Effect of gadolinium trioxide on anode performance of aluminum-air batteries

The self-corrosion rate was analyzed by polarization, impedance, and constant current discharge tests; the anode energy density was calculated by continuous constant current discharge at a current density of 10–180 mA/cm2, and the surface morphology of the electrode was analyzed by scanning electron microscope (SEM) to study the effect of Gd2O3 additives on the aluminum anode performance of aluminum-air batteries in alkaline solution. The positive corrosion potential shift of the polarization curve, the decrease of corrosion current, the decrease of R1 (charge transfer internal resistance) in impedance test, the decrease of hydrogen evolution rate, and the decrease of pitting pits in surface topography analysis showed that the addition of Gd2O3 particles to the pure aluminum anode affected the charge transfer on the anode surface, improved its corrosion resistance, and inhibited the occurrence of hydrogen evolution reaction. The addition of Gd2O3 to pure aluminum has a positive effect on the performance of the battery anode material, which makes the corrosion resistance of the anode material significantly improved, and the discharge is stable and uniform, which is more suitable as an anode material for aluminum-air batteries.


Introduction
Among metal-air batteries, aluminum is a promising anode due to its lightweight, high theoretical voltage, and high theoretical energy density (8100•Wh•kg −1 ) [1].In addition, aluminum is low-cost, environmentally friendly, and recyclable [2][3][4][5].However, one of the main problems that aluminum-air batteries are difficult to use in batteries is the parasitic autocorrosion reaction, that is, the hydrogen evolution reaction [6].The parasitic chemical reaction occurs between the aluminum anode and the electrolyte and is more severe under alkaline electrolyte conditions.The self-corrosion reaction is harmful to aluminum-air batteries, which will cause battery capacity loss and reduce discharge efficiency [7][8][9].Efforts have been made on many fronts to overcome this problem: (1) Select the appropriate aluminum alloy anode electrode [10][11][12], and study some candidate-alloying elements [13], including In, Zn, Sn, Ga, Mg, and Mn.Indium (In) has been shown to shift the anode potential in a positive direction and enhance the hydrogen evolution overpotential.Tin (Sn) increases the dissolution rate of aluminum in an aqueous solution and reduces the corrosion rate of aluminum [14].In addition, zinc (Zn) is known to reduce hydrogen evolution by enhancing the hydrogen evolution reaction (HER) potential; (2) control the parasitic reaction by adding to the electrolyte.In recent years, research has mainly focused on the development of economical and non-toxic corrosion inhibitors [15,16]; (3) coating and ceramic aluminum anode [17,18]; (4) application of gel electrolyte [19]; and (5) use non-aqueous electrolytes [20].But these efforts have had little effect while increasing the complexity of the battery.
At present, to effectively solve the problem of corrosion, the oxidation of aluminum should occur as much as possible in non-aqueous electrolytes, but in non-aqueous electrolytes, the oxygen reduction reaction of the cathode is inhibited.Therefore, a dual electrolyte system that separates the anode and cathode liquid can be used by an anion exchange membrane, which can achieve the flexibility of providing different anode and cathode fluids, and by applying a suitable anode solution, the HER of the anode can be significantly suppressed, resulting in a high-performance battery [21][22][23][24][25][26].
Gadolinium oxide is a metal oxide composed of metal gadolinium and oxides.It is a white powder, colorless, and odorless, with strong resistance to acid and alkali corrosion.It is widely used in catalysis, ceramics, glass industry, solid oxide fuel cells, luminescent materials, polishing materials, and nuclear materials.In battery materials, using gadolinium oxide as the main component of solid fuel cells can significantly increase the stability and corrosion resistance of the battery.In response to the issue of the metal aluminum anode of aluminum-air batteries being prone to corrosion in alkaline solutions, an attempt was made to doping gadolinium oxide in the metal aluminum anode in order to improve its corrosion resistance.

Experimental and characterization analysis
The main steps of the experiment are melting and casting of the anode plate, production of electrode sheets, preparation of electrolyte, electrochemical test, assembly of battery box for discharge test, and corrosion rate test.

Preparation of aluminum anode
The production process of aluminum alloy anode is very rigorous; in the room temperature environment, use an intermediate frequency furnace to preheat the graphite crucible at 200 °C for 5 min, evaporate the water in the crucible, weigh the weighed aluminum block with a purity of 99.99% into the crucible, heat to 760 °C until completely melted, and add a certain amount of metal oxide ( Gd 2 O 3 , purity of 99.9%) wrapped in aluminum foil (included in the weight of aluminum); after it is melted, stir it evenly with graphite rods, and then keep it warm.Immediately after 10 min, the melt in the graphite crucible is poured into the preheated graphite mold, cast, and naturally cooled to room temperature to obtain a rectangular aluminum alloy plate with a thickness of 10 mm.Then, the aluminum anode plate is cold-rolled seven times by a gong press with a total shape change of 80% to make a 2-mm thick plate, and then annealed in a box-type resistance furnace under the condition of 400 °C for 1 h, and then cooled to room temperature with the furnace.The aluminum plate is cut into 1 cm×2 cm sizes with a wire cutting machine and sanded on the grinding machine to remove the surface oxide layer, and after the grinding is completed, the hole is punched, the copper wire is fixed, and the epoxy resin is sealed to make an electrode sheet with an effective working area of 1 cm 2 [27][28][29][30].The prepared electrode sheet is placed in an incubator, treated at a constant temperature at a certain temperature, cooled to room temperature, removed, and placed in a drying room to dry.Then, it is washed sequentially with distilled water, alcohol, and acetone, and wiped with filter paper before the electrochemical test can be performed.

Electrochemical performance test
All electrochemical performance tests were carried out using Metrohm's electrochemical workstation (PGSTAT204) using a three-electrode electrochemical test system consisting of a 1.5 cm× 1.5 cm platinum electrode as the auxiliary electrode, saturated Hg/HgO (1 M KOH) electrode as a reference electrode, and a 1 cm×1 cm working electrode (pure aluminum anode and composite anode); and the electrolyte, working electrode, auxiliary electrode, and reference electrode are assembled in an electrolytic cell.It is then connected to the electrochemical workstation.The electrolyte is 4 mol•L −1 NaOH, open potential, Nyquist curve, and polarization curve measurements for assembled three-electrode systems.The impedance test range is 0.1~10 5 Hz, the polarization curve voltage range is −2~1 V, the scanning speed: is 1 mV/s, and the data fitting software is Metrohm Nova 2.13.

Battery discharge performance test
An aluminum anode, special air electrode, and 4 mol• L -1 NaOH solution are assembled into a full battery using a special battery box to test the discharge performance of the battery.The aluminum anode has a working area of 1 cm 2 , and the cathode has an effective working area of 9 cm 2 .The assembled full battery was connected according to the above figure, and the blue power test system CT2001 (Wuhan Blue Power Electronics Co., Ltd.) was used to carry out a constant current discharge test for 1 h under the conditions of a current density of 10 mA/cm 2 , 20 mA/cm 2 , 40 mA/cm 2 , 60 mA/cm 2 , 80 mA/ cm 2 , 100 mA/cm 2 , 120 mA/cm 2 , 140 mA/cm 2 , 160 mA/cm 2 , and 180 mA/cm 2 .Measurement of discharge voltage, anode mass loss at room temperature, and calculation of constant current discharge energy density, such as Eq.(1).
where w is the energy density (wh/kg); j is the current density (mA/cm 2 ); W 0 is the lost mass (g); U is the discharge voltage (V) corresponding to the current density; S is the active area of the anode plate (cm 2 ); t is the discharge time (min).

Self-corrosion rate test
The composite aluminum anode plate is cut into 1 cm×1 cm samples, sanded (360#-600#-1000#-1500#-2000#) and polished (water throwing) samples with a grinder, cleaned with distilled water, alcohol, acetone, and finally dried.After measuring the mass number of each specimen, the aluminum sample is placed into a drainage gas collection device (consisting of an Erlenmeyer flask, air duct, sink, and an inverted measuring cylinder of appropriate size).Put 4 mol•L −1 NaOH solution which is collected by drainage gas collection method, note that the liquid amount in each Erlenmeyer flask should be consistent and as sufficient as possible; when the sample corrodes to 2 h, record the amount of hydrogen precipitation, take out the aluminum sample, clean, blow dry, and finally weigh.
The average self-corrosion rate and hydrogen evolution corrosion rate of the sample is calculated [31], and the calculation formula is shown as Eqs.( 2) and ( 3).
where VS is the average self-corrosion rate (g•cm -2 •h -1 ); v is the hydrogen evolution corrosion rate (mL•cm -2 •h -1 ); m 0 is the original weight of the specimen (g); m 1 is the weight after corrosion (g); S is the surface area of the specimen (cm 2 ); t is the immersion time (h), V is the volume of hydrogen evolution (mL).

Anode microstructure and pre-and post-discharge morphology test
Scanning electron microscopy (SEM, HitachiTM3000) was used to analyze the morphology of the composite before and after the anode discharge.Before discharging, the anode is sanded with (600#-1000#-1500#-2000#-3000#) sandpaper, and the discharged anode has been treated when weighing.

Analysis of anodic polarization curve of Al-x Gd 2 O 3 composites
The polarization curve of composite anodes with different contents of Gd 2 O 3 particles was tested to explore the polarization performance of composite anodes.Figure 1 is the Tafel polarization curve of the Al-x Gd 2 O 3 composite anode, and the corrosion potential ecorr, corrosion current density jcorr, and polarization resistance in Table 1 are obtained by fitting the Tafel region of the polarization curve by extrapolation.
(2) vs = m 0 -m 1 ∕St It can be seen from Fig. 1 that the corrosion potential of the composite anode with Gd 2 O 3 added to pure Al is forward shifted compared with the corrosion potential of the pure Al anode, where the corrosion potential of the pure Al anode is −1.7227V; the corrosion potential of the Al-1.0Gd 2 O 3 anode is −1.3859V, and the Al-1.0Gd 2 O 3 composite anode is the smallest positive shift in this series of anodes, and the minimum positive shift potential is 0.3368 V.The positive shift of the corrosion potential of Gd 2 O 3 particles in pure Al indicates that the addition of Gd 2 O 3 composites leads to anodic polarization.The occurrence of anodic polarization makes the composite anode more resistant to corrosion in the electrolyte.At the same time, it can be seen from Table 1 that the corrosion current density of Al-x Gd 2 O 3 composite anodes is very small and smaller than that of pure Al anodes, and the current corrosion density of these four composite anodes is very close.The corrosion rate corresponds to the corrosion current density, and the corrosion current density and corrosion rate of the Al-x Gd 2 O 3 composite anode are lower than that of the pure Al anode, which means that the  corrosion resistance of the Al-x Gd 2 O 3 composite anode in alkaline solution has been improved, which is consistent with the corrosion rate, and the self-corrosion of the anode has been suppressed.

Analysis of anodic electrochemical impedance spectroscopy of Al-x Gd 2 O 3 composites
The EIS results of Al-x Gd 2 O 3 composite anode in 4 mol/L NaOH electrolyte are presented in the form of a Nyquist plot as shown in Fig. 2. Figure 3 shows the electrochemical impedance spectroscopy equivalent circuit of the anode.The Nyquist plot parameters of the equivalent circuit are shown in Table 2.As can be seen from Table 2 and Fig. 8, the value of χ 2 is relatively small, indicating that the fitted data is in good agreement with the experimental data.where Rs is the solution resistance; R 1 stands for charge transfer resistance, which determines the difficulty of charge transfer; R 2 is the electrochemical diffusion resistance, which represents the diffusion resistance of the corrosive medium between the films; Q From Fig. 2, it can be seen that the diameter size of the high-frequency capacitor semicircle of pure aluminum is much smaller than that of the Al-x Gd 2 O 3 composite anode; in combination with Table 2, the first high-frequency capacitor loop corresponds to R 1 and Q 1 , and the value of R 1 is much larger than the value of Q 1 and is not on the same order of magnitude; it can be concluded that the diameter of the high-frequency capacitor semicircle is mainly determined by the size of R 1 .The addition of Gd 2 O 3 to pure aluminum affected the surface charge transfer of the composite anode, which improved the corrosion resistance of the composite anode, which was consistent with the positive shift of the polarization curve potential.First of all, the reaction preferentially occurs at the Al interface; Al(OH) 3 is generated in the pores and is easy to depose at the interface; Gd 2 O 3 particles form discontinuous points under the passivation film on the surface of the distribution anode so that Al(OH) 3 is stably attached to the point formed by Gd 2 O 3 and the surface, but the adsorption force of different oxides on the surface of the Al anode is different, which will lead to different shedding times for Gd 2 O 3 particles and the production of Al(OH) 3 .The adsorption force of the Al anode surface to Gd 2 O 3 particles is very poor, and it falls off in a short time.It can be seen from Table 2 that the resistance of the Rs solution of the Al-xGd 2 O 3 composite anode is greater than that of pure aluminum, because the Gd 2 O 3 particles are distributed on the surface of the anode and form voids, and react with the electrolyte on the surface of the Al-x Gd 2 O 3 composite anode; at the same time, the Gd 2 O 3 particles form more and more voids on the anode surface, resulting in Gd 2 O 3 particles and the resulting Al(OH) 3 falling off into the electrolyte to increase the solution resistance Rs.Thus, the reaction between the Al surface and the electrolyte is inhibited, the electrochemical activity of the anode is reduced, and the corrosion resistance is improved.The results show that the addition of Gd 2 O 3 improves the electrochemical activity of the anode.From Nyquist, Fig. 6 and parameter Table 2, the corrosion resistance of the five anodes in alkaline solution is Al-0.5 Gd 2 O 3 >Al-1.0Gd 2 O 3 >Al-1.5Gd 2 O 3 >Al-2.0Gd 2 O 3 >Al, which is completely consistent with the corrosion resistance shown in the polarization curve.

Analysis of anodic discharge performance of Al-x Gd 2 O 3 composite materials
Figure 4 is the discharge curve of the Al-x Gd 2 O 3 composite anode plate at a current density of 10 mA•cm −2 .Table 3 shows the average discharge voltage of the Al-x Gd 2 O 3 composite anode at a current density of 10 mA•cm −2 .As shown by the discharge voltage curve of Fig. 4, pure aluminum suddenly drops at the beginning of the current and then gradually rises until it stabilizes.After some time, as the current density increases and time passes, the battery will slowly rise back to its original level.But that is not to say there are not changes during charging.Instead, there was volatility.The reason for this analysis may be that the polarization of the current at the beginning of discharge causes the working voltage to drop, and then the passivation film generated by polarization is destroyed under the activation of the electrolyte, which promotes the transfer of electrons on the anode surface, thereby increasing the working voltage of the battery.The Al-x Gd 2 O 3 composite anode is gradually rising at the beginning until it is stable, which shows the discharge mode between the two, and the reason for the analysis: because the addition of Gd 2 O 3 particles in Al reduces the initial discharge voltage of the Al anode, but the addition of Gd 2 O 3 particles destroys the compactness and continuity of the oxide film on the surface of Al, improves the discharge activity, and increases the discharge voltage until it is stable.With the discharge process, corrosion products accumulate on the surface of the anode, hindering the effective transfer of electrons; when the damage of the passivation film and the accumulation of corrosion products reach equilibrium, the transfer process of electrons is stable, and the corresponding working voltage reaches a stable state.From Table 3, it can be seen that the Al anode with Gd 2 O 3 added is much higher than the pure anode, possibly due to the presence of Gd 2 O 3 particles, which improves the corrosion resistance of the aluminum anode.From the discharge curve, it can be seen that after the passivation film is destroyed, the strong corrosion resistance of the aluminum anode increases its discharge voltage and enhances its electrochemical performance.
Figure 5 is the relationship between energy density and Gd 2 O 3 mass percentage, from which it can be seen that the energy density of Al-x Gd 2 O 3 composite anode increases with the increase of mass percentage, and the energy density of Al-1.0 Gd 2 O 3 and Al-1.5 Gd 2 O 3 composite anodes is similar.Combined with the discharge voltage diagram, the energy density corresponds to the average discharge voltage.The results show that Al-2.0 Gd 2 O 3 composites have the best anodic discharge performance and anode efficiency.It can be seen from the hydrogen evolution corrosion rate curve that the addition of Gd 2 O 3 will inhibit hydrogen evolution corrosion, of which the most prominent effect is Al-1.0Gd 2 O 3 composite anode, and it is far lower than the hydrogen evolution corrosion rate of pure aluminum, indicating that the addition of a certain amount of Gd 2 O 3 particles significantly improves the corrosion resistance of the composite anode.The corrosion rates of the five anodes given in Table 4 also confirm this conclusion.With the improvement of corrosion resistance, the self-corrosion of the anode is suppressed and the utilization rate of the anode is improved.

Surface morphology analysis of Al-x Gd 2 O 3 composite material before and after anodic discharge
The surface morphology of the pure aluminum anode and Al-x Gd 2 O 3 composite anode before discharge and the surface morphology after discharge in 4mol•L −1 NaOH solution were compared and analyzed.Figure 7 is the surface topography of anodic discharge with different Gd 2 O 3 particle content.In Fig. 7a is the surface topography of pure aluminum plate before anode discharge, Fig. 7a′ is the surface topography after pure aluminum anode discharge.Figure 7 b is the surface topography of Al-0.5 Gd 2 O 3 composite material before anode discharge.Figure 7 b′ is the surface topography of Al-0.5 Gd 2 O 3 composite material after anode discharge.From the surface topography before corrosion, it can be seen that compared with the anode surface topography of Al-x Gd 2 O 3 composite materials, there are more uniformly distributed Gd 2 O 3 particles on the anode surface of Al-x Gd 2 O 3 composites, and the adsorption effect of Al surface on Gd 2 O 3 particles is not as good as that of CeO 2 and TeO 2 particles, so Gd 2 O 3 particles are obvious on the Al surface.It can be seen from the surface topography diagram before corrosion that with the increase of Gd 2 O 3 particles, the agglomeration particles on the surface of the anode increase significantly.Certain pores are generated at the particle-Al matrix hybrid interface, which is of different depths, which affect the corrosion process, and have a dual effect on electrochemical activity and corrosion resistance.For aluminum matrix composites, the aluminum matrix usually appears as an anode, and the reinforcement does not undergo galvanic effects, so Gd 2 O 3 particles are generally not affected by the electrolyte.The surface topography map before corrosion has certain defects, such as rolling deformation defects, no impurity defects, fiber structure defects, and pore defects; these defects are not uniformly distributed on the matrix, they are mainly concentrated at the interface between the particles and the matrix, and it is easier to become the starting point of corrosion in the corrosion process.The corrosion product Al(OH) 3 adheres to the pores and Gd 2 O 3 particles, and the electrochemical reaction process is preferred, thereby reducing the corrosion of the matrix.From Fig. 7a′, it can be seen that pure aluminum has more corrosion pits than other composite anodes, but these five anode corrosion methods are the same, all of which are large corrosion pits and small corrosion pits, which are expanded from small pitting pits, and the corrosion pits are all circular corrosion pits.It can be seen from the figure that the anode corrosion of the Al-1.0Gd 2 O 3 composite is small and not dense.The results show that the addition of Gd 2 O 3 particles will improve the corrosion resistance of the anode, and the Al-1.0Gd 2 O 3 composite anode has the best corrosive performance.

Conclusion
(1) From the polarization curve analysis, it can be seen that the positive shift of the corrosion potential of Gd 2 O 3 in pure Al makes the composite anode more corrosion-resistant in the electrolyte, and in the electrochemical impedance spectrum, the diameter size of the high-frequency capacitance semicircle of pure aluminum is much smaller than that of the Al-x Gd 2 O 3 composite anode, and the addition of Gd 2 O 3 in pure aluminum affects the surface charge transfer of the composite anode, which improves the corrosion resistance of the composite anode, which is consistent with the positive shift of the polarization curve potential.
(2) From the discharge test results, it can be seen that the Al anode with Gd 2 O 3 particles added is much higher than the pure anode, probably because of the agglomeration of a large number of Gd 2 O 3 particles, which concentrates on the defects.As the discharge progresses, these defects may extend to each other, forming a larger corrosion platform, thereby improving the anodic discharge performance of the Al-x Gd 2 O 3 composite.(3) From the discharge test results, it can be seen that the Al anode with added Gd 2 O 3 particles is much higher than the pure anode, which may be due to the presence of a large number of Gd 2 O 3 particles, improving the corrosion resistance of the aluminum anode.As the discharge progresses, when the passivation film is destroyed, the strong corrosion resistance of the aluminum anode increases its discharge voltage and enhances its electrochemical performance.
Author contributions Yiming Zhu and Tianyu Zhao contributed experimental data and paper writing, while Xiaohua Yu, Yanli Zhu, Qingfeng Shen, Rongxing Li, and Gang Xie guided the writing of the paper.
Funding This work has been supported by the General Program of the National Natural Science Foundation of China (51774160).This work has received support from the National Natural Science Foundation of China (52022013, 51974031).This work has received support from the Yunnan Provincial Ten Thousand Talents Plan Project Fund (YNWR-QNBJ-2018-327).

Fig. 1
Fig. 1 Anodic polarization curve of Al-x Gd 2 O 3 composites 1 and Q 2 are CPE (constant phase element); R 3 and L (inductance) represent the induction loop formed by aluminum dissolving in NaOH solution and adsorbing on the surface of the anode.The first high-frequency capacitance loop, corresponding to R 1 and Q 1 , is formed by Al dissolution and hydrogen evolution corrosion, which is also an Al redox reaction.The lowfrequency capacitance ring is created by the dissolution of the hydroxide film on the aluminum surface.

Fig. 2
Fig. 2 Electrochemical impedance spectrum of Al-x Gd 2 O 3 composite anode and Al-x Gd 2 O 3 composite anode AC impedance fitting diagram

Figure 6
Figure 6 is the hydrogen evolution corrosion rate curve of pure Al anode and Al-x Gd 2 O 3 composite anode.Table 3 is the anode corrosion data of Al-x Gd 2 O 3 composite anode in 4 mol/L NaOH solution, and the relevant data obtained by soaking pure Al and Al-x Gd 2 O 3 composite anode in 4 mol/L NaOH solution for 2 h.It can be seen from the hydrogen evolution corrosion rate curve that the addition of Gd 2 O 3 will inhibit hydrogen evolution corrosion, of which the most prominent effect is Al-1.0Gd 2 O 3 composite anode, and it is far lower than the hydrogen evolution corrosion rate of pure aluminum, indicating that the addition of a certain amount of Gd 2 O 3 particles significantly improves the corrosion resistance of the composite anode.The corrosion rates of the five anodes given in Table4also confirm this conclusion.With the improvement of corrosion resistance, the self-corrosion of the anode is suppressed and the utilization rate of the anode is improved.

Figure 7 c
is the surface topography of Al-1.0 Gd 2 O 3 composite material before positive discharge.Figure 7 c′ is the surface topography of Al-1.0 Gd 2 O 3 composite material after positive discharge.Figure 7 d is the surface topography of Al-1.5 Gd 2 O 3 composite material before positive discharge.Figure 7 d′ is the surface topography of Al-1.5 Gd 2 O 3 composite material after positive discharge.

Fig. 5
Fig. 5 Energy density of Al-x Gd 2 O 3 composite anode at a current density of 10 mA•cm −2

Fig. 6
Fig. 6 Hydrogen evolution corrosion rate curve of Al-x Gd 2 O 3 composite anode

Figure 7 e
Figure 7 e is the surface topography of Al-2.0 Gd 2 O 3 composite material before positive discharge, and Fig. 7e′ is the picture which shows the surface topography of Al-2.0 Gd 2 O 3 composite material after positive discharge.From the surface topography before corrosion, it can be seen that compared with the anode surface topography of Al-x Gd 2 O 3 composite materials, there are more uniformly distributed Gd 2 O 3 particles on the anode surface of Al-x Gd 2 O 3 composites, and the adsorption effect of Al surface on Gd 2 O 3 particles is not as good as that of CeO 2 and TeO 2 particles, so Gd 2 O 3 particles are obvious on the Al surface.It can be seen from the surface topography diagram before corrosion that with the increase of Gd 2 O 3 particles, the agglomeration particles on the surface of the anode increase significantly.Certain pores are generated at the particle-Al matrix hybrid interface, which is of different depths, which affect the corrosion process, and have a dual effect on electrochemical activity and corrosion resistance.For aluminum matrix composites, the aluminum matrix usually appears as an anode, and the reinforcement does not undergo galvanic effects, so Gd 2 O 3

Fig. 7
Fig. 7 Surface morphologies of Al-x Gd 2 O 3 composites before and after anodic discharge.a and a′ are the surface topography of pure aluminum anode before and after discharge; b and b′ are the surface topography of Al-0.5 Gd 2 O 3 and before and after discharge; c

Table 1
Fitting parameters of anodic polarization curve of Al-x Gd 2 O 3 composites

Table 2
Al-x Gd 2 O 3 composite anode AC impedance parameter value