Design and test of a flexible electrochemical setup for measurements in aqueous electrolyte solutions at elevated temperature and pressure Gustav K. H. Wiberg, Michael J. Fleige, and Matthias Arenz Citation: Review of Scientific Instruments 85, 085105 (2014); doi: 10.1063/1.4890826 View online: View Table of Contents: Published by the AIP Publishing Articles you may be interested in Electrochemical performance of lithium ion capacitors using aqueous electrolyte at high temperature J. Renewable Sustainable Energy 5, 021404 (2013); 10.1063/1.4798432 Novel inorganic materials for polymer electrolyte and alkaline fuel cells AIP Conf. Proc. 1455, 29 (2012); 10.1063/1.4732468 Investigation into the gas diffusion electrodes of polymer electrolyte membrane fuel cell under long-term durability test J. Renewable Sustainable Energy 2, 013110 (2010); 10.1063/1.3328051 Investigation into polymer electrolyte membrane fuel cell characteristics using four-layer electrode catalyst J. Renewable Sustainable Energy 1, 043102 (2009); 10.1063/1.3167284 In situ measurement of the rate of H absorption by a Pd cathode during the electrolysis of aqueous solutions Rev. Sci. Instrum. 68, 1324 (1997); 10.1063/1.1147901

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Design and test of a flexible electrochemical setup for measurements in aqueous electrolyte solutions at elevated temperature and pressure Gustav K. H. Wiberg,a) Michael J. Fleige, and Matthias Arenza) Department of Chemistry and Nano-Science Center, University of Copenhagen, Universitetsparken 5, 2100 Ø Copenhagen, Denmark

(Received 1 June 2014; accepted 9 July 2014; published online 4 August 2014) We present a detailed description of the construction and testing of an electrochemical cell allowing measurements at elevated temperature and pressure. The cell consists of a stainless steel pressure vessel containing the electrochemical glass cell exhibiting a three electrode configuration. The design of the working electrode is inspired by conventional rotating disk electrode setups. As demonstrated, the setup can be used to investigate temperature dependent electrochemical processes on polycrystalline platinum and also high surface area type electrocatalysts. © 2014 AIP Publishing LLC. [] I. INTRODUCTION

Most electrochemical investigations using aqueous electrolytes are performed at temperatures up to 60 ◦ C.1 This limitation in operation temperature – for example, low temperature fuel cells operate at around 80 ◦ C2 – is mainly due to the fact that unpressurized cells are used.3 Although the temperature could easily be raised to more than 60 ◦ C using such electrochemical cells, at these temperatures considerable electrolyte evaporation would occur during the measurements. The evaporation results in a change in acid concentration and thus a change in the pH of the electrolyte solution. If pH dependent processes such as the hydrogen or oxygen reaction are investigated, a change in pH is a considerable disadvantage. Raising the operation temperature above the boiling point of water is even more challenging. As a consequence, measurements at elevated temperatures (>100 ◦ C) and pressures (>0.1 MPa) are a relatively unexplored area in aqueous electrochemistry.4, 5 Besides basic research, the main interest in investigating electrochemical processes under such extreme conditions has been to perform corrosion studies, as these conditions can be found, for example, in nuclear plants or geothermal brine solutions. One goal in our group is to characterize fuel cell catalysts for high temperature proton exchange membrane fuel cells under conditions close to operation. Fuel cell catalysts are typically characterized using a rotating disk electrode (RDE) setup under ambient conditions.6 Thus, we want to use a setup resembling, as much as possible, this typical setup, but adapted for more challenging conditions. A typical experimental setup to perform measurements under more challenging conditions uses an autoclave with a chemically stable liner into which the electrolyte is poured.7 The most basic cell has the three electrodes placed in the same compartment and connected to the potentiostat via electrical feedthroughs. In order to keep the products from the cathode and anode apart and to limit contamination from corrosion a) Authors to whom correspondence should be addressed. Electronic

addresses: [email protected] and [email protected] 0034-6748/2014/85(8)/085105/5/$30.00

products from the autoclave, a covered 3-compartment cell can be used instead.8 Inspired by the design of the RDE and the above mentioned designs, we present a new setup design for catalyst characterization at elevated temperatures. At the present stage, the application is limited to diffusive mass transport, enabling, e.g., studies concerning the oxidation of alcohols. However, future projects will try to upgrade the design to a RDE setup with controlled mass transport.


As outlined in the Introduction, our work describes the design of an experimental setup that allows our group to perform electrochemical measurements at elevated temperature and pressures. The cell has been tested at temperatures up to 200 ◦ C and pressures up to 10 MPa; and with small amendments in principle, even higher temperatures and pressures can be achieved. The setup consists of a pressure system, an electrochemical (EC) cell, a potentiostat, and a temperature system. A schematic overview of the setup is shown in Fig. 1. In the following paragraphs, we first briefly describe the overall setup before discussing the design of the pressure vessel and the EC cell. The pressure system comprises a gas source, a pressure regulator, a pressure vessel and an exhaust valve. The pressure is provided and adjusted by an external gas cylinder (20 MPa) equipped with a pressure regulator of maximum R The pressure vessel outlet pressure of 10 MPa (Vigour). is more thoroughly described in Sec. II B. The exhaust valve R is connected to a stainless steel outlet tube and (Swagelok) serves for manual pressure regulation and safe depressurization of the chamber.9 The EC cell is placed inside the pressure vessel. The cell is not sealed, i.e., no pressure gradients exist over the cell walls and lid. Cell materials of weaker mechanical strength can therefore be used. This enables great freedom of the cell design and it can be adjusted for any specific purpose. In our

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FIG. 2. The design of the autoclave. The fittings are used as feed through for gas, electrical cables, and temperature sensing element.

FIG. 1. The schematics of the measure system. An exchangeable electrochemical glass cell is placed inside a stainless steel pressure chamber that can be heated. Inside the glass cell, CE, RE, and WE are located in separate compartments and connected to the potentiostat. The electrode compartments are separated by frits.

case, we used replaceable, cheap, and easily modified glass flasks with threads for the lid (see below). The temperature system comprises a hot plate, a heating tape (600 W), and a temperature sensor. The pressure vessel and the EC cell can be heated by the hot plate and heating tape. Attachment of the heating tape all around the chamber minimizes vertical temperature gradients inside the chamber while heating up. It is important to stress that according to our testing this is necessary to prevent evaporated electrolyte from condensing outside of the EC cell and corroding the pressure compartment and contacts. The temperature inside the pressure vessel is probed by a thermocouple, which is, as well as the associated wiring, embedded in a stainless steel sealed tube. The exact position of the sensor can be adjusted. The temperature can thereby either be measured inside the EC cell immerged into the electrolyte or slightly above the electrolyte in order to minimize contamination. In any case, the thermocouple is shielded by means of polytetrafluoroethylene (PTFE)-tubing.

B. Autoclave design

A computer aided design (CAD) rendered image of the autoclave is shown in Fig. 2. The autoclave is designed to handle pressures >20 MPa and elevated temperatures with large safety margins. The vessel body is made out of a stainless steel 316 cylinder (d = 120 mm). A cavity (d = 60 mm, h = 140 mm) has been extruded in its centre. The cavity has a volume of about 0.4 dm3 . Six threaded holes have been drilled for feed through fittings: three for electrical feed connections, two for gas, and the last one as an auxiliary connection. The three electrical feed throughs are intended for the electrical connection to the EC cell. They comprise PTFE coated single strand copper wires, stainless steel ferrules and stainless steel fittings.

The vessel lid is made of a 12 mm thick stainless steel 316 disk. It is bolted to the vessel body by hot M8 stainless steel screws. A Methyl Vinyl Silicone (MVQ) O-ring (MVQ red) ensures that the vessel is hermetically sealed also at elevated temperature. In addition, the lid has a hole dedicated to thermo-couple feed through. C. Electrochemical cell design

Practically any EC cell, which fits to the size confinement of the pressure chamber, can be used for the electrochemical experiments. Here a design of a practical cell is provided to demonstrate the operation of the system. The basic requirement of the EC cell is that it comprises three electrodes, which fit into the confined space and can be easily exchanged. A CAD-drawing of the design is shown in Fig. 3. The cell consists of a 100 ml borosilicate glass laboratory flask with thread (GL45), an in-house produced PTFE lid, and a compartment divider together with working, reference and counter electrodes. We chose the glass flask as electrolyte container due to its inertness to harsh environments, and also due to its exchangeability. The latter point enables post experimental analysis of the electrolyte without inhibiting the ability to conduct a new experiment in parallel. In this case, the PTFE lid is simply exchanged by the original lid and the former put on a new glass flask. The working electrode (WE) consists of a disk embedded in a PEEK cylinder, which is connected to a PEEK covered stainless steel rod. Only the bottom side of the electrode is exposed to the electrolyte, as is typically used in RDE setups. The cylinder can be exchanged and can contain different disk electrodes, e.g., Pt or glassy carbon. The reference electrode (RE) is made of a PEEK cylinder into which a silver wire is placed. A PEEK membrane at the bottom side of the cylinder isolates the cavity inside from external electrolyte. A hole is drilled on the side of the cylinder to avoid a build-up of a pressure gradient. In this setup, the silver wire acts as a pseudo reference electrode. That means the absolute potential of the silver wire depends on the amount of silver ions in close proximity, which are formed while heating the electrolyte to elevated temperatures. As a consequence, the absolute reference potential depends

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Wiberg, Fleige, and Arenz

Rev. Sci. Instrum. 85, 085105 (2014)

WE was platinum disk (d = 5 mm). The RE was a silver wire in the 0.5 M H3 PO4 electrolyte in a compartment separated from the main compartment by PEEK frits. All potentials presented are recalculated with respect to the reversible hydrogen potential (see below), which was measured at each temperature. A glassy carbon rod was used as the CE. The electrochemical experiments were conducted using an in-house developed potentiostat and software.10 The solution resistance was online recorded using a superimposed AC signal (5 mVpp , 5 kHz) and compensated for via an analogue positive feedback scheme. The effective solution resistance was 0.7 VRHE , oxygenated species adsorb and eventually form an oxide. As the temperature increases, it can be seen that the HUPD features are diminished, indicating a lower HUPD coverage.16 At the same time the adsorption of oxygenated species increases, as observed previously by other authors both on polycrystalline and single crystal Pt.17, 18 Reaching temperatures above 100 ◦ C the signature of the adsorption and reduction of oxygenated species changes and in the positive going scan direction a pre-peak develops at about 0.6 VRHE . The pre-peak was reversible upon lowering the temperature and could indicate enhanced Pt dissolution at higher temperatures. A more detailed investigation of the temperature dependent electrochemistry will be published elsewhere, as it is beyond the scope of this journal article.

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Design and test of a flexible electrochemical setup for measurements in aqueous electrolyte solutions at elevated temperature and pressure.

We present a detailed description of the construction and testing of an electrochemical cell allowing measurements at elevated temperature and pressur...
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