Technology Note pubs.acs.org/acscombsci
Fluorophore Metal−Organic Complexes: High-Throughput Optical Screening for Aprotic Electrochemical Systems Sung Hyeon Park,† Chang Hyuck Choi,‡ Seung Yong Lee,§ and Seong Ihl Woo*,†,§ †
Department of Chemical and Biomolecular Engineering and §Graduate School of EEWS (WCU), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea ‡ School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea S Supporting Information *
ABSTRACT: Combinatorial optical screening of aprotic electrocatalysts has not yet been achieved primarily due to H+-associated mechanisms of fluorophore modulation. We have overcome this problem by using fluorophore metal− organic complexes. In particular, eosin Y and quinine can be coordinated with various metallic cations (e.g., Li+, Na+, Mg2+, Zn2+, and Al3+) in aprotic solvents, triggering changes in their fluorescent properties. These interactions have been used in a reliable screening method to determine oxygen reduction/evolution reaction activities of 100 Mn-based binary catalysts for the aprotic Li−air battery. KEYWORDS: combinatorial high-throughput screening, optical screening, oxygen reduction reaction, oxygen evolution reaction, aprotic system, Li−air battery
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salt as a trigger ion to modulate UV activity. The common organic solvents acetonitrile (ACN), dimethyl sulfoxide (DMSO), and tetraethylene glycol dimethyl ether (TEGDME) were used, and fluorescent properties were screened over a 10fold solute concentration gradient. As shown in Figure 1a, eosin Y demonstrated a strong increase in fluorescence with a decrease in LiClO4 concentration. The solute concentration threshold, at which the UV sensitivity was triggered, was strongly dependent on the solvent. Thus, >1 mM LiClO4 was required to eliminate fluorescence of eosin Y in ACN, whereas the threshold in DMSO and TEGDME were within a concentration window of 1 μM to 100 mM (Figure S1). In contrast, the fluorescence of quinine was intensified to increase LiClO4 concentration (Figure 1b), showing a UV sensitivity threshold between 100 mM and 1 M LiClO4 concentration independent of solvent. However, the fluorescence color and intensity were different for different solvents (Figure S2). We further investigated the interactions of fluorophore indicators in ACN with Na+, Mg2+, Zn2+, and Al3+. All acted as fluorescence triggers (Figure S3) with different thresholds for different cations. The nature of the anion had little effect with LiClO4 and Li bis(trifluoromethylsulfonyl)imide (LiTFSI) showing very similar performances (Figures S1 and S2). However, it should be noted that the type of anion is an important determinant of solubility of the trigger ions in the solvent (Figure S4).
atalysts are vital components of devices for electrochemical energy generation, conversion, and storage, such as fuel cells, metal−air batteries, and electrolyzers. Although many highly active transition metal heterogeneous catalysts are known,1−6 finding new and optimized catalysts is challenging due to the boundless variations of combinations and compositions available in diverse catalyst libraries. A variety of combinatorial high-throughput screening technologies7−17 have therefore been developed for the fast assessment of catalyst activities. Optical methods, favored for their ease and speed, rely on the performance of the fluorophore indicators for which ultraviolet (UV) sensitivity is triggered by a change in pH of the electrolyte.7,18−24 These dyes have enabled the rapid screening of electrocatalysts for many H+-involved processes such as oxygen reduction/evolution reactions (ORR/OER, O2 + 4H+ + 4e− ↔ 2H2O)18−21 and methanol oxidation (CH3OH + H2O → CO2 + 6H+ + 6e−).7,22−24 This type of optical screening technology, requiring the ready availability and transfer of protons, is difficult to apply to nonaqueous electrolytes. With the growing demand of high specific energy and energy density, aprotic systems represent promising options because of their wide electrochemical windows and numerous available solutes.25−27 For example, an aprotic rechargeable Li−air battery has been reported to have a much higher energy density (∼3,500 Wh kg−1) than that of an analogous aqueous system (∼1,900 Wh kg−1).26,27 Therefore, development of optical screening methods for systems in aprotic electrolytes would be highly useful. To address this need, we tested two fluorophore indicators, eosin Y and quinine, in various aprotic conditions with LiClO4 © XXXX American Chemical Society
Received: November 5, 2016 Revised: December 28, 2016 Published: January 3, 2017 A
DOI: 10.1021/acscombsci.6b00171 ACS Comb. Sci. XXXX, XXX, XXX−XXX
Technology Note
ACS Combinatorial Science
Figure 1. Fluorescent properties of the eosin Y and quinine indicators. Photographic images of 1 M and 1 μM LiClO4 solutions in ACN including (a) eosin Y (0.1 mM) and (b) quinine (1 mM) indicators. (c) Schematic description of the proposed optical screening mechanisms for eosin Y and quinine indicators.
The proposed fluorescence mechanisms of the indicators in the aprotic systems considering their behavior in aqueous electrolyte28−30 are described in Figure 1c. Eosin Y maintains its nonfluorescent neutral state when the anion O− sites of carboxylate and phenolate ions are occupied by the metallic cations.28,29 The O− anion sites of eosin Y will be vacated when the cation concentration in the electrolyte decreases. Here, eosin Y shifts into its mono- and dianion states, becomes UV sensitive, and fluoresces under UV irradiation. On the other hand, when the lone pairs of electrons at its tertiary amine and pyridyl groups are free, quinine is at its neutral state and is nonfluorescent.30 When the metallic cation concentration increases, quinine shifts to its mono- and dication states as the lone pair sites bond coordinately with the cations and quinine becomes UV sensitive and fluoresces under UV irradiation. The intensity of emission and the UV threshold of salt concentration can be influenced by the experimental variables regardless of the fluorescence color. The selected fluorophore indicators of this fluorescence mechanism were executed under a broad spectrum of aprotic solvents as well as solute cations. This demonstrates its potential applications in high-throughput optical screening for the fast search of suitable electrocatalysts in a wide range of aprotic systems. A promising example is the aprotic Li−air battery system where ORR and OER occur at the cathode during discharge and charge processes, respectively (i.e., 2Li+ + 2e− + O2 ↔ Li2O2). Therefore, assessment of the catalytic performance toward ORR and OER is possible via real-time gradation in fluorescence intensity on the catalyst surface because of the shift in the concentration of Li+ in the aprotic Li−air battery system during discharge/charge. Empirical verification of the aforementioned fluorescence mechanism was performed by fabricating different sets of transition metal-based combinatorial arrays on carbon paper. First, various unary catalysts were used to narrow the scope of candidates for the binary catalysts of over 100 combination and composition spots (Figures S5 and S6). The entire experiment was carried out inside a lab-designed glovebox equipped with a UV lamp and a real-time optical camera (Figure S7). The electrochemical cells were prepared with a 10 mM LiClO4 + 0.1 mM eosin Y electrolyte and a 100 mM LiClO4 + 10 mM quinine electrolyte for ORR and OER, respectively, in an ACN
solvent. The fabricated array was used as a working electrode, and fluorescence emission at each spot was observed for a period of 900 s by chronoamperometry (2.76 and 3.86 V vs Li/ Li+ for ORR and OER, respectively) under UV irradiation (Figures S8 and S9). The intensity of fluorescence emission was digitally quantified for accurate measurements (Figure 2 and
Figure 2. Optical images of fluorescence emissions on the binary combinatorial arrays taken at 900 s for (a) ORR and (c) OER with their digitally converted 2D graphs (b and d), respectively. Detailed composition of each catalyst spot can be found in Figure S6: (top) Mn rich and (bottom) guest element rich.
Figure S10). Note that the detailed physical characterizations of each catalyst spot in the combinatorial array were not described at this optical screening stage (but were separately studied on selected catalysts and discussed afterward) because it deviates from the main objective of the combinatorial study: the rapid search of optimized catalysts. B
DOI: 10.1021/acscombsci.6b00171 ACS Comb. Sci. XXXX, XXX, XXX−XXX
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the carbon electrode maintains a plateau around 2.40 V. The cell potentials were compared at the selected charging time at 5 h due to the differences in capacity and charge plateau of the cathode materials. The cell potentials were around 3.69 (Pt/C), 4.12 (carbon), 4.00 (MnxOy), 3.82 (CoxOy), 3.81 (MC46), and 3.77 V (MR37). The charge profiles of the potential plateau were in the order Pt/C < MR37 < MC46 < CoxOy < MnxOy < carbon, indicating higher OER activities of MR37 and MC46 compared to that of carbon. The order of cell performance and its tendency correspond well with the results from the optical screening analysis for OER. Therefore, the results demonstrate the validity and high reliability of the newly developed optical screening methodology for aprotic electrochemical systems. In conclusion, we studied the fluorescence of metal−organic complexes and consequently demonstrated that eosin Y and quinine as fluorophore indicators have on/off UV sensitivities depending on the concentration of metallic cations in aprotic electrolytes. A novel combinatorial optical screening method was successfully proposed and verified for an aprotic Li−air battery system on a library of 100 Mn-based binary catalysts. This study on real aprotic Li−air battery cells showed an identical tendency of catalytic performances with that of the optical screening results, verifying the validity and reliability of the newly developed combinatorial system. Although this study at its early stage was limited to the aprotic Li−air battery, the wide spectrum of available electrolytes and metallic cations strongly indicates the potential extensibility of the system to various aprotic electrochemical devices. The developed combinatorial system is expected to contribute significantly to opening up new possibilities for the field of electrochemistry, especially in fluorescence quantification for rapid and accurate identification of high potential catalysts.
On the unary catalyst array, Pt/C shows the brightest fluorescence emission in both ORR and OER, indicating the highest catalytic performance among the other 100 candidates. Alternately, Mn- as well as Ru- and Co-based catalysts showed considerable activity in both reactions (Figure S10). These results are in agreement with literature showing that Pt and such metal-derived catalysts (in general, oxide phases) have high performances in the aprotic Li−air battery.1,4,26,27 Mnbased binary combinatorial arrays were prepared from the candidates with the brightest fluorescence emission. Then, the synergetic effects of the various combinations with the other selected 10 elements (which exhibited relatively high catalytic performance in the unary catalyst array) were further investigated. As shown in Figure 2, the spots with the Pt/C exhibit fluorescence similar to that of the binary combinations. The digitalized graph shows that the difference in performance of the catalysts for ORR is almost negligible except for the Mnpoor and carbon catalysts. However, OER performance varied depending on the combination with other elements. The results showed that combinations of Co and Ru with Mn-based catalysts have a higher synergetic effect on OER activity over that of other elements. Real Li−air battery cells with cathodes prepared with the selected binary catalysts were fabricated to verify the validity of these optical screening trends. These catalysts were synthesized via identical experimental conditions to that of the combinatorial array. Taking into account the negligible effect of guest elements on the ORR activity of the Mn-based catalysts, Mn3Ru7- (MR37) and Mn4Co6-based catalysts (MC46) in the weight ratio were selected for the single cell operations as they showed high OER activities from the optical screenings. Pt/C, carbon, and Co- (CoxOy) and Mn-based catalysts (MnxOy) were also fabricated for comparison. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies showed that the presences of oxide phases for Mn, Co, and Ru were dominant (Figures S11−13). The discharge and charge profiles were analyzed with a labdeveloped Swagelok cell in a potential window of 2.0 to 4.2 V (vs Li/Li+) at a current density of 0.1 mA cm−2 (Figure 3). The discharge profiles of the manufactured cathodes exhibited a negligible difference in the discharge plateau except for the carbon electrode (Figure 3). This result is consistent with that from the aforementioned combinatorial study. The catalysts display a plateau around 2.64 V (vs Li/Li+), whereas
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscombsci.6b00171.
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Experimental methods, images of prepared electrolytes, schemes for combinatorial arrays, ORR and OER analyses, and XRD and XPS analyses (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Seong Ihl Woo: 0000-0003-1672-8728 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Energy Efficiency and Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), a granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20112020100110/KIER B52592) This work was supported by the GIST Research Institute(GRI) in 2017.
Figure 3. Charge/discharge profiles of the selected catalysts at a current density of 0.1 mA cm−2. C
DOI: 10.1021/acscombsci.6b00171 ACS Comb. Sci. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acscombsci.6b00171 ACS Comb. Sci. XXXX, XXX, XXX−XXX
