Measurement Techniques for the Study of Thin Film Heterogeneous

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Measurement Techniques for the Study of Thin Film Heterogeneous Water Oxidation Electrocatalysts Michaela Burke Stevens, Lisa J. Enman, Adam S Batchellor, Monty R. Cosby, Ashlee E. Vise, Christina D.M. Trang, and Shannon W. Boettcher Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02796 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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Chemistry of Materials

Measurement Techniques for the Study of Thin Film Heterogeneous Water Oxidation Electrocatalysts Michaela Burke Stevens, Lisa J. Enman, Adam S. Batchellor, Monty R. Cosby, Ashlee E. Vise, Christina D. M. Trang, Shannon W. Boettcher

Department of Chemistry and Biochemistry University of Oregon, Eugene, Oregon 97403, United States *[email protected] Abstract Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They can exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher efficiency catalysts, challenging. We report a series of procedures and measurement techniques that we have adopted or developed to assess each of the above challenges in understanding materials for the OER. These include the targeted synthesis of hydrated oxyhydroxide phases, the assessment and elimination of electrolyte impurities, the use of a quartz crystal microbalance to monitor film loading and dissolution, and the use of an in situ conductivity measurement to understand the flow of electrons from the catalyst active sites to the conductive support electrode. We end with a recipe for the synthesis and characterization of a “standard” Ni(Fe)OxHy catalyst that can be performed in any laboratory with a basic electrochemical setup and used as a quantitative comparison to aid the development of new OER

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catalyst systems. Outline:

1.0 Introduction 2.0 Measurements Suitable to Assess Catalyst Activity 2.1 Electrochemical Cell and Techniques • Cell Setup • Electrolyte • Working Electrodes • Counter Electrodes • Reference Electrodes • Correcting for Uncompensated Series Resistance 2.2 Synthesis of Heterogeneous Catalyst Films • Target Catalyst Phases • Spin Casting • Electrodeposition • Choosing a Deposition Method 2.3 Electrochemical Measurements • Steady-State and Tafel Analysis 2.4 Catalyst Conductivity Comparisons 2.5 Determination of Film Loading 2.5.1 Redox Wave Integration 2.5.2 Capacitance 2.5.3 Quartz Crystal Microbalance Mass Measurements • Validation of the Sauerbrey Equation • Comparison of QCM Data to Peak Integration and Direct Elemental Analysis 2.6 Characterization of Film Stability 2.7 Catalyst Film and Electrolyte Purity • Test for Residual Fe Impurities • Quantifying Fe Impurities in Thin Films 2.8 Calculation of Catalyst Activity Metrics 3.0 Recipe for Making and Testing a “Standard” Ni(Fe)OxHy Catalyst Film 4.0 Conclusions


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1.0 Introduction Measurements in heterogeneous electrocatalysis are in principle simple – one deposits a layer of solid catalyst on a conductive electrode, selects a reference electrode and electrolyte, and uses an inexpensive potentiostat to record the current response as a function of the applied potential (versus the thermodynamic potential for the reaction of interest). In practice, however, a large number of experimental and sample-related factors make interpreting the resulting response, in terms of fundamental processes, difficult. Here we discuss the practices that we use for measuring heterogeneous electrocatalytic activity and related materials parameters. We focus on heterogeneous electrocatalysts for the oxygen evolution reaction (OER), which is the counter electrode reaction involved in overall water splitting (2H2O O2 + 2H2) to make renewable hydrogen (H2) fuel.1 The approaches we discuss, however, are broadly applicable. The efficient production of H2 via water splitting provides one strategy for storing intermittent renewable energy at a massive scale.2–5 This work is meant to supplement the literature, such as the benchmarking publications from the Joint Center for Artificial Photosynthesis, which focus on ranking different catalyst preparations by overpotential and stability,6–9 as well as those that focus on ink preparations.10 We emphasize the use of multiple, simple, widely available, corroborating techniques to develop a physical picture of each catalyst system under study. This enables meaningful activity measurements which can be correlated with composition, structure, and morphology. Materials for the OER are remarkably complex – despite over a century of study they are still not well understood. Part of the challenge is the large variety of catalyst materials (compositions, phases, and morphologies) and the tendency for dynamic changes to the catalyst “surface” under active conditions (the use of quotations around “surface” indicate that the



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surface is not necessarily well defined). Various compositions of metals,11 carbons,12,13 metal oxides,14 selenides15,16, sulfides17,18, and (oxy)hydroxides (sometimes containing borate19,20 or phosphate21,22 counter ions) have been synthesized as catalysts for the OER by electrodeposition, solution casting, solid-state reactions, and hydrothermal methods, among others. Catalysts have been synthesized as single crystals, nanoparticles,8,23,24 monolayers,25 thin films, thick films,26 supported (e.g. with conductive carbon and Nafion) and unsupported inks, and metal-organic frameworks. Most of the OER catalysts that have been studied are probably not structurally “stable” under OER conditions. For example, the sulfides, phosphides, selenides, etc., irreversibly oxidize to form hydrated oxide and oxyhydroxide phases. Many oxides are also not stable. Pourbaix diagrams show the predominate phases are typically oxyhydroxides in neutral to basic oxidizing conditions for many catalyst materials.27,28 The dynamic conversion of many oxide catalysts has been linked to the reactivity of the lattice oxygen and the relatively weak metal oxygen bonds.29– 32

Some oxides appear to be structurally stable,33 although activity has been suggested to

correlate inversely with stability.34

Such dynamic restructuring is problematic for catalyst

testing. If the surface is converting to a new phase, how does one normalize the measured activity? How does one determine the structure – or even the composition – of the phase actually responsible for catalysis? A second major challenge is the lack of information on the electronic properties of electrocatalysts. To drive an electrocatalytic reaction one needs catalyst sites that are electronically wired to the electrode (i.e. the catalyst must be electrically conductive), and ionically wired to the counter electrode through electrolyte solution (i.e. usually accomplished through a liquid electrolyte, except in membrane cells which use a solid ion-conducting polymer


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such as Nafion for ionic wiring). It is therefore essential for any research into the electrocatalytic properties of new materials to also report on the electronic properties of the same material – insulators and lightly doped semiconductors typically cannot function as effective electrocatalysts. Measuring the electronic properties of any dynamically formed surface phase is particularly difficult, but important in order to understand the overall electrocatalytic process. A third major challenge is the convoluting effects of the measurement environment on the observed activity. One major effect is electrolyte impurities. Corrigan, for example, showed in 1987 that Fe impurities in alkaline electrolytes dramatically affect the OER activity of NiOxHy electrodes that he was studying for nickel-metal-hydride batteries.35 We have further shown that Fe incorporates into, and affects the activity of, many materials commonly studied for OER, including those based on Ni (at near-neutral and alkaline pH), Co, and Au.14,36–39 This effect is important in the context of mechanistic OER studies and has led to a re-ordering of the activity trends across the first row transition metals.40 In fact, it now appears that Fe is an essential component of the active site for the OER,38,41 perhaps by activating redox of bridging or terminal oxygen anions.33,42–44 Markovic and coworkers have also reported on the range of other impurities in alkaline electrolyte that influence cathodic processes.45 Any experimental analysis of electrocatalytic activity must assess electrolyte purity and any possible changes to catalyst composition during the electrocatalytic process. Finally, there is recent strong evidence that the substrate on which the catalyst is supported can affect the apparent OER activity. NiOxHy, CoOxHy, Ni(Ce)OxHy, MnOxHy, and FeOxHy have all been reported to be affected by noble metal substrates such as Au. While the detailed interfacial atomic structure and mechanisms of the enhancement are not entirely clear, it is evident that mechanistic catalytic studies should assess whether or not the substrate is affecting



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the apparent activity. Such effects would be more substantial for thin catalyst layers (i.e. of nm thickness) than thick ones (i.e. of µm thickness). We outline a series of simple procedures and measurement techniques that we have adopted or developed in our laboratory to assess each of the above challenges in understanding materials for the OER. For each case we aim to provide a quantitative or qualitative reasoning for the approach, and to describe under what conditions the acquired data is valuable. We discuss 1) our cell setup that minimizes impurities and uncompensated series resistance, 2) the electrochemical characterization that emphasizes steady-state measurements, 3) syntheses that target thin films of thermodynamically favored hydrated oxide or oxyhydroxide phases, 4) the analysis of film loading or electrochemically accessible surface area, 5) the catalyst mechanical and chemical stability, 6) the assessment of electrolyte impurities and their exclusion, and 7) the definition of intrinsic activity metrics, such as turn-over frequency (TOF) and mass activity. We end with a discussion of a Ni(Fe)OxHy catalyst film that we suggest as a “standard” benchmark catalyst preparation for comparing to any new catalysts in alkaline media. The Ni(Fe)OxHy film can easily be prepared in any laboratory and tested under virtually identical conditions to assess measurement technique and relative activity of a new catalyst. We also emphasize that although a standard set of experiments is useful, the diversity of the materials and conditions for the OER requires constant assessment of the validity of the methods used under particular test conditions.

2.0 Measurements Suitable to Assess Catalyst Activity The cell, electrodes, and electrochemical techniques used for catalyst characterization are critical in obtaining reliable activity data that can be correlated with other materials’ properties. Proper care must be taken to correctly assess and address impurity contamination, reference


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electrode stability, uncompensated series resistance, substrate effects, sample instability, and mass-transfer effects.

2.1 Electrochemical Cell and Techniques Cell Setup. A three-electrode electrochemical cell, with working electrode (WE), reference electrode (RE), and counter electrode (CE), allows for measurement or control of the working electrode potential relative to a fixed reference (Figure 1).46 All cell components are plastic (polytetrafluoroethylene, etc.) to mitigate contamination from Fe leaching as glass dissolves in alkaline environments. We find that contamination from external sources can be excluded if only quartz, plastic, or hot glue are directly in contact with the solution. The effects of contamination from the electrolyte, as well as testing electrolytes for contamination, will be discussed in Section 2.7. To limit contamination from previous experiments (Ni, Co and Fe (oxy)hydroxide contamination can lead to discoloration) our cells are thoroughly cleaned or soaked in 1 M sulfuric acid or water as appropriate when not in use. To remove possible SO42- contaminants, the cells are rinsed in water and the alkaline electrolyte to be used in the following experiment prior to use. Although anion effects are not well understood, recent work has also shown that SO42- anions are electrochemically inert and do not affect OER activity.47 Electrolyte. For studies in alkaline media we typically use aqueous 1 M or 0.1 M electrolyte.

During the measurement, high-purity O2 is bubbled through the electrolyte to

saturate it and fix the reversible oxygen potential. Potassium hydroxide is preferred because it has been used in many water electrolysis systems,1,48 but sodium hydroxide is a reasonable alternative. Both are highly soluble and have high ionic conductivity, although the solubility of



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oxygen and diffusion coefficients differ slightly between the two. There is evidence, however, that the cation identity in the alkaline electrolyte affects OER activity, although the mechanism is unclear.49 Recent work by Hunter et al. also shows that dissolved carbonate enhances the activity of Ni(Fe)OxHy films in aq. KOH, and other anion effects may be important.47 All basic electrolytes exposed to air containing carbon dioxide are expected to incorporate dissolved carbonate and replace other inter-sheet anions.47 Carbonate can be removed via the process detailed in reference [47]; we make no attempt to clean our electrolyte of carbonate, but mitigate H2O and CO2 uptake in the unsealed KOH bottles by wrapping with a paraffin film. Bottles not properly closed between uses may have a short half-life. Note KOH pellets are corrosive and should be handled with proper personal protective equipment.

Figure 1. a) Schematic of the cell setup with working electrode (WE), partitioned Pt counter electrode (CE), Hg|HgO reference electrode (RE), aq. 1 M KOH, bubbling oxygen, and rapid stirring to dislodge bubbles. b) Photo of QCM (Stanford Research Systems) WE. c) Photo of


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hand-made Au WE with hot glue covering all wire, epoxy, and glass components.

Working Electrodes. The working electrode is also the substrate supporting the heterogeneous electrocatalyst. In some cases, catalyst/substrate interactions appear to enhance the activity of thin electrocatalyst films.39,50–53 These effects remain poorly understood. To assess the degree of enhancement we typically measure the activity of catalysts on different substrates, including Au, Pt, glassy carbon (GC), and indium tin oxide (ITO) supports. ITO is more resistive, and therefore to prevent significant potential variations across the electrode surface, the electrode area should be made small (~0.1 cm2). The substrate choice will also change the stable potential window during cyclic voltammetry.54 For many experiments we use quartz crystal microbalance (QCM) electrodes (we use an inexpensive resonator circuit from Stanford Research Systems, the QCM200). This allows for measuring the mass loading ex situ and in situ for precise activity measurements. The QCM electrode has a Kynar® polyvinylidene fluoride quartz-crystal holder with Viton® O-rings (see section 2.5.3) (Figure 1b). Due to the size of this electrode, 1.38 cm2, the QCM WE is not used for high loading studies or studies requiring high current densities as any uncertainties in iRu drop scale with total current. Hand-made electrodes (Figure 1c) fashioned with a geometric surface area below 0.5 cm2 are better for measurements at high current density. Conductive films (Au, Pt, etc., ~50 nm in thickness) deposited by e-beam or thermal evaporation onto glass slides (after 25 nm of Ti metal as an oxidatively stable adhesion layer) are used as the electrode substrate. Tinned-copper wire is attached to the substrate with silver paint, covered in epoxy and attached to a glass tube with a liquid tight seal. All tinned copper, silver paint, glass components, and epoxy are covered in hot



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glue for impurity free analyses (Figure 2). We have found that typical epoxies, such as Hysol 1C, contain Fe impurities that affect activity measurements. Hot glue is a commonly available hotmelt polymer adhesive, e.g., obtained from Adhesive Technologies, and is dispensed from an inexpensive heated glue gun. We have found other brands work equally well. Suitable plastic tubing (e.g. polyethylene) could also be used instead of glass to eliminate need to mask the exposed glass from the electrolyte with hot glue.

Figure 2. Step-wise procedure for home-made electrode fabrication using an Au substrate. a) Tinned-copper wire is attached to the electrode with silver paint and covered with epoxy (inset). b) Wire is inserted into a soft glass tube. c) Au substrate is attached to the soft glass tube using epoxy. d) All glass, epoxy, and tinned copper wire are covered with hot glue.


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The mass loading on hand-made electrodes can be determined by redox peak integration, capacitance measurements, or inductively-coupled plasma analysis (see section 2.5). Typically, these electrodes are used with materials that have been first characterized with a QCM electrode and have a prominent and clearly defined redox wave that can be directly correlated with mass loading on the QCM or by elemental analysis. Measurements can also be performed on a Teflon-shrouded rotating-disc electrode (RDE), which provides for a clean cell environment if freshly polished and acid cleaned. We typically use such electrodes for measurements on glassy carbon. We note that for the OER reaction the high mass transport rates afforded by the RDE are not important due to the high concentration of dissolved hydroxide. We use a stir bar positioned at the surface of the WE to remove bubbles and have not seen a dependence on stir speed. For diffusion-limited systems (e.g. the oxygen reduction reaction), a rotating disk electrode is, however, essential.55 Counter Electrodes. The CE is partitioned within a plastic porous container, which we fabricate by drilling small holes into a small polyethylene centrifuge tube. Typical CE partitions are frits which are designed to allow for liquid connection, but minimize bulk solution mixing and keep the H2 bubbles from the CE from saturating the working electrode solution. Many frits are made of silica and release contaminants when slowly dissolved in base. We use a Pt CE because of its high HER activity, stability, and availability.56,57 The geometric surface area is sufficient such that the counter electrode does not limit the current through the cell, and we usually use a CE composed of a Pt mesh or wire with ~ 1 - 2 cm2 of exposed surface area. It should be noted that Pt can be dissolved under anodic conditions and transported electrochemically.58 This is important in the study of non-Pt HER catalysts because Pt can electroplate at the working electrode and is an extremely good HER catalyst. This is not an issue



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for OER testing because Pt is not a good OER catalyst and the CE is under cathodic potentials where Pt does not substantially dissolve. Reference Electrodes. For alkaline systems we use Hg|HgO reference electrodes with a Teflon case and alkaline-stable frit, which are available from CH Instruments (note that Hg is toxic and care should be taken to avoid damaging the inner case). The filling solution in the reference electrode is matched to the electrolyte solution in the cell, eliminating junction potentials or cross contamination. We keep one set of reference electrodes reserved for use in the cleanest, Fe-free electrolyte conditions. Saturated calomel electrodes (SCE) and Ag|AgCl reference electrodes are better suited for near-neutral-pH studies. They are often encased in glass components and use porous glass frits that are not alkaline stable. Thus they can leach Fe (among other contaminates from the glass) or KCl from the filling solution. Cl- is oxidized at competitive potentials to the OER,59 although an intermediate vessel can be used to mitigate solution contamination.60 Conversely if base leaches into the KCl filling solution, the RE electrode potential may shift, for example due to the formation of a second redox couple setting the reference potential (2Ag + 2OH- Ag2O + H2O + e-).28 We calibrate each reference electrode prior to initial use and test periodically for drift. Calibrations are performed using a reversible hydrogen electrode (RHE). An RHE can be easily made with at Pt WE and CE. H2 is bubbled over the WE. The CE is ideally partitioned to mitigate O2 contamination in the cell, but this is not an issue if currents are small. Both Pt electrodes are initially cleaned in aqua-regia and prior to each use cycled in 1 M H2SO4 (+/- 2 V). To perform the calibration, the clean Pt electrode is cycled +/- 200 mV at a slow scan rate (10 mV s-1) around the expected value for the reversible hydrogen electrode (RHE) in the


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electrolyte to be used for the catalyst testing (with good stirring using a stir-bar). Anodic of RHE, mass-transfer-limited hydrogen-oxidation current is measured. Cathodic of RHE, exponentially increasing hydrogen evolution current is measured. The RHE potential, versus the reference electrode under test, is determined from the potential of zero net current, which practically, we take as the average potential of the forward and reverse sweep at zero current. The standard potential for oxygen evolution is 1.23 V anodic of RHE (i.e. 0.3 V vs Hg|HgO in aq. 1 M KOH).46 Correcting for Uncompensated Series Resistance. The flow of electronic or ionic current requires an electrostatic potential drop within the electrochemical cell. Most of this potential drop is compensated for by the use of a reference electrode, which is typically placed near the working electrode within the cell. The potential drop caused by ionic current flow through the electrolyte in-between the WE and RE is uncompensated and must be corrected for. For a simple planar electrode with a uniform current density the uncompensated resistance is given by: =

(1)

where x is the distance from tip of RE to WE, A is the area of the WE, and is the solution conductivity.46 The uncompensated potential is thus given by iRu, where i is the current (not current density) flowing through the electrode. The main components affecting Ru are the position of the WE relative to the RE, the surface area of the WE, and the solution identity and concentration (Eq. 1). Ru can also be affected by the WE conductivity and any resistive components in series with the solution (i.e. bubbles on electrode surface, wires within the electrode).46,61 We minimize Ru by placing the tip of the RE close to the WE surface (typically at a distance of twice the diameter of the reference electrode to minimize current blocking effects),



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using supporting electrolyte at high concentration (~1 M), and using conductive electrode supports. We have explored the use of Luggin-(Haber) capillaries to reduce the RE and WE distance,46 but have found that practically these do not reduce the Ru sufficiently to warrant the extra effort for the moderate current densities (~10 mA cm-2) associated with fundamental studies and small catalyst loadings. We caution that while conducting oxides such as fluorine-doped-tin oxide and indium tin oxide are often used as OER catalyst supports, they are much more resistive than metals which can lead to large potential inhomogeneities across the electrode surface. They also slowly dissolve. When conducting oxides are used, we typically use small electrode sizes, catalyst loadings, and current densities (< 10 mA cm-2). Bubbles can also significantly increase the cell resistance in ways that cannot be simply corrected for. We remove them from the WE surface with rapid stirring. If bubble accumulation is significantly affecting the observed electrochemistry, we recommend reducing the electrode surface area or the catalyst loading. Once Ru has been minimized, it can be measured and used to correct the experimental polarization data after collection (Emeasured - iRu = Ecompensated). While there are a number of methods for measuring uncompensated series resistance, we find potentio-electrochemical impedance spectroscopy the most reliable. Figure 3 shows the impedance response for an Au QCM electrode measured using a Biologic SP200 potentiostat in a three-electrode mode. We apply an OER relevant potential (typically 0.6 V vs. Hg|HgO) and scan between 100 Hz to 1 MHz with a 10 mV AC amplitude. For the simple cell equivalent circuit shown in the Figure 3a inset, one expects that the impedance at the high frequency limit will approach the uncompensated series resistance as the capacitive impedances trend toward zero. However, the


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experimental data show that at high frequency (i.e. above 100 kHz) the imaginary impedance increases, suggesting contributions from cable and instrument capacitances and inductances that are not represented by the simple equivalent circuit. To estimate Ru, then, we equate it to the cell impedance at high frequency where the phase angle is closest to 0 degrees (and thus shows purely resistive circuit behavior). This is typically between 10 to 100 kHz. The results vary slightly (+/- 10-20%), so it is important to consistently use the same analysis technique.46,60 We also emphasize that the magnitude of Ru used for correction must always be reported in the experimental details. We note that many researchers compensate for their uncompensated resistance in situ using an automatic compensation function, typically at 80-85 % of the calculated Ru. We typically avoid compensating in situ (unless we are running chronoamperometry experiments at specific overpotentials) and prefer to collect uncorrected raw data and apply the correction after the experiment. This allows more flexibility with post experiment data analysis and reduces the chance of experimental error.

Figure 3. a) Circuit diagram with Nyquist plot. b) Bode plot for an Au QCM in aq. 1 M semiconductor grade KOH with a Hg|HgO RE at an applied potential of 0.6 V vs. Hg|HgO and AC amplitude of 10 mV. The uncompensated series resistance is estimated from the total impedance of the circuit near the high frequency limit (10-100 kHz) where the phase angle



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approaches 0 degrees and the system is purely resistive.

We measure Ru for every electrode, every time it is introduced into an electrochemical cell, both before and after our standard series of electrochemical characterization experiments (see below). A significant change in Ru before and after the other electrochemical measurements indicates the substrate or substrate connection is degrading.

As previously mentioned, we

recommend reporting the range of Ru for each substrate type measured. We also note that working and counter electrode positions should not be changed in the cell in between measurements as these will affect the magnitude of Ru.

Figure 4. Ru as a function of the RE-to-WE distance where starting distance is > 2d where d is the diameter of the RE to avoid problems with shielding. Data for an Au electrode without a catalyst film and for the same electrode with deposited (Ni0.75Fe0.25OxHy) are shown. Ru is the same with and without the film. The electrolyte is aq. 1 M KOH.

Not consistently accounting for Ru and changing the shape/size of an electrode, the substrate, the RE to WE positioning, or electrolyte concentration can have a significant impact 16 ACS Paragon Plus Environment

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on the data analysis. Small errors in iRu compensation can lead to order-of-magnitude errors in turn-over-frequency (TOF, see section 2.8) estimations, especially at higher currents, due to the exponential dependence of TOF on potential.

We note also that “overcorrection” of the

electrochemical data is relatively common. It can be immediately assessed by redox features that “bend back”, i.e. OER region shows increased current at lower applied potential, which is not physical. Further, if the Tafel slope is decreasing with increased overpotential, this is also usually an indication that the data is over corrected and the measured Ru should be re-assessed. Figure 4 shows the relationship between the WE-to-RE distance and Ru for both an Au electrode (0.8 cm-2) with and without a Ni(Fe)OxHy catalyst film. The dependence on distance from the WE is apparent, and illustrates the need to measure Ru for each electrode, each time the cell is assembled. We also note that for the Ni0.75Fe0.25OxHy catalyst deposited here, the measured Ru is identical whether the film is attached to the electrode or not. This is because the film is porous and permeated with electrolyte.26,62 The impedance measurement does not “see” the electrical resistance of the catalyst layer. This is expected for any electrolyte-permeated or porous catalyst film. If the film is dense, then the electrical resistance of the film will also be measured in series with that of the electrolyte solution.

2.2 Synthesis of Heterogeneous Catalyst Films For fundamental catalyst studies we deposit thin films directly onto a conductive support. To make the films we typically use spin-casting from soluble precursors, followed by annealing,14,63 or electrodeposition,26,36,38,40 as is discussed below. The thin-film architecture is useful for a number of reasons: 1) The catalyst electrical conductivity is less important because the potential drop across the film is minimized as charge carriers only need to move short



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distances (i.e. a few nm).36,40 2) Some catalysts are semiconductors, and thin films (< 5 nm) are not thick enough to support large depletion regions at catalyst-solution and catalyst-electrode Schottky barrier interfaces, which should allow for catalytic comparisons that are not dependent upon carrier concentration and/or interfacial contact properties.64,65 3) Due to the lack of largescale porous architecture, analyses are not substantially affected by differences in surface area (i.e. roughness) or by mass transport of gases and ions through the films.66,67 4) The mass loading can be directly measured in situ through the use of a QCM electrode, as described in section 2.5.3. Target Catalyst Phases. Potential-pH diagrams (e.g. Pourbaix diagrams) indicate the hydroxides or oxyhydroxides are typically the thermodynamically stable phases in alkaline solution at oxidizing potentials for Fe, Co, and Ni,27,28 which are contained in the most-active catalyst compositions in alkaline electrolyte. When starting with a metallic or oxide film, conversion to the hydroxide/oxyhydroxide typically occurs quickly.14,32,39,68 A number of reports also indicate that conversion to hydroxide/oxyhydroxide also occurs with non-oxide catalysts such as sulfides,17,18,69 selenides15,16 and phosphides.70–72 We therefore have directly synthesized hydroxide or oxyhydroxide phases in our studies of first-row transition-metal OER catalysts.62 This approach has been useful because the hydrated oxyhydroxide thin films do not appear to go through dramatic restructuring during the OER and thus we can more-easily assess the number of electrochemically active sites and connect structure and composition to OER activity. [Note: We typically consider the electrochemically active sites to be the metal sites in the mixed metal oxide or (oxy)hydroxide (see section 2.8). Although it is possible and likely that active sites are more complicated than this (see recent studies on film redox chemistry),33,42,44 this simplification allows for a straightforward comparison between different systems.]


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We deposit the catalyst directly onto the conductive electrode substrate without the use of binders or additives. Inks that incorporate catalyst powder, conductive carbon, and a polymer binder, such as Nafion, are often used in other studies.8–10,33 Binders and additives are needed when testing powdered catalysts that cannot be easily deposited onto a conductive substrate (e.g. perovskite oxides). The use of carbon binders in OER catalysis, however, should be undertaken with care. They are not electrochemically stable and oxidize under anodic conditions.73,74 Carbon-supported OER catalysts are thus not relevant for practical application (this likely extends to composites with graphene or carbon nanotubes). The polymer binder/carbon can also introduce impurities, cover up active sites, or otherwise affect the measured performance.75–77 The use of conductive carbon-based binders can mask factors such as poor conductivity that may limit apparent catalytic performance in carbon-free systems,10 and, even if a conductive binder is used, substantial resistive effects may be present if the catalyst particle size is larger than a few nm and it is a non-degenerate semiconductor. Measurements of the pure catalyst conductivity should always be made independently and the magnitude of the conductivity used to assess whether electron transport across the individual catalyst particle is expected to be a significant factor. Spin Casting. The spun-cast films we study are deposited from solution precursors. There are a large variety of possible precursors. For uniform thin-film coating of (oxy)hydroxide catalysts, we have found the following low-temperature recipe to work well. A precursor solution is made with metal nitrate salts in the stoichiometric ratio desired in the final film at a total metals content of 0.05 – 0.1 M. Nitrate is used as a counter-ion because it typically decomposes at low temperatures of 175 - 300 ºC.78 The solvent is a 1:1 mixture of ethanol and water. Ethanol lowers the surface tension of the aqueous solution, improving uniformity. The



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addition of surfactant (e.g. Triton-X 100) is sometimes necessary to improve wetting and film uniformity, as substrate roughness and differences in relative humidity in the spin coater can prevent full coverage.14 The substrates onto which the films are deposited are first cleaned by sonication in an aqueous detergent solution (Contrad 70, Decon Labs, 5% v/v), then rendered hydrophilic by oxygen plasma treatment. A hydrophilic surface for deposition is important to ensure uniform wetting by the hydrophilic precursor solution.79 Annealing of the as-spun film at relatively low temperatures (150 °C) allows for removal of water and some of the nitrate counter ions, but prevents complete conversion to a dense oxide phase. This allows for hydroxide formation upon immersion in base via nitrate-hydroxide ion exchange. Higher temperatures (~ 300 ºC) are needed to decompose the surfactant (if used) and this can lead to the formation of some nanocrystalline oxide phases.14 Electrodeposition. We cathodically electrodeposit hydroxide catalyst films from a 0.1 M (total metal) aqueous solution of transition metal nitrate salt(s).

The likely deposition

mechanism is that, upon application of cathodic current,the pH at the working electrode increases via a nitrate reduction or other related reactions:80 NO + 7H O + 8e → NH + 10OH

(2)

The pH increase causes the insoluble metal hydroxide to precipitate onto the electrode surface:80 M + nOH → M(OH)

(3)

where n is the oxidation state of the metal ion, M. Parameters such as film morphology, substrate coverage, and thickness can be varied via the value of the applied cathodic current density and the amount of time for which it is applied.26,81,82 The current density used also likely affects the deposition mechanism. For example, Merrill et al. report that at certain conditions the dominant mechanisms for Ni(OH)2 deposition was metallic Ni0 formation then immediate


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oxidation by NO3- in the solution.44 Because we use a galvanostatic (constant current) deposition, we typically use a two-electrode cell without a reference electrode. This electrodeposition method works well for first-row transition metals Mn, Fe, Co, and Ni, usually preventing deposition of the metallic form, which can be found in cathodic electrodeposition methods containing ions that cannot be easily electrochemically reduced to consume protons (e.g. nitrate).83 The nitrate salts of Mn, Co, and Ni are used directly, but for iron-containing films, FeCl2 is used as the metal source and NaNO3 as the nitrate source.38,39 We have found that Fe(NO)3 precipitates throughout the solution with time or upon application of cathodic current, likely due to the formation of insoluble FeOOH homogenously in solution.27,28 Deposition solutions containing Fe are therefore purged with N2 before and after addition of FeCl2 to prevent oxidation and precipitation. Anodic electrodeposition can also be used. We have anodically deposited Ni(Fe)OxHy (0 – 20 % Fe) from a ~ pH 9, 0.4 mM (total metals) Ni(NO3)2/FeCl2 solution based on the reports of Nocera and coworkers.19,37 Anodic deposition can be useful because the deposition current directly drives the oxidation of the soluble metal cation to form an insoluble phase. Therefore the deposition charge can in principle be directly related to the film loading,31 although this is complicated in multi-metal-cation systems where each cation has a different reduction potential. It has been also suggested that anodically deposited catalysts have increased connectivity and perform better at higher loadings.84 Using pulsed cathodic deposition is another way to achieve high connectivity at high loading (see below).26 We note that gold substrates are anodically unstable in chloride solutions due to the formation of soluble gold chloride species at the working electrode. Choosing a Deposition Method. There are advantages and disadvantages to each of the



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synthetic methods and therefore one method is typically preferable depending on the question that is to be answered. The main advantage of spin-casting (or similar techniques) is that the stoichiometry of the film can be precisely controlled because it matches that of the precursor solution.14,63 Accurate control of composition is important for studies of catalysts with multiple metals. Because film thickness is dependent on the precursor concentration, solvent, and spin speed,79 there is high reproducibility from sample to sample, even on different substrates. The films are also typically more uniform than the electrodeposited counterparts. The spin-casting method, however, is limited to thin films (~20 nm or less). Low-temperature nitrate removal becomes difficult with increasing thickness. In the cases where surfactants are necessary for wetting the substrate, temperatures of 250 - 300 °C are needed for their removal. Such heating often leads to formation of oxide14 rather than the hydroxide phases, which complicates studies of the active material. Electrodeposition is advantageous for thicker films, and to directly target (oxy)hydroxide phases. For thick films, a pulsed-electrodeposition method rather than a continuous one results in a more uniform composition with thickness, more compact morphology, and films that do not decrease in intrinsic activity with increased loading.26 We assume homogeneous mixing of the electro-precipitated metal cations in the hydroxide phases, but do not have direct experimental evidence. Composition control for mixed-metal films is more difficult for electrodeposition than for spin-casting because the amount of each metal cation depends on its local solubility at a given pH as well as its diffusion to the electrode surface. Therefore, the stoichiometry of the solution is normally different than that in the deposited film.26,36,38,63,85 Typically, we calibrate the desired film composition to the deposition conditions using direct analysis of the film composition and loading using, e.g., elemental analysis (see below). The dependence of the


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deposition process on the solubility of cations rather than the concentration in the solution also requires higher purity salts. Even low-level impurities may be concentrated in the electrodeposited films if they more readily incorporate than the target elements.

2.3 Electrochemical Measurements We routinely use cyclic voltammetry (CV), chronopotentiometry (galvanostatic), and chronoamperometry (potentiostatic) measurements to assess the electrochemical response of the catalyst films. Representative data for a nominal “standard” Ni(Fe)OxHy catalyst film is provided in Section 3.0. While the exponential part of the voltammogram shows a “snap-shot” of the OER current response at a given time, we prefer to use steady state measurements to assess OER activity because many catalysts dynamically change with time and potential. For CV we normalize current by the geometric surface area to yield current density. This allows for quick comparison between films on different sized electrodes, but nominally the same loading. See section 2.8 for how this relates to activity. In cases where we aim to assess the activity from the CV curve, we limit this analysis to systems with little hysteresis and calculate the activity based on an average of the current from the forward and reverse sweep to remove the capacitive background. Voltammetry is useful to assess the electrochemical response of the catalyst prior to the onset of OER, providing insight into the materials’ properties. Many catalysts exhibit redox features associated with the oxidation and reduction of “surface” metal cations (Figure 5). The integrated charge from these redox waves can sometimes be used to assess the fraction of metal cations that are electrochemically active, i.e. in electrical contact with the substrate and ionic contact to the electrolyte. See section 2.5.1 for a detailed discussion of how the integrated



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charge in the pre-wave feature can be used to assess electrochemically active metal cations.

Figure 5. Second cycle of CVs collected at 10 mV s-1 in aq. 1 M KOH for a) NiOxHy, b) CoOxHy, c) MnOxHy and d) FeOxHy films cathodically electrodeposited onto Au QCM crystals and tested in aq. 1 M KOH (Ni- or Co- cleaned for non-Fe catalysts). Adapted from ref [40] with permission.

For an “ideal” surface-attached redox-active film with fast kinetics and no electrical or ion transport limitations, the oxidation and reduction waves are identical in position and shape (with a peak full-width at half height of 90/n mV at 25 ⁰C, where n is the number of electrons transferred in the redox reaction).46 The peak height scales linearly with scan rate (which is why such response is often referred to as pseudocapacitance, classical double-layer capacitive charging currents also scale linearly with scan rate). The total integrated charge and peak width are independent of scan rate. The “ideal” response is Nernstian, that is the concentrations of the reduced and oxidized forms of the surface-attached redox species are in thermodynamic equilibrium with the electrode surface following the Nernst equation:46 !

&

= + "# $% &' (

(4) 24

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Where E is the electrode potential, Co and CR are the concentration of oxidized and reduced species, respectively (in the electroactive film), R is the gas constant, T is the temperature, n is the number of electrons involved in the reaction, F is Faraday’s constant, and Eº’ is the formal reduction potential of the redox couple in the film. Eº’ is equal to both the anodic and cathodic peak potential, Ep,a and Ep,c, in the ideal case. Nernstian response, however, is not observed for the redox features on OER catalysts and the degree of departure provides insight in the underlying electrochemical and material processes. The oxidation or reduction of a metal cation requires the movement of both electrons and ions. Both processes can contribute to non-idealities in the peak shape, including splitting of the anodic and cathodic peak potentials (i.e. Ep,a ≠ Ep,c), broadening of the waves, and the introduction of a scan-rate dependence to the shape and total integrated charge. Assuming that kinetic overpotentials equally shift Ep,a anodically, and Ep,c cathodically, of Eº’ (which is not in general true) then (Ep,a + Ep,c) / 2 can be used as an estimate of Eº’. Figure 5 shows examples of the type of redox behavior observed at a scan rate of 10 mV s-1. Electrodeposited NiOxHy exhibits a sharp redox wave with Ep,a - Ep,c of 73 mV. The splitting is the result of kinetic (electron transfer) and mass transport (i.e. ion motion) overpotentials, which are difficult to separate. The sharpness of the waves suggests a relatively homogeneous environment for the Ni species, presumably predominantly within well-defined nanosheets with the typical brucite structure commonly found for (oxy)hydroxides. Prior to oxidation, the nonfaradaic (capacitive) charging background is of the same magnitude as that of the underlying Au substrate. Electrodeposited CoOxHy has broader redox wave with Ep,a - Ep,c of 24 mV. The decreased splitting compared to NiOxHy is likely the result of faster charge- and ion-transfer kinetics. The increased width of the wave suggests more inhomogeneity in the local



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environments (e.g. possible electrolyte inaccessible regions) and thus a distribution of Eº’, which is consistent with the fact that the integrated charge in the CoOxHy (based on the first cycle) only accounts for ~ 60 % of the total Co in the film. For both NiOxHy and CoOxHy the integrated peak charge is typically larger for the first cycle compared to subsequent cycles; we interpret this in the context of the conductivity switching in the film26 as discussed in section 2.5.1. MnOxHy has a large pre-catalytic pseudocapacitance that is likely due to several redox transitions, each with heterogeneity and kinetic/mass-transport limitations, that are overlapping and indistinguishable (Figure 5c). The FeOxHy sample shows redox features and pseudocapacitance only originating from the Au substrate. FeOxHy is electrically insulating and electrolyte-permeable at these potentials (Figure 5d).

Steady-State and Tafel Analysis We typically collect Tafel (i.e. log current – overpotential) data using constant current analysis (from ~ 0.01 to 10 mA cm-2). Each current step typically requires 3-15 min to reach steady state where the potential is not significantly changing with time. The time to reach steadystate varies per each catalysts’ chemistry. We find it best to characterize the time response and tune the data collection for the specific film composition and testing current density. We typically do not perform long-time stability analyses for fundamental studies, although these are critical for practical applications. Much work is needed to understand catalyst stability and activity degradation during long-time scales of operation at high current densities and temperatures.86 Dissolution, plating of impurities, and catalyst restructuring are all possible deactivation mechanisms. Once the steady-state log(i)-overpotential plot is generated one can extract the intrinsic


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activity at any desired overpotential using the mass loading or electrochemically active surface area of the film. The Tafel slope (from linear regression) is a kinetic descriptor that, in principle, provides information on the electrochemical mechanism. However, we avoid drawing mechanistic conclusions from the Tafel data because we find Tafel slopes to vary widely for similar material classes (i.e. by a factor of two or more). The application of Butler-Volmer theory46 for their mechanistic analysis is probably an oversimplification.

2.4 Catalyst Conductivity Comparisons Electrocatalysis requires electrical conductivity to the active site of the catalyst. It is thus not possible to measure the “intrinsic” activity of an electrically insulating catalyst. Therefore, we measure the electrical conductivity of each catalyst synthesized to understand whether or not the electrical conductivity is affecting the measured OER activity. For a powdered solid catalyst one can measure the electrical conductivity in a dry pressed pellet or in a powder compression cell.87,88 These data are useful as they provide information on the bulk electrical properties of the catalyst phase. Given, however, that many catalysts dynamically restructure under OER conditions, measurement of the electrical conductivity ex-situ is not sufficient. To address this challenge, we have adapted the use of interdigitated array microelectrodes (IDAs) to measure electrical conductivity in situ – i.e. in electrolyte and under OER relevant potentials.36,38,89



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Figure 6. a) IDA with two working electrodes (WE1 and WE2, black) covered in film (brown) with conduction current (white arrows) flowing between WE1 and WE2. b) CoOxHy CV (red) at 10 mV s-1 overlaid with the effective conductivity as a function of potential (black) with a 10 mV offset between WE1 and WE2. c) Effective conductivity of several first row TM (oxy)hydroxides in aq. 1 M KOH. Adapted from Ref. [38] and [40] with permission.

Figure 6a shows the IDA geometry, which consists of two interdigitated working electrodes, coated with a catalyst film. We typically use an IDA with 2 µm gaps between the fingers, as these are the smallest that can be easily commercially obtained. For the IDA, the catalyst coating 28 ACS Paragon Plus Environment

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can be difficult as the catalyst film must be thick enough to uniformly bridge WE1 and WE2. We typically electrodeposit the catalyst films 1-2 µm in thickness, although other methods could be used. To measure the conductivity, we perform a “dual-working-electrode” experiment using a bipotentiostat. This type of measurement would also be possible using a single potentiostat and an appropriate voltage source and current measuring unit (such as a Keithley 2400 source-meter unit). When a bipotentiostat is used, WE1 and WE2 are simultaneously stepped in potential with a small voltage offset (usually 10 mV) to drive electrical conduction between the electrodes. The current on both electrodes is separately measured by the bipotentiostat. The conduction current can be calculated because the current at each working electrode can be considered the sum of the OER current (IOER), the current from catalyst oxidation (Icat), and the through-film conductivity current (Icond).

)*+, = )-+

,

+ )./0, + ).1"2

)*+ = )-+

+ )./0 − ).1"2

)-+

,

≈ )-+

)./0, ≈ )./0 )*+, − )*+ ≈ 2).1"2

The catalyst loading must be similar on both WE1 and WE2 such that the OER currents flowing at each are similar and cancel in the above set of equations. When a secondary voltage source and ammeter is used in conjunction with a singlechannel potentiotstat (instead of using a bipotentiostat), it is important to ensure that the



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potentiostat is operating in floating mode and connected to WE1. The external voltage source is connected between WE1 and WE2 and set to apply an additional 10 mV and an ammeter is used to measure current between the two. Now, however, the potentiostat measures all the current flowing to WE1 and WE2 from solution, so the equations given for calculation of the conductivity must be rearranged. The effective conductivity, σcond, is calculated from Icond by: 6788 =

9:; ?@2∆B

(5)

The symbol w is the IDA gap spacing (2 µm), N is the number of electrodes (130 electrodes), d is the film thickness (measured using a stylus profilometer, usually 1-2 µm). We term this an effective conductivity because Eq. 5 does not take into account the true geometry of the current path from the IDA fingers through the catalyst film.

Figure 7. a) FeOxHy (black) and MnOxHy (purple) CVs overlaid with effective conductivity in aq. 1 M KOH. Note how conductivity onset is coincident with OER current onset for these materials. b) Example of raw conduction data (at η = 500 mV) for FeOxHy conductivity measurements. The voltage-step profile used to correct for the background OER current are shown on the left axis. Adapted from Refs. [38] and [40] with permission.

Ni-based and Co-based materials have an onset of conductivity during the oxidation of


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from M(OH)2 to (nominally) MOOH (Figure 6). In the oxidized state the Co and Ni catalysts have a relatively high conductivity. Other materials, such as FeOxHy and MnOxHy, have very low conductivity and measurements of conductivity in situ are complicated by the fact that the electrical conductivity of the film is not substantial until the film starts to pass OER current (Figure 7a). In order to extract the conductivity current on top of the larger background of OER current we use a potential stepping procedure where the WE1 and WE2 potentials are switched in polarity. The conduction current is given by the current step observed at one electrode (i.e. WE1) when the potential is changed at the other electrode (i.e. WE2) as indicated in Figure 7b. The insulating nature of FeOxHy in particular dramatically affects its ability to perform the OER. This observation of low electronic conductivity explains the previous reports of FeOxHy being a poor OER catalyst. For very thin films on conductive substrates, however, we have shown that FeOxHy is the best first row transition metal catalyst for OER in alkaline media.40

2.5 Determination of Film Loading Quantifying total catalyst loading and determining the fraction that is electrochemically active and participating in catalysis is critical to comparing catalyst activities (see section 2.8). Despite this importance, many catalyst studies do not report the amount of catalyst used. Assessing mass loading of a catalyst is simple when the catalyst is prepared as a powder and made into an ink with known catalyst weight fraction. The surface area of the catalyst can be estimated from gas absorption measurements of the catalyst powder ex situ.66,87,90 This provides a useful estimate of the initial catalyst surface area, but does not report on dynamic changes that may occur due to the reactivity of the oxide surface that may dramatically change the number of



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electrochemically accessible active sites. For thin-film catalysts we use different techniques to estimate loading and the number of electrochemically active sites. These include the integration of redox waves (and correlation of the charge in the redox wave to the number of electrochemically accessible sites and/or mass loading), the measurement of double-layer capacitance, the use of a QCM to measure total mass directly in situ, and the use of inductively coupled plasma optical emission spectroscopy (ICP-OES) to quantify the amount of each element. Multiple methods are needed to assess the total amount of material deposited and to distinguish between electrochemically accessible and inaccessible catalyst. In the following sections we describe our practices for each technique and how we combine them to provide improved analysis. We emphasize that each measurement is best made as a function of mass loading to assess whether the response scales as expected.

2.5.1 Redox Wave Integration. The size of pre-OER redox waves can give insight into electrochemically active surface area.66 However, such analysis can be difficult because 1) the number of electrons per number of metal sites has to be known, 2) thicker films with potentialdependent conductivity switching (see below) may not have fully reversible redox features, 3) the catalysts must have redox features within the electrochemical stability windows of the catalyst and the aqueous electrolyte and 4) at high current densities not all metal sites electrochemically available in the redox wave will be active due to bubble accumulation or other ionic transport limitations. Of the catalysts that we have tested, only films containing Ni or Co have clear pre-catalytic redox features (Figure 8). We focus on NiOxHy or Ni(Fe)OxHy films below.


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Figure 8. Redox features as a function of Fe content (x) in a) Ni(1-x)FexOOH and b) Co(1x)FexOOH in

aq. 1 M KOH. Adapted from Refs. [36] and [38] with permission.

The redox wave corresponds, nominally, to a change in oxidation state of the metal cation (we caution that this simple picture is not entirely correct, these metal oxides/oxyhydroxides are covalent compounds with delocalized energy bands and the electronic details of the redox transitions are more complicated than typically considered).42 The size of the wave can serve as a valuable feature to measure the number of metal atoms participating in the reaction. The current i recorded during a voltammetric sweep is a measure of the charge passed Q over a period of time t: DE

C = D0

(6)

Since the x-axis is a measure of potential E, the relationship linking time to potential is needed: E = υt

(7)

where υ is the scan rate in V/s. Integration of i with respect to t thus yields the amount of charge passed. Note this relationship is only valid when data is not compensated for uncompensated series resistance during data collection and thus the sweep rate does not depend on the current magnitude. 33 ACS Paragon Plus Environment


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There are several assumptions necessary to estimate the number of electrochemically active cations. First, the initial and final redox state stoichiometry must be known or assumed. For Ni (oxy)hydroxides in alkaline media, the redox reaction can be written as: Ni(OH)2 + OH- ⇌ NiOOH + H2O + e-

(8)

In this model, we thus assume the redox is a one electron transfer (see section 2.5.3 for more details on this simplification). The active material, however, may involve Ni in (nominal) oxidation states other than 3+, and the active phase is often represented as NiOxHy to account for mixed valencies. A second simplification is that the measured Ni redox wave accounts for all the electrochemically accessible Ni atoms.

The validity of these assumptions depends on the

loading, scan rates, and choice of anodic or cathodic voltammetric sweeps for integration. First, we measure redox features as a function of scan-rate. For an ideal surface redox process, the integrated charge in the wave is independent of scan rate. Kinetic and (ionic) mass transfer effects, however, can convolute such response. Typically, scan rates of 1 to 20 mV s-1 are sufficiently slow for loadings up to a few tens of µg cm-2 (Figure 9). If the mass loading is low, faster scan rates can be used. In any case, the charge in the wave should be assessed in a regime where it is independent of scan rate.

Figure 9. Dependence of apparent catalyst loading on measurement scan rate for nominally 34 ACS Paragon Plus Environment

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Ni0.85Fe0.15OxHy. To calculate mass loading from the redox wave we assume 1 e- per Ni in the redox transition and a formula unit of Ni0.85Fe0.15OxHy factoring in the fraction the film that is not redox active (i.e. the Fe cations). The loading measured by redox-wave integration (of the first anodic peak, see below) consistently undercounts the actual mass of material deposited as measured by QCM if the scan rate and loading are increased. Figure adapted from Ref. [26] with permission.

We also assess the reversibility of the redox features. As discussed in Section 2.4, many of the (oxy)hydroxide catalysts are conductive under anodic OER potentials, but electrically insulating at their rest potential. If the film is completely reduced, the first anodic sweep will oxidize all the metal cations (Figure 10a). The reverse cathodic sweep, however, only reduces a fraction of the cations in the films because the reduced catalyst (which is nearest to the conductive electrode) becomes electronically insulating and isolates the rest of the catalyst from further reduction.91,92 The next anodic sweep therefore only measures the fraction of the catalyst that was able to be reduced. This “charge-trapping” effect is exacerbated as the film thickness is increased. Figure 10b shows the ratio of the first cathodic sweep (C1) to the first anodic sweep (A1, i.e., which “counts” all the accessible cations) for Ni(Fe)OxHy films as a function of mass loading and deposition method. In order to ensure our voltammetric analysis of the redox wave appropriately counts all of the redox-active cations in the film, we 1) make sure the film is starting in the fully reduced state and 2) measure the redox charge on the first anodic sweep which ends at a potential where the film is in an electrically conductive state. For cathodic electrodepositions, the as-deposited film is fully reduced. For films that have undergone electrochemical treatment, holding the



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potential at -0.5 V vs. Hg|HgO until the light green/transparent color of the film completely returns and reduction currents stabilize (often ~ 10-20 min) is sufficient to return the film to the fully reduced state.

Figure 10. Change in redox wave size due to potential-dependent conductivity switching. In a) the change in redox wave due to charge trapping is highlighted for CVs collected at 10 mV s-1. In b) the ratio of integrated charge of the first cathodic to the first anodic wave as a function of loading is shown for two different deposition methods. The variation in behavior between the two deposition techniques is the result of changes in the microscale architecture of the films.26 Figures adapted from [26] with permission.

The final challenge to ensuring an accurate peak integration is interference from background OER current. For the anodic peak in Figure 10a, the OER current can be subtracted using an exponential background. However, for Ni(Fe)OxHy with high Fe content, the Ni redox wave is underneath the OER current onset (Figure 8a).63 Electrochemical online mass spectrometry of the evolved gas has been employed to separate the OER current from the Ni 36 ACS Paragon Plus Environment

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faradaic current.93 Otherwise one must measure the scan-rate dependence and use a careful background subtraction to assess the faradaic contribution. In summary, to obtain accurate measurements using peak integration, we consider the scan rate employed, the potential-dependent conductivity behavior of the catalyst materials and its effect on the choice of redox waves, and the extent of overlap between redox and OER current.

2.5.2 Capacitance OER catalysts have the ability to act as capacitors and build-up charge at the film/electrolyte interface. Therefore, measuring the (differential) capacitance, Cdl, can be a useful tool for determining the electrochemically active surface area (ECSA), which can be related, with knowledge of the “surface” structure, to the number of accessible cations. Two methods prevalent in literature for obtaining Cdl values are cyclic voltammetry and impedance spectroscopy.6,7 The differential capacitance of a material is a measure of the amount of charge stored for a given change in potential:

2E

GDH = 2+ =

IJK L

(9)

Cyclic voltammetry can be used to obtain the Cdl of the film by measuring the current-voltage response as a function of scan rate (M). A plot of double-layer charging current (idl) vs. M results in a linear plot with a slope equal to the Cdl. Complications in measuring Cdl arise from potential-dependent conductivity (see section 37 ACS Paragon Plus Environment


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2.4) and faradaic redox waves in the potential regions being scanned. To avoid faradaic current, potentials sufficiently cathodic of the OER (and away from metal cation redox features) have been proposed as an appropriate region for Cdl measurement.6–8 However, many catalyst systems are not conductive until they are oxidized, which is often immediately succeeded by the onset of the OER. This problem can be, in principle, overcome by using impedance spectroscopy. As shown in Figure 11, Cdl can be extracted from impedance using a simple equivalent circuit.26 With more complicated circuits, additional parameters can be obtained.94–96

To find an

appropriate potential (range) to collect impedance data, we measure Cdl as a function of potential. Cdl is much higher where the film is conductive (Figure 11a) and it is in this regime were it can be used to estimate ECSA. Figure 11b and 11c show the impedance response from the same catalyst film held at different potentials where it is insulating and conductive, respectively.

Figure 11. Double-layer capacitance data of a Fe-free NiOxHy film showing a several order of magnitude difference in Cdl (and charge transfer resistance Rct values) depending on the potential region in which the data is collected. The apparent uncompensated series resistance Ru values are independent of potential, consistent with an electrolyte-permeated catalyst. In a) the Cdl values are overlaid with a forward (blue) and reverse (red) potential sweep with the equivalent 38 ACS Paragon Plus Environment

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circuit used to model the data shown as an inset. The experimental (squares) and model fit impedance data are shown at b) insulating and c) conducting potential regions of the NiOxHy CV (a). Note the change in unit between b) and c). Figures adapted from [26] with permission.

As shown in Figure 12, the electrode capacitance in a region where the catalyst film is not conductive is simply that of the underlying substrate and is independent of the amount of catalyst deposited. Under these conditions Cdl is not useful for estimated ECSA and should not be used. In Ni(Fe)OxHy systems with even a few Fe %, there is little, or no, potential region where the film is both conductive and no faradaic current is being passed, complicating Cdl assessment.

Figure 12. Double-layer capacitance (Cdl) of Ni(Fe)OxHy films from impedance analysis. Films held near 0.0 V vs Hg/HgO (blue, open symbols) are insulating, with Cdl values that do not scale with loading and are similar to the bare electrodes they are deposited on. Films held at 0.52 V vs. Hg/HgO (red, closed symbols) are conducting, with Cdl that scale linearly with loading and that are three orders of magnitude larger than Cdl of the insulating films. Figures adapted from [26] with permission.



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Once accurate values of Cdl have been measured, these can be converted to ECSA using the areal capacitance of the catalyst. Unfortunately, areal capacitance values are not usually known. For Ni(Fe)OxHy, we have determined an areal capacitance of ~80 µF cm-2 in the charged state. This is similar to (i.e. roughly twice) the average areal capacitance suggested for use in oxide systems.7 This could be due, in part, to both sides of the Ni sheets being capable of participating in (non)faradaic processes. In summary, both impedance spectroscopy and cyclic voltammetry can be used to obtain the Cdl of a catalyst film. However, conductivity switching, faradaic current overlap, and areal capacitance determinations are significant limitations to its use. Because of these challenges, we caution the use of Cdl to determine intrinsic OER activity or for catalyst comparison,6–8 and suggest that it is only used when the catalyst under study is electrically conductive with a welldefined solid/electrolyte interface, or when an alternate technique is viable for confirming the amount of electrochemically active material.

2.5.3 Quartz Crystal Microbalance Mass Measurements We typically use an electrochemical QCM as our primary source of loading analysis. The QCM allows for quick in situ mass analysis to assess both the total loading and possibility of catalyst dissolution or impurity plating during testing. We find QCM analysis is an invaluable, inexpensive tool for electrocatalyst study and development. The use of QCM measurements, however, is not without complications. 1) Films must be rigidly coupled to the substrate to accurately calculate their mass. 2) An analysis of intrinsic activity assumes that the total mass of the film is active, thus returning an activity that can vary


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based on internal surface area and morphology. 3) A conversion factor to calculate total metal sites from the mass has to be defined based on a molecular structure which may not accurately account for strongly absorbed electrolyte ions or water. To account for these possible limitations, we characterize the film-substrate rigidity, electrochemically probe the active “surface” of the film, and both approximate and measure the gram-per-mole conversion factors for the catalyst systems we study. Below we describe how this is performed for NiOxHy and/or Ni(Fe)OxHy. Validation of the Sauerbrey Equation. The QCM is an electrically driven oscillator whose mechanical resonance frequency is related to the mass of the oscillator. One electrode can further serve as a working electrode in an electrochemical cell (i.e. EQCM). The film mass can be calculated by using the Sauerbrey equation: ∆N = −G8 × ∆P

(10)

where ∆N is the experimental resonant frequency change before and after film deposition, P is the mass change per area, and G8 is the sensitivity factor of the 5 MHz AT-cut crystal. G8 can be mathematically expressed by

8QR STU VU

where N is the resonant frequency of the crystal, A is the

piezoelectrically active area, WX is the density of quartz, and YX is the shear modulus of the crystal.97 We calibrate the QCM sensitivity factor periodically using the well-defined electrodeposition of Ag metal.14 When using the Sauerbrey equation to calculate mass, one is assuming that there is uniform thickness across the resonator electrode and that the film is rigidly coupled to the substrate. It is possible to deposit a film where the film viscoelasticity affects the crystal resonance and invalidates the Sauerbrey relationship.98 To assure rigid coupling, we measured the crystal’s conductance (G) as a function of frequency before and after the deposition of a film to directly probe the quality factor (i.e. “sharpness”) of the crystal resonance peak.97 The peak in 41 ACS Paragon Plus Environment


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conductance occurs at the resonant frequency and the full width at half maximum (FWHM) is an indication of degree of viscoelastic damping.

Figure 13. Au quartz crystal (no film, red) and Ni(Fe)OxHy (film, blue) in a) air and b) aq. 1 M KOH. The conductance (G) is given by the real part of the complex admittance Y = 1/Z where Z is the complex impedance, and can be measured directly with any impedance analyzer operating in two-electrode mode. The change in the resonant frequency (maximum conductance) between a film-loaded and bare crystal can be assigned to additional mass of the film because the FWHM of the crystal conductance peak does not change.

For the catalyst films studied here, the observed FWHM is affected primarily by damping of the resonator by the liquid (with and without the catalyst) and not significantly by the deposition of a film (Figure 13). There are no statistical differences in FWHM between a bare Au crystal and a crystal with an electrodeposited Ni(Fe)OxHy film when measured in air or aq. 1 M KOH. The FWHM increases ~20 fold when the crystal is placed in aq. 1 M KOH due to damping from the liquid. The mass of the film is nominally the same when measured in air and solution. These data support a model of a film rigidly coupled to the substrate. Comparison of QCM Data to Peak Integration and Direct Elemental Analysis. A mass measurement from a QCM takes into account the entire mass of the deposited film (with 42 ACS Paragon Plus Environment

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possible contributions from bound/intercalated aqueous species) and is not sensitive to the film composition. Once we have established that the Sauerbrey equation holds (above), we calculate material loading using the sensitivity factor (G8 ) and then the number of active sites using an approximate molecular weight.

Figure 14: Comparison between mass loading calculated from peak integration and from the QCM ∆N for NiOxHy. Loading on the y-axis is calculated from the first anodic Ni redox wave (10 mV s-1) with the assumption of 1 e- per Ni cation in the film.

For cathodically deposited Ni-based catalysts, either Ni(OH)2 or NiOOH (93 or 92 g mol-1) formula units are reasonable (based on supporting diffraction and photoelectron spectroscopy analyses). The number of Ni cations in the films calculated from QCM mass closely matches that obtained from the integration of the Ni redox wave (assuming 1 e- per Ni), supporting a picture of “bulk” or “volume” electrochemical activity where every metal cation is both ionically wired to the electrolyte and electronically to the electrode (Figure 14). The linearity with loading in Figure 14 also further demonstrates the absence of viscoelastic effects affecting the QCM



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measurements.

Figure 15: Comparison of ICP-OES, QCM, and redox wave integration data for determination of Ni loading. a) Moles of e- from CV integration as a function of the moles of Ni from an electrode film dissolved for ICP from cathodically deposited NiOxHy films on home-made Au electrodes in aq. 1 M KOH. b) Mass from the QCM as a function of the moles of Ni determined by peak integration from cathodically deposited NiOxHy film on QCM Au in aq. 1 M KOH assuming 1.53 moles of e- per mole of Ni (from a).

To further check the reliability of the QCM method and develop a fundamental understanding of the molecular entities which compose the films, we dissolve our films and measure the number of Ni atoms using ICP-OES analysis (Figure 15). The dissolution is completed with 2 – 3 mL of 10 % HNO3 over 30 min at room temperature. A 1 mL aliquot is then taken and diluted to 1 % HNO3. Any surfaces on the electrode that are not on the active substrate are wiped clean prior to dissolution. We have had good reproducibility dissolving films from home-made electrodes with sufficient film mass loading (> 5 µg). We can then compare the integrated CV waves from a QCM film to the redox features of NiOxHy on a home-made electrode that was later dissolved for ICP-OES. Using ICP-OES, we find a 1.53 e- per Ni consistent with much of the older literature (Figure 15a).99 Using 1.53 e- per Ni instead of 1 e44 ACS Paragon Plus Environment

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per Ni we calculate a molecular weight of 147 g mol-1 per redox center (Figure 15b). The extra mass relative to the NiOOH formula unit is presumably ions and water. We primarily use the mass calculated from the QCM for our activity and stability comparisons because it is an easy, inexpensive, in situ, and reproducible technique that can be used across the entire OER catalyst composition range. It lacks only the ability to distinguish active from inactive material. For ICP-OES characterization 1) films must be dissolved for analysis, no in situ monitoring is possible, 2) it is non-trivial to setup and film dissolution is based on catalyst composition and structure, and 3) thin films may be below limit of detection.

2.6 Characterization of Film Stability Long term current stability (i.e. the ability of a catalyst to maintain OER current for a specified amount of time) is essential for practical water electrolysis. There are two main deactivation mechanisms, 1) loss of catalyst material (e.g. by dissolution) and/or 2) catalyst conversion to a less-active material (e.g. morphological and/or molecular structure changes).



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Figure 16. In situ stability analysis of Co- and Ni- based materials. a) Electrodeposited Co(Fe)OxHy mass (left, closed squares) and current (right, open right triangles) stability during a polarization at η – iRu = 350 mV in aq. 1 M KOH for 2 h as a function of Fe content. The grey boxes highlight different regions of stability behavior as a function of composition. b) In situ mass change during a ~ 1 h steady-state chronopotentiometry Tafel analysis of spin-cast Ni(M)OxHy (M = Ce, La, Mn, Ti, Fe, ~ 10 %) in aq. 1 M Ni-cleaned KOH. All data are from Au QCM crystal electrodes. Adapted from Reference [38] and [63] with permission.

We simultaneously monitor the OER current and film mass to assess short-term stability. We emphasize that these measurements are not sufficient to make conclusions with regard to practical stability. Such measurements would need to be made under practical operating conditions (e.g. high current, hot electrolytes), in practical cell geometries. Although numerous claims of stability are made in the literature for various OER electrode designs, studies of


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practical stability and degradation mechanisms are in fact sorely missing. An example of our approach, which focuses on gaining fundamental understanding, is given for Co(Fe)OxHy during a 2 h polarization at an overpotential of 350 mV (Figure 16a). The films are deposited by electrodeposition on Au QCM electrodes. There are three composition regions of interest. Films with Fe < 60% have stable masses (composition region 1), but show a large decrease in OER current by the end of the polarization. XPS analysis before and after polarization shows that the oxide character in the film is increasing suggesting a structural conversion occurring that is responsible for a reduction in the number of active sites. In region 2 there is a small compositional range where the current and mass are fairly stable for 2 h. In region 3 with high Fe content, the current increases while the mass decreases. This would be consistent with FeOxHy dissolving that has formed separated domains from the Co(Fe)OxHy. As these (electrically insulating) FeOxHy domains dissolve, more of the Co(Fe)OxHy is activated for OER. We have separately shown that pure FeOxHy films dissolve rapidly in aq. KOH via QCM measurement.39 Although others have used ICP analysis to monitor dissolution of films,34 these measurements have typically not been performed in alkaline media. We have not used ICP analysis of the aq. 1 M KOH due to detection limitations when monitoring the mass change of a thin film into a large volume of solution, and the need to neutralize the large concentration of base before ICP analysis. As a separate example of catalyst stability, Figure 16b shows the change in mass for Ni(M)OxHy (M = Ce, La, Mn, Ti, Fe, ~ 10 %) at a series of different current densities (total measurement time is ~ 1 h). The constant mass observed during the steady-state Tafel analysis confirms that changes in catalyst mass are not affecting Tafel response.



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2.7 Catalyst Film and Electrolyte Purity For accurate measurement of catalyst activities, it is important to use the highest purity electrolyte. Unless otherwise noted, we use semiconductor grade KOH pellets from SigmaAldrich (99.99% trace-metals basis). Even with the highest-purity electrolytes commercially available we find it necessary to remove Fe impurities. Fe impurities are known to affect the activity of both catalyst films (e.g. NiOxHy or CoOxHy)35–38,100 and common electrode substrates (e.g. Au).39,51 When studying catalyst films containing mainly Ni or Co, the corresponding hydroxide can be used to remove Fe impurities from alkaline electrolyte, as described in our previous publications.36–38

First, ~ 2 g of Ni(NO3)2·6H2O (Sigma, 99.999% trace metals basis) or

Co(NO3)2·6H2O (Strem Chemicals, 99.999%) is dissolved in ~ 5 mL of 18.2 MΩ·cm H2O in a 50 mL plastic centrifuge tube. About 20 mL of aq. 1 M KOH (semiconductor grade, Sigma, 99.99% trace metals basis) is added to precipitate Ni(OH)2. After ~ 5 min of shaking, the tube is centrifuged for ~ 10 min and the supernatant decanted. The precipitate is then rinsed by adding ~ 2 mL aq. 1 M KOH, ~ 20 mL 18.2 MΩ·cm H2O, shaken for 5 minutes and centrifuged again. The supernatant is decanted and the rinse repeated a second time. The centrifuge tube is then filled with the electrolyte (50 mL) to be cleaned, shaken vigorously for ~ 10 min and allowed to rest for at least 3 h (we typically let it sit for 24 h). Finally, the tube is shaken again, centrifuged for ~ 10 min (Ni(OH)2) or 1 h (Co(OH)2) and the resulting supernatant is decanted into a plastic container for storage/use (the tubes are previously cleaned with 1 M H2SO4). One caveat of the Ni- or Co-cleaning method is that residual Ni or Co cations remain in the electrolyte from the cleaning process and these can be deposited on the film/electrode during analysis. This can complicate the determination of electrochemically active surface area, catalyst


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stability, and activity – especially if one aims to study systems in the absence of Ni or Co. In these cases, the Ni-cleaned electrolyte can be further purified via a cation-exchange resin, a method adapted from Roger and Symes.101 As shown by Roger and Symes, the resin does not remove Fe ions,101 therefore electrolyte must first be cleaned with Ni(OH)2 to remove Fe impurities and then with the resin to remove residual Ni ions.

It is unclear if residual

iminodiacetate ligands from this cleaning procedure impact the catalytic behavior of the film; other resins may perform better but we have not yet systematically studied them. Test for Residual Fe Impurities. An acid-cleaned metallic Au electrode (see section 2.1) can be cycled in the electrolyte to determine if the electrolyte is free of relevant concentrations of Ni or Fe (Figure 17). The OER current for an Au electrode should remain similar with cycling; a significant increase in OER current with cycling is indicative of Fe contamination. We have shown that monolayers of Fe impurities on Au electrodes dramatically enhance the baseline activity of the Au electrode.39 The Au electrode cycled in Ni- or Cocleaned base will also slowly accumulate Ni or Co ions,38,63 evident by the redox wave that grows in with cycling (not shown in Figure 17). We advocate this simple test with the Au electrode to assess level of Fe impurities in electrolyte before and after cleaning.



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Figure 17. Effects of electrolyte impurities on cyclic voltammetry (20 mV s-1) of an Au electrode in ACS grade (yellow, Sigma), semiconductor grade (blue, Sigma), and Ni(OH)2-cleaned semiconductor grade (pink) aq. 1 M KOH. Cycle 5 is shown for each.

Quantifying Fe Impurities in Thin Films. Even with proper cleaning procedures, we find it important to also conduct a chemical analysis of catalysts after electrochemical characterization. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are particularly useful for compositional analysis of films. We find empirically that XPS and ToF-SIMS are sensitive to Fe concentrations of about 1 and 0.01% of the total metal in a thin film, respectively. For most studies, observing no Fe impurities by XPS using high-resolution scan of the Fe 2p region is sufficient to conclude that Fe impurities are not substantially affecting the electrode OER performance. Consideration of X-ray source, catalyst composition, suspected impurities, and substrate must be taken into account when using XPS to identify impurities. Common first-row transition metals and noble metal substrates used for OER catalysis have overlapping photoelectron and/or Auger lines, which could lead to obstruction or false identification of impurities. For example, with an Al Kα source, both Ni and Co have Auger lines that overlap with the 2p peak for Fe, which is easily resolved by using a Mg Kα source.102 The Fe 2p signal also overlaps with the 4s photoelectron line of Pt and the Mn 2p peak overlaps with the 4s peak of Au.102 As these are not Auger lines, they can only be avoided by measuring a film that is prepared and analyzed in nominally the same way, but on a different substrate.

These interferences should also be

considered when using XPS for compositional analysis in mixed-metal films. Figure 18 shows the Fe 2p spectra of Fe-free, Fe-contaminated, and Fe-co-deposited Ni(Fe)OxHy.


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Figure 18. Fe 2p X-ray photoelectron spectra of NiOxHy films taken with Mg Kα excitation. The as-deposited (black) and cycled (blue) in Ni-cleaned aq. 1 M KOH films show no Fe content. Films in aq. 1 M KOH (TraceSelect, Fluka 99.995%) after 12 min without an applied potential (green) and after 5 cycles (purple) show appreciable Fe content.

An as-deposited

Ni0.75Fe0.25OxHy film is shown in red. Adapted from Ref. [14] with permission.

In some cases, we perform trace analysis of impurities in the solid films using ToF-SIMS, which can detect Fe in films that appear Fe-free by XPS. This level of sensitivity is important when working with Au-based catalysts, which are extremely sensitive to Fe impurities. ToFSIMS, however, requires calibration. To assess the ratio of Fe impurities relative to another metal in the film, a standard with a known ratio must be used.103 The standard we use is typically a film synthesized in the same manner as the sample of interest63 – this ensures comparable sputtering rates by the ion beam. Our standard film, typically Ni(OH)2 with Fe, is prepared such that the concentration of Fe is 2% or less of the total metal in the film. We use XPS to obtain the ratio. This standard film is sputtered and analyzed by ToF-SIMS (during the same run as the sample of interest) to determine a relative sensitivity factor between Ni and Fe



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according to: 9Z[ 9\]

_?I`

^ = _#7`

(11)

where I is the signal intensity from the detector, δ is the sensitivity factor, and [Ni]/[Fe] is known from XPS. This sensitivity factor is then used to obtain a quantitative ratio of Fe to total metal in the sample of interest.

2.8 Calculation of Catalyst Activity Metrics In order to accurately compare catalysts, inform computational efforts, and ultimately design better catalysts, appropriate activity metrics are important. For fundamental studies, we calculate metrics that are normalized in some way to the amount of active catalyst material. The simplest metric is the current passed per unit weight (i.e. mass activity in A g-1) at a given overpotential (i.e. 200 - 400 mV). The weight of the film can be measured directly using the QCM electrode or via ICP analysis using the methods described above. The overpotential should be corrected accurately for iRu, which is different for every cell and electrode. The limitation of this metric is that it does not differentiate between the catalyst that is electrochemically accessible and that which is not, nor does it convey chemical insight into the reaction pathway. A different activity metric is the turnover-frequency (TOF), defined as: cdH e /g

inoomj/#

R TOF = (cdH hijklm gkjmg) = (cdH hijklm gkjmg)

(12)

where F is faraday’s constant (note that although IUPAC defines TOF as molecules reacting per active site, we define it based on the number of catalytic cycles executed per unit time, which is the standard for the field)104. The challenge with the TOF is defining the active site. The simplest method is to take every metal cation as an “active site” and use the total film mass derived from 52 ACS Paragon Plus Environment

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QCM or ICP measurements and the formula unit to determine the moles of metal in the film. This results in a total-metal TOF (TOFtm) which is essentially an average TOF for all the metals in the film (i.e. some metal cations in reality may be inactive or buried in the film and therefore not contribute to the current). We have previously called this simply TOF,14,36,37 TOFmass,38 or TOFin situ mass;40 for clarification purposes we have simplified it to TOFtm.26,39,63 As mentioned previously, it is likely that the real active site identity is more complicated than just a metal site,42,44 but this simplification serves well for activity comparisons. A second method to calculate TOF is to base it on the number of the metal cations that are electrochemically active, for instance by integrating a redox wave or performing a capacitance measurement (under appropriate conditions, as discussed in section 2.5.1-2.5.2). If the catalyst is electrolyte-permeated and electronically conductive, such as the Ni- and Co-based oxyhydroxides, then most of the metals are electrochemically active and the TOF obtained from the redox wave integration is similar (within a factor of 2-3) to TOFtm. It is not possible to compare the TOFtm calculated from redox wave integration across a class of different catalysts if they don’t all have clear redox features. Recently work by Bell, us, and others points to Fe as the possible “active” site in Ni(Fe)OxHy.38,41 One could calculate a TOF assuming only the Fe cations are active. This is useful, for example, if one wanted to compare the activity of Fe cations in different matrices. /ss We have typically termed this the apparent TOF (e.g. pqr#7 ) because we are making an

assumption about the active site chemistry that may not be true. By comparing the TOFtm at different loadings, taking into account the catalyst conductivity, and knowing the precise catalyst composition we have approached a better understanding of the intrinsic catalyst activity. However, none of the above methods actually



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provide an intrinsic or “true” TOF because the active site is not definitively known and the TOF extracted by any of these methods depends on film loading, morphology, and deposition method to some extent.26 Nonetheless, the use of TOF provides a uniform metric to apply to any OER catalyst system and we encourage its use.

TOFs from non-electrode-based systems (e.g.

photocatalysts and biocatalysts) can also be compared,105 but it is difficult to make a direct comparison of activity because the electrochemical driving force is often not well-known.106 Others have also used the current density at a specific overpotential, normalized to the oxide surface area as metric to compare OER catalysts.33,107 While this is suitable for oxide phases that preserve a sharp crystalline surface structure under OER conditions, it is problematic if the surface becomes hydrated and composed of multiple layers of redox active cations. Thus for the systems we study, we avoid the use of this metric. One can also compare current densities, based only on the geometric surface area,6,8 as an activity metric. However, this metric does not account for variations in catalyst loading or internal surface area. In particular, for systems that use inert high-surface-area substrates such as nickel foams, carbon paper, etc., comparisons should be made with care because the macroscopic internal surface area can be many times larger than the geometric one. For this reason, we avoid the use of current density at a given overpotential alone as a metric for electrocatalyst activity. Faradaic efficiency is also a common metric for electrocatalytic systems. Under alkaline conditions the steady-state faradaic efficiency for water oxidation (following film redox) is essentially unity as there are no other species to oxidize and peroxide formation is unfavorable (note that for highly loaded films studied at low current densities, film redox could take a substantial time to come to completion).108,109 For other systems that have several possible byproducts, or for those that use conductive carbon binders susceptible to oxidation, the faradaic


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efficiency is important to measure (e.g. using an oxygen sensor or via mass spectrometry of the cell head space).

3.0 Recipe for Making and Testing a “Standard” Ni(Fe)OxHy Catalyst Film Ni(Fe)OxHy catalysts in alkaline media are among the best-reported catalysts for OER.40,110–112 Ni(Fe)OxHy is also simple to deposit. Given the large number of reports on “new” and “high-activity” OER catalysts in the recent literature, we suggest that all new OER catalysts should be compared with a standard Ni(Fe)OxHy composition that serves as a benchmark that can be measured under virtually the exact same experimental conditions (loading, electrode identity and size, cell geometry, and electrolyte composition). Measured activities for Ni(Fe)OxHy in the literature have varied widely. Such variation can be explained by differences in mass loading and electrochemically active surface area and how they are measured, as well as differences in morphology, Fe content, and catalyst connectivity. Ni-Fe oxides for example, are much less active on a mass basis than Ni-Fe oxyhydroxides, even though the composition may be similar.113 Here we provide a recipe and characterization data for a Ni0.8Fe0.2OxHy catalyst that is simple to make and has well-defined redox characteristics that can be used to assess catalyst loading and intrinsic activity. While higher intrinsic activity (e.g. TOFtm) can be obtained from a Ni(Fe)OxHy (for example with higher Fe content)83 or via a pulsed deposition of thicker films,26 the recipe suggested here has the advantage of showing reversible Ni redox waves that can be easily integrated to assess redox charge, along with good OER activity and reasonable electrochemical stability. The Ni0.8Fe0.2OxHy film is deposited from a 0.095 M Ni(NO3)2 (98 %, Sigma Aldrich)



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and 0.005 M FeCl2 (98 %, Alfa Aesar) aq. solution at a current density of -0.1 mA cm-2 for 30 s using a carbon cloth (Fuel Cell Earth) counter electrode, giving a loading of ~ 4-5 µg cm-2 (or a theoretical thickness of ~ 10-25 nm). These films were deposited onto an Au QCM electrode for mass analysis, but the same results are easily obtained on home-made electrodes. Prior to deposition, CV measurements were performed on the bare Au electrode to check for electrolyte or surface impurities (Figure 19a). The voltammetry of the bare Au QCM electrode shows the presence of Fe impurities in the electrolyte, which is expected as it was not specifically cleaned of Fe. Such electrolyte is suitable for testing catalysts such as Ni0.8Fe0.2OxHy which has much higher activity and contains Fe. After film deposition, the catalyst was electrochemically characterized using the following sequence of experiments: 1) CV, 2) impedance spectroscopy, 3) chronopotentiometry (i.e. Tafel analysis), 4) CV, 5) impedance spectroscopy, and 6) chronoamperometry. For each CV experiment, three voltammograms at 10 mV s-1 from 0 to 0.75 V vs. Hg|HgO (Figure 19b) were collected. The impedance measurements (see Section 2.1) were made at 0.6 V vs Hg|HgO (Figure 19a, inset), and show no change in Ru after analysis, as expected for a stable electrode. The Tafel analysis consisted of chronopotentiometry steps from 0.01 mA cm-2 to 10 mA cm-2 (e.g. 0.01, 0.032, 0.1, 0.32, 1.0, 3.2 and 10 mA cm-2), each held for 3 min except the first two steps which are held for 10 min. These chronopotentiometry steps are then repeated in reverse order at 3 min each. These data were collected on three different films deposited in the same way. The chronoamperometry data (Figure 19d) was collected at 0.6 V vs. Hg|HgO (η = 0.3 V) for 1 h with automatic iRu correction.


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Figure 19. a) CV of Au/Ti QCM electrode substrate (1.38 cm-2) in aq. 1 M KOH prior to electrodeposition. The inset shows the impedance data of the electrode with Ni0.8Fe0.2OxHy before and after the Tafel analysis, b) first two CVs before (blue) and after (red) Tafel analysis. The first anodic peak is filled to show the region of integration for peak analysis. c) Forward (closed) and reverse (open) steps of Tafel analysis, d) current response and corresponding TOFtm (by integration) during chronoamperometry at η = 300 mV that was corrected in situ for iRu.

Data workup includes iRu correction and geometric surface area normalization of all the relevant data (CV, Tafel, etc.). The average Ru of the two measurements is used to correct for uncompensated series resistance. For the Tafel data, the potential recorded during the last minute of each Tafel step is averaged and plotted (Figure 19c). The activity of the catalyst is calculated at a series of overpotentials following the guidelines in Section 2.8 (Table 1). TOFtm are obtained from the Ni oxidation peak integrated charge over the first cycle (Figure 19b inset, 2.2 mC cm-2) and also from the QCM mass. Table



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1 shows that TOFtm varies from 0.03 from 0.08 s-1, depending on how the cation density is measured. For thicker, pulse-deposited Ni(Fe)OxHy we have measured TOFtm of ~0.4 at η = 300 mV. The Tafel slope is 37 ± 3 mV dec-1. These data serve as benchmarks for a moderate activity Ni(Fe)OxHy. By repeating the synthesis and characterization steps outlined above for this system, it can be used to ensure the experimental apparatus (cell, reference electrodes, etc.) are appropriately setup and calibrated. It can also be used as an “internal reference” against which the intrinsic OER activity of other new catalysts can be compared. Any report claiming to have discovered a new catalyst with an intrinsic activity that is meaningfully higher than known systems (in alkaline electrolyte) should have a TOFtm that is, at a minimum, ten times larger than the reference case given. Any claim of a new catalyst reported to have a “low” Tafel slope should have a Tafel slope similar or lower than the 30-40 mV/dec observed for Ni(Fe)OxHy. We finally note that the OER current in the pre- and post-Tafel OER voltammograms are different (Figure 19b), which, along with the chronoamperometry data Figure 19d, indicate some deactivation during polarization as the activity reaches a steady state. We note, however, that the TOF obtained after 60 min of constant voltage polarization at η = 300 mV is the same as that obtained from the steady-state Tafel analysis. Letting the catalyst film sit in electrolyte, or cycling to cathodic potentials, leads to recovery of the some of the initial activity. The details of this process require more study.

XPS and SEM analyses collected before and after

electrochemical analysis show little differences in composition and morphology, respectively, suggesting there is no substantial change in the catalyst due to dissolution or large scale restructuring (Figure 20) and that these do not appear to be mechanisms for reduced activity.


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Table 1: Activity metrics for thin film Ni0.8Fe0.2OxHy on Au from steady-state Tafel analysis Activity

Activity at specific overpotential

Metric

250 mV

300 mV

350 mV

mA cm-2(geometric)*

0.03 ± 0.01

0.7 ± 0.2

15 ± 3

mA g-1(QCM)

7±1

120 ± 20

2400 ± 370

TOFtm (s-) (QCM)

0.001 ± 0.0005

0.033 ± 0.007

0.8 ± 0.3

TOFtm (s-) (integration)

0.003 ± 0.001

0.075 ± 0.006

1.8 ± 0.4

* this metric is not normalized to mass and therefore is not be comparable to systems with different loading

Figure 20. Mg Kα source XP spectra pre- and post- Tafel analysis for the a) Ni 2p and b) Fe 2p regions. SEM images c) pre- and d) post- Tafel analysis of Ni0.8Fe0.2OxHy. The XP spectra of Ni 2p and Fe 2p were used to calculate the ratio of Ni to Fe (it is necessary to use a Mg Kα source to avoid interference from the Auger peaks).



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4.0 Conclusion We have aggregated the best practices that we have developed or adapted in our research group to date for understanding fundamental aspects of heterogeneous OER catalysis. We emphasize the need to accurately measure the quantity of catalyst that is electrochemically active, it’s composition (including impurities), the surface-active phases present under operating conditions, and electrical properties of the catalyst material (both ex situ and in situ if possible). We also reiterate that these techniques are not comprehensive; critical analysis of the underlying physical processes is required for any catalyst system developed to assess which suite of complementary analytical tools are needed. Additional tools to understand the real active site geometries and reaction pathways remain sorely needed. Finally, we presented a simple recipe for a standard Ni(Fe)OxHy catalyst film that can be easily made and compared to new material systems.

Acknowledgements This work was supported by the National Science Foundation Chemical Catalysis program under grant CHE-1566348. A.S.B acknowledges support from the United States Air Force in conjunction with the United States Air Force Academy faculty pipeline program. The authors thank Lena Trotochaud, Adam Smith, Shihui Zou, and Matthew Kast for their contributions to the original research. We acknowledge Steve Golledge for insight into the interpretation of ToF-SIMS and XPS data analysis and Kathy Ayers, Nemanja Danilovic, Julie Renner, and Chris Capuano from Proton OnSite for insightful discussion regarding catalysis in practical electrolyzers. The project made use of CAMCOR facilities supported by grants from


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the W. M. Keck Foundation, the M. J. Murdock Charitable Trust, ONAMI, the Air Force Research Laboratory (FA8650-05-1-5041), the National Science Foundation (0923577 and 0421086), and the University of Oregon. S.W.B. further acknowledges support from the Sloan and Dreyfus Foundations.

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Measurement Techniques for the Study of Thin Film Heterogeneous

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