Surface Redox Pseudocapacitance Boosting Vanadium Nitride for High‐Power and Ultra‐Stable Potassium‐Ion Capacitors

Abstract

Developing a high‐rate and stable battery‐type anode to match the capacitor‐type cathode is a critical issue for potassium ion capacitors (PICs). Surface‐redox pseudocapacitive materials can meet this demand due to their fast surface Faradaic reaction kinetics and superior structure stability during charging–discharging. Herein, a free‐standing anode by growing VN particle‐composed nanosheets on carbon fibers (VN@CFs) is developed. The VN@CFs is endowed with high reversible capacity of 245.8 mA h g–1 at 0.05 A g–1, high rate performance of 102.7 mA h g–1 at 6.0 A g–1, and long‐term stability. Based on the in situ XRD, ex situ XPS and TEM characterizations, and density functional theory calculations, it is proved that the potassium storage of VN derives from a surface‐redox pseudocapacitive mechanism between VN and K+, rather than an intercalation or conversion reaction. As expected, the as‐assembled PICs based on the VN@CFs anode show an ultrahigh power output of 10.9 kW kg–1 when keeping an energy density of 49.2 Wh kg–1 and excellent capacity retention of 86.8% after 15000 cycles.

Efficient Removal of Chlorine Ions by Ultrafine Fe3C Nanoparticles Encapsulated in a Graphene/N-Doped Carbon Hybrid Electrode: Redox and Confinement Effect

“…Based on the above results, it can be speculated that the bulk phase of the Fe 3 C@GNC-700 electrode has not changed significantly due to the Cl − storage process relying on the surface redox pseudocapacitance of the Fe 2+ /Fe 3+ couple at or near the surface of the electrode in the first and possibly the first few dozen cycles. 66 While in the long-term cycle, the phase of the adsorbed electrode behaves as Fe 2 O 3 along with the continuous oxidation of Fe 0 in the bulk, and the valence conversion of the Fe 2+ /Fe 3+ redox couple continues to be maintained during the subsequent adsorption−desorption processes.…”

Confined Assembly of Hydrated Vanadium Oxide into Hollow Mesoporous Carbon Nanospheres for Fast and Stable K+ Storage Capability

“…The Ragone plots (Figure 6f) show that the O d -VOH@C//O d -VOH@C PIC shows outstanding energy output, which is com-parable or superior to some reported PICs. [9,28,35,48,54,[56][57][58][59] The long cycling stability is further investigated at 2.0 A g −1 . The charge/discharge curves are stable (Figure S17, Supporting Information), predicating the high cycling stability.…”

“…The voltage range is determined to be 0.01–3.8 V according to the CV curves of the half‐cells (Figure 6d), and the CV curve of O d ‐VOH@C//O d ‐VOH@C at 1.0 mV s −1 is approximately rectangular with weak redox peaks, showing highly hybrid capacitive characteristics. [ 9 ] Figure 6e displays the charge/discharge curves with the current ranging from 0.5 to 10.0 A g −1 , and the slightly unsymmetrical plots imply a combination of the Faradaic and non‐Faradaic mechanisms. [ 55 ] Moreover, O d ‐VOH@C//O d ‐VOH@C reaches an impressive energy density of 139.6 Wh kg −1 at a power density of 948.3 W kg −1 , which can maintain at 24.8 Wh kg −1 even at a high power density of 22 326.7 W kg −1 .…”

“…In addition, the pseudo‐capacitance behavior mainly occurs on the interfaces of the electrode, producing a fast reaction kinetics, which is comparable to that of electric double‐layer behavior. [ 9 ] Thus, the high‐rate K + storage capability is effectively promoted for the battery‐type electrode. Consequently, the ingenious structures including porous, hollow, core–shelled, and hierarchical structures with enriched interfacial boundaries have been designed to increase the pseudo‐capacitance contribution to the large‐sized K + ion storage as well as accommodate the volume effects.…”

Understanding Pseudocapacitance Mechanisms by Synchrotron X‐ray Analytical Techniques

“…[64] Surface redox pseudocapacitance refers to the process of storing charge on the surface of the electrode material through continuous and reversible redox reactions. [67] Typical examples include transition metal oxides (RuO 2 , MnO 2 ) and some conductive polymers (polyaniline and polypyrrole). [68,69] Intercalation pseudocapacitance, where charge storage occurs not only on the surface, but also in materials with 1D or 2D channels in the crystal structure.…”