Environmentally sustainable facile synthesis of nanocrystalline holmium hafnate (Ho2Hf2O7): Promising new oxide-ion conducting solid electrolyte

Abstract

A 2 B 2 O 7 oxides with defect-fluorite structure are one of the potential candidates for solid oxide fuel cell electrolyte material due to their excessive thermodynamic stability in oxygen potential gradient at elevated temperature between 500 and 900 °C. Holmium hafnate nanoparticles have been synthesised through the Leeds Alginate Process (LAP) using inorganic salts of holmium and hafnium as starting materials immobilized in alginate beads. Ion exchange with sodium alginate and its subsequent thermal treatment have been used to prepare the nanopowder of Ho 2 Hf 2 O 7 . Thermal decomposition of dried beads is carried out at 700 °C for 2 h and 6 h to obtain the nanoparticles of Ho 2 Hf 2 O 7 . This calcination temperature was determined after carrying out simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/ DSC). High Temperature X-ray Diffraction (HT-XRD) was carried out to gain further insight into the thermal decomposition process in static ambient environment. HT-XRD analysis corroborated with the results obtained from TGA/DSC. Nanocrystalline powder of single phase Ho 2 Hf 2 O 7 has been obtained by calcination of oven dried ion-exchanged alginate beads in relatively low temperature range of 500-700 °C. Rietveld refinement of X-ray diffraction (XRD) data confirmed the formation of single phase defect fluorite structure of Ho 2 Hf 2 O 7 . The crystallographic parameters calculated from TEM and XRD analysis are in excellent agreement with each other. Furthermore, TEM-EDX analysis confirms that the Ho 2 Hf 2 O 7 synthesised by the facile alginate process is nearly stoichiometric. Raman spectroscopy gives evidence of the presence of oxide-ion vacancy in holmium hafnate which is supported with ac-impedance spectroscopy measurement at selected three temperatures. The present study suggests that the LAP has the capability of yielding on a large scale single phase defect-fluorite nanoparticles of electrolyte materials for solid oxide fuel cells in environmentally sustainable, economical and energy efficiently manner.

Facile Synthesis of Lanthanum Strontium Cobalt Ferrite (LSCF) Nanopowders Employing an Ion-Exchange Promoted Sol-Gel Process

“…This was validated by the hygroscopic tendency of the material and consequently possesses a tendency to absorb water from the surrounding humid air which could clarify the endothermic drop after the exothermic peak between 200-300 • C. Furthermore, Wang et al [7,11] stated that alginate in some measures offered some of its heat of combustion to the calcination process, hence, the alginate mediated ion-exchange process possibly generated net heat in the process and made this process more energy efficient. This has been shown independently by Sardar et al in their recent work for the synthesis of Ho 2 Zr 2 O 7 [1] and Ho 2 Hf 2 O 7 [12] by an identical process.…”

Microstructure and impedance spectroscopy of high density holmium hafnate (Ho2Hf2O7) from nanoparticulate compacts

“…Nanopowders of holmium hafnate (Ho2Hf2O7) [27] produced by Leeds Alginate Process (LAP), were compacted into pellets and sintered through single step sintering (SSS) and two step sintering (TSS) routes and the effect of processing conditions on the microstructure and ionic conductivity has been analysed. High density Ho2Hf2O7 ceramic is successfully obtained through the TSS process having lower grain growth.…”

“…Nanopowders of high purity single phase holmium hafnate synthesised using LAP has described in our earlier work [27] were pressed into pellets of approximately 13 mm diameter and 1.5 mm thickness in a steel die using a uniaxial press at 5 kN load. A number of pellets in the first set were processed using the conventional single step sintering (SSS) technique in the ambient environment from 1100 °C to 1500 °C with an interval of 100 °C for 2 h at each temperature with the heating and cooling rate of 3 °C/min.…”

“…This process is known to yield reproducibly and consistently homogeneous and pure single phase materials [24,25] with good control over stoichiometry of multicomponent metal oxide nanoparticles [26]. We have recently synthesised for the holmium hafnate (Ho2Hf2O7) nanoparticles, with cubic defect-fluorite structure, below 600°C through LAP and reported its structural and morphological properties [27]. We have also reported on the synthesis and characterisation of holmium zirconate (Ho2Zr2O7) [28] and on the influence of processing conditions on ionic conductivity [29].…”