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Accepted Article Title: Recent advances in the development of anode materials for solid oxide fuel cells utilizing liquid oxygenated hydrocarbon fuels: a mini review Authors: Wei Wang, Jifa Qu, Paulo Sérgio Barros Julião, and Zongping Shao This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Energy Technol. 10.1002/ente.201700738 Link to VoR: http://dx.doi.org/10.1002/ente.201700738

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10.1002/ente.201700738

Energy Technology

Recent advances in the development of anode materials for solid oxide fuel cells utilizing liquid oxygenated hydrocarbon fuels: a mini review Abstract: Solid oxide fuel cells (SOFCs) are the most widely used fuel cells due to their excellent fuel flexibility, high efficiency and low emissions. Although the liquid fuels are easier to handle and transport than hydrogen, their direct use in SOFC leads to serious performance deterioration because of the coke formation on the traditional Ni-based cermet anodes. In this review, the advances in the development of coking resistant anodes and the new liquid fuels such as oxygenated hydrocarbons to solve the problem of coke formation with Nibased anodes are summarized. It is concluded that Ni-based cermets are still the most promising anode materials and some targeted modifications are needed to improve the coking resistance. Several strategies to improve the coking resistance of Ni-based anodes are highlighted. The aim of this review is to provide some helpful guidance and potential directions for the future design of anodes for SOFCs utilizing liquid oxygenated hydrocarbon fuels directly.

1. Introduction Traditional fossil fuels are the main energy sources nowadays and also for the near future. However, fossil fuels suffer from their non-renewable nature, which can cause critical energy crises in the future. In addition, environmental issues accompanied with the use of fossil fuels such as global warming and climate change are causing considerable public concerns. Thus, the demand for safe and sustainable energy sources has stimulated great interest in fuel cells, which are devices that directly convert chemical energy in the fuels to electric power.[1,2] There are several types of fuel cells that are classified by the electrolyte type and the operating temperatures.[3-8] Solid oxide fuel cells (SOFCs) are high-temperature (500-1000 °C) fuel cells that employ a solid ion-conducting electrolyte, which are the cleanest, most efficient and versatile energy conversion systems. Due to their all-solid-state structure, high operational temperatures and no pollutions, SOFCs offer many advantages over conventional power generation systems, even in comparison to other fuel cells, in terms of its high efficiency (without Carnot limitation), reliability, size and fuel flexibilities as well as environmental friendliness.[9-12] The most distinguishing advantages of SOFCs are the high efficiency (up to 60%) and excellent fuel flexibility. In theory, SOFCs can be fueled by any combustible fuels such as gaseous hydrogen, hydrocarbons, liquid oxygenated hydrocarbons and even solid carbon.[13-17] Hydrogen has been demonstrated to be the most environmentally friendly feedstock for fuel cells. Unlike polymer electrolyte membrane fuel cells (PEMFCs) which can only be powered by ultra-pure H2 and simple alcohols, the potential fuels for SOFCs can further include carbon monoxide, biogas, hydrocarbons, etc.[18-21] External reforming and internal

reforming are two important operational modes in SOFCs, which are based on whether the fuel processing part is integrated or not. A typical external reforming system is composed of gas purification systems, catalytic reactors, etc., which increases the cost of the integrated system. SOFC can be fueled by hydrocarbons directly using the internal reforming mode, which afford some advantages such as the system simplicity and low cost. Ni-based cermets are the most widely-used anodes for SOFCs and they display excellent activity and stability for hydrogen electrochemical oxidation reaction.[22] However, the carbon deposition is demonstrated to be a main issue when using Ni-based cermets as the anodes for internal-reforming SOFCs with hydrocarbon fuels. Large amounts of steam/oxidant are widely used as additives into hydrocarbons to suppress the coke formation by increasing the oxygen to carbon (O/C) ratio.[23,24] However, the addition of large amount steam reduced the efficiency and operational stability of SOFCs. One possible solution is to develop new fuels with high O/C ratio for SOFCs. Several factors determine the suitability of a fuel for its potential use in SOFCs directly. The main factor is the possibility of the fuel to form carbon at the operational temperatures (the O/C ratio in the fuel). Other important factors are the energy density and the physical state of the fuel at standard conditions, which determine how easily the fuel can be stored and fed to the SOFC, as well as the availability and cost of the fuel. In this case, the liquid oxygenated hydrocarbons should be suitable, cokingresistant and efficient fuels for SOFCs. Several alternative liquid oxygenated hydrocarbons such as methanol, glycerol, ethanol, ethylene glycol (EG) and acetic acid, have been investigated as fuels for SOFCs through internal reforming/partial oxidation or direct utilization.[25-29] In SOFCs, anode is the place for the fuel oxidation with oxygen ions to generate H2O or CO2. The requirements for the SOFC anode include sufficient porosity, high electronic conductivity, chemical/mechanical stability, compatibility with the other cell components, etc.[30] Only a few metallic or ceramic materials fulfill all these requirements. Of these, the most common SOFC anode is the Ni-based cermet, which exhibits a

[a]

[b]

Dr. W. Wang, P. S. B. Julião, Prof. Z.P. Shao Department of Chemical Engineering Curtin University Perth, WA 6845, Australia E-mail: [email protected] J.F. Qu, Prof. Z.P. Shao Jiangsu National Synergetic Innovation Center for Advanced Material, State Key Laboratory of Materials-Oriented Chemical Engineering Nanjing Tech University Nanjing 210009, China E-mail: [email protected]

This article is protected by copyright. All rights reserved.

Accepted Manuscript

Wei Wang,[a] Jifa Qu,[b] Paulo Sérgio Barros Julião,[a] and Zongping Shao*[a,b]

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Wei Wang obtained his Ph.D. degree in Chemical Engineering at Nanjing Tech University, China, in June, 2013. He is now a postdoctoral fellow in Curtin University, Australia, since December, 2013. His research interests include anode catalytic materials and coke formation mechanism for solid oxide fuel cells (SOFCs) operating on hydrocarbons, fuel selection and application for SOFCs, photocatalysts for degradation of organic substances, photoanodes and photocathodes for dye-sensitized solar cells. He has published over 50 international journal papers.

Zongping Shao received his PhD from Dalian Institute of Chemical Physics, Chinese Academy of Sciences in 2000, after that he has been a visiting scholar at the Institut de Researches Sur La Catalyse, CNRS, France and a postdoc at Materials Science, California Institute of Technology, USA from March 2002 till June 2005. In July 2005, he joined Nanjing Tech University where he was promoted to professor. Currently, he is joint-appointed by Nanjing Tech University and Curtin University. His research interests include solid oxide fuel cells, lithium-ion batteries, supercapacitors, oxygen permeable membranes and polymer-electrolyte membrane fuel cells, photocatalysis and solar cells. He has published more than 400 international journal papers with a total citation of >18000 (Google Scholar) and an H-index of 62 (Google Scholar). He was selected by Thomson Reuters as one of the highly cited researchers in the engineering section (2014). He was also awarded the highest cited Chinese researcher in the energy section by Elsevier China (2015 and 2016).

hydrocarbons in SOFCs are highlighted. Some potential directions of the future research are also outlined, with emphasis on achieving high coking resistance, superior electrocatalytic activity and excellent operational stability of the anodes for liquid oxygenated hydrocarbon fueled SOFCs. This review aims to summarize the strategies to battle the coke formation over Nibased cermet anodes through the functional modification and the fuel system improvement and to review the recent progress in developing alternative anodes for liquid oxygenated hydrocarbon fueled SOFCs.

2. Nickel-based cermet anodes Ni-based cermets are the most widely used anodes in SOFCs due to their superior electronic conductivity and good activity for hydrogen electro-oxidation reaction.[22,31] However, the nickel in these anodes can easily dissolve carbon, leading to volume expansions that can cause structural failure of the anode. Nickel is also demonstrated to be an extremely good catalyst for solid coke formation, which suggests that carbon filaments can be formed, potentially destroying the anode structure and blocking the gas diffusion pathways. As well as causing failure of the cell, this propensity toward carbon formation also renders nickel a poor catalyst for direct oxidation of hydrocarbons, meaning that large quantities of steam need to be used for cells running on even oxygenated hydrocarbons. For instance, the cell performance of a SOFC with Ni-yttria-stabilized zirconia (YSZ) cermet anode operating on glycerol was investigated.[29] At 800 °C, the peak power density (PPD) was 0.265 W cm−2 with a glycerol-steam (1:3) fuel for the SOFC with Ni-YSZ anode, which reached 79% of the PPD obtained by H2 fuel. In addition, a high water/glycerol molar ratio of 3 was needed to obtain a stable cell performance for 100 h at 700 °C, which may decrease the overall efficiency and increase the cost of SOFCs. The carbon deposition problems of YSZ are related to its inertness and consequent inability to mitigate any of the failings of nickel. It has little activity toward electrochemical oxidation or any of the other important catalytic reactions, and possesses extremely low electronic conductivity, meaning that once the nickel has deactivated the cell is useless. It also has no oxygen storage capacity to help improve carbon tolerance. Thus, many researchers have focused on the use of ceria-based electrolyte such as Sm-doped CeO2 (SDC), Gd-doped CeO2 (GDC) as the ceramic phase in Ni-based cermet anode for SOFCs operating on liquid oxygenated hydrocarbon fuels.[46,47] Liu et al. pointed out that direct methanol-fueled SOFC with Ni-SDC anode exhibited relatively stable performance during the long-term testing while the cells with methane and ethanol fuels showed rapid performance degradation with serious carbon deposition on the anode.[25] However, the electrocatalytic activity of Ni-SDC for methanol oxidation was low, as evidenced by the much lower power outputs of SOFCs using methanol fuel as compared with that using H2 fuel.[48] For the SOFCs with Ni-SDC anode, PPDs of 0.56 and 1.09 W cm−2 were achieved with hydrogen at 500 and 600 °C, respectively. The fact that these PPDs decreased severely to 0.26 and 0.82 W cm−2 while operating on methanol (under the same corresponding temperature conditions of 500 and 600 °C) suggests the relatively low electro-activity of NiSDC anode for methanol oxidation.[48] In addition, the poor durability of the SOFC in ethanol fuel suggests that the coking


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good compatibility with electrolytes, an excellent activity for hydrogen electro-oxidation and a sufficient electronic conductivity for electron transfer.[31,32] However, when fed with hydrocarbons, quick carbon deposition over the Ni-based anode may be experienced due to the low coking resistance, and then, a quick cell performance degradation was observed.[33-35] As a result, besides the development of new fuels for SOFCs, the rational design of the coking-resistant anodes is also important for the commercialization of hydrocarbon-fueled SOFCs. Recently, numerous efforts have been devoted to improve the coking resistance of hydrocarbon-fueled SOFCs, e.g., adopting perovskite-based anode materials, applying nonnickel-based anodes and modifying/decorating the Ni-based cermet anodes. Many excellent review papers focusing on the constraints on the design of anodes for SOFCs, anodes for SOFCs operating directly on hydrocarbon fuels, the investigation of the anode kinetics, reaction mechanisms, the anode models and model anodes as well as the cost-effective preparation and fabrication technologies for SOFC anodes have been published in the past decade.[36-45] However, there is still a lack of review articles featuring the topic of the development of the anodes for SOFCs utilizing liquid oxygenated hydrocarbon fuels directly. In this mini review, the recent progress of anode development for SOFCs operating on liquid oxygenated hydrocarbon fuels is presented. The current challenges and future perspectives of the direct utilization of liquid oxygenated

resistance of Ni-SDC anode toward the ethanol fuel should be improved.[25] Coke formation is a serious problem for SOFCs with Nibased cermet anode operating on traditional oxygenated hydrocarbons such as ethanol. Due to the high O/C ratio in the chemical structure of some new oxygenated hydrocarbon fuels (such as formic acid or EG), superior durability of SOFCs with traditional Ni-based cermet anode can still be obtained when these oxygenated hydrocarbon were used as fuels.[28,49] For example, renewable EG was exploited as a potential fuel for SOFCs with Ni-YSZ anodes.[28] The coke formation rates of the various carbon-containing fuels such as EG, glycerol, ethanol and methanol on Ni-YSZ anodes were investigated by oxygentemperature programmed oxidation (O2-TPO). The coke formation rates (displayed in Figure 1) varied from 0.1 to 1.2 (×10-3 mol g-1 min-1) and the coke formation rates were generally correlated with the O/C ratio in the chemical structure of the various hydrocarbon fuels. It suggested that EG should be a highly coking-resistant fuel for SOFCs. A very attractive PPD of 1.2 W cm−2 at 750 °C was obtained by a SOFC single cell operating on EG-steam (1:1) fuel, which reached 86% of the PPD of H2 fuel. The cell with Ni-YSZ anode displayed an excellent durability of 200 h when operating on EG-steam (1:1) fuel without any performance loss.[28] This study confirmed the practical applicability of EG as a direct fuel for SOFCs, which may have a significant effect on the future energy systems.

in an SOFC anode, alloying may reduce the activity for these reactions. It was reported that ruthenium, iron, cobalt, etc., could be added to Ni-based anode to form alloy to suppress the carbon deposition in hydrocarbon fuels.[50-54] Nickel and ruthenium bimetallic catalysts were heterogeneously synthesized via atomic layer deposition (ALD) for use as the anode materials for direct-methanol SOFCs operating in a low-temperature range.[50] The presence of highly dispersed ALD Ru islands over a porous Ni mesh was confirmed and the schematics of direct methanol SOFCs using Pt-GDCNi/ALD Ru configuration and the methanol oxidation process in the triple phase boundary (TPB) region are shown in Figure 2. The differences in the PPDs between samples utilizing Ni/ALD Ru and Pt/ALD Ru (which is the best catalyst for direct methanol SOFCs), were observed to be less than 7% at 300 °C and 30% at 350 °C. The higher electro-activity of the Ni/ALD Ru anode than the Ni anode was assigned to the absence of Ni agglomeration and coke formation, which were harmful to the methanol oxidation reaction. It is expected that such highperformance heterostructured catalyst with low precious metal loading may have applications in several types of electrochemical energy conversion devices. However, this catalyst with such heterostructure prepared by ALD technique is difficult to scale up.

Figure 1. Coke formation rates on the Ni-YSZ anodes after treatment in various carbon-containing atmospheres for 0.5 h at 750 °C. Reproduced with permission from Ref. [28]. Copyright 2015, Elsevier Inc.

3. Modified nickel-based cermet anodes 3.1. Alloying Ni with other coking-resistant metals Coke formation over Ni-based cermet anodes may be suppressed by partially replacing Ni with other metals that are inert for the hydrocarbon cracking reaction. Alloying of Ni can improve coking tolerance by reducing the rate of carbon-carbon bond formation, reducing the amount of the most destructive and deactivating graphitic carbon and/or increasing the rate of competing reactions, such as carbon oxidation. Conversely, since Ni is an excellent electrocatalyst for the oxidation reactions

Figure 2. Schematics of direct methanol SOFCs using Pt-GDC-Ni/ALD Ru and the methanol oxidation process in the TPB region. Reproduced with permission from Ref. [50]. Copyright 2016 American Chemical Society.

In addition, the issue of cost has driven a search for cheaper alternatives and some transition metals such as iron and cobalt have been explored extensively. These metals seem to be effective in reducing the overall coke formation and decreasing the amount of graphitic carbon. For instance, Huang et al. studied the effect of Fe to Ni ratio of the Fe-Ni/scandia stabilized zirconia (ScSZ) composite anodes on the coke formation rate and electrocatalytic activity.[51] Adding iron greatly improved the coking resistance of Ni-based anode, nonetheless,


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3.2. Anode catalyst layer The deposition of a catalyst layer with high catalytic activity for the steam/CO2 reforming and/or partial oxidation reactions of liquid oxygenated hydrocarbons and an excellent coking resistance on the Ni-based cermet anodes was also used to improve the durability and power output of hydrocarbon-fueled SOFCs.[56-60] Barnett and co-workers pioneered the use of RuCeO2 as a catalyst layer for hydrocarbon-fueled SOFCs.[56] This catalyst layer was used to catalyze the reforming of hydrocarbons into H2 and CO, thereby limiting the direct exposure of the Ni-based anodes to the hydrocarbons. In the last ten years, numerous efforts are devoted to the development of highly active and coking resistant anode catalytic materials for SOFCs.[61-65] Ye et al. pioneered the use of Cu-based catalyst layer for SOFCs operating on ethanol in 2008.[66] By optimizing the fabrication condition and the copper to ceria ratio, the cell with Cu-CeO2 catalyst layer calcined at 1100 °C with a Cu to CeO2 weight ratio of 1:2 showed both high power outputs and long-term durability in ethanol-steam fuel with power density maintaining at ~0.35 W cm−2 at 800 oC for 80 h without coke formation. However, the delamination of the catalyst layer from the Ni-based anode was the main reason for performance degradation of SOFCs. To solve this problem, a Ni-CeO2

interlayer was added between Cu-CeO2 and Ni-YSZ layers to enhance the operational durability of SOFCs with Cu-CeO2 catalyst layer.[67] With the help of this Ni-CeO2 interlayer, SOFC single cell showed good stability during 250 h operation in ethanol fuel, as suggested by the scanning electron microscopy (SEM) image of the tight interface connection of Cu-CeO2, NiCeO2 and Ni-YSZ layers (Figure 3). The Ni-CeO2 interlayer matched the thermal expansion coefficients (TEC) of the other two layers and greatly improved the durability of SOFCs operating on ethanol fuel.

Figure 3. The cross-sectional SEM image of fuel cell showing interfaces of three layers in the anode after operation in ethanol for 250 h. Reproduced with permission from Ref. [67]. Copyright 2009, Elsevier Inc.

Besides the Cu-based catalysts for ethanol internal reforming in SOFCs, Ni-based supported catalysts are also demonstrated to be the active anode catalyst layer materials for SOFCs operating on ethanol.[68,69] For example, Ni-Ce0.8Zr0.2O2 catalysts were prepared by a glycine nitrate process (GNP) and an impregnation process (IMP).[68] Ni-Ce0.8Zr0.2O2 (GNP) showed a higher catalytic activity than Ni-Ce0.8Zr0.2O2 (IMP) for ethanol steam reforming, especially at lower temperatures. The superior catalytic activity and coking resistance of Ni-Ce0.8Zr0.2O2 (GNP) were assigned to the small particle size of nickel phase and the strong interaction between nickel and Ce0.8Zr0.2O2, suggesting the importance of synthesis methods on the catalytic activity of Ni-based anode catalysts for steam reforming of ethanol. The cell with Ni-Ce0.8Zr0.2O2 (GNP) catalyst layer yielded a PPD of 0.536 W cm−2 at 700 °C when operating on ethanol-steam fuel, which was comparable to that of H2. Wang et al. demonstrated that the Ce to Zr ratio in Ni-CexZr1-xO2 catalysts played a critical role on the catalytic activity for partial oxidation of ethanol (POE) reaction in SOFCs.[69] Ni/Ce0.8Zr0.2O2 presented the highest POE activity among the various Ni-CexZr1−xO2 catalysts while Ni/CeO2 presented the lowest. The cell with Ni-Ce0.8Zr0.2O2 catalyst layer generated an attractive PPD of 0.692 W cm−2 at 700 °C operating on ethanol-oxygen (4:1) fuel, comparable to that of H2 (0.752 W cm−2). The cell with this catalyst layer also showed excellent operational durability on ethanol-oxygen fuel for 150 h at 700 °C.


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its excessive inclusion decreased the electro-activity of the anode for ethanol oxidation reaction. By considering the balance between the electro-activity and coking resistance, Fe0.5Ni0.5/ScSZ was demonstrated to be an efficient and cokingresistant anode for ethanol-fueled SOFCs. However, the longterm stability of these NiFe alloy-based cermet anodes was a big problem. Very recently, NiCo alloy-SDC composites were investigated as the anodes for SOFCs operated on methanol.[52] The alloying of Co enhanced the interaction between NiO and SDC as well as reduced the particle size and improved the dispersion of Ni1-xCoxO by inhibiting the grain growth of NiO, which resulted in larger TPB length in the anode layer. Besides, Co promoted the growth of (111) face of Ni and decreased the amount and graphitization degree of carbon deposited on the anode in methanol. After carefully tailoring the Ni to Co ratios in the alloy-based anode, the Ni0.9Co0.1-SDC anode displayed the lowest polarization resistance, the highest power output and the most stable performance under a certain current density. In addition to the above-mentioned transition metals, there are some other candidates for increasing the coking tolerance, the most promising of which is tin (Sn). Tin has been trialed in SOFC anodes and appears to confer both carbon and sulfur tolerance.[20,53,55] Farrell and Linic reported that the coke formation on the Ni-based cermet anode in oxygenated hydrocarbons (e.g., ethanol) can be greatly suppressed by alloying Ni with Sn metal with a small amount.[53] Furthermore, it was found that carbon deposition was also affected by the location of the catalyst in the anode and the operating voltage, which were the factors that affect the local O/C ratio and then the coke formation rates. This suggests that the surface chemistry and local O/C ratios were critical factors that should be considered in the design of the coking-resistant anode materials for SOFCs operating on oxygenated hydrocarbons. However, the amount of the secondary metal Sn should be carefully controlled since excessive Sn amount decreased the electro-activity and stability of Ni-based anode sharply.[20]

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Figure 4. Schematic of a SOFC with a catalyst layer during operation on ethanol fuel gas and the proposed mechanism for pyridine-mediated suppression of coke formation on a Ni-Al2O3 catalyst. Reproduced with permission from Ref. [70]. Copyright 2014, Elsevier Inc.

NiCu alloy-GDC composite used as an anode catalyst for SOFCs were studied when ethanol was used as the fuel.[72] A PPD of 0.277 W cm−2 was obtained by cell with this alloy catalyst layer in ethanol-water (1:3) fuel at 800 oC compared with 0.231 W cm−2 for an unmodified cell under the same condition. However, the electrocatalytic activity of this catalyst for ethanol oxidation reaction could be further improved by enhancing the dispersion between NiCu and GDC. 3.3. Surface decoration of other metals or metal oxides Coke formation could be suppressed by introducing another metal/metal oxide as a separate phase to decorate the Ni-based cermet anodes. For instance, the decoration of the Ni-based anode against coking could be achieved by the addition of a small quantity of another metal on the surface of Ni particles.[7375] Among the various metal modifiers, Pd was demonstrated to be a useful promoter to enhance the coking resistance of Nibased anodes of SOFC operating on ethanol fuel.[76] Babaei et al. found that the addition of Pd nanoparticles significantly promoted the electrocatalytic activity and coking resistance of Ni-GDC anode for the electro-oxidation reaction of methane fuel and particularly of methanol and ethanol fuels.[76] It revealed that, different from what was observed in methane, there was no filament carbon formation in methanol and ethanol fuels, suggesting that the presence of hydroxide groups in the molecular structure of alcohols may facilitate the carbon removal from the anode surface. Besides the decoration of metal nanoparticles on the surface of the Ni-based cermet anode, the surface modification by various oxides was proved to be a useful strategy to improve the coking/sulfur tolerance.[77-80] For example, Zr0.35Ce0.65O2-δ (ZDC) was used to decorate the Ni-YSZ anode to improve its coking resistance in methanol fuel.[81] The addition of ZDC not only enhanced the coking resistance due to its oxygen storage capability, but also increased the activity of the anode for fuel electro-oxidation due to the increased conductivity. Furthermore, the addition of ZDC also affected the type of carbon that was formed (which was more reactive), enabling its removal without completely destroying the anode microstructure, highlighting once more the critical role of ZDC in enhancing the coking resistance and operational stability. Very recently, Li et al. decorated the Ni-SDC anode by Mo metal and MoO3 to enhance its electro-activity and coking resistance for SOFCs operating on methanol fuel.[82] By tailoring Ni to Mo ratio, the Ni-3Mo-SDC (a Mo to Ni molar ratio of 0.03:1) composite anode displayed the lowest anodic polarization resistance and the highest power output among the three Momodified Ni-SDC anodes. The durability of the cell was improved with the increased Mo content in the anode, which was mainly assigned to the decreased amount of formed coke with a high graphitization degree. However, the effect of Ni to Mo ratio on the valence state of Mo in the composite anodes and their coking resistance was not clarified clearly, which should be further investigated in the future. 3.4. Replacement of the ceramic phase

Besides the single active-metal supported catalysts, the alloy-based catalysts were also investigated as the active anode catalysts for SOFCs operating on ethanol fuel.[71,72] The crystal structure, microstructure and electro-activities of a nanosized

Considerable research efforts were also devoted to the development of new anodes by changing the ceramic phase in the Ni-based cermet anodes.[83-86] In the hydrocarbon-fueled


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Ni-MgO-Al2O3 catalyst layer was used to tackle the coke formation of Ni-based anodes for SOFCs operated on acetic acid.[27] An attractive PPD of 1.3 W cm−2 was obtained for the SOFC single cell with a Ni-MgO-Al2O3 catalyst layer operating on acetic acid-steam (1:2) at 800 °C and the cell operated stably for 200 h without any performance loss, delamination of the catalyst layer or carbon deposition. As one major composition of the abundant biomass or bio-oil, acetic acid have been proved to be a direct fuel for SOFCs, which should be a promising technique for efficient power generation for the future with great environmental benignity and excellent sustainability. Pyridine as a fuel additive was used by Wang et al. to suppress the coke formation in SOFCs operating on ethanol.[70] Pyridine selectively occupied the acidic sites of the Ni-Al2O3 catalyst layer and significantly reduced the carbon deposition. At 600 °C, by adding 12.5 vol.% pyridine into the ethanol fuel, the carbon deposition amount on Ni-Al2O3 was reduced by 64% without sacrificing the power outputs of the SOFCs. Pyridine occupied the acidic sites of the Ni-Al2O3 catalyst and reduced the amount of carbonium ions formed, thereby reducing the coke formation as shown in Figure 4. Running on ethanol, the cell performance was unstable (lasting for 2.5 h only), yet, when pyridine was added to ethanol, the cell’s operation time was greatly increased to 100 h. This significant enhancement in the durability was assigned to the greatly suppressed coke formation on the Ni-Al2O3 catalyst. This study suggested that pyridine may be applicable to other similar liquid fuels to reduce the coke formation in SOFCs.

SOFCs with oxygen-ion-conducting electrolytes, water was formed in the anode chamber. It was hypothesized that if the anode possesses a water storage capability, the in situ formed water can be stored in the anode to eliminate the formed carbon on the anode. Thus, it was not necessary to add a large amount of steam into the fuel, which may decrease the overall efficiency of the SOFCs. Recently, Liu and co-workers demonstrated that the modification on the Ni surface with BaO could successfully suppress the carbon deposition over a Ni-YSZ anode for SOFCs operating on dry C3H8 and CO fuels.[87] Nanosized BaO with the water-storing capability promoted the water dissociation into OH and H to facilitate carbon gasification at the Ni-BaO interface, also suggesting the anode with a high water storage capability can enhance the coking tolerance. However, nanosized BaO could easily aggregate, evaporate or react with CO2 or other impurities such as sulfur in the fuel at elevated temperatures. To overcome this drawback, a robust proton-conducting perovskite oxide with a water storage capability and a strong interaction with Ni was used to improve the coking resistance of SOFC anode. Proton-conducting perovskite oxides such as BaZr0.4Ce0.4Y0.2O3-δ (BZCY4) have been used as the ceramic phase in Ni-based anode to enhance the coking/sulfur resistance.[84,88] For instance, Wang et al. developed a new Ni+BZCY4 anode with a water storage capability to battle the carbon deposition in SOFCs operating on ethanol.[84] The Ni+BZCY4 anode displayed superior activity for ethanol steam reforming reaction, water storage capability and coking tolerance in comparison to the traditional Ni-YSZ and Ni-SDC anodes. Using Ni+BZCY4 as the anode for ethanol-fueled SOFCs, a high PPD of 0.75 W cm−2 was obtained at 750 °C. The SOFC with Ni+BZCY4 anode operated stably for 180 h without any performance loss in ethanol-steam (1:1) fuel, while SOFCs with Ni-YSZ and Ni-SDC anodes failed in less than 2 h due to the serious coke formation. The proposed mechanism for watermediated carbon removal process on the Ni+BZCY4 anode is shown in Figure 5.

Figure 5. Proposed mechanism for the water-mediated carbon removal process on the Ni+BZCY anode. Reproduced with permission from Ref. [84]. Copyright 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In order to enhance the water storage capability, electroactivity and coking tolerance of the Ni+BZCY4 anode, the reduction in Zr amount and partial Yb doping on Y site were studied by Wang et al. for applications as anodes for ethanolfueled SOFCs.[89] Ni+BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) anode presented superior water storage capability, coking resistance and electro-activity for hydrogen oxidation to those of Ni+BZCY4 and Ni+BaZr0.1Ce0.7Y0.2O3-δ (BZCY7) anodes. A very attractive PPD (0.953 W cm−2) was achieved at 750 °C by a single cell with pure ethanol fuel. Furthermore, a good durability in pure ethanol fuel was obtained at 600 °C for ~100 h by the SOFC single cell with Ni+BZCYYb anode. The much improved catalytic activity for ethanol steam reforming reaction, better coking resistance and higher water storage capability suggested that Ni+BZCYYb cermets can be used as highly active and stable anodes to tackle the carbon deposition problem of SOFCs operated on ethanol fuel.

4. Non-nickel-based anodes 4.1. Copper-based anodes In catalysis, the infiltration of porous structures with metal nanoparticles is a common practice to maximize the active surface while simultaneously hindering carbon deposition by decreasing the area of graphitic carbon growth. In SOFCs, this approach was first used in the preparation of copper-based cermet anodes because the low melting point of copper oxide does not allow Cu/YSZ anodes to be produced by the conventional solid-state route. Some researchers have developed some coking-resistant anodes by substituting Ni with inert Cu for SOFCs operating on hydrocarbons.[90-92] However, due to the low activity of Cu-based cermet anode, CeO2 was also added into the Cu-based anode, which provided a high catalytic activity for hydrocarbon oxidation reactions due to its mixed conductivity.[93,94] Due to the low melting points of copper oxides, a potential method for synthesizing the Cu-YSZ cermets was developed in which a porous YSZ matrix was prepared first, and then Cu and CeO2 were added into it through wet impregnation together, which may increase the complexity for the preparation of the anodes. Cu-ceria-YSZ/ScSZ cermets were used as the anodes for SOFC operating on methanol or ethanol fuel and high power outputs were obtained at temperatures higher than 700 oC.[95,96] However, these anodes suffered from serious coke formation and a sharply reduced cell performance was observed. The coking resistance of the Cu-CeO2-based anodes could be greatly improved by Zr doping in ceria and alloying copper with Co and/or Ru.[97,98] The SOFC single cells with Cu-CeO2, Cu-ZDC and Cu/Ru-ZDC anodes showed much higher power outputs with ethanol fuel than those with H2 fuel and the highest cell performance occurred at ~4 h operation in ethanol, which was assigned to the improvement of the electronic conductivity in the anode due to the carbon deposition while excessive coke formation caused the blocking of the active sites. It was found that the Zr doping led to a better stability, also, the initial activity of the ZDC anodes was recovered after ~1 h of exposure to humidified H2, while the same was not true regarding the ceriabased anodes. Further, the Ru addition of less than 0.5 wt.% enhanced the operational stability by decreasing the coking on


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the anode.[97] More recently, the direct utilization of methanol was investigated on Cu-Co(Ru)/ZDC anodes synthesized by infiltration for SOFCs.[98] The PPDs of the SOFC single cells in pure methanol fuel were only slightly lower than that of humidified H2 and the cell performance loss was comparable to that of pure H2 and little coke formation was found after a continuous operation of 24 h. There are many issues to be resolved with infiltration for the copper-based cermet anodes, especially relating to long-term durability and feasibility of scaling up the process to industrial-sized anodes. 4.2. Noble metal-based anodes Some recent reports have focused on the development of Cubased cermet anodes for direct-ethanol SOFCs (DESOFCs). These DESOFCs can operate at high temperatures with a serious coke formation problem; therefore, numerous efforts have been devoted to develop active anodes for DESOFCs operating at lower temperatures. Pt was demonstrated as the most efficient electrocatalyst for ethanol oxidation reaction in SOFCs, however, CO was formed in this process, which can passivate the Pt catalyst, where CO was hardly converted into CO2 even in the presence of O2 or H2O. To address this problem, Ru has been added to the Pt catalyst since Ru can form hydroxyl groups on the catalyst surface to oxidize CO.[99,100] PtRu alloy-based catalysts were fabricated by ALD technique of Ru surface-coating on sputtered Pt mesh for DESOFCs operating at 300-500 °C.[99,100] The improved surface kinetics and high power output was achieved with the optimized coverage of Ru on Pt surface by controlling the ALD cycling times for use as SOFC anodes. This was attributed to the tradeoff of the different functions of Pt (effective ethanol oxidation, CC cleavage) and Ru (effective CO oxidation). Recently, Pd-based catalysts was developed as alternative anodes for DESOFCs since Pd displayed excellent catalytic activity toward ethanol oxidation at low temperatures and Pd is more abundant and cheaper than Pt. In addition, Pd was more suitable for proton-conducting SOFC application due to its high proton diffusion capability and electronic conductivity.[101] Very recently, Li et al. demonstrated the operation of micro-SOFCs with nanostructured proton-conducting Y-BaZrO3 (BZY) electrolyte to solve the fuel crossover problem for DESOFCs.[102] Nanoporous Pd anode film was synthesized by a pulsed laser deposition (PLD) technique with controlled sputtering pressure. This heterostructured SOFC exhibited PPDs of 72.4 and 15.3 mW cm-2 at 400 °C with H2 and ethanol fuels, respectively. No obvious coke formation was identified, however, the microstructures of Pd anodes after operation showed an obvious difference. Severe agglomeration was originated from the complexity of ethanol oxidation, an issue that deserves attention in future work, with for example alloying Pd with Ru or Pt with a core-shelled structure.

5. Perovskite-based anodes 5.1. Single-phase perovskite Perovskite oxides are a family of complex oxides with a general formula of ABO3, showing excellent thermal and mechanical stability, physical compatibility with typical electrolytes, flexibility

and low cost, making it attractive anode candidates in SOFCs. Recently, various perovskite materials including chromite, vanadate and titanate, etc., were investigated as anodes for SOFCs.[103-108] Among the various single-phase ABO3 perovskite oxides, the most investigated perovskites for the anode materials of SOFCs operated on liquid oxygenated hydrocarbons are based on LaCrO3 system,[109,110] which are reviewed particularly below. La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) perovskite oxide was investigated as anode for SOFCs operating on ethanol.[111] The performance of the electrolytesupported SOFC single cell with humidified H2 fuel was modest with PPDs of 0.165, 0.099 and 0.062 W cm−2 at 850, 800 and 750 °C, respectively. The corresponding PPDs were 0.160, 0.101 and 0.058 W cm−2, respectively, when ethanol was applied as the fuel. No significant performance loss was found by the cell with LSCM anode after 60 h operation in ethanolsteam fuel at 750 °C. However, the electro-activity of LSCM anode was still low, which needed to be improved by adding some active species. Very recently, Sm0.5Ba0.5MnO3-δ (SBM) was studied as an anode for SOFCs operated on H2 and methanol.[112] The low electrical conductivity of the SBM anode layer in H2 (0.1 S cm−1 at 850 °C) results in a poor performance of a single cell with a 300 μm La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte (about 150 mW cm−2 at 850 °C). However, the power output of the cell increased to 415 mW cm−2 with methanol fuel at 850 °C due to the enhanced electrical conductivity of the perovskite anode caused by the moderate carbon deposition. It indicated that SBM was an alternative anode for SOFCs with methanol fuel, notwithstanding it may not be applicable for SOFCs operating on other fuels with more serious coke formation such as methane and ethanol. 5.2. Perovskite-based composite Combining perovskite with other conductors or active catalysts to form a composite anode was demonstrated to be a useful way to improve the conductivity and/or the activity.[113-117] The most widely used method for the perovskite-based composite anode is infiltration/impregnation. Jiang et al. developed a Pd-infiltrated LSCM/YSZ composite anode for the direct utilization of ethanol in SOFCs.[118] Infiltration of Pd nanoparticles significantly improved the activity of LSCM/YSZ anode for ethanol electrooxidation reaction and as a result, the PPD of the cell increased eight times at 800 oC and no coke formation was found. Despite these results, the Pd nanoparticles were easy to agglomerate (from 10-20 to 50-70 nm) at such high operational temperatures, rendering this anode unattractive for large-scale applications. The confinement of Pd nanoparticles into the porous ceria nanocages may be an effective strategy to battle the agglomeration of Pd at high temperatures, which may be a new method to improve the electro-activity of the perovskite anode and stability of Pd nanoparticles.[119] A Ni-modified La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)/GDC composite anode was investigated for SOFCs operating on glycerol.[120] It is interesting that after thermal activation and air treatment at 1100 °C, nickel was mainly presented as ultrafine La2NiO4 particles uniformly dispersed on the perovskite surface. In addition, the thermal activation also led a decoration of perovskite into a lanthanum depleted structure. After the reduction at 800 °C in H2, highly dispersed ultrafine Ni


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Figure 6. Transmission electron micrograph of the as-prepared LSCM-Ru (a) before and (b) after reduction in H2. Reproduced with permission from Ref. [127]. Copyright 2012, Elsevier Inc.

6. Conclusions and perspectives In this review, we summarized recent advances about the anodes for SOFCs operating on liquid oxygenated hydrocarbon fuels. Compared with gas-fueled SOFCs, SOFCs fueled by liquid oxygenated hydrocarbons are in their infancy, but they offer an incremental increase in power outputs and stability. Nowadays, the anode materials for these SOFCs can be classified into four types: 1) Ni-based cermet anodes; 2) modified Ni-based cermet anodes; 3) non-nickel-based anodes and 4) perovskite oxide-based anodes. The anode developments that have been used to enable the successful operation on liquid oxygenated hydrocarbons at intermediatetemperature range (600-800 °C) are summarized in Table 1.

Figure 7. An illustration of the most promising anodes and the relevant design strategies for liquid oxygenated hydrocarbons fueled SOFCs.

An illustration of the most promising anodes and the relevant design strategies for liquid oxygenated hydrocarbons fueled SOFCs was shown in Figure 7. Most of the recent studies on SOFCs fueled with liquid oxygenated hydrocarbons concentrate on the cermet anodes. SOFCs with Ni-based cermet anodes exhibit relatively higher PPDs than those with oxide-based anodes and non-nickel-based anodes without precious metals. Nevertheless, Ni-based cermet anodes still suffer from serious coke formation, thus, their modification is considered the most desirable method to develop highly active and stable anodes for SOFCs. Among the various modification techniques, the use of an anode catalyst layer is the most investigated one for SOFCs operated on ethanol, acetic acid, etc. This strategy can be used for all the liquid hydrocarbon fuel by the careful selection of a suitable reforming/partial oxidation catalyst. However, one challenge should be addressed, that is, the catalyst layer should provide enough electrical conductivity to collect current formed in the anode. Wang et al. reported that the incorporation of inert Cu into the catalyst layer should be a potential solution,[65] and the copper incorporation method should be considered carefully.[14] On the other hand, the development of new liquid oxygenated hydrocarbon fuels and the modification of the existing fuel systems were also critical to address the coking problem of Ni-based cermet anodes. The addition of a basic liquid, such as pyridine, should be a potential way to suppress coke formation of Ni-based anode catalysts in ethanol atmosphere by occupying the acidic sites.[70] This aspect was


Accepted Manuscript

nanoparticles were formed on the surface of the perovskite. A relatively high PPD of 0.327 W cm−2 was achieved for the electrolyte-supported SOFC operating on glycerol fuel in almost dry condition with a steam to carbon (S/C) ratio of 0.2. No obvious coke formation was found on the surface of the composite anode after operating the SOFC with glycerol fuel. Similarly, this anode was also used in syngas or dry methanol fueled SOFCs achieving high power outputs and no significant coke formation.[121] This study may bring a new route to prepare ultrafine metal nanoparticles by using lanthanide oxides with K2NiF4-type structures as the intermediates. Besides the addition of metal or ionic-conducting phase to perovskite by wet-chemistry methods, a phenomenon called exsolution or precipitation of the nanoparticles at the surface of the perovskite anodes was found to be useful to improve the activity of the perovskite anode. The nanoparticles could be particularly interesting for catalytic activities for the oxidation/reforming of the hydrocarbons, but also for the enhancement of the electro-activity of H2 oxidation reaction due to the superior chemical reactivity of the nanoparticles.[122-126] Monteiro et al. reported a pure-phase Ru-doped LSCM perovskite for dry ethanol-fueled SOFCs.[127] Under reducing atmosphere, both the exsolution of Ru nanoparticles to the surface of LSCM grains and enhanced electronic conductivity of LSCM-Ru samples were observed. As shown in Figure 6, after reduction, Ru spherical crystalline nanoparticles with size of ∼5 nm were observed on the surface of larger particles while these spherical nanoparticles were absent in the as-prepared LSCMRu sample. The presence of isolated Ru particles was associated with an enhanced performance of anodes and was pointed out to have interesting properties, such as a homogeneous distribution and limited coalescence at high temperatures, preserving the necessary high surface area for good catalytic activity. It showed that reduced LSCM-Ru anodes displayed higher cell performance with ethanol fuel than that of hydrogen due to the enhanced conductivity and activity. In addition, it was found that Ru addition improved the stability and suppressed coke formation of LSCM anode in ethanol fuel. This study further indicated that partial substitution of a catalytic metal in the structure of perovskite anode and a subsequent nanoparticle exsolution on the surface represents a promising strategy to develop highly-active and coking-resistant anodes for SOFCs.

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control at the large scale production, unlike traditional composite anodes. Another strategy to enhance the power output is to build up perovskite anode-supported SOFCs, which needs several strict requirements of the perovskites such as melting points, mechanical strength, TEC match with the electrolyte, etc. Thus, substantial research and development are required to further develop new perovskite anode materials for SOFCs operating on liquid oxygenated hydrocarbons to enable stable operation with high power output.

Acknowledgements The authors would like to thank the Australia Research Council for supporting the project under contracts DP150104365 and DP160104835. Keywords: solid oxide fuel cell • anode • coking resistance • liquid oxygenated hydrocarbon • direct utilization

Table 1. Performance of SOFCs with typical anode materials operated on liquid oxygenated hydrocarbons at intermediate temperatures

Anode

Electrolyte/Cathode

Fuel

Ni-YSZ

YSZ/LaxSr1-xMnO3(LSM)

Ni-YSZ

PPD -2

Test Period

Ref

(W cm )

(hours)

Glycerol-steam (1:3)

0.265@800 oC and 0.125@650 oC

100@700 oC

29

YSZ/SDC/Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)

EG-steam (1:1)

1.2@750 oC and 0.4@600 oC

200@750 oC

28

Ni-SDC

SDC/Sm0.5Sr0.5CoO3(SSC)-SDC

Methanol

0.698@650 oC and 0.43@600 oC

60@650 oC

25

Ni-YSZ

YSZ/SDC/LSCF

Formic acid

0.571@800 oC and 0.128@600 oC

27@800 oC

49

Ni0.9Co0.1-SDC

SDC-(Li0.67Na0.33)2CO3/lithiated NiO

Methanol

0.675@700 oC

~8@700 oC

52

Sn-Ni-YSZ

YSZ/LSM

Ethanol

0.2@740 oC

20@740 oC

53

Fe0.5Ni0.5-ScSZ

ScSZ/(Pr0.7Ca0.3)0.9MnO3 (PCM)

Ethanol-steam (2:1)

0.26@800 oC and 0.12@700 oC

48@800 oC

51

Cu-CeO2/Ni-YSZ

ScSZ/PCM

Ethanol-steam (2:1)

0.566@800 oC

80@800 oC

66

Cu-CeO2/Ni-CeO2/NiYSZ

ScSZ/PCM

Ethanol-steam (2:1)

0.519@800 oC

250@800 oC

67

Ni-Ce0.8Zr0.2O2/Ni-YSZ

YSZ/SDC/BSCF

Ethanol-steam

0.536@700 oC

/

68

Ni-Ce0.8Zr0.2O2/Ni-YSZ

YSZ/SDC/BSCF

Ethanol-oxygen (4:1)

0.692@700 oC and 0.25@600 oC

150@700 oC

69

Ni-MgO-Al2O3/Ni-YSZ

YSZ/SDC/BSCF

Acetic acid-steam (1:2)

1.3@800 oC and 0.25@600 oC

200@800 oC

27

Ir-GDC/Ni-YSZ

YSZ/LSM

Ethanol

0.4@850 oC

600@850 oC

62

Ni-Al2O3/Ni-YSZ

YSZ/SDC/BSCF

Ethanol-pyridine (3:1)

1.1@750 oC and 0.3@600 oC

100@600 oC

70

NiCu-GDC/Ni-YSZ

YSZ/YDC((Y0.1Ce0.9O2)/LSCF

Ethanol-steam (1:3)

0.334@800 oC

/

71

(2.3:1)


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very interesting and applicable since SOFCs with traditional Nibased cermets still can be used as the anodes without any significant modification of the cell materials. In 2012, it was reported that the liquid formic acid can be directly applied to fuel the SOFCs,[49] while the single cell displayed poor power outputs at lower temperatures. More recently, Su et al. demonstrated a new strategy to battle the carbon deposition in hydrocarbonfueled SOFCs by exploring the synergistic effect provided by mixing the methane and formic acid fuels.[128] The durability of SOFCs operating on methane fuel was greatly enhanced with the addition of formic acid. The formic acid as a fuel additive is expected to be used in various liquid oxygenated hydrocarbons such as acetic acid, etc. For the perovskite-based anodes, the exsolution of the metal elements from the bulk to the perovskite surface is a promising method to enhance the catalytic activity of the perovskite-based anode for fuel electro-oxidation reactions to compete with that of Ni-based cermet anodes. However, the exsolution was strongly limited by the solubility of dopants in the host structure and their density on surfaces seems to be hard to

NiMo-SDC

SDC-(Li0.67Na0.33)2CO3/lithiated NiO

Methanol

0.68@700 oC

5@700 oC

82

Ni-ZDC-YSZ

YSZ/LSM

Ethanol

0.25@800 oC

12@800 oC

81

Ni+BZCY4

SDC/BSCF

Ethanol-steam (1:1)

0.75@750 oC and 0.448@600 oC

180@600 oC

84

Ni-BZCYYb

SDC/BSCF

Ethanol

0.953@750 oC and 0.519@600 oC

95@600 oC

89

Cu-CeO2-YSZ/Ni-ScSZ

ScSZ/PCM

Ethanol-steam (2:1)

0.438@800 oC

50@800 oC

96

Methanol

0.444@800 oC

24@800 oC

Ethanol

0.44@800 oC

20@800 oC

Cu-Co(Ru)-ZDC

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

YSZ/LSM

98

LSCM

LSGM/PCM

Ethanol-steam (2:1)

0.101@800 oC and 0.058@750 oC

60@750 oC

111

LSCM-YSZ

YSZ/LSM

Methanol

0.02@800 oC

100@800 oC

109

SBM

LSGM/SDC/BSCF

Methanol

0.3@800 oC and 0.16@750 oC

20@800 oC

112

Cu-LSCM-ScSZ

ScSZ/PCM

Ethanol-steam (2:1)

0.084@800 oC

12@800 oC

114

LSCM-Ru

YSZ/LSM

Ethanol

0.01@800 oC

/

127

Pd-LSCM-YSZ

YSZ/Pt

Ethanol

0.111@800 oC

/

118

Ni-LSCF-GDC

GDC/LSCF

Glycerol-steam (5:3)

0.327@800 oC

/

120

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Accepted Manuscript

10.1002/ente.201700738

Energy Technology

10.1002/ente.201700738

Energy Technology

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10.1002/ente.201700738

Energy Technology

REVIEW Wei Wang, Jifa Qu, Paulo Sérgio Barros Julião, Zongping Shao* Page No.1 – Page No.12 Recent advances in the development of anode materials for solid oxide fuel cells utilizing liquid oxygenated hydrocarbon fuels: a mini review

Accepted Manuscript

Solid oxide fuel cells (SOFCs) have attracted numerous attentions in the past decades due to their high fuel flexibility, efficiency and low emissions. Recent advances in the deveopment of coking-resistant SOFCs operating on liquid oxygenated hydrocarbons are reviewed with highlights in the design of highly active/stable anode materials and new liquid oxygenated hydrocarbon fuels.
