The new anode environmentally friendly for water electrolysis based on 430 stainless steel coated with recycled cobalt from spent lithium – ion batteries

In this paper, a new anode environmentally friendly for hydrogen production was developed based on 430 stainless steel with an electrodeposited cobalt layer. The novelty of this work is the cobalt source once the electrodeposition bath was obtained from recycling of spent Li-ion batteries cathode with composition LiCoO2. The electrodeposited cobalt behaves as supercapacitor in KOH 1M. In the linear voltammetry in KOH 1M, when the overpotential reaches 370mV, the anodic density current for 430 SS/Co is 19mAcm. Thus, the anode developed in this paper achieves the double of density current with half of production cost if compared with 316SS. Moreover the anode construction described in this paper is an excellent option for Liion battery recycling.


INTRODUCTION
Renewable energy source such as hydrogen is considered crucial for a more sustainable future [1].In fact, since daily processes even space exploration will depend, largely of hydrogen production within few years [2].The hydrogen is considered environmentally friendly once the reaction between H 2 with O 2 molecules (forward direction in Eq.1) has only water molecules as sub-product, beyond high molar enthalpy value (ΔH = 285.8kJ/mol at 1013 mbar and 298K) [2].In this context, the water electrolysis is the most attractive route among the existed processes for very pure hydrogen production (and also O 2 ) in large scale (inverse direction in Eq.1) [1].
H 2(g) +1/2O 2(g) → H 2 O (g) ΔH = 285.8kJ/mol (Eq.1) In water electrolyses, the O 2 production is a limiting factor because the overpotential for oxygen evolution reaction (OER) is responsible for greatest source of energy loss when the electrolysis is performed in alkaline (Eq.2) or acid media (Eq.3) [3,4].However, the alkaline media is preferred because in this condition the catalysts used in water electrolysis are more active and stable [4].The OER also is present in others strategic power sources such as aqueous lithium air batteries [5], NiMH [3], and Zn-air [6].
Many papers are dedicated to investigate the alternative anode for OER with high efficiency [7].The RuO 2 and IrO 2 are suggested as the benchmark for OER in alkaline solution [8].However, these oxides have prohibitively cost for large scale application [8].The commercial electrodes for water electrolyses in industrial scale must have important characteristics such as: high conductivity, high corrosion resistance, high catalytic effect for oxygen evolution reaction, moreover low cost [9].
For Ni foam, the current density for OER is 10mAcm interest is devoted to stainless steel as anode for OER in alkaline solutions [9].The literature revels that the 316 stainless steel (316SS) shows a density current of 10mAcm - 2 in η=0.370mV in KOH 1.0 molL -1 [12].This value is similar to that found for Ni Foam in the same conditions [12].This is expected since the 316 has 12% of Ni in its composition [13].The 430SS has little more than 0.75% of Ni in its composition and approximately the same percentage of others alloys elements [13].The low Ni content makes the 430 SS cheaper, however, also less active for OER.[13].Nevertheless is possible improve the 430SS anodic properties with cobalt electrodeposition.The great advantage of this method is the possibility of cobalt recover from Li-ion batteries recycling [14].
Many papers describe the cobalt recycling from spent Li-ion batteries as a valuable and environmentally friendly method [14][15][16].Moreover, very pure cobalt can be obtained by acidic dissolution of lithium cobalt oxide (LiCoO 2 ) present in spent Liion battery cathode [16].The recycling of LiCoO 2 is important both economically and environmentally [16].In this paper was studied the application of 430SS, coated with recycled cobalt, as anode for OER in 1M KOH solution.The cobalt used was obtained through dissolution of spent cathode of cell phones Li-ion batteries.The electrochemical characterization of this anode was accomplished through cyclic and linear voltammetry and electrochemical impedance spectroscopy (EIS).

EXPERIMENTAL 2.1 Preparation of cobalt electrodeposition bath
The model of Li-ion battery used in this paper was chosen based on the LiCoO 2 presence as cathodic material [16].The Li-ion battery was manually dismantled and physically separated into their different parts: anode, cathode, steel, separators and current collectors.The cathode powder was washed with distilled water under agitation to facilitate the detachment of the active material from current collector.The active material was filtered and washed with distilled water at 40 ºC to remove possible lithium salts such as LiPF 6 and LiCl 4 and dried in air for 24 h.A mass of 250.10 g of positive electrodes was dissolved in an aqueous solution containing 470.00 mL of H 2 SO 4 3.00 mol L -1 and 30.00 mL of H 2 O 2 30 % v/v.The system was maintained under constant magnetic agitation at 80 ºC for 2 h.The pH of electrodeposition bath was maintained equal to 3.0 and the cobalt concentration was 1.00 molL -1 .This high concentration was chosen to increase the electrodeposition efficiency [16].

Electrochemical measurements
Electrochemical measurements were made using an AUTOLAB PGSTAT power supply.The working electrode was made of commercial ferritic stainless steel 430.The steel samples were prepared as rectangular foils with a geometric area of 1.00 cm 2 .The auxiliary electrode, with an area of 3.75 cm 2 , was made of platinum.A saturated calomel electrode (SCE) reference electrode was used.The conversion for reversible hydrogen electrode (RHE) was made using the equation 4 and, the overpotential (η) was calculated using the equation 5 [9] .
  E (SCE)  (0.059 pH  0.242)V 1.23V (Eq.5) The The working electrodes were sanded with 600-grit sandpaper before each measurement and washed with distilled water.All the electrochemical measurements were performed without solution agitation, at 25 °C.The cobalt bath used in cobalt electrodeposition was obtained by acidic dissolution of spent cathode of Li-ion batteries.The Co +2 concentration was adjusted in 1.00mol/L.The pH chosen for cobalt electrodeposition was 4.0.The potential The potential used was -1.0V and the charge density was controlled in 3Ccm -2 [9].The scanning electron microscope JEOL JXA model 8900 RL, equipped with an energy dispersive X-ray detector, was used for surface morphology observations and surface chemical analysis.

Characterization of electrodeposited cobalt onto 430 SS
Figure 1 shows the scanning electron microscopy (SEM) of electrodeposited cobalt.The morphology of electrodeposited cobalt is commonly observed in other papers [15,16].Figure 2 shows the cyclic voltammetry of 430SS with cobalt coating in KOH 1.0molL -1 .The first peak (A 1 ) is due the formation of Co(OH) 2 layer onto cobalt previously electrodeposited [16].The peak A 2 is related to oxidation of Co +2 to Co +3 due formation of monometallic layered double hydroxides (LDH) onto electrodeposited cobalt [17] .The A 2 can also is related with Co 3 O 4 formation [18,19].The peak A 3 is relative to electrochemical reaction related to conversion of Co +3 to Co +4 [16].The formation of Co +4 is important because in alkaline media the OER generally involves the OH -adsorption as first step (Eq.6).Thus, a transition metal ion with multiple valence and strong bonding power is necessary [16].The equation x represent the adsorption of OH -by charged transition metal (M n+ ) in the hydroxide/oxide crystalline net.

Electrochemical performance
The electrochemical interfacial process is better investigated using electrochemical impedance spectroscopy (EIS).

Figure 3
and 4 compares the EIS measures of 430 stainless steel (430SS) and stainless steel with recycled metallic cobalt coating (430 SS/Co) in a KOH 1.0 molL -1 after cyclic voltammetry study.For EIS of 430SS (Figure 3-a) the interface is compatible with Randles circuit (Figure 3-b).However, for 430 SS/Co (figure4-a) the LDH formed onto electrodeposited cobalt behaves as supercapacitor (involving the equilibrium reaction shown in the Eq.3), thus two constant-phase elements must be taking account as showed in figure4-b[17].

Figure 5 Figure 1 -
Figure 5 compares the linear voltammetry of 430SS and 430 SS/Co in a KOH 1.0 molL -1 .When the overpotential reaches 370mV, the density current for 430 SS/Co is 19mAcm -2 against 0.80mAcm -2 for 430SS.The 430SS/Co exhibits the double of

Figure 3 -
Figure 3 -Electrochemical impedance spectroscopy for 430SS in KOH 1M (a) and the