Current collectors are critical components in aqueous batteries, serving as conductive substrates that facilitate electron transfer between electrodes and external circuits. However, aqueous electrolytes introduce unique challenges, primarily due to their corrosive nature and electrochemical instability. The performance and longevity of current collectors in such environments depend heavily on material selection, surface modifications, and compatibility with the electrolyte's pH range.
One of the most significant challenges in aqueous batteries is corrosion of current collectors. Unlike non-aqueous systems, water-based electrolytes are prone to promoting oxidation and dissolution of metallic collectors, especially at high electrode potentials. Common metals like copper and aluminum, widely used in traditional batteries, exhibit poor stability in aqueous media. For example, aluminum forms a passive oxide layer in non-aqueous electrolytes but suffers from severe pitting corrosion in water-based systems, particularly at elevated pH levels. Similarly, copper undergoes rapid dissolution in acidic or neutral aqueous electrolytes, leading to collector degradation and battery failure.
To mitigate corrosion, researchers have explored alternative materials with inherent stability in aqueous environments. Titanium stands out due to its excellent corrosion resistance across a wide pH range. Its passive oxide layer, primarily composed of TiO2, provides a protective barrier against electrolyte-induced degradation. Titanium current collectors demonstrate stability in both acidic and alkaline aqueous electrolytes, making them suitable for various battery chemistries. However, titanium's high cost and lower electrical conductivity compared to copper or aluminum pose economic and performance trade-offs.
Carbon-coated steels have emerged as a cost-effective alternative, combining the mechanical robustness of steel with the electrochemical inertness of carbon coatings. The carbon layer acts as a barrier, preventing direct contact between the steel substrate and the corrosive electrolyte. Additionally, carbon coatings enhance electrical conductivity and reduce interfacial resistance. Studies have shown that carbon-coated stainless steel exhibits minimal corrosion in neutral and mildly alkaline aqueous electrolytes, making it viable for large-scale applications where cost efficiency is critical.
The stability of current collectors in aqueous batteries is also closely tied to the electrolyte's pH window. Different materials exhibit optimal performance within specific pH ranges. For instance, titanium demonstrates superior stability in highly acidic (pH < 2) and highly alkaline (pH > 12) conditions, whereas carbon-coated steels perform best in near-neutral to moderately alkaline environments (pH 7–11). Outside these ranges, material degradation accelerates, leading to increased resistance and capacity fade.
In acidic electrolytes, the hydrogen evolution reaction (HER) becomes a competing process, further complicating current collector performance. Metals with high HER overpotentials, such as titanium, help suppress parasitic reactions, but their adoption is limited by cost. In alkaline media, the oxygen evolution reaction (OER) can oxidize collector surfaces, necessitating materials that form stable passive layers. Nickel and nickel-plated steels are sometimes used in alkaline systems due to their OER resistance, but they are less effective in neutral or acidic conditions.
Another challenge is the mechanical integrity of current collectors under prolonged cycling. Repeated expansion and contraction of electrode materials during charge-discharge cycles can delaminate coatings or fracture brittle substrates. Titanium, while corrosion-resistant, is relatively rigid and may not accommodate volume changes as effectively as more ductile materials. Carbon-coated steels offer a balance of flexibility and durability, but the adhesion strength of the carbon layer must be optimized to prevent peeling under mechanical stress.
Recent advancements focus on hybrid and composite materials to address these limitations. For example, titanium substrates with conductive polymer coatings combine corrosion resistance with improved interfacial conductivity. Similarly, nanostructured carbon coatings on steel enhance both mechanical adhesion and electrochemical performance. These innovations aim to extend the operational lifespan of aqueous batteries while maintaining cost competitiveness.
In summary, the development of current collectors for aqueous batteries requires careful consideration of corrosion resistance, pH stability, and mechanical durability. Titanium and carbon-coated steels represent promising solutions, each with distinct advantages and limitations. Future research will likely focus on optimizing material compositions and coatings to further enhance performance in diverse aqueous electrolyte systems. The choice of current collector ultimately depends on the specific battery chemistry, operating conditions, and economic constraints, highlighting the need for tailored material solutions in this evolving field.
The challenges discussed here underscore the importance of material science in advancing aqueous battery technologies. By addressing corrosion and stability issues, researchers can unlock the full potential of water-based energy storage systems for applications ranging from grid storage to portable electronics. Continued innovation in current collector design will play a pivotal role in improving the efficiency, longevity, and scalability of aqueous batteries.