Significance
Doping is a foundational concept in solid-state semiconductors, enabling precise control over electron and hole transport. However, a comparable approach in liquid or gel-based electrolytes remains elusive, where both cations and anions typically move without discrimination. This work demonstrates a universal electrochemical doping method using electroactive additives to selectively control ionic species in such media, which is essential for the development of advanced energy conversion and storage systems.
Abstract
Doping to control carrier (electron or hole) transport is foundational to modulate the properties of semiconductors, enabling the development of homojunctions and heterojunctions for integrated electronics. Unlike semiconductors with unipolar charge-carrier dominance, both cations and anions in electrolytes are mobile, which is undesirable for many applications. Here, we report a universal strategy to dope electrolytes such that the ion transport can be unipolar by incorporating electroactive polymers within hydrogels that interact discriminately with one type of ion via redox and binding mechanisms, leaving the counterions mobile. This transforms the system into an active, selective conductor that directs ion flow with high precision. We demonstrate the generality of this strategy using a wide range of electroactive polymers and ions. Particularly, we use emeraldine base and leucoemeraldine base, derived from polyaniline to create both n-type and p-type conductors with high ion selectivity. This electrolyte doping strategy has significant implications beyond the developed thermoelectrochemical devices with boosted performance, with potential applications in supercapacitors, batteries, and electrochemical sensors.
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In solid-state semiconductors, including light-emitting diodes, photovoltaics, thermoelectrics, and photo- and electrocatalysts, doping is achieved by immobile dopant atoms that create mobile electrons or holes (1–3). Similar doping strategies, however, do not exist for liquid/gel-state electrolytes which rely on the motion of ions, because both cations and anions can drift or diffuse randomly under electrical fields and concentration gradients. The ability to control transport predominantly by one type of ion (selective ion transport) is highly desirable for many applications, for example, to improve the charge transfer ratio of batteries, supercapacitors, and thermoelectrochemical systems (4–6). In this work, we establish a strategy to enable such control by immobilizing counterions via electrochemical doping in gel-based electrolytes. We validate the effectiveness of this strategy using a wide range of electroactive additives (EAs) to create both n-type and p-type ionic conductors, with the demonstration of low-grade thermal energy conversion into electricity serving as proof of concept.
Although certainly important for batteries, the need for n-type and p-type ionic conductors is well illustrated through ionic thermoelectric cells (iTECs) analogous to inorganic thermoelectric generators (TEG), both of which have been explored for direct conversion of heat into electricity (7–9). Conventional TEGs fabricated from n-type and p-type semiconductors through heavy doping with phosphorus (P) or boron (B) (Fig. 1A) typically exhibit low Seebeck coefficients of ~100 μV K−1 (10–13) and are therefore connected electrically in series and thermally in parallel to boost the voltage output. In contrast, iTECs can achieve thermopower (Si) reaching millivolts per Kelvin (mV K−1) through a combination of the thermogalvanic and thermodiffusion effects (14–16). The thermogalvanic contribution (Stg) arising from the entropy difference of redox couples and the thermodiffusive contribution (Std) originating from differences in mobility and Eastman entropy of transfer between cations and anions (15, 17, 18). The contributions from cations and anions to Std cancel each other since they diffuse in the same direction while carrying opposite Eastman entropy of transfer. Although both n-type and p-type iTEC materials have been reported (17–34), existing materials lack systematic doping control, which makes it difficult to synchronize the signs of Std and Stg.