Aqueous electrolyte metal/air batteries

We discuss below recent progress made in aqueous electrolyte metal/air batteries, non-aqueous electrolyte Li/air batteries and Li/water batteries. These areas have advanced more than the others in recent years. 

Unlike the PEMFCs, the ORR at the air cathode of the aqueous electrolyte metal/air batteries occurs in alkaline environment. A 4-electron pathway is usually observed for highly active catalysts such as Pt, Pd, Rh, Au that are capable of breaking the 0—0 bond, while a 2-electron pathway is observed for low-active catalysts (such as carbon). 

A reaction at the anode of aqueous electrolyte metal/air battery is a reaction of metal oxidation accompanied by metal oxide deposition at the battery s anode. 

FIGURE 4.14 Models of the reaction zones for the ORR. (a) a "triple phase boundary" for aqueous electrolyte metal-air battery and PEMFC and (b) a "two-phase boimdary" for non-aqueous electrolyte Li-air battery. Reprinted from Ref. [170], with permission from Elsevier. 

A rechargeable Li-air battery requires that the ORR and OER be highly reversible. In the aqueous electrolyte, the ORR products (H2O2 and H2O as shown in Eqs. 1-3) are miscible (soluble) with the electrolyte solution, which makes the OER reversible. The aqueous electrolyte Li-air battery can share the same catalyst as those used in the alkaline fuel cells and metal-air batteries, which have been intensively... 

As shown by Eqs. 1 and 5, the ORRs in the aqueous alkaline electrolyte and in the non-aqueous electrolyte share the same two-electron reduction. This means that all catalysts showing the catalytic activity towards the ORR in alkaline fuel cells and metal-air batteries are theoretically suitable for the non-aqueous electrolyte Li-air batteries. Base on the chemical composition of the materials, these catalysts can be briefly classified into the following categories (1) porous carbon and doped carbon materials, (2) transition metal oxides, nitrides, and sulfides, (3) marcocyclic transition metal complexes, (4) non-precious metals and alloys, (5) precious metals and alloys, (6) organic redox mediators. 

Zhang SS, Ren XM, Read J (2011) Heat-treated metal phthalocyanine complex as an oxygen reduction catalyst for non-aqueous electrolyte Li/air batteries. Electrochim Acta 56(12) 4544-4548... 

A number of cells have been developed which make use of the oxygen of the air as the cathodic reagant. These so-called air-depolarized cells are examples of hybrid cells, which are discussed more fully in Chapter 9. Many recent advances in metal-air batteries can be attributed to the research carried out in the 1960s on high current density air electrodes for ambient hydrogen/oxygen fuel cells using aqueous electrolytes. 

Mechanically Rechargeable Batteries. To avoid the time required for electric recharge, the problems of in silu electric recharge, or to utilize anodes that are not electrically rechargeable in aqueous electrolytes, mechanically rechargeable batteries have been studied. These systems arc metal-air couples. Ihe anodes that have received attention arc zinc, lithium, and aluminum. 

With regard to France, in addition to institutional laboratories, EDF has coordinated a National Research Agency project - Z/O. Faisabilite d um batterie Lithium-air (Feasibility of a Lithium-air battery) (project director Ph. Stevens) devoted to the development of a complete aqueous-electrolyte lithium-air cell with all its components. The metal lithium is protected by a first surface layer of LIPON (lithium phosphoms oxynitride) and then by a ceramic membrane of LISICON (Li Super Ionic Conductor LiM2(P04)3 with M=Ti, Zr, Ge, Hf). The declared performances are 100 mAh/cm (sufficient to deliver 500 Wh/kg) for current strength of 2 mA/cm at 30 C, and up to 6 mA/cm at 60°C. The lifetime is 38 cycles with a current strength of 10 mAh/cm. ... 

Wang ZL, Xu D, Xu JJ, Zhang XB (2014) Oxygen electtocatalysts in metal-air batteries from aqueous to nonaqueous electrolytes. Chem Soc Rev 43 7746-7786... 

Most of the metal-air batteries use neutral or alkaline aqueous electrolyte solutions and the oxygen reduction half-cell reaction at the air cathode is ... 

Most of the work in metal-air batteries has been done in aqueous electrolytes that are not suitable for Li-air batteries. Li exothermically reacts with water which prohibits using aqueous... 

In contrast to most other batteries, which must carry both an anode and a cathode inside a storage system, metal-air batteries are unique in that the active cathode material (oxygen) is not stored in the battery. Instead, oxygen can be absorbed from the environment and reduced by catalytic surfaces inside the air electrode. Most metal-air batteries use an aqueous-based electrolyte such as concentrated potassium hydroxide. The typical reaction in a metal-air battery using aqueous-based electrolytes can be expressed by Equation 22.1... 

Figure 22.1 Schematic of reaction processes in metal-air batteries, (a) A battery that uses an aqueous-based electrolyte, where M represents metals (e g., Zn, Al, Mg, Fe, Ca). Hydroxide (OH ) is the ion carrier transferred...

The theoretical data [1] shows that Li and Ca possess very high energy density (13172 and 4560 Ah/kg respectively) but these metals are not suitable to be used as anodes because of their instability in aqueous electrolytes. The theoretical energy densities of Mg and A1 are also high (6846 Wh/kg and 8212 Wh/kg). It is shown that some alloys of Mg and A1 can be successfully used as anodes, especially in metal-air cells with neutral electrolytes. The theoretical energy density of Zn is much lower than that of Li and Ca, but the self-discharge of Zn can be effectively suppressed by the use of suitable inhibitors. That s why the zinc-air batteries with KOH electrolyte are the first metal-air system brought into service. 

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