Electrochemiluminescence Imaging-Based High-Throughput

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Electrochemiluminescence Imaging-Based High-Throughput Screening Platform for Electrocatalysts Used in Fuel Cells Xiaomei Lin, Liyan Zheng, Gongmin Gao, Yuwu Chi,* and Guonan Chen Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection for Food Safety, and Department of Chemistry, Fuzhou University, Fujian 350108, China S Supporting Information *

ABSTRACT: High throughput screening is very important for accelerating the discovery of fuel cell catalysts. In this paper, a novel electrochemiluminescence (ECL, a technology changing electric current into light) imaging-based screening platform for electrocatalysts used in fuel cells has been developed. The ECL imaging-based screening platform consists of bipolar electrode array-bridged electrochemical (EC)/ECL twin cells, by which electrocatalytic reduction currents of O2 can be imaged directly by ECL. The ECL imaging-based screening platform is simple in instrumentation, can image the “current− voltage” dependence directly, reversibly, and sensitively, and may enable the activities of electrocatalysts to be evaluated in a highthroughput way. The developed ECL imaging-based screening platform is envisioned to have promising applications in high throughput combinatorial screening of electrocatalysts for fuel cells.

F

screening methods,13,14 which allows a larger number of catalyst samples to be evaluated one by one but may suffer from much time consumption. Bard and co-workers applied a scanning electrochemical microscope (SECM) technique in screening electrocatalysts for the oxygen reduction reaction in acidic media.15,16 The SECM technique can provide useful thermodynamic and kinetic information about the electrocatalytic reactions but may suffer from time consumption for the catalyst array is imaged serially.17 In 1998, Smotkin and coworkers first used a high-throughput fluorescence-based combinatorial screening method for discovery of better electrocatalysts to dissolve the problem that combinatorial screening of electrochemical catalysts by current−voltage methods can be unwieldy for large sample sizes.18 Since then, the combinatorial optical screening has also been applied for discovering electrochemical catalysts with higher and more stable activity.19−22 However, there exist some difficulties in applying the fluorescence-based combinatorial screening method for systematical, sensitive, and quick characterization of electrocatalysts.11,22 First, the “current−voltage” dependence, one of the most important characteristics for an electrocatalyst, can not be systematically evaluated by the optical screening method.11 This difficulty is due to that most of the fluorescence reagents for combinatorial screening are pH indicators, whose fluorescence intensities are determined by the total protons (or charges) consumed during the electrocatalytic reduction of oxygen rather than the currents at certain potentials.11 Second,

uel cells are the power generation technology of the twenty-first century for they are clean, quiet, and probably the most efficient method of converting fuel into electricity yet devised.1−4 Among the components of a fuel cell, electrocatalysts play a key role in energy conversion and contribute over 55% of the total cost because the precious platinum (Pt) is normally an essential element in constructing cathodic catalysts of high activity.5,6 Therefore, cathodic catalysts, i.e., oxygen reduction reaction (ORR) catalysts, have been extensively studied for achieving both high activity and low cost. Apparently, reducing the precious Pt loading in catalysts without compromising fuel cell performance or even developing non-Pt catalysts is an effective way to meet the cost requirements for fuel cell commercialization.5−7 In the ORR catalyst studies, various shapes of Pt nanostructures,8,9 Pt-based multicomponent materials,10 and non-Pt nanomaterials7 have been synthesized and evaluated. However, it is often quite difficult to obtain superior catalysts via simple designs and preparation procedures or from a limited number of catalyst candidates. Normally, a large number of potential catalyst samples synthesized by various methods and conditions have to be screened to obtain target catalysts with high catalytic activities. Therefore, a high-throughput screening strategy is very important for catalyst identification. Commonly, methods used in electrocatalyst screening mainly include electrochemical and fluorescent technologies.11 Liu and Smotkin developed a parallel electrochemical screening method, which has the advantage of parallel and direct catalyst evaluation (current−voltage dependence) under actual fuel cell conditions at controlled temperature but has the disadvantage of complex instrumentation and limitation in sample size.12 Sullivan et al. and Jiang and Chu reported serial electrochemical © 2012 American Chemical Society

Received: March 31, 2012 Accepted: August 17, 2012 Published: September 4, 2012 7700

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Figure 1. The principle of ECL imaging-based screening platform for electrocatalysts used in fuel cells. (A) The schematic diagram of ECL imagingbased screening platform for electrocatalysts used in fuel cells. (B) The working principle of ECL imaging for electrocatalytic activity: the electrocatalytic reduction current of O2 at the GC cathode is imaged by the ECL reactions of RuII-TPA system at the Pt anode.

the fluorescence-based screening methods suffer from significant interferences between the electrocatalytic reaction and the fluorescent imaging, which leads to a low imaging sensitivity and fails to distinguish small differences in electrocatalytic current at different catalyst spots. The pH values of testing chemical cells are usually limited in a narrow range (typically, pH 3.0−5.0) to ensure the work of both pH indicators and catalysts, which is not consistent with the optimum conditions either for electrocatalytic reduction of O2 or for the fluorescent imaging.11 Consequently, electrocatalytic activities under real fuel cell conditions can not be adequately reflected and sensitively detected. Most recently, Crooker and co-worker reported using the electrodissolution of Ag band electrodes for recording the catalytic currents, which enables rapid screening of ORR catalysts and provides a permanent record of catalyst activity.17 However, this screening technology may suffer from some disadvantages, such as complicated fabrication of microband Ag electrode, irreversible imaging (for irreversible electrodissolution of Ag). In this paper, we first reported a universal, high-throughput, sensitive, and reversible electrochemiluminescence (ECL) imaging-based screening platform for electrocatalysts to overcome the difficulties mentioned above. ECL is a kind of luminescence produced during electrochemical reactions in solutions,23,24 implying that it can be regarded as a technique that converts electrochemical reaction energies (currents) into light forms. In 2001, Manz and co-worker reported using a “U”shape bipolar electrode (or floating Pt electrode called by them) in the separation channel of electrophoresis as a wireless ECL detector to monitor electroreduction currents by ECL.25 The work shows for the first time that it is possible to use ECL intensity of one system (Ru(II) + tripropylamine) for estimating the reduction currents of another system (amino

acids). Since 2002, Crooks and co-workers have reported their research on placing a single “band”-shape bipolar electrode or bipolar electrode array in the same cell or electrochemically connected channels to report currents by ECL imaging.26−28 Their works show that it is very convenient to use ECL imaging (recorded by a CCD) rather than ECL intensity (detected by PMT) for reporting currents. Apparently, the above-mentioned works show that ECL, especially ECL imaging, is a promising method to monitor currents from interesting systems, for the ECL method is sensitive and may be a high-throughput method by imaging mode.25−28 However, up to date, no literature has been reported on the use of a high-throughput ECL imagingbased screening platform for electrocatalysts. Herein, we developed a specially designed ECL imaging-based screening platform for electrocatalysts using bipolar electrode arrayspanned electrochemical (EC)/ECL twin cells. As shown in Figure 1, the ECL imaging-based screening platform consists of one EC cell and one ECL cell. The EC cell has a glassy carbon (GC) cathode array, on which various ORR catalysts for screening are loaded, while the ECL cell has a Pt anode microarray, on which ECL reactions occur. The GC electrodes of the cathode array in EC cell are connected respectively with Pt microelectrodes of the anode microarray in ECL cell by conducting wires to form a bipolar electrode array. Then, applying an adequate potential between the two driving electrodes (i.e., ΔEappl in Figure 1) will drive both the electrocatalytic reductions of O2 at the GC anode array in EC cell (eqs 1a and 1b)5 and ECL reactions of the RuII-TPA system at the Pt cathode array in ECL (eqs 2a−2d).23,24,29,30 Herein, TPA and RuII are tri-n-propylamine and tri-(2,2′bipyridine)ruthenium(II), respectively. O2 + 4H+ + 4e− → 2H 2O 7701

(1a)

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Figure 2. Schematic diagram of ECL imaging-based screening platform for electrocatalysts used in fuel cells.

■

O2 + 2H+ + 2e− → H 2O2 + 2H+ + 2e− → 2H 2O (1b)

Ru(bpy)32 + − e− → Ru(bpy)33 +

(2a)

TPA − e− → [TPA•]+ → TPA• + H+

(2b)

Ru(bpy)33 + + TPA• → Ru(bpy)32 +* + products

(2c)

Ru(bpy)32 +* → Ru(bpy)32 + + hv

(2d)

EXPERIMENTAL SECTION

Fabrication. ECL imaging-based combinatorial screening platform for electrocatalysts consisted of four parts (see Figure 2): an electrochemical reaction cell, an ECL imaging cell, a direct current (DC) power source, and a charge coupled device (CCD) camera. The electrochemical reaction cell (part A in Figure S3, Supporting Information) used for driving electrocatalytic reduction of O2 by catalysts was fabricated as follows: First, four squarely arranged GC cylinders (ϕ 2 mm × 8 mm) were mounted at the center of a 3 cm × 3 cm × 1 cm Perspex block to form a four-GC disk (ϕ 2 mm) cathode array (part a in Figure S3, Supporting Information). Then, the GC cathode array was placed at the bottom of the electrochemical cell by installing on it a sealing Teflon membrane (part b in Figure S3, Supporting Information) and a Perspex cell wall (part c in Figure S3, Supporting Information) with screws (part d in Figure S3, Supporting Information). Finally, a Pt driving anode (part e in Figure S3, Supporting Information) was placed horizontally in the electrochemical cell. The ECL imaging cell (part B in Figure S3, Supporting Information) used for current imaging was fabricated as follows: First, four Pt wires (ϕ 0.2 mm × 50 mm) were squarely arranged and buried in a Perspex tube with epoxy resin to form a four-Pt disk (ϕ 0.2 mm) anode microarray (part f in Figure S3, Supporting Information). Then, the Pt anode microarray was inserted into a cell with a quartz slide as the bottom (part h in Figure S3, Supporting Information) and a drilled Perspex bock as cell wall (part i in Figure S3, Supporting Information). The quartz slide bottom was adhered to the Perspex cell wall with epoxy resin to keep waterproof. ECL lights emitting from the Pt anode microarray were transmitted through the quartz bottom and captured by a CCD microscope (Part D) located under. Finally, a Pt driving cathode (part g in Figure S3, Supporting Information) was placed horizontally in the ECL imaging cell. Each GC electrode in the GC cathode array (2 × 2) was connected with a corresponding Pt microelectrode in the Pt anode microarray (2 × 2) with leads to form a four bipolar electrode array. Electrochemiluminescent Imaging. The ECL imagingbased combinatorial screening platform for electrocatalysts can work after loading catalysts on the GC cathode array, adding an electrolyte solution, and applying an appropriate DC potential

Apparently, for each bipolar electrode, the catalytic reduction current (I) produced at the cathode end is equal to the current provided for the ECL reactions at the anode end (see the bottom section of Figure 1). This means that in this configuration the ECL array can be used for imaging electrocatalytic currents generated at the EC array. The design of the ECL imaging apparatus in this way may have two obvious advantages: (1) the cathode and anode ends of bipolar electrode are placed in two chemically separated cells, which completely eliminates chemical interferences between the EC and ECL cells and enables measurement of both electrocatalytic reactions and ECL imaging under their optimum conditions; (2) the electrode potentials of EC and ECL ends are independent of the length and position of the bipolar electrode (see section B in Figure S4, Supporting Information), and thus, it is easy to control the potentials for electrochemical reactions and ECL imaging. After the above construction, the performance of the ECL imaging-based screening platform and its potential application in high throughput screening of electrocatalysts used in fuel cells were evaluated by investigating the electrocatalytic activities of different Pt-based nanostructures, including a commercial Pt catalyst (JM20% Pt/C), synthesized Pt nanoparticles (PtNPs; see Figure S1, Supporting Information), and Pt nanowires (PtNWs; see Figure S2, Supporting Information).31,32 The validity of the developed ECL imaging-based screening technique for catalysts was verified by the classic current−voltage method. Finally, the advantages of the developed ECL imaging-based screening platform over the classic “current−voltage” methods and fluorescence-based screening techniques were discussed in detail. 7702

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Figure 3. Characterization of Pt anode microarray. (A) Addresses of Pt electrodes in the anode microarray for imaging electrocatalytic activities of catalysts: A(1,1) for the commercial Pt/C catalyst, A(1,2) for the Pt nanowires (PtNWs), A(2,1) for Pt nanospheres (PtNPs), and A(2,2) for noncatalyst. (B) Bright filed micrograph image of Pt anode microarray in Ru(bpy)32+-TPA solution. (C) ECL array imaging for reduction of O2 in the absence of catalyst and upon applying +3.2 V between the EC reaction cell and ECL imaging cell. (D) Corresponding surface plots of ECL array imaging of (C).

and the potential across the bipolar electrode (ΔEonset BE(i,j)) can be obtained as follows (see eqs 4a and 4b):

between the driving anode and driving cathode (parts e and g in Figure S3, Supporting Information) with a DC power source (part C in Figure S3, Supporting Information). ECL array images were recorded by an inverted microscope (Eclipse Ti-U, Nikon, Japan) equipped with a color camera (DS-Ri1 2/3-in., color, 1.50 megapixel).

onset o′ o′ ΔEappl(i,j) = (Edriving anode − Edriving cathode) o′ onset ) + (E Ru − Ecatal(i,j)

■

onset = K + ΔE BE(i,j)

RESULTS AND DISCUSSION Principle of the ECL Imaging-Based Screening Platform for Electrocatalysts. In the ECL imaging-based screening platform (Figure 1), the applied potential across the bipolar electrode array-bridged EC/ECL twin cells, ΔEappl, is used for driving the electrocatalytic reduction of O2 in the EC reaction cell and ECL reactions of the Ru(bpy)32+-TPA system in the ECL imaging cell). When O2 begins to be reduced on the cathodic end of a random bipolar electrode (A(i,j), e.g., bipolar electrode A(1,1), A(1,2), A(2,1), and A(2,2)), the applied potential across the twin cells (ΔEonset appl(i,j)) can be expressed in eq 3 (see Figure S4 and page S6 in Supporting Information for more detailed development of the equation):

onset o′ onset o′ onset Ecatal(i,j) = K + E Ru − ΔEappl(i,j) = E Ru − ΔE BE(i,j)

(4b)

′ ′ The item of K = (E°driving anode − E° driving cathode) can be constants if the solution compositions in the EC reaction cell and the ECL imaging cell are kept unchanged, and the surface areas of the driving anode and cathode are large enough (i.e., approaching onset and nonpolarized electrodes). Apparently, both ΔEappl(i,j) onset ΔEBE(i,j) can be used to estimate the onset potential of O2 reduction catalyzed by the investigated catalyst, Eonset catal(i,j), since there are simple linear relationships between them (see eqs 4a and 4b). ΔEonset BE(i,j) can be regarded as a special “imaging onset potential” (i.e., the onset potential of O2 reduction versus ECL imaging system rather than a normal reference electrode such as Ag/AgCl), while ΔEonset appl(i,j) can be regarded as an “apparent imaging onset potential” since its value can be readily obtained from the applied potential when the imaging electrode is lighted on by ECL reactions. Similarly, we can obtain “imaging maximum potential”, i.e., ΔEmax BE(i,j), and “apparent imaging maximum potential”, i.e., ΔEmax appl(i,j) when the electrocatalytic reduction of O2 by the catalyst reaches its maximum rate or the Pt anode end gives its brightest ECL emission (see eqs 5a and 5b):

onset o′ onset ΔEappl(i,j) = (Edriving anode − Ecatal(i,j)) o′ o′ + (E Ru − Edriving cathode)

(4a)

(3)

′ ′ where E°driving anode and E° driving cathode are, respectively, formal potentials of redox processes occurring at the driving anode and the driving cathode, and Eonset catal(i,j) is the onset potential of O2 reduction catalyzed by the investigated catalyst at the cathodic end of a bipolar electrode (A(i,j)), while E°Ru′ is the formal oxidation potential of Ru(bpy)32+ at the anodic end of the bipolar electrode. By rearranging eq 3, the relationship between onset the onset potential (Eonset catal(i,j)), the driving potential (ΔEappl(i,j)),

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Figure 4. ECL array imaging for electrocatalytic reductions of O2 by Pt-based catalysts at various apparent onset potentials: (A) 1.3 V for the commercial Pt/C catalyst (see A(1,1) at top left); (B) 1.6 V for the synthesized PtNPs catalyst (see A(2,1) at bottom left); (C) 2.4 V for the synthesized PtNWs catalyst (see A(1,2) at top right); and (D) 3.1 V for noncatalyst (see A(2,2) at bottom right). The ECL imaging cell is filled with the solution containing 5.0 mM Ru(bpy)32+, 25.0 mM TPA, and 0.1 M phosphate (pH 7.0), and the electrochemical reaction cell is filled with an airsaturated 0.5 M H2SO4 solution.

for electrocatalytic reduction of O2, while the Pt anode microarray is used for ECL imaging of the electrocatalytic currents transferred from the GC cathode array. The four Pt microelectrodes of the Pt anode array (Figure 3B) were addressed with A(1,1), A(1,2), A(2,1), A(2,2) and used for the ECL imaging of the activities of commercial Pt/C catalyst, synthesized PtNPs catalyst, synthesized PtNWs catalyst, and noncatalyst (bare GC), respectively (Figure 3A). Before measuring the electrocatalytic activities of Pt catalysts, the ECL imaging-based screening platform was tested in the absence of catalysts, i.e., no catalyst was loaded on the four-GC cathode array and electroreduction of O2 was carried out without catalysis. No evident ECL light emission was found at the Pt anode microarray until the applied potential was higher than 3.1 V. A typical ECL array imaging for O2 reduction in the absence of catalyst and upon applying 3.2 V driving potential is shown in Figure 3C. The four Pt anodes in the imaging cell have the same ECL intensity (Figure 3C) or the same column height in ECL emission surface plots (Figure 3D), which indicates that the four bipolar electrodes are equal in the ECL imaging of currents. Further experiments show that the O2 reduction current, one of the parameters for the characterization of catalysis efficiency, can be compared by ECL intensity at different pairs of bipolar electrodes (see Figure S6 and related discussion in the Supporting Information). Evaluation of Pt Electrocatalysts by the ECL ImagingBased Screening Platform. After characterization, the ECL imaging-based screening platform was used to measure the electrocatalytic activities of the target Pt catalysts with the same Pt weight. After loading the Pt catalysts on the GC cathode array, ECL imaging was carried out at various driving potential with a 0.1 V increment. From the ECL images recorded (Figures 4 and 5), the apparent onset potentials (ΔEonset appl(i,j)), the apparent maximum reaction potentials (ΔEmax appl(i,j)), and the maximum currents for the electrocatalytic reduction of O2 by

max o′ o′ ΔEappl(i,j) = (Edriving anode − Edriving cathode) o′ max ) + (E Ru − Ecatal(i,j) max = K + ΔE BE(i,j)

max o′ max o′ max Ecatal(i,j) = K + E Ru − ΔEappl(i,j) = E Ru − ΔE BE(i,j)

(5a) (5b)

Therefore, by recording apparent imaging onset potentials onset (ΔEappl(i,j) ) and apparent imaging maximum reduction potentials (ΔEmax appl(i,j)), the activities of the electrocatalysts used in the fuel cells can be conveniently compared and evaluated. Obviously, the smaller apparent imaging potentials max (ΔEonset appl(i,j) or ΔEappl(i,j)) mean the larger onset or maximum onset potentials (Ecatal(i,j) or Emax catal(i,j)) can be found in “voltage-current” curves (e.g., voltammograms shown in Figures S6−7, Supporting Information), i.e., the better electrocatalytic activity (see Figure S5 and related discussion in the Supporting Information). Additionally, the electrocatalytic reduction currents of O2 can be evaluated by measuring the brightness of ECL since, for each bipolar electrode, the catalytic reduction current is equal to the current for the ECL reactions (see Figure S5, Supporting Information) and is proportional to the ECL intensity.24,25 Characterization of the ECL Imaging-Based Screening Platform for Electrocatalysts. Although the bipolar electrode array used in the screening platform may consist of numerous bipolar electrodes both in theory and future practical applications, a four-bipolar electrode array configuration is provided as an example in this paper for succinctly demonstrating how the ECL imaging-based screening platform for electrocatalysts works. The four-bipolar electrode array consists of a four-GC disk (ϕ 2 mm) cathode array (2 × 2) and a four-Pt microdisk (ϕ 200 μm) anode array (2 × 2). The GC cathode array is used for loading catalysts and providing sites 7704

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Figure 5. ECL array imaging (left column: luminous intensity; right column: intensity surface plot) for electrocatalytic reductions of O2 by Pt-based catalysts at potentials where maximum electrocatalytic currents reach: (A) 2.0 V for commercial Pt/C catalyst (see A(1,1) at top left); (B) 2.1 V for PtNPs catalyst (see A(2,1) at bottom left); (C) 2.8 V for PtNWs catalyst (see A(1,2) at top right); (D) 3.3 V for noncatalyst (see A(2,2) at bottom right). The ECL imaging cell is filled with the solution containing 5.0 mM Ru(bpy)32+, 25.0 mM TPA, and 0.1 M phosphate (pH 7.0), and the electrochemical reaction cell is filled with an air-saturated 0.5 M H2SO4 solution.

the Pt-based catalysts were obtained and shown in Table 1. The values of apparent onset potentials were 1.3 V for the commercial Pt/C catalyst (see A(1,1) at top left of Figure 4A), 1.6 V for the synthesized PtNPs catalyst (see A(2,1) at

bottom left in Figure 4B), 2.4 V for the synthesized PtNWs catalyst (see A(1,2) at top right of Figure 4C), and 3.1 V for noncatalyst (see A(2,2) at bottom right of Figure 4D). On the basis of the apparent onset potentials of O2 reductions, the electrocatalytic activities (per unit weight of Pt) of the four samples were ranked as follows: commercial Pt/C > PtNPs > PtNWs > noncatalyst. Figure 5 shows that the maximum electrocatalytic currents occurred at 2.0 V for the commercial Pt/C catalyst (see A(1,1) at top left of Figure 5A), 2.1 V for the synthesized PtNPs catalyst (see A(2,1) at bottom left in Figure 5B), 2.8 V for the synthesized PtNWs catalyst (see A(1,2) at top right of Figure 5C), and 3.3 V for noncatalyst (see A(2,2) at bottom right of Figure 5D), respectively. This means that the maximum catalytic reduction of O2 by commercial Pt/C catalyst and the synthesized PtNPs catalyst need lower potentials, while the maximum catalytic reduction of O2 by

Table 1. Electrocatalytic Activities of Pt-Based Catalysts Evaluated by the ECL Imaging-Based Screening Platform

arrays

catalysts

apparent onset potentials (V)

A(1,1) A(2,1) A(1,2) A(2,2)

Pt/C PtNPs PtNWs None

1.3 1.6 2.4 3.1

apparent maximum reduction potential (V)

relative maximum reduction rate (%)

2.0 2.1 2.8 3.3

100 98.3 75.5 74.1 7705

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approach usually includes a complicated and expensive multipotentiostat system11,12 and time-consuming data processing.14 Compared with the previously reported fluorescence-based screening methods,6−9 the ECL imaging-based screening platform has the following advantages: (1) it can image the electrocatalytic currents directly and reversibly. Figure S8, Supporting Information, gives an example of the ECL imaging of electrocatlytic reduction currents of O2 in the presence of the commercial Pt/C catalyst at various driving potentials. The imaged currents are clearly potential dependent, and at any given potential, the imaged currents (ECL brightness) obtained during increasing the potentials (from 1.7 to 2.2 V; see section A in Figure S8, Supporting Information) are almost the same as those obtained during a decrease in the potentials (from 2.2 to 1.7 V; see section B in Figure S8, Supporting Information), showing an excellent reversibility of ECL imaging. This allows us to image the important “current−voltage” dependences for catalysts precisely, obtain information of apparent onset potential, maximum catalytic reduction potential, and current, and thus make an accurate evaluation of catalytic activity for catalysts quickly. In contrast, the fluorescence-based screening method evaluates electrocatalytic activity indirectly, slowly, and roughly via observing the total proton changes at the vicinities of catalyst spots over a period of time by the use of a proton indicator. Therefore, the ECL imaging-based screening method overcomes the difficulty of the classic fluorescence-based method in direct, quick, and accurate evaluation of catalytic activities of catalysts. (2) The developed ECL imaging-based screening platform has very high sensitivity and selectivity in imaging electrocatalytic currents for catalysts. The prevailing fluorescence-based screening methods suffer from low sensitivity due to the use of pH fluorescent indicator for imaging electrocatalytic activities of catalysts. The pH of testing chemical cell are limited in a narrow range (typically pH 3.0− 5.0), which is usually not consistent with the optimum conditions for electrocatalytic reduction of O2 and the fluorescent imaging. Furthermore, the fluorescent imaging method has non-negligible background from excitation light and interference of proton diffused from bulk solution to the catalyst spots. Contrarily, for ECL imaging-based screening platform, the electrocatalytic reaction and ECL imaging are carried out in the two separated cells, which means no chemical interference occur except for electric contact between them. Therefore, both the optimum condition for electrocatalytic reduction of O2 and the optimum condition for ECL imaging of current can be simultaneously attained, which enable the ECL imaging-based screening platform to evaluate any electrocatalytic system and achieve a high sensitivity. Moreover, the ECL-based imaging almost has no optical background for no excitation light is needed (i.e., emitting light in dark),23,24 which allows us to obtain ECL images with very high signal-to-noise ratio and thus detect very weak catalytic currents by facilely adjusting the contrast and brightness of the ECL images (as demonstrated in Figure S9, Supporting Information). In addition to the above advantages, the ECL imaging method for catalyst activity characterization exhibits good reproducibility. As demonstrated in Figure S10, Supporting Information, the test samples of same catalyst (e.g., the commercial Pt/C catalyst) prepared at three different time points almost have the same ECL intensity (see sections A, B, and C in Figure S10, Supporting Information) at applying a given driving potential (e.g., 1.5 V), an indication of good reproducibility of this ECL

PtNWs and noncatalyst (bare GC electrode) requires higher potentials. Furthermore, from the relative intensities of the obtained ECL images (the right column in Figure 5), the maximum reduction rates (or currents) of O2 in the presence of the catalysts (with the same weight of Pt) are ranked in the order: Pt/C (100%) > PtNPs (98.3%) > PtNWs (75.5%) > noncatalyst (74.1%). Apparently, both of the maximum catalytic reduction potentials and rates investigated for the above-mentioned four catalytic systems rank the catalytic activities (with the same weight of Pt) as follows: commercial Pt/C > PtNPs > PtNWs > noncatalyst. The catalytic activity order obtained from the maximum reduction potentials and rates is consistent with that from the apparent onset potentials. Unusually, after reaching the maximum intensities (e.g., the commercial Pt/C catalyst has the maximum ECL intensity at 2.0 V; see Figure 5A), ECL intensities were observed to decrease with further increasing potential (e.g., 2.1−3.3 V; see Figure 5B−D) and ECL images became very dim at potentials far away from the maximum reaction potential (see Figure 5D). This unusual phenomenon can be explained by a “Excited-State Quenching” mechanism.33,34 Normally, the electrogengerated Ru(bpy)33+ reacts with TPA or its oxidation intermediates to produce Ru(bpy)32+* that emits ECL light. However, overproduced Ru(bpy)33+ at higher potential may oxidize the excited-state Ru(bpy)32+* (a highly reducing species) into the ground-state Ru(bpy)32+, leading to the quenching of ECL.33,34 The unusual phenomenon makes the ECL imaging technique more convenient to find the maximum reaction potential and current for each catalyst since each catalyst has the brightest ECL image at its own maximum reaction potential while having a much dimmer ECL image at other potentials, where some other catalysts may have the brightest ECL images. Validation of the ECL Imaging-Based Screening Platform. To verify the validity of the developed ECL imaging-based screening technique for catalysts, the catalytic activities of the Pt catalysts were observed by the classic current−voltage method under the same experimental conditions as the ECL imaging (e.g., the equal amounts of Pt catalysts, the same GC electrode area, and reaction media). Figure S7, Supporting Information, shows the voltammograms obtained for O2 reduction in the presence of the Pt catalysts. From these current−voltage curves, the apparent onset potentials, the catalytic reduction peak potentials, and maximum catalytic currents were measured and shown in Table S1, Supporting Information. Apparently, all these data unanimously rank the catalytic activities of the catalysts as follows: commercial Pt/C > PtNPs > PtNWs > noncatalyst. This result obtained by the current−voltage method is well consistent with that obtained by the ECL imaging approach, indicating that the developed ECL imaging-based screening platform is valid in evaluating catalytic activities of electrocatalysts. Advantages of the ECL Imaging-Based Screening Platform for Electrocatalysts. Compared with the conventional current−voltage technique, the ECL imaging-based screening platform has two obvious advantages, including: high-throughput and very simple instrumentation. As a new screening technique, the ECL-based imaging can screen a large number of catalyst samples in one simple test procedure, whereas the current−voltage method can be unwieldy for large sample sizes.18 In instrumentation, the ECL-based imaging system needs only a simple and cheap DC power source and a CCD camera (see Figure 2), while the current−voltage 7706

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Analytical Chemistry

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imaging method. All of the above advantages make the ECL imaging method have attractive applications in combinatorial screening of electrocatalysts.

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CONCLUSIONS In summary, a novel ECL imaging-based screening platform for electrocatalysts used in fuel cells has been developed. The ECL imaging-based screening platform is simple in instrumentation and can image the “current−voltage” dependence directly, reversibly, and sensitively. The bipolar electrode array used in the screening platform may consist of numerous bipolar electrodes by appropriate design and fabrication of electrode arrays, which may enable the activities of electrocatalysts to be evaluated in a high-throughput way. The developed ECL imaging-based screening platform is envisioned to have promising applications in high throughput combinatorial screening of electrocatalysts with high activities from a large catalyst library for its parallel imaging mode, and it also might be applicable in local characterization of nanostructures’s catalytic ability, for example, imaging the catalytic oxygen reduction on the ultramicroelectrode or nanoelectrode with one nanoparticle (nanowire) or few nanoparticles (nanowires), due to the high sensitivity of ECL imaging.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental section, potential and current profiles across the bipolar electrode array, the linear responses between ORR currents and ECL intensities, the validation of the ECL imaging-based screening platform by the classic current− voltage method, reversibility in ECL imaging for “current− voltage” dependence, demonstration on high sensitivity and good reproducibility of ECL imaging method, and Figures S1− S10. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by National Natural Science Foundation of China (21075018), Program for New Century Excellent Talents in Chinese University (NCET-100019), National Basic Research Program of China (2010CB732400), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1116).

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REFERENCES

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dx.doi.org/10.1021/ac300875x | Anal. Chem. 2012, 84, 7700−7707