Graphitic carbon nitride (g-C3N4) has emerged as a promising material for energy storage applications due to its unique properties, including high nitrogen content, thermal stability, and tunable electronic structure. When combined with other materials such as graphene, metal oxides, or conductive polymers, g-C3N4 forms composites that enhance electrochemical performance in supercapacitors and batteries. These composites leverage synergistic effects to improve conductivity, charge storage capacity, and cycling stability.

One of the most studied composites involves g-C3N4 with graphene or reduced graphene oxide (rGO). Graphene provides high electrical conductivity and a large surface area, which compensates for the limited conductivity of pure g-C3N4. In supercapacitors, the combination of g-C3N4 and graphene results in improved specific capacitance due to enhanced charge transfer and pseudocapacitive contributions from nitrogen-rich sites. For example, a g-C3N4/rGO composite demonstrated a specific capacitance of 320 F/g at 1 A/g, significantly higher than pure g-C3N4. The porous structure of the composite also facilitates ion diffusion, contributing to better rate capability.

In lithium-ion batteries, g-C3N4 composites act as effective anode materials. The layered structure of g-C3N4 provides intercalation sites for lithium ions, while conductive additives like graphene prevent aggregation and improve electron transport. A g-C3N4/graphene composite exhibited a reversible capacity of 650 mAh/g after 100 cycles, outperforming many conventional carbon-based anodes. The nitrogen-rich framework also promotes strong interactions with lithium ions, enhancing charge storage kinetics.

Metal oxide-g-C3N4 composites are another class of materials with significant potential. Transition metal oxides such as MnO2, Co3O4, and Fe2O3 introduce redox-active sites that amplify pseudocapacitive behavior in supercapacitors. For instance, a g-C3N4/MnO2 hybrid electrode achieved a specific capacitance of 450 F/g due to the combined effects of double-layer capacitance from g-C3N4 and faradaic reactions from MnO2. The composite structure also mitigates volume expansion issues common in metal oxides, leading to improved cycling stability.

In battery systems, g-C3N4-metal oxide composites enhance lithium or sodium storage performance. A g-C3N4/Fe2O3 composite demonstrated a capacity retention of 85% after 200 cycles in a lithium-ion battery, attributed to the buffering effect of g-C3N4 against Fe2O3 pulverization. Similarly, in sodium-ion batteries, g-C3N4-Co3O4 hybrids showed a stable capacity of 400 mAh/g due to the synergistic interaction between the two components.

Conductive polymers like polyaniline (PANI) and polypyrrole (PPy) are also combined with g-C3N4 to create flexible and high-performance energy storage materials. These polymers contribute high conductivity and additional pseudocapacitance. A g-C3N4/PANI composite exhibited a specific capacitance of 550 F/g in supercapacitors, with 90% retention after 5,000 cycles. The porous g-C3N4 framework prevents polymer chain agglomeration, ensuring efficient charge transport.

The design of g-C3N4 composites often focuses on optimizing morphology and interface engineering. Hierarchical structures with controlled porosity enhance electrolyte accessibility, while strong interfacial bonding between components ensures mechanical stability. For example, 3D interconnected g-C3N4-graphene aerogels provide continuous conductive pathways and prevent restacking, leading to high-rate performance in supercapacitors.

Despite these advantages, challenges remain in scaling up the synthesis of g-C3N4 composites and achieving consistent performance. Variations in precursor composition, reaction conditions, and composite homogeneity can influence electrochemical properties. Future research may focus on standardized synthesis protocols and advanced characterization techniques to better understand structure-performance relationships.

In summary, graphitic carbon nitride composites demonstrate significant potential in energy storage devices by combining the unique properties of g-C3N4 with conductive or redox-active materials. Graphene, metal oxides, and conductive polymers are key components that enhance capacitance, cycling stability, and rate performance. Continued optimization of composite design and fabrication methods will further advance their application in supercapacitors and batteries.