Supercapacitors, also known as electrochemical capacitors, are energy storage devices that bridge the gap between conventional capacitors and batteries. They offer high power density, rapid charge-discharge cycles, and long cycle life. However, like all energy storage systems, supercapacitors are subject to degradation mechanisms that affect their performance over time. The primary degradation mechanisms include electrode corrosion and electrolyte decomposition, which lead to reduced capacitance, increased equivalent series resistance (ESR), and eventual device failure. Understanding these mechanisms and developing mitigation strategies is critical for improving the longevity and reliability of supercapacitors.

Electrode corrosion is a significant degradation mechanism in supercapacitors, particularly those using carbon-based materials such as activated carbon, graphene, or carbon nanotubes. Corrosion occurs due to electrochemical oxidation of the electrode material, especially at high operating voltages or elevated temperatures. The oxidation process generates functional groups on the carbon surface, such as carboxyl, hydroxyl, and carbonyl groups, which alter the electrode's electronic conductivity and reduce its electrochemical active surface area. This results in a decline in capacitance and an increase in ESR. In aqueous electrolytes, corrosion is accelerated by the presence of dissolved oxygen, which promotes oxidative reactions. In organic electrolytes, impurities such as water or acidic residues can catalyze corrosion.

Mitigation strategies for electrode corrosion focus on enhancing the stability of the electrode material. One approach is the application of protective coatings, such as conductive polymers or metal oxides, which act as barriers against oxidative species. For example, thin layers of polyaniline or polypyrrole can be deposited on carbon electrodes to inhibit direct contact between the electrode and corrosive agents. Another strategy involves doping carbon materials with heteroatoms like nitrogen or boron, which modify the electronic structure of the electrode and improve its resistance to oxidation. Additionally, using high-purity electrolytes with minimal impurities reduces the likelihood of corrosion-inducing reactions.

Electrolyte decomposition is another critical degradation mechanism in supercapacitors. The electrolyte, which can be aqueous, organic, or ionic liquid-based, undergoes irreversible chemical reactions during cycling, leading to gas evolution, increased internal pressure, and performance degradation. In aqueous electrolytes, water splitting occurs at voltages exceeding the thermodynamic stability window, producing hydrogen and oxygen gases. This not only depletes the electrolyte but also causes mechanical stress on the device. Organic electrolytes, such as those based on acetonitrile or propylene carbonate, are prone to decomposition at high voltages or temperatures, forming resistive byproducts that increase ESR. Ionic liquids, while more stable, can still degrade under extreme conditions due to anion or cation breakdown.

To mitigate electrolyte decomposition, researchers have explored several strategies. Additives play a crucial role in stabilizing electrolytes. For instance, redox-active additives like hydroquinone or quinone can shuttle protons and electrons, reducing the overpotential for water splitting in aqueous systems. In organic electrolytes, additives such as vinylene carbonate or fluoroethylene carbonate form protective solid-electrolyte interphase (SEI) layers on the electrode surface, preventing further decomposition. Another approach involves using hybrid electrolytes that combine the advantages of different solvent systems, such as mixing organic solvents with ionic liquids to enhance stability. Additionally, operating supercapacitors within their voltage and temperature limits minimizes electrolyte degradation.

Aging tests are essential for evaluating the long-term performance of supercapacitors and identifying degradation mechanisms. Accelerated aging tests subject devices to elevated voltages, temperatures, or current loads to simulate years of operation within a shorter timeframe. Common test protocols include constant voltage holding, where the supercapacitor is held at its maximum rated voltage for extended periods, and cycling tests, where the device undergoes repeated charge-discharge cycles. During these tests, parameters such as capacitance, ESR, and leakage current are monitored to assess degradation. For example, a supercapacitor subjected to 1,000 hours of constant voltage holding at 2.7 V and 70°C might exhibit a 20% capacitance loss and a 50% increase in ESR, indicating significant electrolyte decomposition and electrode corrosion.

Predictive models are valuable tools for estimating supercapacitor lifespan and optimizing design parameters. Electrochemical models based on first principles simulate the reactions occurring at the electrode-electrolyte interface, providing insights into degradation pathways. Empirical models, derived from aging test data, use statistical methods to correlate operating conditions with performance loss. For instance, the Arrhenius equation can predict the temperature-dependent rate of electrolyte decomposition, while linear regression models quantify the relationship between voltage stress and capacitance fade. Machine learning algorithms are increasingly being employed to analyze large datasets from aging tests, identifying patterns and predicting failure modes with high accuracy.

Material coatings and additives are among the most effective mitigation strategies for supercapacitor degradation. For electrode protection, atomic layer deposition (ALD) of metal oxides like Al2O3 or TiO2 creates ultrathin, conformal coatings that prevent oxidative damage while maintaining high conductivity. Polymer coatings, such as poly(3,4-ethylenedioxythiophene) (PEDOT), offer flexibility and adhesion, making them suitable for flexible supercapacitors. On the electrolyte side, additives like nitriles or phosphates scavenge free radicals generated during decomposition, extending the electrolyte's lifespan. Hybrid electrolytes incorporating nanoparticles or gel polymers enhance stability by reducing ion mobility and suppressing side reactions.

In conclusion, degradation mechanisms in supercapacitors, particularly electrode corrosion and electrolyte decomposition, pose significant challenges to their long-term performance. Mitigation strategies such as protective coatings, doping, and additive stabilization have shown promise in enhancing durability. Aging tests and predictive models provide critical insights into degradation processes, enabling the development of more robust supercapacitors. Continued research into advanced materials and innovative designs will further improve the reliability and lifespan of these devices, ensuring their viability for high-performance energy storage applications.