Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications

Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications

Nanocomposites for “nano green energy” applications 12 Liangdong Fan*, Muhammad Afzal†, Chuanxin He*, Bin Zhu†,‡ *Shenzhen University, Shenzhen, PR China, †KTH Royal Institute of Technology, Stockholm, Sweden, ‡Hubei University, Wuhan, PR China Abbreviations ASR BZY EBO GDC HOR H-SOFC LSC LSC113 LSC214 LSGM LSM LTSOFC MCFC MIEC NANOCOFC OCV ORR O-SOFC PAFC PEMFC SDC SEM SOFC STO TEM TPB YDC YSZ area-specific resistance BaZr0.8Y0.2O3Àδ Er0.4Bi1.6O3 Ce0.8Gd0.2O hydrogen oxidation reaction proton-conducting SOFC La0.6Sr0.4CoO3Àδ (La,Sr)CoO3 (La,Sr)2CoO4 La0.9Sr0.1Ga0.8Mg0.2O3Àδ (La,Sr)MnO3Àδ low-temperature SOFC molten carbonate fuel cell mixed ionic and electronic conductor nanocomposite for advanced fuel-cell technology open-circuit voltage oxygen reduction reaction oxygen ionic conducting SOFC phosphoric acid fuel cell proton exchange membrane fuel cell Sm0.2Ce0.8O2Àδ scanning electronic microscopy solid oxide fuel cell SrTiO3 transmission electron microscope triple-phase boundary Y0.2Ce0.8O1.9 Y0.08Zr0.92O2Àδ Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00012-0 © 2017 Elsevier Ltd All rights reserved 422 12.1 Bioenergy Systems for the Future Introduction Green energy resources are urgently demanded in recent years considering the large fossil-fuel consumption and increasing worldwide environmental concerns, especially the developing countries those are facing serious air pollution Though the internal combustion engine has been well-recognized to contribute to the societies for many decades, the low energy utilization efficiency has required for alternative candidates Among several kinds of energy conversion devices and technologies, fuel cell shows unique characteristics of high efficiency and low environmental impact The electric efficiency normally reaches above 40%, which is already higher than the internal combustion engine If combined with the highly valuable exhausted heat due to the internal loss, the energy efficiency could be doubled (Minh, 1993) A fuel cell is an energy conversion device that can directly convert chemical energy into electricity by electrochemical process (Winter and Brodd, 2004) It is distinct from the conventional engine that the chemical energy should be converted to heat first, then to mechanical energy, and finally to the electricity Therefore, the fuel-cell efficiency is independent from the Carnot cycle and much higher than those of other conventional energy conversion devices Generally, a fuel cell contains three key components of porous anode and cathode while among them the sandwiched dense electrolyte (Ormerod, 2003) A schematic illustration of fuel cell is presented in Fig 12.1 The anode is the place where the fuel oxidation reaction takes place, releasing the electrons and producing the protons, while, in the cathode, the oxygen gains the electron and is reduced to oxygen ions Depending on the applied electrolyte, the proton or the oxygen ion transports through the electrolyte and meets with its counterpart H2O Air H e H-SOFC Anode: 4H+ + 4e– 2H2 Cathode: O2 + 4H+ + 4e– H2O + – – e H2O H+ H2O + H2 H2O + N2 (A) Air H2O 2– O e– e– H2O O2– H2O + H2 N2 O-SOFC Anode: 2H2 + 2O2– Cathode: O2 + 4e– 2H2O + 4e– 2O2– (B) Fig 12.1 Schematic inllustration of fuel-cell principle (A) with proton-conducting electrolyte and (B) with oxygen-ion-conducting electrolyte (Fan et al 2013) Nanocomposites for “nano green energy” applications 423 ions, to produce the water The electrons released at the anode move through the external circuit providing the electric power for practical applications The detailed reactions in a fuel cell with proton-conducting electrolyte or with oxygen-ion-conducting electrolyte are also presented in Fig 12.1A and B, respectively There are already several kinds of fuel cell; however, the first discovery of the fuel cell was in 1839 by Sir Grove (Grove, 1839) According to the applied electrolytes, they are divided into alkaline fuel cell (AFC), proton exchange membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), molten-carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC) PAFC was the first fuel-cell type that was successfully employed for commercialization, while the AFC has been the focus at the middle of the 20th century for military/space applications PEMFC and SOFC have been put on the agenda in the recent years because of the significant development of the material and technology evolution (Wachsman and Lee, 2011) The operational temperature of PEMFC with the simple methanol fuel is suitable for vehicle applications and portable or electronic devices, while the combined heat and power resulted in high efficiency and unique characteristics of fuel flexibility make SOFC ready for stationary power plant applications or other auxiliary power unit (APU) Interestingly, there is a contrary development tendency for PEMFC and SOFC The research activity in PEMFC tries to enhance the operational temperature from below 100°C to above 100°C by developing the suitable electrolyte membrane to overcome the constraint of the water-involved proton conduction and to improve the electrode activity While SOFC community intentionally reduces the operation temperature from the conventional 800–1000°C to 500–700°C or even lower to overcome the high-temperature-related problems like electrode microstructure change, element diffusion, and cell performance degradation issues Compared with the PEMFC, SOFC shows the unique property of adoption of various fuels including biogas, natural gas, hydrocarbon fuel, CO, H2S, and solid-state carbon, besides H2 While the PEMFC can only use very limited fuel, such as H2 and methanol (Wachsman and Lee, 2011) Moreover, CO is the poisoning gas for Pt in PEMFC The direct use of hydrocarbon fuel such as natural gas makes SOFC as a candidate of transition power source to replace the current internal combustion engine to reach the hydrogen-powered society in near future Therefore, more and more government and industrial investment and academic research efforts have been put in SOFC field to promote the commercialization of this green energy technology In fact, the demonstration of SOFC for power generation has been presented; the most famous case is the Siemens-Westinghouse tubular SOFC power plant though the project is abandoned after several years’ operation However, the successful demonstration has inspired more effort and continuous input to this promising field Currently, “Bloom Energy” has installed more than 150–200 MW of its fuel-cell systems in the United States since 2001 (Singhal, 2013) The fuel cell uses scandium-stabilized zirconia as electrolyte, operating at 700°C with natural gas as the fuel The customers include Adobe, FedEx, Staples, Google, Coca-Cola, and Wal-Mart Another example of SOFC success application is the Japan ENE-FARM project; there are more than 150,000 fuel-cell units that had been installed for resident application The ENEFARM system is a combined heat and power system; the total system efficiency could 424 Bioenergy Systems for the Future reach to 90%, still using natural gas as the fuel precursor Therefore, SOFC has been the edge of the wide application A recent work by Blum et al reported that the short stack with anode-supported cell based on standard materials of Ni-cermet anode, YSZ electrolyte, and LSM-YSZ cathode presented a continuous operation time of 70,000 h (8 years) without significantly cell performance degradation (Blum et al., 2016) Therefore, we believed that SOFC technology will be the next-generation energy conversion device soon with wide applications One of the agreed targets between PEMFC and SOFC is to improve the fuel-cell performance through developing the novel cell components with improved electric properties and/or optimizing cell microstructure based on the existing materials The latter is even important since the newly exploited material should not only be highly active but also be robust and chemically and thermally compatible with the fuel-cell system Their wide application asks for lengthy, extensive practical atmosphere application Therefore, in parallel to the development of new materials, the research activities on cell component microstructure to improve the ionic conductivity of the presented electrolyte and to enhance the triple phase boundary of the electrode layer are widely performed One of the recognized technologies to optimize the microstructure of the cell comments is the nanotechnology In fact, the application of nanotechnology has been widely used in low-temperature or high-temperature PEMFC (Arico et al., 2005) For example, the state-of-the art electrode catalytic material in PEMFC is platinum because of the high catalytic activities both for oxygen reduction and hydrogen oxidation reactions and high electronic conductivity However, due to the limited material resources, the price of Pt has been increased significantly in the recent years Hence, application of nanosized Pt particle loaded on the porous carbon cloth/particle has been employed to reduce the Pt use while maintaining the electrode activity for fuel-cell power generation The particle size of the Pt has been decreased 500 mW cmÀ2 at 400°C The same group latterly developed even a simple method to prepare SDC@Na2CO3 nanocomposite through one-step coprecipitation (Raza et al., 2010) The nanocomposite was obtained when Sm3+ and Ce3+ were coprecipitated by Na2CO3 solution The newly formed nanoparticle in the precursor slurry absorbed a lot of resident Na2CO3, and it was kept at the final sample Even homogeneous nanoparticle with thinner carbonate-coated nanocomposite was achieved, and the peak output was raised to 1100 mW cmÀ2 at 470°C Such a remarkable fuel-cell performance is still the highest value until to date as per the best knowledge of authors The research of the hybrid ionic conductivity and its possible ionic conduction mechanism is also widely investigated However, it is out of the scope of this work; the readers are suggested to refer to the literature (Maheshwari and Wiemh€ofer, 2016; Wang et al., 2011) and the recent published review article (Fan et al., 2013) Nanocomposites for “nano green energy” applications 435 spectroscopy showed that the interfacial voltage loss has been removed compared to the featured semiarc in electrolyte-based fuel cell, while only one diffusion that resulted in Warburg tail was observed for the newly developed perovskite fuel cell (Zhu et al., 2016a) It is worth to note here that a very recent work from Harvard University (Zhou et al., 2016) presented a fuel cell by using the perovskite nickelate as an electrolyte with the assistance of silicon micromachining processes The free-standing perovskite nickelate is initial a mixed ionic and electronic conductor, while it could be turned into an ionic conductor by hydrogen incorporation in fuel-cell condition through a filling-controlled Mott transition (Zhou et al., 2016) The hydrogenation of SmNiO3 not only leads to the reduction of Ni but also decreases the hole density in the thin film The proton conductivity of PrNiO3 due to the proton incorporation was much higher than those of common proton conductor, such as the current most conductive oxide, BaCe0.8Y0.2O3, and other ionic conductor Therefore, the cell based on the hydrogenated SmNiO3 electrolyte (1.5 μm) and nanoscale thin-film Pt electrode gave a peak power output of 200 mW cmÀ2 at 500°C The above two cases, in fact, presented the same novel fuel-cell type over the conventional pure ionic conductive fuel cell The original mixed ionic and electronic conducting electrolyte gives more opportunity/ design strategy to overcome the constraint of the current pure ionic conductors and promoted the quick application of fuel cell A brief summary of the above section is that the nanocomposite not only overcomes some intrinsic issues with the current electrolyte materials, like the improved stability in fuel-cell condition and enhanced ionic conductivity, but also leads to the breakthrough research result to alter the electrical property significantly, like the change of the conduction from the mixed ionic and electronic conduction to pure ionic conduction The later may leads to the revolution of the fuel-cell knowledge and industrial adoption to promote the commercialization of green, highly efficient fuel-cell technology 12.3 Nanocomposite anodes The commonly used anode material in SOFC is the Ni-YSZ composite, called Ni-cermet, in which Ni serves as the reaction catalyst and electronic conductor, and YSZ as ionic conductive electrolyte and as the matrix phase to support and separate the easily aggregated Ni metal as well as reduce the thermal coefficient mismatch between anode and electrolyte, and most importantly, to extend the triple phase boundary (TBP) length Ni/YSZ shows high catalytic activity toward fuel oxidation However, the oxidation of Ni into NiO could lower down the electronic conductivity and induce the electrode microstructure change/redox instability Ni can be coked under carbon-contained fuel or intermediates In addition, the mature fuel-cell fabrication always leads to the sintering of anode during the densification of the electrolyte layer Therefore, optimization of the electrode microstructure or modification of Ni-cermet electrode and development of novel electrode materials have been widely conducted Among all approaches, the nanocomposite approach shows distinct advantages and application potential 436 Bioenergy Systems for the Future 12.3.1 Optimization of traditional anode material microstructure The electrode reaction rate in fuel cell is highly dependent on the triple phase boundary (TPB) length The TPB is the place where the ionic conductor, the electronic conductor, and the gas phase meet The conventional anodes are made by the mechanical mixing of commercial NiO and YSZ micrometer particles The nonhomogeneous structure limits the TPB length and subsequently the fuel-cell performance Therefore, finding or developing novel methods to synthesize Ni-cermet composite with improved TPB length is urgently needed A nanocomposite of NiO and YSZ was synthesized by Kim et al (2006) by a Pechini-type polymerizable complex method Moreover, the nanocomposite is coconjugated on micrometer grain YSZ powder with the aim of improved performance and durability The nanocomposite showed high thermal cycling resistance, only 11.4% decrease was observed, while it is 56.8% for the commercial mixed sample The area-specific resistance (ASR) of the cell kept stable during more than 100 h testing, and fuel cell presented a quite stable voltage response under a static load of 1.0 A cmÀ2 in reactive gases of H2O-H2/air during more than 550 h durability test Ding et al (2010) developed a coprecipitation method to synthesize NiO-GDC nanocomposite powder for low-temperature operation Coprecipitation method is a simple and promising process to produce homogeneous and small particle size powders The nanocomposite presented an average grain size of 16 nm with a homogeneous phase distribution after the calcination at 600°C for h The nanocomposite anode used SOFC showed a peak power output of 360 mW cmÀ2 at 600°C with GDC electrolyte, about three times higher than the cells with the mechanically mixed conventional composite anode Tremendously improved TPB length in the NiO-GDC nanocomposite is acknowledged NiO-GDC nanocomposite with GDC nanocubes was synthesized by Yamamoto et al (2014) using the organic-ligand-assisted hydrothermal treatment followed by aerosol technique The GDC nanocubes exhibited a highly dispersed cubic shape with an average size of $15 nm and with an orientation (001) crystal facets reflected by (002) facet with a 0.27 nm lattice fringe as shown in Fig 12.8 The improved microstructure of NiO-GDC nanocubes exhibited an impedance of 0.14 Ω cm2 at 600°C, three times less than that of NiO-GDC composite anode Strontium- and magnesium-doped lanthanum gallate (LSGM) electrolyte is represented as one of the most potential electrolytes to replace YSZ to lower down the working of high-temperature SOFC because of the high ionic conductivity and negligible electronic conductivity over a wide oxygen partial pressure range However, electronic conduction or reaction that produce new undesired low-conductivity phases takes place when high-temperature cofiring with Ni-based anodes Nanosize NiOdeposited porous LSGM anode was presented by Zhan et al (2011) using a impregnation method The ASR of the prepared anode nanocomposite was extremely low, 0.026 Ω cm2 at 650°C, substantially lower than that of the target value of 0.15 Ω cm2 In addition, one of the unique properties of impregnated electrode is the super low content of target material As we know, to reach at the percolation value, one of the constituted phases of composite should at least reach to 30 vol% in the composite While in this work, there is only 2.51 vol% requested to get such low ASR The Nanocomposites for “nano green energy” applications 437 0.7 NiO-GDC nanocube 0.6 NiO-GDC composite -Z″ (Ω·cm2) 0.5 0.4 0.3 0.2 0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Z′ (Ω·cm2) Fig 12.8 HR-TEM images of CeO2 nanocubes (left) and EIS performance comparison of NiO-GDC nanocube and composite anodes (right) (Yamamoto et al., 2014) peak power density of single cell with impregnated NiO anode and LSGM electrolyte was 1.2 W cmÀ2 at 650°C with air as the oxidant; it could be improved to 2.0 W cmÀ2 at the same operational temperature using pure oxygen as the oxidant The same group has recently used the same method to develop an active additive impregnated metal support SOFC to improve the mechanical strength and fuel-cell performance of conventional Ni-cermet anode-supported SOFC (Zhou et al., 2015) Cell with trilayer structures of “porous 430L j dense thin scandium-stabilized zirconia electrolyte j porous 430L” was first prepared by laminating and firing tape-casted green sheets at 1325°C The porous 430L stainless steels were then impregnated with nanosized NiO anode and SmBa0.5Sr0.5Co2O5 cathode active catalyst Encouraging power densities of 1.02 W cmÀ2 at 600°C and 0.6 W cmÀ2 at 550°C were obtained The cell also presented 60 h fuel-cell operation duration Such a strategy used metal support configuration, and impregnated active additive suggests a higher robustness and lower material cost for future SOFC successful deployment 12.3.2 Modified Ni-cermet for hydrocarbon application One of the barriers of Ni-cermet anode is its low resistance toward carbon fouling and sulfur resistance when using the current most of hydrocarbon fuel or biogas fuel Ni is highly active to break the CdC bond to cause the carbon deposition, and sulfur contained compound with easily interact with Ni to reduce the active site and reduce the electronic conduction Though there have already developed many kinds of ceramic oxide anodes with much improved cooking and sulfur resistance, their electrochemical performance to oxidize hydrocarbon fuel is much lower than the Ni-based electrode due to the limited catalytic activity and low electronic conductivity (Ge et al., 2012) Yang et al (Liu et al., 2011; Yang et al., 2011) wisely obtained a nanosized barium oxide coating on conventional Ni-YSZ electrode by vapor deposition method The nanosized BaO selectively deposited on Ni site through reacting with NiO then 438 Bioenergy Systems for the Future reduce to isolated BaO-coated Ni on reduction The unique BaO/Ni interface resulted cell did not impede the charge transfer on anode and, therefore, presented similar fuelcell performance compared with bare Ni-YSZ anode-based cell Furthermore, the new cells gave both excellent and 100 h stable fuel-cell performance using C3H8, CO, and gasified carbon fuel versus conventional cell failed in h for C3H8 and