Boosting the Energy Density of Aqueous Batteries via Facile Grotthuss Proton Transport

Abstract

The recent developments in rechargeable aqueous batteries have witnessed a burgeoning interest in the mechanism of proton transport in the cathode materials. Herein, for the first time, we report the Grotthuss proton transport mechanism in α‐MnO2 which features wide [2×2] tunnels. Exemplified by the substitution doping of Ni (≈5 at.%) in α‐MnO2 that increases the energy density of the electrode by ≈25 %, we reveal a close link between the tetragonal‐orthorhombic (TO) distortion of the lattice and the diffusion kinetics of protons in the tunnels. Experimental and theoretical results verify that Ni dopants can exacerbate the TO distortion during discharge, thereby facilitating the hydrogen bond formation in bulk α‐MnO2. The isolated direct hopping mode of proton transport is switched to a facile concerted mode, which involves the formation and concomitant cleavage of O−H bonds in a proton array, namely via Grotthuss proton transport mechanism. Our study provides important insight towards the understanding of proton transport in MnO2 and can serve as a model for the compositional design of cathode materials for rechargeable aqueous batteries.

Oxygen-Deficient β-MnO2@Graphene Oxide Cathode for High-Rate and Long-Life Aqueous Zinc Ion Batteries

“…After discharge (i.e., at point #B in the first cycle, and point #H in the second cycle), two diffraction peaks at 16.35° and 33.95° emerge, corresponding well to the monoclinic MnOOH (orthorhombic, Pnma (62), PDF #88-0648), a typical product of proton conversion in MnO 2 [ 40 , 41 ]. Meanwhile, zinc sulfate hydroxide hydrate by-product (Zn 4 (OH) 6 ·ZnSO 4 ·xH 2 O, abbreviated as “ZSH,” PDF #44-0673) is generated upon discharge, featuring a set of strong diffraction peaks located at 8.12°, 15.08°, 21.56°, and 24.57°, which is a critical evidence for proton intercalation into the lattice framework of MnO 2 [ 20 ]. The presence of ZSH on the electrode can be further confirmed by the morphology evolutions of β-MnO 2 @GO electrodes, (Figs.…”

“…Toward this goal, researchers have adopted various technologies, including pre-intercalation engineering [ 15 ], defect engineering [ 16 , 17 ], interfacial optimization [ 18 , 19 ], and metal-doping [ 20 ], etc. Especially, the incorporation of oxygen vacancies (V O ) is an effective route to improve the rate performance of MnO 2 electrodes.…”

Non‐Metal Ion Co‐Insertion Chemistry in Aqueous Zn/MnO2 Batteries

“…Owing to the bond competition between Mn−O and O−H bonds, the electron density of oxygen atoms in Mn−O bonds is increased (Figure 1 c). [12] As a result, the hydrogen bonds between MnO 2 and NH 4 + are enhanced after NH 4 + insertion, which will reinforce the stabilization of distorted Mn 3+ O 6 octahedra in MnO 2 . Therefore, the co‐insertion of H + /NH 4 + ions in MnO 2 would enhance the electrochemical performance of aqueous batteries.…”

“…In the cases of aqueous batteries with NH 4 + ions, the insertion of NH 4 + ions will be beneficial to the structural stability of host materials due to the formation of hydrogen bonds between the inserted NH 4 + ions and the framework of host materials, resulting in the enhanced cycling performance of the batteries [11] . In the aqueous zinc batteries, the active sites of various charge carriers are often varied in the host materials [12] . As a result, H + ions can insert into the host materials accompanied by the insertion of Zn 2+ ions, realizing the co‐insertion of dual cations in the aqueous zinc batteries [8b, 13] .…”

Grotthuss Proton‐Conductive Covalent Organic Frameworks for Efficient Proton Pseudocapacitors

“…Although much progress has been made for COF‐based supercapacitors, it is still challenging to achieve both high energy density and high power density that is related to the transport of ion charge carriers in the electrode. A recent study has shown that proton Grotthuss transfer in inorganic complexes can facilitate the electrode's redox reaction to enable a high rate capability and energy density for electrochemical devices [39, 40] . But, such a discovery has been not reported for organic materials.…”