Aluminum–air (Al-air) batteries are promising electrochemical storage systems, because of their practicality, low flammability and no risk of explosion, eco-compatibility, and high energy density. ...However, Al-air batteries realized with liquid electrolytes may present safety concerns, because of leakage, and are unsuitable for miniaturized and portable electronic devices. The transition of the electrolytes for Al-air batteries from aqueous to solid form is an inevitable trend for safety, stability, and possible widespread use. Nevertheless, the manufacturing of new solid electrolytes for Al-air batteries is still a new and largely unexplored field. In fact, the most significant results appeared only in the last decade, with the development of new polymer electrolytes. In this Review, we summarize the most significant findings on polymer electrolytes used in Al-air batteries, particularly emphasizing the cell electrochemical performances and how these relate to the characteristics of the electrolyte.
The influence of formic acid on water cluster aggregation has been investigated experimentally by mass spectrometry and tunable UV laser ionization applied to Na‐doped clusters formed in the ...supersonic expansion of water vapors seeded with formic acid (FA) as well as theoretically using high level quantum chemistry methods. The mass spectra of Na−FA(H2O)n clusters show an enlarging of mass distribution toward heavier clusters with respect to the Na−(H2O)n clusters, suggesting similar mass distribution in neutral clusters and an influence of formic acid in water aggregation. Density functional theory and coupled‐cluster type (DLPNO‐CCSD(T)) calculations have been used to calculate structures and energetics of neutral and ionized Na−FA(H2O)n as well as neutral FA(H2O)n. Na‐doped clusters are characterized by very stable geometries. The theoretical adiabatic ionization potential values match pretty well the measured appearance energies and the calculated first six electronic excited states show Rydberg‐type characters, indicating possible autoionization contributions in the mass spectra. Finally, theoretical calculations on neutral FA(H2O)n clusters show the possibility of similarly stable structures in small clusters containing up to n=4–5 water molecules, where FA interacts significantly with waters. This suggests that FA can compete with water molecules in the starting stage of the aggregation process, by forming stable nucleation seed.
Cluster formation: Water aggregation in presence of formic acid at low concentration was investigated by mass spectrometry/photoionization spectroscopy. Experimental data show the formation of larger clusters in the presence of acid molecules. DFT and coupled‐cluster type (DLPNO‐CCSD(T)) calculations support experimental results and suggest that water molecules aggregate preferentially with acid molecule but mainly in the first stage of cluster formation.
The incorporation of aluminum anodes in metal-air galvanic cells is very interesting due to its intrinsic physico-chemical properties that include a high theoretical specific capacity (2.98 Ah g
−1
) ...compared to lithium (3.86 Ah g
−1
), low density, negative standard potential versus the standard hydrogen electrode (SHE), high abundance in the earth's crust, recyclability, and environmental friendliness. In this report, various hydrogels based on xanthan are described that were prepared at different pH values in order to produce solid electrolytes characterized by high ionic conductivity. These electrolytes have been characterized in aluminum-air galvanic cells using Pt/C based air cathodes. The performance of the cells was characterized by discharge tests performed at constant currents across the range from 3 to 6 mA/cm
2
. The ability of recharging the cells was evaluated by determining the electrolyte electrochemical window using cyclic voltammetry (CV) measurements. The ionic conductivity of the hydrogels was determined by electrochemical impedance spectroscopy (EIS). The results evidenced the superior performance of acidic hydrogels in terms of cell capacity and anodic efficiency, due to the capability of xanthan as self-corrosion inhibitor, but the rechargeability was hindered because of the current consuming hydrogen reduction. A larger electrochemical window was observed for neutral hydrogels, but with low cell voltage, and by alkaline electrolytes, but at the expense of the anodic efficiency.
Different commercial carbonaceous materials, two made of activated carbons and one of multiwalled nanotubes, were used to prepare cathodes for primary aluminum-air cells and compared with the more ...expensive platinum-dispersed carbon, usually used as cathode for many types of metal-air cells. The aluminum-air cells used in the electrochemical tests were assembled with alkaline gel polymer electrolytes without any separator. Cells with cathodes made of a cheap activated carbon showed better electrochemical performances than those obtained with platinum-based cathodes. Notably, their discharge capacities were improved and the discharge voltages were always higher than 0.2 V. These improved performances were mainly attributed to the better electrocatalytic activity of the activated carbon as it results from polarization measurements, probably due to the presence of defects, as evidenced from Raman spectra. Three-electrode discharge tests were used to measure the electrode potentials and their impact to the overall cell electrochemical performances. During the discharge, in all cases, an increase of the anodic potential towards more positive values was observed, while the cathodic potential remained almost constant. Thus, the final failure of the cells was mainly due to the degradation of the anodic interface. This indicates the possibility to further increase the cell capacity by adopting suitable mitigation strategies of anodic parasitic reaction or different electrolyte design, with the final aim to realize efficient, cheap, and eco-friendly aluminum-air cells.
Equilibrium geometries and thermodynamic potentials of the neutral and ionised species of the
$ \hbox {Na}\cdots (\hbox {H}_{2}\hbox {O})_{{n}} $
Na
⋯
(
H
2
O
)
n
(
$ {n}=1,..,8 $
n
=
1
,
.
.
,
8
) ...mixed clusters were computed at MN15/def2-TZVPD level of density functional theory (DFT). The vertical and adiabatic ionisation energies and enthalpies were computed and their cluster size dependence was discussed. Laser-induced ionisation involves electronic excitation through Rydberg-type excited states, which have been characterised using the TDDFT method, including the ωB2PLYP double-hybrid exchange-correlation functional. Ab initio molecular dynamics calculations were performed on a time scale of 20 picoseconds. Fluctuations of the charge and the sodium-oxygen atomic distances predict that, the
$ 3\hbox {s}^{1} $
3
s
1
electron of the sodium atom are transferred from the delocalised Rydberg orbitals to the Rydberg orbitals around the water molecules and the sodium atom becomes positively charged with around 0.6e after the first
$ 10\,\hbox {ps} $
10
ps
. On the other hand, some of the water molecules can move away up to 5 Å from the sodium with a significant negative charge on them. It has been shown that non-radiative relaxation cannot be excluded, they can mostly occur for cases
$ n \geqslant 4 $
n
⩾
4
. The results confirm that the adiabatic photo-ionisation can occur on the basis of cluster disintegration.
The feasibility to recharge aluminum‐air cells realized with a dual water‐based electrolyte without separator is demonstrated. The dual electrolyte, made of polyvinyl alcohol and Xanthan gum, has a ...different water content, lower at the anodic interface, where parasitic reactions involving hydrogen production play a crucial role in hindering metal re‐deposition, and higher at the cathode side, where water, depending on the pH, allows an efficient reduction of oxygen during discharge or the oxygen evolution during cell charging. The galvanostatic cycles show in the first discharge and charge cycles, the characteristic plateau trends of secondary batteries. By electrochemical impedance spectroscopy analysis effected after each discharge/charge phase and by measuring anode and cathode potentials during cycling, it is demonstrated that, during the first cycles, the re‐deposition of aluminum is possible by adopting adequate water management in the electrolyte, while the cell malfunctioning in the subsequent cycles is mainly due to the damage of the cathode.
A dual solid electrolyte, prepared frombiodegradable materials and aqueous solutions, is used in aluminum‐air batteries to verify the possibility of recharging, by maintaining minimum water content on the anode and increasing it on the cathode side. The three‐electrode cycling demonstrates that the batteries can be recharged through suitable water management inside the electrolytes.
The processes involved in the photoionization of sodium‐doped clusters are complex, not fully understood for many systems and still strongly debated, especially because of the discrepancy between ...experimental results and predicted cluster structures. We have performed a study on sodium doped formic acid clusters based on UV photoionization spectroscopy and DFT/TDDFT calculations. Apart from the monomer, all the predicted structures show vertical ionization potential values higher than those obtained by the photoionization measurements. We have calculated the absorption spectra and found many Rydberg‐like states near the adiabatic ionization potentials and, crucially, in the UV range where the clusters appearance energies fall. This finding supports the hypothesis of adiabatic contributions in the measured ionization potentials for these clusters.
Low ionization threshold: Why can a cluster be photoionized although its ionization potential is much higher than the ionizing energy?
The Cover Feature shows the natural difference orbital profiles of the first electronic excited states in the UV wavelength range of Na⋅⋅⋅HCOOH⋅⋅⋅(H2O)n clusters. The existence of these Rydberg–type ...electronic transitions could explain the cluster ionization at laser energy lower than their vertical ionization potential, enabling the correct data interpretation on acid‐water cluster formation. More information can be found in the Research Article by Tonia M. Di Palma and co‐workers.
Al‐air batteries assembled with water‐based solid electrolytes can be recharged. As illustrated on the cover, this is possible thanks to the combined use of two solid electrolytes containing a ...different amount of water and contacted with anode and cathode without separators. A valuable feature of these electrolytes is that they are prepared with biodegradable polymers. More details can be found in article number 2101046, Maria F. Gaele and Tonia M. Di Palma.
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