Blood Protein as a Sustainable Bi-functional Catalyst for

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Blood Protein as a Sustainable Bi-functional Catalyst for Reversible Li-CO2 Batteries Jae-yun Lee, Hyun-Soo Kim, Jun-Seo Lee, Chan-Jin Park, and Won-Hee Ryu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03079 • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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ACS Sustainable Chemistry & Engineering

Blood Protein as a Sustainable Bi-functional Catalyst for Reversible Li-CO2 Batteries Jae Yun Lee,1 Hyun-Soo Kim,1 Jun-Seo Lee,1 Chan-Jin Park,2 and Won-Hee Ryu1,* 1

Department of Chemical and Biological Engineering, Sookmyung Women’s University, 100 Cheongpa-

ro 47-gil, Yongsan-gu, Seoul, 04310, Republic of Korea 2

Department of Materials Science and Engineering, Chonnam National University, 77, Yongbongro,

Bukgu, Gwangju, 61186, Republic of Korea

*Corresponding author E-mail: [email protected] (Prof. Won-Hee Ryu)

ABSTRACT. Electrochemical Li–CO2 cells, which provide a sustainable and environmentally friendly pathway away from greenhouse gases, often suffer from sluggish kinetics for the growth and evolution of the cathode species on an electrode. The problematic irreversibility of solid-to-gas conversion reactions can be addressed by introducing efficient catalysts into the Li-CO2 cell. Here, we report the direct utilization of hemoglobin proteins, which are plentiful bio-resources extracted from blood wastes, to effectively boost two-way Li–CO2 reactions. The hemoglobin was immobilized on a cathodic electrode and showed excellent catalytic activity and improved capacity for CO2 reduction and evolution reactions with a desirable weight ratio between the conductive carbons and the hemoglobin catalysts. We also verified the structural characteristics of lithium carbonate product species and the reversibility of the Li– ACS Paragon Plus Environment

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CO2 reaction by ex situ studies. The iron ion active site in a heterocyclic porphyrin ring of hemoglobin can participate in the Li–CO2 reaction as a redox component.

KEYWORDS. hemoglobin, nature-derived catalyst, blood protein, electrochemical carbon dioxide capture and storage, lithium–carbon dioxide batteries

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INTRODUCTION Solving climate change arising from the emission of greenhouse gases is one of the toughest and the most challenging issues for preserving a greener and sustainable global environment.1 The annual amount of evolved carbon dioxide (CO2), which is the predominant greenhouse gas, should be strictly limited to achieve carbon neutrality. Many efforts have been made from the political (e.g., the Paris agreement)2, 3 and engineering aspects (e.g., energy mixes based on alternative fuels and renewable energies)4-6. An effective method that employs fixation and capture of evolved CO2 gas has been investigated to relieve and slow down greenhouse gas emission.7 As a typical method, chemical CO2 absorption using aqueous amine solution has been extensively introduced for reversible carbon capture and storage (CCS).8, 9 CO2 gases react with functional amine groups, and then CO2–amine clusters reversely evolve CO2 by thermal treatment.10 However, huge chemical facilities will be needed to capture and store the CO2 and a quantitatively manageable system is further required for an efficient CCS procedure.11 An electrochemical CO2 capture and storage system, called a Li–CO2 battery, has been recently reported as a promising potential solution for CCS.12-17 Li–CO2 batteries have been built based on inspiration from lithium oxygen (Li–O2) batteries18-21, which allow exceptionally high energy densities that are theoretically comparable to gasoline fuel. Based on Faraday’s law of electrolysis22, 23, our objective was to precisely determine the captured amount of CO2 reactants by controlling the number of electrons participating in the electrochemical reaction. Considering the light molecular weight of gaseous cathodes (O2: 32, CO2: 44) unlike transition metal cathodes (i.e., LiCoO2: ~ 98 for Li-ion batteries), substantial CO2 gases can be stored by reaction with Li ions. The electrochemical reaction between Li+ and CO2 forms solid lithium carbonate products (4Li+ + 3CO2 + 4e- → 2Li2CO3 + C, V0 = 2.82 V) on electrode surfaces, which is analogous to the Li–O2 cell reaction (2Li+ + O2 + 2e- ↔ Li2O2).14, 24, 25 The discharge products are reversely decomposed to Li ions and CO2 gas. Because the discharge species (i.e., Li2CO3 and Li2C2O4) are chemically stable and electrically insulating, considerable product residue remains even after reverse charge, and it is difficult to evolve without catalysts.12, 26 In this regard, recent studies have focused on exploring efficient catalyst alternatives to facilitate the Li–CO2 reaction, such as carbon ACS Paragon Plus Environment

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composites (Mo2C/CNT26, NiO–CNT27, and Ni or Cu nanoparticles on N-doped graphene28, 29), noble metals (Ir30,

31,

Ru24,

32),

metal oxides (Mn2O333, TiO225) polymers (conjugated cobalt

polyphthalocyanine34), organometallic compounds (cobalt phthalocyanine35), and so on. While they have been confirmed to offer catalytic activity for the Li–CO2 cell, most of the aforementioned catalyst candidates are synthesized in a multi-step process and involve rare elements or toxic components in the chemical species. Environmentally friendly and economical production of catalytic materials is an important factor to be considered for future Li–CO2 batteries. Bio-inspired molecular components are preferential to alternatives to develop efficient and sustainable catalysts for facilitating the Li–CO2 reaction. Ryu et al. reported that simple dissolution of heme biomolecules into the battery electrolyte can effectively catalyze reversible oxygen evolution of lithium oxide products in a Li–O2 cell.36 Heme is composed of a common porphyrin molecule containing an Fe center, which is well-known to be a cofactor of the blood protein, hemoglobin.37 A residual oxygen binding site on the Fe center exists in the heme porphyrin, where coordination with the four nitrogen atoms in the center of the ring occurs.37, 38 Moreover, the hemoglobin protein can bi-functionally carry carbon dioxide as well as oxygen in the human body when we breathe in or out. CO2 does not compete for binding on the oxygen binding site.39 It prefers to be separately bound to a histidine group (-NH2) on the protein chains of the hemoglobin structure as simultaneous redox reactions occur with the Fe center.40 Direct utilization of the hemoglobin protein can offer a facile and eco-friendly way to recycle blood bio-wastes (i.e., butchery, food industry, and medical waste) and realize electrochemical and environmental CO2 capture and storage systems.41, 42 Nevertheless, there has not been a report on protein-based catalysts for Li-CO2 batteries thus far. In this work, we report the direct use of the blood protein, hemoglobin, as a catalytic electrode component to boost the electrochemical Li–CO2 reaction (Figure 1). We introduced the hemoglobin into a multi-walled carbon nanotube (MWCNT) electrode at different weight ratios and investigated the electrochemical behaviors under different purging atmospheres. Although high loading of the hemoglobin protein catalysts into the MWCNT electrode can improve the catalytic activity of the Li-CO2 reaction, ACS Paragon Plus Environment

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electron movement in the insulating protein-containing electrode could be impeded. In this regard, we investigated the catalytic behavior in the Li–CO2 cell system and the degree of resistance by incorporating hemoglobin. The structural and morphological features of lithium carbonate products were examined, and the reversible reaction characteristics were successfully confirmed by ex situ characterizations. Our approach to use bio-inspired protein as a catalytic component for the Li–CO2 battery system offers several advantages: (i) economic catalytic materials that use existing natural materials, (ii) environmentally friendly methods for both bio-waste recycling and greenhouse gas deduction, and (iii) manageable CO2 capture and storage by introducing electrochemical battery applications.

EXPERIMENTAL Materials and Chemicals: Hemoglobin from bovine, lithium bis(trifluoromethane)sulfonamide (LiTFSI, 99.95%), diethylene glycol dimethyl ether (DEGDME, anhydrous, 99.5%), 1-methyl-2-pyrrolidinon (NMP, anhydrous, 99.5%), and poly(vinylidene fluoride) (PVDF, Mw ~180,000) were purchased from Sigma-Aldrich (Korea). We used the DEGDME solvent after moisture removal for two weeks by dipping freshly activated molecular sieves (type 4 Å) in the solvent. Preparation of Li–CO2 Cells: The air electrodes were fabricated with different hemoglobin loading amounts. An electrode with a low amount of hemoglobin (l-Hb) was fabricated with 82 wt. % MWCNT, 9 wt. % PVDF, and 9 wt.% hemoglobin dissolved in NMP solution (MWCNT:hemoglobin = 9:1), and the other one with a high amount of hemoglobin (h-Hb) was prepared with 60 wt. % MWCNT, 10 wt. % PVDF, and 30 wt.% hemoglobin dissolved in the same solution (MWCNT:hemoglobin = 2:1). To uniformly mix the hemoglobin with MWCNTs, the MWCNTs were dispersed in an aqueous solution containing hemoglobin with desired amount, and the mixed powder was collected after subsequently evaporating and drying the water solvent. The desired amount of MWCNTs and PVDF binder were mixed with NMP solution to prepare a slurry ink. The slurry was pasted onto a Ni-foam (12 mm in diameter) current collector and dried for 12 h under vacuum at 80 °C. Mass loading of each slurry was approximately 0.4 mg ± 0.05 mg on the spherical Ni foam with a diameter of 13 mm. Then, 1 M of lithium ACS Paragon Plus Environment

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bis(trifluoromethane)sulfonamide (LiTFSI, 99.95%) dissolved in the DEGDME electrolyte for over 24 h under room temperature was used as the electrolyte. A hemoglobin-free electrode fabricated with 90 wt. % MWCNT and 10 wt. % PVDF dissolved in NMP solution was also prepared. A Li metal foil (12 mm in diameter) was used as a counter electrode, and a glass fiber was used as a separator (Whatman GF/A microfiber filter paper). All Li–CO2 cells were assembled in Swagelok cells in an Ar-filled glovebox. The cells were filled with 1 atm of CO2 atmosphere (99.9999%), and before the test, they were stabilized for 48 h in a CO2 atmosphere sealed with a Teflon tape for leakage protection. Electrochemical Characterizations: To confirm the catalytic effect of the samples, a Biologic VSP potentiostat with impedance function was used for cyclic voltammetry experiments and the electrochemical impedance spectroscopy measurements of the cells. All electrochemical experiments were performed at room temperature. Charge/discharge tests were conducted using a potentio-galvanostat (WonATech, WBCS3000, Korea) at a current density of 50 mA g−1 in a voltage range of 2.3 ~ 4.5 V vs Li/Li+. Ex situ Characterizations: To confirm the crystal structures and surface of the electrodes, characterization after cycling were performed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD, Smart lab, Rigaku), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The X-ray source was Al Kα. The surface morphologies of the samples after cycling were analyzed using a scanning electron microscope (SEM). The pH values of the samples were measured by a pH meter (Thermo Scientific™ Orion™ Star A111 pH Benchtop Meter). To measure the pH of the electrodes, each sample was dipped in deionized water. All samples were prepared by galvanostatic discharging and charging at 50 mA g−1. Each electrode was collected after discharging (1st cycle) and after charging (1st cycle). A pristine electrode was prepared to compare each sample. Cells were disassembled in an Ar-filled glove box after discharging/charging in the same manner as for the pristine electrode. Collected electrodes were sealed with Teflon tape to prevent surface oxidation while transferring and measuring.


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RESULTS & DISCUSSION Eco-friendly and economic production of catalytic materials is an important factor for future energy and environmental systems. To allow direct coordination of hemoglobin with CO2 gas reactants, we introduced the hemoglobin protein (from bovine) as a catalytic agent in a MWCNT electrode on a Ni foam substrate. The average diameter of the MWCNTs was approximately 15 nm, and they exhibited uniform and continous morphological features (Figure S1). The hemoglobin involved in the electrode was not dissolved into the electrolyte solution even after 6 months, meaning that it was sufficiently immobilized on the electrode without dissociation into the electrolyte (Figure S2). Figure 2 shows the electrochemical properties of the Li–CO2 cell employing the hemoglobin protein to verify its catalytic efficacy. The cyclic voltammetry (CV) results for the Li–CO2 cell with a pristine MWCNT electrode without hemoglobin after Ar and CO2 purging are shown in Figure 2a. As expected, the current increased after CO2 purging because of the formation of lithium carbonate products compared with an inert environment with Ar gas. A peak at 2.52 V in the cathodic region is related to reduction of CO2 and subsequent formation of the discharge products. The broad anodic peaks indicate reverse decomposition of the products. To verify that the Li–CO2 reaction was facilitated by hemoglobin, Figure 2b and S3 show the CV curves of the hemoglobin-containing cell purged with Ar gas and CO2 gas. For Ar purging, the current value in the CV curve is relatively low, indicating that there was no significant reaction with inert Ar gas. However, an increase in current along the entire voltage region was found in the case of CO2 purging. Thus, the CO2 gas electrochemically reacts as an active material component. We additionally observed cathodic and anodic peaks at 2.82 V and 3.15 V, respectively. These redox peaks are related to the catalytic behavior of hemoglobin during the Li–CO2 cell reaction. While the hemoglobin can catalyze the reaction activity for the Li-CO2 cell, proteins are insulating components, which impede electron transfer in the electrode. Therefore, incorporation of the proper amount of hemoglobin could be a significant factor to realize their catalytic activity. In this regard, we controlled the concentration of hemoglobin in an ink slurry solution and examined the electrochemical properties of the pristine electrode and hemoglobin-containing electrode at a low concentration (l-Hb) and a high concentration (h-Hb) ACS Paragon Plus Environment

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(Figure 2c). The pristine MWCNT electrode exhibited an onset potential of 2.86 V and a peak voltage of 2.52 V for cathodic reaction. For the anodic region, a broad peak appeared near 3.3 V with a low current, and a sudden increase in current was observed above 4 V, corresponding to evolution of discharge products. A cathodic peak shift (2.52 V → 2.81 V) and change in the onset potential to a higher voltage (2.86 V → 3.0 V) occurred in the case of the electrode with a low concentration of hemoglobin. The current for the entire anodic scan further increased with the appearance of a distinct redox peak at 3.16 V. The anodic current increased at a low voltage compared with a pristine cell, meaning a lower overpotential. The CV results of the Li–CO2 cell employing hemoglobin confirm the catalytic effect of the hemoglobin on both the reduction (discharge) and oxidation (charge) for the Li–CO2 cell reaction. With increasing amount of hemoglobin, the current of the cathodic peak at 2.81 V becomes higher because of the catalytic effect of the hemoglobin. However, the current value through the entire voltage region is relatively lower than for the low hemoglobin concentration case. This result demonstrates that a high loading of hemoglobin protein in the electrode could decrease the electronic conductivity of the electrode and consequently hinder the electrochemical reaction kinetics. To evaluate the electrochemical performance of the Li-CO2 cell that employs the hemoglobin catalyst, as shown in Figures 2d and 2e, we recorded charge–discharge profiles with different hemoglobin concentrations and purging gases. Figure 2d shows the charge–discharge behaviors of the hemoglobincontaining electrode with Ar purging and the pristine electrode with CO2 purging. The hemoglobincontaining cell does not work under the Ar purging environment as with the CV result in Figure 2a. The capacity value after CO2 purging increased to 100 mAh g−1 even without hemoglobin. The result directly demonstrates that CO2 is a gaseous cathode material and active material for the Li–CO2 cell. Charge– discharge profiles of the Li–CO2 cell with different hemoglobin amounts are presented to examine the catalytic effects in the Li–CO2 cell, as shown in Figure 2e. While the pristine cell exhibits a low capacity and low voltage reaction for the discharge reaction, the cells that included the hemoglobin catalysts delivered 5 to 8 times higher discharge capacities of 786 mAh g−1 and 512 mAh g−1 for l-Hb and h-Hb, respectively. This significant increase in discharge capacity is mainly due to the catalytic effect of ACS Paragon Plus Environment

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hemoglobin proteins for the Li–CO2 reaction. Both electrodes of l-Hb and h-Hb exhibit higher voltage near 3.0 V at the beginning of discharge compared with that (2.65 V) for the pristine cell, indicating a lower overpotential facilitated by the hemoglobin catalysts. Interestingly, the plateau region of h-Hb near 3.0 V was continuously maintained until the end of discharge compared with that of l-Hb. Although l-Hb showed a relatively shorter plateau at high voltage and subsequently a long low voltage plateau near 2.6 V, the discharge capacity of the cell with l-Hb was much higher than that for h-Hb. These results verify that an optimal amount of hemoglobin catalyst should be introduced into the air electrode because the high loading amount of the hemoglobin protein as an insulating component can deteriorate the electronic conductivity of the entire electrode and deactivate the active surface for electrochemical reaction. As evidence of this argument, the cell employing a low amount of hemoglobin exhibited a lower charge voltage and a higher charge capacity (457 mAh g−1) compared with the high hemoglobin concentration case. To further investigate the concentration effects of hemoglobin in the electrode, we conducted cycling tests of the samples with a capacity limit of 100 mAh g−1 (Figure 2f and S4). The cycling of the Li–CO2 cell employing l-Hb catalysts was stably maintained until 200 cycles compared with pristine and h-Hb electrodes. In the case of h-Hb, sudden degradation in the capacity was observed at the beginning of the cycle, and capacity recovery was subsequently found until 40 cycles. Then, the capacity continuously decreased after 50 cycles. Capacity degradation and recovery during initial cycling could be related to the large amount of insulating protein and the subsequent revealing of the active surface. While hemoglobin materials located near the MWCNT electrode can catalyze the Li–CO2 reaction, a high amount of hemoglobin directly on the MWCNT would electrochemically deactivate the active sites. After product formation and evolution repeatedly occurred during the initial few cycles, a sufficient gap between the MWCNT and hemoglobin appeared, following activation of the electrode (Figure 2f and S4). After recovery, the capacity eventually degraded due to overloading of the insulating protein component in the electrode, implying the importance of a suitable loading amount of hemoglobin. Although excess amount of hemoglobin impedes charge transfer in the electrode, the slow Li–CO2 reaction can be effectively accelerated by adding hemoglobin as the catalyst. We present electrochemical ACS Paragon Plus Environment

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impedance spectroscopy (EIS) profiles at different discharge and charge statuses to elucidate the degree of charge resistance for the Li–CO2 cell reaction (Figures 3a and 3b). The semicircle in the Nyquist plot at high frequencies is associated with charge transfer resistance at the electrode/electrolyte interface.43-46 Prior to cell reaction, the pristine electrode without hemoglobin (~ 200 Ω) showed a relatively lower charge transfer resistance corresponding to the diameter of the first semicircle compared with the hemoglobin-containing electrode (~ 500 Ω). Upon discharge and charge, the charge transfer resistance related to the diameter increased for the pristine electrode. Then, the cell resistance of the pristine electrode continuously increased with increasing repeated cycling. However, the hemoglobin-containing electrode did not show a significant difference in the resistance after discharging and charging. To examine the degree of increase in the charge transfer resistance during cycling, Figures 3c and 3d show the difference in the charge transfer resistance between pristine and discharged or charged electrodes collected at different cycles. For the pristine electrode without hemoglobin, the difference in cell resistance gradually increased with increasing cycle number up to one (a changing degree of impedance is expressed as the cell resistance difference before and after the cycle per the cell resistance value before the cycle). However, there was no significant change for the Hb electrodes in the difference in impedance for both discharging and charging during cycling. From the EIS measurement, we confirmed that incorporation of hemoglobin in the electrode somewhat increased the electronic conductivity; however, it can improve the reaction activity for formation and reversible decomposition of lithium carbonate products in a Li–CO2 cell. This observation shows an analogous trend with the electrochemical results shown in Figure 2. The chemical structure and product characteristics at different electrochemical states were examined by ex situ characterizations. Two different possible reactions for a Li–CO2 battery occur via typical reduction and oxidation of CO2, corresponding to formation and reverse decomposition of Li2CO3 or Li2C2O4 solid products on the electrode surface as written below.24, 26

4Li+ + 3CO2 + 4e− → 2Li2CO3 + C

(1) ACS Paragon Plus Environment

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2Li+ + 2CO2 + 2e− → 2Li2C2O4

(2)

Electrochemically formed products or intermediates are different from chemically formed carbonate products. In the case of a Li–O2 battery, many researchers have observed lithium superoxide (LiO2) and lithium peroxide (Li2O2) instead of lithium oxide (Li2O).47-50 As a possible mechanism, direct reduction of CO2 gas to CO22− could be accompanied by initial electrochemical reduction. In this regard, formation of Li2C2O4 intermediate species makes sense due to acceptance of a smaller number of electrons per CO2 molecule (e.g., 4/3 e/CO2 for reaction (1), 1 e/CO2 for reaction (2)). To verify the existence of lithium carbonate species (Li2CO3 or Li2C2O4) that formed on the electrode surface, we conducted a simple immersion experiment of the ex situ electrodes (pristine, after discharge, after charge) in neutral deionized (DI) water (Figures 4a and 4b). The expected reaction steps for the immersion experiment are described below.

Li2CO3 ↔ 2Li+ + CO32−

(3)

Li2C2O4 ↔ 2Li+ + C2O42−

(4)

CO32− + 2H2O ↔ HCO3− + OH− +H2O ↔ H2CO3 + 2OH− C2O42− + 2H2O ↔ HCOOCO2− + OH− + H2O ↔ HCOOCOOH +2OH−

(5) (6)

Both lithium carbonate (Li2CO3) and lithium oxalate (Li2C2O4) as possible discharge products can be dissociated as CO32− or C2O42−, respectively, into H2O and OH− (alkaline hydroxide ions), thereby increasing the pH of the H2O solution by decreasing the discharged electrode. There is no significant dissolution of hemoglobin, which usually shows the color red when dissolved, indicating that hemoglobin catalysts were effectively immobilized on the electrode (Figure 4a). After the immersion test, we measured the pH values of the H2O solution (Figure 4b). After immersion of the pristine electrode, the solution exhibited a pH of 8.5, and the pH was increased above 10 after immersion of the discharged electrode. For the charged electrode, a lower pH was shown compared with the discharge electrode case; ACS Paragon Plus Environment

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this was because of the reversible decomposition of the lithium carbonate products. To further examine the chemical structure of the discharge products, we conducted attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) of the electrodes that were collected at different electrochemical states (Figure 4c). A peak at 1420 cm−1 related to CO2− was observed after discharge, which disappeared after charging. To identify whether the product state was most similar to Li2CO3 or Li2C2O4, FTIR data of the ex situ electrodes were compared with Li2CO3 and Li2C2O4 reference samples, as shown in Figure S5. Although the peak at 1420 cm−1 partially corresponded to the Li2C2O4 phase, the peak was most similar to the main peak of the Li2CO3 feature. In addition, there were no other peaks for Li2C2O4, meaning that the discharge products are most similar to Li2CO3. However, possible formation of Li2C2O4 as an intermediate species could be involved. In the Li–O2 cell, electrochemical reduction of oxygen gas (O2−) occurred during discharging, followed by formation of amorphous LiO2 and Li2O2 intermediates. After considerable discharging, crystalline Li2O2 products formed. To further examine the crystalline features of the discharge products, Figure 4d and Figure S6 show the XRD results of the electrode collected at different electrochemical states. There was no dominant peak associated with Li2CO3 or Li2C2O4 observed from all samples, suggesting that the carbonate discharge products do not have a crystalline structure and are more amorphous. From the ex situ Raman data, D and G peaks corresponding to MWCNT disappeared after discharging, and the peaks reappeared after charging, indicating that discharge products formed and reversely decomposed during cycling (Figure 4e and Figure S7). To investigate the structural changes in the discharge products on the electrode surface, we conducted X-ray photoelectron (XPS) measurements of the pristine, discharged, and charged electrodes in the C 1s, Li 1s, and Fe 2p spectral regions, respectively (Figures 5a–c). The pristine electrodes were soaked in a LiTFSI + DEGDME electrolyte for 24 h and subsequently dried before XPS measurement to investigate the peak signals. C−O and C−C peaks related to the MWCNT electrode were observed for the all electrodes (Figure 5a). The strong C−O peak appeared after discharge, and it decreased after reverse charge because of the trapped diethylene glycol dimethyl ether (DEGDME) electrolyte.21,

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Simultaneously, the peak representing the lithium bis(trifluoromethane)sulfonamide (LiTFSI) salt increased after discharging. These results mean that electrolyte components (i.e., salt, solvent) incorporated into the discharge products. A peak at 289.5 eV related to Li2CO3 appeared after discharge, and the peak decreased after charging due to decomposition of the discharge products. Peaks representing the PVdF binder for the pristine electrode disappeared after cell operation.30 In the Li 1s spectra, a peak associated with LiTFSI salt (56 eV) was observed for all electrodes (Figure 5b).21 A shoulder peak representing Li2CO3 (55 eV) was present and then disappeared after discharging and subsequent charging, respectively, verifying that the reversibility of the Li–CO2 cell was facilitated by the hemoglobin catalyst. Interestingly, an additional peak at 53 eV related to Li2O was also observed after discharging, and the peak reversely diminished after charging. The peak for Li2O could appear due to (1) chemical dissociation of Li2CO3 to Li2O and CO2 (Li2CO3 → Li2O + CO2) or (2) chemical reaction of Li2CO3 and C to Li2O and CO2 (Li2CO3 + C → Li2O + CO). For both possible reactions, the changes in Gibbs free energy (ΔG) are positive values at different temperatures (15~101 kJ mol−1)51, meaning that non-spontaneous chemical reactions occur. However, the calculation of the Gibbs free energies is based on normal atomic bonding of crystalline Li2CO3 compounds. In this regard, amorphous Li2CO3 products formed upon discharge could be unexpectedly dissociated to solid Li2O and gaseous CO or CO2 (Li2O∙CO2 or Li2O∙CO). Taking into account the full decomposition of the Li2O phase after charging, as shown in Figure 5b, the products were similar to intermediate species, and the unstable products were easily decomposed by reverse charging. To clarify our findings, detailed studies are needed as future work. Hemoglobin possesses an Fe ion center in their heme molecule, and the Fe components in the hemoglobin allow a catalytic redox reaction during discharging and charging. To identify the change in oxidation state of the Fe component in hemoglobin, XPS spectra of the pristine, discharged, and charged electrodes for Fe 2p are respectively shown in Figure 5c. After discharge, the peak for Fe in hemoglobin shifted to a high binding energy as a result of reduction of Fe (712.0 eV → 713.5 eV). The peak recovered to its original state after reverse charging, verifying the redox reaction of Fe for the cell reaction. The incorporation of hemoglobin facilitates electrochemical formation and decomposition of Li2CO3 products. We observed that the ACS Paragon Plus Environment

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discharge products were covered with planar and amorphous shapes on the pristine electrode and had particular shapes, confirming the reversible formation and evolution of the products (Figures 5d and 5e).

CONCLUSION In summary, we report catalytic effects of the blood protein, hemoglobin, as a bio-plentiful and ecofriendly catalyst alternative for facilitating electrochemical Li–CO2 cell reactions. A simple mixture of hemoglobin with a MWCNT electrode exhibited improved electrochemical reactivity for carbon dioxide reduction and oxidation, thereby significantly enhancing cell performance (i.e. capacity and cyclability). We found that there is an optimized ratio of insulating protein catalysts and conductive carbons for the tradeoff relationship between catalytic activity and electron transfer, respectively. Although introduction of hemoglobin into the air electrode slightly increased the charge transfer resistance at the electrochemical interface, the charge transfer resistance values were stably maintained for repeated cycles, unlike the pristine electrode reference. We showed reversible solidification and decomposition of lithium carbonate products by ex situ characterizations. The features of the discharge products are similar to an amorphous Li2CO3 structure with a planar morphology. We also verified that Fe ions in hemoglobin catalyze CO2 reduction and reverse oxidation of the Li2CO3 reaction through reversible redox reaction of the Fe ions. This study offers an interesting direction for seeking efficient and sustainable catalytic materials derived from bio-waste and for subsequently employing biomaterial catalysts for environmentally friendly CO2 reduction technologies based on chemistry for beyond batteries.

ASSOCIATED CONTENT Supporting Information Available: Experimental procedure, SEM and TEM results, photographs of electrolytes, ex situ ATR-FTIR results, ex situ XRD results, ex situ surface morphologies, This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Won-Hee Ryu E-mail: [email protected] (Prof. Won-Hee Ryu)

NOTE The authors declare no competing financial interest. ACKNOWLEDGEMENT

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2019R1C1C1007886). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1A5A1025224).


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Figure 1. Schematic illustration of an air electrode that employs the blood protein, hemoglobin, as a catalytic component and its reaction characteristics in the electrochemical Li–CO2 system


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Figure 2. Cyclic voltammetry (CV) curves of Li–CO2 cells under an Ar and CO2 atmosphere with (a) a pristine electrode and (b) the hemoglobin-containing electrode. (c) CV curves of Li–CO2 cells with and without different concentrations of hemoglobin in air electrodes; low (l-Hb) and high (h-Hb) concentrations. (d) Initial charge/discharge curves of the pristine electrode under an inert Ar atmosphere and h-Hb electrodes under a CO2 atmosphere in the voltage window between 4.5 and 2.3 V at a current density of 50 mA g−1carbon. (e) Initial charge/discharge curves of Li–CO2 cells with and without different concentrations of hemoglobin in the air electrodes in the voltage window between 4.5 and 2.3 V at a current density of 50 mA g−1carbon. (f) Cycle tests of Li–CO2 cells with and without different concentrations of hemoglobin in air electrodes under a specific capacity limit of 100 mA h g−1carbon between 4.5 and 2.3 V at a current density of 50 mA g−1carbon. ACS Paragon Plus Environment

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Figure 3. Electrochemical impedance spectra (EIS) of the pristine electrode and Hb-containing electrode before and after the 1st, 5th, and 10th (a) discharge and (b) charge. The ratio of impedances at different electrochemical statuses for the 1st, 5th, and 10th (c) discharge and (d) charge cycles compared with the impedance of the as-prepared electrode.


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Figure 4. Ex situ measurements of the air electrodes. (a) Photograph of the pristine electrode, the electrodes collected after discharging and charging, and the pristine electrode dipped in deionized water used to measure the (b) pH value. (c) ATR-FTIR results, (d) XRD diffraction patterns, and (e) Raman spectra of the hemoglobin-containing electrodes before and after discharging and charging.


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Figure 5. Ex situ measurements on air electrodes. X-ray photoelectron spectra (XPS) obtained from pristine, discharged, and charged electrodes for (a) C 1s, (b) Li 1s, and (c) Fe 2p. Magnified SEM images of (d) the pristine and (e) discharge products on the Hb-containing electrode.


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Table of contents (TOC) image

Synopsis

Direct utilization of hemoglobin proteins as plentiful bio-catalysts extracted from blood wastes can effectively facilitate reversible Li–CO2 battery reaction.


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Blood Protein as a Sustainable Bi-functional Catalyst for

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