shao. Fuel cells that operate at 300° to 500°C[J]. Science,2020.
APA	Meng Ni,&Zongping Shao.(2020).Fuel cells that operate at 300° to 500°C.Science.
MLA	Meng Ni,et al."Fuel cells that operate at 300° to 500°C".Science (2020).