A bio-inspired coordination polymer as outstanding water oxidation catalyst via second coordination sphere engineering.

TL;DR: Uncoordinated carboxylate is introduced in the second-coordination-sphere of Ni-Fe coordination polymer catalyst as an internal base to promote the water oxidation kinetics by such PCET process.

Abstract: First-row transition metal-based catalysts have been developed for the oxygen evolution reaction (OER) during the past years, however, such catalysts typically operate at overpotentials (η) significantly above thermodynamic requirements. Here, we report an iron/nickel terephthalate coordination polymer on nickel form (NiFeCP/NF) as catalyst for OER, in which both coordinated and uncoordinated carboxylates were maintained after electrolysis. NiFeCP/NF exhibits outstanding electro-catalytic OER activity with a low overpotential of 188 mV at 10 mA cm−2 in 1.0 KOH, with a small Tafel slope and excellent stability. The pH-independent OER activity of NiFeCP/NF on the reversible hydrogen electrode scale suggests that a concerted proton-coupled electron transfer (c-PET) process is the rate-determining step (RDS) during water oxidation. Deuterium kinetic isotope effects, proton inventory studies and atom-proton-transfer measurements indicate that the uncoordinated carboxylates are serving as the proton transfer relays, with a similar function as amino acid residues in photosystem II (PSII), accelerating the proton-transfer rate. Proton-coupled electron transfer (PCET) process is very important for water oxidation catalysis. Here, the authors introduced uncoordinated carboxylate in the second-coordination-sphere of Ni-Fe coordination polymer catalyst as an internal base to promote the water oxidation kinetics by such PCET process.