Two-Step Bipolar Electrochemistry: Generation of Composition

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Two-Step Bipolar Electrochemistry: Generation of Composition Gradient and Visual Screening of Electrocatalytic Activity Hajar Termebaf, Mohsen Shayan, and Abolfazl Kiani* Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran S Supporting Information *

ABSTRACT: Bipolar electrochemistry (BE) is employed for both creating electrocatalysts composition gradient and visual screening of the prepared composition on a single substrate in just two experiment runs. In a series of proof-of-principle experiments, we demonstrate gradient electrodeposition of Ni−Cu using BE; then the electrocatalytic activity of the prepared composition gradient toward the hydrogen evolution reaction (HER) is visually screened in the BE system using array of BPEs. Moreover, the morphology and the chemical composition of the Ni−Cu gradient are screened along the length of the bipolar electrode (BPE). By measuring the potential gradient over the BPE, it is also demonstrated that by controlling the concentration of the metals precursor and the supporting electrolyte, the length of the bipolar electrodeposited gradient can be controlled.

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INTRODUCTION High-throughput parallel screening methodologies provide a very efficient and fast approach for development of new functional materials.1−3 In the field of electrochemistry, highthroughput technologies are being investigated and employed in the fields such as electrocatalysis, batteries, sensor development, corrosion protection, and photocatalysis.4−7 To perform low-cost analysis under the influence of many parameters in a short time, it has been proved that gradient strategy can be a valuable research tool.8,9 Many methods have been reported for the fabrication of gradient surfaces among which electrochemical techniques present many advantages; among the electrochemical approaches, one of the most straightforward and simple methods is bipolar electrochemistry (BE).9 BE has been known for a long time; however, in the recent decade it has attracted considerable attention. Operating principles of BE and its applications have been previously reported.10−13 Briefly, when an electronic conducting substrate is placed in an electrolytic solution and a highly sufficient electric field is applied through the solution by a power supply, faradaic reactions are induced at the two ends of the conductive object. Under these conditions, this substrate acts simultaneously as both anode and cathode, which is called a bipolar electrode (BPE). As the BPE does not require any direct electrical connection to the power supply, a large number of electrodes can be easily employed.14−16 Because of the presences of a potential gradient between the driving electrodes that apply the electric field, an interfacial potential gradient between the solution and the BPE is generated. As a result of this potential gradient, the rate of electrochemical reaction at different locations of the BPE varies in a gradient manner with © XXXX American Chemical Society

the highest rate occurring at the two distal ends of the BPE.10,17This behavior was used to create gradients substrates.17−20 Ulrich and co-workers employed BE for the first time for making molecular gradients.17 Bipolar patterning of a conducting polymer film was also another method used to generate gradient materials.18,19 BE has also been used for the electrodeposition of metals.21−32 Recently, Dorri and coworkers reported formation of a precisely controlled gradient of different copper structure to investigate the wettability of the generated surface.20 In addition to the mentioned applications of BE, bipolar electrodeposition offers a versatile approach to generate chemical compositional gradients.33−35 The key to this technique is the simultaneous electrodeposition of two or more different electroactive materials along the length of a BPE. This principle was first shown by formation of compositional gradients of CdS.33 With a similar approach, gradients of Ag−Au alloys have been synthesized.34 Recently, simultaneous electrodeposition of copper, nickel, and zinc on gold BPE was reported.35 Bipolar generated gradients have also been examined for their catalytic activity. In a recent report, arrays of TiO2 nanotubes were fabricated by bipolar anodization for the screening of the photoelectrochemical activity of the TiO2 nanotube.36 In another report, this bipolar anodization technique was further extended for creating a gradient of photocatalysts, which then was screened for the photocatalytic hydrogen production activity.37 Received: August 7, 2015 Revised: September 30, 2015

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them is coated with an organic dye layer followed by sputtering of a thin layer of a reflective metal. DVD-R was cut by scissors on one edge, and then the top polycarbonate layer was peeled off by hand to expose the metallic reflective layer. Prior to being used as bipolar electrodes, the surface was cleaned with ethanol (organic dye layer was washed by ethanol at this step) and rinsed with distilled water. The electrodes were cut by a scissors into rectangles with the dimensions of 3.0 × 4.5 cm. For patterning electrode on the DVD to make array of electrodes and designing the electrodes’ shape, the surface of the metallic reflective layer was scratched with the help of a pin and a ruler (see the Supporting Information). Measurement of the Electric Potential Gradient. The negative terminal of the voltmeter was connected to the upper part of the BPE which was out of the solution, and its positive terminal was connected to a mechanical pencil lead which acted as a probe for measuring the potential difference. After applying the driving potential to the solution, the pencil lead electrode was placed in the solution near the surface of the BPE and in front of the center of each part of the BPE; by placing the pencil lead electrode in different positions along the BPE in the solution, the potential difference between the bipolar electrode and different positions of the solution over the BPE was measured and the potential gradient diagram was obtained. Fabrication of Cu, Ni, and Ni−Cu Gradients. All the experiments were performed in a home-built cell. This cell was composed of a Teflon container with two pools at both sides (2.0 × 2.0 × 1.0 cm) and a groove at the center. The length of the groove was 4.5 cm with the depth of 0.5 cm that connected the two pools to each other. Before performing the bipolar electrodeposition, the reflective metallic layer of the DVD substrate was carefully scratched (except for the top section of the BPE; see the Supporting Information) with the help of a pin and a ruler to create 14 parallel lines. These lines divide the electrode into 15 discrete areas, 14 of which are equally spaced and identical, and all of the areas are electrically connected to each other through the top section of the BPE that is not scratched. In the bipolar electrodeposition step, the first 14 areas act as cathode and the deposition occurs on them while the last area acts as an anode. In the next step (step 2 in Scheme S1), a sacrificial copper layer was electrodeposited on 1.0 cm of the designated anodic pole of the BPE. For this purpose, by employing the DVD electrode as a cathode in a two-electrode setup, the sacrificial Cu layer was deposited from 0.1 M CuSO4 and 0.1 M H2SO4 solution under the potential of 2.8 V for 5 min. This Cu layer electrodissolves on the anodic pole of the BPE during the bipolar electrodeposition experiment and is thick enough to maintain the length of the BPE constant until the end of the bipolar electrodeposition and prevent the BPE from reduction in length. The as-prepared substrate was placed vertically in the designed groove of the cell in such a way that the rest of the electrode was out of the solution. In order to support the bipolar electrode during the electrodeposition, a plastic wall was inserted vertically into the groove, and the BPE was placed next to it. For bipolar electrodeposition of gradients, in each experiment run, the cell was filled with approximately 10 mL of either solutions of CuCl2, NiCl2, and mixture of CuCl2 and NiCl2; then two Pt electrodes were placed 7.0 cm apart in the pools and were connected to the dc power supply to apply the driving potential. This electrochemical setup is shown in Scheme 1. Parallel Screening of the HER Activity. The array of electrodes was placed at the center of a plastic cell (7.0 cm × 8.5 cm) in which two stainless steel driving electrode sheets were embedded 8.5 cm apart in the cell walls (see the Supporting Information); then the driving potential of 5.0 V was applied to the solution with the dc power supply. In each experiment run, the cell was filled with 25 mL of 0.1 M H2SO4, and the prepared array was placed at the center of the cell. Finally, the array was taken out of the cell and rinsed with distilled water.

Recently, bipolar electrochemistry has also been employed in the field of electrocatalysts screening.38−42 Anodic dissolution of a metal film was the first reproted approach for the screening of the oxygen reduction reaction (ORR) electrocatalyst candidates.38 In another report, bipolar electrochemistry in conjunction with electrogenerated chemiluminescence was employed for screening the ORR activity of electrocatalysts used in fuel cells.40 Applicability of using fluorescence-enabled electrochemical microscopy for developing a platform for highthroughput electrocatalyst screening was also reported.39 Later, the anodic dissolution strategy reported by Crooks group was further developed for parallel screening of electrocatalyst candidates for the ORR and the HER.41,42 Recently, using DVD-R for making low-cost bipolar electrodes, we have employed the anodic dissolution concept and demonstrated its application for sensing and screening purposes.43 In the present study, for the purpose of establishing the feasibility of performing both creation of compositional gradients and screening the electrocatalytic activity of the prepared gradient on a single substratein bipolar electrochemistry, Ni−Cu composition and HER were chosen to investigate in a series of proof-of-concept experiments. In this article, at first, gradient deposition of Ni−Cu is achieved using BE; then the electrocatalytic activity of the prepared composition gradient toward the HER is visually screened in the BE system using array of BPEs. Here, the conductive surface of the DVD-R reflective layer is used just as a substrate for both bipolar electrodeposition and screening purpose in the BE system; however, the described method is not limited to employing DVD-R substrate. The reason for choosing DVD-R is the ease of its application for making bipolar electrodes in our laboratory and creating arrays in a facile manner. As the HER occurs on the cathodic pole of BPEs in the array, the metallic reflective layer of the DVD-R BPEs in the array simultaneously dissolves in anodes, and this is the basis for the visual screening of the catalysts activity on the cathodes. The dissolved length of anodes depends on the efficiency of the associated compositional electrocatalyst at the cathodic pole of BPEs; therefore, by comparing the dissolved lengths of each electrode in the array, electrocatalytic activity of the corresponding electrocatalyst composition in the gradient which is prepared with BE is screened by the naked eye. Therefore, the optimum catalyst composition can be determined. This method is advantageous over the previously reported strategies in providing a simple and fast approach for both creating libraries of electrodeposited electrocatalysts compositions and visually screening the prepared compositions on the same substrate using BE in just two experiment runs.

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EXPERIMENTAL SECTION

Chemicals and Reagents. The deposition solutions were prepared from CuSO4 (purity: 99%), H2SO4 (98%), CuCl2·2H2O, and NiCl2·6H2O. All the materials were from Merck Company. All of the aqueous solutions were prepared using doubly distilled water. Stainless steel sheet and Pt electrodes were used as driving electrodes. HNO3 (purity: 65%) was used for cleaning the Pt electrodes after each experiment. Instruments. A direct current (dc) power supply (MASTECH, HY3005F-3) was used for performing bipolar electrochemistry experiments. Scanning electron micrographs were taken using a field emission scanning electron microscope (model MIRA3 TESCAN). Bipolar Electrode Fabrication. All of the BPEs were made using DVD-R. DVD-Rs are composed of two polycarbonate plates; one of

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RESULTS AND DISCUSSION The morphology and structure of crystals depend on the distance of their creation conditions from equilibrium.44,45 B

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Therefore, because of the presence of a potential gradient along the electrode, bipolar electrochemistry is one of the best methods to create gradient surfaces with different structures and morphologies. The presence of the potential gradient in the solution translates to the presence of a continuum of different available overpotentials for reactions to occur; as a result, there is a continuum of different driving forces along the electrode for electrochemical reactions.10 At the two distal ends of the electrode (the beginning of two poles), overpotential has maximum values; therefore, reactions occurring on these regions experience the maximum distance from the equilibrium condition. By moving along the electrode away from the distal ends, the overpotential is reduced, and somewhere between the two poles it reaches the lowest value (close to the equilibrium condition). This is an inherent advantage of BE which allows screening different conditions in a single experiment. In this study, Ni−Cu gradients are fabricated by bipolar electrodeposition on the BPEs. To create Ni−Cu gradient, the cell was filled with a solution containing CuCl2 and NiCl2, Then, using a dc power supply, a potential of 3.8 V was applied for 5 min to the driving electrodes (Scheme 1). Under these conditions, the electrode is sufficiently polarized to act as a bipolar electrode. Reduction of Ni2+ and Cu2+ ions occur simultaneously at the cathodic pole of the BPE with the simultaneous oxidation of the sacrificial deposited Cu layer on its anodic pole. At the cathodic pole, two competing reduction processes occur during the deposition: copper (Cu2+(aq) + 2e− → Cu(s), E0 = +0.34 V vs SHE) has a more positive reduction potential than Ni (Ni2+(aq) + 2e− → Ni(s), E0 = −0.25 V vs SHE), suggesting that Cu will deposit preferentially. The concentration of Ni2+ in the solution was selected to be relatively higher than the Cu2+ concentration to make up for the thermodynamically less favorable Ni2+ reduction so that Ni would be electrodeposited along with Cu. These competing

Scheme 1. Schematic Configuration of the Bipolar Electrochemical Setup Designed and Used in This Study for the Bipolar Electrodeposition of Cu, Ni, and Ni−Cu Gradients (Each Separately)a

a

Numbers represent the number of each part of the BPE. Red arrows show the potential difference between the BPE and the solution. This scheme is not to scale.

Figure 1. Plot of the potential difference between the BPE and the solution over the BPE as a function of the distance from the cathodic edge along the BPE. The solutions employed are shown in the legend. The inset (a) shows an illustration of the experimental setup used to perform these measurements. The dashed red arrow represents the mechanical pencil lead electrode that was moved in the solution to measure the potential in each position. (b) The diagram schematically shows the faradaic depolarization effect. C

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electrodeposition of copper on a large length of the electrode. When the deposition of Ni is performed in 0.2 M NiCl2, deposition occurs on a larger length compared with 0.02 M CuCl2. It is clear from the diagram in Figure 1 that the supporting electrolyte, due to lowering the faradaic depolarization, has a profound effect on the distribution of potential. Because of this, in the solution of 0.02 M CuCl2 containing 0.4 M KCl, the potential difference (red triangles in Figure 1) increases compared with 0.02 M CuCl2 while for the solution of 0.2 M NiCl2 (yellow crosses), as there is enough high concentration of ions, addition of supporting electrolyte (0.4 M KCl) does not significantly change the potential distribution (dark purple circles). When the electrodeposition is performed in the solution of 0.02 M CuCl2 and 0.2 M NiCl2, the potential difference between the BPE and the solution over the BPE is the highest (blue squares); therefore, the maximum length of deposition is observed under this condition. Surface Morphology Studies. FESEM and EDX analysis were obtained to better understand the structure of the electrodeposited metals and the percentage of each metal in the compositional gradient. For this purpose, the metal percentage and the morphology of the Ni−Cu compositional gradient were compared with those obtained from the Cu and the Ni gradient that were electrodeposited separately from the corresponding solutions. Figure 2a shows the surface morphology of the bipolar electrodeposited Cu which is obtained on three parts of the electrode. EDX analysis results confirm the presence of copper on this electrode (Figure S1 in the Supporting Information). Cu deposited on part 1 of the BPE is formed in a higher overpotential, which means that the reduction and nucleation occurred in a condition with more distance from the equilibrium state. A comparison between the FESEM images from part 1 to 3 shows that with decrease in the overpotential from part 1 to 3, the density of structures decreases and nucleus growth overcome the nucleation such that the morphology changes to cauliflower in part 2 and flower-like in part 3 with larger particles. In remaining parts of the BPE, conditions for nucleation are inappropriate, and the copper is not deposited. Figure 2b shows FESEM images of four parts of the BPE on which Ni is electrodeposited from 0.2 M NiCl2. EDX analysis results confirm the presence of nickel on this electrode (Figure S2). As is shown in Figure 2b, by moving from part 1 to 3, in the first stage of the deposition, a layer of Ni nanoparticles is formed on the electrode; thereafter, the nanoparticles gradually grow into nanoplates.47 In part 5, with the decrease in overpotential, growth of particles is decreased, and nanoplates with lower thickness are formed. In part 7, nucleation decreases, and very low dense structures with larger particles are formed. In the next parts, as it is clear by visual inspection, Ni is not deposited or the amount of deposited Ni is very low, which means that the overpotential is not enough for the reduction of Ni ions over these parts. Figure 3 shows the FESEM images obtained from different parts of the electrode on which Ni−Cu is electrodeposited from a solution containing 0.2 M NiCl2 and CuCl2 0.02 M. With regard to the fact that the Ni:Cu mole ratio is 10:1, the amount of Ni ion is higher than Cu on the surface of the electrode for the reduction reaction; as a result, when the overpotential is appropriate for both of metal ions to electrodeposit (first 3−4 parts), the faster diffusion rate to electrode surface causes the Ni to electrodeposit more than Cu.

processes are exploited to achieve composition variation along the length of the bipolar electrode. In order to estimate the optimum driving potential for the Ni−Cu deposition, first the bipolar electrodeposition was carried out for each ion separately. To determine the minimum driving potential for the deposition of each metal, the deposition process was visually inspected until a change in the color corresponding to the deposited metal on the very edge of the bipolar electrode was observed. The minimum driving potentials to reduce each of the Ni2+ and Cu2+ ions separately from 0.2 M NiCl2 and 0.02 M CuCl2 to metal were respectively obtained 3.0 and 1.8 V. These results are in agreement with the results of traditional electrochemical experiments in a three-electrode setup (recorded LSVs are not shown). Accordingly, with the driving potentials higher than 3.0 V it is possible to deposit both Ni and Cu simultaneously. In order to obtain a compositional gradient with enough thickness and a desired large length, the driving potential should be as large as possible; however, there is an upper limit for the applied driving potential with using BPEs in this study. Driving potential higher than 4.0 V is not practically favorable because with the higher applied potentials, too much hydrogen gas is released on the cathode of the BPE that causes the silver layer to separate from the polycarbonate substrate. Herein, 3.8 V was chosen as the optimum driving potential for the simultaneous electrodeposition of Ni and Cu. With this applied driving potential, metal layer on the cathodic edge of the BPE maintains its adherence to the polycarbonate substrate. The length of the created gradient depends on the nature of cations in the solution and the distribution of electric potential over the bipolar electrode. To prove this, deposition of Ni and Cu was conducted in different conditions of metal ion and supporting electrolyte concentration. Moreover, the distribution of the electric potential was measured through the solution on the bipolar electrode. These results indicate that when the solution contains only 0.02 M CuCl2, the minimum length of deposited metal is observed while the maximum length is contributed to when both 0.02 M CuCl2 and 0.2 M NiCl2 coexist in the solution. Results of measured electric potential distribution (Figure 1) indicate that with a solution of 0.02 M CuCl2 the potential difference between the BPE and the solution is much higher for the first 10 mm of the cathode and changes with a high steep while for the next 30 mm; this potential difference is lower and changes with much lower slope (green squares in Figure 1). On the other hand, when a solution containing 0.02 M CuCl2 and 0.2 M NiCl2 is used, the potential difference between the BPE and the solution is higher (blue squares) than the condition when just 0.02 M CuCl2 is present. Besides, the potential difference distribution is more even and changes approximately linearly over the length of the cathodic pole of the BPE. The reason for this behavior can be explained by the faradaic depolarization, the phenomena of distortion of the electric potential gradient over the bipolar electrode, which is dependent on the fraction of the total current that is passed through the BPE compared with the ionic current through the solution.46 Therefore, faradaic depolarization can occur when an electrolyte with low concentration is employed. The effect of faradaic depolarization on the distortion of potential distribution is schematically shown in the inset of Figure 1. In the solution of 0.02 M CuCl2, the potential difference between the bipolar electrode and the solution is the lowest; therefore, the overpotential is not high enough for the D

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Figure 4. Area chart of the percentage of elements in each part along the bipolar electrodeposited Ni−Cu gradient on the BPE. The bipolar electrodeposition was performed in a solution of 0.2 M NiCl2 and 0.02 M CuCl2 under the driving potential of 3.8 V for 5 min. The presence of Ag is related to the underlying Ag layer of the DVD. As is clear in this chart, by moving from part 1 to 11 by decrease in surface coverage of the deposited Ni−Cu, the percentage of Ag increases. Besides, oxygen in this composition possibly originates from the oxidation of the surface of the Ni and Cu metals during the time interval between making gradient and performing the EDX analysis (about 20 days). Considering the higher coverage and percentage of deposited metals on the first parts, the amount of oxygen is relatively higher on these parts of the electrode.

percentage of Ni is higher than Cu in part 1. Although the reduction of Cu is thermodynamically more favorable than Ni, due to the higher concentration of Ni ions than Cu, nickel is reduced more easily than Cu. As is shown in the FESEM image of part 1, a combination of two different structures of cauliflower and nanoplates can be observed. These structures are a result of high overpotential available for the electrodeposition at this region. The overpotential on part 3 is lower compared to part 1; subsequently, as the EDX analysis results indicate, deposition of Ni occurs less than in part 1. In this area, structures similar to those of part 1 but with smaller sizes are formed. EDX analysis results obtained from part 5 indicate that the percentage of Cu

Figure 2. (a) FESEM images of the bipolar electrodeposited Cu on the first three parts of the BPE. The bipolar electrodeposition was performed in 0.02 M CuCl2 solution. (b) FESEM images of the bipolar electrodeposited Ni on four parts of the BPE. The bipolar electrodeposition was performed in 0.2 M NiCl2 solution. These experiments were performed under the driving potential of 3.8 V for 5 min.

In part 1, the overpotential is in its highest value; therefore, conditions in this region are appropriate for codeposition of Ni and Cu. Results of EDX analysis (Figure 4) indicate that the

Figure 3. FESEM images of the bipolar electrodeposited Ni−Cu on six parts of the BPE. The bipolar electrodeposition was performed in a solution of 0.2 M NiCl2 and 0.02 M CuCl2 under the driving potential of 3.8 V for 5 min. E

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prepared gradient; as a result, the active catalyst is in fact a composition of Ni−Cu−Ag. When a highly enough voltage is applied between the driving electrodes, hydrogen reduction occurs on the cathodes; at the same time, oxidation of the metallic reflective layer of the BPE happens in anodes. The dissolved length of this metallic layer in the anode, which is visually detectable, depends on the activity of the catalyst. Here, the shapes of the anodic poles in the array were designed in a way to improve the sensitivity and the detection limit51 (see the Supporting Information and Scheme 2). Parallel screening of electrocatalysts necessicates the creation of array of electrodes that act electrically independently. As both the electrodeposition and the screening are performed with the BE on the same substrate, an array for the screening was designed on the same susbtrate that the bipolar electrodeposition was performed, but the electrodes in the array were electrically connected in the bipolar electrodeposition step. Before performing the screening step, the nonscratched top section of the electrode, along with the sacrifacial anodic section of the electrode, was cut by a scissors (step 3 in Scheme S1). This process makes the electrodes in the array electrically isolated and independent so that on one end of each (0.5 cm) a composition of Cu−Ni is electrodeposited (this array is shown in Scheme 2). This end of each electrode acts as the cathodic pole for the bipolar screening of the electrocatalytic activity of the compositional gradient for the HER. For screening the HER catalytic activity of the prepared gradients, the array of bipolar electrodes was placed at the center of the designed cell containing 0.1 M H2SO4, and then a potential of 5.0 V was applied between the driving electrodes for 20 s. For screening purpose, the electric field within the solution should be uniform, and all of the electrodes should behave identically under the same conditions. To ensure the generation of a uniform electric field across the array, two stainless steel plates were employed as the driving electrodes which were embedded parallel and with the distance of 7.0 cm from each other (Scheme S2). The performance of the array (that the electrodes in the array behave identical) and the uniformity of the electric field across the array were evaluated in an experiment by subjecting the bare array to the HER in 0.1 M H2SO4 under the driving potential of 5.0 V. Images of the arrays subjected to this test are shown in Figure 5a,b. As it is clear in these images, dissolved lengths of the electrodes in each array are equal which means that both the electric field and the electrodes in the array are uniform. Before performing the parallel screening of the Ni−Cu gradient for the HER electrocatalytic activity, gradients of Cu and Ni that were separately deposited in the BE system were evaluated. Figure 5c shows the array of BPEs after screening the HER catalytic activity of the electrodeposited copper from 0.02 M CuCl2. In comparison with the unmodified array (Figure5a), dissolved lengths of anodes in the first electrodes of the array on cathodes of which Cu is deposited are increased, which shows a higher electrocatalytic activity for the HER (Figure 2a). The array on cathodic poles of which Ni is deposited and subjected to the screening of HER electrocatalytic activity is shown in Figure 5d. Ni was deposited from 0.2 M NiCl2 solution. Similar to the array modified with copper, compared with the unmodified array, HER catalytic activity is increased for the first electrodes that Ni on them is deposited, and the dissolved lengths of anodic poles of the remaining BPEs in the array are approximately equal to those of unmodified array.

is higher than Ni. This translates to the reduction of more Cu ions in comparison with Ni in this area wherein overpotential is not enough for the reduction of Ni ion and conditions for reduction of Cu are more appropriate. In this part the surface coverage by deposited metals is decreased. After this part, the percentage of Ni decreases where in part 11 only copper is deposited. A comparison between the images in Figure 3 reveals that from part 1 to 11, as the overpotential decreases, lower dense structures are formed. Visual Bipolar Screening the Electrocatalytic Activity of Ni, Cu, and Ni−Cu Gradients. High-throughput and screening methods provide the ability to prepare and test catalytic materials having different compositions and structures in a short time. In view of the electrocatalysts screening, BE offers a promising and effective tool in this field. Nickel is an active metal for HER, and due to its high catalytic activity and low cost, it has been investigated as a promising catalyst material. Formation of Ni-transition metal alloy electrodes, such as with Co, V, W, Fe, Mo, Zn, and Cu which are fabricated by electrodeposition, is an approach to enhance the electroactivity of Ni electrodes toward the HER.48 The addition of the transition metals is expected to alter the electrode reaction mechanism leading to a change in the activation energy of the HER.49 The nature and the percentage of the alloying metals and the electrodeposition conditions all influence the physical and chemical properties of the Ni-based alloy electrodes. These characteristics affect electroactivity of Ni-based alloys for the HER.50 As the catalytic activityof a metallic composition material depends on the percentage of each metal and the structure of the composition, the aim of this work is creation of Ni−Cu gradient, which was chosen as a model catalyst, with different structures and percentage using BE, then using one-step screening in bipolar electrochemistry for visualized parallel examination of the catalytic activity of this gradient. This is the first time that both the creation of the compositional gradient and the screening of the electrocatalytic properties of the prepared gradient are evaluated by BE. In this study, as depicted in Scheme 2, the Ni−Cu compositional gradient prepared with BE was subjected to the HER in bipolar electrochemistry setup for the visualized parallel screening of the electrocatalytic activity of the prepared gradient. As one can see in Figure 4, although we electrodeposited the Ni−Cu gradient, Ag is also present in the Scheme 2. Illustration of the Visual Bipolar Screening of the Compositional Gradient for the HER Electrocatalytic Activitya

a

Arrows show the electrodissolved lengths of the anodes of the electrodes in the array. F

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composition gradient can be screened on the same substrate in bipolar electrochemistry. As both the generation and screening of the compositional gradient are performed on a single substrate with bipolar electrochemistry, this approach proposes a fast and efficient method for investigation of composite electrocatalysts. In this study, DVD-R substrate and a simple mechanical method were employed for the fast and easy creation of the array of BPEs to only demonstrate the feasibility of the proposed two-step method. However, the disadvantages to using the DVD-R substrates in electrocatalysts screening must be noted, as the topography of the metal films on DVD-R may influence electrodeposition morphology and, ultimately, the electrochemical response of electrocatalysts. Fortunately, the proposed two-step approach is not limited to DVD BPEs; for example, employing photolithographic methods, ITO substrate can also be patterned to make parts on the substrate and perform the deposition of composition gradient; then in the second step the array of electrodes can be formed by electrically disconnecting the parts and then be employed in the screening experiment.

Figure 5. (a, b) Photographic images of two bare arrays of BPEs after running the HER experiments for evaluation of the electric field uniformity over the arrays and testing that the electrodes in the arrays behave identically. The HER was performed in 0.1 M sulfuric acid under the driving potential of 5.0 V was applied through the solution for (a) 20 s and (b) 60 s. (c, d, and e) Photographic images of the arrays of BPEs after subjecting to the screening of HER electrocatalytic activity of (c) Cu, (d) Ni, and (e) Ni−Cu gradients. The experiments were performed in 0.1 M sulfuric acid under the driving potential of 5.0 V for 20 s.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02945. Procedure of making BPEs; EDX spectra of gradients; effect of time and concentration on the morphology and composition; bipolar screening setup (PDF)

Screening the HER electrocatalytic activity of the composition gradient was performed with the array that is shown in Figure 5e. The Ni−Cu composition gradient was electrodeposited from a solution containing 0.02 M CuCl2 and 0.2 M NiCl2. Compared with the arrays in Figures 5c and 5d, the dissolved lengths of the first four electrodes are increased, which means that the Ni−Cu composition on the first four electrodes has a higher HER activity than the Cu and Ni gradients; furthermore, it is clear from the dissolved length of each electrode in this array (Figure 5e) that the cathodes of the first electrodes exhibit a higher activity for the HER than the remaining electrodes in the array. The reason for this behavior is that as the EDX analysis data indicate, the first electrodes have a higher ratio of Ni:Cu. Ni and Cu have a synergistic effect for hydrogen evolution, and the composition of Ni−Cu enhances the electrocatalytic activity in electrocatalytic evolution of hydrogen. Besides, by moving from electrode 1 to 12, the structure density and also the volume-to-area ratio decrease; as a result, the first electrodes in the array show higher HER activity. To be more quantitative, measuring the electrochemical active surface area would be helpful.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of the Vice chancellorships for research and technology of the University of Isfahan.

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REFERENCES

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CONCLUSIONS We have presented a two-step application of BE. In the first step, creating compositional gradient of electrodeposited materials was achieved, and then the electrocatalytic activity of this composition gradient was screened in the second step. As a proof of concept, Ni−Cu composition gradient was generated using BE, and then the electrocatalytic activity of the Ni−Cu gradient for the HER was visually screened with BE on the same substrate. Moreover, the morphology of the deposited gradients under different conditions was screened and compared along the length of the BPE in a single experiment. The method presented in this work can be extended for electrodeposition of a variety of compositional gradients with catalytic properties for reduction reactions, and then this G

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DOI: 10.1021/acs.langmuir.5b02945 Langmuir XXXX, XXX, XXX−XXX