Nickel / Doped Ceria Solid Oxide Fuel Cell Anodes for Dry Reforming of Methane

A http://dx.doi.org/10.5935/0103-5053.20140245 J. Braz. Chem. Soc., Vol. 25, No. 12, 2356-2363, 2014. Printed in Brazil - ©2014 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Article Nickel / Doped Ceria Solid Oxide Fuel Cell Anodes for Dry Reforming of Methane Renata O. da Fonseca,a,b Andressa A. A. da Silva,b,c Matheus R. M. Signorelli,a,b Raimundo C. Rabelo-Neto,b Fábio B. Noronha,*,b,c Rita C. C. Simõesa and Lisiane V. Mattosa Universidade Federal Fluminense, Departamento de Pós Graduação de Engenharia Química, Rua Passo da Pátria, 156, 24210-240 Niterói-RJ, Brazil a Instituto Nacional de Tecnologia, Divisão de Catálise e Processos Químicos, Av. Venezuela, 82, 20081-312 Rio de Janeiro-RJ, Brazil b Instituto Militar de Engenharia, Departamento de Pós Graduação de Química, Praça General Tibúrcio, 80 - Urca, 22290-270 Rio de Janeiro-RJ, Brazil c Neste trabalho, foi estudado o desempenho catalítico de materiais a base de Ni, suportado em cério dopado com Zr, Pr e Nb para serem usados como anodos em células a combustível de óxido sólido (SOFC) alimentadas com combustíveis contendo metano e CO2. Os anodos foram preparados pelo método hidrotérmico, usando um teor de Ni (14% em volume) menor do que o utilizado em anodos convencionais (30% em volume). Os materiais obtidos foram caracterizados através de análises de difração de raios X, redução à temperatura programada, espectroscopia Raman e termogravimetria. Os resultados mostraram que o material contendo Zr apresentou o menor tamanho de cristalito de níquel metálico, o que levou a um maior valor de atividade inicial na reação de reforma seca do metano a 1073 K. Entretanto, o catalisador Ni/CePr apresentou a menor quantidade de carbono formada. Isto foi atribuído à maior mobilidade de oxigênio do suporte CePr, o que promove o mecanismo de remoção do carbono. In this work, the catalytic performance of Ni supported on ceria doped with Zr, Pr and Nb used as anodes for solid oxide fuel cell (SOFC) operating directly on fuels containing methane and CO2 was studied. The anodes were prepared by a hydrothermal method using a Ni content (14 vol%) lower than that of a conventional SOFC anodes (30 vol%). The materials obtained were characterized by X-ray diffraction, temperature-programmed reduction, Raman spectroscopy and thermogravimetric analyses. The results showed that the sample containing Zr exhibited the lowest Ni crystallite size, which led to a high initial activity on dry reforming of methane at 1073 K. However, the Ni/CePr catalyst showed the lowest carbon formation. This was attributed to the higher oxygen mobility of CePr support that promotes the carbon removal mechanism. Keywords: nickel/doped ceria, SOFC anodes, CO2 reforming of methane Introduction The solid oxide fuel cells (SOFC) technology has been considered as an attractive energy conversion system. It has several advantages such as high efficiency, relatively low sensitivity to impurities, and possibility for operation with an internal reformer. 1,2 In the case of the direct internal reforming solid oxide fuel cell (DIR-SOFC), the complexity and costs of the fuel cell system are reduced, since the available fuels (hydrocarbons or alcohols) can be fed straight to the anode side of SOFC and reformed to *e-mail: H2 without the use of an external reformer. In addition, the conversion of these fuels at the anode side can be promoted due to the H2 consumption by the electrochemical reaction, leading to high conversions and high efficiency.3-5 Associated gas or biogas fueled SOFC system is a potential technology for electric power generation, since it can contribute to reduce CO2 emissions. The associated gas produced in the world’s largest deepwater field in Brazil contains a significant amount of carbon dioxide that has to be removed before its use. In the case of biogas or landfill gas (LFG), this gaseous mixture containing, mainly methane and carbon dioxide, is produced by anaerobic digestion or fermentation of organic matter Vol. 25, No. 12, 2014 Fonseca et al. such as sewage sludge, municipal solid waste (MSW), etc. Therefore, the emission of biogas to the atmosphere may contribute significantly to the greenhouse gas effect. Biogas fuelled SOFC can produce electricity with high efficiency (30‑40%) even in small size power generations (< 20 kW).6 When associated gas or biogas is fed straight to SOFC, H2 and CO are produced by the CO2 reforming of methane, the so called dry reforming of methane (DRM). The Ni/YSZ cermets are typically used as anode for SOFC systems.7-9 Ni provides electronic conductivity, while YSZ provides the ionic conductivity and thermal stability. However, in order to exceed the percolation threshold for electronic conductivity, a high Ni content (above 30 vol.%) is necessary. Since Ni is very active to reforming as well as cracking reactions, such a high volume fraction favors carbon formation when hydrocarbons such as methane are used directly as a fuel in SOFC. The carbon deposits on metal sites of anodes, resulting in the rapid degradation of the cell performance.7,8,10 It is well known that catalyst deactivation due to carbon deposition is one of the main issues of the dry reforming of methane.11,12 Therefore, the development of anodes for SOFC running on associated gas or biogas that exhibit high catalytic activity, high stability and adequate ionic and electronic conductivity at working conditions is still a challenge. Some strategies have been proposed for suppressing carbon deposition over SOFC anodes such as: (i) the addition of an oxidant to the feed; (ii) the decrease in Ni content and; (iii) the use of redox supports. The use of an excess of carbon dioxide in the DRM reaction could avoid carbon deposition on the anodes.13 However, the use of high CO2/CH4 ratios decreases the electrical efficiency of the SOFC by the dilution of fuel, the yield of hydrogen and the system efficiency.13 Decreasing the Ni content of the anode aims at controlling the ensemble size, while keeping the appropriate electronic conductivity. The nickel particle size significantly influences the nucleation rate of carbon.14 Therefore, controlling the number of atoms in an ensemble will most likely suppress the coke formation rate. In this sense, the anode preparation method may play a critical role in determining anode stability during DRM. Some authors15,16 reported the use of SOFC anodes with low Ni content (ca. 10-18 wt.%) prepared by alternative methods. Jasisnky et al.15 showed that the conductivity under H2 of anodes with 14 vol.% of Ni prepared by a net shape procedure was similar to the conductivity of the cermet with 35 vol.% of Ni synthesized by a conventional technique. On the other hand, Moddaferi et al.16 reported that Ni based anodes with low metal content (5 wt.% Ni and 5 wt.% Ru) prepared by the hydrothermal method did not exhibit carbon 2357 formation during oxidative steam reforming (OSR) of propane under SOFC oper (...truncated)