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Научни трудове на Съюза на учените в България-Пловдив, серия Б. Естествени и хуманитарни науки, т. XVIII, ISSN 1311-9192 (Print), ISSN 2534-9376 (On-line), 2018. Scientific researches of the Union of Scientists in Bulgaria-Plovdiv, series B. Natural Sciences and the Humanities, Vol. XVIII, ISSN 1311-9192 (Print), ISSN 2534-9376 (On-line), 2018.
ЕЛЕКТРОКАТАЛИТИЧНИ СВОЙСТВА НА РУТЕНИЙ ЕЛЕКТРООТЛОЖЕН ВЪРХУ СТЪКЛОГРАФИТ И ГРАФИТ: ВЛИЯНИЕ НА УСЛОВИЯТА ПРИ ОТЛАГАНЕ Янна Лазарова, Тотка Додевска Кат. "Органична химия иНеорганична хи мия", УХТ, Пловдив
ELECTROCATALYTIC PROPERTIES OF RUTHENIUM
ELECTRODEPOSITED ON GLASSY CARBON AND GRAPHITE: EFFECT OF THE DEPOSITION CONDITIONS Yanna Lazarova, Totka Dodevska Department of Organic chemistry and Inorganic chemistry, UFT, Plovdiv
Abstract
The article presents results of optimization of two electrochemical procedures for deposition of ruthenium onto glassy carbon and spectroscopic graphite, in order to obtain effective electrocatalysts for reduction of hydrogen peroxide. The effect of the time of electrodeposition has been examined when the process was performed in potentiostatic regime (at a constant potential of -0.4 V vs. Ag/AgCl, 3M KCl). Upon deposition of the metal phase under potentiodynamic conditions (by means of cyclic voltammetry - 1 cycle from -0.5 to +0.5 V), the scan rate has been optimized. The applicability of the modified electrodes for sensitive quantitative amperometric detection of hydrogen peroxide has been demonstrated.
Keywords: electrocatalyst, ruthenium, electrodeposition, hydrogen peroxide reduction, sensor Introduction
In the development of efficient electrode-catalysts, it is particularly important to obtain on chemically inert and electrically conductive carrier metal deposits with high specific catalytic activity in the target reaction. In this connection, dispersing the catalytically active phase onto surface of solid carbonaceous carriers not only repeatedly lowers the price of the produced catalysts but also significantly increases their activity. It should be noted that the catalytic properties of the metal deposits depend on the particles size and shape, and morphology of the metal phase onto the carrier surface. These parameters are determined both by the nature, physical characteristics and pre-treatment of the carrier, as well as by the procedure for deposition. Recent studies proved the electrodeposition as an attractive method for modifying various electrode materials. The great advantage of the electrochemical techniques is the possibility of strict and accurate control allowing high reproducibility of the modification procedure. The required equipment is standard for any electrochemical laboratory and offers a wide range of possibilities electrodeposition to be performed in potentiostatic or potentiodynamic conditions as well as by applying pulse techniques.
Based on the above, the present study deals with the optimization of electrochemical procedures for obtaining a stable ruthenium (Ru) deposits onto carbonaceous matrices (glassy carbon and graphite). In order to obtain Ru-modified electrodes with high catalytic activity in the
reduction of H2O2, two techniques for electrochemical modification are applied and optimized. The effect of the time of exposition of the carrier in the electrolyte solution has been examined when the deposition was performed in potentiostatic regime (via electrolysis) and the scan rate has been optimized upon deposition of the metal phase in potentiodynamic conditions (by means of cyclic voltammetry). With aim to develop a non-enzymatic sensor for sensitive quantitative determination of H2O2 (Chen et al., 2012; Chen et al., 2014; Galvin et al., 2017), the catalytic activity of the so-obtained modified electrodes in the reduction of H2O2 at potentials around and below 0 V (vs. Ag/AgCl, 3M KCl) was investigated.
Materials and Methods
Materials. Two types carbonaceous carriers ware used as working electrodes: 1/ disc from glassy carbon (GC) with diameter of the working surface 3 mm and visible surface area of ca. 7.07 mm2 (Metrohm) and 2/ disc from spectroscopic graphite (Gr) with diameter of the working surface 5.6 mm and visible surface area of ca. 25 mm2 (RWO, Ringsdorf, Germany). RuCl3.nH20, HCl, H2O2 (30% (v/v) aqueous solution), Na2HPO4.12H2O, NaH2PO4.2H2O were purchased from Fluka. 0.1 M phosphate buffer solution (PBS) (pH 7.0) was made of sodium phosphates (monobasic and dibasic) dissolved in double distilled water.
Apparatus and measurements. The electrochemical measurements were performed using computer controlled electrochemical workstation EmStat2 (PalmSens BV, The Nederland), equipped with licensed software PSTrace 2.5.2., in a conventional thermostated three-electrode cell (constant temperature of 25 oC), including working electrode, an Ag/AgCl (3 M KCl) reference electrode, and a platinum auxiliary electrode. To remove oxygen, the background solution was purged with pure argon.
Electrochemical deposition of ruthenium. Before modification, the GC electrode surface was carefully polished with 0.3 and 0.5 ^m alumina slurry on a polishing cloth (LECO, USA); the graphite electrode was carefully polished to mirror-like finish with emery paper with decreasing particle size (P800, P1200 and P2000). After polishing, the electrodes were sonicated in double distilled water for 3 min and allowed to dry at room temperature for few minutes. In order to establish the optimal procedure for electrodeposition of Ru, the metal particles were grown onto the glassy carbon electrode surface by electroreduction of Ru3+ ion from electrolyte containing 2% RuCl3, dissolved in 0.1 M HCl, using following electrochemical techniques (in brackets the corresponding type modified electrode is presented):
1/ electrodeposition under potentiodynamic conditions (by means of cyclic voltammetry, CV): one cycle in the potential range from -0.5 V to +0.5 V at scan rate of: 0.1 V s-1 (Ru/GC1), 0.05 V s-1 (Ru/GC2), 0.01 V s-1 (Ru/GC3), 0.005 V s-1 (Ru/GC4) and 0.003 V s-1 (Ru/GC5); 2/ electrodeposition at constant potential (-0.4 V) with duration: 10 s (Ru/GC6), 60 s (Ru/GC7), 180 s (Ru/GC8) and 600 s (Ru/GC9).
Results and Discussion
In order to obtain a prior information about the catalytic activity in electroreduction of hydrogen peroxide, for all types of modified glassy carbon electrodes from the first series (obtained by means of CV) an amperometric response was recorded in presence of 1.0 mM H2O2 at a constant potential of 0 V in 0.1 M PBS (pH 7.0). The highest reduction current was registered when a modified electrode type Ru/GC4 was used as a working electrode. Comparison of the results of this study showed that increasing the scan rate above 0.005 V s-1 at electrodepositing process, modified electrodes with lower activity in the target reaction are obtained. At the same time, decreasing the scan rate below 0.005 V s-1 does not lead to more effective metal deposition onto the glassy carbon surface. Experimental data suggest that the difference in the catalytic activity of Ru/GC4 and Ru/GC5 type electrodes is within the statistical error.
The stability of ruthenium deposits was tested in the same background electrolyte with continuous cycling of each of the modified electrodes (50 cycles in the range of -1.0 V to +1.0 V at 0.1 V s-1). Loss of metal phase was established at electrodes type Ru/GC1 and type Ru/GC2
(modified at high scan rates) and a partial one at electrode type Ru/GC3. This result confirms that the optimal scan rate for ruthenium deposition (by means of CV in the potential range from -0.5 V to +0.5 V) is 0.005 V s-1. Cyclic voltammogram of the so-modified electrode (Ru/GC4), registered in the potential range from -1.0 V to +0.6 V, confirmed the presence of ruthenium deposits onto the surface of glassy carbon carrier. A pair of well-defined redox peaks was observed - the anode (Epa) and cathode (Epc) peaks are localized at +0.085 V and +0.035 V, respectively and correspond to redox conversion Ru(II)O/Ru(III)O. The ratio Ipa/Ipc = 1.04 and the potential difference AEp = 50 mV indicate a fast reversible redox process.
Analogous studies were carried out with the other four electrodes from the second series (modified under potentiostatic conditions, varying the exposure time of the glassy carbon matrix in the electrolyte solution). It was found that the metal phase deposited onto the surface of the electrodes type Ru/GC6, Ru/GC7 and Ru/GC8 is unstable, therefore further studies were performed using only modified electrode type Ru/GC9.
Based on the aforementioned results, graphite electrodes have been modified using procedures established as an optimal for electrodeposition of Ru: one cycle in the potential range from -0.5 V to +0.5 V at scan rate of 0.005 V s-1 (denoted in the text as modified electrode type Ru/Gr4) and 600 s at constant potential of -0.4 V (denoted as type Ru/Gr9).
The concentration dependence of the amperometric response of the modified electrodes type Ru/GC4, Ru/Gr4, Ru/GC9 and Ru/Gr9 was investigated by means of constant potential amperometry in 0.1 M PBS (pH 7.0). Figure 1a presents the authentic record of the amperometric response of modified electrode type Ru/Gr4 upon addition portions of 0.1 mM and 0.5 mM H2O2 in PBS at an applied potential of -0.05 V. The current-time (I-t) plot is typical - upon successive injection of H2O2 the modified electrode showed increasing reduction currents (staircase current response), corresponding to the electrochemical convertion of the analyte. From the enlarged initial section of the same plot it can also be seen that the electrode responds rapidly to the changes of H2O2 concentration, producing stable signal within 10 s. The background subtracted steady-state signal (Is-Io) of the modified electrode, registered in this study, is presented in Fig. 1b. The linear response was proportional to the H2O2 concentration up to 8.0 mM (correlation coefficient of 0.999) with a sensitivity of 380 ^A mM-1 cm-2 (determined from the slope of the linear portion of the calibration graph).
0,1 mM
0 -200 5 -400 -600 -800
0,5 mM
400
600 800 1000 t, (s)
246 H2O2 concentration, (mM)
Figure 1. (a) Authentic record of the amperometric response of modified electrode type Ru/Gr4 upon additions of 0.1 mM and 0.5 mM H2O2 in 0.1 M PBS (pH 7.0), applied potential of -0.05 V, temperature 25 oC; (b) Dependence of the electrode response on the H2O2 concentration.
The effect of the applied potential on the operational parameters, in terms of electrode sensitivity and linear dynamic range, of the four types modified electrodes was also examined (Table 1). The electrode response was found to increase linearly with the hydrogen peroxide concentration at the applied potentials of 0 V, -0.05 V and -0.1 V, with sensitivity depending on
a
b
both the applied potential and the type of modified electrode. From the data presented in Table 1 it is evident that for all types modified electrodes the sensitivity decreases as the polarization potential become more negative. A pronounced dependence was observed for glassy carbon electrodes modified with Ru (types Ru/GC4 and Ru/GC9) - the electrode sensitivity at potential of 0 V was found to be 1.6 times as high as the sensitivity registered at -0.1 V.
The experimental results show that with both techniques for electrodeposition of ruthenium, a highly active catalyst in the target reaction was obtained using as a carrier graphite electrode. Concerning the operational parameters examined at an applied constant potential of 0 V, the graphite modified with Ru exhibited 1.2 times higher activity than glassy carbon modified using the same electrodeposition method.
As a general rule, on the same carbonaceous carrier (graphite or glassy carbon), the electrodeposition of ruthenium by means of cyclic voltammetry (CV) results in a catalytically active phase, exhibiting 20% higher sensitivity as compared to the electrode, modified using potentiostatic deposition process. In addition, at the same applied potential, the electrodes modified by CV, are distinguished by a longer linear dynamic range of the calibration graph.
Table 1. Operational parameters of modified with ruthenium electrodes, supporting electrolyte PBS (pH 7.0), temperature 25 oC, reference electrode Ag/AgCl (3 M KCl).
Modified electrode type E, V Sensitivity, [iA mM-1 cm-2 Linearity, mM
Ru/GC4 - 0.1 0 200 (^=0.983) 324 (^=0.982) 9.1 8.0
Ru/Gr4 -0.1 -0.05 0 342 (r2=0.99g) 380 (^=0.999) 396 (r2=0.996) 7.1 8.0 7.5
Ru/GC9 -0.1 0 167 (r2=0.98:) 262 (r2=0.988) 8.8 6.0
Ru/Gr9 -0.05 0 224 (r2=0.996) 317 (r2=0.992) 5.4 7.0
Conclusion
Efficient catalysts for electroreduction of hydrogen peroxide at low applied potentials (around and below 0 V vs. Ag/AgCl) have been obtained by modifying carbonaceous matrices (graphite and glassy carbon) with ruthenium using two different electrodeposition techniques. We found that the deposition process of the metal phase has strong effect on the electrocatalytic activity of the modified electrode in the target reaction. The graphite modified with Ru by means of CV (one cycle in the potential range from -0.5 V to +0.5 V at scan rate of 0.005 V s-1) have had an optimal electrode sensitivity of 396 ¡¡A mM-1 cm-2 up to 7.5 mM H2O2 at an applied potential of 0 V.
References
Chen W., Cai S., Ren Q., Wen W., Zhao Y., "Recent advances in electrochemical sensing for hydrogen peroxide: a review", The Analyst, 2012, 137 (1): 49-58.
Chen X., Wu G., Cai Z., Oyama M., Chen X., "Advances in enzyme-free electrochemical sensors for hydrogen peroxide, glucose, and uric acid", Microchim. Acta, 2014, 181: 689-705.
Galvin P., Padmanathan N., Razeeb K., Rohan J., Nagle L., Wahl A., Moore E., Messina W., Twomey K., Ogurtsov V., "Nanoenabling electrochemical sensors for life sciences applications", J. Mat. Res., 2017, 32(15): 2883-2904.
