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Page 6 of 13 Xiao et al. Energy Mater 2023;3:300007 https://dx.doi.org/10.20517/energymater.2022.84

-
2+
movement of the anion OH and prevents the passage of the cation Zn . The redox reactions occurring at
the air cathode and the Zn anode are shown below [59-61] :
Air cathode reaction:

O + 2H O + 4e → 4OH (1)
-
-
2
2
Zn anode reaction:
-
-
Zn + 2OH → ZnO + H O + 2e (2)
2
Overall reaction:

O + 2Zn → 2ZnO (3)
2

As the core part of the air electrode, the activity and durability of the catalyst are the most important
considerations in determining the performance of ZABs [62,63] . Although platinum (Pt) is considered to be
one of the best catalysts for ORR, its scarcity and high price hinder its commercial application. Similarly,
other noble metal catalysts such as iridium oxide (IrO ) and ruthenium oxide (RuO ) also face the same
2
2
dilemma as Pt-based catalysts [64-66] . To reduce cost, transition metal compounds have been reported as
promising ORR and OER electrocatalysts [65,67-69] . Although most of them do not perform as well as Pt-based
catalysts for ORR, their acceptable OER activity makes them better choices for ZABs.

Besides metal-based catalysts, carbon-based materials are also widely used as catalysts in ZABs. They
generally exhibit good chemical stability and electrical conductivity, which facilitates long-term electron
transport during electrochemical processes, while the porous structure with high specific surface area
facilitates the exposure of catalyst active sites and efficient mass transport [70-72] . Therefore, carbon-based
catalysts have gained a lot of attention for developing reversible ZABs. Unfortunately, carbon corrosion
inevitably occurs in alkaline electrolytes, and those catalysts are often prone to loss of structural stability,
leading to a widening of the voltage gap [72,73] . In contrast, metal oxide-based catalysts usually have good
durability, such as excellent durability of 600 h when using Co O nanowire arrays as catalysts
3
4
[Figure 4] [74-76] . Therefore, the rational design of composites of carbon materials and oxides has attracted a
lot of research attention when a pioneering work on N-doped CNT-supported LaNiO was developed for
3
[77]
ZABs .
Flow batteries
Zn-based redox flow batteries usually use Zn as the anode and redox pairs as electrodes, including Zn-Fe,
Zn-Ce, Zn-halogen (Cl , Br , and I ), and Zn-organic couples [Figure 5] [78-86] . Although substantial progress
2
2
2
has been made in Zn-based flow batteries, more efforts are still needed to improve some important
parameters to achieve the goal of commercialization. Current efforts mainly focus on optimization of the
electrolyte, membrane and electrode . This review focuses on the Zn-I flow battery as an example.
[87]
2
[85]
The typical structure of Zn-I is shown in Figure 5B . Basically, repeated Zn /Zn deposition/dissolution
2+
2
reactions occur on the negative side of Zn in acidic or alkaline electrolytes, while conversion reactions
-
between iodide anions (I ) and triiodide anions (I ) occur on the positive side. Despite the advantages of
-
3
I /I couples in terms of cost and water solubility, the low utilization of iodine originates from the formation
-
-
3
[88]
of insoluble I during charging and discharging severely hinders the application of Zn-I batteries .
2
2