Electrocatalytic water oxidation using (bpy)₂Co-based precursors

Abstract

As an earth-abundant and inexpensive first-row transition metal, cobalt is a good candidate for the development of water oxidn. catalysts. The (bpy) ₂Co (bpy = 2,2'-bipyridine) mol. motif offers stable and functional electrocatalytic water oxidn., while substituents on the bipyridyl group offer a ready means for attachment to materials. Data are consistent with the Co(II) complex (bpy) ₂Co(NO3)₂ (1-H) serving as an active and robust catalyst in neutral aq. conditions with a catalytic potential (E_(cat)) of 1.3 V vs NHE at a scan rate of 20 mV s⁻¹ in 0.1 M phosphate buffer and an estd. turnover frequency of 40 s⁻¹. A series of (Ybpy)₂CoX₂ complexes (Ybpy = 4,4'-disubstituted bpy ligands with Y = NH₂ or NO₂; X = NO₃ or Cl) were synthesized and tested for electrocatalytic water oxidn. to det. the effect of electron-withdrawing and electrondonating substituents on electrocatalytic activity. Addnl., the diamino-substituted complex (4,4'-NH2bpy)₂Co(NO3)₂ (1-NH₂) serves as an avenue of approach for the functionalization of carbon-based material supports and the development of heterogeneous water oxidn. catalysts. Complex 1-NH₂ was utilized to append the (bpy)₂Co motif to carbon-materials derived from the hydrothermal carbonization (HTC) of monosaccharide starting materials via diazotization. Alternatively, addn. of 4,4'-divinyl-2,2'-bipyridine during the HTC synthesis provided a carbon material with pre-ligated coordination sites, which allowed for direct metalation with simple metal salts such as CoCl₂.bul.6H₂O or CoNO₃.bul.6H₂O.