… the practical energy density of aprotic Li−air batteries is limited by the deposition of the insoluble discharge products (Li₂O₂) clogging the air electrodes, the decomposition of organic electrolytes, and the need for excess packaging to keep the battery in a dry oxygen environment. To overcome these problems, a hybrid system has been developed, composed of a nonaqueous electrolyte in the anode side (oxidation of Li to Li⁺) and an aqueous electrolyte in the cathode side (reduction of oxygen in the presence of Li⁺), separated by a solid lithium-ion-conducting electrolyte. However, the cycle life of such hybrid systems is limited. The problem lies mainly in the sluggish kinetics of the oxygen reduction reaction (ORR) and poor durability of the cathode catalyst, which actually is also the major limiting factor for other energy conversion and storage technologies, such as fuel cells and metal−air batteries. Therefore, it is desirable to develop highly active and stable electrocatalysts for the ORR in aqueous media.

Pt and Pt-based alloys remain the most efficient ORR catalysts, but their high cost and scarcity have made them increasingly unattractive for such applications.… Recent studies have shown that supported Pd and Pd-based alloys exhibit ORR activity comparable to that of Pt/C in alkaline media. However, most reports have focused mainly on the disordered Pd-based alloy catalysts, which have varying surface composition and thus randomly distributed active sites. … Herein we report an ordered Pd₃Fe intermetallic catalyst that exhibits significantly enhanced activity and durability for the ORR relative to disordered Pd₃Fe/C, Pd/C, and Pt/C.

—Cui et al.

In contrast to disordered materials, in an ordered intermetallic phase, every crystallographic site is occupied by only one type of atom. An ordered phase can provide predictable control over structural, geometric, and electronic effects. Further, the researchers noted, as the order in intermetallic phases arises from the high enthalpy of mixing, a high chemical and structural stability can be expected. Ordered intermetallic catalysts also appear to possess different catalytic properties in comparison to their disordered alloys. The discovery of ordered Pt-based catalysts inspired the UT team to investigate Pd-based intermetallic catalysts.

Left. Crystal structure of ordered Pd3Fe. Right. Crystal structure of disordered Pd3Fe. Credit: ACS, Cui et al. Click to enlarge.

The UT team compared the catalytic performances of ordered Pd₃Fe/C and conventional Pt/C in a rechargeable hybrid Li−air battery. The polarization curve for ordered Pd₃Fe/C almost overlapped that for Pt/C, indicating that these two catalysts have similar activities for the ORR in hybrid Li−air cells.

The team calculated the maximum power densities to be 11.7 mW cm⁻² for ordered Pd₃Fe/C and 12.3 mW cm⁻² for Pt/C. For the battery with ordered Pd₃Fe/C as the ORR catalyst, the round-trip overpotential increased from 0.9 V at the first cycle to 0.92 V at the 55^th cycle and to 0.98 V at the 220^th cycle. In the case of the cell with Pt/C as ORR catalyst, the round-trip overpotential is 0.9 V initially, and it increases to 1.31 V at the 55^th cycle.

The small (2.2%) increase relative to the initial round-tip overpotential in the the battery with ordered Pd₃Fe/C compared to the much larger increase (45.5%) for the battery with Pt/C indicated that the ordered Pd₃Fe/C catalyst significantly enhances the cycling stability of hybrid Li−air batteries.

Resources

• Zhiming Cui, Longjun Li, Arumugam Manthiram, and John B. Goodenough (2015) “Enhanced Cycling Stability of Hybrid Li–Air Batteries Enabled by Ordered Pd3Fe Intermetallic Electrocatalyst” Journal of the American Chemical Society doi: 10.1021/jacs.5b03865