Solid oxide fuel cell (SOFC) is an energy conversion device with high efficiency and flexibility of fuel such as H2, CmHn, CO due to its high operation temperature (700-1000°C). Generally the fuel electrode is composed of Ni/oxide cermet, but the electrochemical oxidation or carburization of Ni to NiO or NiCx cause degradation and failure of electron conductive path . Metal-free electrodes are also investigated but their low electron conductivity limit their power generation performances. On the other hand, carbon nanotubes (CNTs) are extensively investigated as a conductive material in various electrochemical devices due to their flexible and fine conductive networks. It is promising to solve the issues of anode degradation by applying CNTs in SOFC. However, there is no report of SOFC with CNTs because carbon materials has a risk to burn out under high temperature.
By focusing on the analogy of the conditions between the CNT growth and the SOFC power generation, we propose a new concept to introduce CNTs into the pores of porous anode of SOFC directly (Fig. 1). In SOFC, fuels are supplied into electrode with metal and oxide at 700-1000°C. In CNT growth, carbon source are supplied to metal nanoparticle on oxide support at 700-900°C . Therefore, we expect CNT can be directly fabricated by growing them inside the SOFC anode if appropriate metal nanoparticles are supported on oxide under hydrocarbon gas supply. By this method, we also expect CNT network keeps the conductivity even if the Ni conductive path failed and CNTs can also promote electrochemical reaction. This concept is available also for Ni-free electrodes. The CNT growth and power generation can be seamlessly switched by only changing gas without changing temperature. Additionally, even if CNTs burn out, they can be regrown repeatedly as desired and sustainable SOFC will be achieved. In this study, CNTs were grown directly inside SOFC anode and power generation characteristics of the electrodes with and without CNTs were investigated.
CNTs were grown on three different powder: Y2O3-stabilized ZrO2 (YSZ) and Gd doped CeO2 (GDC) as common O2- conductor and γ-Al2O3 as common substrate of CNT growth. These oxides are coated by 5×10-3 mol/g of metal precursor by dipping into FeNO3 (aq) or NiSO4 (aq) and drying in air. The samples were heated up to 800°C under H2 5%/Ar atmosphere to form Fe and Ni nanoparticles. Then CNTs were grown under the supply of C3H8 5%/H25%/Ar and at 800°C for 10 min to grow CNTs.
CNT growth on anode followed by power generation were carried out (Fig. 2). The anode with 3 mol% Fe coated Al2O3, 97 mol% GDC and the cathode with LSM/ScSZ were coated on an YSZ disk electrolyte (0.25 mm thickness, 20 mm diameter). 1%H2O-99%H2 gas supply to anode at 200 sccm for power generation without CNTs. Then the anode gas changed to C3H8 5%/H2 5%/Ar for 1min to grow CNTs and then to 1%H2O-99%H2again for power generation with CNTs. The operation temperature was 900°C and pure oxygen was supplied to cathode at 60 sccm.
RESULT AND DISCUSSION
1. CNT growth on oxide powder
CNT growth could be observed by SEM on all six samples and large amount of thin CNTs with 10-20 nm in diameter were grown on Fe/Al2O3 and thick CNTs with 100-200 nm grown on Ni/GDC regardless of CNT growth conditions such as C3H8 pressure, metal amount and temperature. In SOFC electrodes, thick CNT cannot make fine conductive network and they will cause a damage to anode structure, so we applied the CNT growth in SOFC on Fe/Al2O3mixed GDC.
2. Power generation characteristic of SOFC with CNT growth anodes
Maximum power density before CNT growth was 173 mW/cm2 at 900°C in H2 fuel. After CNT growth, it increased to 232 mW/cm2. Clear positive effect of CNTs was obtained by a power generation test on the same anode. CNT growth with ~10 nm in diameter and uniform distribution inside pore of the anode was obtained after the supply of carbon source gas. The effect of CNT amount on the power density of Ni-free GDC anode and the repeated growth of CNTs will be also discussed.
We proposed a new concept to apply CNT directly on SOFC anode. CNTs were successfully grown directly inside the Ni-free GDC electrode and the improve of maximum power density more than 30% was achieved.
 H. Shimada, F. Ohba, X. Li, A. Hagiwara and M. Ihara, Journal of Electrochemical Society, 159(7), 2012, F360-367.