Graphene has emerged as a promising material for energy storage applications, particularly in supercapacitors and lithium-ion (Li-ion) battery anodes, due to its exceptional electrical conductivity, high surface area, and mechanical flexibility. The performance of graphene in these applications is heavily influenced by doping and porosity, which can be engineered to enhance capacity, rate capability, and cycling stability.
### **Doping Effects on Graphene for Energy Storage**
Doping graphene with heteroatoms such as nitrogen (N), sulfur (S), boron (B), or phosphorus (P) modifies its electronic structure, improving charge transfer kinetics and electrochemical activity. Nitrogen doping, in particular, has been widely studied due to its ability to introduce additional active sites for charge storage.
In **supercapacitors**, nitrogen-doped graphene exhibits enhanced pseudocapacitance due to Faradaic reactions at the doped sites. Studies have shown that N-doped graphene can achieve specific capacitances exceeding 300 F/g in aqueous electrolytes, compared to ~200 F/g for pristine graphene. The improved performance is attributed to the introduction of pyridinic and pyrrolic nitrogen, which contribute redox-active sites.
For **Li-ion battery anodes**, doping enhances lithium adsorption and diffusion. Nitrogen-doped graphene demonstrates higher reversible capacities (~1000–1200 mAh/g) compared to undoped graphene (~500–700 mAh/g) due to stronger Li-ion binding at defect sites. However, excessive doping can lead to irreversible structural changes, reducing cycling stability.
Sulfur and boron doping also influence performance. Sulfur-doped graphene introduces larger interlayer spacing, facilitating Li-ion intercalation, while boron doping enhances electronic conductivity but may reduce capacity due to stronger Li-ion binding, leading to slower kinetics.
### **Porosity Engineering in Graphene**
Porosity plays a critical role in determining the electrochemical performance of graphene-based electrodes. Controlled porosity increases the accessible surface area, improves electrolyte infiltration, and accommodates volume changes during charge-discharge cycles.
In **supercapacitors**, porous graphene structures (e.g., with mesopores of 2–50 nm) enable efficient ion transport, reducing internal resistance. Hierarchically porous graphene, combining micropores (<2 nm) and mesopores, maximizes charge storage by balancing electric double-layer capacitance (EDLC) and ion diffusion rates. Such materials have demonstrated capacitance retention above 90% after 10,000 cycles.
For **Li-ion anodes**, porous graphene mitigates volume expansion issues common in conventional graphite anodes. Three-dimensional (3D) porous graphene networks provide mechanical stability, preventing electrode pulverization during cycling. Studies report that porous graphene anodes maintain capacities above 800 mAh/g over 500 cycles, significantly outperforming non-porous counterparts.
### **Synergistic Effects of Doping and Porosity**
Combining doping and porosity optimization yields superior electrochemical performance. For instance, nitrogen-doped porous graphene exhibits both improved conductivity and increased active sites, leading to higher energy and power densities in supercapacitors. Similarly, sulfur-doped porous graphene anodes demonstrate enhanced Li-ion storage due to synergistic effects of expanded interlayer spacing and defect-mediated adsorption.
However, trade-offs exist. Excessive porosity can reduce mechanical integrity, while high doping levels may introduce irreversible side reactions. Balancing these factors is crucial for optimizing performance.
### **Challenges and Future Perspectives**
Despite progress, challenges remain in scalable synthesis of doped and porous graphene with precise control over structural parameters. Future research should focus on:
- Developing cost-effective doping techniques without compromising structural integrity.
- Optimizing pore size distribution for specific electrolytes (organic/aqueous) in supercapacitors.
- Enhancing interfacial stability between graphene and electrolytes in Li-ion batteries.
Advances in these areas will further establish graphene as a leading material for next-generation energy storage systems.