wileyonlinelibrary.comelectrical power each year, energy conversion effi ciency is, therefore, a key factor in reducing the environmental impacts and utilizing the valuable natural resources responsibly. In comparison with the conventional electricity generation methods, fuel cells are perhaps among the most promising alternatives, showing excellent environmental-friendliness, high efficiency and reasonable energy density. [1][2][3][4] Practically, in each type of fuel cells, the rate-determining step is always believed to be the oxygen reduction reaction (ORR), due to the lack of the affordable yet high-performance electrocatalyst. [5][6][7][8] For instance, the high cathodic overpotential loss is a major factor in undermining the performances of the state-of-the-art solid oxide fuel cells (SOFCs) at temperatures between 500 and 700 °C. [ 7 ] Similarly, in the researches on proton exchange membrane fuel cells (PEMFCs), desperate efforts are devoted to the development of highly active ORR catalysts to replace the stateof-the-art carbon supported platinum. [ 8 ] Indeed, the noble metal contained materials, including Pt and its alloys, [ 8 ] were proven to be superior ORR catalysts. However, the problem is the very limited precious metals resources on this planet for large scale commercialization of the electrochemical devices referred above, not to mention their high prices and the ease of suffering degradations due to poisoning and/or leaching of metals. Alternatively, non-precious metals and compounds [ 9,10 ] as well as metal-free materials, [11][12][13] e.g., organometallic compounds and heteroatom-doped carbon materials, were developed that showed good ORR performances. In spite of that, the gradual deactivations of these catalysts during the stability test were commonly observed, and the complicated synthesis protocols posted another challenge regarding the practical application. Recently, studies on perovskite oxides, [4][5][6][7][14][15][16][17][18][19][20][21][22][23][24] such as La 0.5 Sr 0.5 CoO 2.91 (LSC), [ 16 ] Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF), [ 17,18 ] PrBa 0.5 Sr 0.5 Co 2-x Fe x O 5+δ (PBSCF), [ 19 ] and Sr 0.95 Ag 0.05 Nb 0.1 Co 0.9 O 3-δ , [ 20 ] offered a new dimension in tackling these challenges. Specifi cally, the double perovskite catalyst with a general formula of AA′B 2 O 5+δ demonstrated much higher oxygen ion diffusion rate and surfaceexchange coeffi cient relative to the ABO 3 -type, contributing to a faster ORR. [ 4,7,[19][20][21][22][23][24][25][26][27][28][29] The effi ciencies of a number of electrochemical devices (e.g., fuel cells and metal-air batteries) are mainly governed by the kinetics of the oxygen reduction reaction (ORR). Among all the good ORR catalysts, the partially substituted double perovskite oxide (AA′B 2 O 5+δ ) has the unique layered structure, providing a great fl exibility regarding the optimization of its electronic structures and physicochemical properties. Here, it is demonstrated that the double perovskite oxide, i.e., NdBa 0.75 Ca 0.25 Co 1.5 Fe 0....
