A route for large-scale preparation of multifunctional superhydrophobic coating with electrochemically-modifie kaolin for efficien corrosion protection of magnesium alloys

Xing Liu,Tin C.Zhng,Yuxin Zhng,Jinsong Ro,?,Shojun Yun,?

a Low-carbon Technology &Chemical Reaction Engineering Lab,College of Chemical Engineering,Sichuan University,Chengdu 610065,China

b Civil &Environmental Engineering Department,University of Nebraska-Lincoln,Omaha,NE 68182-0178,USA

c State Key Laboratory of Mechanical Transmissions,College of Materials Science and Engineering,Chongqing University,Chongqing 400044,China

Abstract Superhydrophobic coating has been widely studied for its great applicational potential,such as for corrosion protection of magnesium alloys while it has been restrained by expensive materials,sophisticated preparation process and infir rough structures.In this study,the electrochemical method was adopted by using a two-electrode system for rapid hydrophobic modificatio to obtain superhydrophobic kaolin.By mixing the modifie superhydrophobic kaolin with commercial epoxy resin and polydimethylsiloxane glue,a paint can be formed and easily used on various substrates for preparation of superhydrophobic coating via spraying method.The influenc factors on wettability of the modifie kaolin and the mixing ratio of each component of the coating were explored.Also,the wettability,durability and anticorrosion of the prepared coating were evaluated comprehensively.The coating was able to maintain superhydrophobic after immersed in HCl solution at pH 1,the NaOH solution at pH 14,and 3.5 wt.% NaCl solution for 16,21,30 days,respectively.In addition,the coating exhibited 4A grade adhesion,high hydrophobicity after abraded for 200 cycles on a 600-mesh sandpaper with 100 g weight,and 99.86% anticorrosion efficien y after soaked in 3.5 wt.% NaCl solution for 20 days,demonstrating a good robustness and anti-corrosion property.Furthermore,the coating showed good transparency,fl xibility and was easy to make in a large scale by the spraying method,which is of great significanc to promote the practical application of superhydrophobic coatings and the anticorrosion Mg alloys.

Keywords: Superhydrophobic coating;Large-scale application;Magnesium alloys;Corrosion protection.

1.Introduction

In the past few years,research on the preparation and application of superhydrophobic coatings has increased rapidly due to their applicational potential in self-cleaning,waterproofing antifouling,corrosion prevention,oil-water separation,drag reduction,energy transfer,etc.[1–4].Some of these studies were concentrated on the exploration of new application field of superhydrophobic coatings,while others were aimed at the improvement of the deficien y of superhydrophobic coatings in practical application [1,5].Over the past decades,studies have shown that the preparation of superhydrophobic coatings needs to meet two conditions: one is appropriate roughness and the other is low enough surface energy[6–8].Based on this principle,a typically applicational example of superhydrophobic coatings is the corrosion protection of magnesium alloys,which have been proved effective in improving the corrosion resistance by orders of magnitude [9–11].However,in practice,it has been found that the rough structures of superhydrophobic coatings are vulnerable to wear,which makes the surface lose super hydrophobicity and thus greatly limits the application of the coating in the real environment [12].Moreover,to enhance the stability of the coating requires simultaneously improving the adhesion,wear resistance and chemical stability of the coating;solving these problems at the same time is quite challenging.

In recent years,some studies have made great progress by mixing superhydrophobic modifie organic/inorganic materials with adhesives,and then applied them on the surface of substrate materials by using the mature coating preparation methods in industry,such as spraying,brushing and dip-coating [13–15].Peng et al.[16] used heptafluoro utyric acid modifie diethylenetriamine as an epoxy resin curing agent and prepared all-organic coatings by mixing with epoxy resin,perfluoropolyethe and polytetrafluoroet ylene particles,which were then applied to different substrate surfaces by spraying and brushing methods.Results showed that after 30 times of adhesion test,the water contact angle (CA) and rolling angle (SA) of the prepared superhydrophobic surface were still greater than 155° and less than 5°.The coating can be immersed in aqua regia for 60 min and 1 M sodium hydroxide solution for 12 h.Furthermore,Zhang et al.[17]used melting technology to fully mix polytetrafluoroet ylene(PTFE)nanoparticles with epoxy resin adhesive to enhance their binding force,and sprayed them on the substrate as the firs layer.Then,by an electrospray method,the superhydrophobic particles prepared byin-situsynthesis of silica on the graphene oxide surface and then superhydrophobic modificatio with perfluorosilane were applied to the top layer to produce a double-layered reinforced superhydrophobic coating.The coating remained superhydrophobic after wear on a 1000-mesh sandpaper for 2000 cycles or soaking in 0.1 M HCl solution for 30 min.As is seen in the above studies,the use of epoxy resin adhesive greatly increased the adhesion of the coating.The doping of hydrophobic organic/inorganic particles or superhydrophobic modifie inorganic particles help to enhance the mechanical robustness and chemical stability of the coating.Also,it is convenient to prepare coating by the spraying method,which is conducive to the realization of large-scale coating preparation.

However,when the stability of superhydrophobic coating is improved,great efforts are also necessary to optimize the cost of coating preparation,such as lowering material price and toxicity.According to a SciFinder search,more than 33%of the studies published on the preparation of superhydrophobic coatings used fluoride as a low surface energy substance over the last fi e years,which,to some extent,should be reduced for its expensiveness and carcinogenic toxicity.Based on this consideration,some reports have been focused on fluoride-fre superhydrophobic coatings [18–20].Liu et al.[21] fabricated a robust superhydrophobic coating by using dodecyltrimethoxysilane (DTMS) modifie magnesium silicate nanotubes,which provided a fir and impact-resistant surface with low-cost and environmentally benign materials.For all this,the time and energy consumptions of the prepared process should also be considered.

Among the reports on the preparation of superhydrophobic coatings,electrodeposition has been one of the most commonly used methods due to its advantages of green fluorine free materials,simple process and short time consumption[10].For example,Liu et al.[22] successfully prepared a superhydrophobic coating on the surface of magnesium alloy sheets by using the ethanol solution of cerium nitrate and tetradecanoic acid as the electrolyte in a two-electrode system.The whole deposition process can be completed within 60 min.Similarly,Zhang et al.[18] successfully prepared a superhydrophobic coating on the surface of aluminum alloy using the ethanol solution of cerium nitrate and stearic acid as the electrolyte by constant piezoelectric deposition in a two-electrode system.The deposition time was only 10 min,and the water contact angle (CA) of the coating reached 168.6±2.5°.Nevertheless,the coating prepared by electrodeposition is usually thin and has poor mechanical properties,which makes the long-term stability of the coating,such as the corrosion resistance,unfavorable in application [23].To strengthen the electrodeposited coating,Wu et al.[24] used the pre-polymer of tetraethyl orthosilicate (TEOS) and dodecyl trimethoxy silane (DTMS) as the electrolytes to prepare superhydrophobic sol-gel film by cathodic electrodeposition.On the one hand,the thickness of the electrodeposition coating was increased.On the other hand,the condensation time of the traditional sol-gel was reduced.Furthermore,Zhang et al.[25] doped trivalent cerium ions and benzotriazole corrosion inhibitors in the process of preparing a sol-gel superhydrophobic fil by the electrodeposition method,which not only made full use of the advantages of the sol-gel electrochemical method for preparing superhydrophobic coating,but also made up for the shortcomings of rough and porous superhydrophobic surface to some extent.However,the electrodeposition method may be difficul to be applied to nonplanar materials and the inner surface of materials,and is not applicable to non-conductive materials such as ordinary glass,paper,sponge,etc.,thus limiting its application range in preparing superhydrophobic surface to some extent.

Based on the above discussions,it can be found that the organic/inorganic hybrid superhydrophobic coating prepared by the spraying method has good chemical stability and mechanical properties,but the cost of the materials used is high and the process is time-consuming.The method of preparing superhydrophobic coating by electrodeposition has the advantages of low material cost,green process and short time consumed,but the mechanical property of the coating is undesirable,and it cannot be applied to non-conductive materials.Therefore,we hypothesized that combining the two methods would complement each other and thus allow the preparation of a superior superhydrophobic coating.

Herein,we developed an electrochemical method for the firs time to modify inorganic particles instead of depositing coating on electrodes,which aimed to simplify the process of superhydrophobic modificatio of inorganic materials for expanding their application ranges on various substrates with industrial mature methods rather than just limiting them on electrode surfaces.Thus,kaolin,a common inorganic material,was electrochemically modifie to obtain a bulk of superhydrophobic kaolin powder firs and then mixed with adhesives to prepared anticorrosion coatings.The method developed here by combination of organic/inorganic hybrid superhydrophobic coatings and electrodeposition coatings has two main advantages,one is that electrochemical modificatio in a two-electrode system greatly reduces the modificatio time(only 1.5 h required) and makes full use of the relatively expensive low surface energy material,and the other is that mixing of materials with adhesives can enhance the overall adhesion and stability of the coating,and is convenient for the coatings to be massively employed in various substrates with a facile and low-cost process.The as-prepared superhydrophobic coating has excellent waterproof,antifouling and antiseptic properties as well as good mechanical and chemical stability.In addition,the selected raw materials are low cost,green and environment-friendly,satisfying the requirements of practical application.

2.Experimental

2.1.Materials

Kaolin powders (kaolin,CP),hexadecyltrimethoxysilane(HDTMS,85%) and tetraethyl orthosilicate (TEOS,98%)were purchased from Aladdin (Shanghai,China).Sylgard 184 polydimethylsiloxane (PDMS) prepolymer and the curing agent were purchased from Dow Corning Corporation(Jiangsu,China).Commercial epoxy resin (E51) and curing agent (W93) were purchased from Kunshan Jiulimei Electronic Materials Co.(Jiangsu,China).All other agents including xylene,glacial acetic acid,sodium nitrate were analytical grade and acquired from Kelong Chem.Co.,(Chengdu,China).All the above chemical agents were used directly.Ultrapure water with electrical conductivity of 18.25 MΩcm was self-made by a reverse osmosis(RO)workstation (2YCG7-2-40L,China).

2.2.Superhydrophobic modificatio of kaolin

As shown in Fig.1,kaolin was hydrophobically modifie by electrochemical means to obtain superhydrophobic kaolin powder (kaolin/HDTMS),and then kaolin/HDTMS was mixed with adhesives (E51 and PDMS) to form a coating mixture,which was applied on the substrate surfaces by spraying method.The specifi operation steps are as follows.A predetermined amount of kaolin was added in anhydrous ethanol (4–10 wt.%) and transferred to a 150 mL custommade electrolytic cell (Tianjin ida,China).The mixture was dispersed by ultrasonic (200 W,40 KHz) for 10 min.TEOS and HDTMS were added and stirred by magnetic force for 5 min.Then,an aliquot 30 mL of 0.67 M NaNO3aqueous solution was added into the mixed solution with glacial acetic acid (3.25 mL,17.5 mol/L) being added to adjust the pH to around 4.The mixture was continuedly stirred for 30 min before applied a constant voltage in a range of 15-40 V by a DC power supply(Huatai Power Supply HAP03-100)with the assistance of stirring.After the reaction,the solution was treated by ultrasound (200 W,40 KHz) and washed by centrifugation at 8000 rpm for 5 min with a mixed solution of anhydrous ethanol/water (v/v = 1: 1) for three times.After collected and dried at 60 °C,the HDTM modifie superhydrophobic kaolin was obtained.Under the same other conditions,different reaction times (30,60,90,120,150 min) and reaction voltages(15,20,25,30,35,40 V) were carried out.The wettability of the products under each condition was recorded.For reading convenience,the modifie superhydrophobic kaolin under better conditions is denoted as kaolin/HDTMS.

2.3.Preparation of epoxy resin composite coating

Typically,the E51 epoxy resin and PDMS adhesives were added into 6 mLm-xylene firs and underwent ultrasonic treatment(200 W,40 KHz)for 10 min.Afterwards,the curing agents of epoxy resin and PDMS were added according to the main agent and curing agent mass ratio of 10:3 and 10:1,respectively,stirring for 20 min to form an evenly mixed paint.Then it was sprayed on substrates such as Mg alloys,glass slides,filte paper,etc,by a spray gun (BAIMA,China) to prepared coatings.The spraying pressure was 0.4 MPa and the spraying distance was 15 cm.Finally,the coating was dried at 80 °C for standby,and the resulting coating was denoted as EP.To investigate a proper blending content of PDMS,the change of CA of the coating was recorded with PDMS doping quality being 0,25,50,75 and 100% of epoxy resin adhesive.For analytical comparison,pure epoxy resin coating without PDMS was prepared in the same way and denoted as E51.

2.4.Preparation of kaolin/HDTMS doped superhydrophobic composite coating

The preparation procedure of superhydrophobic coating was similar to that of EP,except that selected kaolin/HDTMS superhydrophobic powder was added after the addition of epoxy resin,and the dosage of PDMS was fi ed at 50% of the mass of epoxy resin.The wettability changes of coatings prepared with kaolin/HDTMS doping amounts of 30,50,65,80,and 100% were investigated,where the percentage refers to the sum of the mass of kaolin/HDTMS relative to E51 and PDMS.For example,given that the mass sum of E51 and PDMS is 1.5 g,when 0.45 g kaolin/HDTMS was added,the doping amount of superhydrophobic powder was denoted as 30%.The coating obtained under a proper doping amount of kaolin/HDTMS was denoted as kaolin/HDTMS@EP.It should be noted that when kaolin/HDTMS@EP coating was applied on the surface of magnesium alloys for anticorrosion,the substrate should be sprayed with a thin EP layer in advance to avoid the defect of rough and porous superhydrophobic surface

2.5.Surface characterization

The surface and cross section morphology of the samples were analyzed by scanning electron microscopy (SEM,JEOL JSM-7610 SEM,Toyoko,Japan),together with the elemental composition analysis by energy dispersive spectroscopy(EDS).Compositions of bare kaolin and kaolin/HDTMS were analyzed by X-ray photoelectron spectroscopy (XPS,Escalab 250Xi,USA).By means of Fourier transform infrared spectrometer (FT-IR),the powder products under different reaction times and reaction voltages were measured by the tablet pressing method while the solid surface of AZ31B Mg,E51,EP and kaolin/HDTMS@EP coatings were determined by total reflect vity spectroscopy (ATR-FTIR).The thermogravimetric analysis (TG,NETZSCH STA 449F3 Jupiter,Germany) of kaolin,kaolin/HDTMS,E51,EP and kaolin/HDTMS@EP was performed in N2atmosphere at a heating rate of 10 °C/min from room temperature to 800 °C.The wettability of the samples was tested using a water contact angle meter(SZ-CAMC11,SUNZERN,China).All water contact angles (CA) and sliding angles (SA) were measured with 18.25 MΩcm ultrapure water droplets (unless otherwise stated) after stabilization on the surface for approximately 15 s and average curves with error bars were plotted by testing each sample at fi e different locations.

Fig.1.Schematic diagram of preparation of superhydrophobic coating.

2.6.Anti-fouling and transparency test

To figur out the anti-fouling ability,the anti-dust experiment was carried out on the magnesium alloy sheet and glass sheet coated with kaolin/HDTMS@EP coating,and the specifi operation method was the same as our previous study[21].In addition,the anti-pollution property of the filte paper with and without the superhydrophobic coating was compared by immersing it in methyl orange solution.The transmittance of the coating was tested by an Ultraviolet visible diffuse reflectanc spectrometer (UV-Vis,Lambda 750S R,USA) in a range of 200-800 nm.Besides,the optical photos of 5-40 μL methyl orange droplets and various drinks on the surface of the coating were recorded.

2.7.Chemical stability test

In order to comprehensively evaluate the chemical stability of the prepared superhydrophobic coating,the samples coated with kaolin/HDTMS@EP were tested in high-temperature furnace,salty,acidic and alkaline solution.In the temperature resistance experiment,the samples were placed in a tubular furnace and kept at 160 °C for 2-48 h.After a certain period of time,the samples were taken out and cooled to room temperature in the air to determine CA and SA.Besides,the coating was soaked in HCl solution of 3.5 wt.%NaCl,pH = 1,respectively.After a certain time,the sample was taken out,slightly washed,and dried with anhydrous ethanol before recording the wettability changes of the coating.In the alkaline resistance test,the coating was immersed in NaOH solution with pH = 14.The wettability difference of the soaked samples was recorded before and after slightly rinsed with anhydrous ethanol and dried at 80 °C for 30 min.A high-speed camera (FastCam Mini WX100,resolution 1024 × 1024) was used to record the bouncing behavior of water droplets on the coating surface after the above chemical stability tests.The shooting speed was 1080 pfs,with 20 μL water droplets falling down from around 2 cm height.

2.8.Abrasion and adhesion test

To determine the robustness of the superhydrophobic coating,sandpaper abrasion test was carried out first In abrasion test,the coated magnesium alloys were put on a 600-mesh metallographic sandpaper with 100 g weight on the surface and pushed back and forth along the horizontal direction.Every 10 cm distance was recorded as one abrasion cycle.Besides,the coating was abraded in fl wing water.In the fl w friction test,150 mL of deionized water was placed in a 250 mL beaker and stirred by a magnetic stirrer at a speed of 500 rpm.The superhydrophobic sample was vertically suspended in the deionized water to be rubbed by the water in the cup.The changes of the contact angle of the coating with the friction time were recorded.For the adhesion test,3 M Scotch tape was placed onto the surface of the superhydrophobic coating and wiped fla with a pencil.After holding for 30 s,the tape was slowly teared off.The wettability change was recorded after every 20-time repeat.In addition,X-cut test was carried out according to ASTM D3359-02 standard [26] to evaluate the adhesion between the coating and Mg alloy substrate.

2.9.Anti-corrosion test

In order to investigate the corrosion resistance of the prepared superhydrophobic coating,the Tafel polarization curves and electrochemical impedance spectra (EIS) of the samples were measured in 3.5 wt.% NaCl etching solution.In the three-electrode system,saturated calomel electrode(SCE),self-made sample with exposed area of 1 cm2and square platinum sheet of 1 cm2were used as reference electrode,working electrode and counter electrode,respectively.The Tafel curves were scanned in the range of open circuit potential(OCP)±250 mV.TheEcorrandicorrwere obtained by extrapolating from the cathodic and anodic region of Tafel curves,and anticorrosion efficien y was calculated in the same way as our previous study [27].The EIS was detected at the frequency of 105-10?2Hz with a scanning speed of 5 mV/s.All EIS tests were repeated three times to ensure the reliability of the results.The average impedance modulus at 0.01 Hz with error bars was given for comparison.The impedance data of the coatings were fitte with Zview software to further analyze the corrosion resistance of the coatings.Besides,continuous neutral salt spray test was carried out in a salt spray box (HD-E808-60,China).Typically,5 wt.% NaCl solution was used as spray solution with pH at 6.5-7.2.The test temperature was set at 35.0 ± 1.0 °C,and the sedimentation rate of salt spray was adjusted to 1-2 mL/80 cm2h.The inclination of all samples was around 20°.

3.Results and discussion

3.1.Effect of reaction time and voltage

As shown in Fig.2a,the powder products obtained by deposition for 30 min show hydrophobicity,with CA of 130-150° while the CA of untreated kaolin is close to 0°,showing superhydrophilicity according to the water contact angle testing results.The transfer of kaolin from superhydrophilicity to hydrophobicity indicates the starting polycondensation of TEOS and HDTMS on the surface of kaolin surface;yet the degree of polycondensation may not be enough.With an increase in electrodeposition time to 60 min,the powders were superhydrophobic and the CA was about 155°.When the electrodeposition time was 90 min,CA of the product could reach 157±1.56°.However,upon further prolonging the deposition time to 150 min,the CA changed little.Similar phenomenon was reported in previous study upon electrodepositing directly on the cathode [24].With increasing the reaction time during the initial period,the CA underwent an noticeable increase,yet after a certain range of time (such as 200 s) it did not change significantl [24].These above results can be attributed to the full hydrolysis and condensation reactions of TEOS and HDTMS on the kaolin surface after 90 min,whilst further prolonging the electrodeposition time is no longer effective in promoting the reactions.This can be proved by the following FTIR characterization.Also,the change of wettability of the product under different voltages was investigated with the constant time of 90 min.

Fig.2b shows that the products obtained show obvious hydrophobicity with CA>145°at the voltage range of 15-40 V.It can be found that the CA of powder products increases with an increase in reaction voltage in the range of 15-30 V.When the electrodeposition voltage was 30 V,the product obtained the best super hydrophobicity.However,when the deposition voltage continued to increase,the CA failed to increase significantl ,which was similar to the phenomenon with an increase in deposition time under a constant voltage.This may be due to the increasing condensation degree of TEOS and HDTMS on kaolin surface with an increase in deposition time and voltage in an appropriate range,thus lowering the surface energy and offering foundation for super hydrophobicity.The voltage here serves as a driving force to facilitate the generation rate of OH-,promoting the hydrolysis and condensation reaction on the kaolin surface [24].As a result,a higher voltage is coupled with shorter reaction time.Whereas,excessive voltage would cause the reaction to proceed too fast,resulting in the failure of uniform dispersion of condensation polymer and thus the agglomeration of the products.

By comparing and analyzing the infrared spectra of products with different reaction times and voltages in Fig.2c and d,it can be found that the obtained products have similar infrared characteristics.Among them,the absorption peaks at 2923,2854,1468 and 1384 cm?1can be attributed to C-H vibration absorption peaks of methyl and methylene in longchain alkanes.In addition,the absorption peaks near 1070,794 and 463 cm?1can be classifie as the stretching vibration of Si-O-Si [28].Compared with the infrared spectra of the original kaolin,the above results indicated that TEOS and HDTMS were successfully condensed and polymerized on the surface of kaolin.In addition,it is noted that when reaction occurred at constant voltage of 30 V for 30-90 min or at 15-30 V for 90 min,the absorption intensity of peaks at 2854 and 2923 cm?1increazed with an increase in deposition time and voltage,possibly duo to the condensed degree of TEOS and HDTMS on kaolin surface [29].While the reaction time and voltage beyond the two nodes,peaks at 2923 and 2854 cm?1showed no obvious change,which was similar to the wettability characteristics shown in Fig.2 a and b.Therefore,it can be concluded that the main function of the voltage is to provide the driving force required for the reaction,while the deposition time is to ensure that the reaction is fully carried out for superhydrophobic modification The product kaolin/HDTMS obtained by the reaction at 30 V for 90 min was selected for further tests/discussion.

Fig.2.(a) CA and (c) FTIR spectra of products obtained through electrochemical synthesis for 30-150 min at a constant voltage of 30 V;(b) CA and (d)FTIR spectra of products obtained through electrochemical synthesis for 90 min at constant voltages of 15-40 V.

Fig.3.Effects on (a) wettability and (b) yield of products obtained at different mass fractions of kaolin.

3.2.Effect of mass fraction of kaolin

For other conditions being the same,the effect of kaolin doping mass fraction (relative to the mass sum of TEOS and HDTMS,i.e.Mkaolin/(MTEOS+MHDTMS) × 100%) was investigated on the wettability and yield of the product.Since TEOS and HDTMS were the main parts of raw material cost,the mass of the product (MP) accounted for the sum of the mass of TEOS and HDTMS (MTEOS+MHDTMS) was used to evaluate the yield of the product.As shown in Fig.3a,by comparing the CA of the products obtained,it can be found that with the mass fraction of kaolin being increased from 60 to 120%,the CA obtained fluctuate at around 155°.When the mass fraction of kaolin increases to 135%,the CA is greater than 150°,but it decreases somewhat compared with the products obtained by other doping amounts.As shown in Fig.3b,with the mass fraction of kaolin being increased from 60 to 120%,the amount of products produced presents an increasing trend and the yield of products (denoted as kaolin/HDTMS) reaches the maximum with 120% kaolin.If possible,the more kaolin is added,the better,meaning that more hydrophobic materials can be prepared at a lower cost.However,the results showed that the yield would decrease with an increase in kaolin doped,which is understandable owing to the limitation of the constant mass of TEOS and HDTMS.

Fig.4.(a) Effects of PMDS blending content on the CA of EP;(b) effects of kaolin/HDTMS loading amounts on the wettability of kaolin/HDTMS@EP(insert is the corresponding optical image);(c),(d1-d2),(e1-e4) are the CA,SA and water adhesion tests of kaolin/HDTMS@EP with a 65% kaolin/HDTMS loading amount.

3.3.Wettability of the prepared EP and kaolin/HDTMS@EP coatings

As shown in Fig.4a,without PDMS,the epoxy resin coating was hydrophilic with a CA of about 70°.When 25%PDMS was doped,the coating became hydrophobic with a CA of 91-104°.When the doping amount of PDMS is 50%,the CA of the coating is stable at about 104° while an excess amount of PDMS cannot increase the hydrophobicity of the coating,indicating that the doping amount of 50%has fully exhibited the wettability of PDMS.Fig.4b shows that when the doping amount of kaolin/HDTMS is 30%,the CA of the coating is about 119°,which is slightly larger than that of the EP coating yet far from superhydrophobicity.This is because the doping of superhydrophobic powder increased the roughness of the coating to some extent,but the amount is so small that kaolin/HDTMS is covered by adhesive.With kaolin/HDTMS being increased to 50%,the CA of coating is ～154°,but the SA only reaches ～14°.By contrast,with 65% kaolin/HDTMS,the coating is endowed the best superhydrophobic performance with CA of ～156° and SA less than 5°,whilst more addition of kaolin/HDTMS is superfluou and cannot help to improve the super hydrophobicity further.Fig.4c and d1-d2 show,respectively,the images of CA and SA of the coating with 65% kaolin/HDTMS(kaolin/HDTMS@EP).It can be seen that the water droplet is approximately spherical on the surface of the coating and can be rolled off with a slight tilt angle,indicating that the coating has a low adhesion force.This can be intuitively seen from the adhesion test in Fig.4e1–e4.When the droplet on the needle contacted the coating surface,external force was applied to squeeze until deformation,and then slowly lifted the droplet.It can be found from Fig.4e4 that the droplet is completely lifted,and there are no residual droplets on the coating surface.

3.4.Morphology and composition analysis

As shown in Fig.5a,b,the morphology of the original kaolin is mainly platy,with particle size ranging from hundreds of nanometers to microns,and its surface is relatively smooth.However,after superhydrophobic modification some nanoparticles agglomerate to some extent and form micronano sized lumps.This is due to the polycondensation reaction of silanes on the surface of kaolin and the formation of polymer film According to the change of morphology,it can be inferred that the roughness of the modifie kaolin increases,and the surface energy decreases because of the polymer fil on the surface,which provides two necessary conditions for the preparation of superhydrophobic coating[30].As can be seen from Fig.5c,d,the surface of EP coating is fla and smooth.However,after doping with superhydrophobic modifie kaolin powder,the surface of the coating forms a rough micro-nano convex structure,which provides a good structural basis for the hydrophobicity of the coating.As can be seen from Fig.5e and f,the coating EP and kaolin/HDTMS@EP are controlled at a similar thickness and are tightly bound to the substrate.

Fig.5.SEM images of different materials and coatings: (a) kaolin;(b) kaolin/HDTMS;(c) EP;(d) kaolin/HDTMS@EP;(e) and (f) the cross-section images of EP and kaolin/HDTMS@EP.

I n composition analysis,EDS characteristics (Fig.S1)show that the original kaolin mainly contains Al,Si and O elements as it is mainly composed of aluminum oxides,silicon oxides and bound water[31].After hydrophobic modification kaolin/HDMS has a large amount of C elements and a higher percentage of Si than Al element,which is mainly attributed to the condensation and polymerization of HDTMS on kaolin surface.C element comes from the long chain carbon on the surface of the modifie kaolin,and the increased Si element comes from the O-Si bond at the long chain end of HDTMS[32].This can be confirme by XPS characteristics.

As shown in Fig.6a,b,the original kaolin powder mainly contains O,Al,Si,and a small amount of C.After HDTMS modification the peak of C1s was obviously enhanced,indicating that the content of C element was increased.The peak separation fittin of the C1s spectrum shows the peak of C-H/C-C at 284.7 eV and the peak of C-Si at 284.3 eV,indicating that the large increase in C content is mainly due to the condensation of HDTMS on kaolin surface [19].Further analyzed by ATR-FTIR spectrum as shown in Fig.6c,there is no obvious absorption peak of the original magnesium alloy AZ31B Mg.After coated with epoxy resin,the absorption peaks of the sample E51 at 1607 and 1508 cm?1can correspond to the C=C and C-C bonds of aromatic hydrocarbons in the epoxy resin,respectively.For the EP coating,an obvious absorption peak appears at 1035 cm?1,which can be attributed to the Si-O-Si bond in PDMS.Compared with the infrared spectra of E51 and EP,the prepared superhydrophobic coating has absorption peaks at 2923 and 2854 cm?1,which can be attributed to the C-H bond stretching vibration of long-chain silane in HDTMS [32].The above analysis confirme the successful modificatio of HDTMS on kaolin surface.

To figur out the thermal stability of the prepared material and coatings,TG curves were showed in Fig.6d.It can be seen that the mass loss of kaolin is about 5% from room temperature to 800 °C,which is mainly due to the gasificatio of physisorbed water,the decomposition of hydroxyl groups and bound water [31].This indicates that kaolin itself has a good heat resistance.In contrast,the superhydrophobic modifie kaolin underwent more mass loss at 300-700 °C,with the remaining mass fraction decreasing from 97.19 to 82.68%.The mass loss of 14.51% could be attributed to the decomposition of surface long-chain alkanes [29].It can be seen from curve E51 that there is a small step between 100 and 300 °C,which is due to the loss of moisture absorbed by the epoxy resin during curing [33].The mass of E51 decreases rapidly due to thermal decomposition between 300 and 500 °C,and the carbon residue is about 10.86% at 800 °C.In comparison,the mass loss rate decreased after doping with PDMS,indicating that the heat resistance of the coating was slightly enhanced,which was due to the good thermal stability of Si-O bond in PDMS [34].Moreover,the thermal stability of the coating was significantl improved after the doping of the superhydrophobic kaolin,which was due to the better heat resistance of kaolin/HDTMS and thus enhanced the overall thermal stability of the coating.

Fig.6.Compositional analysis of materials and coatings: (a) XPS total spectrum;(b) XPS C1s spectrum;(c) ATR-FTIR curves;(d) TG curves.

3.5.Antifouling and light transmittance analysis

As shown in Fig.7a,the composite coating with 65%kaolin/HDTMS doping is sprayed on the glass sheet,filte paper,and copper mesh substrate.The coating can achieve good superhydrophobicity with CA ～155° and SA ～5°,showing that the prepared coating with good general applicability not only can be used for anti-corrosion of metal substrates,but also has the potential to be applied to other substrates to promote the application of super-hydrophobic surfaces.For example,as shown in Fig.7b1,b2,when immersed in an aqueous solution,the bare filte paper quickly absorbed water and soaked through,while the filte paper modifie by the coating,as shown in Fig.7c1–c2,kept dry though it was repeatedly immersed in the solution and then lifted up.Moreover,the coated filte paper can maintain its waterproof and antifouling ability even if it was slightly rubbed,indicating that the prepared coating has certain fl xibility.The superhydrophobic coating can also endowed good antifouling properties to other substrates such as Mg alloys and glass slides.As shown in Fig.7d1-d3,as water drops on the surface of the coating,the water drops would quickly roll off and take away the surface soil due to the superhydrophobicity of the coating.For the coated glass substrate as shown in Fig.7e1-e3,similar antifouling performance can also be observed.

In addition,the prepared coating also has good light transmittance.As shown in Fig.8a,within the visible wavelength range of 400-800 nm,the light transmittance of the blank glass sheet is about 90%,and remains ～80% after 10 s spraying with the prepared coating.Interestingly,when the spraying time was shorter,such as 4 s,the transmittance of the glass increased rather than decreased,reaching 92%.This is due to the bare glass surface is smooth,light reflectio occur mainly on the surface.After painted with super hydrophobic coating,glass obtained rough surface structure.The structure may have the similar function as some of the compound eyes of insects,where the light scattering phenomenon occurs mainly on the surface.Thus,the coated glass has "transmittance increasing effect",namely to reduce the light reflectio to increase the light transmittance [35].Studies have shown that this performance occurs when the thickness of the coating is 1/4 of the wavelength of light incident,and the refractive index is equal to the square root of the product of the coating and the substrate refractive index [36].The good transmittance of the coating can also be intuitively observed through optical images.As shown in Fig.8b1-b4,the sprayed glass sheet was placed on the surface of words,and the words below can be clearly identified Moreover,methyl orange dye drops of different sizes ranging from 5 to 40 μL and various beverage droplets are approximately spherical on the surface of the coating.The surface is resistant to various kinds of beverages including coke,milk,coffee,tea,etc.More details are shown by snapshots in Fig.S2.In addition,the coating can be easily applied to the preparation of superhydrophobic surface in a large area by spraying method.As shown in Fig.8c,the composite coating can be conveniently applied to the 250 × 250 mm2magnesium alloy plate,so that the surface has superior hydrophobicity and maintains the luster of the metal to some extent due to the good light transmittance of the coating.The above results show that the prepared coating can be easily used to prepare superhydrophobic surface on various substrates and has good water resistance,antifouling ability and light transmittance.

Fig.7.Wettability and anti-fouling ability of the composite coating applied on different substrates: (a) surface wettability of coated glass,fil er paper and copper mesh;(b1,b2) blank filte paper;(c1,c2) coated filte paper;(d1-d3) coated Mg alloy;and (e1-e3) coated glass slide.

3.6.Chemical stability

As shown in Fig.9a,during the calcination of the coating at 160°C for 48 h,the wettability remained relatively stable with CA remained at 155° and SA fluctuate below 10°,indicating that the superhydrophobic property of the coating had good thermal stability.This is consistent with the aforementioned thermogravimetric testing results in Fig.6d.It should be noted that the coating turned yellow after calcination as shown in the inner illustration,which was due to the aging of the adhesive at a higher temperature,indicating that the prepared superhydrophobic powder should be used in conjunction with the high-temperature aging resistant adhesive as needed at a higher temperature for a long time.As shown in Fig.9 b,during the immersion process in 3.5 wt.% NaCl solution for 30 days,despite a slight fluctuatio of the CA and SA,the coating maintained stable superhydrophobicity overall.According to the inserted optical pictures,it can be found the coating showed obvious reflectio phenomenon in both early and late soaking in solution.The enlarged images can be seen in Fig.S3a-b.This is due to the air layer captured on the coating surface and thus forming a liquid-vapor interface between the superhydrophobic coating and the liquid,which effectively restrains the immersion of corrosive solution [37].

英國遠征軍1914年8月開赴法國時僅有827輛汽車（包括747輛征用的車）和25輛摩托車，到戰(zhàn)爭結束前幾個月，英國陸軍車輛達到了5.6萬輛卡車、2.3萬輛汽車、3.4萬輛摩托車和機動腳踏車。此外，1917年4月，參戰(zhàn)的美國帶到法國5萬輛汽油驅動車。這些車輛根據(jù)部隊的需要，將部隊和補給從一地迅速運到另一地。而德國占據(jù)優(yōu)勢的火車因為缺乏機動性，在戰(zhàn)爭中漸漸失去優(yōu)勢。

In contrast,the wettability of coating appears to suffer greater effect in HCl solution at pH 1.As shown in the left inset in Fig.9c,after the coating was immersed 24 h,the phenomenon similar to silver mirror on the surface disappeared,indicating that certain changes had taken place on the surface of the coating.The solution might slowly penetrate into the air layer,causing the degraded hydrophobicity of the coating.However,after drying at 80 °C for 30 min or natural drying in the air for longer time,the mirror-like phenomenon appeared again.Even after soaking for 16 days,the coating maintained good superhydrophobicity (right inset),indicating that the prepared superhydrophobic coating has a certain selfhealing ability.The enlarged images are available in Fig.S3c,d.When the coating soaking in the NaOH solution at pH 14,the coating showed a similar healing property,that is,soaked in the solution for several days later,the coating transferred the super hydrophobic to hydrophilic,whilst after heating or natural drying at room temperature,coating restored the super hydrophobic as shown in Fig.9d.Thus,20 days later,the coating could still maintain its super hydrophobic,showing that the prepared super hydrophobic coating has strong healing capacity.

Fig.8.(a) Transmittance of kaolin/HDTMS@EP coated glass slides;(b1-b4) optical of coated glass slides;(c) coating applied on 250 × 250 mm2 Mg alloy plate.

Fig.9.Wettability changes of the prepared composite coating in different environments: (a) calcination at 160 °C in the air;(b) 3.5 wt.% NaCl solution;(c)HCl solution with pH of 1;(d) NaOH solution with pH of 14.

The change in the wettability of the coating may be due to the fact that the surface of the coating could be affected by the ions of the solution during the immersion process,resulting in changes in structure and chemical composition,thereby increasing the surface energy of the coating and causing the hydrophobicity decrease.After the coating is heated,the coating network structure could undergo rearrangement,and the surface carbon chains would migrate,so that the surface energy of the coating was reduced,and the superhydrophobicity was restored [19].According to the surface characterization after the coating continuously immersed in NaOH solution with pH 14 for 7 days,the surface of the coating was slightly etched to smooth (Fig.S4),while the rough structure was restored after heating.The O/C atomic ratio decreased from 0.54 to 0.52 after heating for healing.Studies have shown that this may be due to the formation of hydrophilic -OH during the soaking process,and the PDMS composite coating swelled after being washed by the organic solvent and heated to restore the rough structure,through which the hydrophobic carbon chain could migrate to replace the hydrophilic groups,so that the superhydrophobic properties of the coating were restored [38].

To further characterize the wettability,a high-speed camera was used to record the bounce behavior of water droplets before and after the stability test of the super-hydrophobic composite coating.As shown in Fig.10a-e,after the aforementioned chemical stability testing,the surface of the composite coating still has a similar water drop bounce behavior to the untreated composite coating.When the water droplets fell on the surface of the coating,they would spread out due to the impact,forming a fla shape and fully contact the surface after 4-6 ms.After another 11-16 ms,they could regroup and bounce off the coating surface.This indicates that the composite coating remained good superhydrophobic properties after being calcined or immersed in acidic,alkaline,and salty solution,which is consistent with the results of aforementioned coating wettability tests as shown in Fig.9.

3.7.Durability and adhesion

The wear resistance of the superhydrophobic coating is one of the important factors that affect the life of the coating in application.The wear test is usually carried out by using the sandpaper friction test coating [39,40].As shown in Fig.11a,during the abrasion process on 600 grit sandpaper under a weight of 100 g,the CA gradually decreased and SA increased.After firs 20 abrasion cycles,the CA remained relatively stable,but the SA in some locations reached more than 10°.The coating can stand up to 40 cycles with CA greater than 150° and SA ～33°,and the mass loss of the coating was 12%.With abrasion cycles increased to 200 cycles,the wettability changed from super-hydrophobicity to hydrophobicity with a CA of 125°,which is slightly larger than the CA (104°) of the EP coating.This can be expected as the coating would inevitably suffer mass loss during the wear process,resulting in the destruction of the rough surface structure.As shown in Fig.S5,the protruding part of the coating surface has been worn after 200 times of sandpaper abrasion.It is relatively flat so that the hydrophobicity of the coating was reduced and close to that of the composite epoxy resin coating.

Furthermore,running water friction was used to wear the coating.As shown in Fig.11b,the CA of the coating remained relatively stable during two weeks of continuous friction in running water under stirring at 500 rpm,while SA tended to increase.After two weeks of friction,SA increased to ～17°,which means that the surface of the coating was abraded under the friction of running water for a long time.It can also be seen from the inset that the surface of the coating became rougher,but the coating still maintained high hydrophobicity.Combined with the sandpaper abrasion experiment,it can be found that the prepared superhydrophobic surface can withstand repeated deliberate abrasion,showing favorable robustness in application and also reflectin good adhesion to some extent.

Fig.11c shows that the CA of the coating remains stable after 100 peeling cycles of 3M tape despite a slight increase in SA,indicating a reliable adhesion of the superhydrophobic coating.The X-cut method is commonly used in the industry to test the adhesion of the coating to the substrate and to evaluate the adhesion level according to the peeling condition of the coating [26].As shown in the inset of Fig.11c,by comparing the optical pictures before and after the adhesion test,only little peeling traces can be found at the scratches after the X-cut test,indicating 4A level of the coating adhesion.

3.8.Anticorrosion performance

As shown in Fig.12a,after 5 days of immersion in the accelerated corrosion solution,the electrochemical impedance of the coating EP remained stable at ～105.3ohm cm2.When the soaking time was extended to 10 days,the coating impedance decreased significantl ,and two obvious time constants appeared in the frequency vs.phase angle diagram (Fig.12b),indicating that the corrosive media may have penetrated the coating to damage the substrate.By further extending the soaking time to 13 days,it can be found that the impedance at high frequencies decreases by nearly two orders of magnitude,and the third time constant began to appear with a total impedance of 104.7ohm cm2at low frequency.In contrast,as shown in Fig.12c,d,the superhydrophobic composite coating sustained stable impedance at the firs 5 days soaking and showed one time constant.After soaking for 10 days,it can be found that the frequency impedance curve began to shift down,and the second time constant seemed to appear in the range of 0.1-10 Hz,which became more obvious after the prolonged soaking time increased to 15 days.After 20 days,the coating was affected to some extent in the process of soaking in corrosive solution,though the overall corrosion resistance maintained relatively high with an impedance value of 104.8ohm cm2.The coating impedances of EP and kaolin/HDTMS@EP at a frequency of 0.01 Hz can also be seen in Fig.S6.

Fig.10.Bounce off images of droplets on superhydrophobic coating: (a) untreated;(b) after calcined at 160 °C for 48 h;(c) after immersed in 3.5 wt.%NaCl for 30 days;(d) after immersed in pH = 1 HCl solution for 16 days;(e) after immersed in pH = 14 NaOH solution for 20 days.

Results of Tafel curves were consistent with that of the above EIS.For EP coating as shown in Fig.12e,the corrosion current fluctuate slightly at the initial 5 days immersion,but remained at a similar level (see Table S1).With an increase in immersion time to 13 days,the corrosion current increased from 1.85 × 10?7to 1.64 × 10?6A cm?2,indicating that EP coating had acceptable stability when being immersed in 3.5 wt.% NaCl for a short time,and can be used as an adhesive for superhydrophobic powder.By combining with superhydrophobic powders,new functions are given to the coating,and,on the other hand,corrosion resistance could be enhanced.As shown in Fig.12f,during 20 days immersion,Tafel curves of superhydrophobic coating tended to move downward to the lower right,which means that the corrosion potential of the sample decreased and the corrosion current increased.However,the corrosion resistance of the coating remained stable at the initial 10 days immersion in the accelerated corrosion solution with anti-corrosion efficien y above 99.95% (Table 1).According to the previous research results,it is known that the bare magnesium alloy would undergo serious corrosion after being immersed in 3.5 wt.% NaCl solution for several hours,withicorrof 4.71 × 10?4A cm?2[21].By comparison,after soaking for 20 days,the superhydrophobic sample maintained a low corrosion current density of 1.19 × 10?6A cm?2,with anti-corrosion efficien y of 99.86%,showing reliable corrosion resistance.

Table 1 Tafel polarization parameters of superhydrophobic composite coating after immersed in 3.5 wt.% NaCl aqueous for 1,5,10,15 and 20 days.

Furthermore,as seen in Fig.12g,the equivalent circuit diagrams of one-,two- and three-time constant were used to fi the obtained impedance spectrum of the coating,so as to evaluate the corrosion resistance of the coating.As shown in Fig.12h,with the extension of the immersion time,the coating impedance of both EP and kaolin/HDTMS@EP showed a gradual decline,but maintained a relatively high corrosion resistance.It can be found that after EP soaking for 13 days,RcandRctdecreased by an order of magnitude,while kaolin/HDTMS@EP showed enhanced corrosion resistance after soaking for 20 days,withRcof 104.5andRctof 105.1ohm cm2.The fitte impedance is close to the testing impedance at 0.01 Hz.The effectively improved corrosion resistance of the superhydrophobic composite coating can be due to the "air cushion effect" [41] and the "capillary effect" [42] of the superhydrophobic surface.For one aspect,the rough structure of the superhydrophobic surface can trap air to form an air layer reducing the contact area between the liquid and the solid surface as proved by the aforementioned mirror-like phenomena.For another aspect,the surface micronano structure and low surface energy enable the coating to form capillary that has a repulsive force against the corrosive liquid immersion,thereby reducing the corrosion rate of the metal substrate.

Fig.11.Wettability changes of the prepared composite coating with: (a) sandpaper abrasion cycles;(b) water abrasion time and (c) tape-peeling cycles (inserts are the optical pictures of adhesion test by X-cut method).

Neutral salt spray test is one of the most commonly used methods to evaluate the corrosion resistance of coatings as it can not only greatly shorten the time of corrosion test,but also provide convictive reference value for practical application.Also,salt spray droplets are usually small and in constant fl w,so they can be more destructive than salt solutions in some cases [43].As shown in Fig.13,the surface of AZ31B Mg has been severely corroded after 24 h salt spray test,forming magnesium hydroxide corrosion products attached to the surface [44].However,the corrosion products of magnesium alloy have no protective effect.After 168 h,the surface corrosion intensifie and many obvious pits have been formed,which is caused by the intensifie surface pitting corrosion due to the existence of Cl?.For EP sample,there is no obvious change on the surface 24 h,indicating that the coating has a certain blocking effect on water mist and chloride ions.After 72 h,a small number of black spots appears at the bottom of the coating,illustrating pitting has occurred on the surface of the substrate.After 168 h,the substrate has been corroded in a large area and the coating surface has perforated in many places,which is probably caused by the release of hydrogen associating with the corrosion of magnesium alloys[45].In contrast,no corrosion can be found on the surface of kaolin/HDTMS@EP coating after 72 h salt spray test.After 120 h,a small amount of black spots begin to appear at the bottom of the coating,indicating that salt spray permeated the coating to the substrate.However,after 168 h,the number of black spots only increased slightly,demonstrating the corrosion was significantl reduced compared with AZ31B Mg and EP coating.This is mainly due to the water-repellency effect of the superhydrophobic surface,which plays a better role in blocking the salt spray,thus slowing down its entering speed and extending the service life of the substrate.

Fig.12.Electrochemical impedance spectra and Tafel curves of (a,b,e) EP coating and (c,d,f) kaolin/HDTMS@EP coating after immersed in 3.5 wt.% NaCl solution for different times;(g) fittin model of the electrochemical impedance spectra, Rs represents the electrolyte resistance,CPE non-ideal capacitance of the surface, Rc the coating impedance, Rct the impedance of electronic double layer, Rdiff the diffusion impedance,mainly controlled by the sediment diffusion process of magnesium alloy corrosion products [46];(h) values of Rc,Rct of coatings.

To comprehensively understand functionality of the asprepared superhydrophobic coating in this study,a brief comparison of superhydrophobic coatings (including prepared methods,main materials,consuming time,chemical stability,durability andicorr) on Mg alloys corrosion protection previously reported in literatures was summarized in Table 2.One should be noted is that the time listed in columns of chemical stability andicorrrefer to the immersion time in 3.5 wt.% NaCl solution,and the durability mainly refers to the abrasion test on sandpaper with different weight on sample surfaces if no extra notations.From Table 2,it is easy to fin out that the materials used here are green without any fluorides and the fabrication time at around 2 h is relatively facile and time-saving.Moreover,the chemical stability,durability and corrosion protection effect of the as-prepared coatings demonstrates satisfactory superiority in comparison with most of previously-reported coatings.

Fig.13.Optical pictures of AZ31B Mg,EP coating and kaolin/HDTMS@EP coating during 168 h neutral salt spray test.

Table 2 Preparation method and performance comparison of superhydrophobic coatings on Mg alloys corrosion protection.

4.Conclusion

A novel electrochemical method was applied to modify kaolin to obtain superhydrophobic powder (kaolin/HDTMS)in a low-cost and time-saving way.By mixing kaolin/HDTMS with commercial adhesives,superhydrophobic coating(kaolin/HDTMS@EP) was easily formed on various kinds of substrates via spraying method.The superhydrophobic coating with CA ～155° and SA ～5° exhibited favorable antifouling ability,light transmittance,chemical stability and thermal healing capacity,which can sustain super hydrophobicity after soaking in pH = 1 HCl solution,pH = 14 NaOH solution and 3.5 wt.% NaCl solution for 16,20 and 30 days,respectively.Besides,the kaolin/HDTMS@EP can bear 40 cycles sandpaper abrasion,100 cycles 3 M scotch peeling test,and the 4A adhesion level by the X-cut method.Moreover,the coated Mg alloys can remain anti-corrosion efficien y of 99.86% after immersed in 3.5 wt.% NaCl solution for 20 days.These results demonstrate that the method and products developed here are in favor of the fabrication and application of superhydrophobic coatings in various field for anti-fouling,anti-corrosion and light transmittance increasing usages.

Experimental information of Tafel polarization parameters of EP coating (Table S1),EDS spectra of kaolin and kaolin/HDTMS (Fig.S1),Snapshots of the superhydrophobic coating resistant to beverage contamination (Fig.S2),Optical images of the superhydrophobic coating after immersed in 3.5 wt.% NaCl solution and in HCl solution with pH of 1 (Fig.S3),SEM and EDS spectra of the superhydrophobic coating before (a) and after (b) heating at 80 °C for 30 min after immersed in pH = 14 NaOH solution for 7 days (Fig.S4),SEM images of the prepared composite coating after 200 cycles of sandpaper abrasion test (Fig.S5),Electrochemical impedance of EP and kaolin/HDTMS@EP coatings at 0.01 Hz.(Fig.S6).

Acknowledgments

The authors appreciate the financia support of the National Natural Science Foundation of China (Grant No.21978182),the measurement of UV-vis and SEM by the Institute of New Energy and Low Carbon Technology of Sichuan University,and the FTIR,TGA and electrochemical measurements technical assistance by the Engineering Teaching Center,School of Chemical Engineering,Sichuan University,and the XPS characterization by Zhang San from Shiyanjia Lab(www.shiyanjia.com)

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.07.001.