Review pubs.acs.org/cm
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. 2017.29:120-140. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/17/19. For personal use only.
Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States 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 catalyst systems.
1.0. INTRODUCTION Measurements in heterogeneous electrocatalysis are in principle simpleone 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. © 2016 American Chemical Society
Materials for the OER are remarkably complexdespite 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” indicates that the surface is not necessarily well-defined). Various compositions of metals,11 carbons,12,13 metal oxides,14 selenides,15,16 sulfides,17,18 and (oxy)hydroxides (sometimes containing borate19,20 or phosphate21,22 counterions) 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 Special Issue: Methods and Protocols in Materials Chemistry Received: July 8, 2016 Revised: August 31, 2016 Published: September 8, 2016 120
DOI: 10.1021/acs.chemmater.6b02796 Chem. Mater. 2017, 29, 120−140
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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 turnover 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.
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 structureor even the compositionof 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 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 materialinsulators 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 the 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 reordering 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 co-workers 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 the apparent activity. Such effects would be more substantial for thin catalyst layers (i.e., of nanometer thickness) than thick ones (i.e., of micrometer 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 are valuable. We discuss (1) our cell setup that minimizes impurities and uncompensated series resistance, (2) the electrochemical
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 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
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 handmade Au WE with hot glue covering all wire, epoxy, and glass components. 121
DOI: 10.1021/acs.chemmater.6b02796 Chem. Mater. 2017, 29, 120−140
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Chemistry of Materials
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 glue for impurity free analyses (Figure 2). We have found that typical epoxies,
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 wellunderstood, recent work has 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 or 0.1 M electrolyte. During the measurement, highpurity 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 solubilities of 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 intersheet anions.47 Carbonate can be removed via the process detailed in ref 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. 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 densities. 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
Figure 2. Step-wise procedure for homemade 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.
such as Hysol 1C, contain Fe impurities that affect activity measurements. Hot glue is a commonly available hot-melt 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 the need to mask the exposed glass from the electrolyte with hot glue. The mass loading on handmade 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 122
DOI: 10.1021/acs.chemmater.6b02796 Chem. Mater. 2017, 29, 120−140
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Chemistry of Materials
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 the RE is uncompensated and must be corrected for. For a simple planar electrode with a uniform current density the uncompensated resistance is given by
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 a 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 hydrogenevolution-reaction (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 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 an 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 + 2e−).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 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 sweeps 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
Ru =
x κA
(1)
where x is the distance from tip of RE to WE, A is the area of the WE, and κ 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), 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 distance46 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 fluorinedoped 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 in base. When conducting oxides are used, we typically use small electrode sizes, catalyst loadings, and current densities (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.
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° (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. 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. 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. Not consistently accounting for Ru and changing the shape/ size of an electrode, the substrate, the RE to WE positioning, or the electrolyte concentration can have a significant impact on the data analysis. Small errors in iRu compensation can lead to order-of-magnitude errors in turnover 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., the 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 are overcorrected and the measured Ru should be reassessed. Figure 4 shows the relationship between the WE-to-RE distance and Ru for a Au electrode (0.8 cm−2) both 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 spincasting from soluble precursors, followed by annealing14,63 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 distances (i.e., a few nanometers).36,40 (2) Some catalysts are semiconductors, and thin films (5 μg). We can then compare the integrated CV waves from a QCM film to the redox features of NiOxHy on a homemade 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 e− 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, and no in situ monitoring is possible; (2) it is nontrivial to set-up, and film dissolution is based on catalyst composition and structure; and (3) thin films may be below the 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). We simultaneously monitor the OER current and film mass to assess short-term stability. We emphasize that these measurements are not suf f icient 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 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
