Using Hemoglobin as a Performance Enhancer in Rechargeable

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Using Hemoglobin as a Performance Enhancer in Rechargeable Lithium-Oxygen Batteries Rudra Narayan Samajdar, Sweta M. George, and Aninda Jiban Bhattacharyya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b08164 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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The Journal of Physical Chemistry

Using Hemoglobin as a Performance Enhancer in Rechargeable Lithium-Oxygen Batteries

Rudra N. Samajdar†, Sweta M. George‡, and Aninda J. Bhattacharyya†* †Solid

State and Structural Chemistry Unit, Indian Institute of Science, Bangalore: 560012

INDIA ‡Interdisciplinary

Center for Energy Research, Indian Institute of Science, Bangalore: 560012

INDIA

Corresponding Author * [email protected]

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ABSTRACT:

We demonstrate here that hemoglobin, a biological oxygen binder / transporter can be used as a performance-enhancing additive in non – aqueous lithium oxygen batteries. In a fashion similar to the way hemoglobin binds and transports oxygen in the human blood, it can bind and transport oxygen in the electrolyte solution in a conventional lithium oxygen battery. Binding and transport of oxygen into soluble electrolyte phase enhances the efficiency of oxygen reduction reactions (ORRs) occurring at the air cathode by preventing accumulation of solid insulating discharge products at the cathode site. We observe stable galvanostatic cycling, high specific capacity, as well as low polarization in the cell in the presence of hemoglobin. EIS indicates low interfacial resistance even after several rounds of galvanostatic charge discharge cycles. We thus propose the use of oxygen binding natural biomolecules as possible redox mediators for energy harvesting systems utilizing oxygen electrochemistry in the future.

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INTRODUCTION: In the twenty first century, a major concern for science is to provide clean, efficient energy such that the human civilization continues to thrive for many more generations. Batteries, which derive electrical energy from chemical reactions, are an important source of clean, green energy.1 Known for nearly two centuries since the demonstration of the voltaic pile in 1800, batteries have come a long way in terms of identification of efficient chemistries and improvement of fabrication technology towards ‘building better batteries’ for modern society.2 Conventional lithium ion batteries function on the principle of intercalation chemistry.3 However, identification of oxygen as a high energy density storage material marked a paradigm shift from solid-state intercalation chemistry based energy storage. Abraham and co – workers demonstrated a rechargeable metal oxygen battery three decades ago.4 Lithium-oxygen batteries display a theoretical energy density of 11,140 Wh/Kg (considering the weight of metallic lithium only, since oxygen can be obtained from the atmosphere).5 This value is higher than even the best-known intercalation compounds. However, lithium – oxygen electrochemistry is non-trivial. The multistep electron transfer redox processes are influenced by choice of the electrolyte. In nonaqueous electrolytes, depending on the choice of solvents and salts, the fundamental mechanism can be a 2e- oxygen reduction reaction (ORR) involving the formation of superoxide, LiO2 and peroxide, Li2O2. However, superoxide is a metastable phase and peroxide is the more stable

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discharge product.6 Depending on the conditions, ORR can also occur through a 4e- process forming Li2O. The reversibility of the transformation of O2 to LixOy is crucial for stable battery performance. The reversibility is affected by the sluggish nature of the oxygen evolution reaction (OER) and formation of insoluble oxides (LixOy) during ORR. The high polarization derived from poor ORR kinetics during charging process leads to undesirable side reactions in the electrolyte and air cathode.7 This lowers the efficiency of the battery. There has been intense research on different components of the lithium – oxygen battery system over the last two decades. One strategy is develop efficient electrocatalysts immobilized on the porous air cathode to facilitate ORR/OER during the charge – discharge processes. However, accumulation of insulating Li2O2 on the catalyst surface blocks the air cathode pores. With progress in charge – discharge cycling, the insulating oxide layer thickens, inhibiting electron transfer, reducing the battery efficiency.8 Redox mediators (RM) are a class of compounds that have been explored for enhancing the cyclability and columbic efficiency of lithium – oxygen batteries.9

RMs like TTF

(tetrathiafulvalene) and TEMPO (2,2,6,6 – tetramethylpiperidin-1-yl-oxyl) participate in the OER by oxidizing itself through electron transport to cathode. This oxidized state chemically reacts with lithium peroxide to oxidize it to molecular oxygen. Organometallic redox mediators (e.g. phthalocyanines, terpyridines) can also bind with molecular oxygen and change its oxidation state, thus facilitating the ORR and OER.10 Incorporating a redox mediator dissolved in the electrolyte rectifies the issue of clogging of oxygen inlet pores and catalytic poisoning. It also helps in reducing interfacial resistance contributing to ohmic polarization of the cell. However, soluble redox mediators initiate redox shuttle phenomena leading to lower cycling efficiency and anode

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passivation. Ideally, redox mediation in solution phase in lithium-oxygen systems can be achieved by placing bulky mediators placed at the cathode – electrolyte interface, which bind reversibly to molecular oxygen, and thus facilitate oxygen evolution as well as reduction in charge and discharge cycles respectively. Phthalocyanines and terpyridines have been earlier reported to bind reversibly to molecular oxygen and function as redox mediators11. Porphyrins, which are closely related to phthalocyanines and are much more abundant in nature than phthalocyanines, are yet to become popular in the battery field. In 2016 hemin , an iron – porphyrin complex, which is essentially the cofactor in most heme proteins, was demonstrated as a redox mediator in lithium oxygen systems12. The bare cofactor hemin is characterized by a reversible FeII/III couple. However, its ability to complex with molecular oxygen is limited primarily due to its simple planar structure. Nature has been harvesting chemical energy from oxygen since aerobic life evolved nearly two billion years ago – a fascinating chemical phenomenon known simply as respiration13. We therefore explore the chemistry of respiration for identifying redox mediators for lithium – oxygen systems. In mammals, hemoglobin is a tetrameric protein present in the blood responsible for transport and storage of molecular oxygen. It has an iron – porphyrin active site, surrounded by alpha helical amino acid chain that gives rise to its secondary structure; four such similar subunits make up the tertiary structure of the protein. Both respiration and lithium-oxygen batteries require a reversible oxygen-binding agent. Hemoglobin, which has been shouldering the responsibility of binding molecular oxygen for life and thus binding life to aerobic earth over the last two billion years, becomes an obvious choice for a redox mediator. We have already studied redox transformations between + II/III states of iron in hemoglobin (vis a vis hemin) under electrode surface confinement14-15. We now utilize its oxygen complexing

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capability as well, in conjunction with its redox cyclability, to propose its usage as a redox mediator in lithium-oxygen battery. While the active site analogue, hemin, has already been reported in the literature, we report here a protein as an additive in the non – aqueous electrochemical environment of a battery. The bulky nature of the protein makes its diffusion within the electrolyte slow, thus negating parasitic redox shuttling effects between cathode and anode. As consequence of the oxygen complexing ability of hemoglobin, the reaction center for lithium–oxygen electrochemistry is expected to shift to the protein active site, which is in solution. This would prevent build – up of insulating discharge products on the cathode surface (which happens if the reaction site is the electrocatalyst on the cathode surface) which kills off the cell in the end. While the redox reversibility of the active site analogue is more efficient than the protein, the oxygen complexing ability of the protein is much more efficient than the bare active site. As an efficient redox mediator requires both efficient redox cycling and dedicated oxygen complexing ability, we infer that the protein will be more effective as a redox meditator compared to a heme complex. To the best of our knowledge, this hypothesis has not been made in the literature till now. Materials and Methods: TEGDME, LiClO4, and metallic Lithium are of battery grade, purchased from Sigma Aldrich, and used as received. Lyophilized form of human hemoglobin is also purchased from Sigma Aldrich, and used as received. Ultra high pure oxygen (99.85%) is purchased from Chemix Gases, Bangalore, INDIA. All galvanostatic experiments (charge / discharge cycling and GITT) are performed in an Arbin electrochemical workstation, Arbin Instruments, USA and in a SP 300 electrochemical workstation (BioLogic) Electrochemical impedance spectroscopy within the frequency range 0.01 – 106 Hz at

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constant cell voltage are performed in the SP 300 (BioLogic) workstation. Cyclic voltammograms are performed in a CHI 645 D electrochemical workstation, CH Instruments, USA. UV-vis spectra are collected in a Lambda 750 UV/Vis/NIR spectrometer, Perkin Elmer. Separator UV data is collected in diffuse reflectance mode through integrating sphere. The X-ray diffraction data are collected on a PANanalytical diffractometer using Cu Kα radiation (λ = 0.1542 nm). The Raman spectra are recorded on a Renishaw Micro-Raman 2000 spectrometer operated at LabRam HR with a diode pumped solid state laser, 532 nm laser power 35 mW with a beam spot size of about 2 µm. Fabrication of air cell: Nickel foam, pre-treated with dilute HNO3, is used as the porus substrate for the air cathode. The air cathode is made by drop casting a slurry of open ended MWCNT and PVDF binder in the ratio 90:10 on the nickel foam. The electrolyte consists of a saturated dispersion of hemoglobin in 1M LiClO4 – TEGDME solution prepared by constant stirring at room temperature under argon atmosphere in an MBraun LabStar MB 10 compact glove box. The lithium air cell is assembled in a Swagelok joint with one side open for O2 flow through the air cathode and the other side containing lithium closed with a stainless-steel piston. The cell assembly is done in the glove box under argon. The entire Swagelok setup is housed in a glass jar. Post assembly, the jar is taken out of the glove box and a flow of high purity oxygen (99.85 %; Chemix Gases, Bangalore, INDIA) is allowed to pass through the jar for 20 minutes at a flow rate of 100mL/min beyond which the system is isolated and self-sustained throughout the course of the experiments. Results and Discussion:

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UV–visible spectroscopy: The absorption spectra of porphyrin molecules is characterized by a set of transitions from the a1u and a2u to the egx state (Q bands, having weak oscillator strength) and from the a1u and a2u to the egy states (Soret band, strong oscillator strength). The Soret band occurring at around 400 nm is strongly dependent on the spin state and coordination environment around the prophyrin metal center.16 The position of the Soret band is a good marker for following oxygen binding to the heme. The Soret band for hemoglobin in water occurs at around 406 nm for the Fe (III) state. Reduction to Fe (II) causes a red shift of this band to 426 nm. On binding with molecular oxygen, the band shows a blue shift again to 413 nm. The protein in organic electrolyte shows a slight red shift of the Soret band (419 nm) compared to that in aqueous buffer. This red shift arises as a consequence of binding of electrolyte anion to the iron center (Figure S2).17 After battery cycling, there is a slight blue shift of the Soret band (416 nm) which is a consequence of O2 binding to the heme center (Figure 1). The Soret band for hemin, on the other hand shows a large shift of about 30 nm before and after battery cycling indicating change in oxidation state of the iron center on battery cycling and hence lesser stability as a redox mediator (Figure S1). The Soret band of hemoglobin stored in organic electrolyte for three weeks shows no shifts, indicating the long-term stability of the protein in the cell. (Figure S3).

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Figure 1: (Left panel): Variation in the Soret band of Hemoglobin on oxidation state change and oxygen binding – de binding. (Top) Fe (III); (Middle) Fe (II) obtained after reduction with NaBH4 (Bottom) Fe (II) on binding with molecular O2. (Right panel): (Top) Protein in organic electrolyte (on separator) before cycling (Top) and after cycling (Bottom).

Electrochemical Impedance Spectroscopy: An effective redox mediator is expected to increase the efficiency of the air cell by preventing buildup of insulating discharge products (Li2O2, Li2O, LiO2) at the air cathode. In a Nyquist plot, typically the anode-electrolyte interfacial resistance appear in the high frequency regime whereas for the air cathode – electrolyte interface appear at lower frequencies18-19. We measured the impedance spectra of the air cell post first discharge and

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charge (Figure 2A) and observed an interesting feature of double slope in the diffusion regime of the Nyquist plot. To investigate the origin of this profile in the Nyquist plot, we assembled few control cells explained in (Scheme S1). We studied the interface between protein electrolyte and lithium as well as protein electrolyte and air cathode separately (Using EIS from Li|Hb|Li and CNT|Hb|CNT respectively. Here Hb denotes the protein hemoglobin dissolved in the electrolyte as a redox mediator). We observe that the electrolyte – air cathode interface (diffusion) dominates the low frequency region of the Nyquist spectrum of the full cell. As the diffusion signature is obtained in the symmetric air cathode cell (CNT|Hb|CNT, Figure 2B) and not in the symmetric anode (Li|Hb|Li, Figure 2C) cell, we propose that the diffusion signal is due to movement of molecular oxygen mediated by hemoglobin during discharge at the air cathode20-21. This supports the hypothesis of hemoglobin acting as a redox mediator. Additionally, the buildup of insulating discharge products after several rounds of galvanostatic charge discharge cycling is not observed in the presence of hemoglobin, but seen in the cells assembled without any redox mediator and with hemin as a redox mediator (Figure S5). The equivalent circuit fit of the porous air cathode is shown in Figure S4. This diffusion component, characteristic of a CPE restricted linear diffusion at the porous air cathode22, is not observed in the absence of the protein or in the presence of hemin (Figure S5). This is because the porosity of the air cathode is lost after repeated battery cycling due to build up of insulating discharge products. The presence of insulating Li2O2, Li2O is confirmed through post cycling Raman spectra and X-ray diffraction of the air cathode from cells assembled without hemoglobin. (Figure S9,S10).The post cycling XRD of air cathode from cell assembled with hemoglobin in the electrolyte, shows no peaks corresponding to these discharge products indicating good cycling reversibility, validating the observation from EIS.

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Figure 2: Electrochemical impedance spectroscopy of (A) assembled lithium air cell after first charge (blue) and discharge (orange). (B) symmetric CNT – electrolyte – CNT cell to understand the cathode – electrolyte interface, and (C) symmetric Li – electrolyte – Li cell to understand the anode – electrolyte interface. Battery Cycling: Figure 3 shows the first galvanostatic charge and discharge cycles of air cell assembled with hemoglobin (compared to that assembled in the presence of hemin, and in the presence of the bare electrocatalyst CNT). The cell with hemoglobin shows a specific capacity of 4200 mAh/g (of loaded CNT) whereas the one with hemin shows a specific capacity of 3100 mAh/g. The cell assembled without any redox mediator shows a capacity of approximately 2200

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mAh/g (of loaded CNT). When cycled at a fixed capacity, the hemoglobin cell shows stability and constant polarization over a large number of cycles, indicating good battery stability.

Figure 3: (Left panel): Galvanostatic charge discharge cycles of lithium air in the presence of redox mediators hemoglobin (red), hemin (green); and in the absence of redox mediator i.e. bare CNT (brown); (Right panel): Voltage profile of Lithium air cell containing hemoglobin cycled with a capacity limit of 500 mAh/gCNT between 4.5 and 1.75V at a current density of 100 mA/gCNT.

Galvanostatic Intermittent Titration Technique: We study the polarization in the lithium air cells using the galvanostatic intermittent titration technique (GITT). One of the principal causes of failure of the lithium air cells over long cycles is the development of insulating discharge products at the cathode, which in turn give rise to a very high polarization and overpotentials.23 We see from the GITT data (Figure 4) that the polarization of the cell is lower in the presence of hemoglobin (1.04 V in initial cycle followed by gradual decrease with increasing cycle number) compared to that in the cell assembled without hemoglobin. The protein is expected to reduce the polarization by preventing the formation of insulating discharge products at the air cathode, thus making the

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cycling more efficient. We note that the polarization of the cell in the presence of hemoglobin is lower compared to that observed in the presence of hemin (polarization ~ 1.19 V; Figure S6). This suggests that hemoglobin can function as a more efficient redox mediator compared to hemin, presumably due to its more efficient oxygen binding properties.

Figure 4: Galvanostatic intermittent titration technique (GITT) as applied to lithium air cells assembled (orange) in the presence of hemoglobin as the redox mediator; and (blue) in the absence of the redox mediator (with only CNT as the electrocatalyst on the cathode). The cells with the redox mediator exhibit lower polarization and hence more cycling stability. Conclusions: In this paper, we demonstrate the usage of an oxygen carrying protein i.e. hemoglobin as a possible redox mediator in a lithium – oxygen cell. The FeII/III redox couple lies in between the Li2O formation and decomposition window thus, confirming its possible usage as a redox mediator for the lithium – oxygen electrochemistry at the air cathode (Figures S7, S8). The protein is dissolved

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in the electrolyte and electrochemical and battery cycling experiments are studied. We compare the data with bare heme (here hemin), which was reported earlier.12 Hemoglobin is observed to retain its electronic structure and local chemical environment around the iron center even after the galvanostatic charge – discharge cycles. This is evidenced from the retention of the porphyrin Soret band of the electrolyte soaked separator obtained from the disassembled cell before and after galvanostatic cycling. Impedance spectra indicate lower interfacial resistance at the cathode in the presence of hemoglobin, even after several rounds of galvanostatic charge discharge cycling. For cells without hemoglobin, the resistance is affected at the low frequency region where one sees the contribution due to insulating discharge products formed at the air cathode. Lower interfacial resistance indicates efficient formation and decomposition of lithium oxygen compounds during the galvanostatic charge discharge cycles, which prevents precipitation of insulating products at the cathode, and maintains cathode porosity for better redox reversibility. Only an efficient redox mediator that controls the air cathode reactions during charge and discharge can bring about this effect. Galvanostatic charge discharge cycling data show reasonably high specific capacity during charge / discharge cycles (~ 4200 mAh/g of CNT), and a stable voltage window over several rounds of cycling. We also employ the galvanostatic intermittent titration technique (GITT) to demonstrate that the battery exhibits lower polarization in the presence of hemoglobin with the overpotential decreasing with increase in cycle number. Thus hemoglobin, and heme proteins in general, can be proposed as redox mediators for lithium air batteries. The usage of a natural oxygen carrier as a redox mediator in a lithium oxygen cell is attractive as it open up a host of possibilities where one can use bio-inspired natural molecules for efficient application in energy storage and harvesting.

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ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

Comparison of UV – visible spectra of hemin adsorbed on the separator after battery assembly – before and after battery cycling; Comparison of UV – visible spectra of hemoglobin dissolved in water and in organic electrolyte in the presence of Li ion; Stability of hemoglobin in organic electrolyte observed through UV – visible spectroscopy; Modified Randles circuit fit to impedance spectra of symmetric cell CNT|Hb|CNT for analyzing cathode – electrolyte interface; EIS of cells assembled with and without redox mediators after several rounds of galvanostatic charge discharge cycling; Schematic representation of different kinds of symmetric air cells used for understanding anode / cathode electrolyte interface using EIS; Galvanostatic intermittent titration technique (GITT) data for cells assembled with and without hemin as the redox mediator; Cyclic voltammogram of hemoglobin dissolved in organic electrolyte solution; Cyclic voltammogram of lithium air cell assembled with hemoglobin as the redox mediator; Post cycling Raman spectra of air cathode from cell assembled without hemoglobin, Post cycling XRD of air cathode taken from cells assembled with and without hemoglobin; Discussion on impedance spectroscopy data analysis. (PDF)

ACKNOWLEDGEMENTS We thank Dr. G. R. Dillip for help with impedance data analysis. We also thank Dr. Suman Das and Dr. Farheen N. Sayed for help with the galvanostatic intermittent titration technique (GITT) measurements. We acknowledge the DST-SERB for financial support under CRG/2018/002242.

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