Metal-Organic Framework and Carbon Black Supported Mofs As Dynamic Electrocatalyst for Oxygen Reduction Reaction in an Alkaline Electrolyte

J. Chem. Sci. (2021) 133:34 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12039-021-01900-xSadhana(0123456789().,-volV)FT3](0123456789().,-volV) REGULAR ARTICLE

Metal-Organic Framework and Carbon Black supported MOFs as dynamic electrocatalyst for oxygen reduction reaction in an alkaline electrolyte

VRUSHALI RAUTa, BAPI BERAb, MANOJ NEERGATb and DIPANWITA DASa,* aDepartment of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400 019, India bDepartment of Energy Science and Engineering, Indian Institute of Technology Bombay (IITB), Powai, Mumbai 400 076, India E-mail: [email protected]

MS received 15 October 2020; revised 18 January 2021; accepted 25 February 2021

Abstract. The Pt-based expensive catalysts and sluggish kinetics at cathode in oxygen reduction reaction (ORR) hinder the rapid commercialization of fuel cells. The quest for cheap, non-noble metal catalysts to replace Pt-based catalysts has thus become a critical issue in the field of fuel cells. The carbon black (CB) and CB supported catalyst have been explored with the ultimate goal of finding a substitute for Pt-based catalysts in fuel cells. In the present work, we synthesized Zn-based MOF (1), 1 selectivity gives H2O2 followed by two-electron pathways. However, sample 1 modification might be needed to enhance its selectivity for the generation of H2O. Two composites of MOFs with carbon black and 1 were prepared to increase the H2O yield, called 1.CB and 1.SCB. The electrochemical generation of H2O2 was analyzed by the rotating ring disk electrode (RRDE) using catalyst 1. Following the addition of CB, H2O2 yields decreased from above 93% (1) to 59% and 75% for 1.CB and 1.SCB, respectively. CB modified catalysts moved towards four- electron pathways due to the conductive nature of CB. Electrochemical Impedance Spectroscopy (EIS) has also been performed to study in detail the conductivity effect of CB and kinetic behavior of ORR in alkaline electrolyte. This research opens up a new path for ORR to advance non-precious metal catalysts based on MOFs.

Keywords. Non-noble cathode catalyst; low-temperature fuel cell; oxygen reduction reaction; metal- organic frameworks; carbon black; electrochemistry; rotating ring-disc electrode; hydrogen peroxide.

1. Introduction applications. Pt-alloys have been extensively resear- ched to tackle the cost problem thus optimizing the use Oxygen reduction reaction (ORR) has become even of Pt, with some alloys showing much better catalytic more important with the advent of electrochemical efficiency than commercial Pt/C. A variety of Pt energy storage and conversion devices such as fuel substitutes were also considered, including Pd cata- 1–5 cells, metal-air batteries. The ORR by two-electron lysts and non-noble metals (Fe, Cu, Mn, and Co are the pathway or a four-electron direct path, or two two- materials most studied) and carbon materials func- electron pathways, depending on the electrode mate- tionalized catalysts.11–16 The most widely used carbon 6–10 rial and its surface properties. Over the years many materials have been used for cathode catalysts. Long- electrode materials have been tested for the ORR to term interests have been acquired in the electro- improve electrode efficiency. In recent decades, chemical oxygen reduction on these carbon-based or among the other catalysts, platinum-based materials modified carbon electrodes. Most mediators were used have been proven to the most effective catalysts for as electrocatalysts to reduce oxygen for the production ORR. But its high cost and scarcity of this metal of H2OorH2O2. Even if the ORR mechanism is not greatly hinder the wider use of large-scale fully clear, the reduction of oxygen in carbon-based

*For correspondence Supplementary Information: The online version contains supplementary material available at https://doi.org/10.1007/s12039-021- 01900-x. 34 Page 2 of 9 J. Chem. Sci. (2021) 133:34 cathodes is generally considered to include hydrogen electrochemical impedance spectroscope (EIS) study peroxide formation in acidic and alkaline mediums. alongside electrode kinetic parameter. Carbon blacks (CB) are the most commonly used supports catalysts for fuel cells in many studies and 2. Experimental commercial applications. Electrically conductive CB has received special attention in the manufacture of 2.1 Materials conductive polymer composites.17 Low cost, high conductivity and easy availability of CB help reduce Zinc nitrate hexahydrate (Zn(NO3)26H2O, 99%), ter- the overall cost of the fuel cell. CB has been identified ephalic acid (H2BDC, 99%), Sucrose (99.5%) and as a carbon support catalyst for its specific character- NafionÒ (5 wt% solution in lower aliphatic alcohols- istics and for the application of various preparedness H2O) were purchased from Sigma-Aldrich. Iso- treatments, such as CoFeN/C, Au/C, Ag/C, Co-Ppy/C, propanol (C H OH, 99.5%) and potassium hydroxide 18–22 3 7 FePc/PANI/C or MnOOH/C. Recent electro- (KOH, 99%) from Merck. Carbon black Super P was chemical studies, including that CB oxidation provides procured from Alfa Aesar. N, N-dimethylformamide carbon supporting surface groups which facilitate Pt (Dry DMF, 99%), ethanol (absolute 99.9%) and deposition, another study found Vulcan XC-72 treated Chloroform were purchased from SD Fine chemicals. with citric acid caused smaller Pt particles to be All solutions used in electrochemical experiments 23 deposited and the porosity of CB has also been were prepared with Millipore water obtained from shown to play a role in Pt – Ru catalyst action towards Direct-Q Millipore de-ionizer. methanol oxidation.24 Yuan et al., investigated PPy, carbon black (Ketjen black) and cobalt supported catalyst to enhance catalytic activity and stability and 2.2 Synthesis of MOF-5 (1) also found a change in the catalytic ORR mechanism from a two-electron to a four-electron transfer MOF-5 (1) has been synthesized using the literature 45 process.25 procedure as reported by Huanhuan Li et al.. On the contrary, porous metal-organic frameworks Zn(NO3)2 6H2O (1.043 g, 3.05 mmol) and H2BDC (MOFs) have received a lot of attention, emerging as a (0.2086 g, 0.701 mmol) were dissolved in 100 mL of new class of crystalline pore materials with multiple DMF at room temperature. The mixture was trans- functions.26–29 MOFs have gained intensive attention ferred into a 200 mL Teflon-lined autoclave, which ° in various fields of research and industrial applications, was sealed and maintained at 120 C for 24 h. The including gas separation, storage, sensing, magnet and yield was white cube-shaped crystals. After cooling heterogeneous catalysis.30–33 MOFs can be synthesized the autoclave to room temperature, the obtained solid by an electrochemical viewpoint based on the facts that product was decanted and washed with DMF (3-4 ° MOFs can usually be considered to consist of metal times). The product was dried under vacuum at 120 C ions and ligands, repeated metal complex units and for 6 h. specific categories of metal complexes that are electrochemically active. Thus, MOFs with high electro- 2.3 Synthesis of 1.CB chemical properties and electrocatalytic activities can be built in electrochemically functional frameworks. 1.CB was prepared by the following method Although the outstanding structural properties in MOFs (Scheme 1). Synthesized 1 (8 mg) and 2 mg carbon could make it interesting and promising for electro- black was dispersed in a mixture of 1 mL ethanol and chemical applications.34–36 Recently few papers, 37 1 mL de-ionized water. The mixture was ultrasoni- regarding to Pyrolyzed MOF, MOF heteroatom- cated at room temperature for 30 min. The well-dis- doping (S, O, N, B, P),38 post modification of 39–44 persed homogeneous suspension was dried at room MOF have been reported as ORR catalysts. temperature. After drying, black colored compound In the present study, we combined the metal-organic was obtained. framework and CB towards the ORR investigation. The zinc-based MOF and its CB composite were successfully synthesized and shows potential catalysts 2.4 Synthesis of 1.SCB in the reduction of oxygen in alkaline media. The electrochemical performance and electrocatalytic 1.SCB was prepared in the following manner activity of the MOF and CB modified MOFs were (Scheme 2). Carbon black (0.15 g) and sucrose (0.8 g) examined by cyclic voltammetry, RRDE and in chloroform were mixed in a 50 mL vial with stirring J. Chem. Sci. (2021) 133:34 Page 3 of 9 34

Scheme 1. Synthesis of 1 by using solvothermal method and 1.CB by ultrasonication method.

Scheme 2. Synthesis of 1.SCB. for 2 h. Then, 1 (0.5 g) in chloroform was added to the temperature. A standard Ag/AgCl (saturated KCl) mixture. The colloidal suspension was heated at 200 °C electrode and a carbon rod (*5 mm diameter) were with stirring for 12 h for the carbonization of sucrose. used as a reference electrode, counter electrode The obtained 1.SCB was dried under vacuum. respectively. For the preparation of the working electrode, 10 mg catalyst dispersed in 5 mL of DI water, and 20 ll Naﬁon and the mixture was sonicated for 2.5 Physical characterization 15 min. After that, 10 mL isopropanol was dispersed in a mixture and it was sonicated for another 15 min to X-ray diffraction patterns (XRD) were recorded on a obtain a homogeneous ink. A well-dispersed homo- D/max 2500VL/PC diffractometer equipped with Cu a geneous suspension ink was loaded on the glassy K radiation and the corresponding work voltage and carbon electrode (rotating disk electrode (RDE; disk the current was 40 kV and 30 mA, respectively. FTIR area = 0.196 cm2) and rotating ring-disk electrode spectra were obtained using KBr pellets on a Perk- (RRDE; disk area = 0.196 cm2, ring inElmer Spectrum 400 FT-IR/FT-NIR spectrometer. area = 0.109 cm2)) to get a loading of 128 lgcm-2 Surface morphologies of the MOFs were examined by and allowed to dry at room temperature for 2 h. scanning electron microscope (SEM, MIRA 3 TES- Cyclic voltammetry (CV) curves were recorded in CAN) on an acceleration voltage of 10 kV. The ther- N2-orO2-saturated 0.1 M KOH aqueous solution from mogravimetric analysis (TGA) was performed using a 0.2 to -1.0 V (vs. Ag/AgCl) at a scan rate of 100 mV thermogravimetric analyzer (Perkin Elmer, USAA -1 ° s . The ORR linear sweep voltammetry (LSV) was TG/DTA) from 30 to 800 C with a heating rate of recorded in N - and O -saturated 0.1 M KOH aqueous ° -1 2 2 10 C min in air atmosphere. solution from 0.0 to -1.0 V (vs. Ag/AgCl), sweep rate of 10 mV s-1 at rotational speeds of 500 to 2000 rpm. 2.6 Electrode preparation and electrochemical The electron transfer number (n) was calculated by - 38,40,41 characterization using the Koutecky Levich (K–L) equation: 1 1 ¼ 1 þ 1 ¼ x1=2 þ 1 ð Þ All the electrochemical experiments were carried out J JL Jk B Jk 1 at room temperature. The electrolytes were saturated with O or N at least 30 min before electrochemical 2=3 1=6 2 2 B ¼ 0:62nFCO2ðÞDO2 m ð2Þ measurements and ﬂow was maintained during the recording. Electrochemical measurements were per- Jk ¼ nFkCO2 ð3Þ formed in 0.1 M KOH aqueous solution using a Wave Driver 20 Bipotentiostat (Pine Instrument Company, where J is the measured current density, Jk is the Grove City, PA) and a three-electrode cell at room kinetic current density, JL is the diffusion limiting 34 Page 4 of 9 J. Chem. Sci. (2021) 133:34

Figure 1. PXRD patterns of synthesized (a) 1, 1.CB and 1.SCB, TGA curves of the (b) 1, 1.CB and 1.SCB sample (10°C/min at N2 atmosphere) and FT-IR spectra of (c) 1, 1.CB and 1.SCB. current density, B is Levich slope, n is the number of heating conditions.44 Sucrose was used as a carbon electrons transferred per O2 molecule, x is the angular black (CB) binder in this reaction. CB has a higher frequency of rotation, F is the Faraday constant, k is hydrophobic characteristic than other carbon materi- the rate constant of the ORR, m is the kinetic viscosity, als. CB layer was applied on 1 by carbonization of CO2 is the concentration of oxygen in 0.1 mol/L KOH sucrose to improve the hydrostability of the resulting –6 3 electrolyte (1.21 9 10 mol/cm ) and DO2 is the material. Synthesized MOFs were characterized for -5 diffusion coefficient of O2 in 0.1 M KOH (1.9 9 10 crystallinity using x-ray diffractogram (XRD) shown cm s-1). The K–L curves have been shown a linear in Figure 1a. The PXRD pattern of 1 matches with the relationship between J–1 and x–1/2. The quantitative reported structural data obtained by Li et al.,45 1 analysis of H2O2 generated at the electrode could be diffraction peaks ca. 6.8 2h and 9.6 2h show the accurately determined by using RRDE, a platinum ring (200)cubic and (220)cubic reflections, respectively. It where the hydrogen peroxide is detected. The hydro- indicates the successful synthesis of 1 in the cubic 46 gen peroxide production (I(H2O2)%) was determined crystalline phase. The data represents a unique by the following equations:42,43 crystal phase that is in good agreement with the sim- ulated patterns of the single-crystal data.47 The = 200Ir N observed diffraction peak at 6.8 (2h) present in sample IðH2O2Þ% ¼ ð4Þ Id þ Ir=N 1, also exists in 1.CB and 1.SCB. The splitting of 9.6 (2h) peak indicates that structural change takes where N is the current collection efficiency of the Pt place due to CB incorporation and coating of CB in the ring (N = 0.2), Id and Ir the measured currents for the sample 1.CB and 1.SCB, respectively. Moreover, disk and ring electrodes, respectively. The impedance we have observed that the incorporation and coating of spectra were collected by sweeping the frequency CB in 1 framework retains the crystallinity of 1.CB from 10 kHz to 50 mHz at 10 points per decade with and 1.SCB. an AC amplitude of 5 mV rms. All these data were Figure 1b shows the TGA pattern obtained for the recorded in a single sine mode. Impedance spectro- samples. 1 shows two steps weight loss zones. The first scope was fitted using ‘‘ZSimpWin’’ software from step of weight loss in the temperature range of Solartron with complex non-linear squares (CNLS) 150-200 °C can be attributed to the loss of guest method. The experimental error in the collected data is molecules and moisture loss. The second step is due to * 5%. the decomposition of organic linkers. Due to the incorporation of CB into MOFs, 1.CB and 1.SCB 3. Results and Discussion shows increasing mass and mass loss occurs at low temperature because of the low thermal stability.48 3.1 Structure and morphology of materials 1.CB shows three steps weight loss with the first step due to guest molecules whereas the second step was 1 was synthesized by using the solvothermal method attributed to the decomposition of carbon and the third in Teflon-lined autoclave according to Li et al., by step was due to organic linker’s decomposition. mixing approach.45 1.CB was prepared by as-syn- 1.SCB shows two steps in weight loss where the thesized 1 along with CB at room temperature by the second step was attributed to the decomposition of CB ultra-sonicating method. 1.SCB prepared by CB and organic linkers simultaneously, therefore, it was layer, coated on 1 using sucrose in chloroform under shown only two steps for decomposition. J. Chem. Sci. (2021) 133:34 Page 5 of 9 34

Figure 2. Scanning electron microscope (SEM) images of 1(a), 1.CB (b) and 1.SCB (c).

FTIR patterns of the 1, 1.CB and 1.SCB samples solutions, which suggest pronounced electrocatalytic (Figure 1c) show the same functional groups. The activity of the catalysts for ORRs. 1, 1.CB, 1.SCB 3411 cm-1 (±7cm-1) peak represents the O–H and CB shows peak potential at -0.43 V, -0.40 V, vibration of water. The absence of a peak near -0.46 V, and -0.41 V, respectively and current 1700 cm-1 is indicative of the deprotonation of the density -0.48, -2.76, -0.66 and -0.71 j(mAcm-2), carboxylate groups in the H2BDC ligand. The peaks at respectively. Although synthesized MOFs are not 1586 and 1382 cm-1 (±6cm-1) are associated with showing a substantial difference in the peak potential the asymmetric and symmetric vibrations of the car- but show a significant difference in current density. boxyl groups of H2BDC ligand, respectively. The Without CB containing MOF, 1 shows less current as peaks at 815 and 737 cm-1 could be attributed to the compared to CB and CB containing MOFs. 1.CB C–H vibrations. shows a higher current as compared to CB and other SEM images of MOFs and CB containing MOFs are MOFs. This indicates 1.CB has higher conductivity shown in Figure 2. The 1 (Figure 2a) morphology is and electrocatalytic activity for ORR (Figure 3). characterized by a well-defined cubic crystals structure Linear sweep voltammetry (LSV) with a rotating in the size of 20-200 lm. After making a composite ring disk electrode was performed to analyze the ORR of 1 with CB, the structural change takes place as activity of samples more precisely. Synthesized MOF shown in Figure 2b, where the obtained crystal breaks catalysts exhibit very high catalytic performance in into small particles and forms a composite with carbon alkaline electrolyte. Figure 4a indicates the currents black. 1.CB sample undergoes ultrasonication treat- measured in the ring current (top sections) and disk ment, which provides sufficient pressure to 1 crystal, current (bottom sections) for ORR. Disk current of 1, leading to the breakage of crystal size in the range of 1.CB and 1.SCB samples show an onset potential 3-5 lm. 1.SCB (Figure 2c) shows the CB layer on of -0.33, -0.28, -0.33 V and saturated current the surface of 1 by carbonization of sucrose. The CB layer coated on 1 did not damage the structure of MOF and the crystallite framework prevented from rupture.

3.2 Electrochemical characterization

The electro-catalytic performances of 1, 1.CB and 1.SCB were compared with CB and evaluated by cyclic voltammetry (CV) in 0.1 M KOH aqueous solution saturated with N2 or O2. The CV scans were recorded within a potential range of -1.0 to 0.2 V (versus Ag/AgCl) at a scan rate of 100 mVs-1.As shown in Figure S1, featureless voltammetric currents were observed in N2-saturated 0.1 M KOH solutions for 1, 1.CB and 1.SCB materials. In comparison, Figure 3. CV of 1, 1.CB, 1.SCB and CB in O2 -1 noticeable reduction peaks emerged for O2-saturated saturated 0.1 M KOH solution at a scan rate of 100 mVs . 34 Page 6 of 9 J. Chem. Sci. (2021) 133:34 density of -1.7, -2.8, -1.8 j(mAcm-2) (versus Ag/ limitations. The electron transfer numbers of 1, 1.CB AgCl), respectively. and 1.SCB were estimated from slopes of the K–L Compared to all synthesized catalyst, 1.CB shows plots to be 1.90, 2.40, 1.95 at -0.9V, respectively. K–L slightly less negative onset potential and high satu- equation shows that reduction of O2 occurs mainly rated current density at the rotating speed of 1500 rpm. through the two-electron pathway on 1, 1.CB and Apparently, 1.CB efficiency is better for ORR reac- 1.SCB electrodes in alkaline medium, suggesting tion than other synthesized catalyst. To understand the that the reduction of O2 produces hydrogen peroxide reaction mechanism and catalytic efficiency of 1.CB, as the final product. the sample was analyzed by obtaining LSV data at To determine the quantitative analysis of hydrogen different rotation speeds as shown in Figure 4c, d and peroxide production, the per cent of (I(H2O2)) was Figure S2 (Supplementary Information). The Tafel accurately determined by using ring current (equation plots of sample 1, 1.CB and 1.SCB were extrapo- 4).40,42,52–54 All the samples generated ring current at lated from corresponding Koutecky-Levich (K–L) onset potential for the ORR. The n values are similar equation (equations 1-3) to further clarify reaction to 2 for all samples (Figure 5a), suggesting a two- kinetics and reaction pathway (Figure 4e and Fig- electron transfer pathway has been taken. There can be ure S3a, b, Supplementary Information).49–51 K–L a possibility for further electrochemical reduction of curves of 1, 1.CB and 1.SCB from -0.5 to -1.0 V peroxide to water on these disk electrodes. For that and current density show a good linear relationship reason, oxygen reduction reaction on non-precious under each potential with x-1/2, intercept b is much catalyst surface can occur via 2?2 electron transfer greater than zero, suggesting that the experimental method. The H2O2 production was calculated 93%, current density consists of both a kinetic current den- 59% and 75% for 1, 1.CB and 1.SCB, respectively sity and diffusion current density. It was proclaimed (Figure 5b). Catalyst 1 exhibits higher H2O2 produc- that the ORR process of 1, 1.CB and 1.SCB sam- tion as compared to other catalysts. After adding CB in ples were determined by both kinetics and diffusion 1, the 1.CB and 1.SCB composite indicated a substantial improvement to produce H2O in ORR activity as the production of H2O2% decreases compared to 1. For 1.CB, the electron transfer number (n) was obtained was high and the yield of hydrogen peroxide also low compared to catalyst 1, indicates that catalyst 1.CB can be a very efficacious electrocatalyst in ORR. Hence, the addition of CB increases the conductivity of catalyst and increase the percentage of H2O production, indicating that CB plays an important role in ORR. Electrochemical impedance spectroscopy (EIS) was used to explore the electron transfer kinetics and conductivity effect after the addition of carbon black in synthesized MOFs.55,56 In the high and low-frequency regions, respectively, semicircle arcs and the linear part corresponding to the charge transfer and mass transfer processes are visible. Figure 6, shows the resulting Nyquist plot of CB, 1, 1.CB and 1.SCB at -450 mV potential in O2-saturated 0.1 M KOH solution. Nuquist plots indicate semicircular arcs in high- frequency regions and low-frequency regions corresponding to the process of charge transfer and mass transfer, respectively. The resistance to charge transfer can be measured with a primary semicircle and sec- Figure 4. RRDE of 1, 1.CB and 1.SCB in O2-saturated -1 ondary semicircle analyses the resistance to mass 0.1 M KOH solution with a sweep rate of 10 mV s at transfer during ORR. The change in semicircular arc 1500 rpm (ring current (a), disk current (b)), LSV of 1.CB -1 diameter indicates the existence of transfer of elec- in O2-saturated 0.1 M KOH solution at 10 mV s with different rotation rates (c, d) and K–L plots of 1.CB trons between dissolved oxygen and the electrodes (e) from -0.5V to -1.0V. studied. 1.CB semicircle diameter is significantly J. Chem. Sci. (2021) 133:34 Page 7 of 9 34

Figure 5. (a) Electron-transfer numbers of 1, 1.CB and 1.SCB from -0.5 to -1.0 V. (b) Peroxide percentage at 1500 rpm in an O2-saturated 0.1 M KOH electrolyte.

after the addition of CB, it is slowly leading to a four- electron pathway. 1.CB catalyst shows the highest selectivity for H2O production due to the ultrasonication method compared to 1 and 1.SCB. This is attributed to the better adhesion between 1 and CB, which improves the conductivity of the electrocatalyst. Synthesized MOFs can be used as a low-cost electrode material because of the advantages such as ease of synthesized, cheaper starting materials, exceptional electrocatalytic activity and good stability.

Supplementary Information (SI) Figures S1-S3 are available at www.ias.ac.in/chemsci. Figure 6. EIS patterns of carbon black, 1, 1.CB and 1.SCB in O2 saturated 0.1 M KOH solution at -450 mV. Acknowledgements The symbols and solid lines show the experimental and fitted data, respectively. DD is grateful for the financial support received from the Department of Science and Technology INSPIRE Faculty award/research grant (IFA–13 CH–94). VR is grateful to smaller than for 1, 1.SCB and CB. 1, 1.CB and UGC-BSR, Govt. of India for the research fellowship. 1.SCB have different time constants for both pro- Sophisticated Analytical Instrument Facility (SAIF) at IIT cesses (charge transfer and mass transfer). In the case Bombay is acknowledged for TGA analysis of the samples. of catalyst 1, the kinetic of reaction is slower than the BB and MN acknowledge the Department of Science and 1.CB and 1.SCB. This indicates that the addition of Technology (DST), India for the financial support of this CB support significantly increases the rate of electron project through Grant SR/S1/PC-68/2012. transfer (rate of reaction) for 1.CB which enhances catalytic activity and conductivity in ORR. It confirms References from the EIS study that the increase in conductivity is due to the addition of CB. 1. Morozan A, Jousselme B and Palacin S 2011 Low- platinum and platinum-free catalysts for the oxygenre- duction reaction at fuelcell cathodes Energy Environ. 4. Conclusions Sci. 4 1238 2. Ellis MW, Spakovsky MRV and Nelson DJ 2001 Fuel In summary, carbon black composited with 1, samples cell systems: efficient, flexible energy conversion for (1.CB and 1.SCB) has been successfully synthe- the 21st century Proc. IEEE 89 1808 sized by using facile ultrasonication and heating 3. Niakolas DK, Daletou M, Neophytides SG and Vayenas CG 2016 Fuel cells are a commercially viable alterna- method, respectively. 1 indicates that oxygen reduc- tive for the production of ‘‘‘clean’’’ energy Ambio 45 tion occurs to give H2O2 by two-electron pathway, S32 34 Page 8 of 9 J. Chem. Sci. (2021) 133:34

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