gtag('config', 'UA-114241270-1');
Your browser is out-of-date!

Update your browser to view this website correctly. Update my browser now

×

Discipline
Chemical
Keywords
Formic Acid
Voltage Inversion
Fuel Cell
Observation Type
Standalone
Nature
Standard Data
Submitted
Mar 3rd, 2017
Published
Jun 5th, 2017
  • Abstract

    Fuel cells are electrochemical devices that convert chemical into electrical energy with comparatively high efficiency. In this report, we present the occurrence of inversion in the cell voltage that accompanies the self-organized voltage oscillations in a fuel cell fed with formic acid and oxygen, the so-called direct Direct Formic Acid Fuel Cell (DFAFC).

  • Figure
  • Introduction

    Oscillations in electrocatalytic reactions have a long history in electrochemistry, and example include many oxidation reactions which are relevant in the interconversion between chemical and electrical energies. Beyond half-cell, laboratory experiments, kinetic instabilities have been also found in polymer electrolyte membrane fuel cells (PEMFC) fed with CO contaminated H2 and, more recently, methanol and formic acid. Under oscillatory regime, most systems are expected to result in higher efficiency. Motivated by the recent reports of oscillatory kinetics in fuel cell fed directly with carbon-containing fuels, we have investigated the occurrence of voltage oscillations in a Direct Formic Acid Fuel Cell (DFAFC). Herein we report the inversion of voltage that results of the emergence of self-organized oscillations in a DFAFC.

  • Objective

    The current study aims at explaining the voltage inversion that follows the kinetic instabilities of formic acid electro-oxidation in a Direct Formic Acid Fuel Cell.

  • Results & Discussion

    The dynamic behavior of the DFAFC was initially studied via slow potential and current sweeps, and characteristic results are presented in figure 1A. When the potential is the control parameter, a typical stationary response is reached, and the cell voltage decreases monotonically with the increase of the current. However, under galvanostatic control, the DFAFC presented voltage oscillations above a certain current threshold, has been already reported for the cell fed with H2/CO and directly with methanol and formic acid. Additionally, it is observed in figure 1A that the oscillations during the electro-oxidation of formic acid start at approximately 0 V, have an amplitude of about 0.6 V and that an inversion in the cell voltage occurs. To better assess the dynamics of these oscillations, it was selected a current value where the system presents kinetic instability, 0.076 A cm-2, and a chronopotentiometry was performed to evaluate how the voltage evolves over time, figure 1B. It can be seen that between about 0.13 and -0.05 V there is a slow poisoning of the anode electrode due to carbon monoxide adsorbed (COad) from the dehydration of weakly adsorbed formic acid. Another stable intermediate that also contributes to the loss of electrode activity is the bridged-bonded formate (HCOOad) which, like COad, inhibits the direct oxidation pathway of formic acid. Eventually the electrode reaches a critical coverage of catalytic poisons and, in order to keep the current constant, the anode overpotential increases (therefore the cell voltage decreases) leading to the formation of oxygenated species on platinum. These species react with COad via the Langmuir-Hinshelwood mechanism, releasing free sites which causes the anodic overpotential to decrease and the new cycle begins again.

    The emergence of oscillatory kinetics during the electro-oxidation of formic acid has been extensively studied with the aid of techniques such as infrared spectroscopy and numerical modeling. Mota et al. have showed kinetic instabilities in a DFAFC using H2 on the cathode instead of O2. An intriguing aspect to be understood in this contribution consists of the inversion in the DFAFC voltage observed during oscillations. This fact adds a difficulty in studying this system, since cells operating at negative voltages results in degradation of the carbon support contained in the catalytic layer as well as in the diffusion layer. Lopes et al. observed this polarity inversion during the oscillatory dynamics in a PEMFC with PdPt/C anodic catalyst. They assumed that this phenomenon is caused by the abrupt increase of the anodic overpotential due to the dynamic of the oscillations with H2/CO. Nevertheless, this voltage inversion may be better understood as follows.

    Figure 1C shows a schematic description of the current-potential curves for the electro-oxidation of formic acid (anodic reaction), and for the oxygen reduction (cathodic reaction). The amplitude of the potential oscillations are comparable to that found in half-cell experiments. In the present case, i.e. a DFAFC fed with oxygen in the cathode, the sluggish kinetic of the cathodic reaction and its consequent huge overpotential, contributes to the voltage inversion. In fact, as presented in figure 1C, the overpotential (Ecell in Fig. 1A) remains positive for lower applied currents and when potential oscillations set in, the higher current required causes a higher overpotential in both electrodes, that is, anode and cathode, and consequently the voltage inversion.

  • Conclusions

    Herein we communicate the occurrence of voltage inversion that follows the oscillatory dynamics in a low temperature fuel cell fed with formic acid (anode) and oxygen (cathode). The voltage inversion was suggested to result of the sluggish kinetics in the oxygen reduction reaction that considerably reduces the potential at the cathode. The present finding adds to the understanding of the individual contributions of anodic and cathodic reactions in a direct liquid fuel cell operating under oscillatory regime. This reasoning is crucial for the exploration of oscillatory instabilities to, for instance, improve the overall performance of PEM reactors and other practical systems.

  • Methods

    The methodology used for the preparation of the membrane-electrodes assembly (MEA) is available elsewhere. Commercial Pt/C 50 wt.% (Alfa Aesar) was used as both anodic and cathodic electrocatalyst, with metal loading of 1 mgPt cm-2 in both cases. The MEA was prepared with a Nafion 115 membrane (electrolyte) and had a geometric area of approximately 4.6 cm-2. The flow of fuel in the anode was controlled by a Watson Marlow 520Di peristaltic pump, the flow was fixed in 2 mL min-1, and the experiments were carried out at 30°C. The unit cell was fed with an aqueous solution of 8 mol L-1 of formic acid (Panreac, 98%) at the anode and with excess of either pure oxygen or hydrogen at atmospheric pressure at the cathode. The electrochemical experiments were performed with an Autolab/Eco Chemie PGSTAT320N potentiostat.

  • Funding statement

    J.A.N. and H.V. acknowledge São Paulo Research Foundation (FAPESP) for the scholarship (grant #2015/09295-9) and financial support (grants #2012/21204-0, and #2013/16930-7). H.V. (#306151/2010-3) acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.

  • Acknowledgements

    We thank Dr. Valdecir Paganin for technical assistance.

  • Ethics statement

    Not Applicable.

  • References
  • 1
    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum
    2
    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum
    3
    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum
    4
    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum
    5
    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum ipsum

    Lorem ipsum Lorem ipsum Lorem ipsum
    Matters7.5/20

    Voltage Inversion Caused by Self-organized Oscillations in a Direct Formic Acid Fuel Cell

    Affiliation listing not available.
    Abstractlink

    Fuel cells are electrochemical devices that convert chemical into electrical energy with comparatively high efficiency. In this report, we present the occurrence of inversion in the cell voltage that accompanies the self-organized voltage oscillations in a fuel cell fed with formic acid and oxygen, the so-called direct Direct Formic Acid Fuel Cell (DFAFC).

    Figurelink

    Figure 1.

    (A) Polarization curve of the DFAFC recorded at 1 mV s-1 (black curve) and galvanodynamic sweep at 17 µA cm-2 s-1 (red curve).

    (B) Cell voltage time-series during galvanostatic (i = 0.076 Acm-2) operation of the DFAFC.

    (C) Schematic current-potential curves for formic acid oxidation in DFAFC obtained by either a current (red curve) or potential (black curve) sweep, and for oxygen reduction (blue curve). The curves for formic acid oxidation were obtained using H2 in the cathode instead of O2, which allows to read directly the anodic overpotential.

    Introductionlink

    Oscillations in electrocatalytic reactions have a long history in electrochemistry[1][2], and example include many oxidation reactions which are relevant in the interconversion between chemical and electrical energies. Beyond half-cell, laboratory experiments, kinetic instabilities have been also found in polymer electrolyte membrane fuel cells (PEMFC) fed with CO contaminated H2[3][4][5] and, more recently, methanol[6] and formic acid[7]. Under oscillatory regime, most systems are expected to result in higher efficiency[2][3][4][8][9]. Motivated by the recent reports of oscillatory kinetics in fuel cell fed directly with carbon-containing fuels[6][7], we have investigated the occurrence of voltage oscillations in a Direct Formic Acid Fuel Cell (DFAFC). Herein we report the inversion of voltage that results of the emergence of self-organized oscillations in a DFAFC.

    Objectivelink

    The current study aims at explaining the voltage inversion that follows the kinetic instabilities of formic acid electro-oxidation in a Direct Formic Acid Fuel Cell.

    Results & Discussionlink

    The dynamic behavior of the DFAFC was initially studied via slow potential and current sweeps, and characteristic results are presented in figure 1A. When the potential is the control parameter, a typical stationary response is reached, and the cell voltage decreases monotonically with the increase of the current. However, under galvanostatic control, the DFAFC presented voltage oscillations above a certain current threshold, has been already reported for the cell fed with H2/CO[3][4][5][8][9] and directly with methanol[6] and formic acid[7]. Additionally, it is observed in figure 1A that the oscillations during the electro-oxidation of formic acid start at approximately 0 V, have an amplitude of about 0.6 V and that an inversion in the cell voltage occurs. To better assess the dynamics of these oscillations, it was selected a current value where the system presents kinetic instability, 0.076 A cm-2, and a chronopotentiometry was performed to evaluate how the voltage evolves over time, figure 1B. It can be seen that between about 0.13 and -0.05 V there is a slow poisoning of the anode electrode due to carbon monoxide adsorbed (COad) from the dehydration of weakly adsorbed formic acid[10]. Another stable intermediate that also contributes to the loss of electrode activity is the bridged-bonded formate (HCOOad) which, like COad, inhibits the direct oxidation pathway of formic acid[10][11]. Eventually the electrode reaches a critical coverage of catalytic poisons and, in order to keep the current constant, the anode overpotential increases (therefore the cell voltage decreases) leading to the formation of oxygenated species on platinum. These species react with COad via the Langmuir-Hinshelwood mechanism, releasing free sites which causes the anodic overpotential to decrease and the new cycle begins again[9].

    The emergence of oscillatory kinetics during the electro-oxidation of formic acid has been extensively studied with the aid of techniques such as infrared spectroscopy[12] and numerical modeling[13][14]. Mota et al.[15] have showed kinetic instabilities in a DFAFC using H2 on the cathode instead of O2. An intriguing aspect to be understood in this contribution consists of the inversion in the DFAFC voltage observed during oscillations. This fact adds a difficulty in studying this system, since cells operating at negative voltages results in degradation of the carbon support contained in the catalytic layer as well as in the diffusion layer. Lopes et al.[8] observed this polarity inversion during the oscillatory dynamics in a PEMFC with PdPt/C anodic catalyst. They assumed that this phenomenon is caused by the abrupt increase of the anodic overpotential due to the dynamic of the oscillations with H2/CO. Nevertheless, this voltage inversion may be better understood as follows.

    Figure 1C shows a schematic description of the current-potential curves for the electro-oxidation of formic acid (anodic reaction), and for the oxygen reduction (cathodic reaction). The amplitude of the potential oscillations are comparable to that found in half-cell experiments. In the present case, i.e. a DFAFC fed with oxygen in the cathode, the sluggish kinetic of the cathodic reaction[16] and its consequent huge overpotential, contributes to the voltage inversion. In fact, as presented in figure 1C, the overpotential (Ecell in Fig. 1A) remains positive for lower applied currents and when potential oscillations set in, the higher current required causes a higher overpotential in both electrodes, that is, anode and cathode, and consequently the voltage inversion.

    Conclusionslink

    Herein we communicate the occurrence of voltage inversion that follows the oscillatory dynamics in a low temperature fuel cell fed with formic acid (anode) and oxygen (cathode). The voltage inversion was suggested to result of the sluggish kinetics in the oxygen reduction reaction that considerably reduces the potential at the cathode. The present finding adds to the understanding of the individual contributions of anodic and cathodic reactions in a direct liquid fuel cell operating under oscillatory regime. This reasoning is crucial for the exploration of oscillatory instabilities to, for instance, improve the overall performance of PEM reactors and other practical systems.

    Methodslink

    The methodology used for the preparation of the membrane-electrodes assembly (MEA) is available elsewhere[17]. Commercial Pt/C 50 wt.% (Alfa Aesar) was used as both anodic and cathodic electrocatalyst, with metal loading of 1 mgPt cm-2 in both cases. The MEA was prepared with a Nafion 115 membrane (electrolyte) and had a geometric area of approximately 4.6 cm-2. The flow of fuel in the anode was controlled by a Watson Marlow 520Di peristaltic pump, the flow was fixed in 2 mL min-1, and the experiments were carried out at 30°C. The unit cell was fed with an aqueous solution of 8 mol L-1 of formic acid (Panreac, 98%) at the anode and with excess of either pure oxygen or hydrogen at atmospheric pressure at the cathode. The electrochemical experiments were performed with an Autolab/Eco Chemie PGSTAT320N potentiostat.

    Funding Statementlink

    J.A.N. and H.V. acknowledge São Paulo Research Foundation (FAPESP) for the scholarship (grant #2015/09295-9) and financial support (grants #2012/21204-0, and #2013/16930-7). H.V. (#306151/2010-3) acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.

    Acknowledgementslink

    We thank Dr. Valdecir Paganin for technical assistance.

    Conflict of interestlink

    The authors declare no conflicts of interest.

    Ethics Statementlink

    Not Applicable.

    No fraudulence is committed in performing these experiments or during processing of the data. We understand that in the case of fraudulence, the study can be retracted by ScienceMatters.

    Referenceslink
    1. M. Hachkar, B. Beden, C. Lamy
      Oscillating electrocatalytic systems: Part I. Survey of systems involving the oxidation of organics and detailed electrochemical investigation of formaldehyde oxidation on rhodium electrodes
      Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 287/1990, pages 81-98 DOI: 10.1016/0022-0728(90)87161-cchrome_reader_mode
    2. Krewer Ulrike, Vidakovic-Koch Tanja, Rihko-Struckmann Liisa
      Electrochemical Oxidation of Carbon-Containing Fuels and Their Dynamics in Low-Temperature Fuel Cells
      ChemPhysChem, 12/2011, pages 2518-2544 DOI: 10.1002/cphc.201100095chrome_reader_mode
    3. Mota Andressa, Eiswirth Markus, Gonzalez Ernesto R.
      Enhanced Efficiency of CO-Containing Hydrogen Electroxidation with Autonomous Oscillations
      The Journal of Physical Chemistry C, 117/2013, pages 12495-12501 DOI: 10.1021/jp311185cchrome_reader_mode
    4. Richard Hanke-Rauschenbach, Michael Mangold, Kai Sundmacher
      Nonlinear dynamics of fuel cells: a review
      Reviews in Chemical Engineering, 27/2011, pages 23-52 DOI: 10.1515/revce.2011.001chrome_reader_mode
    5. Kirsch S., Hanke-Rauschenbach R., Stein B.,more_horiz, Sundmacher K.
      The Electro-Oxidation of H2, CO in a Model PEM Fuel Cell: Oscillations, Chaos, Pulses
      Journal of the Electrochemical Society, 160/2013, pages F436-F446 DOI: 10.1149/2.002306jeschrome_reader_mode
    6. Jéssica A. Nogueira, Ivonne K. Peña Arias, Richard Hanke-Rauschenbach,more_horiz, Kai Sundmacher
      Autonomous Voltage Oscillations in a Direct Methanol Fuel Cell
      Electrochimica Acta, 212/2016, pages 545-552 DOI: 10.1016/j.electacta.2016.07.050chrome_reader_mode
    7. Andressa Mota-Lima, Djalma R. Silva, Luiz H.S. Gasparotto, Ernesto R. Gonzalez
      Stationary and Damped Oscillations in a Direct Formic Acid Fuel Cell (DFAFC) using Pt/C
      Electrochimica Acta, 235/2017, pages 135-142 DOI: 10.1016/j.electacta.2017.03.056chrome_reader_mode
    8. Lopes Pietro P., Ticianelli Edson A., Varela Hamilton
      Potential oscillations in a proton exchange membrane fuel cell with a Pd–Pt/C anode
      Journal of Power Sources, 196/2011, pages 84-89 DOI: 10.1016/j.jpowsour.2010.07.034chrome_reader_mode
    9. Zhang Jingxin, Fehribach Joseph D., Datta Ravindra
      Mechanistic and Bifurcation Analysis of Anode Potential Oscillations in PEMFCs with CO in Anode Feed
      Journal of The Electrochemical Society, 151/2004, page A689 DOI: 10.1149/1.1688795chrome_reader_mode
    10. Chen Yan Xia, Heinen Martin, Jusys Zenonas, Behm Rolf Jürgen
      Kinetics and Mechanism of the Electrooxidation of Formic Acid-Spectroelectrochemical Studies in a Flow Cell
      Angewandte Chemie International Edition, 45/2006, pages 981-985 DOI: 10.1002/anie.200502172chrome_reader_mode
    11. Samjeské Gabor, Osawa Masatoshi
      Current Oscillations during Formic Acid Oxidation on a Pt Electrode: Insight into the Mechanism by Time-Resolved IR Spectroscopy
      Angewandte Chemie International Edition, 44/2005, pages 5694-5698 DOI: 10.1002/anie.200501009chrome_reader_mode
    12. Samjeské Gabor, Miki Atsushi, Ye Shen,more_horiz, Osawa Masatoshi
      Potential Oscillations in Galvanostatic Electrooxidation of Formic Acid on Platinum:  A Time-Resolved Surface-Enhanced Infrared Study
      The Journal of Physical Chemistry B, 109/2005, pages 23509-23516 DOI: 10.1021/jp055220jchrome_reader_mode
    13. Mukouyama Yoshiharu, Kikuchi Mitsunobu, Samjeské Gabor,more_horiz, Okamoto Hiroshi
      Potential Oscillations in Galvanostatic Electrooxidation of Formic Acid on Platinum:  A Mathematical Modeling and Simulation
      The Journal of Physical Chemistry B, 110/2006, pages 11912-11917 DOI: 10.1021/jp061129jchrome_reader_mode
    14. Perini Nickson, Batista Bruno C., Angelo Antonio C. D.,more_horiz, Varela Hamilton
      Long-Lasting Oscillations in the Electro-Oxidation of Formic Acid on PtSn Intermetallic Surfaces
      ChemPhysChem, 15/2014, pages 1753-1760 DOI: 10.1002/cphc.201301186chrome_reader_mode
    15. Andressa Motaa, Ernesto Rafael Gonzaleza
      Enhanced Efficiency with Autonomous Oscillations: Challenges for DAFC
      ECS Transactions, 58/2013, pages 1879-1884 DOI: 10.1149/05801.1879ecstchrome_reader_mode
    16. Shao Minhua, Chang Qiaowan, Dodelet Jean-Pol, Chenitz Regis
      Recent Advances in Electrocatalysts for Oxygen Reduction Reaction
      Chemical Reviews, 116/2016, pages 3594-3657 DOI: 10.1021/acs.chemrev.5b00462chrome_reader_mode
    17. V. A. Paganin, E. A. Ticianelli, E. R. Gonzalez
      Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells
      Journal of Applied Electrochemistry, 26/1996, pages 297-304 DOI: 10.1007/bf00242099chrome_reader_mode
    Commentslink

    Create a Matters account to leave a comment.