Understanding Multi-Stage Charge Storage on Nanoporous Carbons in Zn-Ion Hybrid Capacitors

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Zinc-ion hybrid capacitors (ZIHCs) offer high power but have poorly understood charge storage mechanisms, including proton influence.
• Optimizing carbon materials is crucial for advancing ZIHC performance.

Purpose of the Study:

• To elucidate the charge storage mechanisms in ZIHCs by investigating the role of pore size in porous carbons.
• To understand the impact of pore architecture on capacity and cycling stability.
• To explore the influence of proton storage and zinc hydroxysulfate hydrate formation on ZIHC performance.

Main Methods:

• Synthesis of model porous carbons with controlled pore sizes but similar surface area and chemistry.
• Electrochemical characterization, including galvanostatic charge-discharge and cyclic voltammetry.
• Electrochemical Quartz Crystal Microbalance (EQCM) analysis to study in-situ mass changes.
• Analysis of zinc hydroxysulfate hydrate (ZHS) formation and its correlation with pore structure.

Main Results:

• Supermicropores and small mesopores (0.86-4 nm) enabled a high capacity of 198 mAh g^-1 (446 F g^-1).
• Larger mesopores (4-13 nm) significantly improved cycling stability, exceeding 0.6 million cycles.
• A 4-stage charge storage mechanism involving Zn²⁺, proton, and water interactions was identified, influenced by pore size and pH-driven ZHS precipitation.

Conclusions:

• Pore size engineering in carbons is a viable strategy to optimize ZIHC performance, balancing capacity and longevity.
• The study clarifies the complex charge storage mechanism in ZIHCs, highlighting the interplay between ion adsorption, water reduction, and ZHS formation.
• Controlled pore architecture can mitigate detrimental ZHS precipitation, enhancing overall device stability.