Testing mixed metal bimetallic, and monometallic, cryptates for electrocatalytic hydrogen evolution

Abstract

Appropriately designed catalysts help to minimise the energy required to convert the energy-poor feedstock H₂O into energy-rich molecular H₂. Herein, two families of pyridazine-based cryptates, mononuclear [M^IILⁱ](BF₄)₂ and mixed metal dinuclear [M^IICu^ILⁱ](BF₄)₃ (M = Fe, Co, Cu or Zn; Lⁱ is the Schiff base cryptand made by 2 : 3 condensation of tris(2-aminoethyl)amine and 3,6-diformylpyridazine), are investigated as potential electrocatalysts for the hydrogen evolution reaction (HER) in MeCN with acetic acid as the proton source. The synthesis and structures of a new mixed metal cryptate, [Zn^IICu^ILⁱ](BF₄)₃, and the tetrafluoroborate analogue of the previously reported perchlorate salt of the mono-zinc cryptate, [Zn^IILⁱ](BF₄)₂·0.5H₂O, are reported. Electrocatalytic HER testing showed that a deposit forms on the glassy carbon working electrode during electrolysis and it is the active species responsible for the very modest activity observed. The deposits formed by the heterobinuclear cryptates had higher activities (2.0 < TON_2h < 3.5) than the deposits formed by the mononuclear cryptates (TON_2h < 0.75). But unfortunately the control, using Cu^I(MeCN)₄BF₄, had a similar TON_2h (2.3) to those seen for the heterobinculear cryptates, which indicates that it is the deposit formed by the Cu^I cation present in the heterobinuclear cryptates that is likely responsible for the observed, very modest, HER activity.