The invention concerns the use as a redox a catalyst and/or mediator in a fuel cell catholyte solution of the compound of Formula (I) wherein: X is selected from hydrogen and from various functional groups; R1-8 are independently selected from hydrogen and various functional groups; wherein R1 and X and/or R5 and X may together form an optionally substituted ring structure; wherein R1 and R2 and/or R2 and R3 and/or R3 and R4 and/or R4 and R8 and/or R8 and R7 and/or R7 and R6 and/or R6 and R5 may together form an optionally substituted ring structure; wherein (L) indicates the optional presence of a linking bond or group between the two neighbouring aromatic rings of the structure, and when present may form an optionally substituted ring structure with one or both of R4 and R8; and wherein at least one substituent group of the structure is a charge-modifying substituent.