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Supercapacitors are a perfect example of a soft meets hard interface: they consist of two nanoporous electrodes submerged in a liquid electrolyte. When a voltage is applied across the electrodes, the ions in the electrolyte move accordingly and form electric double layers at the electrode-electrolyte interfaces to balance the charge. This interfacial charge separation enables rapid, reversible charge storage through dominantly electrostatic (non-faradaic) processes, thereby supporting high power density and long cycle life. Accordingly, supercapacitors are widely used for high power energy storage applications. More recently, however, it has been discovered that exposing one supercapacitor electrode to a CO2 containing gas also enables reversible and largely selective CO2 capture upon charging, known as Supercapacitive Swing Adsorption (SSA) [1].
This electrochemical CO2 capture approach inherits several attractive features from supercapacitors, including long cycle lifetimes, inherent tolerance to humidity in the gas stream, the use of abundant and sustainable materials, and low energy consumption without the need for thermal regeneration. However, despite growing interest and extensive empirical optimisation, the fundamental mechanism governing CO2 adsorption and desorption in supercapacitors has remained unresolved.
Here, we identify the mechanism of CO2 capture in aqueous supercapacitors, showing that it is governed by an electrochemical pH swing at the electrode-electrolyte interface. This pH swing modulates carbon speciation and the dissolved inorganic carbon content in the electrolyte, driving CO2 uptake and release from the gas phase. Using operando monitoring in a dedicated supercapacitor setup for SSA, we quantify local pH changes during charging and discharging. Negative electrode polarisation induces more basic conditions, driving CO2 uptake, while positive polarisation leads to local acidification and consequently CO2 release. Calculated changes in dissolved inorganic carbon as a function of pH are in excellent quantitative agreement with experimentally measured CO2 uptake and release. Furthermore, suppressing pH changes using a buffered electrolyte almost completely suppresses CO2 capture, confirming that pH is the controlling variable.
By identifying local pH changes as the governing factor for CO2 capture and release in supercapacitors, this work resolves the long-standing lack of mechanistic understanding in SSA and provides a first foundation for controlling and improving performance in supercapacitor-based electrochemical CO2 capture systems.
[1] B. Kokoszka, N. K. Jarrah, C. Liu, D. T. Moore and K. Landskron, Angew. Chem. Int. Ed., 2014, 53, 3698–3701