Snowflake-like Cu2S as visible-light-carrier for boosting Pd electrocatalytic ethylene glycol oxidation under visible light irradiation

Journal Pre-proof Snowflake-like Cu2S as visible-light-carrier for boosting Pd electrocatalytic ethylene glycol oxidation under visible light irradiation

Haifeng Gao, Chunyang Zhai, Chen Yuan, Zhao-Qing Liu, Mingshan Zhu PII:

S0013-4686(19)32085-7

DOI:

https://doi.org/10.1016/j.electacta.2019.135214

Reference:

EA 135214

To appear in:

Electrochimica Acta

Received Date:

04 April 2019

Accepted Date:

03 November 2019

Please cite this article as: Haifeng Gao, Chunyang Zhai, Chen Yuan, Zhao-Qing Liu, Mingshan Zhu, Snowflake-like Cu2S as visible-light-carrier for boosting Pd electrocatalytic ethylene glycol oxidation under visible light irradiation, Electrochimica Acta (2019), https://doi.org/10.1016/j. electacta.2019.135214

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Graphical Abstract

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Snowflake-like Cu2S as visible-light-carrier for boosting Pd electrocatalytic ethylene glycol oxidation under visible light irradiation Haifeng Gao a, Chunyang Zhai a, Chen Yuan a, Zhao-Qing Liu c, Mingshan Zhu*,a,b a

School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, P.R. China b

c

School of Environment, Jinan University, Guangzhou 511443, P.R. China

School of Chemistry and Chemical Engineering/Institute of Clean Energy and

Materials/Guangzhou Key Laboratory for Clean Energy and Materials, Guangzhou University, 510006, P. R. China *E–mail: [email protected] Abstract: Photo-assisted electrocatalytic oxidation alcohol molecule holds promise for both sustainable energy generation and energy conversion in direct alcohol fuel cells (DAFCs). Herein, we demonstrate that snowflake-like Cu2S on which Pd nanoparticles are grown acts as an excellent catalyst for ethylene glycol oxidation reaction (EGOR) under visible light irradiation. The peak current density of Pd-Cu2S electrode reaches 3254 mA mg-1Pd under visible light irradiation, which is 1.7 and 5.5 times than that of Pd-Cu2S electrode and commercial Pd/C electrode under dark condition. In the chronopotentiometry experiments, the sustained time of Pd-Cu2S electrode under visible light illumination is 2.9 and 5.0 times longer than that of Pd-

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Cu2S electrode and commercial Pd/C electrode in dark. These results indicate that PdCu2S catalyst exhibits excellent catalytic activity and stability for EGOR under photoelectric cooperation. Meanwhile, the present work indicates that Cu2S snowflake as the traditional noble metal electrocatalyst carrier demonstrates promising prospects in efficient and stable solar fuel conversion and fuel cell reaction. Key words: cuprous sulfide; ethylene glycol oxidation; visible light; snowflake-like structure; Pd electrocatalyst 1. Introduction Ethylene glycol can be obtained through indirect conversion of related biomass cellulose with high yield, and its environmentally friendly properties can well cater to the development of modern society in the field of direct ethylene glycol fuel cells (DEGFCs) [1-2]. Unfortunately, it is a great obstacle to find a highly efficient anode catalyst for the complete oxidation of ethylene glycol. Initially, precious metals, such as Pd, Pt, are basically made into nanoparticles (NPs) and used as catalysts for DEGFCs. But due to the skyrocketing price and poor anti-toxicity, the commercial development of Pd catalysts is seriously hindered in DEGFCs [3]. To this end, it is imperative to explore high-effective and low-cost anode electrocatalysts. As we all known, the catalytic activity of precious metal electrocatalysts is enhanced by hybridization with a support because charge transfer can be effectively

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improved during electrochemical reaction between precious metal and support [4-6]. Moreover, the carrier can reduce the agglomeration of precious metals and improve the utilization of precious metals, thereby improving the catalytic reaction performance

[7].

Apart

from

these,

the

developments

of

inexpensive

metal/semiconductor electrocatalysts to reduce the amount of precious metals have significant economic benefits on the energy industry. Recently, large amount of semiconductor materials are used as the support to deposit the precious metal electrocatalysts to improve their electrocatalytic performance [8-11]. On the other hand, semiconductors are generally used as the photo-activated functional materials in the solar energy conversion. In the recent research reports, photoelectrochemical technique has been considered as a promising tool to improve the activity of electrocatalytic alcohol oxidation in the direct alcohol fuel cells (DAFCs) by using metal/semiconductor composites [12-18]. The synergistic effect of photocatalysis and electrocatalysis on metal/semiconductor electrode results in the enhanced alcohol oxidation performance under light irradiation. Cuprous sulfide (Cu2S), a narrow band gap semiconductor of 1.2 eV with broad absorption in visible region has received extensive attention. However, the main applications of Cu2S are limited in the solar cells and photocatalysis. [19-23]. Herein, in order to expand its new application area and to expect the high performance of

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anode electrocatalyst in DEGFCs, a snowflake structure of Cu2S was synthesized and firstly used as the carrier for the decoration of Pd NPs. Combining the optical and photocatalytic properties of Cu2S with the excellent electrocatalytic activity of Pd NPs, electrocatalytic oxidation of ethylene glycol was evaluated under visible light irradiation. Cyclic voltammetries (CVs) and chronopotentiometry (CP) results showed that the as-prepared Pd-Cu2S exhibited outstanding catalytic performance and stability under visible light relative to commercial Pd/C in conventional electrocatalytic process. The unique snowflake structure and wide wavelength absorption of Cu2S provide a new insight into improving the traditional electrocatalytic application and Cu2S snowflake structure is demonstrated as an ideal photoactive carrier for fuel cell reaction in DAFCs. 2. Experimental 2.1. Materials and reagents Copper chloride (CuCl22H2O), thiourea (CH4N2S), ethylenediamine (EDA), NN dimethylformamide (DMF), ethylene glycol (C2H6O2), potassium hydroxide (KOH) chloro palladium acid (H2PdCl4) and ethanol (C2H5OH) were procured from Sinopharm Chemical Reagent Co and used without further purification. All chemical reagents are of analytical grade and used directly. 2.2. Synthesis of pure Cu2S snowflakes

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Pure Cu2S snowflakes were prepared via a simple solvothermal method [24]. Typically, 0.171 g CuCl22H2O were dispersed into 30 mL EDA under sonication until obtaining a clear solution. Then 0.218 g CH4N2S was slowly added under stirred vigorously to ensure a good dispersion of reactants. Subsequently, the above homogeneous solution was transferred into 50 mL Teflon autoclave and kept at 80 oC for 10 h. After cooling down to room temperature naturally, the precipitates were obtained by centrifugation (8000 rpm, 10 min) and washed with deionized water and absolute ethanol several times. Subsequently, the obtained black powders were dried at 60 oC in vacuum oven overnight. 2.3. Preparation of Pd NPs loaded on Cu2S snowflakes (Pd-Cu2S) The Pd NPs loaded Cu2S snowflakes (Pd-Cu2S) were prepared by a simple solvothermal method. 40 mg as-synthesized Cu2S snowflakes were dispersed into 20 mL DMF, and were further sonicated for 30 min to obtain a suspension. Then 0.42 mL H2PdCl4 (0.226 mol L-1) was added and stirred vigorously for 2 h. After that, the suspension was transferred into a 25 mL Teflon-lined autoclave and maintained at 160 oC

for 6 h. The as-obtained raw products were centrifuged and washed several times

with deionized water and ethanol, and then dried at 60 oC in vacuum overnight. Thus the 20 wt% contents of Pd in Pd-Cu2S composites were obtained. The Pd-Cu2S composites with different Pd NPs loading amounts: 5 wt%, 10 wt% and 30 wt% were

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harvested by controlling the mass of H2PdCl4 with similar procedure. 2.4. Photoelectrochemical measurements All electrochemical/photoelectrochemical properties tests were carried out at room temperature by a CHI 760E electrochemical workstation (Shanghai Chenhua Instrumental Co., Ltd., China) under visible light assembled by a 500W Xe lamp with UV cut-off filter (>420 nm). Saturated calomel electrode (SCE), a piece of Pt wire, and L-glassy carbon electrode (L-GCE) with diameter (d =0.3 cm) were used as reference, counter and the working electrodes, respectively. The working electrode was prepared as follow: 2 mg as-prepared Pd-CuS2 samples were dispersed into 1 mL water/ethanol mixtures (Vwater:Vethanol = 1) including 20 μL Nafion (DuPond, USA) and then under ultrasonication for 60 min. Then 5 µL catalyst ink (2 mg mL-1) was dropped

onto

a

pre-polished

L-GCE

surface

and

dried

naturally.

The

photoelectrocatalytic activities of Pd-Cu2S electrodes were evaluated by cyclic voltammetry (CV) in 1.0 M ethylene glycol+1.0 M KOH. Chronopotentiometry curves (CP) of the working electrodes under dark and visible-light illumination were measured at a 10 µA. Photocurrent responses of Pd-Cu2S and pure Cu2S samples were tested at a -0.15 V. The electrochemical impedance spectroscopy (EIS) measurements were taken in 1.0 M ethylene glycol+1.0 M KOH from 100 kHz to 1 Hz at -0.15 V. In addition, 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6]+0.1 M KCl mixture solution as a redox

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probe was measured for CV and EIS measurements with and without visible light irradiation. 2.5. Characterization Scanning electron microscope (SEM, JSM-6330FT) was used to analyze the morphology and microstructure of samples. The attached energy dispersive X-ray spectroscopy (EDS) detector was applied for elemental analysis. UV-vis-near-infrared (NIR) spectrophotometer (Lambda 950) was used to obtain the UV-vis diffuse reflectance spectra (UV-DRS). X-ray photoelectron spectroscopy (XPS) was detected on an ESCALab220i-XL electron spectrometer. X-ray diffraction (XRD) patterns of the products were collected by a PANalytical X' Pert PRO MRD system with Cu Ka radiation (k =1.54056 Å) operated at 40 kV and 30 mA. 3. Results and discussion 3.1. Characterization of the as-prepared samples

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Fig. 1. SEM images of pure Cu2S (A and B) and Pd-Cu2S (C and D) samples. As a classic bidentate ligand, EDA is used to bind with metal cations. In the process of the synthesis of Cu2S samples, Cu2+ can completely transform to the [Cu (En)2]2+ complex in EDA solvent at a controlled temperature, which is benefit with the formation of snowflakes structures. The morphologies and microstructures of the as-synthesized pure Cu2S and Pd-Cu2S samples were characterized by SEM. It can be clearly observed from Fig. 1A and B that the as-obtained pure Cu2S sample displays an unique six-branch dendritic snowflake structure with smooth and clean surface, and the length of six dendritic trunks is about 3-5 µm. Meanwhile, each dendritic trunk has splitted into more than ten subbranches. When Pd NPs were deposited on the surface of Cu2S snowflakes through solvothermal method, firstly, the morphology

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of Cu2S snowflake did not significantly change at 160 ℃ (Fig. 1C and D), indicating Cu2S snowflakes with excellent thermal stability. Secondly, the surface of Cu2S snowflakes became very rough compared with pure Cu2S snowflakes, and many tiny particles were observed on the surface of Cu2S snowflakes (Fig. 1D). This result indicates that Pd NPs were loaded on the Cu2S snowflakes successfully. In order to confirm the elements composition of Pd-Cu2S, SEM, EDX and corresponding elemental mappings of Pd-Cu2S were given in Fig. 2. These results clearly showed the uniform distribution of Cu, S and Pd elements in as-prepared Pd-Cu2S samples.

Fig. 2. (A) SEM, (B) EDX and corresponding elemental mappings (C-F) of asprepared Pd-Cu2S composite.

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Fig. 3. (A): XRD patterns of of pure Cu2S and Pd-Cu2S samples. XPS spectra of Cu 2p (B), S 2p (C), and Pd 3d (D) of pure Cu2S (a) and Pd-Cu2S (b). The phase compositions and structures of the obtained Cu2S and Pd-Cu2S samples were measured by XRD. As shown in Fig. 3A, the main characteristic diffraction peaks of snowflakes match well with the standard pattern of the Cu2S phase (JCPDS 02-1294) [24,25], indicating that pure Cu2S snowflakes were successfully synthesized. After loading Pd NPs on the surface of Cu2S snowflakes, a distinct signal peaks appeared at 40.1°, which was assigned to the (111) phase of Pd (JCPDS 46-1043) [26]. This result indicates the existence of Pd NPs in the Pd-Cu2S composites. The surface composition and elemental states of Cu2S and Pd-Cu2S samples were performed by XPS and the results were shown in Fig. 3B-D. In the pure 10

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Cu2S sample, the spectrum of Cu 2p shows two peaks located at 932.4 and 952.5 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. There is no satellite peak between Cu 2p3/2 and Cu 2p1/2 (Fig. 3B-a), indicating that Cu+ is the sole form of Cu element in Cu2S sample [27-28]. Fig. 3C-a shows that the S 2p XPS exists two broad peaks of S 2p3/2 and S 2p1/2 at binding energies of 161.2 and 162.4 eV, respectively, which means the value of S of sample with S2- [24,28]. Besides, a doublet peak centered at 335.1 and 340.7 eV were observed, which agree well with the value of Pd0 state [29] (Fig. 3D). These results reveal the generation of Pd-Cu2S composites. More intresting, compared to the pure Cu2S, the positions of Cu 2p and S 2p peaks in PdCu2S sample are slightly shifted. This shift might be due to the surface electrons of Cu2S transferring to Pd NPs by hybridization with Pd [29].

Fig. 4. (A): UV-vis diffuse reflectance spectra of pure Cu2S and Pd-Cu2S. The insert is Mott-Schottky plot of Cu2S. (B): Photocurrent responses of pure Cu2S (a) and PdCu2S (b) samples in the solution of 1.0 M ethylene glycol+1.0 M KOH at -0.15V. The optical properties of pure Cu2S and Pd-Cu2S samples are tested by UV-vis

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diffuse reflectance. In Fig. 4A, pure Cu2S snowflakes displayed a broad light absorption in the visible light region, attributing to the basic optical property of semiconductor Cu2S snowflakes. After Pd NPs deposition, Pd-Cu2S composites exhibit a similar absorption region with pure Cu2S snowflakes. Meanwhile, the MottSchottky (M-S) analysis is measured to obtain the band values of Cu2S snowflakes. The M-S analysis shows that the plot of Cu2S has a negative slope, suggesting the behavior of p-type semiconductor, and the flat band potential is ca. 0.85 eV vs. NHE. According to previous reports [29], the band gap (Eg) of Cu2S is 1.28 eV. In general, the valance band (VB) of p-type semiconductor is more positive (ca. 0.2 eV) than Fermi level which is same as flat band potential. Therefore, the EVB of Cu2S is ca. 1.05 eV vs. NHE, while ECB of Cu2S is ca. -0.23 eV according to the empirical formula (Eg = EVB-ECB) [30]. It's well known that photocurrent is considered as a useful method to assess the separation efficiency of photogenerated electron-hole pairs on the catalyst surface [31,32]. Photocurrent densities of Cu2S and Pd-Cu2S samples were measured with visible light on/off every 50 s. As shown in Fig. 4B, the photocurrent intensity of PdCu2S composites reached to 0.025 mA cm-2 under visible light illumination, while the photocurrent intensity of pure Cu2S was 0.008 mA cm-2 under the same condition. Such higher photocurrent response could be ascribed to the significantly enhanced

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seperation efficiency of photogenerated electron-hole pairs in Pd-Cu2S composites. In addition, the photocurrent responses of Cu2S and Pd-Cu2S samples are repeatable during on/off cycles under visible light illumination, suggesting the stability of the samples under light irradiation. 3.2. Ethylene glycol oxidation on Pd-Cu2S composites catalyst Thanks to the unique Cu2S snowflakes structure with superior optical and electrical properties, the results that Pd NPs decorated on the surface of Cu2S snowflakes are expected to display the better electrocatalytic performances towards liquid fuel oxidation under visible light illumination. Accordingly, EGOR was selected to evaluate the photoelectrocatalytic properties of as-obtained samples. The CV measurements were conducted in 1.0 M ethylene glycol +1.0 M KOH at the potential range from -0.7 to 0.2 V with the scanning rate of 50 mV s-1 under dark condition and visible light irradiation. Firstly, two characteristic peaks locate at 0.1 to -0.3 V in positive sweep peak and -0.2 to -0.35 V in reverse sweep peak were observed and presented in Fig. 5A. The oxidation peaks in the positive sweep are ascribed to the direct oxidation of ethylene glycol, meanwhile, the another anodic peak in the reverse sweep is attributed to the removal of the COads and other CO-like intermediates left over from the positive sweep [33-35]. Generally, the intensity of the positive peak current density is applied to evaluate the catalytic performance of

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catalyst. Fig. 5A shows the corresponding CVs of ethylene glycol in alkaline medium of Pd-Cu2S and commercial Pd/C electrodes. The mass catalytic activity of Pd-Cu2S electrodes (1887 mA mg–1Pd) is obviously higher than that of commercial Pd/C (597 mA mg–1Pd) under electrocatalytic condition, which indicates Cu2S snowflakes can be a promising metal carrier in DEGFC. More importantly, under visible light irradiation, the positive peak current density of Pd-Cu2S electrode is 3254 mA mg–1Pd, which is 1.7 and 5.5 times than that of Pd-Cu2S electrode and commercial Pd/C electrodes in traditional electrocatalytic process, respectively. The noteworthy enhancement was attributed to the synergistic effect of photocatalysis and electrocatalysis of Pd-Cu2S catalyst in EGOR under visible light illumination. To further study the influence of the content of Pd nanoparticles on catalytic activity of Pd-Cu2S composites, the catalytic performance of different weight amount of Pd loaded on Cu2S snowflakes were investigated and the results were shown in Fig. 5B. It shows that 20 wt% Pd NPs deposition to the Cu2S surface is the most suitable ratio in the process of photo-assisted electrocatalytic oxidation of ethylene glycol. When further increased the amount of Pd, the mass catalytic activity decreased significantly.

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Fig. 5. (A): CVs of Pd-Cu2S (a, b) and commercial Pd/C (c) electrodes with (a) and without (b, c) visible-light irradiation in the solution of 1.0M ethylene glycol+1.0M KOH. (B): The histogram of activities of different electrodes for EGOR under visible light illumination and dark condition.

Fig. 6. (A): Change trend of positive peak current intensity on Pd-Cu2S and commercial Pd/C electrodes with increasing number of scanning cycles. (B): The positive peak current intensity of different weight ratios of Pd in Pd-Cu2S for EGOR under visible light irradiation. 3.2 Stability of Pd-Cu2S composites catalysts The superior durability of catalyst is another important factor to assess the

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performance of catalyst in DEGFCs. Basically, a catalyst with better durability means a longer lifetime, which is essential for practical commercial applications. To measure the durability of the catalysts, the scan cycling experiments were carried out in the EGOR. As shown in Fig. 6A, for commercial Pd/C, the forward oxidation peak current density performs a maximum at the initial stage, and then decreased gradually with increasing the number of scans. This result indicates that the lifetime of the commercial Pd/C is short. But to Pd-Cu2S catalyst, with the increase of the number of scans, the peak current density significantly increased, and reaches a maximum of 1887 mA mg–1Pd at about 150-cycle at electrocatalytic condition. At the end of the test, the peak current density is 1739 mA mg–1Pd, which is slightly decreased compared to the maximum value. When the Pd-Cu2S catalyst is irradiated under visible light, the change trend of the peak current density is similar, while the peak current density enhanced obviously compared with the Pd-Cu2S catalyst under dark condition at the same number of scans. Moreover, there is a slight change of pH before (pH=13.15) and after (pH=13.05) reaction, which means the hydroxide ion concentration is only slight consumed during the experiment. These conclusions reveal that the new PdCu2S catalyst has superior durability of EGOR not only under dark condition but also under visible light irradiation. Meanwhile, in Fig. 6B, the analogous durability test is also used to perform catalytic properties and stabilities of the different weight ratio Pd

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in Pd-Cu2S composites under visible light assistance. These observations clearly support the fact that 20 wt% Pd-Cu2S catalyst exhibited prominent catalytic activity and stability performance at the same condition. To further certificate the capability of catalyst’s poisoning resistance toward EGOR, CP measurements were recorded for all catalysts at a bias current density of 10 µA, which are illustrated in Fig. 7. In general, the potential is gradually increased with the polarization time, suggesting the better anti-poisoning abilities of catalyst. The catalyst turns to be poisoned when the voltage enhanced quickly [35]. As shown in Fig. 7A, the Pd-Cu2S electrode can withstand poisoning species for about 4662 s before the potential jumped to the higher potential, which is almost 1.7 times than that of commercial Pd/C electrode (2747 s) under dark condition. The result means that the Pd-Cu2S electrode has better poisoning resistance ability compared to the commercial Pd/C electrode, which might be contributed to the synergistic effect of uniformly distributed ultrasmall Pd NPs and unique snowflakes structure of Cu2S. Secondly, the sustained time of Pd-Cu2S composites electrode is about 13620 s under visible light illumination before the electrode potential jumped to the higher potential, which is 2.9 and 5.0 times longer than the Pd-Cu2S modified electrode and commercial Pd/C electrode under dark condition. The above phenomenon further indicates that the anti-poisoning performance of Pd-Cu2S can be extremely enhanced

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when the electrode is exposed under visible light, due to the synergistic effect of uniformly distributed ultrasmall Pd NPs and the perfect optical property of Cu2S snowflakes under visible light irradiation.

Fig. 7. Chronopotentiometry curves of Pd-Cu2S (a, b) and commercial Pd/C (c) electrodes with visible light (a) and dark condition (b, c) in 1.0 M ethylene glycol+1.0 M KOH solution at 10 µA. 3.3. EIS spectra EIS is regarded as a valid electrochemical approach to analyze the electrontransfer efficiency of electrode. Generally, the faster mobility of interfacial charger transfer will produce smaller diameter of theimpedance arc (DIA), which corresponds to the smaller charge transfer resistance (Rct) [36,37]. The Nyquist plots of EIS for PdCu2S and commercial Pd/C in the solution of 1.0 M ethylene glycol+1.0 M KOH at 0.15 V under different conditions were shown in Fig. 8. According to Fig. 8A, the DIA of Pd-Cu2S electrode is smaller than that of commercial Pd/C. When the

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electrode was placed under visible light, the DIA of Pd-Cu2S electrode became smaller. This result reveals that the Pd-Cu2S electrode exhibited the highest interfacial charge transfer efficiency during the photo-irradiation process. To visually show the parameter of charge transfer resistance, equivalent circuit was used for simulating the impedance spectra and the results were shown in Fig. 8B. Rct represents the chargetransfer resistance at electrode/solution interface, Q is the electrode double-layer capacitance formed at electrode/solution interface and Rs is associated with the electrolyte resistance. The corresponding parameters of Rct are summarized in Table 1. The Rct of Pd-Cu2S electrode and commercial Pd/C electrode is 176 Ω cm2 and 622 Ω cm2 under dark, respectively. When Pd-Cu2S electrode is exposed under visible light illumination, Rct is decreased to 57.2 Ω cm2. These results suggest that Pd-Cu2S exhibits higher electrocatalytic activity for EGOR compared to commercial Pd/C under electrocatalytic process. Meanwhile, photo-electric synergistic effect can effectively improve the interface charge transfer rate of Pd-Cu2S catalyst.

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Fig. 8. (A): EIS spectra of of Pd-Cu2S (a, b) and commercial Pd/C (c) electrodes with (a) and without (b, c) visible light irradiation in 1.0 M ethylene glycol+1.0 M KOH. (B): Equivalent circuit was used for simulating the impedance spectra. To

investigate

the

charge

transfer

efficiency

of

samples,

the

K3[Fe(CN)6/K4[Fe(CN)6]+KCl electrolyte was used as redox probes. Fig. 9A displays the CVs of the reversible redox couple of Fe(CN)64-/3-. The redox peak current intensity of Pd-Cu2S was enhanced with assistance of visible light illumination compared with the same electrode under dark condition, and commercial Pd/C displayed the lowest peak current intensity. Moreover, EIS spectra of the electrodes were also measured in 2.5 mM K3[Fe(CN)6/K4[Fe(CN)6]+0.1 M KCl electrolyte at a potential of 0.2 V with and without visible light illumination. As shown in Fig. 9B, the similar phenomenon was obtained, which Pd-Cu2S electrode with visible light irradiation showed the smallest DIA. All above results imply that the presence of visible light illumination facilitates the interfacial charge-transport rate between redox couples in the mixture solution and electrode surface.

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Fig. 9. (A): CVs and (B): EIS spectra of Pd-Cu2S (a, b) and commercial Pd/C (c) electrodes with (a) and without (b, c) visible light irradiation in 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6]+0.1 M KCl solution at a potential of 0.2 V. The inset shows the equicalent circuit to fit the impedance spectra. Table 1. EIS fitting parameters of Rct (1) and Rct (2) from equivalent circuits for different samples under different electrolyte. Electrode

1.0 M ethylene glycol +1.0 M KOH

Electrode

2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] +0.1 M KCl

Rct (1)

Rct (2)

Pd-Cu2S-Light

57 Ωcm2

432 Ωcm2

Pd-Cu2S-Dark

176 Ωcm2

872 Ωcm2

Commercial Pd/C

623 Ωcm2

2813 Ωcm2

3.4 Proposed mechanism of photoelectrocatalytic oxidation ethylene glycol on PdCu2S composites According to the above results, the Pd-Cu2S catalyst reveals excellent character in EGOR process when the catalyst electrode is exposed on visible light irradiation. The proposed mechanism is shown in Scheme 1. In traditional electrocatalytic 21

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process (i), the as-obtained Cu2S snowflakes are acted as a carrier, which prohibit the aggregation of Pd NPs, resulting in Pd NPs to be more evenly distributed on the surface of Cu2S snowflakes. Then, Pd NPs are used as active sites and the ethylene glycol molecules adsorbed on the surface of Pd NPs are electrooxidized into CO2 [38,39]. The above pathways were described in following. Eqs. (i): (1)-(7): Pd+(CH2OH)2solotionPd-(CH2OH)2ads

(1)

Pd-(CH2OH)2ads+4OH- Pd-(HCO)2ads+4H2O+4e-

(2)

Pd-(HCO)2ads+4OH- Pd-(HCOO)2ads+ 2H2O+4e-

(3)

Pd-HCOOads + e- Pd-COads+ OH-

(4)

Pd-(HCO)ads + e- Pd-COads+ OH-

(5)

Pd-OH- Pd+OHads+ e-

(6)

Pd-COads+Pd+OHads 2Pd+CO2+ H++e-

(7)

Scheme 1. Proposed scheme illustration for photoelectrocatalytic oxidation of

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ethylene glycol on Pd-Cu2S electrode. In the photocatalytic process (ii), Cu2S snowflakes possess a broad visible light absorption and can be excited to generate electrons (e-) and holes (h+) on the CB and valence band (VB), respectively, when it exposed to visible light (>420 nm) irradiation. The holes on the VB have strong oxidative properties, which can convert surface adsorbed H2O/OH- to hydroxyl radicals (·OH) with strong oxidizing properties [40]. The adsorbed ethylene glycol molecules on the surface of Pd-Cu2S composites catalysts would be oxidized by ·OH, resulting in photoelectrocatalytic EGOR process. Meanwhile, the as-synthesized Pd-Cu2S composites perform effective interfacial charge transfers between Cu2S snowflakes and Pd NPs. Photogenerated electrons flow to the external circuit through the Pd NPs intermediate, preventing the combination of electron and holes. The above procedures were described in Eqs (ii): (8)-(11): Cu2S + hv  Cu2S + e- + h+

(8)

h+(Cu2S) + OH-  OH

(9)

(CH2OH)2 + 10(•OH) 2CO2 +8H2O +5e-

(10)

Intermediates (COads) + •OH  CO2 + H+ + e-

(11)

4. Conclusions In summary, unique Cu2S snowflake structure with outstanding optical and

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electrical properties were firstly synthesized. Then, Pd NPs loading on as-prepared Cu2S snowflake was conducted into EGOR. Results show that Pd-Cu2S composites catalyst exhibited excellent activity and stability for EGOR under photo/electric catalytic cooperation than that of the same electrode and commercial Pd/C in the electrocatalytic process. Photocurrent response, chronopotentiometry, and EIS results confirmed that the presence of visible light illumination could facilitate the interfacial charge-transport rate between the solution and Pd-Cu2S composites electrode surface. This work is significant for the development of new eletrocatalyst with high activity for the application of alcohol fuel cell with assistance of visible light. Acknowledgments The authors appreciate to the National Natural Science Foundation of China (Grant 21603111 and 51702173). This work was also sponsored by K.C. Wong Magna Fund in Ningbo University. References [1] A. Serov, C. Kwak, Recent achievements in direct ethylene glycol fuel cells (DEGFC), Appl. Catal. B: Environ. 97 (2010) 1-12. [2] L. An, R. Chen, Recent progress in alkaline direct ethylene glycol fuel cells for sustainable energy production, J. Power Sources 329 (2016) 484-501. [3] S. Li, J. Lai, R. Luque, G. Xu, Designed multimetallic Pd nanosponges with

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Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.