ANL team demonstrates improved Li-O2 performance with iron-nitrogen-carbon composite cathode material
|Discharge/charge voltage profiles of Li−O2 cells using α- MnO2/XC-72 and Fe/N/C as cathode catalysts. Credit: ACS, Shui et al. Click to enlarge.|
A team at Argonne National Laboratory (ANL) has demonstrated improved performance of a rechargeable Li−O2 battery when an iron−nitrogen−carbon (Fe/N/C) composite is used as the cathode catalyst.
In side-by-side studies reported in the Journal of the American Chemical Society, they found that such a catalyst could reduce overpotentials during both discharge and charge processes when compared with the benchmark metal oxide catalyst, such as α-MnO2 or high-surface-area carbon. (High overpotentials in discharge and charge result in low efficiences, and are one of the known obstacles needing resolution for the commercialization of Li-air batteries.)
|Cycling performance of cells with catalysts Fe/N/C and carbon black (BP) as cathode catalysts. Current was 0.05 mA with duration of 5 h. Credit: ACS, Shui et al. Click to enlarge.|
Detailed studies also suggested that the chemically modified carbon selectively promoted the decomposition of lithium peroxide over that of the electrolyte. This improved selectivity led to an enhanced battery lifespan under controlled cycling (decomposition of the electrolyte harms lifespan), with 50 discharge−charge cycles achieved.
The study, concluded the team, emphasizes the importance and promise of developing efficient electrocatalysts for Li−O2 battery application, particularly those that can promote the oxygen evolution reaction through better mass and electronic transfers.
The rechargeable Li−air battery represents an attractive energy storage device for electric vehicle applications due to its high theoretical energy storage capacity. The use of an active cathode catalyst would reduce both discharging and charging overpotentials by facilitating the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charge, thereby increasing the overall energy storage efficiency.
...From a rational design point of view, an ideal cathode catalyst in the Li−O2 battery should have highly active catalytic centers densely populated over the support surface, with minimum separation between individual sites, to achieve maximum interaction with the solid precipitate, such as Li2O2. The active sites should also be easily accessible to the electrons necessary to complete the electrochemical reactions. One such material is the transition metal−nitrogen−carbon composite prepared by thermolysis of transition metals (Fe, Co, etc.) ligated by nitrogen-containing organic compounds over high-surface-area carbon support.
For example, iron−nitrogen−carbon (Fe/N/C) catalysts have been synthesized and studied extensively as low-cost alternatives to Pt for ORR in both acidic and alkaline fuel cells. Significant improvements in performance and durability have been reported recently for the Fe/N/C material, rendering it a benchmark for the nonprecious metal catalysts in fuel cell application. Nonetheless, the nature of the active site and catalytic mechanism involved remain to be fully understood. One representative Fe/N/C catalyst is the material prepared by pyrolysis of supported iron(II) acetate and 1,10-phenanthroline that has demonstrated excellent activity toward ORR in the aqueous phase. Since it is low-cost and easy to make, it is particularly attractive if such a catalyst can be used in promoting cathodic reactions in the Li−O2 battery.—Shui et al.
The team synthesized an atomically dispersed Fe/N/C composite to study its role in controlling the ORR during Li–O2 battery charging with the use of a tetra(ethylene glycol) dimethyl ether-based electrolyte.
In addition to finding that Li–O2 cells using Fe/N/C as the cathode catalyst showed lower overpotentials than α-MnO2/carbon catalyst and carbon-only material, they found that gases evolved during the charge step contained only oxygen for Fe/N/C cathode catalyst, whereas CO2 was also detected in the case of α-MnO2/C or carbon-only material—presumably generated from electrolyte decomposition.
Jiang-Lan Shui, Naba K. Karan, Mahalingam Balasubramanian, Shu-You Li, and Di-Jia Liu (2012) Fe/N/C Composite in Li–O2 Battery: Studies of Catalytic Structure and Activity toward Oxygen Evolution Reaction. Journal of the American Chemical Society doi: 10.1021/ja3042993