A critical overview of the latest developments in the aluminum battery technologies is reported. The substitution of lithium with alternative metal anodes characterized by lower cost and higher abundance is nowadays one of the most widely explored paths to reduce the cost of electrochemical storage systems and enable long-term sustainability. Aluminum based secondary batteries could be a viable alternative to the present Li-ion technology because of their high volumetric capacity (8040 mAh cm(-3) for Al vs 2046 mAh cm(-3) for Li). Additionally, the low cost aluminum makes these batteries appealing for large-scale electrical energy storage. Here, we describe the evolution of the various aluminum systems, starting from those based on aqueous electrolytes to, in more details, those based on non-aqueous electrolytes. Particular attention has been dedicated to the latest development of electrolytic media characterized by low reactivity towards other cell components. The attention is then focused on electrode materials enabling the reversible aluminum intercalation-deintercalation process. Finally, we touch on the topic of high-capacity aluminum-sulfur batteries, attempting to forecast their chances to reach the status of practical energy storage systems.
Gas diffusion electrodes (GDE) composed of: Toray carbon paper gas diffusion layer (unteflonated and teflonated with 30%wt PTFE, respectively) and MnO 2 supported on Vulcan XC72R catalyst layer with 20%wt PTFE, were investigated in 6 M KOH without and with O 2 , respectively. Four sources of MnO 2 (Sigma-Aldrich, Tronox, Riedel and Merck Inc.) were comparatively studied by electrochemical methods accompanied by XRD characterization. Cyclic voltammetry scans of GDE in N 2 -purged electrolyte were used to estimate the Tafel slopes for Mn(IV) reduction and to identify active Mn(IV) sites that play an important catalytic role. Two oxygen reduction reaction (ORR) mechanisms were identified by porous rotating disk electrode (PRDE) polarization as a function of electrode potential. At high potentials (above −300 mV vs. MOE) the O 2 /HO 2 − step is mainly catalyzed by the quasi-unreduced MnO 2 surface and active sites on the carbon support, while at potentials more negative than −300 mV, the redox catalysis by Mn(III) prevails. The main catalytic sites for the second step HO 2 − /OH − , are Mn(III) sites. The hydrophobic property of the porous electrode is important due to the effect on peroxide desorption/readsorption on the electrode surface. PRDE and flow cell experiments revealed the Sigma-Aldrich γ-MnO 2 was the most active for ORR.
The stability of bifunctional oxygen electrodes used in electrically rechargeable zinc/air batteries was investigated as a function of the concentration in the feed gas. Using pure oxygen as feed gas, the oxygen CO 2 evolving electrode was life limiting being stable up to 2500 h. However, by increasing the in CO 2 -concentration synthetic air up to 10 000 ppm, the life limiting electrode is changed to the oxygen reducing electrode and the lifetime is reduced due to poisoning by carbonate precipitation inside the pores of the gas di †usion electrode. Furthermore, the Ðltering efficiency of several alkaline Ðlter materials has been tested. It has been found CO 2 that the Ðlter capacity was strongly dependent on the humidity ratio of the feed gas. 9.2 g of LiOHÈCa(OH) 2 mixture were able to Ðlter 2500 l ambient air (90% RH, 409 ppm with a rest concentration of less than 20 CO 2 ) ppm.
A new type of zinc/air fuel cell comprising a Hg/Pb free Zn foam anode, a PVA/KOH electrolyte membrane and a MnO 2 /SiOCbased cathode was developed in this work. The electrochemical activity of the zinc foam and air electrode was investigated in 6 M KOH under half-cell conditions. The pristine ZnO layer of the foam matrix favoured direct oxidation of the zinc particles to zinc oxide in 6 M KOH. In the laboratory cell, a specific energy of about 500 mWh g -1 zinc was measured at 5 mA discharging current with a zinc foam, a PVA/KOH/H 2 O membrane and a MnO 2 +Vulcan/carbon paper cathode. A correlation between cell performances and porosity of the zinc foam was found. However, stability of the Zn foam and SiOC GDL materials towards KOH should be improved.
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