Carbon nanomaterials, including graphene, carbon nanotubes, and porous carbon structures, are widely used in energy storage devices such as supercapacitors and batteries due to their high conductivity, large surface area, and chemical stability. However, degradation mechanisms during electrochemical cycling can compromise their performance. Key degradation pathways include oxidation, pore collapse, and solid-electrolyte interphase (SEI) formation, each contributing to capacity fade and increased resistance. Mitigation strategies, such as protective coatings and electrolyte additives, aim to enhance durability. Advanced characterization techniques, including in-situ X-ray diffraction (XRD) and impedance spectroscopy, provide insights into aging processes.

Oxidation is a primary degradation mechanism in carbon-based electrodes, particularly in aqueous or oxygen-rich environments. During cycling, reactive oxygen species or electrochemical potentials can lead to the formation of oxygen-containing functional groups (e.g., carboxyl, hydroxyl, or epoxy groups) on the carbon surface. This process reduces electrical conductivity and active surface area, impairing charge storage capacity. In supercapacitors, oxidation is more pronounced at high operating voltages, where water decomposition or electrolyte decomposition generates oxidative byproducts. In batteries, oxidation can occur at the anode during overcharging or due to electrolyte instability. Mitigation strategies include the use of nitrogen or sulfur doping, which introduces heteroatoms that stabilize the carbon structure against oxidative attack. Additionally, operating within a stable voltage window minimizes oxidative degradation.

Pore collapse is another critical issue, particularly in highly porous carbon materials used in supercapacitors. The repeated insertion and extraction of ions during cycling exert mechanical stress on the carbon framework, leading to structural deformation or pore closure. This reduces the accessible surface area for charge storage and increases ion transport resistance. In batteries, similar effects occur when carbon anodes undergo volume changes during lithiation and delithiation. To counteract pore collapse, researchers have developed hierarchical carbon structures with reinforced pore walls or incorporated secondary phases, such as carbon nanotubes, to provide mechanical support. Optimizing the pore size distribution also helps maintain structural integrity by preventing excessive stress concentration.

SEI formation is a dominant degradation pathway in lithium-ion batteries where carbon materials serve as anodes. The SEI layer forms during initial cycles due to electrolyte reduction at the electrode surface. While a stable SEI is essential for preventing further electrolyte decomposition, excessive or unstable SEI growth consumes active lithium ions and increases interfacial resistance. In supercapacitors, a similar phenomenon occurs when organic electrolytes decompose at high voltages, forming resistive surface films. Strategies to control SEI formation include the use of electrolyte additives, such as vinylene carbonate or fluoroethylene carbonate, which promote the formation of a more stable and conductive SEI layer. Pre-passivation techniques, where the electrode is treated to form an artificial SEI before cycling, have also shown promise in reducing irreversible capacity loss.

Protective coatings are an effective approach to mitigate degradation. Thin layers of metal oxides (e.g., Al2O3 or TiO2) or conductive polymers can shield carbon surfaces from oxidative attack while maintaining ion accessibility. In batteries, atomic layer deposition (ALD) has been used to apply conformal coatings that stabilize the SEI and prevent electrolyte penetration. For supercapacitors, graphene or carbon nitride coatings have been explored to enhance chemical stability without compromising porosity. The choice of coating material depends on compatibility with the electrolyte and the operating voltage range.

Electrolyte additives play a crucial role in suppressing degradation. Additives such as lithium bis(oxalato)borate (LiBOB) or cesium hexafluorophosphate (CsPF6) can stabilize the electrode-electrolyte interface by forming protective layers or scavenging reactive species. In supercapacitors, ionic liquid additives have been shown to reduce gas evolution and oxidation at high voltages. The concentration and type of additive must be optimized to avoid adverse effects on ion transport or wettability.

Characterization techniques are essential for understanding degradation mechanisms and evaluating mitigation strategies. In-situ XRD provides real-time monitoring of structural changes in carbon electrodes during cycling, revealing phase transitions or amorphization due to oxidation or pore collapse. Impedance spectroscopy tracks interfacial resistance evolution, offering insights into SEI growth or pore blockage. Post-mortem analysis using scanning electron microscopy (SEM) or transmission electron microscopy (TEM) can visualize morphological changes, while X-ray photoelectron spectroscopy (XPS) identifies chemical modifications on the carbon surface.

Quantitative studies have demonstrated that unprotected carbon anodes in lithium-ion batteries can lose up to 20-30% of their initial capacity after 500 cycles due to SEI growth and oxidation. In contrast, electrodes with protective coatings or optimized electrolytes exhibit capacity retention above 90% under similar conditions. For supercapacitors, pore collapse can reduce capacitance by 15-25% after 10,000 cycles, but hierarchical carbon designs mitigate this loss to below 10%.

In summary, degradation pathways in carbon nanomaterials during cycling are a major challenge for energy storage devices. Oxidation, pore collapse, and SEI formation each contribute to performance decay, but targeted mitigation strategies can significantly improve longevity. Protective coatings and electrolyte additives are among the most effective solutions, while advanced characterization techniques enable precise monitoring of aging processes. Continued research into material design and interfacial engineering will further enhance the durability of carbon-based electrodes in supercapacitors and batteries.