IN-SITU X-RAY STUDY OF LEAD SULFATION IN SULFURIC ACID ENVIROMENT L. CHLADIL, P. VANÝSEK, O. ČECH, AND P. BAČA

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1 IN-SITU X-RAY STUDY OF LEAD SULFATION IN SULFURIC ACID ENVIROMENT L. CHLADIL, P. VANÝSEK, O. ČECH, AND P. BAČA LABAT, Varna June 14 th,

2 Content Importance of the in-situ studies Ex-situ experiments In-situ: instrumentation Experiments Directions of the future research 2

3 Importance of the XRD In-situ Observations Lead-acid is a mature electrochemical system Many issues have been well resolved Currently there is an effort to resolve the PCL3 effect This effect relates to NAM and in more detail it is affected by the processes of nucleation and growth of sulfate crystals inside an electrode In the context of resolving the PCL3 effect we need to get more information about PbSO 4 nucleation, its growth and redistribution Figure 1: Image of the sulfated Pb electrode surface 3

4 Capabilities of XRD Measurements Monitoring of the electrochemical processes in the context of structural changes of electrode materials. Ability of qualitative and quantitative analysis. Qualitative analysis identifies specific material phases. Quantitative analysis calculates the mass ratio of the identified phases. XRD can also monitor preferred orientation of particles or preferred position of particles in respect to the electrode surface. Methods also exist (with some restriction) to monitor the average particle size of the electrode material. 4

5 XRD Ex-situ Experiments Monitoring of the PbSO 4 crystal growth in mild acidic environment Area of reproducible sulfates growth Oxidation in H 2 SO 4 at ph 2 (7 mmol/l H 2 SO 4 ), and 33 % H 2 SO 4 Several electrodes exposed (ex-situ) to sequentially increasing oxidation times at E = 0.0 V vs. MSRE Washing of electrodes in distilled water and drying in the oven at 60 C MSRE Cd ref. Figure 2: Lead electrodes prepared for oxidation 33 % H 2 SO 4 Figure 3: Pourbaix diagram for Pb in H 2 SO 4 with marked potentials of the most often used reference electrodes 5

6 IN-SITU X-RAY STUDY / LABAT 2017 XRD Ex-situ Experiment 10 min. Figure 5: Morphology of PbSO4 after 10 min. of anodic oxidation Intensity (cps) 2.0e e e e e (2 0 0) (0 2 0) Lead Sulfate Lead Oxide Sulfate Hydrate (1 2 3) (1 2-1) (1 1 2) (1 0 2 (2 1 1) (1 2(02)2-1 (2 0 2) (0 2 2) (2 1 0) (0(0023)0) (2 0 1) (1 1 1 (2 0 0) (0 1 1 (1 1 1) Lead, syn theta (deg) Figure 4: Three-electrode set up for anodic oxidation Figure 6: Composition of the analyzed layer formed during a 5-min oxidation in mol/l H2SO4 6

7 XRD Ex-situ Experiments Figure 8: Phase composition of untreated Pb electrode, and electrode oxidized for 5 min., 10 min. and 15 min. in mol/l solution of H 2 SO 4 electrolyte Figure 7: Particle size distribution of PbSO 4 for 5, 10 and 15 min. of anodic oxidation in mol/l H 2 SO 4 7

8 Advantages Summary of the Ex-situ Experiments Possibility of a full sample analysis (even SEM observation), exact study of particle size distribution and observation of crystallite growth Easy sample preparation Disadvantages It is necessary to prepare many samples issue with getting the same and reproducible surface Metal lead samples exhibit preferred orientation because of the rolling processes Possible effect of post-processing - washing up, drying -> possible generation of parasitic phases Time-consuming and laborious experiments Other possibilities: In-situ analysis of electrodes in an electrochemical cell 8

9 Equipment for In-situ Analysis Electrochemical cell Diffractometer with installed in-situ cell Figure 9: Layout of XRD ECC-Opto-Std cell by EL-CELL company (left) and internal connection of the in-situ cell inside of a diffractometer Rigaku Miniflex 600 HR (right) 9

10 Optimized XRD Cell G) C) F) B) A) D) E) Figure 10: modified parts of in-situ cell before assembling A) Working electrode (WE) B) glass separator C) counter electrode D) reference Cd electrode E) spacer ring F) gold plated wire for connection of working electrode G) polyimide window cover layer 10

11 In-situ Cell Optimization The requirements are defined by: Corrosion resistance of the cell Used devices and their geometry 2 1,5 1 0,5 0 1,5 1 Polyimide 240 µm Table 1: Comparison of Pb/PbSO 4 electrodes measured on standard cell holder and in in-situ cell Standard holder In-situ cell DS 0.1, IHS 5 Method WPPF WPPF PbSO 4 (%) 44.7(3) 44.2(3) Lead (%) 55.3(3) 55.8(4) 0,5 Polyimide 25 µm 0 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 Figure 11: Comparison of diffraction spectra for different polyimide cover layer 11

12 In-Situ Experiments For the purpose of studying of PbSO 4 crystal growth on Pb surface the following experiments were prepared: 1) Observation of the PbSO 4 growth on surface Pb substrate at equilibrium condition (at OCV) 2) Monitoring or sulfate content evolution during cycling 12

13 Working Electrodes Figure 12: Perforated lead electrode 13

14 In-Situ Experiments PbSO 4 growth on surface of Pb substrate at OCV intensity (a.u.) 3 2,5 2 1,5 1 0,5 1 hour 2 hours 3 hours 4 hours 5 hours 0 25, , , ,5 2theta Figure 13: Diffraction spectra with detail for 1 to 5 hour anodic oxidation in 33 % electrolyte H 2 SO 4 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 7,3 8,7 12,7 14,3 16,6 17,4 17,8 19,4 20,9 92,7 91,3 87,0 85,7 83,4 82,6 82,2 80,6 79,1 25,8 71, t [hod] Figure 14: Bar chart for time evolution of the phase composition of electrodes (lead and lead sulfate) during first 16 hours at OCV Pb PbSO4 33,4 66,6 14

15 In-Situ Experiments PbSO 4 growth on surface of Pb substrate at OCV % PbSO 4 = t PbSO 4 [%] time at OCV [hr] Figure 15: Dependence of lead sulfate growth on the Pb surface at OCV 15

16 In-situ Electrode Cycling I [A] 0, ,00025 Discharging 2 Oxidation peak 0, , OCV -0, Charging 6-0,00015 Reduction peak -0, ,15-0,1-0,05 0 0,05 0,1 0,15 0,2 0,25 0,3 U [V] Figure 16: Cyclic voltammetry of Pb/PbSO 4 electrode in 33 % H 2 SO 4 solution, SR 0.25 mv/s 16

17 Electrode Cycling State pure Pb charged discharged 1 st cycle 5 th cycle 10 th cycle Pb [%] PbSO 4 [%] % 0,11 2,66 90% 80% 70% 60% 43,8 51,7 53,8 54,1 50% 99,9 97,3 40% 30% 20% 10% 56,2 48,3 46,2 45,9 0% Pb nabito vybito 1 cyklus 5 cyklů 10 cyklů Pb PbSO4 Figure 17: Bar chart for evolution of the phase composition of electrodes (lead and lead sulfate) during the first ten cycles 17

18 Comparison of the Charged and Discharged States 2 C) intensity (a.u.) 1,5 1 0,5 (1 1 1) (2 0 0) (2 2 0) (3 1 1) Pb/PbSO₄ after 15 cycles D) intensity (a.u.) 0,5 Pb/PbSO₄ at 0V vs Cd theta-2theta Figure 18: Diffraction spectra of Pb/PbSO 4 electrode after cycling (15 cycles) in discharge state (up) and discharge state (bottom) 18

19 Preferred Orientation Table 2: Results of preferred orientation of PbSO 4 layer and Pb substrate obtained from Rietveld analysis of pure Pb, Pb electrode after initial oxidation and after resting at OCV Pure Pb Pb in 33% H₂SO₄ Pb/PbSO₄ after 15 cycles Pb/PbSO₄ at 0V vs. Cd/Cd²+ Lead sulfate h k l March (19) (16) 0.506(2) coefficient Lead base h k l March coefficient 1.704(16) 2.132(7) 2.125(7) 1.871(4) 19

20 Evaluation of In-situ Experiments Measurements inside of the in-situ cell have higher demands on equipment and on the electrode preparation but provide continuous results. During the first experiments the monitoring of the lead sulfate evolution on Pb substrate under equilibrium conditions were obtained and also during the subsequent 15 voltammetric cycles. Results pointed to the fact that the sulfate layer grows mainly during the first several cycles and afterwards the growth almost stops. Even when the reduction potential 0 V vs. Cd/Cd 2+ is applied (charging NAM) the sulfate layer is not fully converted to metal lead (possible insulation of electrode collector) this issue is important mainly in the context of PCL3 Preferred orientation of the substrate was identified and also a certain preferred orientation of the prepared PbSO 4 layer -> possibly the created sulfate layer is structurally determined by the Pb substrate. The precise analysis of the particle size evolution is difficult because of the large crystals in the final stage of the electrode cycling and also because of difficult determination of the preferred orientation. 20

21 Directions of the Future Work Detailed study of the relation between preferred crystallographic orientation of the substrate of the sulfate layer Possible removal of the preferred orientation by heat treatment of the Pb substrate and evaluation of its influence on crystal growth In-situ experiments with powdered electrode material and the study of possible additive benefits 21

22 Acknowledgement This research work has been carried out in the Centre for Research and Utilization of Renewable Energy (CVVOZE). Authors gratefully acknowledge financial support from the Ministry of Education, Youth and Sports of the Czech Republic under NPU I programme (project No. LO1210) and project 1618 BS_CNP1 and by ALABC Project 1618 BS_CNP1. 22

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