Fuel battery research based on mixed conductive functional layer
Abstract: Solid oxide fuel battery is a high energy density output clean energy conversion device. One of its characteristics is that the battery operation must reach the required high temperature. Developing low temperature fuel batterys is an important aspect to solve its commercial bottleneck. No electrolyte The diaphragm layer fuel battery is a new type of battery different from the traditional solid oxide fuel battery structure. This type of battery is simple to manufacture and has high performance output at low temperature. This paper uses secondary solid phase method to synthesize electrons with layered structure. Conductive material - LiNi0.8Co0.15Al0.05O2-δ (LNCA), and it is combined with iridium-doped ytterbium oxide of ion-conducting material to obtain a composite material with electron-ion mixed conductivity. The electrolyte-free diaphragm layer fuel battery. The functional layer simultaneously acts as a catalyst and ion conduction. We studied the effect of the thickness of the functional layer and the ratio of the electron-ion conductive material on the device performance, and explained the mechanism of the effect. The maximum power output of 937 mW cm-2 is obtained at 550 °C, and it is feasible to operate at lower temperatures.
Keywords: Hybrid conduction; no electrolyte membrane layer; functional layer; fuel battery
Abstract: Solid oxide fuel battery (SOFC) is a kind of green energy conversion device with high power density output. It needs high operation temperature. Developing low temperature SOFC is a key issue to overcome the bottle neck problems to achieve commercialization. Electrolyte layer free fuel battery (EFFC) differs greatly from the conventional SOFC in structure. Its fabrication process is easy and it can obtain high power output under low temperature. We use solid-state reaction method to electron anion ion-electron conductor (MIEC) And it was used as the functional layer of EFFC devices. The functional layer is supposed to act as both catalyst and ion transport media. We investigated the influences of the functional layer thickness as well The device achieved a maximum power output of 937 mW cm-2 at 550 oC, and can work under lower temperature.
Keywords: Mixed conductivity; electrolyte layer free; functional layer; fuel battery
Developing green energy technology is an important measure to alleviate energy crisis and environmental problems. Solid oxide fuel battery (SOFC) is currently the most efficient energy conversion method for energy conversion. The traditional SOFC is based on an ionic electrolyte and is an electrochemical device that includes a three-layer structure of an anode, an electrolyte, and a cathode. In recent decades, Korea, Japan, and developed countries in Europe and the United States have invested in SOFC. A large amount of funds and manpower to promote its commercial development. China's fuel battery research has also made important breakthroughs with the support of the state. However, as far as the current situation is concerned, SOFC technology is not completely complete in performance, life and cost. One of the reasons for achieving commercialization is its core The Y2O3 doped ZrO2 (YSZ) material of the electrolyte layer needs to reach a sufficiently high oxygen ion conductivity (0.1 S/cm) at a high temperature of 800 to 1 000 °C. The thermal expansion coefficients of the anode, electrolyte and cathode materials at high temperatures. The matching is the key, otherwise it will easily lead to the failure of the device. However, this leads to a relatively high cost of the battery.
Lowering the operating temperature of the SOFC can reduce the decomposition of the battery itself and the thermal matching between related materials, etc. Low temperature operation can greatly reduce the cost of the required materials. Therefore, lowering the operating temperature becomes the development trend of SOFC. To make the conductivity of the material meet the requirements of medium and low temperature operation, it is necessary to have superionic conductivity of the electrolyte material. Thin film of electrolyte material is used to increase its conductivity at low and medium temperatures ; on the other hand, new low temperature and high ionic conductivity materials such as Sm doped yttrium oxide (SDC) can be developed . In addition, many studies It shows that the two-phase interface of nanocomposites can provide exbatteryent performance [3-4].
In fuel battery reactions, effective catalysis mainly occurs in three phases. At the interface, in order to increase the three-phase reaction boundary (TPB) of the battery, a composite material of an ionic conductor and an electrode material is often used as an electrode. This type of material is called a mixed ion-electron conductor (MIEC, Mixed ion). -electron conductor). In the MIEC electrode, the ion conductive material can effectively transport the catalyzed ions to the electrolyte layer, thereby improving the performance of the battery. In 2011, ZHU et al [5-6] found that nanocomposites using low temperature SOFC The electrode material can realize the full function of the fuel battery. This type of battery is called the electrolyte-free diaphragm fuel battery (EFFC). The output current density of the battery exceeds 1 000 mA/cm2 at 550 °C, and the output power can reach 600 mW/ Cm2. The research results were selected by Nature Nanotechnology as the highlight of the 2011 study. The article was published in the article "FUEL CELL: Three in one". The main part of the battery is a composite with mixed ion-electron conductivity as the functional layer. The vast majority of the thickness of the entire fuel battery. Both sides of the functional layer are foamed nickel coated with a material with high electronic conductivity. It is simple and can be integrated and pressed. And because the device works at low temperature, it also avoids the thermal expansion problem caused by high temperature.
Unlike the MIEC material used for the electrode, the MIEC ion conductive material as a functional layer The mass ratio is higher than that of electronically conductive materials. It has been found that doped yttrium oxide and conventional electrode materials in solid oxides can be used as functional layers of EFFC to obtain higher performance output. These composite materials include LiNiZnO2-δ- SDC [5,7], SFMO-SDC , LSCT-SDC , LSCF-SCDC , etc. In addition, lithium battery electrode materials can also achieve higher performance output in EFFC, such as LiMnO2 ?δ, LCN, LNFO, etc.
LiNi0.8Co0.2O2-δ(LNC) is an exbatteryent lithium battery electrode material with high specific capacity and layered The structure facilitates the insertion and extraction of Li. At the same time, the research  shows that LNC has good catalytic activity, and the layered structure material is beneficial to the proton transport . Al doping can further increase the chemical stability of LNC.  found that LiNi1-yCoyO2 materials have good electrons above 500 °C Electrical properties, especially when the content of Ni is \u003e0.75.
This paper synthesizes LiNi0.8Co0.15Al0.05O2-δ (LNCA) with a layered structure by secondary solid phase method, as an electronic conductive material, and LNCA It is compounded with SDC material  with high ionic conductivity at low and medium temperatures. Based on this material, EFFC is studied. The effect of the ratio of LNCA and SDC in the composite on the performance of the battery and the thickness of the functional layer on the EFFC performance are studied. Impact.
1.1 Synthesis of LNCA
The reaction initials Ni(OH)2, LiOH·H2O, Co3O4, Al2O3 are weighed and mixed in a stoichiometric ratio, and then subjected to secondary high temperature solidification. Phase synthesis of LiNi0.8Co0.15Al0.05O2-δ. The first reaction temperature was 520 °C, and the holding time was 5 h. After the natural product was naturally cooled, it was ground uniformly for the second time, and kept at 810 °C for 5 h. The right amount of air is introduced. The product of the second high temperature reaction is naturally cooled and then ground to obtain the final product.
1.2 Fuel battery production
The prepared LNCA and terpineol are mixed to the appropriate viscosity and brushed. Apply to the surface of the foamed nickel and wait for it to dry After that, it is used as the electrode of the EFFC battery. Then, LNCA and SDC are mixed uniformly in different proportions as the functional layer of the battery. The above substances are pressed into a 13 mm diameter disc by a powder tableting machine in the order of the electrode-functional layer-electrode. The pressure is 8-10 MPa, and the effective area of ??the battery is 0.64 cm2.
1.3 Material and battery test
The phase structure analysis of the material uses Bruker AXS D8 advanced X-ray diffractometer (XRD), Cu target, scanning speed 6o/min, scanning range 10°～90°. The surface morphology of the material is analyzed by JSM7100F field emission scanning electron microscope (FESEM). The fuel battery performance test uses IT8500 as the electronic load, the test temperature is 550 °C, and the dry H2 is used as fuel. Air as oxygen source.
2 Results and discussion
2.1 Material characterization
Figure 1 shows the XRD spectrum of the synthesized LNCA, which shows that it has a layered structure of α-NaFeO2.
(a) and (b) are SEM topographies of the synthesized LNCA. As can be seen from Fig. 2(a), the synthesized LNCA is spherical and has a diameter of 1 to several tens of micrometers. The surface of the spherical particles consists of many small particles. Composition, can provide The specific surface area is favorable for the occurrence of catalytic reaction. Figure 2 (c) shows that the SDC used is composed of some nano-sized particles. Figure 2 (d) shows the composite material with mass fraction of 30% LNCA/70% SDC. As a functional cross-section of the device after fabrication, it can be seen from Figure 2(d) that the spherical LNCA is evenly distributed between the SDCs. The SOFC device must be airtight, and the conventional SOFC requires a dense electrolyte layer to avoid fuel. Short circuit inside the device with oxidant. There is no dense electrolyte layer in the EFFC device. The functional layer pressed in the figure shows denser, no obvious gap, and the pores are in the nanometer size, which is much smaller than the flameout distance (about 1 mm). .
Therefore, even if hydrogen and oxygen meet in the microchannels of the nanostructures, there is no possibility of explosion. At the same time, the thickness of the device also ensures that gas diffusion exists only in the thinner surface of the battery. Br\u003e 2.2 Fuel battery performance and its influencing factors
For EFFC devices, the main functional part is MIEC composite material. The thickness of the composite material not only affects the molding process during the pressing process of the device, but also Will affect the internal resistance of the device, thus affecting the output performance of the device. Therefore, the effect of the thickness of the functional layer on the device performance is first studied. Since the same pressure is used in the pressing process, the thickness of the functional layer is related to the quality of the powder used. The thickness of 0.35 g powder after pressing is about 0.7 mm. LNCA and SDC are mixed in a ratio of 30% and 70% by mass, and EFFC is constructed with 0.35, 0.5 and 0.7 g as functional layers respectively.
Output of battery performance The curve is shown in Fig. 3. As can be seen from Fig. 3, when the mass of the intermediate layer is 0.5 g, the maximum output power of the battery is slightly larger than 0.35 g; when the functional layer mass is 0.7 g, the battery output is the lowest, and the open circuit voltage is the highest. The open circuit voltages of .0.35 g and 0.5 g are almost the same, and are kept above 1.0 V. Note that the use of LNCA-SDC material with electron-ion mixed conductivity as the functional layer (intermediate layer) of the device does not cause short circuit of the battery. At the same time, it can be seen from the polarization curve that the 0.7 g functional layer battery has the largest polarization resistance (about 0.53 Ωcm 2 ), and the 0.5 g battery has the lowest polarization resistance (about 0.29 Ω cm 2 ).
Next, the effects of different LNCA and LNCA mass fractions on device performance were investigated, which were 10% LNCA/90% LNCA, 30% LNCA/70% LNCA, 40% LNCA/60% LNCA, and 60% LNCA/40% LNCA, respectively. The performance output curve of the battery is shown in Figure 4. From the figure we can see that the output performance of the battery varies greatly with the composition, and 30% of the LNCA/70% SDC batteries have the best performance output, which is more than 1.2. The open circuit voltage of V and the maximum power output of about 900 mW cm-2. The difference in battery performance is derived from the difference in the proportion of electron-ion conductive materials in the functional layer. With the increase of the electronic conductive material LNCA, the electronic conductivity of the composite material Gradually enhanced. Previous studies  showed that in two-phase composites, when the electronic conductivity and ionic conductivity of the material are balanced, the battery can obtain the maximum performance output. When LNCA accounts for 10% and 40% of the mass fraction. When the open circuit voltage of the battery is reduced, but the battery performance does not change much. When the proportion of LNCA exceeds SDC, that is, the proportion of electronic conductive material is higher than that of the ion conductor, the open circuit voltage of the battery drops to 0.9 V. This is because Internal electronic conductivity of the material Dominant, the electrons generated by the anode catalysis easily move to the cathode through the battery, causing a short circuit.
For fuel batterys, the operating temperature is the key to affecting the internal ionic conductivity of the device. For traditional yttria-stabilized zirconia (YSZ) electrolytes It is necessary to reach 1 000 °C to obtain higher conductivity. In the EFFC device, the electron-ion mixed conductive functional material can obtain higher conductivity at low temperature due to the two-phase compounding. At the same time, it is better. The battery output performance, the anode and cathode must also have high catalytic activity, which is also closely related to the operating temperature.
Figure 5 shows the performance of the battery at different operating temperatures.
It is found that the battery has 550 °C Highest performance output. Battery performance is attenuated with decreasing temperature. However, even at 450 °C, the open circuit voltage of the battery remains above 1 V and the maximum power density exceeds 400 mW cm-2. It still has high catalytic activity and ionic conductivity. This means that the fuel battery has the ability to operate at low operating temperatures.
3 Conclusions\u003cbr \u003e (1) LNCA material was synthesized by solid phase method, which has α-NaFeO2 layered structure.
(2) LNCA was mixed with SDC with ion conductivity to obtain mixed electron-ion conductive material, and EFFC is constructed as a functional layer. The functional layer simultaneously functions as electrode catalysis and ion conduction.
(3) The thickness of the functional layer and the proportion of electron-ion conductive material in the composite material are affected to affect the performance of the battery. Output. When the mass fraction is 30% LNCA/70% SDC and the functional layer mass is 0.5 g, the battery achieves a maximum power output of 937 mWcm-2 at 550 °C. Moreover, the battery maintains a high open circuit voltage at 450 °C and Power output, indicating its potential to work at low temperatures.
 PERGOLESI D, FABBRI E, D'EPIFANIO A, et al. High proton conduction in grain-boundary-free yttrium- Doped barium zirconate films grown by pulsed laser deposition [J]. Nat Mater, 2010, 9(10): 846-852.
LIM M, DING D, BAI Y, et al. An efficient SOFC based on Samaria-doped ceria SDC) electrolyte [J]. J Electrochem Soc, 2012, 159(6): B661-B665.
WANG X, MA Y, LI S, et al. Ceria-based nanocomposite with simultaneous proton and oxygen ion Conductive for low-temperature solid oxide fuel batterys [J]. J Power Sources, 2011, 196(5): 2754-2758.
XIA C, LI Y, TIAN Y, et al. A high performance composite Ionic conducting electrolyte for intermediate temperature fuel battery and evidence for ternary ionic conduction [J]. J Power Sources, 2009, 188(1): 156-162.
ZHU B, RAZA R, ABBAS G, et al An electrolyte-free fuel battery constructed from one homogenous layer with mixed conductivity [J]. Adv Funct Mater, 2011, 21(13): 2465-2469.
ZHU B, RAZA R, QIN H, et Al. Fuel batterys based on electrolyte and non-electrolyte separators [J]. Energ Environ Sci, 2011, 4(8): 2986-2992.
ZHAO Y, HE Y, FAN L, et al. Synthesis Of hierarchically porous LiNiCuZn -oxide and its electrochemical performance for low-temperature fuel batterys [J]. Int J Hydrogen Energy, 2014, 39(23): 12317-12322.
LIU Y, TANG Y, MA Z, et al. Flowerlike CeO(2) microspheres coated with Sr(2)Fe(1.5)Mo(0.5)O(x) nanoparticles for an advanced fuel battery [J]. Sci Rep, 2015, 5: 11946.
DONG W, YAQUB A, JANJUA NK, et al. All in One Multifunctional Perovskite Material for Next Generation SOFC [J]. Electrochim Acta, 2016, 193: 225-230.
ZHU B, HUANG Y, FAN L , et al. Novel fuel battery with nanocomposite functional layer designed by perovskite solar battery principle [J]. Nano Energy, 2016, 19: 156-164.
ZHU B, FAN L, HE Y, et al. A commercial lithium battery LiMn-oxide for fuel battery applications [J]. Materials Letters, 2014, 126: 85-88.
ZHU B, LUND PD, RAZA R, et al. Schottky Junction Effect on High Performance Fuel Cells Based on Nanocomposit e Materials [J]. Adv Energy Mater, 2015, 5(8): 1401895.
ZHU B, FAN L, DENG H, et al. LiNiFe-based layered structure oxide and composite for advanced single layer fuel Cells [J]. J Power Sources, 2016, 316: 37-43.
FAN L, SU PC. Layer-structured LiNi0.8Co0.2O2: A new triple (H+/O2?/e?) Conducting cathode for low temperature proton conducting solid oxide fuel batterys [J]. J Power Sources, 2016, 306:369-377.
LAN R, TAO S. Novel Proton Conductors in the Layered Oxide Material LixAl0.5Co0 .5O2 [J]. Adv Energy Mater, 2014, 4(7): 1301683.
 Zhang Huangqiu An, Zhu Bin. Fuel Cell Challenges and New Opportunities [J]. Sustainable Energy, 2012, 2 (4): 89-96.
 WILK P, MARZEC J, MOLENDA J. Structural and electrical properties of LiNi1?yCoyO2 [J]. Solid State Ion, 2003, 157(1–4): 109- 114.
 DONG X, TIAN L, LI J, et al. Single layer fuel battery based on a composite o f Ce0.8Sm0.2O2?δ–Na2CO3 and a mixed ionic and electronic conductor Sr2Fe1.5Mo0.5O6?δ [J]. J Power Sources, 2014, 249(0): 270-276.