Design and test of a flexible electrochemical setup for measurements in aqueouselectrolyte solutions at elevated temperature and pressureGustav K. H. Wiberg, Michael J. Fleige, and Matthias Arenz

Citation: Review of Scientific Instruments 85, 085105 (2014); doi: 10.1063/1.4890826 View online: http://dx.doi.org/10.1063/1.4890826 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/8?ver=pdfcov 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-termdurability 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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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 085105 (2014)

Design and test of a flexible electrochemical setup for measurementsin 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 allowingmeasurements at elevated temperature and pressure. The cell consists of a stainless steel pressurevessel containing the electrochemical glass cell exhibiting a three electrode configuration. The de-sign of the working electrode is inspired by conventional rotating disk electrode setups. As demon-strated, the setup can be used to investigate temperature dependent electrochemical processes onpolycrystalline platinum and also high surface area type electrocatalysts. © 2014 AIP PublishingLLC. [http://dx.doi.org/10.1063/1.4890826]

I. INTRODUCTION

Most electrochemical investigations using aqueous elec-trolytes are performed at temperatures up to 60 ◦C.1 This lim-itation in operation temperature – for example, low temper-ature fuel cells operate at around 80 ◦C2 – is mainly dueto the fact that unpressurized cells are used.3 Although thetemperature could easily be raised to more than 60 ◦C usingsuch electrochemical cells, at these temperatures consider-able electrolyte evaporation would occur during the measure-ments. The evaporation results in a change in acid concentra-tion and thus a change in the pH of the electrolyte solution.If pH dependent processes such as the hydrogen or oxygenreaction are investigated, a change in pH is a considerabledisadvantage.

Raising the operation temperature above the boiling pointof water is even more challenging. As a consequence, mea-surements at elevated temperatures (>100 ◦C) and pressures(>0.1 MPa) are a relatively unexplored area in aqueouselectrochemistry.4, 5 Besides basic research, the main inter-est in investigating electrochemical processes under such ex-treme conditions has been to perform corrosion studies, asthese conditions can be found, for example, in nuclear plantsor geothermal brine solutions.

One goal in our group is to characterize fuel cell catalystsfor high temperature proton exchange membrane fuel cellsunder conditions close to operation. Fuel cell catalysts aretypically characterized using a rotating disk electrode (RDE)setup under ambient conditions.6 Thus, we want to use asetup resembling, as much as possible, this typical setup, butadapted for more challenging conditions.

A typical experimental setup to perform measurementsunder more challenging conditions uses an autoclave with achemically stable liner into which the electrolyte is poured.7

The most basic cell has the three electrodes placed in the samecompartment and connected to the potentiostat via electricalfeedthroughs. In order to keep the products from the cathodeand anode apart and to limit contamination from corrosion

a)Authors to whom correspondence should be addressed. Electronicaddresses: gustav.wiberg@gmail.com and m.arenz@chem.ku.dk

products from the autoclave, a covered 3-compartment cellcan be used instead.8

Inspired by the design of the RDE and the above men-tioned designs, we present a new setup design for cata-lyst characterization at elevated temperatures. At the presentstage, 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 aRDE setup with controlled mass transport.

II. INSTRUMENTATION

A. System setup

As outlined in the Introduction, our work describes thedesign of an experimental setup that allows our group to per-form electrochemical measurements at elevated temperatureand pressures. The cell has been tested at temperatures up to200 ◦C and pressures up to 10 MPa; and with small amend-ments in principle, even higher temperatures and pressurescan be achieved. The setup consists of a pressure system, anelectrochemical (EC) cell, a potentiostat, and a temperaturesystem. A schematic overview of the setup is shown in Fig. 1.In the following paragraphs, we first briefly describe the over-all setup before discussing the design of the pressure vesseland the EC cell.

The pressure system comprises a gas source, a pressureregulator, a pressure vessel and an exhaust valve. The pres-sure is provided and adjusted by an external gas cylinder(20 MPa) equipped with a pressure regulator of maximumoutlet pressure of 10 MPa (Vigour R©). The pressure vesselis more thoroughly described in Sec. II B. The exhaust valve(Swagelok R©) is connected to a stainless steel outlet tube andserves for manual pressure regulation and safe depressuriza-tion of the chamber.9

The EC cell is placed inside the pressure vessel. The cellis not sealed, i.e., no pressure gradients exist over the cellwalls and lid. Cell materials of weaker mechanical strengthcan therefore be used. This enables great freedom of the celldesign and it can be adjusted for any specific purpose. In our

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085105-2 Wiberg, Fleige, and Arenz Rev. Sci. Instrum. 85, 085105 (2014)

FIG. 1. The schematics of the measure system. An exchangeable electro-chemical glass cell is placed inside a stainless steel pressure chamber thatcan be heated. Inside the glass cell, CE, RE, and WE are located in separatecompartments and connected to the potentiostat. The electrode compartmentsare separated by frits.

case, we used replaceable, cheap, and easily modified glassflasks with threads for the lid (see below).

The temperature system comprises a hot plate, a heatingtape (600 W), and a temperature sensor. The pressure vesseland the EC cell can be heated by the hot plate and heatingtape. Attachment of the heating tape all around the chamberminimizes vertical temperature gradients inside the chamberwhile heating up. It is important to stress that according toour testing this is necessary to prevent evaporated electrolytefrom condensing outside of the EC cell and corroding thepressure compartment and contacts. The temperature insidethe pressure vessel is probed by a thermocouple, which is, aswell as the associated wiring, embedded in a stainless steelsealed tube. The exact position of the sensor can be adjusted.The temperature can thereby either be measured inside the ECcell immerged into the electrolyte or slightly above the elec-trolyte in order to minimize contamination. In any case, thethermocouple is shielded by means of polytetrafluoroethylene(PTFE)-tubing.

B. Autoclave design

A computer aided design (CAD) rendered image of theautoclave is shown in Fig. 2. The autoclave is designed tohandle pressures >20 MPa and elevated temperatures withlarge safety margins.

The vessel body is made out of a stainless steel 316 cylin-der (d = 120 mm). A cavity (d = 60 mm, h = 140 mm) hasbeen extruded in its centre. The cavity has a volume of about0.4 dm3. Six threaded holes have been drilled for feed throughfittings: three for electrical feed connections, two for gas, andthe last one as an auxiliary connection. The three electricalfeed throughs are intended for the electrical connection tothe EC cell. They comprise PTFE coated single strand cop-per wires, stainless steel ferrules and stainless steel fittings.

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

The vessel lid is made of a 12 mm thick stainless steel316 disk. It is bolted to the vessel body by hot M8 stainlesssteel screws. A Methyl Vinyl Silicone (MVQ) O-ring (MVQred) ensures that the vessel is hermetically sealed also at ele-vated temperature. In addition, the lid has a hole dedicated tothermo-couple feed through.

C. Electrochemical cell design

Practically any EC cell, which fits to the size confinementof the pressure chamber, can be used for the electrochemicalexperiments. Here a design of a practical cell is provided todemonstrate the operation of the system. The basic require-ment of the EC cell is that it comprises three electrodes, whichfit into the confined space and can be easily exchanged. ACAD-drawing of the design is shown in Fig. 3.

The cell consists of a 100 ml borosilicate glass labora-tory flask with thread (GL45), an in-house produced PTFElid, and a compartment divider together with working, refer-ence and counter electrodes. We chose the glass flask as elec-trolyte container due to its inertness to harsh environments,and also due to its exchangeability. The latter point enablespost experimental analysis of the electrolyte without inhibit-ing the ability to conduct a new experiment in parallel. In thiscase, the PTFE lid is simply exchanged by the original lid andthe former put on a new glass flask.

The working electrode (WE) consists of a disk embeddedin a PEEK cylinder, which is connected to a PEEK coveredstainless steel rod. Only the bottom side of the electrode isexposed to the electrolyte, as is typically used in RDE setups.The cylinder can be exchanged and can contain different diskelectrodes, e.g., Pt or glassy carbon.

The reference electrode (RE) is made of a PEEK cylin-der into which a silver wire is placed. A PEEK membraneat the bottom side of the cylinder isolates the cavity insidefrom external electrolyte. A hole is drilled on the side of thecylinder to avoid a build-up of a pressure gradient. In thissetup, the silver wire acts as a pseudo reference electrode.That means the absolute potential of the silver wire dependson the amount of silver ions in close proximity, which areformed while heating the electrolyte to elevated temperatures.As a consequence, the absolute reference potential depends

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085105-3 Wiberg, Fleige, and Arenz Rev. Sci. Instrum. 85, 085105 (2014)

FIG. 3. The electrochemical cell assembly comprising a glass flask, a com-partment divider, as well as the connection to RE and WE. The CE is notdepicted. The potentiostat wires are connected to the WE-connector (WC),RE-connector (RC), and CE-connector (CC). In the assembled cell, the REis placed at position (R), the CE at (C), and gas inlet tube at (G) of the com-partment divider.

on the pre-conditioning of the Ag wire and heating ramp andin order to compare the data to other systems and setups, areversible reaction, such as the reversible hydrogen reaction,must be measured in situ. All the potentials are then related toit instead of the used reference electrode (see Sec. IV B). Eventhough other reference electrodes, e.g., pre-loaded Pd/H2 ref-erence electrodes can easily be used as well, the advantageof the chosen Ag reference electrode is that always a stable(in time) potential is formed under the different, but constantoperation conditions. By comparison employing a pre-loadedPd/H2 reference, we observed a stable reference potential atelevated temperatures when using diluted acid electrolyte, butnot in concentrated acid electrolyte. In the latter case the elec-trode de-gassed leading to a constant shift of potential.

As counter electrode (CE, not shown), a Pt mesh or aglassy carbon rod was tested.

The compartment divider is used to create three separatecompartments inside the glass flask; one for each electrode.The divider is made out of a Teflon block from which twocompartments have been extruded. At the lower end of thedivider, two holes are placed connecting each compartmentto the external compartment. A PEEK frit inserted inside theholes limits the diffusion between the compartments.

III. EXPERIMENTAL

All electrochemical experiments we discuss in the fol-lowing were conducted using the described setup.

The electrolyte solution, 0.5 M H3PO4, was preparedfrom conc. H3PO4 (Merck Suprapur R©) and ultrapure water(MilliQ, 18.2 M� cm−1). The electrolyte was purged and thesystem pressurized by Ar, 99.999% pure (Air Liquide). The

WE was platinum disk (d = 5 mm). The RE was a silver wirein the 0.5 M H3PO4 electrolyte in a compartment separatedfrom the main compartment by PEEK frits. All potentials pre-sented are recalculated with respect to the reversible hydrogenpotential (see below), which was measured at each tempera-ture. A glassy carbon rod was used as the CE.

The electrochemical experiments were conducted usingan in-house developed potentiostat and software.10 The solu-tion resistance was online recorded using a superimposed ACsignal (5 mVpp, 5 kHz) and compensated for via an analoguepositive feedback scheme. The effective solution resistancewas <5 �.

IV. OPERATION AND FIRST RESULTS

To test the capabilities of the setup, we electrochemi-cally characterized a polycrystalline (PC) platinum disk elec-trode at different temperatures (25–180 ◦C) and elevated pres-sure (9 MPa) in a dilute aqueous electrolyte (0.5 M H3PO4).The electrochemical techniques used were chrono voltamme-try and cyclic voltammetry (CV). In Secs. IV A, IV B, andIV C, we discuss the experimental procedures and electro-chemical data we obtained.

A. Heating and pressure control

The first step in the experimental protocol is to de-aeratethe electrolyte solution before closing the pressure vessel.Then, the pressure vessel is closed and the system is pressur-ized with Ar gas to 9 MPa using the pressure regulator. Thepressure vessel is thereafter heated using the hot plate andthe heating tape. As mentioned above, in initial tests, it wasobserved that it is important to uniformly heat up the pres-sure vessel in order to avoid condensation of the electrolyteat colder parts and concomitant corrosion of contacts and cellwalls.

As the temperature increases, the pressure inside thechamber increases. In order to keep the pressure constant, theexhaust valve must frequently be opened to reduce the excesspressure down to the set value of the regulator. The pressureadjustment upon cooling the cell works accordingly.

B. Dynamic determination of reversible hydrogenelectrode potential

The potential of the WE interface is measured as a volt-age difference (E) between WE and RE (Ag-wire). In temper-ature dependent experiments, where the temperature of bothelectrodes is increased, we have to take into account that allelectrochemical interface potentials (φ) exhibit temperaturedependency and that the voltage-temperature relation of eachinterface is a function of the electrode material as well as theelectrolyte composition and concentration. In other words, themeasured temperature dependency of the voltage differencebetween WE and RE originates from both electrodes.

EWE (T ) = φWE (T ) − φRE (T ) . (1)

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085105-4 Wiberg, Fleige, and Arenz Rev. Sci. Instrum. 85, 085105 (2014)

For reason of comparability to other experiments and tomitigate possible instabilities of the silver wire in the volt-age measurement during heating, often it is of advantage torelate the measured potential to a more defined electrochemi-cal reaction, which is measured in situ. In our work presentedhere, we chose the reversible hydrogen reaction. It should bementioned that, as an alternative, we also successfully testedadding a small amount (10−3 M) of silver ions into the REcompartment. The silver ions stabilize the reference poten-tial as it becomes independent to small amounts of Ag ionsformed during the heating procedure. At the same time nocontaminations of silver ions were observed in the main mea-surement compartment as the typical characteristics of Ag de-position on Pt were absent.11 However, this reference poten-tial is inconvenient for data comparison, as it is scarcely usedin electrochemistry.

The reversible potential of the hydrogen reaction dependson the experimental conditions. The two most commonly usedstandard conditions are to measure the potential on a platinumelectrode while bubbling H2-gas or under continuous hydro-gen production by the reference electrode itself using a de-fined current. In the former case, the potential of the referenceelectrode defines the reversible hydrogen electrode (RHE),12

while the latter case defines the dynamic hydrogen electrode(DHE).13 The obvious disadvantage of both the RHE andDHE is the background of hydrogen, which can affect theelectrochemical response of the electrode to be characterized.Additionally, the DHE slightly depends on the applied cur-rent density. Thus, to mitigate these disadvantages, we applieda slightly modified version of the DHE to obtain the RHE,where we only shortly evolved hydrogen to locally saturatethe solution and then switched to open circuit conditions. Thepotential of the platinum electrode after opening the controlcircuit (i.e., no current flows through the electrode) definesthe reversible hydrogen electrode (RHE).

More explicitly, the RHE is measured by first evolvingH2 for 30 s (−5 mA cm−2) and then switching to open cir-cuit, while recording the potential as a function of time. Thisis demonstrated in Fig. 4, where the recorded WE potentialis shown as function of time after opening of the controlcircuit.

As seen in Fig. 4, the WE potential changes initiallyquickly with time, but soon reaches a steady rate of change.

FIG. 4. A chronovoltammogram of the Pt-electrode at open circuit directlyafter evolving hydrogen at −5 mA cm−2 for 15 s.

FIG. 5. The dynamically determined reversible hydrogen electrode potentialversus the potential of the Ag wire as function of temperature. The pressurein the cell was 9 MPa.

The initial change is due to capacitive effects, whereas thesteady change is due to the decreasing concentration of H2close to the electrode surface. The RHE can now be obtainedby a linear extrapolation along the line back to the ordinate att = 0 s.

Using this procedure, we measured the RHE potential vsthe potential of the Ag wire during the heating process of thepressure vessel. The results are shown in Fig. 5.

It can be seen that the ERHE versus the Ag RE in-creases linearly with temperature (0.99 mV K−1 ± 0.04 mVK−1) over a temperature range from 50 to 160 ◦C. On thebasis of equation (1) we can elucidate which of the elec-trodes (RE or WE) contributed most to the observed tem-perature dependency. The well-known RHE can be treatedtheoretically based on the experimental conditions. That is,the RHE in an electrolyte with a pH value of 1.3 is ex-pected to change by 0.58 mV/K versus a non-temperature de-pendent electrode (From the Nernst equation we get: dE

dT=

dE0

dT− ln 10 ∗ R

FpH , where dE0

dTis the temperature change of

the standard potential,19 R the gas constant and F is Fara-day’s constant. dE

dT= 0.84 − 0.26 = 0.58[mV

K].). The temper-

ature dependency of the Ag wire RE can now be estimated to−0.4 mV/K. This value corresponds surprisingly well to thetemperature dependency of EAg/Ag+ under standard conditionswhich is −0.5 mV/K.14

C. CV of PC Pt versus temperature

During the heating process, we performed cyclic voltam-metry (CV) applying different scan rates from 2 Vs−1 to0.1 Vs−1. We thereby adjusted the applied potential windowsuch that it stayed constant versus the RHE. In Fig. 6, a selec-tion of voltammograms recorded at different temperatures isshown.

Here we do not intend to discuss all aspects of the ob-served temperature dependency of the polarization curves, butinstead only describe our observations. At the moment dif-ferent measurement series changing the electrolyte are per-formed, which will be discussed elsewhere.

At room temperature the well-known features of cleanpolycrystalline Pt in H3PO4 were observed.15 No indicationsof major contaminations are visible. As control we conducted

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085105-5 Wiberg, Fleige, and Arenz Rev. Sci. Instrum. 85, 085105 (2014)

FIG. 6. Polarization curves of polycrystalline platinum in 0.5 M H3PO4recorded at different temperatures are shown. Scan rate: 0.3 Vs−1. The pres-sure was, at all times, 9 MPa in a pure Ar atmosphere.

the same experiment with Pt wires as working, reference, andcounter electrode resulting in the same features.

In the potential range between 0 and 0.4 VRHE the fea-tures seen in the CV are attributed to the reversible under-potential deposition of protons (HUPD). By comparison, themeasured current between 0.4 and 0.7 VRHE is purely capaci-tive, i.e., the double layer region is observed. Whereas above,>0.7 VRHE, oxygenated species adsorb and eventually forman oxide. As the temperature increases, it can be seen thatthe HUPD features are diminished, indicating a lower HUPDcoverage.16 At the same time the adsorption of oxygenatedspecies increases, as observed previously by other authorsboth on polycrystalline and single crystal Pt.17, 18

Reaching temperatures above 100 ◦C the signature of theadsorption and reduction of oxygenated species changes andin the positive going scan direction a pre-peak develops atabout 0.6 VRHE. The pre-peak was reversible upon lower-ing the temperature and could indicate enhanced Pt disso-lution at higher temperatures. A more detailed investigationof the temperature dependent electrochemistry will be pub-lished elsewhere, as it is beyond the scope of this journalarticle.

V. CONCLUSION

We present a setup for electrochemical measurements atelevated pressures and temperatures, which allows us to inves-tigate reactions in aqueous electrolytes. We used a flexible de-sign comprised of an external pressure vessel and a glass cellfor the actual electrochemical measurements, which is placedinside the pressure chamber. As no pressure gradient over thecell walls exists, the cell material is not limited to mechan-ically strong materials. Thus, great freedom is given to thecell design. Herein we show an example of a cell design us-ing standard laboratory glass flasks with a modified lid. Oursetup enables easy access to experimental conditions, whichare typically not explored. In particular, we intend to employit for studies related to the relatively new field of high temper-ature proton exchange fuel cells.

ACKNOWLEDGMENTS

The research leading to these results has received fundingfrom the European Union Seventh Framework Programme(FP7/2007-2013) under Grant Agreement No. 309741. Weacknowledge fruitful discussion about the cell design withAlessandro Lavacchi of ICCOM, Italy. We also give thanksto Michael Bernt, NBI workshop, Copenhagen, for the manu-facturing of the designed setup.

1T. J. Schmidt, V. Stamenkovic, M. Arenz, N. M. Markovic, and P. N. Ross,Electrochim. Acta 47, 3765 (2002).

2H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner, Appl. Catal.B Environ. 56, 9 (2005).

3K. J. J. Mayrhofer, K. Hartl, V. Juhart, and M. Arenz, J. Am. Chem. Soc.131, 16348 (2009).

4G. G. Wildgoose, D. Giovanelli, N. S. Lawrence, and R. G. Compton, Elec-troanalysis 16, 421 (2004).

5A. Camargo, B. Previdello, H. Varela, and E. Gonzalez, Quim. Nova33(10), 2143 (2010).

6K. J. J. Mayrhofer, S. J. Ashton, J. Kreuzer, and M. Arenz, Int. J. Elec-trochem. Sci. 4, 1 (2009).

7D. G. de Jones and H. G. Masterson, in Advances Corrosion Science Tech-nology, edited by M. G. Fontana and R. W. Staehle (Springer US, Boston,MA, 1970), pp. 1–49.

8V. M. Tsionskii and L. B. Kriksunov, J. Electroanal. Chem. Interfacial Elec-trochem. 204, 131 (1986).

9See supplementary material at http://dx.doi.org/10.1063/1.4890826 for billof material.

10G. K. H. Wiberg, Ph.D. thesis, Technische Universität München, Munich,2010; available at http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:91-diss-20101008-993285-1-2.

11L. H. Mascaro, M. C. Santos, S. A. S. Machado, and L. A. Avaca, J. Chem.Soc. Faraday Trans. 93, 3999 (1997).

12D. A. MacInnes and L. Adler, Proc. Natl. Acad. Sci. U.S.A. 5, 160 (1919).13J. Giner, J. Electrochem. Soc. 111, 376 (1964).14A. Lewenstam and F. Scholz, Handbook of Reference Electrodes (Springer,

Berlin, 2013).15S. J. Clouser, J. C. Huang, and E. Yeager, J. Appl. Electrochem. 23, 597

(1993).16W. Vogel and J. Baris, Electrochim. Acta 23, 463 (1978).17J. L. Cohen, D. J. Volpe, and H. D. Abruña, Phys. Chem. Chem. Phys. 9,

49 (2007).18N. Garcia-Araez, V. Climent, and J. M. Feliu, Russ. J. Electrochem. 48, 271

(2012).19B. E. Conway, H. Angerstein-Kozlowska, and W. B. A. Sharp, J. Chem.

Soc., Faraday Trans. 1 74, 1373 (1978).

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