Bi-functional Electrocatalyst Of LaNiO₃Perovskite Oxide And Its Application

In this paper, we selected perovskit oxide LaNiO3as the target. LaNiO3electrocatalyst was synthesized by a sol-gel method using citric acid as complex agentand ethylene glycol as thickening agent. At the same time, LaNiO3with the partialsubstitution at B sites with Mg or Fe cation and graphene oxide-coated LaNiO3weresynthesized to enhance the electrocatalytic performances. The crystalline structure,morphology and surface composition for each as-prepared material were characterizedby XRD, FESEM and XPS techniques. The intrinsic catalytic properties of thesematerials toward the oxygen reduction reaction (ORR) and oxygen evolution reaction(OER) were studied in0.1M alkaline potassium hydroxide solution using the rotatingdisk electrode technique. Also Li-air primary batteries on the basis of Mg-dopedperovskite oxides LaNi1-xMgxO3(x=0,0.08,0.15) and nonaqueous electrolyte werefabricated and tested.The ORR and OER polarization curves revealed that the bifunctional catalyticperformances were improved in the order of XC-72Carbon, LaNiO3, LaNi0.98Mg0.02O3and LaNi0.85Mg0.15O3. The ORR overall electron transfer numbers were also increasedin the order of XC-72, LaNiO3, LaNi0.92Mg0.08O3and LaNi0.85Mg0.15O3nanoparticles,which could be drawn from the Koutecky-Levich plots. And the overall ORR electrontransfer number of LaNi0.85Mg0.15O3was comparable to the theoretical value (4) inalkaline solution. The discharge capacity densities of nonaqueous Li-air primary batteryusing LaNiO3, LaNi0.95Mg0.05O3and LaNi0.85Mg0.15O3as the cathode catalyst were,respectively,300mAh/g,490mAh/g and620mAh/g, which were consistent with thebifunctional catalytic performances in alkaline solution. The results of XPS indicted anincrease of absorbed hydroxyl on the surface of the catalyst, a stronger covalency ofB-O2bond, and a higher Ni3+/Ni2+ratio with larger Mg-doping. It was notable that thelatter two peaks become stronger. It should be noted that Ni4+oxide at approximately859eV was found after Mg-doping, and the peak of LaNi0.85Mg0.15O3was stronger thanthat of LaNi0.95Mg0.05O3and LaNi0.92Mg0.08O3. The improved ORR and OER catalyticperformances were related to the existence of Ni4+ion and the stronger B-O2bond forthe Mg-doped perovskite oxides.For LaNi1-yFeyO3(y=0.1,0.2,0.3,0.4,0.5,0.6,1.0) perovskite oxides, LaFe0.8Fe0.2O3performanced the best bifunctional catalytic performance, the ORR onset potential, limiting current density and overall electron transfer number of which were-0.077V,4.3mA/cm2and4, respectively. LaNi0.8Fe0.2O3showed the highest OER limitimgcurrent density (65mA/cm2) and the most negative OER onset potential (0.35V).LaNi0.6Fe0.4O3had the similary OER limitimg current density with LaFe0.8Fe0.2O3, butwith a more positive OER onset potential. From XPS spectrum, we found thatperovskite oxides had a higher Ni3+/Ni2+ratio with a minor replacement of Ni cationwith Fe cation. However, when the doping content was too high (y>0.2), the Ni3+/Ni2+ratio decreased with increasing the doping content. For LaNi1-yFeyO3(x=0,0.1,0.2,0.6,1.0) catalysts, increasing the doping content could induce an increase of absorbedhydroxyl on the surface of the catalyst, a stronger covalency of B-O2bond, and a higherNi3+/Ni2+ratio.The results of XRD and FESEM revealed that NR-GO-coated LaNiO3had the samestructure with LaNiO3and a better conductive network than LaNiO3. At the same time,NR-GO could constrain the growth of LaNiO3particles.15wt%NR-GO-coatedLaNiO3performanced the best bifunctional catalytic activity, indicating graphenecoating was a practical method to improve the catalytic property of LaNiO3.