Recent improvements in PbO2 nanowire electrodes for lead-acid battery

Recent improvements in PbO2 nanowire electrodes for lead-acid battery

Accepted Manuscript Recent Improvements in PbO2 Nanowire Electrodes for Lead-Acid Battery Alessandra Moncada, Salvatore Piazza, Carmelo Sunseri, Rosal...

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Accepted Manuscript Recent Improvements in PbO2 Nanowire Electrodes for Lead-Acid Battery Alessandra Moncada, Salvatore Piazza, Carmelo Sunseri, Rosalinda Inguanta PII:

S0378-7753(14)01808-4

DOI:

10.1016/j.jpowsour.2014.10.189

Reference:

POWER 20109

To appear in:

Journal of Power Sources

Received Date: 30 September 2014 Accepted Date: 30 October 2014

Please cite this article as: A. Moncada, S. Piazza, C. Sunseri, R. Inguanta, Recent Improvements in PbO2 Nanowire Electrodes for Lead-Acid Battery, Journal of Power Sources (2014), doi: 10.1016/ j.jpowsour.2014.10.189. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Recent Improvements in PbO2 Nanowire Electrodes for Lead-Acid Battery Alessandra Moncada, Salvatore Piazza, Carmelo Sunseri, Rosalinda Inguanta*

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Laboratorio di Chimica Fisica Applicata, Dipartimento di Ingegneria Chimica Gestionale Informatica Meccanica, Università di Palermo, Viale delle Scienze - 90128 Palermo (Italy)

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GRAPHICAL ABSTRACT

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Corresponding author: Tel.: +39-0912386567232; fax: +39-09123860841. E-mail address: [email protected] (R. Inguanta)

Presented at the LABAT’ 2014 conference, Albena, Bulgaria, 10-13 June 2014 1

ACCEPTED MANUSCRIPT

Recent Improvements in PbO2 Nanowire Electrodes for Lead-Acid Battery Alessandra Moncada, Salvatore Piazza, Carmelo Sunseri, Rosalinda Inguanta*

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Laboratorio di Chimica Fisica Applicata, Dipartimento di Ingegneria Chimica Gestionale Informatica Meccanica, Università di Palermo, Viale delle Scienze - 90128 Palermo (Italy)

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Abstract

Lead oxide nanowires are an attractive alternative to conventional pasted electrodes, owing to their

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high surface area leading to high specific energy batteries. Here, we report the performance of template electrodeposited PbO2 nanowires used as positive electrodes. Nanostructured electrodes were tested at constant charge/discharge rate from 2C to 10C, with a cut-off potential of 1.2V and discharge depth up to 90% of the gravimetric charge. These new type of electrodes are able to work at very high C-rate without fading, reaching an efficiency of about 90% with a very good cycling

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stability. In particular, after an initial stabilization, a specific capacity of about 200 mAh g-1, very close to the theoretical one of 224 mAh g-1, was drained for more than 1000 cycles at a C-rate

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higher than 1C with an efficiency close to 90%. This behaviour significantly distinguishes PbO2 nanostructured electrodes from the conventional ones with pasted active material. In addition,

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discharge at a quasi-constant voltage of about 2.1 V, without reaching the cut-off potential also at high C-rate, occurs. This implies a quasi-constant energy supply during fast discharge. According to these findings, innovative applications as hybrid or electrical mobility or buffer in renewable energy plants can be envisaged.

Keywords: PbO2 nanowires; Template electrosynthesis; Lead-acid battery; High C-rate; Cycle-life.

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Corresponding author: Tel.: +39-0912386567232; fax: +39-09123860841. E-mail address: [email protected] (R. Inguanta)

Presented at the LABAT’ 2014 conference, Albena, Bulgaria, 10-13 June 2014 1

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Introduction In the field of lead-acid batteries, many efforts have been made to improve cycling stability

and power performance, in order to make them suitable for renewable energy storage and hybrid electrical vehicles [1-2]. Specific energy of the commercial lead-acid batteries is about 30 Wh kg-1,

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and this low value is mainly due to the low utilization degree of active pastes. In fact, during discharge, a non-conductive lead sulphate phase covering inner active paste is formed so that its reaction with the electrolyte is inhibited. Different additives, both conductive and non-conductive,

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are used in order to increase the conductive network of the active paste or decrease the inactive paste weight. For instance, addition of different sized carbon nanoparticles [3], carbon black [4-6]

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activated carbon [5-6], carbon nanotubes [7] was tested. A coating of a sugar derived carbon was also used with remarkable improvement in rate capability, active material utilization, cycle performance and charge acceptance [8]. In order to improve electrolyte storage and porosity of the electrodes, Sorge et al. [9-10] added porous-hollow glass microspheres to the active paste, and they

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showed that volume loading of microspheres is a crucial parameter to avoid the breakage of the conductive network of the active materials.

In this frame, a good strategy seems to be the substitution of the conventional plates with

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innovative electrodes having high capacity and good stability. To achieve this goal, it is necessary to fabricate electrodes with high surface area, enhancing the electrochemical reaction kinetics at

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electrode/electrolyte interface, and with the entire active material in contact with the current collector, ensuring electrical conductivity during charge/discharge cycle [11]. Up to date, the two principal approaches adopted to achieve this goal are both based on the use of nanostructured materials. In the first approach, high surface area was obtained by using 3D nanostructured current collector in place of the conventional lead grid. Carbon foam [12-14], 3D porous titanium [11] and lead foam [15] were used that were filled with active materials following different methods. For instance, Zang et al. [11] proposed 3D porous titanium covered with PbO2 by electrodeposition. They obtained a specific capacity of 132 mAh g-1 with an active material utilization of 57%. Very 2

ACCEPTED MANUSCRIPT interesting is also the approach proposed by Ji et al. [15] who prepared lead foam by lead electrodeposition on a copper-foam substrate. By finite element analysis, they showed that the 3D connected network of the lead foam for positive electrode reduces current density, polarization resistance, and ohmic resistance of the battery owing to the large contact area with the active

(19-36 % higher than the conventional battery).

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material. As a result, the lead foam battery has a higher utilization of the positive active material

The second method to achieve a very high surface area is based on the fabrication of active

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material in nanostructured form. Following this approach, Morales et al. fabricated β-PbO2 nanoparticles [16-17], and their electrodes delivered a capacity of 160 mAh g-1 at 1C for 280 cycles.

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In [18], a mixture of α and β-PbO2 nano-micro-particles with different morphologies were used, and a discharge capacity of about 155 and 134 mAh g-1 was obtained at 0.2C and 0.4C, respectively, whilst, at 1C, a capacity of about 97 mAh g-1 was delivered. Nanoparticles of β-PbO2 also tested in mini-tubular electrodes [19] showed a utilization degree of the active mass higher than

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45% with an excellent cycling stability.

To enhance the electrochemical active surface area it is possible to use uniform array of nanostructures (nanowires or nanotubes) instead of nanoparticles. These types of nanostructures, if

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firmly attached to a current collector, provide a very large surface area that enhances the specific

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capacity of the battery. In a recent work [20], we have shown that PbO2 nanostructured electrodes, obtained by template electrodeposition, are able to deliver at 1C an almost constant capacity of about 190 mAh g-1 (85% of active material utilization), close to the theoretical value (224 mAh g-1). Besides, PbO2 nanowires showed a very good cycling stability for more than 1000 cycles. These findings indicate that this new type of electrode might be a promising substitute of positive plates in lead-acid battery. Here, we will show that these electrodes are capable to work also at high C-rate, from 2C up to 10C. The aim was to check cycling stability, and power performance of the PbO2 nanostructured electrodes under severe operative conditions in order to assemble a battery, with PbO2 and Pb 3

ACCEPTED MANUSCRIPT nanostructured electrodes, whose preliminary performance was reported in [21]. It is important to highlight that the C-rates here tested are far higher than those of commercial batteries, whose highest operative rate is C/5, but with short cycle life [1].

Experimental

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The experimental procedure was detailed elsewhere [20-23]. Briefly, commercially tracketched polycarbonate membranes (Whatman, Cyclopore 47) were used as template with pore

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diameters ranging from 180 to 250 nm and a thickness of about 16±0.65 µm, according to our SEM

membrane to make it conductive.

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analysis. Prior to deposition, a very thin layer of Au was sputtered onto one surface of the

Nanostructured electrodes were obtained by a two-step procedure from an aqueous solution of 1M Pb(NO3)2 and 0.3M HNO3 using a PAR Potentiostat/Galvanostat (mod. PARSTAT 2273). In the first step, a current collector of PbO2 was electrodeposited onto the Au-coated side of the

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membrane. The modified Sovirel® cell used for electrodeposition allowed [24] the growth of PbO2 nanowires inside membrane channels during the second step performed at constant potential. After electrodeposition, polycarbonate membrane was dissolved in CHCl3. Nanowire mass was evaluate

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by gravimetric measurements using a Sartorius microbalance (mod. Premium Microbalance

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ME36S). Morphology of the lead oxide nanowires was examined before and after electrochemical testing by SEM analysis, as detailed in [25-27]. Nanostructured electrodes were tested in a 5M sulphuric acid aqueous solution, in a zerogap configuration using commercially available negative plate and separator (AGM type). Negative plates, that have a higher capacity with respect to the nanostructured electrodes, were replaced every 200 cycles. Charge/discharge cycles were carried out using a multi-channel cell test system (Solartron, 1470E) at different C-rates, from 2C to 10C, with a cut-off potential of 1.2 V. The cell was discharged up to 90% of the gravimetric charge of PbO2 electrode.

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Results and Discussion As reported in our previous work [20], PbO2 nanostructured electrodes can deliver at 1C a

capacity of 190 mAh g-1 (close to the theoretical value of 224 mAh g-1) for over than 1000 cycles. Starting from this encouraging result, nanostructured electrodes were also tested at higher C-rate.

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Morphology of the as prepared PbO2 nanowires is well detailed in Figure S1: images show that electrode consists of nanowire array well anchored to the current collector, having a regular cylindrical shape with diameters very close to 250 nm and a length of about 15 µm. PbO2 nanowires

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orientation is random, reflecting geometry of template channels. Besides, array is characterized by the presence of large voids between nanowires. We have found that this morphology is a key

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parameter for good performance of the nanostructured electrodes, because it ensures permeation of the electrolyte during the charge/discharge process, and also it allows to easier accommodate volume expansion/contraction during cycling, as shown in [23]. As above mentioned, nanowires are attached to a PbO2 current collector about 80 µm thick, which acts also as mechanical support.

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Specific tests were performed in order to ascertain that current collector was not involved in the conversion reactions occurring under cycling.

Nanostructured electrodes were cycled at different C-rates, from 2C up to 10C. Two

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different procedures of cycling were adopted: according to the first one, electrode was cycled at high C-rate since beginning, whilst according to the second one, the first 100 cycles were carried

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out at 1C, then the C-rate was raised to the desired value. These different tests were performed to check the influence of the initial stabilization on the life time of the nanostructured electrode. As reported in [20], at 1C stabilization is accomplished in approximately 50 cycles, reaching a specific capacity of about 190 mAh g-1. Since this initial stabilization is usual for lead-acid batteries and it is due to a change in the active material morphology [28], it was important to verify whether it depends on initial C-rate. In all cycling tests, the first charge was conducted according to the multi current step procedure, proposed in [20], consisting in a step-wise increase of current from C/5 up to either 1C 5

ACCEPTED MANUSCRIPT or higher in dependence on the final C-rate at which tests were conducted. The first slow charge was fundamental for the mechanical stability of our nanostructured electrodes, because it avoids the reaching a high cell voltage at which gas evolution should occur. All electrochemical tests were performed with a cut-off potential of 1.2V, even though

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different cut-off potential can be set for each C-rate [1]. We have maintained 1.2 V for better comparing results at different C-rates and to avoid an excessive stress of the nanostructured electrodes owing to a deeper discharge. However, it is important to highlight that this cut-off value

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is lower in comparison with that employed in commercial battery, where a cut-off voltage between

lead to a rapid death of the battery.

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1.75 and 1.8 V is always set in order to avoid a deep sulphation of the active paste, which would

Figure 1 shows discharge capacity and efficiency vs. cycle number for tests conducted at 2C. In the case of electrode firstly tested at 1C for 100 cycles, similarly to what discussed in [20], during stabilization a rapid improvement of battery performance occurred and just after the 40th

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cycle an efficiency of about 80% was achieved, which increased further after a short plateau. When the cycling rate was increased from 1C to 2C, a drop down to about 80% occurred, followed by a rapid increase up to a 90% efficiency at 112th cycle, which remained stable for the remaining

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cycles. The charge/discharge process of this electrode was stopped after 1180 cycles for analyzing its morphology by SEM.

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Also for the direct cycling at 2C (triangles in Fig.2), a stabilization about 100 cycles long was found. After this phase, an efficiency of 90% was measured. During the charge/discharge process of this electrode, several unintentional interruptions of power supply occurred, some of them lasting some days (from 2 to 5). Nevertheless the recovery of efficiency after the interruptions was significantly high, it remained below 90%, at values between 85-87%. This reveals that nanostructured electrodes adapt quite well to discontinuous operational conditions of battery. As for the charge/discharge curves, no remarkable changes were found in the first 100 cycles for the electrode firstly tested at 1C, in comparison to what shown in [20]. On the other hand, 6

ACCEPTED MANUSCRIPT when the C-rate was stepped from 1C to 2C, in the charge curves, two principal variations were observed after the increase at 2C. Figure 2a shows that the potential peak at the beginning of charge was disappearing with cycling and it was no more present after the 30th cycle, likely due to the formation of an easily re-oxidable lead sulphate during discharge. After this cycle, cell voltage

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quickly increased at the beginning of the charge process, then it continued to increase with time but more slowly up to 1200 sec, after which a steeper increase occurred. The second observed variation was the small voltage increase (only 30 mV from cycle 50 to 100) during the first 100 cycles, whilst

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no relevant changes could be envisaged in charge curves (Figure 2b) for the subsequent cycles. This is a very important finding, because it indicates a stable behaviour of the electrode under cycling.

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Figure 2a shows that after the 30th cycle from the transition to 2C, battery was discharging without reaching the cut-off voltage; during these first cycles the rapid increase of battery performance up to 90% occurs (Figure 1), while discharge curves present a small drop of about 100 mV. From 30th up to 1000th cycle, discharge curves show a quasi-constant voltage without

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appreciable differences (Figure 2b). Since negative electrode mass is largely in excess, the voltage plateau during discharge shown in Figure 2a, and b can be attributed to the conversion reaction of PbO2 to PbSO4 [29-30]. This finding is very important in the field of the energy storage, because it

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implies a quasi-constant energy supply during fast discharge, at variance with conventional leadacid battery. This behaviour is attributable to the high surface/volume ratio of nanostructures,

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ensuring that conversion reaction involves almost entirely the electrode active mass determining high discharge efficiency even at high C-rate. When nanostructured electrodes were cycled directly at 2C (Figure S2) the charge/discharge curves show that almost 100 cycles were necessary before reaching a stable performance. In comparison to curves of Figure 2, the main difference is that for the first 20 cycles, charge and discharge occurred at a voltage respectively higher and lower than that measured for the electrodes initially stabilized at 1C. This behaviour may be related to an initial difficulty of the electrode to withstand the conversion reactions at high C-rate; in fact a very low efficiency was measured for 7

ACCEPTED MANUSCRIPT the first cycles. In general, when the electrode worked directly at 2C, discharge occurred to a potential slightly lower than that measured for electrodes initially stabilized at 1C (Figure S3). As reported in [20], the efficiency increase under cycling can be attributed to increase of wettability of the active material likely due to hydration of the surface coupled with morphological

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change under cycling. Really, Figure 3 shows that morphology of the electrode after 1080 cycles is quite different in comparison with the as-prepared electrode (Figure S1). This morphology change is likely due to the continuous volume variation coupled with the conversion of PbO2 to PbSO4 and

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vice-versa. Another relevant aspect evidenced by micrographs of Figure 3a, and b is that the current collector appears unchanged, without cracks or any other morphological modification, confirming

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that it did not undergo any volume change and therefore it was not involved in the electrochemical reactions. Figure 3b also shows that, even if the morphology was changed, the nanostructures coming from the initial nanowires were still well attached to the current collector. Figures 3c, and d show the presence of macro-crystals of lead sulphate on the electrode surface, due to the fact that

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electrode had already reached the end of 1080th discharge. Figure 3d clearly show that nanowire cylindrical shape was transformed in a shorter conical one. Besides, in this last image, it can be seen that a free space separates neighbouring cones. This morphological variation entails additional

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advantages for the active material, which maintains both a high surface area, for the occurrence of the electrochemical reactions, and a high degree of porosity, that facilitates transport of both

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sulphate ions and water towards the surface of the active material. Similar conclusions were found also by other authors in the case of nanostructured PbO2 thin-films obtained by electrodeposition on glassy-carbon [31] or by galvanostatic oxidation of pure Pb metal [32]. In particular, the interesting results by Chen et al [32], showed that PbO2 layers, with high surface area and good connectivity between PbO2 nanoparticles, gave a discharge capacity of about 206 mAh g-1. According to Figure 3, we can say that a progressive transformation of the nanowires occurred under cycling, with formation of a typical micro-, nano-porosity not achievable with other procedures. But, although

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ACCEPTED MANUSCRIPT nanowires were modified under cycling, their initial presence led to progressive formation of a morphology determining peculiar performances never reached by pasted electrodes. Figure 4 shows performance of nanostructured electrodes cycled at 5C. Obviously, no important changes are present in the initial stabilization conducted at 1C. After the increase to 5C,

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the efficiency collapses down to 71% and then almost instantaneously it reaches 90%, keeping this value up to the interruption of the test, after about 1400 cycles. Test was stopped for analysing the electrode by SEM. In the case of tests directly carried out at 5C, a very low efficiency was

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measured in the first cycles, then efficiency was increasing and after the 50th cycle a value of 90%, stable up to about 1000 cycles, was achieved. Also in this case, during the test of the electrode,

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different interruptions of power supply occurred, but they did not cause any performance decay of the electrode.

About the charge/discharge curves, Figure 5 shows that the first charge occurred at higher potential with irregular trend probably due to the periodic removal of gas gradually accumulating

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on the electrode. The other charge curves show similar features: an initial steep increase of voltage up to about 2.33 V, which was kept almost constant for about 500 sec, then voltage slowly increased to about 2.45 V at the end of the charge cycle. As cycle number increased (Figure S4), no

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relevant differences could be envisaged with an initial voltage peak appearing on charging curves only after the 700th cycle. For the discharge curves, Figure 5 shows that the cut-off voltage was

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reached only in the first cycle. The improvement of the battery performance under cycling is also revealed by the final discharge voltage, passing from 1.96 V, after 10 cycles, up to 2.03 V, after 100 cycles. After the initial transient, about half of discharge occurred at a quasi-constant voltage of about 2.1 V, then voltage slowly decreased with time. No relevant changes were observed at increasing cycle number. Only after the 700th cycle, the final discharge voltage begun to decrease, reaching 2.03 V after 1200 cycles. This value was lower than that measured at 2C, where discharge occurred at a voltage higher than 2.1 V also after 1000 cycles.

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ACCEPTED MANUSCRIPT In the case of electrodes tested directly at 5C, we evidence that some tests failed owing to several voltage peaks during the charge phase, likely originated by breakage of the nanostructured electrodes due to gas development. However, in this case, charge/discharge curves showed an irregular shape up to the 40th cycle; in particular, charge and discharge occurred at a voltage higher

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than 2.6 V and lower than 2 V, respectively. Besides, although until the 40th cycle discharge curves reached the cut-off potential of 1.2 V, the delivered capacity was increasing under cycling. From 40th cycle, charge/discharge curves became regular, with charges occurring for a large part at 2.35

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V, and only at the end values between 2.48 and 2.57 V were achieved, whilst discharge voltage was higher than 2.1 V up to about 250 sec, then it decreased gradually down to 1.96V.

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Considering the low initial wettability of nanostructures [20], failed tests and, in general, the initial irregular trend recorded in the case of the direct charge/discharge at 5C were probably due to the limited conversion depth of nanostructured PbO2 to PbSO4. At increasing C-rate, conversion to PbSO4 was occurring only in the outermost layer in contact with the electrolyte. This highly

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resistive layer caused a sudden decrease of the discharge voltage, leading to the drop toward the cut-off value. During the subsequent phase of charging, PbSO4 was quickly converted into PbO2, and gas begun to develop on the electrode surface, whose accumulation led to the interruption of

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electric continuity between nanowires and electrolyte (typical bubble effect), with the consequent instantaneous increase of cell voltage, that in some cases caused electrode breakage. The initial

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stabilization at 1C of the electrodes to be cycled at 5C avoided this drawback because, as reported in [20], during this period a more gradual change of electrode morphology occurred, leading to an increase of its wettability. In particular, the new morphology was characterized by a high degree of porosity that greatly facilitates ion and water transport towards the surface of the active material. Thanks to these features, the nanostructured electrode becomes able to withstand high charge/discharge current without damage, and to work at the maximum efficiency for a very high cycle number. Consequence is the longer lifetime electrodes, as can be seen from Figures 1 and 4.

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ACCEPTED MANUSCRIPT Figure 6 shows that the mean discharge voltage, calculated from discharge curves, is almost constant at the value of about 2.1 V. As mentioned above, this finding is very important in the field of the energy storage, because it implies a quasi-constant energy supply also during fast discharge. Figure 7 shows SEM images of the PbO2 nanostructured electrode after about 1400

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charge/discharge cycles at 5C. It is possible to see that also at this C-rate a strong modification of electrode morphology occurred. Figure 7a shows that the entire electrode surface is covered by PbSO4 micro-crystals under which the array of nanostructures is yet present, as best evidenced in

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Figure 7b, where the underlying morphology is detailed at higher-magnification. Figure 7c shows a well-detailed morphology of the nanostructure evidencing that the initial cylindrical shape of

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nanowires was transformed in a very irregular profile extremely rough, forming a highly porous mass that is perfectly able to bear charge/discharge cycle at high C-rate. At 10C, we observed a behaviour similar to that at 5C. Figure 8 shows that for samples conditioned at 1C for the first 100 cycles, efficiency is almost identical to that of electrodes cycled

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at 2C, and 5C (Figures 1, and 4, respectively). After the increase from 1C to 10C, efficiency dropped down to 60%, then it increased quickly, reaching the maximum efficiency of 90% after further 10 cycles. Electrode efficiency remained stable up to almost 1400 cycles, even if electrodes

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show damage signs after about 1100 cycles. In fact, charging voltage was increasing with cycle number while discharging one was decreasing (Figure S5 and S6). Just after the 1200th cycle

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charging voltage was higher than 2.6 V, while discharging one was lower than 1.95 V. It is interesting to note that as the C-rate increased, the efficiency drop, soon after the change from 1C to C-rate, increased also, as summarized in Table 1, where it is possible to also compare the values of charge/discharge voltage at different C-rate. Table I shows that: -

after 100 cycles at 1C, additional cycles, dependent on C-rate, needed for achieving 90% efficiency;

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the final charge voltage was always slightly higher than the stationary one;

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symmetrically, the final discharge voltage was always slightly lower than the stationary one;

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stationary charging and discharging voltages did not change significantly passing from 2C and 10C rate (i.e., in the presence of a current 5 times greater). This

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implies that ohmic drop was low so as to be practically negligible.

Very few tests were carried out on electrodes cycled directly at 10C, because many of them failed for the reasons discussed above. Furthermore, as it is evident in Figure 8, the electrode has

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performance very unstable for the first 100 cycles. Few electrodes that went beyond this critical stage were able to charge/discharge with an efficiency of 90% for about 1000 cycles. In this case

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charging occurred at 2.41 V, reaching a value of 2.58 V at the end of charge, whilst discharge occurred at a mean voltage of about 2.06 V (Figure S6). After the charge/discharge process, electrodes had a morphology similar to that of Figure 7, characterized by the presence of very rough nanostructures with an irregular shape (Figure S7).

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According to these results, we can conclude that, differently from the case of 2C rate, where behaviour and cycle-life were not influenced by history of the cycling process, at higher C-rate the initial stabilization at 1C is fundamental in order to obtain a longer cycle-life and good stable

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performance since beginning of charge/discharge process. In fact, as mentioned above, morphology modification with formation of a high porous structure, and increase of wettability guarantees that

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almost the entire active material is easily accessible to electrolyte; thus electrode becomes able to bear the sulphation/desulphation reaction also at high current. This procedure seems suitable in industrial production, considering that initial stabilization at 1C was about 8 days long, i.e. comparable to curing and formation time of commercial plate [33].

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Conclusions PbO2 nanostructured electrodes are able to be cycled at high C-rate with very good

performance and long cycle-life. The nanostructured electrodes were obtained by template 12

ACCEPTED MANUSCRIPT electrodeposition and were tested in a 5M H2SO4 solution at 2C, 5C and 10C. The results are very interesting because nanostructured electrode enables to work at very high charge-discharge rates for more than 1000 cycles, with very high specific capacity and energy density, although these tests were performed in very stressing conditions (low cut-off voltage, and discharge up to 90% of

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gravimetric charge).

At each C-rate, electrodes were tested with two different methods of charge/discharge: some electrodes were cycled directly at high C-rate, whilst others were firstly cycled at 1C for 100 cycles,

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then the rate was increased to the desired value (2C or more). We found that at 2C, only the cycle life was different. In fact, in all conditions, electrode reached an efficiency of 90%, which

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corresponds to a specific capacity of about 200 mAh g-1, close to the theoretical one for PbO2 (224 mAh g-1). At 5C and 10C, similar very good results were obtained but only when electrodes were tested with initial stabilization at 1C for 100 cycles. In fact, also in this case electrodes were able to deliver a discharge capacity of about 200 mAh g-1 after few cycles from the 1C transition to higher

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C-rate.

Many tests carried out directly at 5C or 10C failed, especially at 10C, during the initial cycles. We have attributed this behaviour to the very low initial wettability of nanostructured

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electrodes, which implies sulphation and subsequent desulphation only of the outer layer. During this stage, owing to both the low quantity of PbSO4 to be converted into PbO2 and the high charge

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current, a large amount of gas accumulates on electrode surface, interrupting the electrical continuity with the electrolyte with consequent spike voltage, leading to the breakage of nanostructured electrodes. This phenomenon did not occur when electrodes were initially stabilized at 1C because during this phase an important modification of their morphology happened. In particular, we have found an increase of roughness and porosity of the electrodes coupled to an increase of the active mass wettability, making them able to work at high charge/discharge current. According to these findings, we can conclude that initial stabilization of the electrodes at 1C is fundamental for working at high C-rate, avoiding rapid failure during charge/discharge process 13

ACCEPTED MANUSCRIPT and obtaining good and stable performance since beginning of the electrochemical test. Besides, we have found that the electrodes firstly stabilized at 1C have a longer lifetime at all C-rates. Another important finding is that at each C-rate, after few initial cycles, discharge occurs without reaching the cut-off potential of 1.2 V. Besides, a quasi-constant voltage of about 2.1 V was

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recorded for almost all discharge curves; that implies a quasi-constant energy supply during discharge at very fast C-rate.

Here presented and discussed results indicate that template electrodeposited PbO2 nanowires

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are an attractive alternative to the currently used pasted electrodes, because the nanostructured surface typically formed under cycling guarantees performances not achievable by usual pasted

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electrodes.

Further work is in progress concerning nanostructured Pb electrodes in order to assembly a battery with both nanostructured electrodes, in view of innovative technological applications in the

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immediate future.

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ACCEPTED MANUSCRIPT References [1] D. Linden, T. B. Reddy Edt.s, Handbook of Batteries, 4th Edition, McGraw-Hill, New-York, 2010

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[13] Y. Chen, B-Z. Chen, X-C. Shi, H. Xu, W. Shang, Y. Yuan, L-P. Xiao, Electrochimica Acta 53

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[14] Y. Chen, B-Z. Chen, L-W. Ma, Y. Yuan, Electrochem. Comm. 10 (2008) 1064–1066. [15] K. Ji, C. Xu, H. Zhao, Z. Dai, J. Power Sources 248 (2014) 307-316. [16] J. Morales, G. Petkova, M. Cruz, A. Caballero, Electrochem. Solid State Lett. 7 (2004) A75A77. [17] J. Morales, G. Petkova, M. Cruz, A. Caballer, J. Power Sources 158 (2006) 831–836. [18] N. Fan, C. Sun, D. Kong, Y.Qian, J. Power Sources 254 (2014) 323-328. [19] M. Bervas, M. Perrin, S. Geniès, F. Mattera, J. Power Sources 173 (2007) 570–577.

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ACCEPTED MANUSCRIPT [20] A. Moncada, M.C. Mistretta, S. Randazzo, S. Piazza, C. Sunseri, R. Inguanta, J. Power Sources 256 (2014) 72-79. [21] R. Inguanta, S. Randazzo, A. Moncada, M.C. Mistretta, S. Piazza, C. Sunseri, Chem. Eng. Trans. 32 (2013) 2227-2232.

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[22] G. Ferrara, C. Arbizzani, L. Damen, M. Guidotti, M. Lazzari, F. Vergottini, R. Inguanta, S. Piazza, C. Sunseri, M. Mastragostino, J Power Sources 211 (2012) 103-107.

[23] G. Ferrara, L. Damen, C. Arbizzani, R. Inguanta, S. Piazza, C. Sunseri, M. Mastragostino, J

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Power Sources 196 (2011) 1496-1473.

[24] R. Inguanta, F. Ferrara, P. Livreri, S. Piazza, C. Sunseri, Curr. Nanosci. 7 (2011) 210-218.

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[25] R. Inguanta, F. Vergottini, G. Ferrara, S. Piazza, C. Sunseri, Electrochimica Acta 55 (2010) 8556-8562.

[26] R. Inguanta, E. Rinaldo, S. Piazza, C. Sunseri, J Solid State Electrochem. 16 (2012) 39393946.

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[27] R. Inguanta, F. G. Vergottini, G. Ferrara, M. C. Mistretta, C. Sunseri, S. Piazza, International Patent, 2012, WO2012117289.

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[32] Y. Chen, B-Z. H. Huang, H. Ma, D. Kong, Electrochimica Acta 88 (2013) 79-85. [33] D. A. J. Rand, P. T. Moseley, J. Garche, C. D. Parker, Edt.s, Valve-Regulated Lead-Acid Batteries, Elsevier, Amsterdam, 2004.

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ACCEPTED MANUSCRIPT Figure Captions

Figure 1. Discharge capacity and efficiency at 2C vs. cycle number. Asterisks indicate the

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interruption of power supply.

Figure 2. Charge-discharge curves recorded at 2C after initial stabilization at 1C for 100 cycles; (a)

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sectional view; (c), and (d) top-view.

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Figure 3. SEM images of PbO2 nanostructured electrode after 1010 cycles at 2C: (a), and (b) cross-

Figure 4. Discharge capacity and efficiency at 5C vs. cycle number. Asterisks indicate the

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interruption of power supply.

Figure 5. Charge-discharge curves recorded at 5C after initial stabilization at 1C for 100 cycles.

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Figure 7. Average discharge voltage at 5C vs. cycle number.

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Figure 7. SEM images of PbO2 nanostructured electrode after 1298 cycles at 5C, after an initial stabilization at 1C for 100 cycles.

Figure 8. Discharge capacity and efficiency at 10C vs. cycle number.

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Table I. Values of some parameters for the nanostructured PbO2 electrode tested at different C-rate,

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after an initial stabilization at 1C for 100 cycles. (η 100th cycle at 1C= 83%±2%).

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η 101 cycle Cycle for achieving η=90% Steady charging voltage End charging voltage Steady discharging voltage End discharging voltage

5C 71% 103th 2.330±0.010 V 2.450±0.01 V 2.11±0.01 V 1.995±0.035

10C 59% 105th 2.380±0.010V 2.510±0.010 V 2.100±0.025 1.970±0.030

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2C 80% 112th 2.310±0.030 V 2.355±0.025V 2.185±0.025V 2.125±0.005

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Recent Improvements in PbO2 Nanowire Electrodes for Lead-Acid Battery Alessandra Moncada, Salvatore Piazza, Carmelo Sunseri, Rosalinda Inguanta*

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Laboratorio di Chimica Fisica Applicata, Dipartimento di Ingegneria Chimica Gestionale Informatica Meccanica, Università di Palermo, Viale delle Scienze - 90128 Palermo (Italy)

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HIGHLIGHTS PbO2 nanowires were tested at high C-rate, from 2C to 10C;

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The particular morphology permits to obtain high specific energy battery;

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Electrodes can delivered a capacity of about 200 mAh g-1 for more than 1000 cycles;

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Nanostructured electrodes discharge at a quasi-constant voltage of about 2.1V;

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These new lead-acid battery can be used in hybrid and electrical cars.

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*

Corresponding author: Tel.: +39-0912386567232; fax: +39-09123860841. E-mail address: [email protected] (R. Inguanta)

Presented at the LABAT’ 2014 conference, Albena, Bulgaria, 10-13 June 2014 1

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Recent Improvements in PbO2 Nanowire Electrodes for Lead-Acid Battery Alessandra Moncada, Salvatore Piazza, Carmelo Sunseri, Rosalinda Inguanta*

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Laboratorio di Chimica Fisica Applicata, Dipartimento di Ingegneria Chimica Gestionale Informatica Meccanica, Università di Palermo, Viale delle Scienze - 90128 Palermo (Italy)

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Supplementary Data

Figure S1. SEM images of as-prepared PbO2 nanostructured electrode: (a) and (b) cross-sectional view; (c) and (d) top-view.

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Corresponding author: Tel.: +39-0912386567232; fax: +39-09123860841. E-mail address: [email protected] (R. Inguanta)

Presented at the LABAT’ 2014 conference, Albena, Bulgaria, 10-13 June 2014

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Figure S2. Charge-discharge curves recorded direct at 2Cfrom cycle 1 to cycle 300.

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Figure S3. Mean discharge voltage at 2C vs. numbers of cycles.

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Figure S4. Charge-discharge curves recorded at 5C after a initial stabilization at 1C for 100 cycles:

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from cycle 200 to cycle 1200.

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Figure S5. Charge-discharge curves recorded at 10C after an initial stabilization at 1C for 100 cycles; (a) from cycle 1 to cycle 100 (b) from cycle 200 to cycle 1100.

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Figure S6. Mean discharge voltage at 10C vs. numbers of cycles.

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Figure S7. SEM images of PbO2 nanostructured electrode after 1288 cycles at 10C with an initial

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stabilization at 1C for 100 cycle.