Synthesis and Characterization of La[sub 0.6]Ca[sub 0.4]Co[sub 0.8]Ru[sub 0.2]O[sub 3] for Oxygen Reduction Reaction in an Alkaline Electrolyte

Abstract

We employed an amorphous citrate precursor ͑ACP͒ method to synthesize stoichiometric La 0.6 Ca 0.4 Co 0.8 Ru 0.2 O 3 powders. Result from X-ray diffraction demonstrated a perovskite La 0.6 Ca 0.4 CoO 3 lattice, indicating the incorporation of Ru 3+ at the Co 3+ sites. Image from the scanning electron microscope revealed a foamlike microstructure with various pores at 0.5-3 m. La 0.6 Ca 0.4 Co 0.8 Ru 0.2 O 3 exhibited a higher H 2 O 2 decomposition rate in KOH solution as opposed to that of ACP-derived La 0.6 Ca 0.4 CoO 3 , which suggested an improved catalytic ability for oxygen reduction reaction ͑ORR͒. A similar behavior was observed during the ORR current-potential polarization using La 0.6 Ca 0.4 Co 0.8 Ru 0.2 O 3 supported on Black Pearl 2000 as a gas diffusion electrode. In addition, impedance spectra at the open-circuit voltage and selective cathodic overpotentials also confirmed a smaller charge-transfer resistance for La 0.6 Ca 0.4 Co 0.8 Ru 0.2 O 3 . In galvanostatic measurements and lifetime evaluation, La 0.6 Ca 0.4 Co 0.8 Ru 0.2 O 3 demonstrated steady voltage profiles with negligible degradation.Oxygen reduction reaction ͑ORR͒ is a critical step in fuel cell electrochemistry because a significant overpotential is often required to activate the relatively stable oxygen molecules. 1 Because conventional fuel cells involve an acidic electrolyte, noble metals and alloys such as Pt and Pt 3 Ni are selected for their chemical inertness. 2,3 However, to reduce the system cost, it is necessary to adopt less expensive materials with comparable performances. 4,5 The ORR in an alkaline electrolyte reveals less polarization loss, leading to a fuel cell with better efficiency. 6 Furthermore, without concerns over acids, less noble metals including Ag, Ni, and Co, as well as oxides of perovskite ͑ABO 3 ͒ and spinel ͑AB 2 O 4 ͒ can be employed as electrocatalysts. 7-11 Among them, lanthanum cobaltate ͑LaCoO 3 ͒ and its derivatives have attracted considerable attention for their simple synthesis and impressive performances. 12,13 In LaCoO 3 the catalytic ability for the ORR is attributed to Co 3+ . To date, a rich variety of transition-metal ions have been explored for partial replacements at the Co 3+ sites to further improve the ORR kinetic. 14-16 Recently, we synthesized La 0.6 Ca 0.4 CoIr 0.25 O 3.5−␦ and La 0.6 Ca 0.4 Co 0.8 Ir 0.2 O 3 and reported improved performances for both the ORR and oxygen evolution reaction over those of La 0.6 Ca 0.4 CoO 3 . 17,18 We realize that the successful incorporation of Ir 4+ at the Co 3+ sites is responsible for the catalytic enhancements. Because metallic Ru is known as an ORR electrocatalyst, 19,20 a similar strategy can be employed to introduce Ru 3+ at the Co 3+ sites for possible catalytic actions. The perovskite lattice is expected to provide a stable platform for hosting Ru 3+ . As a result, a reduced amount of Ru is used, as opposed to the metallic form.In an alkaline electrolyte, the ORR is understood to proceed in two distinct ways. 21 The straightforward route is ...

Perovskite materials in energy storage and conversion

“…This strategy of enhancing the catalytic activity of the noble metal was also reported by other groups (e.g. Ir or Ru‐doped La 0.6 Ca 0.4 CoO 3 and Ru‐doped CaMnO 3 ) …”

“…Owing to their low cost, high efficiency and flexible structures, perovskite oxides have recently drawn tremendous attention as promising alternatives to high‐efficiency OER electrocatalysis in alkaline media . Most notably, BSCF, which was formerly known as a high‐performance perovskite in oxygen permeation membranes and SOFCs, has been identified by Shao‐Horn and co‐workers as a highly active OER catalyst that even outperforms the gold‐standard IrO 2 in terms of intrinsic activity .…”

Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices

“…In the series of La 0.8 Sr 0.2 Co 1– y B′ y O 3 (B′ = Cu, Ni, Fe, and Cr), the authors found that La 0.8 Sr 0.2 Co 0.9 Ni 0.1 O 3 and La 0.8 Sr 0.2 Co 0.9 Fe 0.1 O 3 showed the highest electrocatalytic activity in 1 M KOH solution, whereas the partial substitution of Co with Cu or Cr exhibited detrimental effects on the electrocatalytic activity . Doping with precious metals can also lead to synergistic effects that improve electrocatalytic activities (e.g., the incorporation of Ir into La 0.6 Ca 0.4 CoO 3 to form La 0.6 Ca 0.4 Co 0.8 Ir 0.2 O 3 and Ru-doped La 0.6 Ca 0.4 CoO 3 with the composition of La 0.6 Ca 0.4 Co 0.8 Ru 0.2 O 3 ); however, the high cost of these elements has hindered this development.…”

Oxygen electrocatalysts in metal–air batteries: from aqueous to nonaqueous electrolytes

“…Further work led to catalyst materials of the general composition La 1Àx A x Co 1Ày B y O 3 (A = Ca; B = Mn, Fe, Co, Ni, Cu) synthesized by the different methods. [148][149][150][151][152][153][154][155][156] A strong composition dependence on catalyst performance was observed for both A and B-site doping. For example, Sr-substitution showed better performance at oxygen evolution, whereas, the Ca substituted material showed slightly better performance upon oxygen reduction.…”