Characterization of NiO–Al₂O₃ composite and its conductivity in biogas for solid oxide fuel cell

Metal nitrates were used as an oxidizer and glycine as a fuel for the combustion synthesis. Pure chemicals, Ni(NO₃)₂·6H₂O, Al (NO₃)₃·9H₂O, and C₂H₅NO₂ (glycine) of analytical grade, all from Alfa Aesar were used. The stoichiometric composition of the redox mixture was calculated based on the principles of propellant chemistry, keeping the O/F ratio around unity [20]. The chemicals in required amounts were dissolved in double distilled water. The intimate solution of metal nitrate and glycine was obtained using a magnetic stirrer with heating to evaporate the excess water content in the solution. After formation of gel, it was kept in preheated furnace (600 °C) where it was auto-ignited with the rapid evolution of large volume of gases to produce fine powder. Assuming complete combustion, the chemical reactions at optimum oxidant to fuel ratio are as given below:

and

Within a very short duration, the local reaction temperature rose from 1200 °C to 1500 °C as it was confirmed by an infrared pyrometer. The adiabatic flame temperatures were calculated using equation 4 [21] and the thermodynamic data have been collected in Table 1:

The calculated values of the adiabatic flame temperatures are 978 °C for NiO and 1323 °C for α-Al₂O₃, respectively.

The as-obtained NiO and Al₂O₃ powders were heat treated at 600 and 1200 °C for 2 h to remove any type of carboneous impurity and hence, pure and well crystalline powders were obtained. The heat treated powders (30 % NiO and 70 % α-Al₂O₃) were further physically mixed and pelletized in circular shapes. The pellets were sintered at different temperatures.

Structural studies of the samples were carried out by XRD (Phillips-3710 powder X-ray diffractometer) in the 2θ range of 10 to 90° using CuKα₁ radiation (λ = 1.54056 Å). The morphology of the powder was studied using scanning electron microscopy (SEM, JEOL JSM-6360). The particle size of a sample prepared at stoichiometric ratio was studied with a transmission electron microscope (TEM, PHILIPS CM200). The DC conductivity of sintered pellets was measured in air, hydrogen and biogas atmospheres in the temperature range from room temperature to 600 °C. The data were collected during cooling to ensure temperature stabilization and to avoid any change due to in-situ reduction of surface during heating. The NiO–Al₂O₃ was screen printed on both sides of pre-sintered NiO-GDC and its conductivity was evaluated as anode functional layer.

A complete cell was formed by using NiO-GDC, GDC and LSCF-GDC as anode, electrolyte and cathode, respectively and NiO–Al₂O₃ as anode functional layer. The open circuit voltages (OCV) of the cells were also measured in biogas.

The X-ray diffraction patterns of as-synthesized and calcined powders of NiO and α-Al₂O₃ nanoparticles are shown in Fig. 1 and Fig. 2, respectively. Fig. 1a depicts the XRD pattern of as-obtained NiO, which shows peaks arising from NiO and also nickel nitrate, indicating that combustion is incomplete. The combustion of this residual occurs during subsequent calcination at 600 °C for 2 h and phase pure cubic NiO is obtained (Fig. 1b). The calcined powder has crystallites of 31 nm and lattice parameter of 4.17 Å.

Fig. 1

Fig. 2

On the other hand, the as-obtained Al₂O₃ is amorphous (Fig. 2a). It needs high calcination temperature to obtain crystalline α-Al₂O₃ particles. Therefore, the as-obtained powder was calcined at 1200 °C for 2 h. Its XRD pattern (Fig. 2b) shows broad peaks indicating presence of small size particles (~34 nm). The α-Al₂O₃ nanoparticles have crystallized into the rhombohedral structure with lattice parameters; a = b = 4.76 Å and c = 12.76 Å. The elemental analysis of the calcined powders also showed the peaks due to either Ni and oxygen or Al and oxygen. No impurities were detected. The analysis data are tabulated in Table 2.

The morphology of the as-obtained (Fig. 3a) and calcined (Fig. 3b) NiO powder shows almost the same features and is foam-like with highly porous structure. The porosity has little decreased after calcination. The insets in the figures reveal roughly spherical grains. The TEM image of NiO nanoparticles (Fig. 3c) shows the nearly spherical particles, 30 nm in size. The SEM images of the as-obtained and calcined Al₂O₃ nanoparticles are shown in Fig. 4a and 4b. The amorphous-like structure without well-defined particle shapes is observable from SEM image of as-obtained Al₂O₃ (Fig. 4a). The amorphous powder is constituted of micrograins with a large size distribution. After calcination, the grain growth occurs and highly silky soft powder is seen in the SEM image (Fig. 4b). The higher magnified image of α-Al₂O₃ is shown in the inset of Fig. 4b. It reveals the platelet morphology of α-Al₂O₃. The TEM of Al₂O₃ (Fig. 4c) shows a mixture of different kinds of particles: platelets, spherical and needles but most crystallites exhibit hexagonal platelet morphology as shown in the inset of Fig. 4c. The average size of the platelet is 70 nm.

Fig. 3

Fig. 4

The green pellets of NiO–Al₂O₃ were finally sintered at 900 °C and 1200 °C for 2 h and their structural evaluation is shown in Fig. 5a and Fig. 5b. It is observed that the peaks from cubic NiO and rhombohedral α-Al₂O₃ overlap appreciably. The diffraction peaks in 2θ range of 60° to 70° show presence of peaks at 63.21° and 67.08° corresponding to NiO and Al₂O₃, which undoubtedly evidence the absence of NiAl₂O₄. Thus, no intermediate phases are formed in the composite sintered at 900 °C, which suggests no solid solution among α-Al₂O₃ and NiO. However, higher sintering temperature (1200 °C) results in intermediate NiAl₂O₄ spinel phase as shown in Fig. 5b. The sharp intense peaks indicate that the material is in crystalline form, but the crystallite sizes have not improved considerably. They are in the range of 33 nm to 35 nm for NiO and 37 nm to 40 nm for α-Al₂O₃.

Fig. 5

The presence of NiAl₂O₄ in the composite sintered at 1200 °C for 2 h is calculated by using following formula:

It is seen that only about 3 % spinel is present in the NiO–Al₂O₃ composite. The lattice parameters of α-Al₂O₃ and NiO phases are collected in Table 3.

On the contrary, Wang et al. [22] observed that Ni loading up to 20 wt.% was soluble in Al₂O₃ forming spinel NiAl₂O₄, and NiO loading above 20 % formed Ni–Al₂O₃. This shows that NiO species reacted with the Al₂O₃ to form a spinel NiAl₂O₄ phase. Moreover, Hao et al. [23] reported that the NiO phase started to appear in Ni–Al₂O₃ prepared by an aerogel method at nickel contents exceeding about 20 wt.%. These results indicate that the degree of reaction between nickel and the Al₂O₃ in the Ni–Al₂O₃ catalysts is dependent on the preparation method.

The EDS analysis of NiO–Al₂O₃ nanocomposite shows that the atomic percent of Ni in the pellets sintered at 900 °C and 1200 °C is 22.99 and 19.51, respectively, contrary to the theoretically expected value of 17.65. This demonstrates slight inhomogeneous distribution of Ni. Similarly, the atomic percent of Al in the pellets sintered at 900 °C and 1200 °C is 77 and 80.49, respectively, contrary to the theoretically expected value of 82.35.

SEM images of NiO–Al₂O₃ nanocomposites are shown in the Fig. 6. Fig. 6a reflects non uniform grain distribution with poor crystalline structure in the sample sintered at 900 °C. It exhibits platelet like structures and it is composed of Al₂O₃ as observed in Fig. 4b. After sintering at 1200 °C, the grain growth is observed and spherically shaped pores appear.

Fig. 6

The DC conductivity of NiO–Al₂O₃ was measured in the temperature range of 200 °C to 610 °C. The plots of DC conductivity (σ) versus reciprocal of temperature in air, hydrogen and biogas atmospheres are shown in the Fig. 7. The conductivity is observed to increase with temperature, suggesting thermally activated process. One can observe that the lowest conductivity values are found in air in the whole studied temperature range. Four times enhancement of the conductivity occurs in hydrogen. Further, the conductivity in air and biogas is almost the same up to 380 °C.

Fig. 7

In air, initiation of ionic conduction due to hopping of oxide ions is observed above 415 °C while electronic conductivity is negligible. In hydrogen, due to the catalytic activity of NiO, it gets oxidized to protons and electrons (equation 1). These electrons are effectively transported by electronic phase (NiO) in the composite. This gives high conductivity with the least activation energy of 0.23 eV. The catalytic reactivity is improved with a rise in temperature giving a considerable increase in conductivity. The slight change in the slope of conductivity is attributable to the change in defect mechanism.

On the contrary, in biogas, substantial increase in conductivity above 415 °C is observed. The biogas being a mixture of CH₄ and CO₂, undergoes dry reformation by NiO–Al₂O₃ according to reaction:

A little amount of air has also been introduced during the measurements giving partial oxidation of CH₄ according to reaction:

In addition, NiO–Al₂O₃ oxidizes the hydrogen obtained in this processes and plays a dual role. However, the comparatively less conductivity in biogas than in hydrogen and high activation energy of 0.67 eV signify that a less quantity of hydrogen is available. Nonetheless, propensity of CO₂ cracking may also be the reason of this lower conductivity. The conductivity and activation energy values in different atmospheres are tabulated in Table 4.

The FT-IR spectra of NiO–Al₂O₃ exposed in biogas at 400 °C, 500 °C and 600 °C for 2 h are shown in Fig. 8a. All the spectra show broad bands in the region 3000 to 3700 cm^–1 and a strong band at 2341 cm^–1. The bands near 3000 cm^–1 are assigned to C–H stretching vibrations (from methane) and those around 3500 cm^–1 are due to presence of moisture. The band at 2341 cm^–1 is due to asymmetric stretching vibration of C=O (CO₂). The intensity of this band is observed to decrease with the temperature. The band at 864 cm^–1 is observed to shift to 890 cm^–1 and then to 989 cm^–1 with an increase in temperature, shifting the M–O vibrations to higher wave number side.

Fig. 8

Moreover, the bands in the region 1300 to 1800 cm^–1 are observed for the anode exposed at 400 °C and 500 °C. These are due to CH₄ adsorbed on the surface. This band disappears at 600 °C and two distinct peaks, attributed to O–H vibrations and vibrations of adsorbed water molecule are observed at 1343 cm^–1 and 1543 cm^–1, respectively. The water molecules originate from the cracking of CO formed in reaction 6 and 7 by reaction:

If this is the case, carbon would be present in the sample but it has not been detected in XRD. This is because carbon is very sensitive to water in the atmosphere. Hence, it forms O–H functional groups showing a band at 1343 cm^–1. Thus, the analysis of FT-IR spectra clearly shows the presence of surface adsorbed CH₄ and CO₂ at 400 °C and 500 °C. Generally, biogas contains methane (50 % to 70 %), carbon dioxide (30 % to 40 %) and other compounds such as hydrogen sulfide (in ppm), water, and other trace gas compounds. The H₂S is easily soluble in water. The absorption process is purely physical. Therefore, sulfur is not deposited on the surface of the material and has not been detected by FT-IR spectroscopy.

The XRD patterns of the samples exposed to biogas at temperatures of 400 °C, 500 °C, and 600 °C are shown in Fig. 8b. It shows all the peaks corresponding to NiO and α-Al₂O₃. Further, no Ni peaks have been observed what means that reduction of NiO has not occurred.

The OCV of single cells NiOGDC/ GDC/LSCF-GDC and NiO–Al₂O₃–NiOGDC/ GDC/LSCF-GDC were measured in biogas at 500 °C. The OCV of a cell formed with NiO-GDC anode was 0.64 V while that of NiO–Al₂O₃–NiO-GDC anode was 1.11 V, close to the theoretical value of 1.18 V.