Low‐Overpotential Electrocatalytic Water Splitting with Noble‐Metal‐Free Nanoparticles Supported in a sp³ N‐Rich Flexible COF

Abstract

Covalent organic frameworks (COFs) are crystalline organic polymers with tunable structures. Here, a COF is prepared using building units with highly flexible tetrahedral sp3 nitrogens. This flexibility gives rise to structural changes which generate mesopores capable of confining very small (<2 nm sized) non‐noble‐metal‐based nanoparticles (NPs). This nanocomposite shows exceptional activity toward the oxygen‐evolution reaction from alkaline water with an overpotential of 258 mV at a current density of 10 mA cm−2. The overpotential observed in the COF‐nanoparticle system is the best in class, and is close to the current record of ≈200 mV for any noble‐metal‐free electrocatalytic water splitting system—the Fe–Co–Ni metal‐oxide‐film system. Also, it possesses outstanding kinetics (Tafel slope of 38.9 mV dec−1) for the reaction. The COF is able to stabilize such small‐sized NP in the absence of any capping agent because of the COF–Ni(OH)2 interactions arising from the N‐rich backbone of the COF. Density‐functional‐theory modeling of the interaction between the hexagonal Ni(OH)2 nanosheets and the COF shows that in the most favorable configuration the Ni(OH)2 nanosheets are sandwiched between the sp3 nitrogens of the adjacent COF layers and this can be crucial to maximizing their synergistic interactions.

Low Band Gap Benzimidazole COF Supported Ni3N as Highly Active OER Catalyst

“…Unlike in previous reports, [9] here the FE-SEM of the Ni 3 N loaded phase, shows majority of the nanoparticle being loaded within the COF and hardly any is seen on the surface ( Figure 2D). The Ni 3 N nanoparticle sizes are very similar to the sizes of Ni/Co hydroxide nanoparticles grown on a mesoporous N-rich COF support, [12] which further supports the nanoscopic confinement these COFs can provide. Evidently, the nanocrystallites do not seem to coalesce to undesirable sizes nor do they seem to grow to unfavorably large particles.…”

“…To evaluate the electronics of the composite, since microscopy suggested extremely thin layers, a model was developed wherein 1.4 nm hexagonal sheets of Ni 3 N (P6 3 22→P6/m) were introduced into the COF lattice (Note: the bulk form of Ni 3 N typically crystallizes with a hexagonal sheet structure). Interestingly, the observed conductivity values are still three-orders of magnitude higher than the current observed in a recently reported ambipolar COF, wherein the conduction was promoted by donor-acceptor nodes [12] and such μA level currents have been observed also in a C-60 doped conjugated COF. A dispersion corrected DFT minimization provided a most probable solution, wherein the Ni 3 N clusters resided in the interlayer spaces of the COF, occupying the large mesopores present in them (Figure 5).…”

“…Recently, such layered COFs have been explored as supports for metal nanoparticles, resulting in composites with high catalytic/electrocatalytic activities. [12] COFs have been built from components rich in electronic character (conjugated π-clouds, donor-acceptor nodes or high polarizing moieties such as lone pairs, acidic or basic sites) resulting in conducting or semiconducting materials. [12] COFs have been built from components rich in electronic character (conjugated π-clouds, donor-acceptor nodes or high polarizing moieties such as lone pairs, acidic or basic sites) resulting in conducting or semiconducting materials.…”

Ag Nanoparticles Supported on a Resorcinol‐Phenylenediamine‐Based Covalent Organic Framework for Chemical Fixation of CO2

“…COFs find application in diverse fields of research such as gas separation and storage, heterogeneous catalysis, energy storage and conversion, biological application and optics . Earlier, in our group, we have developed efficient heterogeneous COF‐based catalysts for organic transformations and electrocatalytic oxygen evolution reaction . Several experimental and theoretical inputs reveal that the COF not only provides support for the growth of small‐sized nPs but also delivers a synergistic structural and electronic influence on the catalytic process .…”

Gold Nanocluster‐Decorated Nickel Nitride as Stable Electrocatalyst for Oxygen Evolution Reaction in Alkaline Media

“…As such, surface functionalization has been previously demonstrated to be a way forward of solving this challenge. Enhanced OER activity is obtained due to synergistic effect between the Ni 3 N and N‐doped COF/pure COF . However this solution results in catalysts with low durability (with ∼15 % initial activity loss); also this approach requires complex synthesis approaches.…”