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Bipolar electrochemistry for concurrently evaluating the stability of anode and cathode electrocatalysts and the overall cell performance during long-term water electrolysis Vera Essmann, Stefan Barwe, Justus Masa, and Wolfgang Schuhmann Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02393 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016
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Analytical Chemistry
Bipolar electrochemistry for concurrently evaluating the stability of anode and cathode electrocatalysts and the overall cell performance during long-term water electrolysis Vera Eßmann, Stefan Barwe, Justus Masa, Wolfgang Schuhmann* Analytical Chemistry - Center for Electrochemical Sciences (CES); Ruhr-Universität Bochum, Universitätsstr. 150; 44780 Bochum; Germany, *Fax: + 49 234 321 4683, mail:
[email protected] ABSTRACT: Electrochemical efficiency and stability are among the most important characteristics of electrocatalysts. These parameters are usually evaluated separately for the anodic and cathodic half-cell reactions in a three-electrode system or by measuring the overall cell voltage between the anode and cathode as a function of current or time. Here, we demonstrate how bipolar electrochemistry can be exploited to evaluate the efficiency of electrocatalysts for full electrochemical water splitting while simultaneously and independently monitoring the individual performance and stability of the half-cell electrocatalysts. Using a closed bipolar electrochemistry setup, all important parameters such as overvoltage, half-cell potential, and catalyst stability can be derived from a single galvanostatic experiment. In the proposed experiment, none of the half-reactions is limiting on the other, making it possible to precisely monitor the contribution of the individual half-cell reactions on the durability of the cell performance. The proposed approach was successfully employed to investigate the long-term performance of a bifunctional water splitting catalyst, specifically amorphous cobalt boride (Co2B), and the durability of the electrocatalyst at the anode and cathode during water electrolysis. Additionally, by periodically alternating the polarization applied to the bipolar electrode (BE) modified with a bifunctional oxygen electrocatalyst, it was possible to explicitly follow the contributions of the oxygen reduction (ORR) and the oxygen evolution (OER) half-reactions on the overall long-term durability of the bifunctional OER/ORR electrocatalyst. Introduction The design of efficient, stable, low-cost, abundant, and scalable catalysts is essential for many reactions in electrochemical energy storage, conversion and supply. Novel electrocatalysts have to be rigorously characterized. The chemical structure and thus structure-activity correlations as well as mechanistic insights are usually obtained by a combination of ex situ and in situ characterization techniques using e.g. XPS, Raman spectroscopy, and X-ray absorption spectroscopy (EXAFS and XANES) among others. SEM and TEM provide information about the morphology, whereas crystallinity is determined via XRD. Certainly, proper assessment of the electrochemical performance of a new catalyst is as important as understanding its physico-chemical properties. In most cases, linear sweep voltammograms (LSVs) are recorded to determine the overpotential required to attain or deliver a specific current density for a given catalyzed reaction.1-3 However, catalytic activity is also associated with stability, and as a matter of fact, despite most research presently is aiming on finding electrocatalysts with decreased overpotentials, long-term stability is not of secondary importance. Typically, the long-term performance of an electrocatalyst is investigated by galvanostatic or chronoamperometric measurements in a three-electrode setup where the reaction at the counter electrode is presumed to be non-limiting.4-7 Alternatively, the anode and cathode are modified with a catalyst and deployed in a two-electrode cell, where the cell voltage is recorded as a function of current or time. In this
case, the measured cell voltage constitutes of the sum of the anodic and cathodic overpotentials and the potential drop across the liquid or membrane electrolyte.8,9 However, the limiting reaction cannot be determined following this approach. We demonstrate how to concurrently monitor the long-term activity of the anodic and cathodic half-cell reactions in parallel to their overall performance during electrolysis using bipolar electrochemistry. Stimulated by the application of a constant electric field between two feeder electrodes, BPE invokes oxidation and reduction reactions at the opposite poles of a conductive object in solution. Compared to conventional electrochemical techniques, this method has several advantages, which have been utilized in various fields of electrochemistry.10,11 The fact that the conductive object, i.e. the bipolar electrode (BE), does not have a direct electrical connection facilitates the use of microscopic and even nanoscopic objects.12,13 Since oxidation at the anodic pole and reduction at the cathodic pole of the BE occur simultaneously and to the same extent, an optical readout reaction may be applied to draw conclusions on the bipolar current and therefore on a redox-active analyte.14-16 Moreover, the gradient in interfacial potential difference along the BE was used for its heterogeneous modification or the analysis of corrosion gradients.17-21 Moreover, BPE has previously been applied for the characterization of electrocatalysts.14,22 The activity of an ORR catalyst can for instance be correlated with the amount of anodically dissolved Ag.23 Fosdick et al. observed that the bet1
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ter the catalyst, the higher the activity at a given applied electric field and thus, the more Ag is oxidized. However, these approaches only allow obtaining qualitative comparison of different catalysts or compositions, but not quantitative information about the actual performance, such as the overpotential at a certain current density or the overvoltage between oxidation and reduction reaction at a specific current density. In this study, a closed bipolar electrochemistry (BPE) setup is used with the two extremities of the bipolar electrode being physically separated (Scheme 1).
Scheme 1. Schematic of the experimental setup for independently assessing the stability of a catalyst at the anodic and cathodic pole of a BE connected to a full electrolysis system. RE = reference electrode, BEC = cathodic pole of the bipolar electrode, BEA = anodic pole. The solution and rotation speed depend on the respective reaction. Two rotating disk electrodes (RDE) modified with the respective catalyst of interest are placed in two separate vessels and connected to a BE. A constant current is applied between two feeder electrodes, each placed in one of the vessels. Since the BE is the only connection between the two feeder electrodes in this closed bipolar configuration, the current through the BE is equivalent to the applied current and can thus be adjusted to a desired current density. This concept has already been applied for bipolar sensors and in fundamental studies.24,25 In our approach, the interfacial potential difference at the opposite anodic and cathodic BE extremities, BEA and BEC, are continuously monitored using two reference electrodes (RE). Importantly, the overpotential is indicative for the catalyst’s activity at the respective pole and does not depend on the electrochemical performance of the counter extremity. With this method, we concurrently measured the overpotentials required for water oxidation and reduction at a bifunctional water splitting catalyst, namely Co2B.8 In addition, the absolute overvoltage for water electrolysis can be evaluated simultaneously without considering any additional contributions such as e.g. a membrane resistance. Furthermore, the reversibility of a bifunctional, non-precious catalyst for reversible oxygen electrodes6 was investigated by periodically alternating the current direction and consequently the polarization of the BE. It is also possible to analyze the effect of different solutions on the oxidation and reduction reaction respectively, or to use electrode geometries other than RDEs.
Experimental Section
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Chemicals. Potassium hydroxide was purchased from Roth (Karlsruhe, D), ethanol and Nafion® perfluorinated resin solution from Sigma-Aldrich (Steinheim, D). All chemicals were used as received. Solutions were prepared with ultrapure water (SG water, Hamburg, D). The synthesis and characterization of the catalysts, Co2B and Mn-Co oxide catalysts partially embedded in N-doped CNTs (designated as NCNT), has been published previously.6,8 Conventional electrochemical characterization. For all measurements with the Co2B catalyst, glassy carbon (GC) rods (Ø 3 mm) were used as working, and later on as bipolar electrodes. Prior to the measurements, the surface was polished on 3 µm and 1 µm lapping film (3M, Neuss, D), thoroughly rinsed with ultrapure water and laterally insulated with Teflon tape.26 Measurements with the M/NCNT-catalyst were carried out using Teflon-sealed GC-electrodes (Ø 3.8 mm) that were pretreated in the same way. Afterwards, the electrodes were modified with 0.21 mg mL-1 of the respective catalyst (5 mg mL-1 stock solution of 49% ultrapure water, 49% ethanol and 2% Nafion®) and let to dry at room temperature for 20 min. Before collecting any activity data, the Co2B-modified electrodes were cycled at a scan rate of 0.1 V s-1 from 0.53 to -0.57 V vs. RHE for the cathodic pole and from 1.0 to 2.0 V vs. RHE for the anodic pole. The experiments were performed in Ar-saturated 1 M KOH as electrolyte and at a rotation speed of 1600 rpm to avoid accumulation of gas bubbles at the electrode surface. For investigation of the hydrogen evolution reaction (HER) in Ar-saturated 0.5 M H2SO4, the potential was cycled from 0.27 to -0.87 V. In the case of the M/NCNT-catalyst, conditioning was performed at 0.1 V s-1 from 0.97 to -0.02 V vs. RHE for the cathodic pole in O2-saturated 0.1 M KOH and from 1.0 to 2.0 V vs. RHE for the anodic pole in Ar-saturated 0.1 M KOH at a rotation speed of 900 rpm until reproducible voltammograms were obtained. LSVs for each reaction were recorded in the same solution and in the same potential range at a scan rate of 0.005 V s-1. All three-electrode measurements were carried out using a Ag/AgCl (3 M KCl) reference electrodes and stainless steel plates as counter electrodes, which were used as feeder electrodes in the related bipolar measurements. BPE measurements. The setup for galvanostatic BPE was assembled as shown in Scheme 1 using the same solutions as for the conventional electrochemical catalyst characterization. For the bifunctional water-splitting catalyst Co2B, a current of 0.72 mA was applied to the stainless steel feeder electrodes corresponding to a current density of 10 mA cm-2 at the BE, the two connected GC-RDEs. The local interfacial potential difference between the BE and the solution at BEA and BEC were recorded over several hours using two multimeters (CEM instruments, Bremen, D) and two Ag/AgCl (3 M KCl) reference electrodes, that were positioned close to the RDEs. LSVs were recorded after the electrochemical conditioning procedure, in between, and after the bipolar measurements, as described above. For the analysis of the bifunctional oxygen catalyst, a current of 0.072 mA was applied to the feeder electrodes, which corresponds to a current density of 1 mA cm-2 at the BE. The 0.1 M KOH solution at the BEC was saturat2
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Analytical Chemistry
ed with O2. Investigating the catalyst reversibility was realized by alternating the feeder current between 0.072 mA and -0.072 mA after each 30 min. In this case, both solutions were purged with O2. Also here, LSVs were recorded once in between and after the measurement.
Results and Discussion To investigate the long-term activity of both reactions of a bifunctional water-splitting catalyst, namely amorphous cobalt boride (Co2B), and simultaneously the overall electrolytic performance, a closed bipolar system featuring two GC-RDEs modified with the said model
catalyst was set up as shown in Scheme 1. For all experiments discussed hereafter, the catalyst on both poles was initially activated by cyclic voltammetry, employing the stainless steel feeder electrodes as counter electrodes. In order to evaluate the activity in the conventional three-electrode configuration as reference for comparison with the bipolar results, polarization curves for the two poles were recorded before, in between and after the bipolar measurements. In this study, we investigated the electrocatalytic performance of Co2B for the H2 evolution reaction (HER) and the O2 evolution reaction (OER) at a current density of 10 mA cm-2 in 1 M KOH.8
Figure 1. Stability assessment of the water-splitting catalyst, Co2B. a) Measurement of the interfacial potential difference at BEA (OER reaction, red) and at BEC (HER reaction, black) for approx. 14 h with both poles modified with Co2B-500 and rotating at 1600 rpm in 1 M KOH. 3-electrode LSVs at 0.005 V s-1 in the same solution before the bipolar measurement (black), after half of the time (red) and afterwards (blue) for both poles (b) HER and c) OER). From the polarization curves, it can be seen that HER at the electrode intended to serve as the cathodic extremity of the BE attains a current density of -10 mA cm-2 at a potential of about -0.5 V vs. RHE (Figure 1b, black). At the other RDE where the OER occurs, 10 mA cm-2 is reached at approx. 1.6 V vs. RHE (Figure 1c, black). Finally, both RDEs were connected to form the BE and a constant current was applied to the feeder electrodes. The same current has to flow through the BE as this is the only connection between the two feeder electrode compartments. With identical surface areas of the two BE extremities, the current density at both poles is equal and set to be 10 mA cm-2. Depending on the reactions at the feeder electrodes, the ones at the BE, the potential drop at the interfaces and in solution, the voltage between the feeder electrodes continuously changes to satisfy the galvanostatically demanded current (Figure S 1). In order to understand which potential drop occurs in which part of the cell, all potential differences and voltages have once been measured exemplarily during galvanostatic BPE (Scheme S 1). The sum of all measured potential differences in the cell is equal to the overall applied voltage between the feeder electrodes. However, only the interfacial potential differences between the BE and the two solutions (∆EBE-Sol) are of interest, which are constantly monitored with two reference electrodes positioned in close proximity to the RDEs. The potential of the BE is determined by the solution potentials it is exposed to and adjusts to one where anodic and cathodic bipolar currents are equal (iC = iA). At the beginning of the galvanostatic bipolar experiment, the potential difference at BEC was measured to be -0.49 V and at the BEA 1.60 V vs. RHE, so almost
exactly the same values as those determined from the LSVs. After 7 h and 14 h of continuous polarization the potentials have changed to -0.26 V and 1.62 V, and -0.24 V and 1.62 V vs. RHE, respectively, whereas the LSV showed potential values of -0.27 V and 1.61 V, and -0.24 V and 1.64 V vs. RHE, respectively. Also the overvoltage difference between OER and HER can be derived from this experiment by simply summing up the individual measured overpotentials, in this case approx. 1.88 V. To proof that the measured potential difference ∆EBE-Sol only depends on the activity of the catalyst at the respective pole control experiments were performed. During a bipolar measurement, the catalyst layer of one RDE was deliberately removed resulting in a sudden increase in the applied voltage as well as in the overpotential at this particular pole while the overpotential at the opposite end of the BE remained unchanged. This means that measuring the potential difference for each pole during galvanostatic BPE is as precise as determining the potential for a catalytic reaction by means of LSV. The closed BPE configuration can be imagined as two electrolysis cells where the currents at the feeder anode and BEC and at the feeder cathode and BEA, respectively, have to be equal. However, in this configuration, the potentials of BEC and BEA are controlled by the potential of the feeder electrodes via the solution potential and not directly by a power supply as in conventional electrolysis. Consequently, when the catalyst at one BE extremity loses activity, the potential of the feeder electrode in that “half-cell” has to change and thereby the solution potential as well as ∆EBE-Sol are modulated. ∆EBE-Sol in the other “half-cell” is unaffected, because the potential of the BE (EBE) remains unchanged as iC = iA must remain valid. 3
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From this experiment, it can be seen that the anodically polarized Co2B catalyst slowly loses activity over time as a consequence of a decline in stability. In contrast, the Co2B-catalyzed HER reaction improves with time. Surface oxides naturally envelop amorphous metal boride particles in a core-shell structure27-30 which are not suitable for HER catalysis being favored on an oxide-free metal surface. In the course of the bipolar experiment, the oxide film is continuously reduced at the cathodic BE extremity with concomitant improvement of the water reduction activity. Since the poles of the BE are physically separated, the influence of different solvents on the individual half-cell reactions may be analyzed independently. On this basis, the effect of an acidic pH on the HER was studied while pH 14 was maintained for OER. Whereas the OER occurred at an overpotential of ≈1.60 V vs. RHE, proton reduction required significantly higher overpotentials in more acidic solution (see Figure S 2). At the beginning of the HER measurements, ∆EBEC -Sol was determined to be -0.65 V vs. RHE as compared to -0.49 V in 1 M KOH. After 7 h, the catalyst performance improved and the reaction occurred at -0.53 V vs. RHE. After the bipolar measurement, the interfacial potential difference at the cathodic BE pole was -0.44 V vs. RHE and hence 0.2 V more negative than in alkaline solution. This is consistent with previous observations that transition metal electrocatalysts perform better in alkaline media.31 Only very few materials, typically involving molybdenum, show good activity under acidic conditions.32 For some applications, for example in metal-air batteries33,34 and reversible fuel cell systems35 bifunctionality and reversibility of electrocatalysts for the ORR and OER are key requirements. Both metal-air batteries and regenerative fuel cells employ a reversible oxygen electrode. Here, the catalyst has to be efficient for ORR during the discharging process and for OER during the charging process. We employed the proposed closed galvanostatic BPE technique to investigate the stability of a previously reported bifunctional catalyst in a reversible oxygen electrode, comprised of Mn-Co spinel
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oxides partially embedded in nitrogen-doped carbon nanotubes6, hereafter denoted as M/NCNT. Both GC-RDEs were modified with the M/NCNT catalyst and evaluated for long-term stability by periodically reversing the polarity of the poles (current direction), between the OER (1 mA cm-2) and the ORR (-1 mA cm-2). BPE was started with the same polarization as the preceding LSVs. The current direction and thus the polarization of the BE was reversed every 30 min then causing the ORR to occur at the previous anodic pole and vice versa (applied feeder voltage vs. time shown in Figure S 3). The potential difference shown in orange in Figure 2a always corresponds to that specific GC-RDE that was initially anodically polarized. To follow the initial BEC, the green trace has to be considered. The LSVs predicted an anodic overpotential of 1.56 V vs. RHE for the OER and 0.83 V vs. RHE for the ORR. At the beginning of the bipolar measurement, the overpotentials were measured to be 1.60 V and 0.83 V vs. RHE, respectively. After ten alternations of the current direction, the overpotential for OER at the initially anodically polarized catalyst layer ∆EBEA, initial -Sol increased to 1.68 V vs. RHE and ∆EBEC, initial -Sol was 0.57 V vs. RHE, respectively. It was 1.67 V and 0.60 V vs. RHE based on the LSV data. Interestingly, the overpotential for ORR is significantly higher for the electrode that had been polarized anodically in the CV and LSV prior to the bipolar experiment (BEA, initial, orange). The active sites for ORR include cobalt coordinated to nitrogen (Co-Nx) groups, nitrogen doped carbon groups, including pyrrolic, pyridinic and graphitic groups, and possibly, cobalt oxide. At highly oxidizing potentials, the (Co-Nx)-groups are likely irreversibly transformed to oxide groups, which reduces the available sites for the ORR and thus increases the overpotential. In the second half of the experiment, the catalysts at both BE poles continue to slowly lose activity. The final overpotential at a current density of 1 mA cm-2 at the initial BEC was 0.55 V vs. RHE, while it was 1.70 V vs. RHE at BEA which compared well with 1.70 V vs. RHE obtained from LSV.
Figure 2. Durability measurement for the bifunctional M/NCNT oxygen catalyst. a) Measurement of the interfacial potential difference at the initial BEA (orange, OER and ORR alternating) and at the initial BEC (green, ORR and OER alternating) with alternating current directions for approx. 11 h, both poles were modified with M/NCNT and rotated at 900 rpm in 0.1 M KOH. 3-electrode LSVs prior to the bipolar measurement (black), after half of the time (red) and after the bipolar experiment (blue) for both poles (b) ORR and c) OER) with 0.005 V s-1 in 0.1 M KOH. For comparison, the measurement was repeated without alternating the current direction (Figure S 4). At constant polarization, the catalyst layers are stable and active for the entire measuring time of almost 14 h. At the
beginning, ∆EBEA -Sol was 1.65 V vs. RHE and ∆EBEC -Sol was 0.83 V vs. RHE, whereas the values from the LSVs data were 1.64 V and 0.83 V vs. RHE, respectively. These values barely changed within the time of the experiment, 4
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Analytical Chemistry ORR and OER during stability assessment of the NCNTcatalyst at a constant current direction.
i.e. to ∆EBEA -Sol = 1.64 V vs. RHE and ∆EBEC -Sol = 0.82 V vs. RHE (1.66 V and 0.82 V vs. RHE from the LSVs). In this case, the active sites for ORR are not oxidized during the measurement. Consequently, the activity for both reactions only depends on the intrinsic stability of the catalyst and additionally on the stability of the prepared catalyst film. Besides investigating the effect of different electrolytes on the reduction and oxidation half-cell reactions independently, it should also be possible to use electrode geometries for the BE extremities other than RDEs. Moreover, by changing the ratio of the surface areas ABEA /ABEC , the anodic and cathodic current densities can be varied independent of each another.
All measurements were conducted and the manuscript was written by V.E. with continuous discussion with S.B., J.M. and W.S. The used catalysts were synthesized by J.M. All authors have given approval to the final version of the manuscript.
Conclusions
ACKNOWLEDGMENT
To characterize the activity of electrocatalysts, the overall cell performance, their stability and the influence of various electrolytes, temperatures etc. in the conventional way multiple measurements are needed. We suggest that all these information can be derived from a single measurement using a simple and flexible BPE setup involving a current source and two multimeters to drive and read out the reactions. In this way, the overpotentials for HER and OER catalyzed by the bifunctional water-splitting catalyst Co2B were recorded in parallel over 14 h, thereby enabling concurrent evaluation of the long-term cell performance of the catalyst-modified electrodes and the durability of the individual half-cell reactions. Moreover, it is possible to investigate the reversibility of catalysts that are designed to be applied in polaritychanging systems like metal-air batteries or regenerative fuel cells. The reversibility of a bifunctional oxygen catalyst, consisting of Mn-Co oxide nanoparticles partially embedded in N-doped CNTs, was studied by alternating the current direction in the bipolar system and thus the polarization of the catalyst. Since the two ends of the BE are located in separate vessels and the reactions are balanced by the feeder electrodes, no semi-permeable membrane is necessary so that the overvoltage between oxidation and reduction only contains the individually measured overpotentials of the two reactions. Moreover, the effect of different electrolytes on the half-cell reactions can be evaluated separately as shown for the Co2B catalyst. Though, only bifunctional catalysts have been used in this study, it is certainly also possible to analyze the performance of different catalysts for the oxidation and reduction reaction, respectively, and to use geometries other than RDEs.
The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Cluster of Excellence Resolv (EXC1069) and the Bundesministerium für Bildung und Forschung (BMBF) in the frameworks of the project “Mangan” (FKZ 03EK3548). V.E. expresses her gratitude to the German Chemical Industries Association (VCI) and the German National Academic Foundation for their support.
ASSOCIATED CONTENT Supporting Information Overview of measured potential differences and voltages in the bipolar setup during stability assessment of the Co2Bcatalyst, feeder voltage vs. time during stability assessment of the Co2B-catalyst, reference electrode data, feeder voltage, LSVs for ORR and OER during stability assessment of the Co2B-catalyst with acidic conditions at the cathode, feeder voltage vs. time for the reversibility test of NCNT500, and reference electrode data, feeder voltage, LSVs for
AUTHOR INFORMATION Corresponding Author *Phone: + 49 234 322 6200. Fax: + 49 234 321 4683. E-mail:
[email protected].
Author Contributions
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