A Robust One‐Compartment Fuel Cell with a Polynuclear Cyanide Complex as a Cathode for Utilizing H 2 O 2 as a Sustainable Fuel at Ambient Conditions

A robust one‐compartment H 2 O 2 fuel cell, which operates without membranes at room temperature, has been constructed by using a series of polynuclear cyanide complexes that contain Fe, Co, Mn, and Cr as cathodes, in sharp contrast to conventional H 2 and MeOH fuel cells, which require membranes and high temperatures. A high open‐circuit potential of 0.68 V was achieved by using Fe 3 [{Co III (CN) 6 } 2 ] on a carbon cloth as the cathode and a Ni mesh as the anode of a H 2 O 2 fuel cell by using an aqueous solution of H 2 O 2 (0.30  M , pH 3) with a maximum power density of 0.45 mW cm −2 . The open‐circuit potential and maximum power density of the H 2 O 2 fuel cell were further increased to 0.78 V and 1.2 mW cm −2 , respectively, by operation under these conditions at pH 1. No catalytic activity of Co 3 [{Co III (CN) 6 } 2 ] and Co 3 [{Fe III (CN) 6 } 2 ] towards H 2 O 2 reduction suggests that the N‐bound Fe ions are active species for H 2 O 2 reduction. H 2 O 2 fuel cells that used Fe 3 [{Mn III (CN) 6 } 2 ] and Fe 3 [{Cr III (CN) 6 } 2 ] as the cathode exhibited lower performance compared with that using Fe 3 [{Co III (CN) 6 } 2 ] as a cathode, because ligand isomerization of Fe 3 [{M III (CN) 6 } 2 ] into (FeM 2 )[{Fe II (CN) 6 } 2 ] (M=Cr or Mn) occurred to form inactive FeC bonds under ambient conditions, whereas no ligand isomerization of Fe 3 [{Co III (CN) 6 } 2 ] occurred under the same reaction conditions. The importance of stable Fe 2+ N bonds was further indicated by the high performance of the H 2 O 2 fuel cells with Fe 3 [{Ir III (CN) 6 } 2 ] and Fe 3 [{Rh III (CN) 6 } 2 ], which also contained stable Fe 2+ N bonds. The stable Fe 2+ N bonds in Fe 3 [{Co III (CN) 6 } 2 ], which lead to high activity for the electrocatalytic reduction of H 2 O 2 , allow Fe 3 [{Co III (CN) 6 } 2 ] to act as a superior cathode in one‐compartment H 2 O 2 fuel cells.