Electrochemical Capacitors with Confined Redox Electrolytes and Porous Electrodes

Abstract

Electrochemical capacitors (ECs), including electrical‐double‐layer capacitors and pseudocapacitors, feature high power densities but low energy densities. To improve the energy densities of ECs, redox electrolyte‐enhanced ECs (R‐ECs) or supercapbatteries are designed through employing confined soluble redox electrolytes and porous electrodes. In R‐ECs the energy storage is based on diffusion‐controlled faradaic processes of confined redox electrolytes at the surface of a porous electrode, which thus take the merits of high power densities of ECs and high energy densities of batteries. In the past few years, there has been great progress in the development of this energy storage technology, particularly in the design and synthesis of novel redox electrolytes and porous electrodes, as well as the configurations of new devices. Herein, a full‐screen picture of the fundamentals and the state‐of‐art progress of R‐ECs are given together with a discussion and outlines about the challenges and future perspectives of R‐ECs. The strategies to improve the performance of R‐ECs are highlighted from the aspects of their capacitances and capacitance retention, power densities, and energy densities. The insight into the philosophies behind these strategies will be favorable to promote the R‐EC technology toward practical applications of supercapacitors in different fields.

Coupling W18O49/Ti3C2Tx MXene Pseudocapacitive Electrodes with Redox Electrolytes to Construct High‐Performance Asymmetric Supercapacitors

“…It has been reported recently that coupling redox electrolytes with a porous capacitor electrode is one of the most efficient ways to alter capacitive performance of a capacitor electrodes. [46][47][48][49][50] For example, redox couples of small-sized bromides have high redox potentials, and their redox reactions are reversible. As a case study, FeBr 3 was selected as the redox species and dissolved in 1 m H 2 SO 4 solution.…”

Rational Design of Diamond Electrodes

“…Further matching of redox electrolytes (e.g., type, amount) with diamond electrodes (e.g., size and density of pores/channels) is expected to construct diamond supercabatteries with even higher performance. 56…”

“…Especially, the E g and E v values of diamond R-SCs are higher than those of most SCs, while their P g and P v values are bigger than those of batteries. , Such R-SCs were also named battery-like SCs or supercabatteries, − ,, of which high charge storage capacity was explained by fast faradaic reactions of redox species occurring at diamond surface or confined redox species inside diamond electrodes. Further matching of redox electrolytes (e.g., type, amount) with diamond electrodes (e.g., size and density of pores/channels) is expected to construct diamond supercabatteries with even higher performance …”

Low-Grade Thermal Energy Harvesting and Self-Powered Sensing Based on Thermogalvanic Hydrogels

“…In recent years, significant progress has been made in liquid thermogalvanics by improving electrodes, electrolytes, and redox species [ 30 , 31 , 32 , 33 , 34 ]. In particular, studies on quasi-solid thermogalvanics have eliminated the leakage risk of liquid electrolytes by introducing physically crosslinked networks [ 15 ].…”

“…Inexpensive and environmentally friendly electrode and electrolyte materials form simple device structures, as well as excellent thermoelectric power performance [ 30 , 31 , 32 ]. As shown in Figure 1 , the number of publications and citations using hydrogels to harvest and sense thermal energy has increased from 2000 to 2022 [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , ...…”