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CuO nanosheets grown on cupper foil as the catalyst for H2O2 electroreduction in alkaline medium
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 6 1 1 e1 3 6 1 5
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
CuO nanosheets grown on cupper foil as the catalyst for H2O2 electroreduction in alkaline medium Yunhu Li, Dianxue Cao*, Yao Liu, Ran Liu, Fan Yang, Jinling Yin, Guiling Wang Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Nantong St #145, Harbin 150001, PR China
article info
abstract
Article history:
The growth of CuO nanosheet arrays on Cu foil was demonstrated. The morphology and
Received 23 November 2011
structure of the CuO were examined by scanning electron microscopy and X-ray diffraction
Received in revised form
spectroscopy. The catalytic performance of the obtained CuO/Cu electrode for hydrogen
5 January 2012
peroxide electroreduction in 3.0 mol dm3 KOH was evaluated by means of cyclic vol-
Accepted 10 January 2012
tammetry and chronoamperometry. The CuO/Cu electrode shows an onset potential for
Available online 22 March 2012
H2O2 electroreduction comparable to Co3O4 nanowire arrays grown on Ni foam and around 100 mV more negative than precious metal catalysts, such as Pt and Pd, demonstrating its
Keywords:
good catalytic activity for H2O2 electroreduction. The stabilized mass current density for
Cupper oxide
H2O2 electroreduction on the CuO/Cu electrode at 0.3 V reached about 57% of that on
Nanosheet arrays
Co3O4 nanowire arrays grown on nickel foam. Compared to conventional fuel cell elec-
Hydrogen peroxide reduction
trodes fabricated by mixing active materials with conducting agents and polymer binders,
Electrocatalyst
this electrode of CuO nanosheet arrays directly grown on Cu has superior mass transport property, which combining with its low-cost and facile preparation, make it a promising electrode for fuel cell using H2O2 as the oxidant. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells for use in air-free environments (space and underwater) require liquid or compressed oxygen as oxidant. The bulky tank for carrying oxygen significantly reduces the energy density and safety standard of fuel cell systems. Recently, H2O2 has been investigated as an alternative oxidant in replacing of oxygen. H2O2 is a powerful oxidizer. Its electroreduction takes place via a two-electron transfer involving the breakage of single dioxygen bonds rather than the breakage of double bonds for O2 reduction. So H2O2 electroreduction has a much lower activation barrier and thus much faster kinetics than O2 electroreduction. H2O2 is a liquid and its easy storage and handling enable it to be conveniently used
in underwater or space fuel cells. Also, the solid/liquid reaction zone at the H2O2 cathode is more realizable than the solid/liquid/gas region (gas-diffusion electrode) at the O2 cathode. For all these reasons, the use of H2O2 as a fuel cell oxidant has caught attentions recently. Several types of fuel cells using H2O2 as oxidant have been tested, including direct borohydride fuel cells [1e6], metal semi fuel cell [7e11], direct hydrazine fuel cells [12,13], direct methanol fuel cells [14], direct formic acid fuel cells [15], and direct hydrogen peroxide fuel cells [16e18]. Compared to the research on electrocatalysts for O2 reduction, studies on electrocatalysts for H2O2 reduction are relative sparse. Electrocatalysts investigated so far include metals and alloys (Pd, Au, Ag, Cu, PdeIr, PdePt, PdeRu, PgeAg)
* Corresponding author. Tel./fax: þ86 451 82589036. E-mail address: [email protected] (D. Cao). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.038
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 6 1 1 e1 3 6 1 5
[3,4,6e8,11,19e25], macrocycle complexes of transition metals (Fe- and Co-porphyrin, Cu-triazine) [26e28] and metal oxides (Co3O4, NiCo2O4, La1xSrxMnO3) [29e32]. Noble metal catalysts usually show high activity for H2O2 electroreduction, but they significantly catalyze the chemical decomposition of H2O2 to oxygen, leading to the reduction of H2O2 utilization. Besides, they are expensive. Recent studies demonstrated that cheap transition metal oxides (e.g., Co3O4) exhibit superior activity and stability for H2O2 electroreduction in alkaline medium and the chemical decomposition of H2O2 on these catalysts can be minimized at low concentration of H2O2 [29e32]. As part of our ongoing research of low-cost electrocatalysts for H2O2 based fuel cell, we report, in this study, the preparation and catalytic performance of CuO nanosheets freely standing on Cu foil for H2O2 electroreduction. The electrochemically active CuO nanosheets were grown from the Cu foil substrate in a single step, and the Cu foil served as both the substrate of CuO nanosheets and the current collector of the electrode. So the obtained electrode can be directly used without additional processing.
2.
Experimental
2.1. Preparation and characterization of CuO nanosheets on Cu foil The preparation of CuO nanosheets on Cu foil was carried out as follows. Typically, a Cu foil (length width thickness ¼ 10 10 1 mm, 99.99% in purity) was degreased with acetone, etched with 6.0 mol dm3 HCl for 15 min and rinsed with distilled water extensively. The treated Cu foil was then immediately immersed into a solution containing 400 cm3 water, 1 cm3 ammonia (28 wt.%) and 7 cm3 NaOH solution (1 mol dm3). After reaction for 120 h at room temperature, the sample was removed from the solution, washed with distilled water thoroughly and dried in air at room temperature. The shiny Cu foil surface turned to complete black after the reaction. The obtained electrode (denoted as CuO/Cu) has a CuO loading of 2.9 mg cm2, which was determined by dissolving CuO in 6 mol dm3 HCl. The morphology of the CuO/Cu electrode was examined by a scanning electron microscope (SEM, JEOL JSM-6480), and its structure was analyzed using an X-ray diffractometer (Rigaku TTR III) with Cu Ka radiation (l ¼ 0.1514178 nm).
2.2.
electrolyte was 3.0 mol dm3 aqueous KOH solution. All solutions were made with analytical grade chemical reagents and Milli-Q water (Millipore, 18 MU cm). The reported current densities were normalized to the mass of CuO (2.9 mg in the 1 cm2 electrode).
3.
Results and discussion
3.1.
Characterization of CuO/Cu
Fig. 1 shows the XRD patterns of the CuO/Cu electrode and CuO powder scratched down from Cu foil surface. The peaks at 2q of 43.4 , 50.6 and 74.3 for the CuO/Cu electrode are due to the substrate Cu foil according to the standard crystallographic spectrum of Cu (JCPDS Card No.85-1326). The broad diffraction peaks located at 2q ¼ 35.4 and 38.6 for both CuO/ Cu and CuO powder are well indexed to (1111)e(002) and (111)e(200) planes of monoclinic CuO structure (JCPDS Card No.80-1916), respectively. So it can be concluded that pure phase monoclinic CuO was formed on Cu foil. The broad feature of CuO diffraction peaks is an indication of small crystallite size of CuO. The average crystallite size of the CuO is w10 nm estimated from the (111) to (200) peak of CuO using Scherrer equation. The chemical reactions involved in the growth of CuO on Cu foil at room temperature might be as follows [33,34]: 2þ Cu þ O2 þ H2 O þ 4NH3 / CuðNH3 Þ4 þ2OH
(1)
2þ CuðNH3 Þ4 þ2OH /CuðOHÞ2 þ 4NH3
(2)
CuðOHÞ2 þ 2OH /CuðOHÞ2 4
(3)
CuðOHÞ2 4 /CuO þ 2OH þ H2 O
(4)
The synthesis of Cu(OH)2 nanoribbon, nanoneedle and nanotube from Cu metal substrate in alkaline solution via reaction 1 and/or 2 has been confirmed [35,36]. Cu(OH)2 is a metastable phase which can easily transform into more stable CuO in NaOH solutions at room temperature [34,37]. The transformation of Cu(OH)2 into CuO in aqueous solution was believed to be a reconstructive transformation involving a dissolution reaction via the intermediate of Cu(OH)2 4
Electrochemical measurements
Cyclic voltammetric and chronoamperometric experiments were performed in a conventional three-electrode electrochemical cell using a computerized potentiostat (Autolab PGSTAT302, Eco Chemie). The CuO/Cu electrode acted as the working electrode. A glassy carbon rod behind a D-porosity glass frit was employed as the counter electrode to minimize the effect of H2O2 decomposition. Two counter electrodes were used and placed on each side of the CuO/Cu electrode to make sure the uniform current distribution. A saturated Ag/ AgCl, KCl electrode served as the reference. All potentials were referred to the reference electrode. All electrochemical measurements were performed at room temperature. The
Fig. 1 e XRD patterns of the CuO/Cu electrode and CuO powder scratched down from Cu foil.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 6 1 1 e1 3 6 1 5
followed by the precipitation of CuO. That is, two OH ions react with one Cu(OH)2 molecule to form Cu(OH)2 4 (eq. (3)), which is unstable, only exists momentarily and rapidly liberates two OH ions leading to the formation of CuO (eq. (4)). Fig. 2 shows the SEM image of the CuO/Cu electrode. As can be seen that CuO presents as nanosheets, which roughly aligned perpendicular to the copper surface. The nanosheets form clusters and cover the copper surface completely and compactly. The thickness of nanosheets is less than approximately 30 nm measured at their upper edges and the nanosheet layer on Cu foil is around a few micrometers thick. Conventional fuel cell electrodes are generally fabricated by mixing powder of active materials with conducting materials (e.g., carbon black) and polymer binders (e.g., polytetrafluoroethylene) to form pastes and then applying the pastes to current collectors. Such obtained electrodes usually suffer drawbacks of low active material utilization because some catalysts are unable to contact with current collector and/or electrolyte. The CuO/Cu electrode has higher utilization efficiency of active material because all CuO are in electrical contact (directly or indirectly through other particles) with the Cu foil current collector. Besides, the open spaces between CuO nanosheets allow for easy transportation of H2O2 reactant and electrolyte into the inner region of the electrode and thus diminish mass transport control [30,38].
3.2. Electrocatalytic performance of the CuO/Cu electrode for H2O2 reduction Fig. 3 shows the cyclic voltammograms (CVs) of the asprepared CuO/Cu electrode in 3 mol dm3 NaOH solution at a scan rate of 5 mV s1 in the potential range of 0.65 to 0.1 V, which is the range at which H2O2 electroreduction occurs. The insert is the stabilized CV of the CuO/Cu electrode measured in the potential range of 0e0.4 V. The stabilized CV in the potential window of 0e0.4 V (the insert) displayed a pair of broad anodic/cathodic peak. The cathodic peak can be attributed to the reduction of CuO to Cu2O and/or CuOH. The anodic peak can be assigned to the oxidation of Cu2O and/or CuOH to CuO and/or Cu(OH)2 [39e42]. The CVs for the freshly prepared CuO/Cu electrode in the potential range of 0.65 to 0.1 V exhibits a strong anodic peak and a very weak cathodic peak. The peak currents decreased with the increase of potential cycling number and became remarkably week after 30 cycles. The anodic peak can be associated with the formation of Cu2O from oxidation of Cu foil, which was not
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Fig. 3 e Cyclic voltammograms of the as-prepared CuO/Cu electrode in 3 mol dmL3 KOH solution at 5 mV sL1 in the potential range of L0.65 to L0.1 V. Insert is the stabilized CV of the CuO/Cu electrode measured in the potential range of 0e0.4 V.
covered by CuO nanosheets. The cathodic peak corresponds to the reduction of Cu2O surface film [39,43,44]. It is well established that Cu2O is a poorly conducting semiconductor, so the thin film of Cu2O formed Cu surface during potential cycling acts as a barrier to electron transfer and protects Cu to be further oxidized. Consequently, the anodic currents were reduced by repetitive potential cyclings [43]. Fig. 4 shows CVs of the CuO/Cu electrode in 3 mol dm3 KOH solution containing H2O2 with various concentrations. The electrode was subjected to potential cycling between 0.65 and 0.1 V for 30 times in KOH solution prior to H2O2 electroreduction measurements. The potential scans start from the open circuit potential and go negatively. As can be seen that the cathodic currents are much larger in the presence of H2O2 than that without H2O2, and the currents increase with the increase of H2O2 concentration up to 0.5 mol dm3. This demonstrated that electroreduction of H2O2 occurred on the CuO/Cu electrode. The cathodic peak current density exhibited an approximate linear relationship with the H2O2 concentration up to 0.5 mol dm3, which implies that the reduction reaction at the peak potential was controlled by H2O2 diffusion. Notably, further increase in the H2O2 concentration leads to no increase of peak current. This
Fig. 2 e SEM images of the CuO/Cu electrode.
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Fig. 4 e Cyclic voltammograms of the CuO/Cu electrode in 3 mol dmL3 KOH solution containing H2O2 with various concentrations.
unexpected behavior is likely due to the severe chemical decomposition of H2O2 because it is well known that H2O2 is unstable in strong alkaline electrolyte, it usually undergoes chemical decomposition to oxygen and the decomposition rate is faster at higher H2O2 concentration. We noticed that when H2O2 concentration is higher than 0.5 mol dm3, a significant amount of gas bubbles evolved from the electrode surface. Since chemical decomposition of H2O2 to oxygen results in the reduction of H2O2 utilization efficiency, using low concentration H2O2 is essential for keeping high H2O2 utilization efficiency. The onset potential of H2O2 electroreduction on the CuO/Cu electrode is w0.1 V, which is nearly the same as that on Co3O4 nanowire array electrode (w0.1 V) [30] and slightly more negative than that on Pt, Pd and Au electrodes (0e0.06 V) [45]. The comparable overpotential for H2O2 electroreduction on the CuO/Cu electrode with that on other electrocatalysts suggested that CuO has fairly good catalytic activity for H2O2 electroreduction.
The stability of CuO/Cu electrode for H2O2 electroreduction was investigated by chronoamperometric measurements. Fig. 5 shows chronoamperometric curves of H2O2 electroreduction on the CuO/Cu electrode. Prior to measurements, the electrode was potential cycled for 30 times between 0.65 and 0.1 V in KOH solution to transfer the exposed Cu to Cu2O (Fig. 3). The potentials were selected in the region from open circuit potential to the peak potential according to the CVs (Fig. 4). The reduction currents reached steady-state quickly after constant potentials were applied and displayed no sign of decrease within the test period, indicating that the CuO/Cu electrode has a good stability for catalyzing H2O2 electroreduction. The stabilized current density at 0.3 V is around 4.3 mA mg1 for the CuO/Cu electrode, which is about 57% of that for Co3O4 nanowire arrays grown on nickel foam (7.6 mA mg1) reported previously [30]. Even though the activity of the CuO/Cu electrode is lower than that of Co3O4 nanowire array electrode, the lowcost, abundant resource and easy preparation of CuO electrode still makes it a potential electrode for fuel cells using H2O2 as oxidants.
4.
Conclusions
CuO nanosheet arrays grown on Cu foil was investigated as the electrocatalyst for H2O2 reduction in alkaline medium. The onset potential of H2O2 electroreduction on the CuO/Cu electrode is close to that on Co3O4 nanowire arrays grown on nickel foam, and slightly more negative than that on Pt, Pd and Au electrodes, indicting that CuO has good catalytic activity for H2O2 electroreduction. The stabilized current density for H2O2 electroreduction on the CuO/Cu electrode at 0.3 V is about 57% of that on Co3O4 nanowire arrays. However, due to the low-cost, abundant resource and easy preparation of CuO/Cu electrode, it is a promising electrode for fuel cells using H2O2 as oxidants. The catalytic performance of CuO could be further improved by tuning its morphology.
Acknowledgments We gratefully acknowledge the financial support of this research by National Nature Science Foundation of China (20973048) and Ph.D. Programs Foundation of Ministry of Education of China (20102304110001).
references
Fig. 5 e Chronoamperometric curves for H2O2 electroreduction on the CuO/Cu electrode measured in 3.0 mol dmL3 KOH D 0.4 mol dmL3 H2O2.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
CuO nanosheets grown on cupper foil as the catalyst for H2O2 electroreduction in alkaline medium Yunhu Li, Dianxue Cao*, Yao Liu, Ran Liu, Fan Yang, Jinling Yin, Guiling Wang Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Nantong St #145, Harbin 150001, PR China
article info
abstract
Article history:
The growth of CuO nanosheet arrays on Cu foil was demonstrated. The morphology and
Received 23 November 2011
structure of the CuO were examined by scanning electron microscopy and X-ray diffraction
Received in revised form
spectroscopy. The catalytic performance of the obtained CuO/Cu electrode for hydrogen
5 January 2012
peroxide electroreduction in 3.0 mol dm3 KOH was evaluated by means of cyclic vol-
Accepted 10 January 2012
tammetry and chronoamperometry. The CuO/Cu electrode shows an onset potential for
Available online 22 March 2012
H2O2 electroreduction comparable to Co3O4 nanowire arrays grown on Ni foam and around 100 mV more negative than precious metal catalysts, such as Pt and Pd, demonstrating its
Keywords:
good catalytic activity for H2O2 electroreduction. The stabilized mass current density for
Cupper oxide
H2O2 electroreduction on the CuO/Cu electrode at 0.3 V reached about 57% of that on
Nanosheet arrays
Co3O4 nanowire arrays grown on nickel foam. Compared to conventional fuel cell elec-
Hydrogen peroxide reduction
trodes fabricated by mixing active materials with conducting agents and polymer binders,
Electrocatalyst
this electrode of CuO nanosheet arrays directly grown on Cu has superior mass transport property, which combining with its low-cost and facile preparation, make it a promising electrode for fuel cell using H2O2 as the oxidant. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fuel cells for use in air-free environments (space and underwater) require liquid or compressed oxygen as oxidant. The bulky tank for carrying oxygen significantly reduces the energy density and safety standard of fuel cell systems. Recently, H2O2 has been investigated as an alternative oxidant in replacing of oxygen. H2O2 is a powerful oxidizer. Its electroreduction takes place via a two-electron transfer involving the breakage of single dioxygen bonds rather than the breakage of double bonds for O2 reduction. So H2O2 electroreduction has a much lower activation barrier and thus much faster kinetics than O2 electroreduction. H2O2 is a liquid and its easy storage and handling enable it to be conveniently used
in underwater or space fuel cells. Also, the solid/liquid reaction zone at the H2O2 cathode is more realizable than the solid/liquid/gas region (gas-diffusion electrode) at the O2 cathode. For all these reasons, the use of H2O2 as a fuel cell oxidant has caught attentions recently. Several types of fuel cells using H2O2 as oxidant have been tested, including direct borohydride fuel cells [1e6], metal semi fuel cell [7e11], direct hydrazine fuel cells [12,13], direct methanol fuel cells [14], direct formic acid fuel cells [15], and direct hydrogen peroxide fuel cells [16e18]. Compared to the research on electrocatalysts for O2 reduction, studies on electrocatalysts for H2O2 reduction are relative sparse. Electrocatalysts investigated so far include metals and alloys (Pd, Au, Ag, Cu, PdeIr, PdePt, PdeRu, PgeAg)
* Corresponding author. Tel./fax: þ86 451 82589036. E-mail address: [email protected] (D. Cao). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.038
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 6 1 1 e1 3 6 1 5
[3,4,6e8,11,19e25], macrocycle complexes of transition metals (Fe- and Co-porphyrin, Cu-triazine) [26e28] and metal oxides (Co3O4, NiCo2O4, La1xSrxMnO3) [29e32]. Noble metal catalysts usually show high activity for H2O2 electroreduction, but they significantly catalyze the chemical decomposition of H2O2 to oxygen, leading to the reduction of H2O2 utilization. Besides, they are expensive. Recent studies demonstrated that cheap transition metal oxides (e.g., Co3O4) exhibit superior activity and stability for H2O2 electroreduction in alkaline medium and the chemical decomposition of H2O2 on these catalysts can be minimized at low concentration of H2O2 [29e32]. As part of our ongoing research of low-cost electrocatalysts for H2O2 based fuel cell, we report, in this study, the preparation and catalytic performance of CuO nanosheets freely standing on Cu foil for H2O2 electroreduction. The electrochemically active CuO nanosheets were grown from the Cu foil substrate in a single step, and the Cu foil served as both the substrate of CuO nanosheets and the current collector of the electrode. So the obtained electrode can be directly used without additional processing.
2.
Experimental
2.1. Preparation and characterization of CuO nanosheets on Cu foil The preparation of CuO nanosheets on Cu foil was carried out as follows. Typically, a Cu foil (length width thickness ¼ 10 10 1 mm, 99.99% in purity) was degreased with acetone, etched with 6.0 mol dm3 HCl for 15 min and rinsed with distilled water extensively. The treated Cu foil was then immediately immersed into a solution containing 400 cm3 water, 1 cm3 ammonia (28 wt.%) and 7 cm3 NaOH solution (1 mol dm3). After reaction for 120 h at room temperature, the sample was removed from the solution, washed with distilled water thoroughly and dried in air at room temperature. The shiny Cu foil surface turned to complete black after the reaction. The obtained electrode (denoted as CuO/Cu) has a CuO loading of 2.9 mg cm2, which was determined by dissolving CuO in 6 mol dm3 HCl. The morphology of the CuO/Cu electrode was examined by a scanning electron microscope (SEM, JEOL JSM-6480), and its structure was analyzed using an X-ray diffractometer (Rigaku TTR III) with Cu Ka radiation (l ¼ 0.1514178 nm).
2.2.
electrolyte was 3.0 mol dm3 aqueous KOH solution. All solutions were made with analytical grade chemical reagents and Milli-Q water (Millipore, 18 MU cm). The reported current densities were normalized to the mass of CuO (2.9 mg in the 1 cm2 electrode).
3.
Results and discussion
3.1.
Characterization of CuO/Cu
Fig. 1 shows the XRD patterns of the CuO/Cu electrode and CuO powder scratched down from Cu foil surface. The peaks at 2q of 43.4 , 50.6 and 74.3 for the CuO/Cu electrode are due to the substrate Cu foil according to the standard crystallographic spectrum of Cu (JCPDS Card No.85-1326). The broad diffraction peaks located at 2q ¼ 35.4 and 38.6 for both CuO/ Cu and CuO powder are well indexed to (1111)e(002) and (111)e(200) planes of monoclinic CuO structure (JCPDS Card No.80-1916), respectively. So it can be concluded that pure phase monoclinic CuO was formed on Cu foil. The broad feature of CuO diffraction peaks is an indication of small crystallite size of CuO. The average crystallite size of the CuO is w10 nm estimated from the (111) to (200) peak of CuO using Scherrer equation. The chemical reactions involved in the growth of CuO on Cu foil at room temperature might be as follows [33,34]: 2þ Cu þ O2 þ H2 O þ 4NH3 / CuðNH3 Þ4 þ2OH
(1)
2þ CuðNH3 Þ4 þ2OH /CuðOHÞ2 þ 4NH3
(2)
CuðOHÞ2 þ 2OH /CuðOHÞ2 4
(3)
CuðOHÞ2 4 /CuO þ 2OH þ H2 O
(4)
The synthesis of Cu(OH)2 nanoribbon, nanoneedle and nanotube from Cu metal substrate in alkaline solution via reaction 1 and/or 2 has been confirmed [35,36]. Cu(OH)2 is a metastable phase which can easily transform into more stable CuO in NaOH solutions at room temperature [34,37]. The transformation of Cu(OH)2 into CuO in aqueous solution was believed to be a reconstructive transformation involving a dissolution reaction via the intermediate of Cu(OH)2 4
Electrochemical measurements
Cyclic voltammetric and chronoamperometric experiments were performed in a conventional three-electrode electrochemical cell using a computerized potentiostat (Autolab PGSTAT302, Eco Chemie). The CuO/Cu electrode acted as the working electrode. A glassy carbon rod behind a D-porosity glass frit was employed as the counter electrode to minimize the effect of H2O2 decomposition. Two counter electrodes were used and placed on each side of the CuO/Cu electrode to make sure the uniform current distribution. A saturated Ag/ AgCl, KCl electrode served as the reference. All potentials were referred to the reference electrode. All electrochemical measurements were performed at room temperature. The
Fig. 1 e XRD patterns of the CuO/Cu electrode and CuO powder scratched down from Cu foil.
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followed by the precipitation of CuO. That is, two OH ions react with one Cu(OH)2 molecule to form Cu(OH)2 4 (eq. (3)), which is unstable, only exists momentarily and rapidly liberates two OH ions leading to the formation of CuO (eq. (4)). Fig. 2 shows the SEM image of the CuO/Cu electrode. As can be seen that CuO presents as nanosheets, which roughly aligned perpendicular to the copper surface. The nanosheets form clusters and cover the copper surface completely and compactly. The thickness of nanosheets is less than approximately 30 nm measured at their upper edges and the nanosheet layer on Cu foil is around a few micrometers thick. Conventional fuel cell electrodes are generally fabricated by mixing powder of active materials with conducting materials (e.g., carbon black) and polymer binders (e.g., polytetrafluoroethylene) to form pastes and then applying the pastes to current collectors. Such obtained electrodes usually suffer drawbacks of low active material utilization because some catalysts are unable to contact with current collector and/or electrolyte. The CuO/Cu electrode has higher utilization efficiency of active material because all CuO are in electrical contact (directly or indirectly through other particles) with the Cu foil current collector. Besides, the open spaces between CuO nanosheets allow for easy transportation of H2O2 reactant and electrolyte into the inner region of the electrode and thus diminish mass transport control [30,38].
3.2. Electrocatalytic performance of the CuO/Cu electrode for H2O2 reduction Fig. 3 shows the cyclic voltammograms (CVs) of the asprepared CuO/Cu electrode in 3 mol dm3 NaOH solution at a scan rate of 5 mV s1 in the potential range of 0.65 to 0.1 V, which is the range at which H2O2 electroreduction occurs. The insert is the stabilized CV of the CuO/Cu electrode measured in the potential range of 0e0.4 V. The stabilized CV in the potential window of 0e0.4 V (the insert) displayed a pair of broad anodic/cathodic peak. The cathodic peak can be attributed to the reduction of CuO to Cu2O and/or CuOH. The anodic peak can be assigned to the oxidation of Cu2O and/or CuOH to CuO and/or Cu(OH)2 [39e42]. The CVs for the freshly prepared CuO/Cu electrode in the potential range of 0.65 to 0.1 V exhibits a strong anodic peak and a very weak cathodic peak. The peak currents decreased with the increase of potential cycling number and became remarkably week after 30 cycles. The anodic peak can be associated with the formation of Cu2O from oxidation of Cu foil, which was not
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Fig. 3 e Cyclic voltammograms of the as-prepared CuO/Cu electrode in 3 mol dmL3 KOH solution at 5 mV sL1 in the potential range of L0.65 to L0.1 V. Insert is the stabilized CV of the CuO/Cu electrode measured in the potential range of 0e0.4 V.
covered by CuO nanosheets. The cathodic peak corresponds to the reduction of Cu2O surface film [39,43,44]. It is well established that Cu2O is a poorly conducting semiconductor, so the thin film of Cu2O formed Cu surface during potential cycling acts as a barrier to electron transfer and protects Cu to be further oxidized. Consequently, the anodic currents were reduced by repetitive potential cyclings [43]. Fig. 4 shows CVs of the CuO/Cu electrode in 3 mol dm3 KOH solution containing H2O2 with various concentrations. The electrode was subjected to potential cycling between 0.65 and 0.1 V for 30 times in KOH solution prior to H2O2 electroreduction measurements. The potential scans start from the open circuit potential and go negatively. As can be seen that the cathodic currents are much larger in the presence of H2O2 than that without H2O2, and the currents increase with the increase of H2O2 concentration up to 0.5 mol dm3. This demonstrated that electroreduction of H2O2 occurred on the CuO/Cu electrode. The cathodic peak current density exhibited an approximate linear relationship with the H2O2 concentration up to 0.5 mol dm3, which implies that the reduction reaction at the peak potential was controlled by H2O2 diffusion. Notably, further increase in the H2O2 concentration leads to no increase of peak current. This
Fig. 2 e SEM images of the CuO/Cu electrode.
13614
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 6 1 1 e1 3 6 1 5
Fig. 4 e Cyclic voltammograms of the CuO/Cu electrode in 3 mol dmL3 KOH solution containing H2O2 with various concentrations.
unexpected behavior is likely due to the severe chemical decomposition of H2O2 because it is well known that H2O2 is unstable in strong alkaline electrolyte, it usually undergoes chemical decomposition to oxygen and the decomposition rate is faster at higher H2O2 concentration. We noticed that when H2O2 concentration is higher than 0.5 mol dm3, a significant amount of gas bubbles evolved from the electrode surface. Since chemical decomposition of H2O2 to oxygen results in the reduction of H2O2 utilization efficiency, using low concentration H2O2 is essential for keeping high H2O2 utilization efficiency. The onset potential of H2O2 electroreduction on the CuO/Cu electrode is w0.1 V, which is nearly the same as that on Co3O4 nanowire array electrode (w0.1 V) [30] and slightly more negative than that on Pt, Pd and Au electrodes (0e0.06 V) [45]. The comparable overpotential for H2O2 electroreduction on the CuO/Cu electrode with that on other electrocatalysts suggested that CuO has fairly good catalytic activity for H2O2 electroreduction.
The stability of CuO/Cu electrode for H2O2 electroreduction was investigated by chronoamperometric measurements. Fig. 5 shows chronoamperometric curves of H2O2 electroreduction on the CuO/Cu electrode. Prior to measurements, the electrode was potential cycled for 30 times between 0.65 and 0.1 V in KOH solution to transfer the exposed Cu to Cu2O (Fig. 3). The potentials were selected in the region from open circuit potential to the peak potential according to the CVs (Fig. 4). The reduction currents reached steady-state quickly after constant potentials were applied and displayed no sign of decrease within the test period, indicating that the CuO/Cu electrode has a good stability for catalyzing H2O2 electroreduction. The stabilized current density at 0.3 V is around 4.3 mA mg1 for the CuO/Cu electrode, which is about 57% of that for Co3O4 nanowire arrays grown on nickel foam (7.6 mA mg1) reported previously [30]. Even though the activity of the CuO/Cu electrode is lower than that of Co3O4 nanowire array electrode, the lowcost, abundant resource and easy preparation of CuO electrode still makes it a potential electrode for fuel cells using H2O2 as oxidants.
4.
Conclusions
CuO nanosheet arrays grown on Cu foil was investigated as the electrocatalyst for H2O2 reduction in alkaline medium. The onset potential of H2O2 electroreduction on the CuO/Cu electrode is close to that on Co3O4 nanowire arrays grown on nickel foam, and slightly more negative than that on Pt, Pd and Au electrodes, indicting that CuO has good catalytic activity for H2O2 electroreduction. The stabilized current density for H2O2 electroreduction on the CuO/Cu electrode at 0.3 V is about 57% of that on Co3O4 nanowire arrays. However, due to the low-cost, abundant resource and easy preparation of CuO/Cu electrode, it is a promising electrode for fuel cells using H2O2 as oxidants. The catalytic performance of CuO could be further improved by tuning its morphology.
Acknowledgments We gratefully acknowledge the financial support of this research by National Nature Science Foundation of China (20973048) and Ph.D. Programs Foundation of Ministry of Education of China (20102304110001).
references
Fig. 5 e Chronoamperometric curves for H2O2 electroreduction on the CuO/Cu electrode measured in 3.0 mol dmL3 KOH D 0.4 mol dmL3 H2O2.
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