Electronic Tuning of CO₂ Interaction by Oriented Coordination of N‐Rich Auxiliary in Porphyrin Metal–Organic Frameworks for Light‐Assisted CO₂ Electroreduction

Abstract

The efficient CO2 electroreduction into high‐value products largely relies on the CO2 adsorption/activation or electron‐transfer of electrocatalysts, thus site‐specific functionalization methods that enable boosted related interactions of electrocatalysts are much desired. Here, an oriented coordination strategy is reported to introduce N‐rich auxiliary (i.e., hexamethylenetetramine, HMTA) into metalloporphyrin metal organic frameworks (MOFs) to synthesize a series of site‐specific functionalized electrocatalysts (HMTA@MOF‐545‐M, M = Fe, Co, and Ni) and they are successfully applied in light‐assisted CO2 electroreduction. Noteworthy, thus‐obtained HMTA@MOF‐545‐Co presents approximately two times enhanced CO2 adsorption‐enthalpy and electrochemical active surface‐area with largely decreased impedance‐value after modification, resulting in almost twice higher CO2 electroreduction performance than its unmodified counterpart. Besides, its CO2 electroreduction performance can be further improved under light‐illumination and displays superior FECO (≈100%), high CO generation rate (≈5.11 mol m−2 h−1 at −1.1 V) and energy efficiency (≈70% at −0.7 V). Theoretical calculations verify that the oriented coordination of HMTA can increase the charge density of active sites, almost doubly enhance the CO2 adsorption energy, and largely reduce the energy barrier of rate determining step for the boosted performance improvement. This work might promote the development of modifiable porous crystalline electrocatalysts in high‐efficiency CO2 electroreduction.

Exfoliation of a Two-Dimensional Metal–Organic Framework for Enhanced Photocatalytic CO2 Reduction

“…MOFs are porous crystalline materials built through coordination bonds between metal or metal clusters and organic linkers. − Compared to other solid-state materials, MOFs have unique advantages with regard to the capability of precise structure control and prediction, giving MOF materials great potential in various applications such as gas storage, , separation, − sensing, − catalysis, − and so on. In particular, these features endow MOFs with the unique opportunity to integrate light-harvesting and catalytic components in a single solid in an orderly manner. − This approach has been validated in several reports of MOF-based integrated photocatalysts for CO 2 reduction. − However, exciton and mass transfer through the framework remains a challenge for MOF photocatalysis despite the porous catalyst structure.…”

Photocoupled Electroreduction of CO2 over Photosensitizer-Decorated Covalent Organic Frameworks

“…36,37 Notably, it is difficult to integrate these advantages in a MOF, due to the energetically unfavored ligand-to-node charge transfer. 18,46 To date, there is, to the best of our knowledge, no report of donor−acceptor (D−A) characteristic 2D COFs with a giant built-in electric field for light-coupled CO 2 electroreduction.…”

“…In view of the key scientific issues above, it is recommended to integrate an acceptor with strong CO 2 reduction ability, as well as powerful donor characterized by long-lived triplet excited states, into a crystalline material to establish a built-in electric field. − This approach can thus effectively extend the excited-state lifetime of the CO 2 reduction sites, thereby augmenting the efficacy of photoinduced excited-state CO 2 electrocatalysis. As an emerging class of crystalline porous materials constructed by functional organic building blocks with covalent bonds, two-dimensional (2D) COFs would be promising platforms to incorporate CO 2 reduction sites and light harvesters into their backbones. − Furthermore, the ordered π-array structures in highly crystalline 2D COFs can provide preorganized channels for high-rate intra-framework excited-electron transfer. , Notably, it is difficult to integrate these advantages in a MOF, due to the energetically unfavored ligand-to-node charge transfer. , To date, there is, to the best of our knowledge, no report of donor–acceptor (D–A) characteristic 2D COFs with a giant built-in electric field for light-coupled CO 2 electroreduction.…”

“…In recent years, a few efforts have been directed toward incorporating metalloporphyrin or metallophthalocyanine, serving as both photosensitive unit and redox active site, into the metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) for visible light-coupled CO 2 RR. − As proved, the introduction of a visible-light field can effectively induce the active site from ground states to highly reactive excited states, leading to superior performance in photo-coupled CO 2 electroreduction. In particular, the metalloporphyrin moieties have a lower photosensitivity and CO 2 electrochemical activation capacity than metallophthalocyanine, so the former is harder to show improved photocoupled CO 2 RR performance over a wide potential range than the latter.…”

“…Therefore, it is urgent to take actions to improve the photo-coupled CO 2 RR performance of metalloporphyrin. Notably, the efficient excited-state lifetime of the active site, which is an important influencing factor in visible photo-coupled electrocatalysis, has long been neglected. − First, the excited state of these previously reported photosensitizing MOFs/COFs mainly populated on the LMCT state (ligand-to-metal charge transfer) of a mono-photosensitive units, where no extra photosensitive donor with long-lived triplet excited state to prolong the excited-state lifetime of the active sites. , Additionally, the lack of strong intra-framework driving force with specific direction, such as a built-in electric field, also often leads to rapid relaxation of excited state during the delivery process. , Moreover, the excited state at the active site is also easily dissipated through luminescence, heat release, and electron transfer to nearby species, etc . Consequently, the resulting short excited-state lifetime is difficult to continuously provide assistance for the electrocatalytic CO 2 RR, which inevitably compromises their activity in photo-coupled CO 2 electroreduction. , Although Lan’s group designed double-excited three-dimensional (3D) COFs for improving the performance of photocatalytic hydrogen (H 2 ) evolution, there is still a lack of in-depth research in photo-coupled CO 2 electrocatalysis.…”