Formation of electroactive colloids via in situ coprecipitation under electric field: Erbium chloride alkaline aqueous pseudocapacitor

• Novel ErCl3–KOH aqueous pseudocapacitor system was demonstrated.
• In situ crystallization of ErCl3 in alkaline electrolyte leads to highly electroactive ErOOH colloids.
• Electric field assisted chemical coprecipitation and Faradaic redox reactions occur at the same time and same electrode.
• ErCl3 pseudocapacitors can show ultrahigh capacitance of 1811 F/g.

For the first time, a new ErCl3 alkaline aqueous pseudocapacitor system was demonstrated by designing commercial ErCl3 salt electrode in alkaline aqueous electrolyte, where the materials synthesis and subsequently integrating into practical electrode structures occur at the same spatial and temporal scale. Highly electroactive ErOOH colloids were in-situ crystallized via electric field assisted chemical coprecipitation of ErCl3 in KOH aqueous electrolyte. These electroactive ErOOH colloids absorbed by carbon black and PVDF matrix were highly redox-reactive with higher cation utilization ratio of 86 % and specific capacitance values of 1811 F/g, exceeding the one-electron redox theoretical capacitance (Er3+ ↔ Er2+). We believe that additional two-electron (Er2+ ↔ Er) or three-electron (Er3+ ↔ Er) reactions can occur in our designed ErCl3 alkaline aqueous pseudocapacitor system. The specific electrode configuration with ErOOH colloids grown among the carbon black/PVDF matrix can create short ion diffusion and electron transfer length to enable the fast and reversible Faradaic reactions. This work shows promising for finding high-performance electrical energy storage systems via designing the colloidal state of electroactive cations with the utilization of in-situ crystallization route.

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