Metal-Free Electrocatalytic Alkaline Water Splitting by Porous Macrocyclic Proton Sponges

Abstract

Macrocycles are unique as they encapsulate and transfer guest molecules or ions and facilitate catalytic processes. Although metalated macrocycles are pivotal in electrocatalytic processes, using metal-free analogs has been rare. Following the strategy of Kanbara et al., we synthesized an azacalixarene macrocycle— N, N’, N’’-tris(p-aminophenyl)azacalix[3](2,6)pyridine (CalixNH2). The macrocycle encapsulates a proton in its cavity, maintaining the protonation even in highly alkaline media. Notably, it retains almost 50% protonated form in 1 M KOH (~pH 14)—acting as a proton sponge. As hydrogen evolution is complex in alkaline media owing to sluggish water dissociation, we implemented the proton sponge (CalixNH2) in an alkaline hydrogen evolution reaction. Conjugated Porous polymers, TpCalix and DhaCalix, have been synthesized from the triamine—CalixNH2. The most efficient catalyst, TpCalix, has shown excellent performance in alkaline HER and OER in 1 M KOH (~pH 14), with low overpotentials of only 112(±2) and 290(±2) mV at 10 mA cm-2, respectively, and durable up to 24 hours. A full-cell reaction using TpCalix in both the cathode and anode exhibited a low full-cell voltage of 1.73 V and was stable for 12 hours. DFT calculations verified the tripyridinic core, which acts as the principal site for proton abstraction and binding.