Direct internal reforming solid oxide fuel cell (DIRSOFC) fueled by biogas has been investigating with the motivation of downsizing and cost reduction of SOFC systems. While chemical energy of biogas composed of CH₄ and CO₂ can be directly converted to electricity without an external fuel reformer, this direct conversion can cause carbon deposition within Ni-stabilized zirconia cermet anode, hence SOFC performance degrades rapidly [1,2]. To solve this issue, paper-structured catalyst (PSC), flexible catalyst made up of inorganic fibre (Cf) network in which metallic catalysts for fuel reforming such as Ni, Ru, etc. are dispersed, is now being studied [3,4].

In this work, three types of Ni-loaded PSCs were prepared via the simple paper making process [3,4]. Fig. 1 shows the SEM images of Ni-loaded PSCs, (a) without the dispersion of support oxide particles in the fibre network (Ni-PSC), (b) with the dispersion of (Mg,Al)O derived from hydrotalcite (Ni/(Mg,Al)O-PSC) and (c) with the dispersion of (Ce,Zr)O₂ (Ni/(Ce,Zr)O₂-PSC).

In the case of Ni-PSC, because Ni particles are loaded on the smooth surface of amorphous Cf, Ni coarsening was significantly promoted resulting in Ni particle size of 200-500 nm. On the other hand, when the oxide particles are dispersed in the fibre network, fine Ni particles < 10 nm were formed on the oxide particles to have higher contact area with gaseous reactants and prevent Ni particles from agglomeration at high operation temperature. The results of the tests of CH₄ dry reforming (CH₄ + CO₂ → 2H₂ + 2CO) carried out at 750 ^oC with GHSV of 3500 h^-1 are shown in Fig. 2. Compared to Ni-PSC and Ni/(Mg,Al)O-PSC, Ni/(Ce,Zr)O₂-PSC exhibited higher CH₄ conversion close to 90 %. Moreover after the 15 h test, carbon deposition was not observed inside of the Ni/(Ce,Zr)O₂-PSC, whereas formation of whisker carbon was noticeable for the Ni/(Mg,Al)O-PSC. This indicates that lattice oxygen in the (Ce,Zr)O₂ contributed to oxidization of carbonaceous species deposited on the catalyst surface to regenerate active sites for dry reforming reaction. Now, the performances of SOFC with and without PSCs are being evaluated in the direct feed of simulated biogas mixture at 750 ^oC.

Keywords: Paper-structured catalyst, ceria, internal reforming, biogas, dry reforming.

Reference:

[1] Wang S., Lu G. Effects of promoters on catalytic activity and carbon deposition of Ni/γ-Al₂O₃ catalyst in CO₂ reforming of CH₄. J. Chem. Technol. Biotechnol. (2000), 75, 589-595.

[2] Ke K., Gunji A., Mori H., Tsuchida S., Takahashi H., Ukai K et al. Effect of oxide on carbon deposition behaviour of CH₄fuel on Ni/ScSZ cermet anode in high temperature SOFCs. Solid state ionics (2006), 177, 541-541.

 [3] Shiratori Y., Sakamoto M. Performance improvement of direct internal reforming solid oxide fuel cell fuelled by H₂S-contaminated biogas with paper-structured catalyst technology. Journal of Power Sources (2016), 332, 170-179.

[4] Shiratori Y., Q. T. Tran, Sasaki. K. Performance enhancement of biodiesel fueled SOFC using paper-structured catalyst. Int. J. Hydrogen Energy (2013), 38 (23), 9856-9866.