Bioinspired Carboxylate–Water Coordination Polymers with Hydrogen-Bond Clusters and Local Coordination Flexibility for Electrochemical Water Splitting

Coordination
polymers (CPs) have attracted considerable attention
for use in noble-metal-free electrochemical water splitting, for example,
by the hydrogen/oxygen evolution reactions (HER/OER). Transition-metal
sites with aqueous accessibility and local coordination flexibility
are highly desirable in CPs for use as coordination-polymer-based
electrocatalysts, but the challenge of structural assembly has limited
their development. By analogy with the hydrogen bond cluster around
the water-splitting center in photosystem II, the polycarboxylate
aryl ether ligand and first-row transition-metal (Co and Ni) salts
were employed to assemble CP-based electrocatalysts for water splitting.
The obtained monometallic CPs, Co–HL (1) {[Co2(HL)­(H2O)5]·3H2O}n, Ni–HL (2) {[Ni2(HL)­(H2O)5]·3H2O}n, and bimetallic CPs, Co/Ni–HL, with
different ratios of Co/Ni ions had semirigid isostructural frameworks
and carboxylate–water hydrogen bond clusters around the transition-metal
sites. The susceptibility of the hydrogen bond cluster and response
of the local coordination environment to stimuli were examined by
investigating the thermo-/solvatochromism performance of the CPs.
The local hydrogen bond system within the CPs contributes to electron
injection and the synergistic bimetallic effect to support electrochemical
applications. Among the mono- and bimetallic CP materials, Co/Ni–HL(5:5),
with a Co/Ni ratio of 5:5, exhibited the optimal water splitting performance.
This bimetallic CP was grown in situ on FeNi foam to improve the conductivity,
and the composite material exhibited excellent performance, with overpotentials
of 258 mV for the OER and 90 mV for the HER at 10 mA cm–2, a small Tafel slope, and high faradaic efficiency. Overall water
splitting generated a current density of 10 mA cm–2 at a voltage of 1.52 V with excellent durability; its performance
was comparable to that of state-of-the-art materials for this application.