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Standardized benchmarking of water splitting catalysts in a combined electrochemical flow cell/ICP-OES setup. Ioannis Spanos, Alexander A Auer, Sebastian Neugebauer, Xiaohui Deng, Harun Tuysuz, and Robert Schloegl ACS Catal., Just Accepted Manuscript • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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Standardized benchmarking of water splitting catalysts in a combined electrochemical flow cell/ICP-OES setup

Ioannis Spanos*1, Alexander A. Auer2, Sebastian Neugebauer1, Xiaohui Deng3, Harun Tüysüz3, Robert Schlögl1 1

Max Planck Institute for Chemical Energy Conversion, Department of Heterogeneous

Reactions, Stiftstrasse 34-36, Muelheim an der Ruhr, 45470, Germany 2

Max Planck Institute for Chemical Energy Conversion, Department of Molecular Theory and

Spectroscopy, Stiftstrasse 34-36, Muelheim an der Ruhr, 45470, Germany 3

Max Planck Institute für Kohlenforschung, Department of Heterogeneous Catalysis and

Sustainable Energy, Kaiser-Wilhelm-Platz 1, Muelheim an der Ruhr, 45470, Germany Email: [email protected]

Abstract The oxygen evolution reaction (OER) is the limiting step in splitting water into its constituents, hydrogen and oxygen. Hence research on potential OER catalysts has become the focus of many studies. In this work, we investigate capable OER catalysts but focus on catalyst stability, which is, especially in this case, at least equally as important as catalyst activity. We propose a specialized setup for monitoring the corrosion profiles of metal oxide 1 ACS Paragon Plus Environment

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catalysts during a stability testing protocol, which is specifically designed to standardize the investigation of OER catalysts by means of differentiating between catalyst corrosion and deactivation, oxygen evolution efficiency and catalyst activity. For this purpose, we combined an electrochemical flow cell (EFC) with an oxygen sensor and an Inductively Coupled Plasma – Optical Emission Spectrometer (ICP-OES) for the simultaneous investigation of catalyst deactivation, activity and faradaic efficiency of catalysts. We tested various catalysts, with IrO2 and NiCoO2 used as benchmark materials in acidic and alkaline environment, respectively. The scalability of our setup will allow the user to investigate catalytic materials with supports of higher surface area than typical for micro-electrochemical flow cells, thus in conditions closer to commercial electrolysers.

Keywords: Oxygen evolution, electrochemical flow cell, ICP-OES, catalyst benchmarking, electrochemistry, faradaic efficiency

Introduction Renewable energy demand has raised over the years due to limited fossil fuel based energy resources like oil, coal, etc. As such, sustainable chemical energy conversion via e.g. water electrolysis has been extensively investigated in order to design new and efficient oxide catalyst materials like: IrO2, RuO2, MnOx, NiO, etc. for acidic or alkaline applications

1–5

,

however, with greater emphasis on catalyst activity than stability. In acidic environment, RuO2 dominates in regards of catalytic activity 6,7, however RuO2 conversion to soluble RuO4 decreases catalyst stability

8,9

. As a consequence, very active and most importantly durable

materials like IrO2 are required for a viable electrolysis setup10,11, as high potential is 2 ACS Paragon Plus Environment

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necessary for significant oxygen evolution. This becomes a limiting factor for most OER catalysts, since at high potential values catalyst corrosion is predominant. The most desirable intermittent operation of electrochemical devices fed with volatile renewable electricity causes additional strain on the stability of electrode materials having to operate under frequent potential changes. Such operation is critical in applications where the source of electricity is fluctuating like solar power. Hence, research on OER catalysts should focus more on the stability of the many materials that can potentially become OER catalyst candidates. In order to design novel OER catalysts with superior stability, the first step is to set up benchmark tests in order to be able to compare possible catalysts under identical conditions. Furthermore, universalization of experimental conditions under which the catalysts are compared to is essential for comparability of the results. Thus, a consistent methodology for a more robust catalyst investigation is necessary, where both stability and activity are studied using the same set of parameters and with a distinct focus on stability, in order to sufficiently understand the underlying mechanisms of electrochemical reactions which explain catalytic activity and catalyst corrosion 12–16. In this work, we propose parallel analysis of catalyst corrosion, activity evaluation and oxygen evolution during electrochemical measurements by combining an electrochemical flow cell with a Clark electrochemical oxygen sensor and an ICP-OES. This setup allows for a quantitative transient analysis of catalyst corrosion products like dissolved metal species during an electrochemical stress test. An added advantage of our method is simplicity and that similarly to techniques like Electrochemical Quartz Crystal Microbalance (EC-QCM)17–21, ICP-OES can monitor fast surface changes that lead to catalyst corrosion due to the electrochemical conditions and identify corrosion of individual metals in alloyed catalysts, i.e. NiCoO2 etc. However, EC-QCM coupling with a flow cell22–25, which will allow the investigation of larger surface area electrodes is certainly advantageous for a future setup 3 ACS Paragon Plus Environment

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design and more in-depth OER catalysts investigations. An additional design principle behind the setup was the scalability of the system, which can then simulate electrolysers, where electrodes of larger surface area are applied. Furthermore, the EFC design is also aimed at versatility in the investigation of different types of catalysts (C-based materials, nano powder catalysts, molecular catalysts, thin films, pellets, etc.). Catalyst supporting materials like glassy carbon, gold, titanium etc. of different sizes can be used, while control of the electrolyte flow is also possible. The chemical evolution of both the bulk and the terminating surface layer of an electrocatalyst during its operation under realistic load conditions may contain dynamic phenomena with significant impact on both its stability and activity that can only be recognized by in-situ observation. In order to investigate this dynamic behavior, a standardization time-resolved catalyst corrosion protocol was designed. Our protocol aims at unraveling corrosion processes and associating the catalyst corrosion profile with the applied electrochemical conditions, which alter the structure and properties of the nominal catalyst into a continuously changing ‘actual’ catalyst during the electrochemical processes. Furthermore, in order to visualize those changes inflicted upon the catalyst we use spider-graph plots to compare and categorize OER catalysts in terms of activity and stability.

1. Experimental All measurements were conducted either in 1M KOH or in 0,1M HClO4 depending on the catalytic material, with internal resistance (IR)-correction. A coil-shaped platinized platinum wire (PT-5W, 125µm diameter, 99.99%, Science Products GmbH), placed along the flow channel following the electrolyte flow, was used as the counter electrode (CE), while the reference electrodes (RE) (SCE, CH Instruments Inc., CHI150, reference potential +241mV 4 ACS Paragon Plus Environment

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vs. NHE) and (Hg/HgO, CH Instruments Inc., CHI152, reference potential +0,098Vvs NHE) were inserted perpendicular to the electrolyte outlet channel. All potentials are expressed vs the RHE potential scale. Catalysts were drop casted on glassy carbon support measuring a surface area of 0.196cm2, previously polished with fine 0.05µm and 1.0µm alumina powder and ultra-sonicated for 15min in MilliQ water. Catalyst inks were prepared by mixing by ultra-sonication 49% H2O, 49% ethanol and 2% Nafion solution for 30min. Subsequently a certain amount of catalyst ink was drop casted on the glassy carbon working electrode (WE) until a loading of 100µg/cm2 was achieved. The catalyst ink was dried on the glassy carbon support under an Argon stream for 30min and finally the working electrode sample holder was inserted into the flow cell. The potentiostat used for the electrochemical measurements is a Bio-Logic SP-150, while the embedded EC-Lab software was used to electrochemically monitor the catalysts. The resulting electrolyte stream is continuously fed into the ICP-OES (Spectroblue EOP, Ametek) by means of a peristaltic pump at a flow rate of 0.18 - 4.67ml/min, through a quartz nebulizer operating at nebulizer gas flow rates of 0.85 L min−1 (Ar, purity 99,999%). All stability tests and oxygen evolution faradaic efficiency measurements in this work have been performed in chrono-potentiometric mode, by applying constant 10mA/cm2. Rotating Ring Disc Electrode (RRDE) measurements for oxygen detection and comparison with similar EFC measurements were performed on a Bio-Logic VMP3 potentiostat, equipped with a Pine research E6R1 RRDE tip and Pine research, AFMSRC electrode rotator. Constant 0.4VRHE potential at the ring was applied during a LSV from 1.2-1.6VRHE at 5mV/s scan rate in 0.1M HClO4 at 0.86ml/min and the ring current was collected. For all catalysts electrochemical characterization, a flow rate of 0.86ml/min was used, because it provides a good balance between oxygen gas removal from the catalyst surface and 5 ACS Paragon Plus Environment

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sufficient detection of the catalyst corrosion products in the ICP-OES. Lower flow rates (see section 2.2 in SI) prohibit reproducible experimental conditions due to excessive oxygen bubble formation at such high current densities. Transient signals of Ir, Ni, Co and Fe were recorded continuously with an integration interval of 100ms and 2 sweeps per reading and detection limits are 0.12ppb for Ni and Co, 0.08ppb for Fe and 1ppb for Ir according to the manufacturer. Fast integration interval is the reason for the persistent noise effect on all transient ICP analysis, with amplitude of ~0.1x10-2 monolayers s-1 on all samples. Although this problem can be alleviated by higher integrations intervals of 1sec, fast corrosion product analysis was more desirable. However, our detection limits are very close to values reported in the literature (for more information see SI)26,27. Additionally, in order to perform a background correction on the ICP-OES data a 300sec time window before and after the 2h chronopotentiometric analysis, without any current passing through the cell, was used in order to certify that no signal drift was observed. For this reason ICP-OES signal is recorded over a total time of 7800sec in contrast to the 7200sec of the chrono-potentiometric stress tests. Calibration was performed using 7 standard solutions (100, 50, 10, 5, 1, 0.5 and 0 (as a blank solution) ppm metal, prepared from Merck CertiPUR®). The RF power was set to 1400 W with a plasma gas flow rate of 15 L min−1. Typically in ICP-OES analysis an internal standard of known concentration is used during transient signal collection for fast correction of any signal drift detected during measurements and normalization of the acquired data. However, this technique was not used in our case due to minimal drift which was less than 0.5x10-4 monolayers s-1 in the course of the 2h stress tests, well within the noise width. The catalysts used in this work were for alkaline applications a commercial mixed oxide (Sigma Aldrich, 99% metal basis, >150nm particle size) and ordered mesoporous Co3O4 and Fe-Co3O4 oxides, while for acidic applications a commercial IrO2 (Iridium(IV) Oxide, Premion, 99.99% metal basis, Ir 84.5% min) respectively. For the synthesis of mesostructured Co3O4 and Fe-Co3O4 6 ACS Paragon Plus Environment

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and for material characterization (SAXS, XRD, BET, SEM and TEM) we refer the reader to the supporting information.

2. Electrochemical flow cell investigations 2.1

Cell design

The basic design principle of the experimental setup for detailed investigations of OER catalyst performance presented here is the combination of the ability to measure efficiency and degradation with a flexible design, as a broad variety of catalyst materials, like powders, pellets, wafers sputtered with catalytic materials etc. exist, should be compared under reproducible conditions. For this purpose, an electrochemical flow cell setup in conjunction with an ICP-OES and an online-oxygen sensor has been chosen. The alternative, the traditional rotating disc electrode (RDE) cell setup, has certain difficulties to combine with an ICP-OES due to the very large electrolyte volume which would lead to high dilution of the corrosion products during the stability measurements in solution. Thus metal concentration profiles would be very difficult to interpret. The experimental setup is comprised of an electrochemical flow cell made out of polyether ether ketone (PEEK), a chemical-resistant material, which is specifically chosen to avoid any oxygen adsorption and permeation through the walls of the cell, which would otherwise cause an O2 – ‘’contamination’’ and affect the oxygen evolution measurements. The cell body and all channels (diameter 2 mm) were mechanically manufactured from PEEK, and the electrolyte inlet and outlet were connected by Tygon® tubing (2 mm inner diameter) with electrolyte container and the ICP-OES respectively. Both channels were positioned in an angle of 60° allowing for better circulation of the electrolyte on the surface of the catalytic material. The working electrode at the bottom of the cell was encircled with a thin silicone 7 ACS Paragon Plus Environment

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gasket to avoid electrolyte escape while protecting the working electrode from external contaminations. The thickness of the gasket was chosen to be 0.2mm, minimizing the excess volume of electrolyte due to the gasket thickness. Cell design can affect electrochemical data collected during activity and stability measurements. Thus placing inlet and outlet channels in a 60o angle enhances oxygen removal from the catalyst surface, compared to a design where both channels are placed in an 1800 angle, which would significantly hinder oxygen removal. Counter electrode placement and flow direction have been carefully chosen so that corrosion products during stability measurements are not deposited on the counter electrode, which would otherwise hinder full corrosion products detection by the ICP-OES apparatus. A polytetrafluoroethylene (PTFE) bottle equipped with a gas-inlet to saturate the electrolyte with the required gas during operation is used as electrolyte reservoir. Furthermore, the setup contains gas tight PEEK nuts for the counter and reference electrodes. The design of the flow channels allows for the removal of the gas formed on the counter electrode and prevents any gas bubbles from blocking the reference electrode. All connections between the parts are made of PEEK tubes (ID 2 mm). In addition, all parts in contact with the electrolyte are made of PEEK due to its chemical resistance. An identical EFC with the top part made of acrylic (Fig. 1) was used in order to study gas formation and removal during operating conditions. However, an EFC made completely of PEEK was used throughout our experiments. The oxygen sensor has been placed outside the cell and between the outlet channel and the ICP – OES in order to measure all oxygen evolved by the OER catalysts. The Clark electrode used as oxygen sensor in our design measures dissolved oxygen in the solution. Gaseous products however cannot be measured with this design. Oxygen bubbles produced during electrochemical measurements are measured as 100% oxygen concentration. Additionally the 8 ACS Paragon Plus Environment

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design of the flow cell requires that the flow of the electrolyte is in the direction of CE→WE→RE which can cause some gaseous products from produced at the CE, i.e. H2 to pass through the WE leading to H2 and O2 recombination into H2O. As such, we do not expect to measure near unity faradaic efficiency for the catalysts investigated in this work besides the fact that some of the current goes into catalyst activation (see section 4.1).Increasing the flow rate to 0.86ml/min, gas bubbles are removed faster during stress tests and faradaic efficiency measurements, where a high current density of 10mA/cm2 was chosen. Higher flow rates would compromise the sensibility of corrosion products detection during a stress test from the ICP-OES due to very low corrosion products concentration in the solution. Another solution would be to invert the flow direction into RE→WE→CE. However that would cause an additional problem: A proportion of the corrosion products during the stability measurements would be deposited on the CE, thus decreasing the sensitivity of the ICP-OES. However, the scope of this work is catalyst comparison not pure quantitative analysis of gaseous products evolved by the catalysts used (See Section 4.1). The EFC setup is designed to use the hydrostatic force to induce the electrolyte flow. As the hydrostatic force – and hence the electrolyte flow – is dependent on the height difference of the electrolyte reservoir and the cell, the peristaltic of the ICP-OES pump is used in withdraw mode, thus controlling a defined electrolyte flow rate.

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Figure 1. Transparent acrylic version of the electrochemical flow cell for performance and corrosion evaluation. An identical cell made of PEEK was used throughout our experiments to guarantee chemical stability of the cell material during electrochemical investigations.

3. Standardization protocol A series of sequential electrochemical measurements divided into three distinct sections form our standard protocol for OER catalysts evaluation (Fig. 2) 28. The pre-analysis section of the standardization protocol (marked in green in Fig. 2) includes open circuit potential (OCP) measurements, with information about the initial state of the catalyst surface and a preconditioning treatment of the catalyst. Pre-conditioning is crucial for the catalyst activation. However, it can be different depending on the catalytic material used. Usually, it involves potential cycling between certain oxidative potential windows. Noble metals are known to corrode when cycled between oxidative and reductive potential values in acid

29,30

. Metal-

oxides, formed at high positive potential are swiftly removed at reductive potential values removing at the same time catalyst material, thus corroding the catalyst itself. Catalytic materials like NiCoO2 and Co3O4 show remarkable activity enhancement after potential cycling

31

. Pre-conditioning can significantly alter the surface properties of the catalysts,

while at the same time redox processes in the electrode material emerge, which may be the


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reason for the activity enhancement 32,33. As a result, the catalyst reaches its final state before the start of the second section of the standard protocol, which includes both activity and stability measurements. Between each individual step of the protocol gentle, potentiodynamic or galvanodynamic measurements are necessary. This is to avoid any sudden increase of the applied potential from the open circuit potential to the operating potential, which would otherwise deactivate the catalyst faster, rendering our stability measurements problematic, since the observed corrosion would be also a result of deteriorating electrochemical conditions mishandling during the stability test. The second section of the protocol is catalyst evaluation (marked in blue in Fig. 2), which includes activity, faradaic efficiency and stability measurements. Linear voltammetry sweeps (LSV) between 1.2-1.7VRHE are used for the evaluation of catalytic activity before and after stability measurements. Faradaic efficiency measurements for the evaluation of the electron conversion into oxygen evolution is performed by chronopotentiometry (CP) or chronoamperometry (CA) measurements at 10mA cm-2 and at 1.6VRHE until the value of the total charges passed through the cell reaches 1C. Stability measurements include similarly CP or CA measurements at 10mA/cm2 and 1.8VRHE respectively for 2h. Taking into consideration that OER catalysts are supposed to operate at the above conditions for much longer periods, the duration of the stability can be increased to 6h or even 12h. However, initial screening of stable materials is possible even during a shorter period of time. Finally, after catalyst evaluation, post-analysis measurements (marked in red in Fig. 2) are necessary in order to compare catalytic activity and oxygen evolution after the stability measurements with the use of carefully chosen activity and stability markers (see section 4.2). Thereafter catalyst specific data are uploaded to a database allowing for easy comparison and screening of catalysts. 11 ACS Paragon Plus Environment

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Figure 2. Activity, stability and oxygen evolution measurements of the standardization protocol for OER catalysts evaluation.

4. Results and Discussion 4.1

Catalyst evaluation

We continue our discussion with a series of representative examples for the stability and activity evaluation of OER catalysts. Both acidic and alkaline environment catalysts were characterized under similar conditions for comparison. For catalyst activity comparison, geometric surface area normalization was chosen even though specific activity is traditionally used for catalyst performance evaluation. Using Pt-based catalysts, electrochemically active surface (ECSA) can be easily determined by CO oxidation measurements

34

. However, for 12

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non-noble metal catalysts lack of knowledge on the active sites before and especially after stability measurements hinders the evaluation of real catalytically active surface area. For non-noble metal catalysts, double layer (DL) capacitance measurements can be performed to calculate the ECSA of catalysts (See supporting information). DL capacitance was evaluated in a potential window around the open circuit potential of each catalyst. Subsequently the roughness factor was calculated and the experimental current density during activity measurements was normalized. One must take into account that materials of high porosity can suffer from slow masstransport of OH− into and O2 out of the pores which can cause interior active sites within the porous film to become catalytically inaccessible during OER operation. As such, materials of very high geometric activity may show significantly lower specific activity due to the very large surface area (see for example IrO2 in supporting information). It should be noted that ECSA measurements on catalysts of unknown active sites cannot be used for direct comparison of the turnover frequency of catalysts but only for specific activity comparison. As such, we used geometric surface area normalization for catalyst comparison, even though this might also be suboptimal due to the dissimilar electrochemical active surface area of catalysts. As the relation between the geometric electrode area and the surface area of the catalyst exposed depends inter alia critically on how the electrode was manufactured, we note that “performance” measurements are to be taken with great care both within a series of selfmade electrodes and even more so when comparing literature data.

Catalysts for alkaline conditions In the following we will discuss catalyst characterization in acidic and alkaline conditions. Nibased OER catalysts are considered as the most active and stable catalysts in alkaline conditions

35

. In this work we tested commercial bulk NiCoO2, nanostructured ordered 13 ACS Paragon Plus Environment

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mesoporous Fe-Co3O4 and Co3O4 as alkaline catalyst candidates. Activity was investigated by typical linear sweep voltammetry from 1.2-1.7VRHE at a scan rate of 5mV/s and a flow rate of 0.86ml/min, before and after the chronopotentiometry stability test at 10mA/cm2 for 2h (Fig. 3). Commercial NiCoO2 was used as our benchmarking alkaline OER catalyst. Cyclic voltammetry from 0.7-1.6VRHE was performed on the NiCoO2, revealing a NiII → NiIII and CoII → CoIII transition at onset potentials of ~1.3VRHE and a NiIII → NiIV and CoIII → CoIV transition at potentials 36,37 ~1.45VRHE (Fig. 3A). Activation of Ni-based catalysts is common 38,39

due to Fe contamination of commercial KOH solutions. However, since no KOH

purification was performed on the electrolyte used throughout these experiments, we expect an activity increase for the NiCoO2 catalyst. In order to compare all catalysts under the same conditions, we extended the preconditioning by adding an extra 2h chronopotentiometric step until the potential reached a plateau. After these initial 2h the chronopotentiometric stress test was only allowed to begin. For this reason the total stress test time was 4h (Fig. 3C). The catalyst showed significant initial corrosion (Fig. 3B) during the first 2h of preconditioning but still high stability after the 2h preconditioning (Fig 3D) and at end of the chronopotentiometry stress test for 2h at 10mA/cm2 (Fig. 3E). We can only assume that surface cleaning and inactive sites removal takes place for the first 2h of steady potential preconditioning, while catalyst corrosion slowly begins after the initial 2h. This is an indication that a different protocol is necessary for Ni-based catalysts and that KOH purification is necessary to eliminate Fe impurities effect on catalyst activity. Transient ICP analysis of the same catalyst reveals initial catalyst corrosion during the first minutes of the stress test; however surface passivation starts after around 10minutes and no more Ni or Co corrosion can be detected.


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Figure 3. Activity and stability evaluation of commercial NiCoO2 OER catalyst. A) Cyclic voltamograms from 0.7-1.60VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis during chronopotentiometry preconditioning at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH for the first 2hours test. E) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH after the 2h preconditioning and 2h stress test.


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Co3O4 is also reported to be a very active OER catalyst material

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40,41

. Ordered mesoporous

Co3O4 was tested in a similar manner to the NiCoO2 catalyst. Cyclic voltammetry from 0.71.6VRHE (Fig. 4A) reveal, as in the case of NiCoO2, a CoII → CoIII transition however with higher potential at~1.3VRHE and a CoIII → CoIV transition at 1.45VRHE

42

. Similarly, two

reduction peaks at 1.15 and 1.3V reveal a CoIII →CoII and a CoIV → CoIII transition respectively. After 50 cycles, the intensity of the peaks increases, suggesting that the number of surface CoII and CoIII species, which can be electrochemically oxidized, increases. Chronopotentiometry stress test and transient ICP analysis at 10mAcm2 for 2h reveal high stability of the Co3O4 catalyst similar to NiCoO2 (Fig. 4B-D). ICP-OES data suggest that metal corrosion was minimal during chronopotentiometric analysis, which comes into contrast with electrochemical analysis where a small activity drop was observed, suggesting a different deactivation mechanism compared to NiCoO2. The corrosion profile of the Co3O4 catalyst is also different than the one for NiCoO2. Instead of an initial fast corrosion and a subsequent surface passivation, the Co3O4 shows no corrosion during the stability test. Instead minimal corrosion was detected approximately after ~2h, meaning that longer stress tests are required for a thorough stability analysis of OER catalysts.


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Figure 4. Activity and stability evaluation of Co3O4 OER catalyst. A) Cyclic voltamograms from 0.7-1.60VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis during chronopotentiometry at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH.

Finally, we tested an ordered mesoporous Fe-Co3O4 OER catalyst with a Co:Fe ratio of 64:1 43

. This catalyst shows remarkable activity and stability (Fig.5A-D). Accordingly, after a

chronopotentiometry stress test at 10mA/cm2 for 2h, the oxygen evolution overpotential only slightly increased, at ~11mV (Fig. 5C). Similar to the Co3O4 catalyst oxidation peaks appear at 1.25VRHE and 1.45VRHE indicating CoII → CoIII and CoIII → CoIV transitions respectively. The ICP-OES signal is consistent with the metal content of the catalyst, as only Co corrosion could be detected, while no appreciable Fe corrosion could be detected due to the extremely


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low Fe content of the catalyst. Another explanation for this very low Fe amount detected is possible contamination of the KOH electrolyte from Fe species.

Figure 5. Activity and stability evaluation of Fe-Co3O4 OER catalyst. A) Cyclic voltamograms from 0.7-1.60VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis during chronopotentiometry at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH.

Catalysts for acidic conditions IrO2, a well-established OER catalyst in acidic conditions

44–46

was tested in 0.1M HClO4.

Likewise with the previous samples, cyclic voltamograms between 0.7-1.45VRHE were recorded (Fig. 6A). The potential window was specifically chosen for the IrO2 catalyst due to the lower OER onset potential compared to alkaline catalysts. Chronopotentiometry stability 18 ACS Paragon Plus Environment

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test at 10mA/cm2 for 2h in combination with transient ICP-OES analysis reveal the same initial metal dissolution and surface passivation, however in the case of IrO2 the amount of Ir dissolved is lower compared to OER catalysts used in alkaline electrolyte, indicating higher stability additional to the already significantly higher activity (Fig. 6B-C). Signal magnification (Fig. 6B) shows minimum catalyst corrosion for the IrO2 catalyst. The potential spikes observed in the chronopotentiometric profile during the stability test, are an indication of increased gas bubble formation compared to alkaline catalysts, covering the surface of the catalyst. Linear sweep voltammetry between 1.2-1.6VRHE was performed for the IrO2 catalyst (Fig. 6D). We chose to sweep only up to 1.6V for the IrO2 catalyst since it is well established that IrO2 catalysts have much lower oxygen evolution onset potential compared to transition metal-based OER catalysts.

Figure 6. Activity and stability evaluation of commercial IrO2 OER catalyst. A) Cyclic voltamograms from 0.05-1.45VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis 19 ACS Paragon Plus Environment

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during chronopotentiometry at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.21.6VRHE at 5mV/s in 0.1M HClO4. Inset: ICP-OES signal magnified from 0-1000sec.

4.2 Standardization protocol data analysis In order to compare the OER catalysts investigated in this work, we propose certain activity and stability markers (Table 1). Activity markers

EJ=10mA/cm2: The potential at a current density value of 10mA/cm2. EOER, onset: Oxygen evolution reaction onset potential. Stability markers

∆JLSV%: The relative difference in percentage of the maximum current density during a LSV before and after the chrono-potentiometric stress test analysis.

nCP,2h%: The relative difference in percentage of the oxygen evolution overpotential at 0h and at 2h of the chrono-potentiometric stress test analysis at 10mA/cm2.

nFaradaic: Faradaic efficiency before the stress test. ∆corCP,2h: Total metal corrosion in monolayers, calculated by integrating the surface area underneath the corrosion peaks taken by ICP-OES analysis during the chrono-potentiometric stress test between the 300th and 1200th sec (see section 4.1).


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These markers can be plotted in spider graphs and categorize catalysts according to their activity and stability performance by observing the respective shape of each spider graph. In order to illustrate this analysis, we compare hypothetical model catalysts (Fig. 7 & Table 1) with the catalysts investigated in this work in the next paragraphs (Fig.8 & Table 2). The experimental conditions for the above measurements are described in section 4.1. In summary, EOER,

onset

is a measure of the overpotential of oxygen evolution. It is an

important marker but we believe that EJ=10mA/cm2 is more important since a current density of 10mA/cm2 has been traditionally used for OER catalysts stability evaluation measurements. As a result lower overpotential in a 10mA/cm2 galvanostatic mode of operation for an electrolyser is very important for catalyst comparison. For stability measurements we have chosen 4 individual markers. Markers like faradaic efficiency and the relative difference in overpotential and maximum current density in an LSV before and after a chronopotentiometry stability test, nCP,2h% and ∆JLSV% respectively, are the most important markers. Even a small difference in the values of these 3 markers after a stability test can be detrimental on the stability of a catalyst. As an example a faradaic efficiency loss or maximum current density in a LSV of even 1% after 2h of stability test, if we assume a stable drop rate, means that in less than 100 hours such a catalyst is insufficient for oxygen evolution. However for such an evaluation an extended stress test is necessary since 2 hours are sufficient only for an initial catalyst evaluation. In order to categorize catalysts according to their stability and activity we chose 5 different hypothetical model catalysts (catalysts #1 - #5), each one favoring activity, stability or both. The values of each marker for these model catalysts were chosen carefully to present each category of catalysts. A mostly-active catalyst, (catalyst #1, Fig. 7A) or a mostly-stable catalyst (catalysts #2 - #4, Fig. 7B-D) can be easily distinguished from an ideally active and stable catalyst (Catalyst #5, 21 ACS Paragon Plus Environment

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Fig. 7E) simply by observing the shape of each spider graph. Consequently a spider graph’s shape can be used for catalyst categorization according to activity and stability.

Table 1. OER markers for theoretical model catalysts were ideal and non-ideal values were chosen for each marker to represent different catalyst categories of high or weak activity/stability.

OER catalysts

Catalyst #1

Catalyst #2

Catalyst #3

Catalyst #4

Catalyst #5

EJ=10mA cm-2geo ∆JLSV% nFaradaic EOER, onset nCP, 2h% ∆corCP, 2h

1,5 50 80 1,45 10 5

1,75 50 100 1,70 10 0

1,75 0 100 1,70 0 0

1,75 0 80 1,70 0 5

1,50 0 100 1,45 0 0

Plotting all markers in a single spider-graph, not only can we compare catalysts my means of activity and stability but also distinguish between different corrosion pathways. For instance, catalysts #2 and #4 both have low activity but they favor stability.


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Figure 7. Pairs of performance indicators plotted for hypothetical OER catalysts showing different catalyst properties like activity, stability and faradaic efficiency for oxygen evolution and how different indicators influence catalyst performance. Figure 11 D shows the ideal combination of indicators for a hypothetically perfect OER catalyst of high activity and stability.


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Catalyst #2 shows minimum corrosion in terms of ∆corCP, 2h and very high faradaic efficiency. On the other hand, for catalyst #4 even though it is heavily corroded, its stability is high. This characterizes two different scenarios - degradation by actual catalyst corrosion (dissolution) and degradation by catalyst deactivation without actual corrosion, where active sites are nevertheless deactivated/destroyed. Even though the scope of this work is not to study different corrosion mechanisms, it is evident by the data that different corrosion mechanisms exist and that spider graphs are a very useful tool for the fast recognition of trends in the development of active and stable catalysts. Hence, we commence by discussing the spider-graph plots for the real catalysts discussed in section 4.1.

Table 2. OER markers collected from section 4.1. *Negative value for the overpotential difference due to catalyst activation during the stress test of NiCoO2 catalyst. OER catalysts EJ=10mA cm-2geo ∆JLSV% nFaradaic EOER, onset nCP, 2h% ∆corCP, 2h

NiCoO2 1.684 1.3 92 1.638 -2.8* 1.8

Co3O4 1.61 15.6 93 1.56 6.4 ~0.01

Fe-Co3O4 1.60 8.1 95 1.559 0.72 0.45

IrO2 1.524 37.5 95 1.526 0.74 0.08

Similarly, out of the four different catalysts we investigated in our manuscript we believe that the Fe-Co3O4 catalyst is the most promising one in alkaline environment due to a balance in stability and activity. Co3O4 even though it showed very good corrosion behavior, with nondetectable catalyst corrosion, still some activity drop was observed due to catalyst deactivation. NiCoO2 although very stable and showed minimum activity drop still it showed the highest catalyst corrosion, while some small activity drop was observed during the 24 ACS Paragon Plus Environment

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extended 4h stress test for the specified catalyst. IrO2 although very active initially deteriorates fast after a stability test meaning that this specific IrO2 catalyst investigated here cannot be advised as an OER catalyst in acidic environment.

Starting with the IrO2 catalyst, it is evident that it is a very active catalyst in acidic environment, with the lowest EOER, onset and EJ=10mA/cm2 values, resembling a catalyst of high activity but of low stability due to the corrosion that takes place during the stability test which impacts stability, resulting in the high value of ∆JLSV% (Fig. 8A). Co3O4 is a very active catalyst for alkaline environment; however slightly deactivated during the chronopotentiometric stability test, resembling a catalyst of high stability but of lower activity (Fig. 8B). Overpotential decrease during the chronopotentiometric stability test reveals the high stability properties of the NiCoO2 mixed oxide OER catalyst, resembling a catalyst of high stability but of lower activity (Fig. 8C). Comparing NiCoO2 and Co3O4 one can also detect different corrosion mechanisms as in the case of the hypothetical catalysts #2 and #4 respectively. Finally, Fe-Co3O4 is a very stable and active catalyst resembling an ideal catalyst (Fig. 8D). Under materials design aspects one could now use this test protocol to further optimize catalyst materials, increase the durability of active systems or try to increase the activity of stable systems. The analysis allows going even further, as it can distinguish between corrosion and deactivation degradation, and further investigations can be directed towards an understanding of the underlying mechanisms.


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Figure 8. Spider graph of steady-state and ‘before-after’ parameters of OER catalysts at a flow rate of 0.86ml/min.


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Important questions to address are, for example: Why does a material deactivate without dissolution and why are there materials that dissolve to some extent while not losing activity? Both, regeneration of active sites on the surface or uncovering other active sites during dissolution might be possible explanations. However further investigations are necessary to answer these questions and are subject of a future work. In conclusion, this setup can be applied to a broad spectrum of catalyst materials in order to have a basis for comparison and improvement by means of standardized experiments and lends itself optimally for application in large research and development projects that require benchmarking of a broad variety of catalysts under conditions close to electrolyser setups47.

5. Conclusions In this work, we demonstrate our novel setup of an EFC combined with an ICP-OES and a Clark electrode as an oxygen sensor, for standardizing the evaluation of stability, activity, oxygen evolution and corrosion of a multitude of OER catalysts in acidic and alkaline environment. Our system combines accurate measurements and flexibility since the aforementioned EFC can be combined with any mass spectroscopic method for gaseous products characterization (Online Mass Spectrometry, HPLC etc.). Transient analysis of catalyst corrosion during the electrochemical stress test with our apparatus has the advantage that it can closely monitor and correlate material corrosion with the electrochemical conditions applied. Our standardization protocol, through spider graph visualization of activity and stability trends of catalyst materials, allows us to categorize different OER catalysts by means of activity and stability and distinguish different deactivation mechanisms. Our method is simple while a broad range of catalysts can be investigated, from acidic to alkaline environment and from pellets to powder catalysts. This is the first system to our knowledge that provides such capabilities, thus it allows for a fast screening of OER catalysts in regards of stability and activity, thus it should be used as a guideline for any user who wishes catalyst stability and activity comparison performed on the same footing.


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ACKNOWLEDGMENT

The authors would like to acknowledge Norbert Pfänder, Marius Podleska, Maike Hashagen and Frank Girgsdies for material characterization and the MAXNET Energy consortium of Max Planck Society for the financial support. The project was initiated in the framework of the BMBF project MANGAN.

Supporting Information Available: In this document, material characterization information, oxygen sensor optimization data, electrochemical surface area measurements and ICP-OES data are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Transparent acrylic version of the electrochemical flow cell for performance and corrosion evaluation. An identical cell made of PEEK was used throughout our experiments to guarantee chemical stability of the cell material during electrochemical investigations. 641x427mm (96 x 96 DPI)


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Figure 1. Transparent acrylic version of the electrochemical flow cell for performance and corrosion evaluation. An identical cell made of PEEK was used throughout our experiments to guarantee chemical stability of the cell material during electrochemical investigations. 254x169mm (96 x 96 DPI)


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Figure 2. Activity, stability and oxygen evolution measurements of the standardization protocol for OER catalysts evaluation. 416x342mm (96 x 96 DPI)


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Figure 3. Activity and stability evaluation of commercial NiCoO2 OER catalyst. A) Cyclic voltamograms from 0.7-1.60VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis during chronopotentiometry preconditioning at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH for the first 2hours test. E) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH after the 2h preconditioning and 2h stress test. 284x199mm (300 x 300 DPI)


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Figure 4. Activity and stability evaluation of Co3O4 OER catalyst. A) Cyclic voltamograms from 0.71.60VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis during chronopotentiometry at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH. 284x199mm (300 x 300 DPI)


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Figure 5. Activity and stability evaluation of Fe-Co3O4 OER catalyst. A) Cyclic voltamograms from 0.71.60VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis during chronopotentiometry at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.2-1.7VRHE at 5mV/s in 1M KOH. 284x199mm (300 x 300 DPI)


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ACS Catalysis

Figure 6. Activity and stability evaluation of commercial IrO2 OER catalyst. A) Cyclic voltamograms from 0.05-1.45VRHE at 50mV/s for 50 scans. B-C) Transient ICP analysis during chronopotentiometry at 10mA/cm2 for 2h. D) Linear sweep voltammetry from 1.2-1.6VRHE at 5mV/s in 0.1M HClO4. Inset: ICP-OES signal magnified from 0-1000sec. 284x199mm (300 x 300 DPI)


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ACS Catalysis

Figure 7. Pairs of performance indicators plotted for hypothetical OER catalysts showing different catalyst properties like activity, stability and faradaic efficiency for oxygen evolution and how different indicators influence catalyst performance. Figure 11 D shows the ideal combination of indicators for a hypothetically perfect OER catalyst of high activity and stability. 290x218mm (96 x 96 DPI)


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Figure 8. Spider graph of steady-state and ‘before-after’ parameters of OER catalysts at a flow rate of 0.86ml/min. 245x224mm (96 x 96 DPI)


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ACS Catalysis

For table of contents only. Electrochemical Flow Cell/ICP-OES setup for the combined static and dynamic evaluation of OER catalysts. 312x235mm (96 x 96 DPI)


Standardized Benchmarking of Water Splitting ... - ACS Publications

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