Superhydrophobic Air-breathing Cathode for Efficient

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Superhydrophobic Air-breathing Cathode for Efficient Hydrogen Peroxide Generation through Two-electron Pathway Oxygen Reduction Reaction Qian Zhao, Jingkun An, Shu Wang, Yujie Qiao, Chengmei Liao, Cong Wang, Xin Wang, and Nan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09942 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Date:

June 7, 2019

Submitted to:


Superhydrophobic

Air-breathing

Cathode

for

Efficient

Hydrogen Peroxide Generation through Two-electron Pathway Oxygen Reduction Reaction Qian Zhao a, Jingkun An a, Shu Wang a, Yujie Qiao a, Chengmei Liao b, Cong Wang a, Xin Wang b, Nan Li *a a

Tianjin Key Lab Indoor Air Environmental Quality Control, School of Environmental

Science and Engineering, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin 300072, China b MOE

Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key

Laboratory of Environmental Remediation and Pollution Control, Nankai University, No. 38 Tongyan Road, Jinnan District, Tianjin, 300350, China * Corresponding authors: E-mail: [email protected]

Abstract Electrochemical catalysis of carbon-based material via two-electron pathway oxygen reduction reaction (ORR) offers great potential for in-situ hydrogen peroxide (H2O2) production. In this work, we tuned catalyst mesostructure and hydrophilicity/ hydrophobicity by adjusting polytetrafluoroethylene (PTFE) content in graphite/carbon black/PTFE hybrid catalyst layer (CL), aimed to improving the 2-electron ORR activity for efficient H2O2 generation. As the only superhydrophobic CL with initiating contact angles of 141.11 °, PTFE0.57 obtained the highest H2O2 yield of 3005 ± 58 mg L−1 h−1 (at 25 mA cm−2) and highest current efficiency (CE) of 84% (at 20 mA cm−2). Rotating ring disk electrode (RRDE) results demonstrated that less PTFE content in CLs would 1

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result in less electrons transferred and better selectivity towards 2-electron ORR. Though the highest H2 concentration (2 μmol L−1 at 25 mA cm−2) was monitored from PTFE0.57 which contained the lowest PTFE, the CE decreased inversely with increase of PTFE, which proved that H2O2 decomposition reaction was the major side reaction. Higher PTFE content increased the hydrophilicity of CL for excessive H+ and insufficient O2 diffusion, which induced H2O2 decomposition into H2O. Simultaneously, electroactive surface area of CLs decreased with higher PTFE content, from 0.0041 m2 g−1 of PTFE0.57 to 0.0019 m2 g−1 of PTFE4.56. Besides, higher PTFE content in CL leads to the increase of total impedance (from 14.5 Ω of PTFE0.57 to 18.3 Ω of PTFE4.56), which hinders the electron transfer and ORR activity furtherly.

Keywords Superhydrophobic, hydrogen peroxide (H2O2), oxygen reduction reaction (ORR), polytetrafluoroethylene (PTFE), air-breathing cathode

1. Introduction Hydrogen peroxide (H2O2) is a versatile and environmentally benign chemical without the generation of hazardous residues upon decomposition.1-3 It is highly valuable for many manufacturing industries, such as pulp bleaching and textile, as well as for electronic industries, wastewater treatment, chemical oxidation (including the largescale production of propene oxide from propene oxidation) and so on. Traditionally, H2O2 is obtained by the anthraquinone oxidation (AO) process on an industrial scale. Nevertheless, the AO process involves the sequential hydrogenation and oxidation of an alkylanthraquinone precursor dissolved in a mixture of organic solvents, followed by liquid–liquid extraction to recover H2O2. The multistep method with significant energy input and waste generation can hardly be regarded a green method.4 Furthermore, the transport, storage, and handling of bulk H2O2 exist potential hazards and additional costs.5 Therefore, greener and cleaner methods for H2O2 in-situ generation and 2


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utilization are extremely demanded. Electrochemical reduction of O2 to H2O2 via 2-electron oxygen reduction reaction (ORR) offers a great potential for in-situ H2O2 production.6-10 Over the years, significant endeavors have been concentrated on developing a suitable electrocatalyst with high selectivity for the reduction of O2 to H2O2. Based on a combination of density functional theory (DFT) calculations and experiments, it is found that activity and selectivity can be described by the *HOO binding energy and the geometric arrangement of the catalyst sites. Consequently, Ag-Hg and Pd-Hg alloy exhibited excellent activity and selectivity towards H2O2 production.8 Likewise, PtHg4 was also proven to be highly selective, active, and stable for the 2-electron ORR through theoretical model and tests.11 Unfortunately, the sophisticated fabrication, high costs, and poor durability of these materials will be bottleneck in their practical application.12 Recently, the carbonaceous material was regarded as an alternative for noble metals or transition metals.13-14 Generally, carbon-based materials (e.g., activated carbons, fullerene, carbon nanotubes (CNT), carbon nanowalls, carbon aerogels, graphite, graphene, graphdiyne, etc.), have a variety of existence forms including powders, ﬁbers, aerogels, composites, sheets, monoliths, tubes and so on.15 Carbon-based material was amply used as catalytic material due to its large surface area, good electrical conductivity, corrosion resistance, global abundance, and low price.16 In recent years, several metal-free heteroatom-doped carbon-based materials (e.g., nitrogen,17 fluorine,5 sulfur,18 boron19) have been proved to exhibit the ORR activity. Additionally, a growing community of researchers have synthesized mesopores carbon materials by means of doping nitrogen into carbon materials for 2-electron ORR electrocatalyst.6,

20-21

Therefore, the carbon-based materials have good application prospect and better economic value in ORR. A typical air-cathode, composed of a hydrophobic gas diffusion layer (GDL) facing to the air and a submerged catalyst layer (CL) facing to the electrolyte, maintains O2 diffusion path from air and then enable oxygen reduction on the cathodes.22 In previous researches, different air-cathodes were developed for 2-electron ORR to generate 3



H2O2.23-24 Our group reported a heteroatom-free carbon black/graphite (CB&G) hybrid air-breathing cathode using PTFE as binder, which was proven to be efficient for H2O2 production.25-26 It is well-known that an ideal balance of hydrophilicity and hydrophobicity can result in a steady three-phase interfaces (TPIs) among the electrolyte solution, O2 and catalytic sites in the CL for the sustainable electrogeneration of H2O2.27-28 The solid phase offers electron and catalyst, the gas phase is responsible for gas diffusion, whereas liquid phase supplies proton as well as transfers products. However, an ideal balance of hydrophilicity and hydrophobicity has hardly been investigated yet for air-cathodes. Furthermore, it is regarded that the electrocatalytic behavior can be influenced by the surface and porous architecture.5 In addition to enhancing the catalytic performance, an excellent porous architecture would ensure diffusion of reactants and products. Nevertheless, the relationships between the porous architecture and activity towards 2-electron ORR for H2O2 production have not been studied clearly. No matter for submerged electrode, gas diffusion electrode or aircathode, the binder (Nafion, polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyvinylidene fluoride (PVDF), etc.) is indispensable for fabrication of molding electrode from powder material.29 Thereinto, PTFE is more popular because of its low cost, hydrophobic property, excellent thermal and chemical stability.30 Moreover, the use of PTFE would tune the hydrophilicity/hydrophobicity of the electrode, which determines the establishment of TPIs. In the present work, we investigated the effects of hydrophilicity on the TPIs equilibrium and 2-electron ORR activity. Catalyst porosity character and hydrophilicity/hydrophobicity of air-breathing cathodes were tuned through adjusting PTFE content in CLs. The characterization techniques including scanning electron microscopy (SEM), nitrogen adsorption–desorption, and contact angle were applied to investigate the mechanism behind the influence of hydrophilicity/hydrophobicity to ORR activity. Furthermore, the electrochemical techniques including rotating ring disk electrode (RRDE), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were employed to analyze 4


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electrochemical properties of different air-breathing cathodes. 2. Materials and methods 2.1 Preparation of electrodes The CB&G air-breathing cathode was a typical sandwich structure with CL, the stainless-steel mesh (SSM) as current collector and matrix, and GDL. The CL with gradient content of PTFE were made by following procedures. The mixture of 5 g graphite powder (40 μm, HTF0325, >99.9%, Huatai Chemical Reagent Co. Ltd., Qingdao, China) and 1 g carbon black (CB, 30 nm, Vulcan XC-72R, >99.9%, Cabot Corporation, US) was dispersed into 45 mL ethyl alcohol in a beaker with ultrasonic stirring for 10 min, followed by drops adding different volume of PTFE emulsion (60%, Horizon, Shanghai, China)(0.57, 1.14, 2.28, 3.42, and 4.56 mL, individually). After another 10 min ultrasonic agitation, the mixture was stirred at 80 °C to get a dough-like paste as previous work.31 The paste was rolled onto one side of the 0.3 mm SSM (Type 304N, 60 meshes, Detiannuo Commercial Trade Co. Ltd., Tianjin, China) to be a flat sheet in thickness of 0.5 mm.32 Similarly, the GDL was prepared parallelly by the mixture of CB and PTFE with mass ratio of 4:9, followed by roll-pressing onto the other side of the SSM to form the air-breathing cathodes with total thick of 1 mm.26 The air-breathing cathodes with gradient PTFE content were marked as PTFE0.57, PTFE1.14, PTFE2.28, PTFE3.42, and PTFE4.56, respectively. It is worth noting that the PTFE volume is not authentic PTFE content used for H2O2 electro-generation which was performed in the following section. The PTFE mass percent was showed as below (Table 1): Table 1: The corresponding PTFE content in CLs of five air- breathing cathodes. Cathode samples

PTFE0.57

PTFE1.14

PTFE2.28

PTFE3.42

PTFE4.56

PTFE mass percent

12.5%

22.2%

36.3%

46.1%

53.3%

2.2 Cathode characterization The surface morphology of CLs with different PTFE content was characterized using SEM (Hitachi SU8010, Japan) analysis at magnification of 10 K.33 Nitrogen adsorption–desorption isotherms were measured at 77 K on an Autosorb-iQ (Quantachrome, USA). The specific surface area distributions were determined based 5



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on the Brunauere-Emmette-Teller (BET) method, whereas pore volume distributions were analyzed using Barrett-Joyner-Halenda (BJH) model. The contact angles were commonly used to embody the hydrophilic and hydrophobic properties of materials and then predict their wetting behavior. The contact angle is defined as the angle between the liquid–vapor and solid–liquid interfaces of a solid–liquid–vapor system.34 Water contact angles were measured using a contact angle meter (JC2000D, China) with a water drop volume of 0.2 μL. The contact angle was measured at the moments of 10 and 60 seconds since the droplet contacted material surfaces. 2.3 Reactor construction and operation All the H2O2 production experiments and electrochemical tests were performed in an undivided chamber cubic reactor with diameter of 3 cm and length of 4 cm (net volume of 28 mL) unless special illustration.22 A magnetic stirring apparatus (SP88857106; Thermo Scientific, USA) was set under the reactor. Different air-breathing cathodes with surface area of 7 cm2 were assembled on the side of reactors with CL facing to electrolyte and GDL facing to ambient air. The Pt sheet anode (1 cm2) was placed 2 cm away from air breathing cathode in parallel. Galvanostatic mode was applied with a DC power (KD3005D digital-control, Korad, Shenzhen, China) at current range of 35 – 175 mA. The electro-generation of H2O2 was conducted in 50 mM Na2SO4 electrolyte, and hydrogen (H2) content variation was monitored real-time using microsensors connected to a micromanipulator and a multimeter (MM-Meter, Unisense, Aarhus N, Denmark).35 The H2 microsensor probe was perpendicularly set in chamber close to air-breathing cathode around 0.2 cm. During the H2O2 electro-generation, 2 mL solution was sampled at every 20 min and measured at 400 nm by Multiscan Spectrum (Spark 10M, TECAN, Switzerland) using potassium titanium (IV) oxalate method.36 The current efficiency (CE, also called Faradaic efficiency) of the electrolytic cell for H2O2 generation, defined as the cathodic H2O2 generation divided by the charge production,37 was calculated with Eq. (1). 𝐶𝐸 =

𝑛𝐹𝐶H2O2𝑉 𝐼𝑡

(1)

× 100% 6


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Where n is the number of electrons transferred for oxygen reduction to H2O2, F is the Faraday constant (96,486 C mol-1), CH2O2 stands for the concentration of H2O2 (mol·L1),

V is the bulk volume (L), I is the current (A), and t is the time (s). The air-breathing cathodes with gradient PTFE content in CL were used repeatedly

at 20 mA cm−2 current density in order to examine their catalytic stability. The H2O2 concentration was measured and the electrolyte was renewed per hour. The experiments were carried out at room temperature (25 ± 1 ºC) unless otherwise specified. All the experiments were carried out at least in triplicate and the mean values were reported with the standard deviation as error value. 2.4 Electrochemical measurement LSV and EIS analysis were conducted in the 28 mL reactor using a potentiostat (Autolab PGSTAT 302N; Metrohm, Switzerland) and companion software, NOVA in 50 mM Na2SO4 solution. The air-breathing cathodes, platinum sheet (1 cm2) and Ag/AgCl electrode (3.5 M KCl, 0.197 V vs. SHE) were used as working electrode, counter electrode and reference electrode, respectively. The potentials mentioned here were all versus Ag/AgCl except as noted. LSV was proceeded at a scan rate of 10 mV s−1 over a potential range of 0.3 – −3.0 V. At the end of LSV measurement, the concentration of H2O2 in the electrolyte was analyzed.36 EIS was implemented at −0.5 V in the frequency from 100 kHz to 0.1 Hz with the voltage amplitude of 10 mV. RRDE (Metrohm Autolab B.V., Utrecht, The Netherlands) was carried out in a threeelectrode system attached to the potentiostat/galvanostat (Autolab PGSTAT 302N; Metrohm, Switzerland), with a theory collection efficiency of 24.9%. RRDE Pt-GC (glassy carbon), which consisted of a 5 mm-diameter disk and a 375 μm-thickness ring, was used as working electrode. Different catalysts with gradient PTFE volume were made for RRDE investigation. For preparing the ink electrode, 50 mg graphite powder and 10 mg CB powder were suspended in a mixture containing 3 mL isopropyl alcohol, 0.2 mL Nafion (5%, Hesen DE520, Shanghai, China) and 0.8 mL deionized water, following with adding 5.7, 11.4, 22.8, 34.2 and 45.6 μL PTFE emulsion, respectively corresponding to the CB&G air-breathing cathodes with different PTFE content. At the 7



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same time, the commercially available CB (30 nm, Vulcan XC-72R, >99.9%, Cabot Corporation, US), which is commonly employed as benchmark electrocatalyst for the ORR, was used as a reference for 2-electron-pathway generating H2O2.38 A 4.5 μL of the mixture was pipetted onto the well-polished 5 mm-diameter disk after a quick shake for a while and then dried at room temperature. A solution of 0.1 M KOH (300 mL) was used as electrolyte. For all RRDE tests, the electrolyte was aerated with oxygen for 20 min before measurements and the oxygen flow was maintained on the electrolyte during the tests. RRDE tests were carried out from 0.2 to −0.9 V at scan rate of 10 mV s−1 with the ring potential at 1.0 V (vs. Ag/AgCl)39, and the working electrode rotation rate from 400 to 3600 rpm. At the same time, steady state polarization curves in N2saturated 0.1 M KOH solution were obtained for PTFE0.57, PTFE1.14, PTFE2.28, PTFE3.42, PTFE4.56, and Vulcan XC 72R. The ORR activity was determined by subtracting the current obtained in an N2-saturated electrolyte from that obtained in an O2-saturated electrolyte.40 The experiments were controlled and analyzed with Metrohm Autolab B.V. NOVA 2.1.3 software. The selectivity of H2O2 production was assessed by calculating the electron transfer numbers from the slopes of the Koutecky–Levich (KL) plots according to the following three equations (Eqs. (2, 3, and 4))21, 41: 1 𝑗

1

1

(2)

= 𝑗𝑘 + 𝐵 × 𝜔1/2

―1/6 B = 0.62 × n × F × C𝑂2 × 𝐷2/3 𝑂2 × 𝑣

(

(3)

)

𝑛

X(𝐻2𝑂2) (%) = 2 ― 2 × 100

(4)

Where j and jk are the responsive current density and kinetic current density, respectively (mA cm-2), B is the slope of the KL plot, ω is the angular velocity (rpm), n is the electron transfer numbers, CO2 is the bulk concentration of O2 (mol cm−3), DO2 is the diffusion coefficient of O2 in the electrolyte solution (cm2 s−1), v is the kinematic viscosity (cm2 s−1), and X(H2O2) is selectivity percentage towards H2O2 generation. CV was used to determine the electroactive surface area of different air-breathing cathodes in a potential range of 0.8 to −0.2 V at a scan rate of 10 mV s−1. CV was performed in a three-electrode cell filled with solution of 10 mM K3[Fe(CN)6] and 1.0 8


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M KCl with air-breathing cathode as working electrode.42. The electroactive surface area was calculated on the basis of Randles-Sevcik formula (Eq. (5))2: 𝐼p = 2.69 × 105 × 𝐴𝑒𝑐𝑎 × 𝐷1/2 × 𝑛3/2 × 𝛾1/2 × 𝐶

(5)

where Ip is the peak current (A), n is the number of electrons involved in the redox reaction (n = 1), Aeca is the area of the electrode (cm2), D is the diffusion coefficient of the molecule in solution (7.6 × 10−6 cm2 s−1), C is the concentration of the probe molecule in the bulk solution (1 × 10−5 mol cm−3), and γ is the scan rate of the CV (0.01 V s−1). 3. Results 3.1 Electrocatalytic activity of cathodes in H2O2 generation For all air-breathing cathodes, an apparent increase in H2O2 yields was observed with increase of j (Figure 1A), whilst CE showed a tendency of first increase and then decline as a function of j (Figure 1B). The H2O2 yields of PTFE0.57 at 5, 10, 15, 20, and 25 mA cm-2 were 521 ± 1, 1059 ± 6, 1835 ± 7, 2666 ± 66, and 3005 ± 58 mg L−1 h−1, corresponding to CE of 66%, 67%, 77%, 84%, and 76%, respectively. Previous studies reported different carbon-based electrodes (including CB-carbon fiber, carbon-PTFE, PTFE-carbon cloth and CNT electrodes) can electrochemical generate H2O2 at 20 – 30 mA cm−2 with yield of 147 – 762 mg L−1 h−1 and CE of 30% – 57%.43-46 Obviously, both the H2O2 yields and CE of PTFE0.57 were much higher than before-mentioned researches at similar j. Besides, cathodes submerged into the electrolyte such as graphite felt were air aeration into the electrolyte, and most of gas diffusion electrodes previously reported always use high pressure air flow as oxygen supply. Therefore, exogenous air supplies no matter air aeration or high-pressure air flow was ineluctably. However, the CB&G air-breathing cathodes were working by free air diffusion.

9



Figure 1. The H2O2 yields (A) and CE (B) as a function of j for PTFE0.57, PTFE1.14, PTFE2.28, PTFE3.42, and PTFE4.56 at different current densities. H2 content as a function of time for PTFE0.57 during electrolysis. (C) H2 content as a function of time for different cathodes at 25 mA cm-2. (D) It can be seen that H2O2 yields increased with decrease of PTFE content at each j (Figure 1A). The H2O2 yields of PTFE0.57 was 22% higher than PTFE4.56 at 5 mA cm−2, while 41% higher at 25 mA cm−2. Higher deviation was observed with the increase of j. H2 content in the electrolyte was real-time monitored at various current densities. Trace content of H2 (0.48 μmol L−1) was detected for PTFE0.57 at 5 mA cm−2, then increased to 0.62, 0.85, 1.39, and 2.02 μmol L−1 at 10, 15, 20 and 25 mA cm−2, respectively. (Figure 1C). In fact, we have tried a lower PTFE content in CL. However, the lower PTFE content than PTFE0.57 would hardly form dough-like paste in the 10


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fabrication of air-breathing cathode. Therefore, the air-breathing cathode PTFE0.57 performed best for H2O2 generation on the premise that the air-breathing cathode can be formed. As for catalytic stability of air-breathing cathodes, it can be seen from Figure S1 that the H2O2 concentration decreased 0.6%, 1.3%, 4.4%, 15%, and 21% with increasing PTFE content after six rounds. It shows that the catalytic stability for H2O2 generation decreased with increase of PTFE content in CL. It is worth noting that the cathodes at j of 20 mA cm−2 exhibited the highest CE towards H2O2 production. The j was the major driving force of electron transfer, which determine the kinetics and efficiency in electrolytic processes. The electron transfer rate was limited with j less than 20 mA cm−2, which resulted in part of electrode capacity underused.47 Higher j can accelerate the electron transfer and ion migration rate, which further promote the 2-electron ORR for higher yields of H2O2. On the other hand, the CE decline with j higher than 20 mA cm−2 was partially attributed to the electron consumption in side reactions (Eqs. (6, 7, and 8)). 2H + +2𝑒 ― →𝐻2

(6)

𝐻2𝑂2 +2𝐻 + +2𝑒 ― →2𝐻2𝑂

(7)

1

(8)

𝐻2𝑂2→2𝑂 + 𝐻2𝑂 2

3.2 Structural characterization Surface morphology and pore structural properties. The main structure of CLs with different PTFE content was cross-linked networks of graphite flake and CB particles bound by filamentary PTFE, forming a rough and porous surface (Figure 2 A1-A5). Besides, various transport channels were formed in all of the CLs. For PTFE0.57, the CL surface was interspersed with little PTFE silk, whereas the surface of PTFE4.56 was covered by large amount of PTFE silk. Apparently, the interlaced PTFE networks became more complicated with higher content of PTFE (Figure 2 A1-A5). After electrogeneration of H2O2, the main structure of CLs was hardly changed. (Figure 2 B1-B5) It can be considered that all of the CLs were stable in structure.

11



Figure 2. SEM images of the CLs of fresh air-breathing cathodes and after electrogeneration of H2O2 with five different PTFE content at magnification of 10 K. (Fresh CLs: (A1) PTFE0.57, (A2) PTFE1.14, (A3) PTFE2.28, (A4) PTFE3.42, and (A5) PTFE4.56; used CLs: (B1) PTFE0.57, (B2) PTFE1.14, (B3) PTFE2.28, (B4) PTFE3.42, and (B5) PTFE4.56.) Pore surface area (C) and pore volume (D) distribution of different CLs with different PTFE content. The PTFE dosage of 0 mL represented CB&G powder without PTFE. The CLs with different PTFE content were further characterized by N2 adsorptiondesorption measurements at 77K. (Figure S2) Adsorption–desorption isotherms of all the CLs belongs to Type Ⅲ with weak affinities between the solid surface and N2 molecule but strong intermolecular force.48 Moreover, the hysteresis loops are type H3 with a steeper shape and two capillary condensation steps at higher relative pressures, demonstrating more macropore adsorption of Lamellar granular material.49 The total BET surface area decreased slightly with 12.2, 11.9, 11.2, 10.9, and 10.5 m2 g − 1 for PTFE0.57, PTFE1.14, PTFE2.28, PTFE3.42, and PTFE4.56, respectively. (black squares in 12


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Figure 3B) As shown in Figure 2C and 2D, pores of 3 – 10 nm were dominated in CB/graphite powder, which account for 45% of the total pore surface area. After adding gradient PTFE emulsion (0.57, 1.14, 2.28, 3.42, and 4.56 mL) following by rolling, the pore surface area of 3 – 10 nm decreased from 5.88 to 2.08, 1.17, 0.96, 0.85, and 0.62 m2 g−1 for PTFE0.57, PTFE1.14, PTFE2.28, PTFE3.42, and PTFE4.56, respectively. (Figure 3B) The coverage of PTFE against the carbon sites leaded to the pore surface area of 3 – 10 nm decreased, while increased surface area of 30 – 60 nm and 60 – 130 nm could be attributed to making pores after adding PTFE and rolling press. Simultaneously, comparing to CB&G powder, pore volume of 3 – 10 nm mesoporous in rolling CLs decreased by 69%, 77%, 87%, 89%, and 92%, respectively.

Figure 3. Electrocatalytic activity towards [Fe(CN)6]3−/[Fe(CN)6]4− redox couple by CV of the five air-breathing cathodes. (A) Comparison of BET surface area including the pores at the range of 3 – 10 nm (red dots) and total pores (black squares), and electrochemical active surface area (blue triangle) resulted from N2 adsorptiondesorption and CV in the ferrocyanide system, respectively. (B) Hydrophilicity and hydrophobicity. To illustrate the wettability of the CLs, the water contact angles were tested. Though all of the CLs were all hydrophobic, hydrophobicity decreased with higher PTFE content, accompanied by the decrease of initiating contact angles from 141.11° of PTFE0.57 to 107.12° of PTFE4.56. (Figure 4) After 50 seconds, the contact angle of PTFE0.57 decreased by only 2.2% to 138.05°. However, the contact angle of PTFE4.56 decreased by 25.5% to 79.81°, accounted for only 57.8% of PTFE0.57 13



at the same time. The water permeability of CLs increased with higher PTFE content in CLs. It is worth noting that sintering of PTFE in the CL could result in an ‘over hydrophobicity’.28 PTFE will melt and crystallize during the thermal treatment above 320 °C. However, the unsintered PTFE in the CL is hydrophilic and the CL in this work was unsintered. Therefore, the hydrophobicity decreased with increasing PTFE content in CL.

Figure 4. The contact angle of five CLs with different PTFE content measured at the moments of 10 and 60 seconds since the droplet contacted material surfaces. 3.3 Electrochemical characteristics Electroactive surface characterization. As showed in Figure 3A, both of oxidation and reduction peaks in CV plots were due to the diffusion limitation, while the reversible redox peaks indicated the freely exchanging of electrons on the electrode surface. The reduction peak current of PTFE0.57, PTFE1.14, PTFE2.28, PTFE3.42, and PTFE 4.56 were 8.59, 6.65, 5.34, 4.50, and 3.89 mA, respectively. A 55% decrement of reduction peak current was observed from PTFE0.57 to PTFE4.56. According to Randles14


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Sevcik formula (Eq. 5), the electroactive surface area of CLs decreased with higher PTFE content, from 0.0041 m2 g−1 of PTFE0.57 to 0.0019 m2 g−1 of PTFE4.56, which was in direct proportion to the specific surface area of the 3 – 10 nm mesoporous. (red dots in Figure 3B) ORR performance. To determine the ORR pathway, the electrocatalytic activity, electron transfer number (n) and selectivity of different electrocatalysts were investigated by a RRDE system. The polarization curves of different air-breathing cathodes presented similar shape and plateau of the well-defined limiting current at negative potential values of 0.5 – 0.6 V (Figure S3), which indicated that the ORR is controlled by mass transfer in this region. It is worth noting that all of the ring current didn’t start from 0. It could be attributed to the background value from Pt ring. Similar phenomena were found in other articles.50-52 PTFE0.57 showed the highest ring current, which was even 30% higher than the reference carbon material for 2-electron ORR (Vulcan XC 72R) after removing background value, indicating the best catalytic activity for H2O2 electro-generation. In addition, both the disk and ring current of coated-disk electrodes exhibited a tendency of PTFE0.57 > PTFE1.14 > PTFE2.28 > PTFE3.42 > PTFE4.56. The transient current response with respect to the cathodic potential was obtained from the polarization curves of the CL in deoxygenated (N2 aeration and GDL blocked) and oxygenated (no aeration in the electrolyte and GDL unblocked) conditions (Figure 5A). The short dash in Figure 5A showed ORR was inhibited and only hydrogen evolution reaction (HER) occurred in deoxygenated condition. The current of HER in deoxygenated condition exhibited similar trends with HER at galvanostatic model, which was in the order of PTFE0.57 > PTFE1.14 > PTFE2.28 > PTFE3.42 > PTFE4.56. PTFE0.57 showed the highest HER response current. Higher polarization currents were obtained under oxygenated condition for all the five air-breathing cathodes, which was attributed to the contribution of ORR. At −1.0 V, the highest net current (the deviation between deoxygenated current and oxygenated current) of 0.045 A was observed for PTFE0.57, which suggested the highest ORR activity for H2O2 electrogeneration.53 The 15



net current of PTFE0.57 was 34.8% higher than PTFE1.14 and 37.5%, 73.3%, 86.5% than PTFE2.28, PTFE3.42 and PTFE4.56, respectively. The H2O2 yield during LSV test presented similar trend as: PTFE0.57 (106.8 ± 7 mg L−1) > PTFE1.14 (95.2 ± 1 mg L−1) > PTFE2.28 (70.5 ± 2 mg L−1) > PTFE3.42 (55.1 ± 3 mg L−1) > PTFE4.56 (46.7 ± 4 mg L−1). (Figure 5B) The above results suggested the 2-electron ORR activity impoved with the decline of PTFE content in CLs.

Figure 5. LSV of five air-breathing cathodes with different PTFE content in CLs at the scan rate of 10 mV s−1 obtained in 50 mM Na2SO4. (A: Line + symbol) Before LSV, the system was poised at −1.0 V for 10 min with deoxygenated electrolyte and blocked cathode. (A: Short dash) And H2O2 concentration measured at the end of LSV of five air-breathing cathodes with different PTFE content in CLs. (B) 4.

Discussion

As shown in Figures 1A and 1B, both of the H2O2 yields and CE increased with decease of PTFE content in CL. PTFE0.57 achieved the highest H2O2 yields of 3005 ± 58 mg L−1 h−1 at 25 mA cm−2 corresponding to CE of 76%, while the highest CE of 84% was obtained at 20 mA cm−2. According to the LSV results, the highest net current and H2O2 concentration of PTFE0.57 indicated the best 2-electron ORR performance. HER (Eq. (6)) and H2O2 decomposition reaction (Eq. (7)) are two main side reactions that consume cathodic electrons and reduce CE. Though H2 content increased with the decrease of PTFE content, trace H2 production (2 μmol L-1 at 25 mA cm−2) was negligible compared with H2O2 production (3005 ± 58 mg L−1 at 25 mA cm−2). (Figure 16


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1D) Owing to the decomposition of H2O2 into H2O at higher PTFE content CLs, the production of H2O2 varied significantly, which is the main reason for the decrease of CE. (Eq. (7)) As a result, the highest CE of PTFE0.57 was 84 % at 20 mA cm-2, 15%, 27%, 33%, and 42% higher than PTFE1.14, PTFE2.28, PTFE3.42, and PTFE4.56, respectively. As the electron transfer number of ORR was inversely proportional to the slope of the KL plot, the PTFE0.57 presented lowest electron transfer number of 2.89 and highest H2O2 selectivity of 55.5% to get the best 2-electron ORR performance (Figure 6) (Eqs. 2, 3, and 4). Compare to the CE (84%) of PTFE0.57 at 20 mA cm−2 during H2O2 production the lower selectivity of 55.5% was attributed to as follows: plenty of active sites were created by rolling method for air-breathing cathodes, however, there’s only a thin film on the surface of RRDE and not enough active sites provided at RRDE test.54 The electron transfer number of PTFE1.14, PTFE2.28, PTFE3.42, and PTFE4.56 increased to 3.0, 3.16, 3.21 and 3.29, respectively, with H2O2 selectivity decrease to 50%, 42%, 39.5%, and 35.5%. (Table 2) The RRDE results of ink electrodes confirmed that lower PTFE content in CLs was benefit to 2-electron ORR, which was in agreement with operational results of air-breathing cathodes in electrochemical cells. It is interesting found that the selectivity (X) toward 2-electron reduction of CB was so high with 93% at RRDE test. Nevertheless, it has been reported that pure CB or graphite and PTFE as CL directly for H2O2 generation was not better than the CB&G and PTFE hybrid as CL in our previous work.22

17



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Figure 6. Koutecky−Levich (KL) plots for PTFE0.57, PTFE1.14, PTFE2.28, PTFE3.42, PTFE4.56, and Vulcan XC 72R in oxygen-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1. Ring current Er = 1.0 V (vs. Ag/AgCl). Table 2 Electron transfer number (n) and selectivity (X) toward 2-electron reduction. PTFE0.57

PTFE1.14

PTFE2.28

PTFE3.42

PTFE4.56

Vulcan XC 72R

Selectivity (X (%))

55.5%

50%

42%

39.5%

35.5%

93%

Electron transfer number (n)

2.89

3.00

3.16

3.21

3.29

2.14

According to the pore analysis, the variation trend of H2O2 yields as a function of PTFE content was consistent with the pore volume and pore surface area of 3 – 10 nm pores. As shown in Figure S4, the Aeca was linear positive correlated to both of BET surface area of 3 – 10 nm pore and total BET surface area. Because of R12 (0.9322) > R22 (0.8894), it is confirmed that the Aeca was more relevant to the specific surface area of the 3 – 10 nm mesoporous rather than the total BET surface area. The Aeca decreased with increase of PTFE content from 0.0041 m2 g−1 of PTFE0.57 to 0.0019 m2 g−1 of PTFE4.56 (blue triangles in Figure 3B). Obviously, BET surface area of 3 – 10 nm pores 18


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as well as the Aeca were inversely proportional to PTFE content in CLs, but directly proportional to H2O2 yields. Larger BET surface area of 3 – 10 nm pores resulted in more ORR catalytic sites exposure on electroactive surface area, which further promoted electro-generation of H2O2.55 Park et al.21 reported similar phenomenon that the well-ordered mesopores carbon-based materials with diameters of 3.4 – 4.0 nm presented high catalytic activities as well as high selectivity toward H2O2. The hydrophobicity of the CL was mainly due to CB and graphite because unsintered PTFE in the CL is hydrophilic. As a result, higher PTFE content will decrease the hydrophobicity of CLs. Simultaneously, increase of hydrophilicity with higher PTFE content will lead to excessive supply of H+, which induced the H2O2 decomposition to H2O. (Eq. (7)) Furthermore, higher PTFE content in CL can induced leakage of water which leading to catalyst flooding.28 Besides, the amount of O2 diffused to catalytic sites decreased with higher PTFE content. For PTFE4.56, insufficient O2 and superfluous H+ on catalytic sites impeded H2O2 generation and facilitated reduction of H2O2 to H2O. Compared with initial 10 seconds, the decreased contact angle after 50 seconds showed water permeability of CL. It is presented that the water permeability of PTFE4.56 was higher than PTFE0.57, which demonstrated PTFE4.56 will be easier water flooding in the H2O2 generation. This can be the reason why the catalytic stability decreased with increasing of PTFE content in CL. As is well known, superhydrophobic surfaces are defined as water forms contact angles of 150 ° and larger, with only a few degrees of contact angle hysteresis (or sliding angle), < 5 – 10 °.56 Among all the air-breathing cathodes in this work, PTFE0.57 was the only superhydrophobic cathodes with a contact angle of 141.11 °. Superhydrophobic surface of PTFE0.57 provide enough O2 and proper H+ on the catalytic sites, which favored 2-electron ORR for efficient H2O2 generation. (Eq. (9)) 2𝐻 + +2𝑒 ― + 𝑂2→𝐻2𝑂2

(9)

Base on the EIS analysis, the small semicircle of Nyquist plot represented the charge transfer process (Rct) and the large one was regarded as diffusion impedance (Rd) at electrolyte solution diffusing process. The Nyquist plot of both PTFE0.57 and PTFE1.14 19



were only one large semicircle, because the Rct was too little to be observed. (Figure S5) However, Rct of 0.61, 0.66, and 0.86 Ω was calculated for PTFE2.28, PTFE3.42 and PTFE4.56, respectively. The solution resistance (Rs) increased with higher PTFE content, from 10.9 Ω of PTFE0.57 to 13.6 Ω of PTFE4.56. (Figure 7) It can be attributed to that higher content of PTFE in CL will bring larger resistance due to the intrinsic insulativity of PTFE. The better electrical conductivity with less PTFE content in CLs led to higher net current in LSV (Figure 5A), representing more excellent ORR performance.

Figure 7 Resistance components of different air-breathing cathodes with different PTFE content in CL. 5.

Conclusions

In the electrochemical system for in-situ H2O2 production, we found that superhydrophobic air-breathing cathode PTFE0.57 with lowest PTFE content achieved the highest H2O2 yield of 3005 ± 58 mg L−1 h−1. Both H2O2 yield and CE decreased with higher PTFE content in CLs. 3 – 10 nm mesoporous was the major catalytic active 20


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sites for 2-electron ORR. The electroactive area of CLs was proportional to the surface area of 3 – 10 nm mesoporous, which decreased with higher PEFE content. H2O2 decomposition reaction was the major side reaction which consumed cathodic electrons and reduced CE. Overall, PTFE0.57 exhibited superhydrophobicity, highest electroactive area and electrical conductivity, as well as cost effectiveness, which has broad implications for in-situ efficient electro-generation of H2O2.

Supporting information Catalytic stability, N2 absorption–desorption isotherms, polarization curves using RRDE system, correlation between active surface area and BET surface area, and Nyquist plots.

Notes The authors declare no competing financial interest.

Acknowledgements We greatly acknowledge for financial support of this work by the National Natural Science Foundation of China (No.51778408 and No.21577068). References 1.

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Superhydrophobic Air-breathing Cathode for Efficient

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