Unleashing the potential of π-conjugation in organic framework materials for electrochemical energy storage and conversion

The depletion of nonrenewable energy and the speedy increase in toxic gas emissions have been driving the laden exploration of green energy. There is an urgent demand for the advancement of clean, affordable, and efficient alternative energy sources [[1], [2], [3]]. Nowadays, there is continuous global progress in the development of green and clean energy resources, including wind, ocean, solar, and biomass energy. Electrochemical ESC technologies are also considered important solutions for addressing global energy needs and environmental concerns.

The performance of facilities in ESC hinges significantly on the active materials utilized [[4], [5], [6]]. Therefore, it becomes increasingly important for developing ideal active materials to optimize the electrochemical performance. So far, the main materials employed in electrochemical storage and conversion are precious metal-based materials and inorganic materials, such as the Pt group and Ru/Ir-based benchmark for the photoelectron-catalysis [7,8] as well as the hydroxide [9,10] and oxide materials [11,12] for supercapacitors. However, the high cost of precious metals and the poor durability of inorganic materials greatly limit the widespread practical application of renewable energy technologies. Specifically, the reserves of precious metals on Earth are small and unevenly distributed. This scarcity is at the heart of the high price of precious metals. Certain inorganic materials (e.g., glass, ceramics, etc.), while hard, have low toughness and are susceptible to cracking or shattering on impact. Given these limitations, there is a pressing demand to innovate electroactive materials with low cost, high activity, and long-term enduringness.

The overall efficiency of the materials in ESC depends on the specific structure and performance of the incorporated functional compounds [13,14]. Pi-conjugation refers to the interaction and sharing of multiple π-electrons in a molecule to form a relatively stable electron cloud that can be distributed among multiple atoms. This effect is the basis of many chemical reactions and molecular properties, and is widely found in aromatic compounds, certain olefins, and other organic compounds containing conjugated multiple bonds. This conjugated structure is conducive to improve the efficiency of energy storage and conversion by adjusting the electronic structure and enhancing the electron transfer. Carbon materials show significant promise in advancing ESC technologies [15,16]. Among these, graphene has garnered significant attention owing to the specific two-dimensional (2D) structure, which consists of a single atomic layer of sp²-bonded carbon atoms densely arranged in a honeycomb lattice through π-π interactions [17]. Because of the distinctive π-conjugated effect, graphene exhibits remarkable physico-chemical properties, like high intrinsic carrier mobility (200,000 cm² V⁻¹ s⁻¹) [18].

Metal-Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) are both highly ordered porous materials. Due to their unique advantages in structural design and performance tuning, they have become widely studied materials in recent years. MOFs are ordered porous materials formed by the coordination of metal ions or metal clusters with organic ligands. COFs are porous crystalline materials made of organic units connected by covalent bonds [[19], [20], [21]]. Unlike MOFs, the nodes and linkers in COFs are organic molecules without metallic elements. Conjugated MOFs and COFs have shown broad prospects in diverse fields such as gas storage and separation [22,23], catalysis [24,25], sensing [26,27], and drug delivery [28,29]. Due to the delocalization of in-plane charges and the extended π conjugation, the 2D conjugated framework exhibits high charge carrier mobility [30]. As a result, they often exhibit excellent electrochemical performance. For example, Wang et al. demonstrated a 2D d-π cMOF (Cu-HATNH) with dual active sites based on hexaazanonaphthalene (HATN), and encapsulated it on the surface of carbon nanotubes to serve as a cathode material for potassium-ion batteries (PIBs) [31]. This synthetic strategy promotes the utilization of active sites while accelerating electron transfer, resulting in Cu-HATNH@CNT a good initial capacity and cycling stability as a cathode material for PIBs. Furthermore, some π-conjugated molecules can make good results by directly coating on no π-conjugated MOF. In our previous work, the π-conjugated organic ligand HHTP (2,3,6,7,10,11-hexahydroxytriphenylene) was designated as the coating of the parent ZIF-67 [32], which can enhance the conductivity. Under the synergistic effect of π-conjugated framework and ZIF-67 porous structure, the electrochemical catalytic performance of the obtained material has been greatly improved compared to the original ZIF-67. The above facts indicate that π-conjugated molecules have presented significant application in ESC in recent years, owing to their high charge mobility.

Herein, in this review, we first sum up the construction of conjugated systems in term of electron orbital types. As shown in Fig. 1, it mainly involves the π-π conjugation, p-π conjugation, and d-π conjugation. Secondly, taking MOFs and COFs containing π-conjugated structures as examples, the specific applications of π-conjugated structures in ESC are presented. This includes detailed discussions on their performance in areas like energy storage (lithium‑sulfur batteries, ion batteries, and supercapacitors) and energy conversion (solar energy conversion, photocatalysis, and electrocatalysis). At last, the prospects and challenges of π-conjugation in molecules in future development are discussed.