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Pd nanoparticles anchored to nano-peony CoMn2O4 as an efficient catalyst for H2O2 electroreduction
Journal Pre-proof Pd nanoparticles anchored to nano-peony CoMn2O4 as an efficient catalyst for H2O2 electroreduction
Congying Song, Xinhang Li, Lin Zhang, Peng Yan, Chenlin Xu, Kai Zhu, Kui Cheng, Ke Ye, Jun Yan, Dianxue Cao, Guiling Wang PII:
S1572-6657(19)30979-8
DOI:
https://doi.org/10.1016/j.jelechem.2019.113711
Reference:
JEAC 113711
To appear in:
Journal of Electroanalytical Chemistry
Received date:
27 July 2019
Revised date:
5 November 2019
Accepted date:
1 December 2019
Please cite this article as: C. Song, X. Li, L. Zhang, et al., Pd nanoparticles anchored to nano-peony CoMn2O4 as an efficient catalyst for H2O2 electroreduction, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/j.jelechem.2019.113711
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© 2019 Published by Elsevier.
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Pd nanoparticles anchored to nano-peony CoMn2O4 as an efficient catalyst for H2O2 electroreduction Congying Songa, Xinhang Lia, Lin Zhanga, Peng Yan*, a, b, Chenlin Xua, Kai Zhua, Kui Chenga, Ke Yea, Jun Yana, Dianxue Caoa, Guiling Wang*, a a Key Laboratory of Superlight Materials and Surface Technology of Ministry of
of
Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001, P.R. China
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Email: [email protected]
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b College of Materials and Chemical Engineering，Heilongjiang Institute of
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Technology，Harbin 150050，China
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Abstract
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Email: [email protected] (Peng Yan)
Bimetallic oxide CoMn2O4 with nano-peony structure is prepared on Ni foam by
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a traditional hydrothermal process. Then constant-potential electrodeposition is applied to anchor Pd nanoparticles on CoMn2O4 to form an electrode of Pd nanoparticles modified CoMn2O4 (PCMN electrode) for H2O2 electroreduction. The combination of Pd and CoMn2O4 effectively reduces the dosage of noble metal. Besides, no binder is involved in the preparation which cuts the electrode cost and avoids the poor stability of traditional electrodes produced with binders. Scanning electron microscopy, X-ray diffraction and transmission electron microscopy are operated to investigate the structure and composition of the electrode. And the electrochemical behavior of the electrode is characterized by cyclic voltammetry and
Journal Pre-proof chronoamperometry. In 0.7 mol L-1 H2O2 and 3 mol L-1 NaOH, a reduction current density of 580 mA cm-2 (normalized by geometric area) at -0.8 V on the electrode is obtained which reveals large capacity for actual application in H2O2-based fuel cell. Keywords: Nano-peony CoMn2O4; Palladium nanoparticles; Hydrothermal process; Catalysts; H2O2 electroreduction
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1. Introduction Nowadays, fossil fuel shortage and environmental pollution are two severe
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problems human beings are facing. Electric energy is an economical, practical and
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clean energy compared to fossil fuels. As a type of energy conversion device, fuel
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cells are able to convert chemical energy stored in fuels directly into electric energy.
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Due to the advantages of environmental friendly design and higher energy conversion
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efficiency[1, 2], fuel cells including hydrogen-oxygen fuel cell[3, 4], hydrogen-air fuel cell[5, 6], direct borohydride-hydrogen peroxide fuel cell[7, 8], direct methanol
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fuel cell[9, 10] and metal-air battery[11, 12] have obtained a good deal attention from researchers around the world. Among these fuel cells, direct borohydride-hydrogen peroxide fuel cell is a new type of liquid fuel cell which solves the problems of difficult storage and transportation of oxygen and hydrogen in traditional fuel cell. Therefore, direct borohydride-hydrogen peroxide shows great application prospects in oxygen-free environment like space and underwater. As oxidant, the reaction of H2O2 reduction does not involve any intermediate or by-product which will poison the catalysts. Besides, the two-electron reaction of H2O2 reduction possesses faster kinetics than the four-electron process of O2
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reduction[13-16]. Moreover, compared to O2, liquid H2O2 is capable to infiltrate the electrode surface better and the reduction of H2O2 happens at the solid-liquid interface. As for the O2 reduction, it requires a three-phase interface of solid-liquid-gas which may not be conducive to increase the reaction rate and volume power density[17-18]. At present, electrode materials applied to H2O2 reduction mainly includes three
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types: (1) noble metals like Au, Pt and Pd[19-21]; (2) transition metals and their oxides like Fe, Co, CuO, MnO2 and Co3O4[22-24]; (3) compounds of metals and
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organics[25-27]. Traditionally, catalyst powders were usually mixed with conductive
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carbon materials to obtain a slurry and then coated onto current collectors using some
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polymer binders. Santos’s group has used Pt powders supported on Vulcan XC-72
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carbon black to catalyze the reduction of H2O2[20]. And Wang’s group prepared a
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carbon-supported Au solid nanoparticles cathode for direct borohydride-hydrogen peroxide fuel cell[21]. Though noble metals possess excellent catalytic activity
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toward H2O2 reduction, the high price limits their large-scale application. Reducing the dosage of noble metals means reducing the cost of electrodes. Therefore, it is an effective method to reduce the dosage of noble metals to combine noble metals with other low-cost materials. Transition metal oxides with low cost have been studied extensively for H2O2 reduction. Like Co3O4, it can be prepared with unique three-dimensional
structure
morphology-controlled
Co3O4
and
our
group[28]
nanowires
with
has
previously
considerable
prepared
electrochemical
performance. However, most recent researches focus on investigating the catalytic activity of transition metal oxides with single metal element. The catalytic activity of
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bimetallic transition metal oxides toward H2O2 reduction is relatively little studied. As we all know, the essence of chemical reaction is the breakage and formation of chemical bonds. The difference of electronegativity between different metal elements in bimetallic transition metal oxides will be good for breaking the bonds. Therefore, in this work, Mn element was introduced to synthesize a bimetallic
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transition metal oxide CoMn2O4 with a unique peony-like structure. And then a composite of Pd nanoparticles modified CoMn2O4 was obtained by a simple
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electrodeposition process. Pd is a kind of noble metal with wider source and lower
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price compared to Pt and also possesses good catalytic activity, selectivity and
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durability for H2O2 electroreduction[27, 29]. In this design, because of the
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combination of Pd and CoMn2O4, the dosage of noble metal could be reduced. In
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addition, no polymer binder was involved in the preparation which also cut the cost. The three-dimensional structure of CoMn2O4 efficaciously dispersed the distribution
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of Pd nanoparticles and then avoided the decline of noble metal utilization. Furthermore, the difference of electronegativity among Co, Mn and Pd could also improve the performance of the as-prepared electrode. The following parts introduced the detailed characterization and catalytic behavior of the PCMN electrode. 2. Material and methods The PCMN electrode was prepared through a hydrothermal method followed by a treatment of electrodeposition and Fig. 1 shows the whole experimental process. 2.1 Preparation of CoMn2O4 supported on Ni foam (CMN) electrode The CMN electrode was synthesized using a traditional hydrothermal process.
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Firstly, a piece of Ni foam (size: 2 cm×4 cm×1.1 mm) was immersed into acetone, hydrochloric acid (6 mol L-1) and deionized water respectively with ultrasonic treatment aiming to dissolve the oxide layer and oil on Ni foam. Secondly, 0.4657 g Co(NO3)2, 0.1776 g NH4F, 0.576 g CO(NH2)2 and 0.1259 mL Mn(NO3)2 were added into 40 mL deionized water to get a transparent solution. Then, the treated Ni foam
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and solution were put into a autoclave (50 mL) and maintained at 100℃ for 5 h. After the autoclave cooled down to ambient temperature, the precursor was taken out from
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the autoclave and washed with deionized water for several times. At last, the Ni foam
was obtained.
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2.2 Preparation of PCMN electrode
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with precursor was calcined in a 300℃ muffle furnace for 4 h and the CMN electrode
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As depicted in Fig. 1, PCMN electrode was acquired through an electrodeposition operation. In a three-electrode system, the CMN electrode served as
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the working electrode, a carbon rod served as the counter electrode and a Ag/AgCl electrode served as the reference electrode. The electrodeposition process was conducted in a solution of 50 mL 1 mmol L-1 PdCl2 with an imposed potential of -1.2 V for 30 min. The final PCMN electrode was also washed by deionized water and dried in a vacuum oven.
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Figure 1. Schematic diagram of the preparation process
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2.3 Physical characterization and electrochemical tests of PCMN electrode
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Composition and structure are two key factors which can influence the catalytic
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activity of an electrode. Herein, we employed X-ray diffraction (XRD, Rigaku TTR III), scanning electron microscopy (SEM, JEOL JSM-6480) as well as transmission
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electron microscopy (TEM, FEI Teccai G2 S-Twin, Philips) to study the composition
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and structure of the PCMN electrode. Besides, X-ray photoelectron spectroscope with Al Kα radiation (XPS, Thermo ESCALAB 250) was used to explore the valence states of Co and Mn in CoMn2O4. And the Pd loading was measured by an inductive coupled plasma emission spectrometer (ICP, Xseries II, Thermo Scientific). At the same time, electrochemical tests including cyclic voltammetry (CV) and chronoamperometry (CA) were used to study the electrochemical behavior of the PCMN electrode commanded by an electrochemical workstation (EC-lab VMP3/Z). All the tests were done in a three-electrode system in which the PCMN electrode was regarded as the working electrode. A Ag/AgCl electrode and a carbon electrode were regarded as the counter electrode and reference electrode, respectively.
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3. Results and discussion 3.1 Physical characterization of PCMN electrode In order to confirm the composition of the as-prepared electrode, XRD pattern of PCMN electrode was measured and shown in Fig. 2. On the pattern of PCMN electrode, there are two groups of diffraction peaks. Thereinto, one group of peaks
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appeared at around 31.2°, 32.8°, 36.4°, 38.9°, 44.8°, 52.8°, 54.4°, 56.6°, 59.0°, 60.7° and 64.9°, 70.7°, 75.0°, 77.3° and 78.5° correspond to the (220), (113), (311), (004),
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(400), (332), (205), (333), (511), (404), (440), (610), (533), (622) and (444) lattice
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planes of CoMn2O4 (JCPDS card No. 18-0408). And another group of peaks at 40.1°,
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46.7° and 68.2° can be indexed to the (111), (200) and (220) lattice planes of Pd
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(JCPDS card No. 65-6174) which demonstrates that Pd was formed on CoMn2O4
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surface successfully through electrodeposition.
Figure 2. XRD pattern of PCMN electrode
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Fig. 3 shows the SEM images of CMN(Fig. 3a, c and e) and PCMN(Fig. 3b, d and f) electrodes, respectively. As seen from Fig. 3a, CoMn2O4 was formed on skeletons of Ni foam evenly and it is obvious to see from Fig. 3c and 3e that the prepared CoMn2O4 presents a peony-like structure like what was shown in the insert of Fig. 3e. Meanwhile, each CoMn2O4 peony was made up of many nanosheets. Many
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gaps existed among the peonies and the nanosheets, which provide more sites for the following deposition of Pd and larger contact surface area between electrolyte and
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electrode. After Pd was electrodeposited on the CMN electrode(Fig. 3b), it is obvious
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that the formed Pd failed to form a whole film covering the CoMn2O4 peony but
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anchored on the nanosheets dispersedly in a form of particle without distinct
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agglomeration. And from the SEM images with bigger magnification (Fig. 3d and 3f),
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the size of Pd particles is closed to 150 nm. The deposition of Pd did not damage the original structure of the CMN electrode and CoMn2O4 still maintains the peony-like
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structure well. The detailed morphology of the PCMN electrode and Pd particles was further shown in the following TEM characterization.
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Figure 3. SEM images of CMN(a, c, e) and PCMN(b, d, f) electrodes To further study the morphology of PCMN electrode, TEM and HRTEM images were shown in Fig. 4. From Fig. 4a and b, it is not hard to see that the as-prepared CoMn2O4 shows a sheet-like structure and Pd was formed on the surface of the sheets with a particle-like morphology dispersedly. The size distribution of Pd particles was shown in Fig. S1. Furthermore, we can confirm the composition of PCMN electrode on basis of the HRTEM results displayed in Fig. 4c and d. It can be roughly distinguished that there are four kinds of lattice fringes. To identify the lattice spacing more clearly, three regions marked by pink rectangle were enlarged and shown in the insert of Fig. 4c and d. The values of d-spacing of 0.202, 0.231 and 0.247 nm are
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index to the (400), (004) and (311) lattice planes of CoMn2O4 (JCPDS card No. 18-0408). Meanwhile, the d-spacing of 0.224 nm in Fig. 4d correspond to the (111) lattice plane of Pd (JCPDS card No. 65-6174). These results are well matched with
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the XRD results which demonstrate the successful synthesis of PCMN electrode.
Figure 4. TEM (a and b) and HRTEM (c and d) images of PCMN electrode X-ray photoelectron spectroscopy (XPS) was also operated to study the valence states of Mn, Co, O, Pd in the PCMN electrode. Results show that the atomic Co/Mn ratio is 1:1.7 which is closed to the empirical formula 1:2. Fig. 5 shows the XPS spectra of Mn 2p, Co 2p, O 1s and Pd 3d. From Fig. 5a, it can be seen that two main
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peaks at binding energies of 642.0 and 653.7 eV conform to Mn 2p3/2 and Mn 2p1/2, respectively. According to the Lorentzian-Gaussian curve fitting method, the fitted peak energies of 641.5 eV (2p3/2) and 653.5 eV (2p1/2) were indexed to the binding energy of Mn2+. Meanwhile the two peaks at 642.9 eV (2p3/2) and 654.5 eV (2p1/2) were correspond to Mn3+[30]. Besides, in Co 2p spectrum, the peaks at 782.7 eV (2p3/2), 796.8 eV (2p1/2) and 780.4 eV (2p3/2), 795.5 eV (2p1/2) were correspond to
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Co2+ and Co3+. Two satellites at 786.3 eV and 803.3 eV also appeared in the spectrum
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of Co 2p revealing the coexistence of Co2+ and Co3+[31]. As for the spectrum of O 1s,
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there are three fitted peaks located at 529.8, 531.0 and 532.4 eV designated to
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oxygen-metal bonds, oxygen in hydroxyl and oxygen in water molecules adsorbed on
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surface.[32] Meanwhile, the peaks on Pd 3d spectrum at binding energies of 340.38
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and 335.20 eV were assigned to Pd 3d3/2 and Pd 3d5/2 which reveals that Pd exists in
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the electrode in metallic Pd form[33].
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Figure 5. XPS spectra of (a) Mn 2p (b) Co 2p (c) O 1s (d) Pd 3d in PCMN electrode
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3.2 Electrochemical tests of PCMN electrode
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In order to compare the catalytic property of different electrodes and study the influence of CoMn2O4 and Pd nanoparticles on the electrochemical performance, cyclic voltammograms (CVs) of the current collector(Ni foam), CMN and PCMN electrodes in 0.7 mol L-1 H2O2 and 3 mol L-1 NaOH at a scan rate of 20 mV s-1 were tested in Fig. 6. It can be seen that on the CV curve represents for Ni foam, the current density in the whole potential range is around 0 showing that the current collector has little catalytic performance for the electroreduction of H2O2. When CoMn2O4 was prepared on the current collector(Ni foam), CMN electrode exhibits a distinctly better catalytic activity than Ni foam. At a potential of -0.8 V, a reduction current density of 200 mA cm-2 (normalized by geometric area) on the CMN electrode was achieved.
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Compared to CMN electrode, the reduction current density of H2O2 on PCMN electrode was 580 mA cm-2 (normalized by geometric area) at a potential of -0.8 V that is 2.9 times as that on CMN electrode revealing a further improvement of catalytic activity. The remarkable catalytic activity of PCMN and CMN electrode can be explained by Fig. 7a and b. As shown in Fig. 7a, the peony-like structure of
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CoMn2O4 can effectively disperse Pd particles and the gaps among CoMn2O4 highly increased the contact area between H2O2 and the catalysts. According to the previous
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literature[24], the conversion of Mn(II)/Mn(III) and Co(II)/Co(III) are believed to be
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responsible for the reduction of H2O2. When H2O2 transferred to the surface of
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CoMn2O4, it is likely to form three types of transition states including
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Co(II)-O-O-Co(II), Mn(II)-O-O-Mn(II) and Co(II)-O-O-Mn(II). After that, the O-O
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bond was broken by obtaining electrons from Co(II) and Mn(II) with a result of the formation of Co(III) and Mn(III). Among the situations, the O-O bond in the
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transition state of Co(II)-O-O-Mn(II) is easier to be broken because of the differences of electronegativity between Co(II) and Mn(II). In the same way, when H2O2 moles were adsorbed on the surface of PCMN electrode, besides Co(II)-O-O-Co(II), Mn(II)-O-O-Mn(II) and Co(II)-O-O-Mn(II), another three transition states including Co(II)-O-O-Pd, Mn(II)-O-O-Pd and Pd-O-O-Pd(shown in Fig. 7b) can also be formed. As we all know, precious metal Pd possesses good catalytic activity toward H2O2 reduction. In addition, the more significant differences of electronegativity between Pd and Co(II) or Pd and Mn(II) will further improve the catalytic activity of PCMN electrode toward H2O2 electroreduction. Therefore, after the electrodeposition of Pd,
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the current density of H2O2 reduction increased drastically. Moreover, ICP measurement was conducted to investigate the Pd loading which is 0.0783 mg in a piece of PCMN electrode (1cm×1cm). It can be concluded that the addition of small amount Pd highly promoted the electrochemical performance of PCMN electrode due
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to the synergy effect of Co, Mn and Pd.
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Figure 6. CVs of Ni foam, CMN and PCMN electrode in 0.7 mol L-1 H2O2 and 3 mol L-1 NaOH (scan rate: 20 mV s-1).
Figure 7. Schematic diagram of reaction mechanism of H2O2 reduction on PCMN electrode.
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Electrocatalysis
is
a
kind
of
reaction
happening
on
a
solid(electrode)-liquid(electrolyte) interface. Therefore, surface area (SA) is a key factor which can influence the catalytic property of an electrode. In view of this, we calculated the SA of CMN and PCMN electrodes respectively. Fig. 8a shows the CVs of CMN electrode within a narrow range of potential from -0.1 to 0 V (in which only
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nonfaradaic process happened) in 3 mol L-1 NaOH with different scan rates. The relationship between current densities at -0.05 V and scan rates of CMN electrode was
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shown in Fig. 8b to obtain the double layer capacitance (Cd). According to the
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following Eq. 1 and 2 (where SA represents for the surface area; Cd represents for the
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double layer capacitance; j represents for the current density and v represents for the
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scan rate; C* represents for the capacitance of the unite true specific surface area of
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the electrode used to be 60 μF cm-2[34-36]), the SA value of CMN electrode was estimated to be 116.17 cm2 which is much larger than some reported values[36, 37]. In
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addition, according to the previous reported method to calculate the surface area of a composite of metal oxide and noble metal [34], the SA of PCMN electrode can be calculated by two steps. Firstly, we can obtain the SA of the CMN electrode through the “capacitance” method and then the SA of the deposited Pd particles is determined by measuring the charge Q(C) from the Pd surface oxide reduction peak based on Eq. 3. Fig. 8c shows the CV curve of PCMN electrode in 3 mol L-1 NaOH within a potential of -1.2 to 0.6 V. In this range, there is an oxidation peak at around of -0.8 V which is related to the surface hydrogen desorption[38]. Besides, there is a pair of broad redox peaks appearing at around 0.3 to 0.5 V. The formation of this pair of
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PCMN electrode is a sum of the SA of CMN electrode (116.17 cm2 calculated by the
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“capacitance” method) and Pd particles (63.68 cm2). As a result, the SA of the PCMN
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electrode (1cm×1cm) is 179.85 cm2. Of course, it is worth noting that, this value is
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just an estimate value. Except for the error caused by the calculation methods, the
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effect of Pd particles deposition on the SA of CMN electrode was also not taken into
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account. In the following part, the current densities used are normalized by the calculated SA value of PCMN electrode.
(1)
SA Cd / C *
(2)
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dj Cd dv SAPd
Q 424
(3)
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Figure 8. CVs of CMN electrode in narrow potential range of -0.1 ~0 V in 3 mol L-1 NaOH at different scan rates (a); plots of averaged current density at -0.05 V against
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scan rates of CMN (b); CV curve of PCMN electrode in 3 mol L-1 NaOH (potential
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range: -1.2~0.6 V; scan rate: 20 mV s-1) (c); local enlargement of the reduction peak marked by red rectangle in Fig. 8c (d) For a chemical reaction, the concentration of reactant and product exhibit great influence on the electrode performance. Therefore, investigating the influence of H2O2 and NaOH concentration is of great significance to obtain a best electrode performance. Fig. 9 were the CVs and CAs of PCMN electrode in solutions of 0.7 mol L-1 H2O2 and x mol L-1 NaOH (x=1, 2, 3, 4), x mol L-1 H2O2 and 3 mol L-1 NaOH (x=0.4, 0.5, 0.6, 0.7, 0.8). As seen from Fig. 9a and b, there is a change rule that the current densities at all potentials increase first and then decrease as the NaOH concentration increases. According to the CAs (Fig. 9b), the current densities at -0.4 V
Journal Pre-proof are 0.98, 1.18, 1.95 and 1.05 mA cm-2 respectively under the conditions of 1, 2, 3 and 4 mol L-1 NaOH. It can be concluded that when the NaOH is 3 mol L-1, the current density reaches its maximum. This phenomenon may be a result of the combined action of electrolyte conductivity, viscosity and ion concentration. NaOH as a strong electrolyte, its concentration greatly affects the conductivity of electrolyte. In a certain
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range, the conductivity of NaOH solution increase with increasing NaOH concentration[44-46]. On the other hand, increase of NaOH concentration also leads
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to a increase of solution viscosity which may decrease the availability of ions
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reaching the electrode surface. Besides, when the NaOH concentration is much higher,
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more OH- ions will compete with HO2- ions to adsorb on the electrode surface.
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Relatively, the active electrode surfaces are covered by more OH- ions compared with
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in the situation of low NaOH concentration[47]. In this work, when the NaOH concentration is lower than 3 mol L-1, the effect of solution conductivity increase may
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be greater than that of viscosity increase and OH- adsorption. Thus, the current density increased with the NaOH concentration increasing. When NaOH concentration is higher than 3 mol L-1, the effect of competitive adsorption of OHions may be greater than the effect of electrolyte conductivity and viscosity, so the current density decreases. In addition, as products, the increase of OH- ions will inhibit the H2O2 reduction. As the same as NaOH, with the H2O2 concentration changing from 0.4 to 0.8 mol L-1(shown in Fig. 9c and d), reduction current density on PCMN electrode also shows a trend of increase before decrease. At -0.4 V (Fig. 9d), the reduction current densities of H2O2 are 0.97, 1.18, 1.45, 1.95 and 1.60 mA
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solutions of lower H2O2 concentrations due to the instability of H2O2 in alkaline solution. And the self-decomposition reaction of H2O2 produces more O2 gas bubbles
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that attach on the surface of the electrode and then decrease the effective surface
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area[44, 48, 49]. When the effect of self-decomposition is greater than that of the
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increase of H2O2, the current density reduced and fuel waste was caused. From what
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was discussed above, appropriate concentration of NaOH and H2O2 should be chosen
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in practice to obtain the best electrode performance. In this work, the PCMN electrode
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shows the best catalytic activity in 3 mol L-1 NaOH and 0.7 mol L-1 H2O2.
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Figure 9. CVs (scan rate: 20 mV s-1) (a) and chronoamperometry curves (CAs) (potential: -0.4 V）(b) of PCMN electrode in 0.7 mol L-1 H2O2 and x mol L-1 NaOH
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(x=1, 2, 3, 4); CVs (c) and CAs (d) of PCMN electrode in x mol L-1 H2O2 and 3 mol
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L-1 NaOH (x=0.4, 0.5, 0.6, 0.7, 0.8)
Chronoamperometry (which can simulate the working conditions of fuel cell) was still employed to study the stability of PCMN electrode in 3 mol L-1 NaOH and 0.7 mol L-1 H2O2 at -0.2, -0.4, -0.6 and -0.8 V, respectively (Fig. 10). As seen, the current density of H2O2 reduction on PCMN electrode at each potential reached a stable value in several seconds after the test started and increased significantly when the potential goes negatively. Current densities at -0.2, -0.4, -0.6 and -0.8 V are basically stable at 0.92, 1.82, 2.53 and 3.02 mA cm-2 respectively. When the potential shifted to -0.6 and -0.8 V, the CA curves exhibit a slight fluctuation which can be attributed to the disturbance of O2 bubbbles produced by H2O2 decomposition.
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Whereas the current density did not show any attenuation during the test. CAs results at different potentials reveal that PCMN electrode possesses favourable stability
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toward H2O2 reduction.
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Figure 10. CAs of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH at different potentials (-0.2, -0.4, -0.6 and -0.8 V)
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Temperature is another factor which has significant impact on the electrode performance owing to its obvious effects on the diffusion rate of ions in the solution. Fig. 11 shows the polarization curves of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH under conditions of different temperatures, from which we can obtain the exchange current density j0 of H2O2 reduction through the extension cord of tafel area and the horizontal line where overpotential is always 0. In Table. 1, the values of j0 at different temperatures were shown to be 0.79, 0.87, 1.01, 1.07 and 1.16 mA cm-2. The increasing trend of j0 with temperature rising is a consequence of the slight raise of diffusion coefficient[50, 51]. According to the values of j0 at different temperatures
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and Arrhenius formula (Eq. 4) (in which j is the current density; T is the thermodynamic temperature; R is the molar gas constant; Ea is the activation energy), we can obtain the value of activation energy of H2O2 electroreduction on the PMCN electrode, which is 7.57 kJ mol-1. This is a much smaller value than those in reported literature[53-55] and indicates a lower barrier of H2O2 reduction and excellent
ln j Ea T RT 2
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(4)
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catalytic activity of PCMN electrode.
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Table 1. Values of j0 (exchange current density) (mA cm-2) of H2O2 electroreduction
j0(mA cm-2)
0.79
303
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293
0.87
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T (K)
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under conditions of different temperatures in 3 mol L-1 NaOH and 0.7 mol L-1 H2O2
313
323
333
1.01
1.07
1.16
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Figure 11. Polarization curves of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH at different temperatures (293.15, 303.15, 313.15, 323.15 and 333.15 K) (a~e) and Arrhenius plots (f) of exchange current density j0 of H2O2 reduction.
4. Conclusions Using facile hydrothermal process and electrodeposition, a peony-like structured CoMn2O4 was in-situ formed on the conductive framework Ni foam and an original electrode of PCMN was synthesized for H2O2 electroreduction. The unique peony
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structure provides large surface area and plenty of active sites for H2O2 reduction. Besides, it can also effectively disperse Pd nanoparticles and avoid agglomeration, which has a negative effect on the excellent performance of noble metals. The as-prepared PCMN electrode exhibits excellent catalytic activity and stability toward H2O2 reduction because of the addition of Pd and the synergy effect of Pd, Co and Mn. In 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH, the current density reached almost 600 mA
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cm-2 at -0.8 V and the activation energy is only 7.57 kJ mol-1 which demonstrate a
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large application possibility of PCMN electrode in hydrogen peroxide based fuel
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cells.
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Acknowledgements
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We gratefully acknowledge the financial support of this research by the PhD
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Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities (GK6530260034), the Ph.D. Scientific Research Initiation Fund
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Project of Heilongjiang Institute of Technology (2017BJ20), the National Natural Science Foundation of China (51572052).
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Figure Captions Figure 1. Schematic diagram of the preparation process Figure 2. XRD pattern of PCMN electrode Figure 3. SEM images of CMN(a, c, e) and PCMN(b, d, f) electrodes Figure 4. TEM (a and b) and HRTEM (c and d) images of PCMN electrode Figure 5. XPS spectra of (a) Mn 2p (b) Co 2p (c) O 1s (d) Pd 3d in PCMN electrode
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Figure 6. CVs of Ni foam, CMN and PCMN electrode in 0.7 mol L-1 H2O2 and 3 mol
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L-1 NaOH (scan rate: 20 mV s-1)
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Figure 7. Schematic diagram of reaction mechanism of H2O2 reduction on PCMN
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electrode
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Figure 8. CVs of CMN electrode in narrow potential range of -0.1 ~0 V in 3 mol L-1
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NaOH at different scan rates (a); plots of averaged current density at -0.05 V against scan rates of CMN (b); CV curve of PCMN electrode in 3 mol L-1 NaOH (potential
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range: -1.2~0.6 V; scan rate: 20 mV s-1) (c); local enlargement of the reduction peak marked by red rectangle in Fig. 8c (d) Figure 9. CVs (scan rate: 20 mV s-1) (a) and chronoamperometry curves (CAs) (potential: -0.4 V）(b) of PCMN electrode in 0.7 mol L-1 H2O2 and x mol L-1 NaOH (x=1, 2, 3, 4); CVs (c) and CAs (d) of PCMN electrode in x mol L-1 H2O2 and 3 mol L-1 NaOH (x=0.4, 0.5, 0.6, 0.7, 0.8) Figure 10. CAs of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH at different potentials (-0.2, -0.4, -0.6 and -0.8 V) Figure 11. Polarization curves of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1
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NaOH at different temperatures (293.15, 303.15, 313.15, 323.15 and 333.15 K) (a~e)
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and Arrhenius plots (f) of exchange current density j0 of H2O2 reduction
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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A binder-free electrode of Pd nanoparticles anchored to nano-peony CoMn2O4 was prepared and the synergy effect of Co, Mn and Pd highly improved the catalytic
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activity toward H2O2 reduction.
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Highlights ► Pd modified CoMn2O4 electrode with special nano-peony structure was synthesized ► The special nano-peony structure provides more active sites for H2O2 reduction
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►The synergy effect of Co, Mn and Pd highly improved the catalytic activity
Congying Song, Xinhang Li, Lin Zhang, Peng Yan, Chenlin Xu, Kai Zhu, Kui Cheng, Ke Ye, Jun Yan, Dianxue Cao, Guiling Wang PII:
S1572-6657(19)30979-8
DOI:
https://doi.org/10.1016/j.jelechem.2019.113711
Reference:
JEAC 113711
To appear in:
Journal of Electroanalytical Chemistry
Received date:
27 July 2019
Revised date:
5 November 2019
Accepted date:
1 December 2019
Please cite this article as: C. Song, X. Li, L. Zhang, et al., Pd nanoparticles anchored to nano-peony CoMn2O4 as an efficient catalyst for H2O2 electroreduction, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/j.jelechem.2019.113711
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© 2019 Published by Elsevier.
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Pd nanoparticles anchored to nano-peony CoMn2O4 as an efficient catalyst for H2O2 electroreduction Congying Songa, Xinhang Lia, Lin Zhanga, Peng Yan*, a, b, Chenlin Xua, Kai Zhua, Kui Chenga, Ke Yea, Jun Yana, Dianxue Caoa, Guiling Wang*, a a Key Laboratory of Superlight Materials and Surface Technology of Ministry of
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Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001, P.R. China
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Email: [email protected]
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b College of Materials and Chemical Engineering，Heilongjiang Institute of
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Technology，Harbin 150050，China
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Abstract
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Email: [email protected] (Peng Yan)
Bimetallic oxide CoMn2O4 with nano-peony structure is prepared on Ni foam by
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a traditional hydrothermal process. Then constant-potential electrodeposition is applied to anchor Pd nanoparticles on CoMn2O4 to form an electrode of Pd nanoparticles modified CoMn2O4 (PCMN electrode) for H2O2 electroreduction. The combination of Pd and CoMn2O4 effectively reduces the dosage of noble metal. Besides, no binder is involved in the preparation which cuts the electrode cost and avoids the poor stability of traditional electrodes produced with binders. Scanning electron microscopy, X-ray diffraction and transmission electron microscopy are operated to investigate the structure and composition of the electrode. And the electrochemical behavior of the electrode is characterized by cyclic voltammetry and
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1. Introduction Nowadays, fossil fuel shortage and environmental pollution are two severe
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problems human beings are facing. Electric energy is an economical, practical and
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clean energy compared to fossil fuels. As a type of energy conversion device, fuel
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cells are able to convert chemical energy stored in fuels directly into electric energy.
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Due to the advantages of environmental friendly design and higher energy conversion
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efficiency[1, 2], fuel cells including hydrogen-oxygen fuel cell[3, 4], hydrogen-air fuel cell[5, 6], direct borohydride-hydrogen peroxide fuel cell[7, 8], direct methanol
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fuel cell[9, 10] and metal-air battery[11, 12] have obtained a good deal attention from researchers around the world. Among these fuel cells, direct borohydride-hydrogen peroxide fuel cell is a new type of liquid fuel cell which solves the problems of difficult storage and transportation of oxygen and hydrogen in traditional fuel cell. Therefore, direct borohydride-hydrogen peroxide shows great application prospects in oxygen-free environment like space and underwater. As oxidant, the reaction of H2O2 reduction does not involve any intermediate or by-product which will poison the catalysts. Besides, the two-electron reaction of H2O2 reduction possesses faster kinetics than the four-electron process of O2
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reduction[13-16]. Moreover, compared to O2, liquid H2O2 is capable to infiltrate the electrode surface better and the reduction of H2O2 happens at the solid-liquid interface. As for the O2 reduction, it requires a three-phase interface of solid-liquid-gas which may not be conducive to increase the reaction rate and volume power density[17-18]. At present, electrode materials applied to H2O2 reduction mainly includes three
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types: (1) noble metals like Au, Pt and Pd[19-21]; (2) transition metals and their oxides like Fe, Co, CuO, MnO2 and Co3O4[22-24]; (3) compounds of metals and
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organics[25-27]. Traditionally, catalyst powders were usually mixed with conductive
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carbon materials to obtain a slurry and then coated onto current collectors using some
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polymer binders. Santos’s group has used Pt powders supported on Vulcan XC-72
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carbon black to catalyze the reduction of H2O2[20]. And Wang’s group prepared a
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carbon-supported Au solid nanoparticles cathode for direct borohydride-hydrogen peroxide fuel cell[21]. Though noble metals possess excellent catalytic activity
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toward H2O2 reduction, the high price limits their large-scale application. Reducing the dosage of noble metals means reducing the cost of electrodes. Therefore, it is an effective method to reduce the dosage of noble metals to combine noble metals with other low-cost materials. Transition metal oxides with low cost have been studied extensively for H2O2 reduction. Like Co3O4, it can be prepared with unique three-dimensional
structure
morphology-controlled
Co3O4
and
our
group[28]
nanowires
with
has
previously
considerable
prepared
electrochemical
performance. However, most recent researches focus on investigating the catalytic activity of transition metal oxides with single metal element. The catalytic activity of
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bimetallic transition metal oxides toward H2O2 reduction is relatively little studied. As we all know, the essence of chemical reaction is the breakage and formation of chemical bonds. The difference of electronegativity between different metal elements in bimetallic transition metal oxides will be good for breaking the bonds. Therefore, in this work, Mn element was introduced to synthesize a bimetallic
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transition metal oxide CoMn2O4 with a unique peony-like structure. And then a composite of Pd nanoparticles modified CoMn2O4 was obtained by a simple
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electrodeposition process. Pd is a kind of noble metal with wider source and lower
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price compared to Pt and also possesses good catalytic activity, selectivity and
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durability for H2O2 electroreduction[27, 29]. In this design, because of the
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combination of Pd and CoMn2O4, the dosage of noble metal could be reduced. In
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addition, no polymer binder was involved in the preparation which also cut the cost. The three-dimensional structure of CoMn2O4 efficaciously dispersed the distribution
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of Pd nanoparticles and then avoided the decline of noble metal utilization. Furthermore, the difference of electronegativity among Co, Mn and Pd could also improve the performance of the as-prepared electrode. The following parts introduced the detailed characterization and catalytic behavior of the PCMN electrode. 2. Material and methods The PCMN electrode was prepared through a hydrothermal method followed by a treatment of electrodeposition and Fig. 1 shows the whole experimental process. 2.1 Preparation of CoMn2O4 supported on Ni foam (CMN) electrode The CMN electrode was synthesized using a traditional hydrothermal process.
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Firstly, a piece of Ni foam (size: 2 cm×4 cm×1.1 mm) was immersed into acetone, hydrochloric acid (6 mol L-1) and deionized water respectively with ultrasonic treatment aiming to dissolve the oxide layer and oil on Ni foam. Secondly, 0.4657 g Co(NO3)2, 0.1776 g NH4F, 0.576 g CO(NH2)2 and 0.1259 mL Mn(NO3)2 were added into 40 mL deionized water to get a transparent solution. Then, the treated Ni foam
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and solution were put into a autoclave (50 mL) and maintained at 100℃ for 5 h. After the autoclave cooled down to ambient temperature, the precursor was taken out from
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the autoclave and washed with deionized water for several times. At last, the Ni foam
was obtained.
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2.2 Preparation of PCMN electrode
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with precursor was calcined in a 300℃ muffle furnace for 4 h and the CMN electrode
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As depicted in Fig. 1, PCMN electrode was acquired through an electrodeposition operation. In a three-electrode system, the CMN electrode served as
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the working electrode, a carbon rod served as the counter electrode and a Ag/AgCl electrode served as the reference electrode. The electrodeposition process was conducted in a solution of 50 mL 1 mmol L-1 PdCl2 with an imposed potential of -1.2 V for 30 min. The final PCMN electrode was also washed by deionized water and dried in a vacuum oven.
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Figure 1. Schematic diagram of the preparation process
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2.3 Physical characterization and electrochemical tests of PCMN electrode
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Composition and structure are two key factors which can influence the catalytic
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activity of an electrode. Herein, we employed X-ray diffraction (XRD, Rigaku TTR III), scanning electron microscopy (SEM, JEOL JSM-6480) as well as transmission
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electron microscopy (TEM, FEI Teccai G2 S-Twin, Philips) to study the composition
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and structure of the PCMN electrode. Besides, X-ray photoelectron spectroscope with Al Kα radiation (XPS, Thermo ESCALAB 250) was used to explore the valence states of Co and Mn in CoMn2O4. And the Pd loading was measured by an inductive coupled plasma emission spectrometer (ICP, Xseries II, Thermo Scientific). At the same time, electrochemical tests including cyclic voltammetry (CV) and chronoamperometry (CA) were used to study the electrochemical behavior of the PCMN electrode commanded by an electrochemical workstation (EC-lab VMP3/Z). All the tests were done in a three-electrode system in which the PCMN electrode was regarded as the working electrode. A Ag/AgCl electrode and a carbon electrode were regarded as the counter electrode and reference electrode, respectively.
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3. Results and discussion 3.1 Physical characterization of PCMN electrode In order to confirm the composition of the as-prepared electrode, XRD pattern of PCMN electrode was measured and shown in Fig. 2. On the pattern of PCMN electrode, there are two groups of diffraction peaks. Thereinto, one group of peaks
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appeared at around 31.2°, 32.8°, 36.4°, 38.9°, 44.8°, 52.8°, 54.4°, 56.6°, 59.0°, 60.7° and 64.9°, 70.7°, 75.0°, 77.3° and 78.5° correspond to the (220), (113), (311), (004),
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(400), (332), (205), (333), (511), (404), (440), (610), (533), (622) and (444) lattice
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planes of CoMn2O4 (JCPDS card No. 18-0408). And another group of peaks at 40.1°,
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46.7° and 68.2° can be indexed to the (111), (200) and (220) lattice planes of Pd
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(JCPDS card No. 65-6174) which demonstrates that Pd was formed on CoMn2O4
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surface successfully through electrodeposition.
Figure 2. XRD pattern of PCMN electrode
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Fig. 3 shows the SEM images of CMN(Fig. 3a, c and e) and PCMN(Fig. 3b, d and f) electrodes, respectively. As seen from Fig. 3a, CoMn2O4 was formed on skeletons of Ni foam evenly and it is obvious to see from Fig. 3c and 3e that the prepared CoMn2O4 presents a peony-like structure like what was shown in the insert of Fig. 3e. Meanwhile, each CoMn2O4 peony was made up of many nanosheets. Many
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gaps existed among the peonies and the nanosheets, which provide more sites for the following deposition of Pd and larger contact surface area between electrolyte and
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electrode. After Pd was electrodeposited on the CMN electrode(Fig. 3b), it is obvious
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that the formed Pd failed to form a whole film covering the CoMn2O4 peony but
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anchored on the nanosheets dispersedly in a form of particle without distinct
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agglomeration. And from the SEM images with bigger magnification (Fig. 3d and 3f),
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the size of Pd particles is closed to 150 nm. The deposition of Pd did not damage the original structure of the CMN electrode and CoMn2O4 still maintains the peony-like
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structure well. The detailed morphology of the PCMN electrode and Pd particles was further shown in the following TEM characterization.
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Figure 3. SEM images of CMN(a, c, e) and PCMN(b, d, f) electrodes To further study the morphology of PCMN electrode, TEM and HRTEM images were shown in Fig. 4. From Fig. 4a and b, it is not hard to see that the as-prepared CoMn2O4 shows a sheet-like structure and Pd was formed on the surface of the sheets with a particle-like morphology dispersedly. The size distribution of Pd particles was shown in Fig. S1. Furthermore, we can confirm the composition of PCMN electrode on basis of the HRTEM results displayed in Fig. 4c and d. It can be roughly distinguished that there are four kinds of lattice fringes. To identify the lattice spacing more clearly, three regions marked by pink rectangle were enlarged and shown in the insert of Fig. 4c and d. The values of d-spacing of 0.202, 0.231 and 0.247 nm are
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index to the (400), (004) and (311) lattice planes of CoMn2O4 (JCPDS card No. 18-0408). Meanwhile, the d-spacing of 0.224 nm in Fig. 4d correspond to the (111) lattice plane of Pd (JCPDS card No. 65-6174). These results are well matched with
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the XRD results which demonstrate the successful synthesis of PCMN electrode.
Figure 4. TEM (a and b) and HRTEM (c and d) images of PCMN electrode X-ray photoelectron spectroscopy (XPS) was also operated to study the valence states of Mn, Co, O, Pd in the PCMN electrode. Results show that the atomic Co/Mn ratio is 1:1.7 which is closed to the empirical formula 1:2. Fig. 5 shows the XPS spectra of Mn 2p, Co 2p, O 1s and Pd 3d. From Fig. 5a, it can be seen that two main
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peaks at binding energies of 642.0 and 653.7 eV conform to Mn 2p3/2 and Mn 2p1/2, respectively. According to the Lorentzian-Gaussian curve fitting method, the fitted peak energies of 641.5 eV (2p3/2) and 653.5 eV (2p1/2) were indexed to the binding energy of Mn2+. Meanwhile the two peaks at 642.9 eV (2p3/2) and 654.5 eV (2p1/2) were correspond to Mn3+[30]. Besides, in Co 2p spectrum, the peaks at 782.7 eV (2p3/2), 796.8 eV (2p1/2) and 780.4 eV (2p3/2), 795.5 eV (2p1/2) were correspond to
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Co2+ and Co3+. Two satellites at 786.3 eV and 803.3 eV also appeared in the spectrum
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of Co 2p revealing the coexistence of Co2+ and Co3+[31]. As for the spectrum of O 1s,
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there are three fitted peaks located at 529.8, 531.0 and 532.4 eV designated to
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oxygen-metal bonds, oxygen in hydroxyl and oxygen in water molecules adsorbed on
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surface.[32] Meanwhile, the peaks on Pd 3d spectrum at binding energies of 340.38
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and 335.20 eV were assigned to Pd 3d3/2 and Pd 3d5/2 which reveals that Pd exists in
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the electrode in metallic Pd form[33].
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Figure 5. XPS spectra of (a) Mn 2p (b) Co 2p (c) O 1s (d) Pd 3d in PCMN electrode
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3.2 Electrochemical tests of PCMN electrode
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In order to compare the catalytic property of different electrodes and study the influence of CoMn2O4 and Pd nanoparticles on the electrochemical performance, cyclic voltammograms (CVs) of the current collector(Ni foam), CMN and PCMN electrodes in 0.7 mol L-1 H2O2 and 3 mol L-1 NaOH at a scan rate of 20 mV s-1 were tested in Fig. 6. It can be seen that on the CV curve represents for Ni foam, the current density in the whole potential range is around 0 showing that the current collector has little catalytic performance for the electroreduction of H2O2. When CoMn2O4 was prepared on the current collector(Ni foam), CMN electrode exhibits a distinctly better catalytic activity than Ni foam. At a potential of -0.8 V, a reduction current density of 200 mA cm-2 (normalized by geometric area) on the CMN electrode was achieved.
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Compared to CMN electrode, the reduction current density of H2O2 on PCMN electrode was 580 mA cm-2 (normalized by geometric area) at a potential of -0.8 V that is 2.9 times as that on CMN electrode revealing a further improvement of catalytic activity. The remarkable catalytic activity of PCMN and CMN electrode can be explained by Fig. 7a and b. As shown in Fig. 7a, the peony-like structure of
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CoMn2O4 can effectively disperse Pd particles and the gaps among CoMn2O4 highly increased the contact area between H2O2 and the catalysts. According to the previous
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literature[24], the conversion of Mn(II)/Mn(III) and Co(II)/Co(III) are believed to be
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responsible for the reduction of H2O2. When H2O2 transferred to the surface of
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CoMn2O4, it is likely to form three types of transition states including
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Co(II)-O-O-Co(II), Mn(II)-O-O-Mn(II) and Co(II)-O-O-Mn(II). After that, the O-O
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bond was broken by obtaining electrons from Co(II) and Mn(II) with a result of the formation of Co(III) and Mn(III). Among the situations, the O-O bond in the
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transition state of Co(II)-O-O-Mn(II) is easier to be broken because of the differences of electronegativity between Co(II) and Mn(II). In the same way, when H2O2 moles were adsorbed on the surface of PCMN electrode, besides Co(II)-O-O-Co(II), Mn(II)-O-O-Mn(II) and Co(II)-O-O-Mn(II), another three transition states including Co(II)-O-O-Pd, Mn(II)-O-O-Pd and Pd-O-O-Pd(shown in Fig. 7b) can also be formed. As we all know, precious metal Pd possesses good catalytic activity toward H2O2 reduction. In addition, the more significant differences of electronegativity between Pd and Co(II) or Pd and Mn(II) will further improve the catalytic activity of PCMN electrode toward H2O2 electroreduction. Therefore, after the electrodeposition of Pd,
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the current density of H2O2 reduction increased drastically. Moreover, ICP measurement was conducted to investigate the Pd loading which is 0.0783 mg in a piece of PCMN electrode (1cm×1cm). It can be concluded that the addition of small amount Pd highly promoted the electrochemical performance of PCMN electrode due
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Figure 6. CVs of Ni foam, CMN and PCMN electrode in 0.7 mol L-1 H2O2 and 3 mol L-1 NaOH (scan rate: 20 mV s-1).
Figure 7. Schematic diagram of reaction mechanism of H2O2 reduction on PCMN electrode.
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Electrocatalysis
is
a
kind
of
reaction
happening
on
a
solid(electrode)-liquid(electrolyte) interface. Therefore, surface area (SA) is a key factor which can influence the catalytic property of an electrode. In view of this, we calculated the SA of CMN and PCMN electrodes respectively. Fig. 8a shows the CVs of CMN electrode within a narrow range of potential from -0.1 to 0 V (in which only
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nonfaradaic process happened) in 3 mol L-1 NaOH with different scan rates. The relationship between current densities at -0.05 V and scan rates of CMN electrode was
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shown in Fig. 8b to obtain the double layer capacitance (Cd). According to the
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following Eq. 1 and 2 (where SA represents for the surface area; Cd represents for the
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double layer capacitance; j represents for the current density and v represents for the
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scan rate; C* represents for the capacitance of the unite true specific surface area of
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the electrode used to be 60 μF cm-2[34-36]), the SA value of CMN electrode was estimated to be 116.17 cm2 which is much larger than some reported values[36, 37]. In
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addition, according to the previous reported method to calculate the surface area of a composite of metal oxide and noble metal [34], the SA of PCMN electrode can be calculated by two steps. Firstly, we can obtain the SA of the CMN electrode through the “capacitance” method and then the SA of the deposited Pd particles is determined by measuring the charge Q(C) from the Pd surface oxide reduction peak based on Eq. 3. Fig. 8c shows the CV curve of PCMN electrode in 3 mol L-1 NaOH within a potential of -1.2 to 0.6 V. In this range, there is an oxidation peak at around of -0.8 V which is related to the surface hydrogen desorption[38]. Besides, there is a pair of broad redox peaks appearing at around 0.3 to 0.5 V. The formation of this pair of
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PCMN electrode is a sum of the SA of CMN electrode (116.17 cm2 calculated by the
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“capacitance” method) and Pd particles (63.68 cm2). As a result, the SA of the PCMN
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electrode (1cm×1cm) is 179.85 cm2. Of course, it is worth noting that, this value is
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just an estimate value. Except for the error caused by the calculation methods, the
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effect of Pd particles deposition on the SA of CMN electrode was also not taken into
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account. In the following part, the current densities used are normalized by the calculated SA value of PCMN electrode.
(1)
SA Cd / C *
(2)
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dj Cd dv SAPd
Q 424
(3)
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Figure 8. CVs of CMN electrode in narrow potential range of -0.1 ~0 V in 3 mol L-1 NaOH at different scan rates (a); plots of averaged current density at -0.05 V against
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scan rates of CMN (b); CV curve of PCMN electrode in 3 mol L-1 NaOH (potential
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range: -1.2~0.6 V; scan rate: 20 mV s-1) (c); local enlargement of the reduction peak marked by red rectangle in Fig. 8c (d) For a chemical reaction, the concentration of reactant and product exhibit great influence on the electrode performance. Therefore, investigating the influence of H2O2 and NaOH concentration is of great significance to obtain a best electrode performance. Fig. 9 were the CVs and CAs of PCMN electrode in solutions of 0.7 mol L-1 H2O2 and x mol L-1 NaOH (x=1, 2, 3, 4), x mol L-1 H2O2 and 3 mol L-1 NaOH (x=0.4, 0.5, 0.6, 0.7, 0.8). As seen from Fig. 9a and b, there is a change rule that the current densities at all potentials increase first and then decrease as the NaOH concentration increases. According to the CAs (Fig. 9b), the current densities at -0.4 V
Journal Pre-proof are 0.98, 1.18, 1.95 and 1.05 mA cm-2 respectively under the conditions of 1, 2, 3 and 4 mol L-1 NaOH. It can be concluded that when the NaOH is 3 mol L-1, the current density reaches its maximum. This phenomenon may be a result of the combined action of electrolyte conductivity, viscosity and ion concentration. NaOH as a strong electrolyte, its concentration greatly affects the conductivity of electrolyte. In a certain
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range, the conductivity of NaOH solution increase with increasing NaOH concentration[44-46]. On the other hand, increase of NaOH concentration also leads
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to a increase of solution viscosity which may decrease the availability of ions
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reaching the electrode surface. Besides, when the NaOH concentration is much higher,
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more OH- ions will compete with HO2- ions to adsorb on the electrode surface.
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Relatively, the active electrode surfaces are covered by more OH- ions compared with
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in the situation of low NaOH concentration[47]. In this work, when the NaOH concentration is lower than 3 mol L-1, the effect of solution conductivity increase may
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be greater than that of viscosity increase and OH- adsorption. Thus, the current density increased with the NaOH concentration increasing. When NaOH concentration is higher than 3 mol L-1, the effect of competitive adsorption of OHions may be greater than the effect of electrolyte conductivity and viscosity, so the current density decreases. In addition, as products, the increase of OH- ions will inhibit the H2O2 reduction. As the same as NaOH, with the H2O2 concentration changing from 0.4 to 0.8 mol L-1(shown in Fig. 9c and d), reduction current density on PCMN electrode also shows a trend of increase before decrease. At -0.4 V (Fig. 9d), the reduction current densities of H2O2 are 0.97, 1.18, 1.45, 1.95 and 1.60 mA
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solutions of lower H2O2 concentrations due to the instability of H2O2 in alkaline solution. And the self-decomposition reaction of H2O2 produces more O2 gas bubbles
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that attach on the surface of the electrode and then decrease the effective surface
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area[44, 48, 49]. When the effect of self-decomposition is greater than that of the
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increase of H2O2, the current density reduced and fuel waste was caused. From what
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was discussed above, appropriate concentration of NaOH and H2O2 should be chosen
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in practice to obtain the best electrode performance. In this work, the PCMN electrode
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shows the best catalytic activity in 3 mol L-1 NaOH and 0.7 mol L-1 H2O2.
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Figure 9. CVs (scan rate: 20 mV s-1) (a) and chronoamperometry curves (CAs) (potential: -0.4 V）(b) of PCMN electrode in 0.7 mol L-1 H2O2 and x mol L-1 NaOH
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(x=1, 2, 3, 4); CVs (c) and CAs (d) of PCMN electrode in x mol L-1 H2O2 and 3 mol
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L-1 NaOH (x=0.4, 0.5, 0.6, 0.7, 0.8)
Chronoamperometry (which can simulate the working conditions of fuel cell) was still employed to study the stability of PCMN electrode in 3 mol L-1 NaOH and 0.7 mol L-1 H2O2 at -0.2, -0.4, -0.6 and -0.8 V, respectively (Fig. 10). As seen, the current density of H2O2 reduction on PCMN electrode at each potential reached a stable value in several seconds after the test started and increased significantly when the potential goes negatively. Current densities at -0.2, -0.4, -0.6 and -0.8 V are basically stable at 0.92, 1.82, 2.53 and 3.02 mA cm-2 respectively. When the potential shifted to -0.6 and -0.8 V, the CA curves exhibit a slight fluctuation which can be attributed to the disturbance of O2 bubbbles produced by H2O2 decomposition.
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Whereas the current density did not show any attenuation during the test. CAs results at different potentials reveal that PCMN electrode possesses favourable stability
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Figure 10. CAs of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH at different potentials (-0.2, -0.4, -0.6 and -0.8 V)
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Temperature is another factor which has significant impact on the electrode performance owing to its obvious effects on the diffusion rate of ions in the solution. Fig. 11 shows the polarization curves of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH under conditions of different temperatures, from which we can obtain the exchange current density j0 of H2O2 reduction through the extension cord of tafel area and the horizontal line where overpotential is always 0. In Table. 1, the values of j0 at different temperatures were shown to be 0.79, 0.87, 1.01, 1.07 and 1.16 mA cm-2. The increasing trend of j0 with temperature rising is a consequence of the slight raise of diffusion coefficient[50, 51]. According to the values of j0 at different temperatures
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and Arrhenius formula (Eq. 4) (in which j is the current density; T is the thermodynamic temperature; R is the molar gas constant; Ea is the activation energy), we can obtain the value of activation energy of H2O2 electroreduction on the PMCN electrode, which is 7.57 kJ mol-1. This is a much smaller value than those in reported literature[53-55] and indicates a lower barrier of H2O2 reduction and excellent
ln j Ea T RT 2
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(4)
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catalytic activity of PCMN electrode.
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Table 1. Values of j0 (exchange current density) (mA cm-2) of H2O2 electroreduction
j0(mA cm-2)
0.79
303
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293
0.87
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T (K)
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under conditions of different temperatures in 3 mol L-1 NaOH and 0.7 mol L-1 H2O2
313
323
333
1.01
1.07
1.16
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Figure 11. Polarization curves of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH at different temperatures (293.15, 303.15, 313.15, 323.15 and 333.15 K) (a~e) and Arrhenius plots (f) of exchange current density j0 of H2O2 reduction.
4. Conclusions Using facile hydrothermal process and electrodeposition, a peony-like structured CoMn2O4 was in-situ formed on the conductive framework Ni foam and an original electrode of PCMN was synthesized for H2O2 electroreduction. The unique peony
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structure provides large surface area and plenty of active sites for H2O2 reduction. Besides, it can also effectively disperse Pd nanoparticles and avoid agglomeration, which has a negative effect on the excellent performance of noble metals. The as-prepared PCMN electrode exhibits excellent catalytic activity and stability toward H2O2 reduction because of the addition of Pd and the synergy effect of Pd, Co and Mn. In 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH, the current density reached almost 600 mA
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cm-2 at -0.8 V and the activation energy is only 7.57 kJ mol-1 which demonstrate a
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large application possibility of PCMN electrode in hydrogen peroxide based fuel
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cells.
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Acknowledgements
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We gratefully acknowledge the financial support of this research by the PhD
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Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities (GK6530260034), the Ph.D. Scientific Research Initiation Fund
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Project of Heilongjiang Institute of Technology (2017BJ20), the National Natural Science Foundation of China (51572052).
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Figure Captions Figure 1. Schematic diagram of the preparation process Figure 2. XRD pattern of PCMN electrode Figure 3. SEM images of CMN(a, c, e) and PCMN(b, d, f) electrodes Figure 4. TEM (a and b) and HRTEM (c and d) images of PCMN electrode Figure 5. XPS spectra of (a) Mn 2p (b) Co 2p (c) O 1s (d) Pd 3d in PCMN electrode
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Figure 6. CVs of Ni foam, CMN and PCMN electrode in 0.7 mol L-1 H2O2 and 3 mol
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L-1 NaOH (scan rate: 20 mV s-1)
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Figure 7. Schematic diagram of reaction mechanism of H2O2 reduction on PCMN
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electrode
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Figure 8. CVs of CMN electrode in narrow potential range of -0.1 ~0 V in 3 mol L-1
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NaOH at different scan rates (a); plots of averaged current density at -0.05 V against scan rates of CMN (b); CV curve of PCMN electrode in 3 mol L-1 NaOH (potential
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range: -1.2~0.6 V; scan rate: 20 mV s-1) (c); local enlargement of the reduction peak marked by red rectangle in Fig. 8c (d) Figure 9. CVs (scan rate: 20 mV s-1) (a) and chronoamperometry curves (CAs) (potential: -0.4 V）(b) of PCMN electrode in 0.7 mol L-1 H2O2 and x mol L-1 NaOH (x=1, 2, 3, 4); CVs (c) and CAs (d) of PCMN electrode in x mol L-1 H2O2 and 3 mol L-1 NaOH (x=0.4, 0.5, 0.6, 0.7, 0.8) Figure 10. CAs of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1 NaOH at different potentials (-0.2, -0.4, -0.6 and -0.8 V) Figure 11. Polarization curves of PCMN electrode in 3 mol L-1 H2O2 and 0.7 mol L-1
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NaOH at different temperatures (293.15, 303.15, 313.15, 323.15 and 333.15 K) (a~e)
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and Arrhenius plots (f) of exchange current density j0 of H2O2 reduction
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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A binder-free electrode of Pd nanoparticles anchored to nano-peony CoMn2O4 was prepared and the synergy effect of Co, Mn and Pd highly improved the catalytic
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Highlights ► Pd modified CoMn2O4 electrode with special nano-peony structure was synthesized ► The special nano-peony structure provides more active sites for H2O2 reduction
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►The synergy effect of Co, Mn and Pd highly improved the catalytic activity