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Energy Procedia 63 (2014) 4031 - 4034
GHGT-12
Electrochemical detection of underwater CO2 using amine-
functionalized electrode
Hiroshi Sato*
Research Laboratory, IHI Corporation, 1, Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan
Abstract
In this paper, fabrication of amino group-modified sensor electrode and electrochemical detection of CO2 in the saline solution will be reported. Electrochemical detection of CO2 was carried out using cyclic voltammetry technique in redox active potassium ferricyanide aqueous solution. Peak currents of oxidation and reduction reaction in cyclic voltammograms have changed according to the concentration of CO2 molecules in the measurement solutions. The calibration curve for the CO2 concentration was obtained by plotting oxidation peak currents. The sensor electrode prepared in this study can estimate differences between the concentration of CO2 in normal seawater and that of up to 20 times.
©2014TheAuthors.PublishedbyElsevierLtd. Thisis an open access article under the CC BY-NC-ND license
(http://creativecommons.Org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Organizing Committee of GHGT-12
Keywords: Carbon dioxide; Seawater; Electrochemical detection; Chemical sensor; Functionalized electrode
1. Introduction
Carbon Capture and Storage (CCS) technologies have received considerable attention because of the possible applications to prevent global warming by reducing greenhouse gas such as carbon dioxide (CO2) [1]. The CCS process consists of four processes of the capture process, the transportation process, the injection process, and the storage process. In these processes, the storage process takes the longest period of the whole CCS process. Because of that, long-term maintenance and monitoring is required to guarantee of safety. The detection technique of CO2 which exists in underwater environment is one of the key component technologies required for practical application of CCS process. In order to perform continuous monitoring of underwater CO2 in ocean storage parts, indirect
* Corresponding author. Tel.: +81-45-759-2819; fax: +81-45-759-2208. E-mail address: hiroshi_sato@ihi.co.jp
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Organizing Committee of GHGT-12
doi:10.1016/j.egypro.2014.11.434
detection techniques, such as pH measurement, have been used commonly. For the future, however, direct detection technique will be required for performing more precise and sensitive monitoring of underwater CO2.
Chemical sensors are applied to many scientific and industrial fields, such as medical treatment, food and environmental monitoring; because of the sensor has high sensitivity and selectivity. The chemical sensor consists from two parts: the detector part and the transducer part. Detector parts recognize the molecules to be measured by functional organic groups. On the other hand, transducer parts convert molecular recognitions to signals such as electrochemical signals, surface plasmon resonance, change of mass, etc. Since surface modification techniques for the surface of the electrode are very common, electrochemical measurement is widely used for transducer part.
It is known that organic molecules having amino groups can react with CO2 molecules forming carbamate ions. Recently, for example, various alkanolamines and its blends have been studied as solvents for capture process of CCS [2-4]. It is supposed to be able to fabricate a chemical sensor that can directly measure CO2 molecules using amino groups as a detector part of the sensor. In this paper, electrochemical detection of CO2 in the saline solution using amine-functionalized electrode will be reported. Electrochemical responses were measured in order to evaluate combinations of CO2 molecules to the amino groups immobilized on the surface of the electrode. The calibration curve is obtained by plotting electrochemical responses for CO2 concentration.
2. Materials and Methods
2.1. Reagents
Aminoethantiol and potassium hexacyanoferrate (III) were purchased from Tokyo Kasei Co. (Tokyo, Japan) and Kanto Kagaku Co. (Tokyo, Japan), respectively. Sodium chloride and Sodium bicarbonate were obtained from Showa kagaku Co. (Tokyo, Japan). All the materials were of the highest grade available and were used as received.
2.2. Preparation oof Amine-modified Electrodes
A gold (Au) disk electrode (diameter, 3.0 mm) was used. First, the surface of the electrode was polished thoroughly with aqueous slurries of alumina paste and then rinsed with water, gently. The Au electrode was dipped in a freshly prepared aminothiol aqueous solution (10 mM) overnight to fabricate self-assembled monolayer (SAM) which has amino groups in the end of molecular chain, and then the electrode was rinsed with water, gently.
Detector part of chemical sensor
Fig. 1. Fabrication of amino group terminated SAM on the Au electrode; immersion set-up of the electrode (left) and surface of the electrode immersed overnight (right).
2.3. Electrochemical Evaluations
Electrochemical measurements of the amine-functionalized electrode were carried out on a conventional three-electrode system using a platinum wire as the counter electrode and an Ag/AgCl electrode as the reference electrode. All measurements were performed at room temperature.
Fig. 2. Experimental set-up for electrochemical evaluations of the sensor electrode.
As a measurement solution of electrochemical evaluations, an aqueous solution containing potassium hexacyanoferrate (III) (5 mM) and sodium bicarbonate (0 to 50 mM) was prepared using a 500 mM sodium chloride aqueous solution. Concentrations of CO2 solved in the solution were adjusted by changing the concentration of sodium bicarbonate, which can produce CO2 molecules by their chemical equilibrium in an aqueous solution as shown below.
NaHC03(s) ^ Na+ + HC03" (1)
HC03 + H+ —> H2C03{aq) (2)
H2C03(aq) —> H20(aq) + C02(aq) (3)
3. Results and discussions
Electrochemical detection of CO2 was carried out using cyclic voltammetry technique in redox active potassium ferricyanide aqueous solution. Figure 3 shows cyclic voltammograms (CVs) and the calibration curve obtained. As shown in the left part of Figure 3, oxidation and reduction current of the ferricyanide anions were obtained. Moreover, both peak currents of oxidation and reduction have changed according to the concentration of the added sodium bicarbonate; increasing concentrations of CO2 molecules by adding sodium bicarbonate leads decreasing of both oxidation and reduction peak currents. The calibration curve for the CO2 concentration (shown in the right part of Figure 3) was obtained by plotting oxidation peak currents of CVs. As shown in the plot, it is clear that the sensor electrode prepared in this study can detect differences between normal concentration of CO2 in seawater (0.25 ppm) and 10 times or 20 times higher concentration of that.
The mechanisms of change in electrochemical responses depending CO2 concentration of the measurement solutions, are illustrated in Figure 4. In the case of absence of CO2 molecules (left part of Figure 4), ferricyanide anions can diffuse to electrode surface easily, due to the amino groups which are neutral in their equilibrium. However, in the case of presence of CO2 molecules (right part of Figure 4), negative-charged carbamate anions are produced by combining CO2 and amino groups on the electrode surface. Then, by the electrostatic repulsion between ferricyanide and carbamate anions, it is expected that oxidation and reduction current are decreased.
Fig. 3. Cyclic voltammograms of amino group-modified sensor electrodes at 100 mV s-1 of scan rate (left) and the calibration curve obtained by plotting electrochemical responses for CO2 concentration (right).
Surface of the electrode
Carbamate ions
Fe(III)(CN)63
-S" -S"
H „N
Fe(II)(CN)64
Fe(III)(CN)63'
Fig. 4. Schematic illustrations of electrochemical reactions on the electrode surface at absence (left) and presence (right) of CO2 molecules in the solution.
4. Conclusions
Electrochemical detection of CO2 in the saline solution was performed using amino group-modified Au electrode. Oxidation and reduction current of the ferricyanide anions changed depending on the concentration of CO2 molecules in the measurement solutions. The sensor electrodes prepared in this study are able to estimate differences between the concentration of CO2 in normal seawater and that of up to 20 times.
References
[1] IEA, CCS 2014, Insight series 2014. http://www.iea.org/publications/insights/insightpublications/Insight_CCS2014_FINAL.pdf.
[2] Sato H, Yoshihisa K, Kubota N, Takahashi K, Matsumoto A, Yamanaka Y, Furukawa Y. Lab-scale characterization of CO2 absorbents containing various amine species. Energy Proceida 2013; 37:431-435.
[3] Nakamura S, Yamanaka Y, Matsuyama T, Okuno S, Sato H. IHI's Amine-Based CO2 Capture Technology for Coal Fired Power Plant. Energy Proceida 2013; 37:1897-1903.
[4] Nitta M, Hirose M, Abe T, Furukawa Y, Sato H, Yamanaka Y, 13C-NMR spectroscopic study on chemical species in piperazine-amine-CO2-H2O system before and after heating. Energy Proceida 2013; 37:869-876.
