On How Experimental Conditions Affect the Electrochemical

Aug 12, 2016 - An electrochemical conditioning protocol was developed for studying amorphous phases of nickel hydroxide films (NiOx) that renders cong...
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On How Experimental Conditions Affect the Electrochemical Response of Disordered Nickel Oxyhydroxide Films Rodney D. L. Smith, Rebecca S. Sherbo, Kevan E. Dettelbach, and Curtis P. Berlinguette* Departments of Chemistry and Chemical & Biological Engineering, The University of British Columbia, 2036 Main Mall, Vancouver, BC V6T1Z1, Canada S Supporting Information *

ABSTRACT: An electrochemical conditioning protocol was developed for studying amorphous phases of nickel hydroxide films (NiOx) that renders congruent and reproducible redox behavior that enables more standardized mechanistic studies. Dynamic oxidative behavior was measured for the conditioned films, where the initial peak associated with the oxidation of NiOx contains 1.3−2.0 times the charge for that measured in subsequent oxidative scans and the corresponding reductive peak. It is shown that charge trapping does not fully account for this excess charge. The magnitude of excess anodic charge is affected by pH, OH− diffusion through the film, and voltammetric scan rates.

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the “resting state” and the starting point of the catalytic cycle.13 The assignment of the “precatalytic” oxidation process is therefore crucial to elucidating OER mechanisms. Departing from this conventional assignment of either one or two electrons per nickel site comprising Ep,a, we have previously presented the possibility that four electrons per nickel site account for the Ep,a signal in amorphous phases of NiOx.23 Further, identification of a minor oxidation process at voltages corresponding to the onset of electrocatalytic OER suggests that a secondary minor surface species may be responsible for electrocatalytic OER. We showed that the amount of charge passed through Ep,a during the first oxidative scan (Ep,a1) is approximately twice that measured in the anodic peak than for all subsequent scans (Ep,a2) during successive cyclic voltammograms (CVs). This dynamic oxidative behavior, where Ep,a changes position and shape during successive scans, is typically ascribed to the trapping of oxidized nickel sites within the film.24,25 We have experimentally ruled out charge-trapping being the sole factor in the dynamic behavior23 and contend that two different two-electron redox steps are the primary source of Ep,a1 containing twice the number of electrons as Ep,a2 (Figure 1). Whereas the first scans of CV experiments are often ignored in the nickel OER literature, the variable behavior of NiOx during voltammetric cycling has been highlighted.15 Our attention to the electrochemical behavior in the early stages of oxidation has helped us unify a series of seemingly disparate experimental data in the literature with a mechanistic description of NiOx that is more closely aligned with the reaction steps followed by well-defined homogeneous catalysts (Figure 123).26,27

INTRODUCTION Nickel anodes are widely used in alkaline electrolyzers, and there is a growing body of literature showing how mixed-metal catalysts containing nickel yield state-of-the-art (photo-)electrocatalysis efficiencies.1−7 Defining the mode of action for nickel-based oxygen evolution reaction (OER) catalysts is therefore an important step toward realizing scalable energy storage schemes involving hydrogen fuels.8−13 An accurate mechanistic description of the OER at nickel electrodes is made complicated, however, by the electrochemical response of the material being exquisitely sensitive to phase changes,14−16 conditioning,17 and infinitesimally small amounts of impurities.18,19 Drawing meaningful structure−property relationships for amorphous phases of nickel-based materials is particularly challenging given the lack of acute structural information known for these systems. Nickel oxyhydroxide (NiOx) OER electrocatalysts generally exist as layered double hydroxides (LDHs) and display different electrochemical behavior that depends on the interplanar spacing that exists between the metal oxygen layers. The interplanar spacing affects both the position and shape of the anodic peak (Ep,a) that is presented prior to rapid OER catalysis.20,21 The broader Ep,a signal for the denser β-phase characterized by shorter interplanar distances is thought to involve the reversible oxidation of nickel(II) to nickel(III). This assignment does not necessarily hold for amorphous phases of nickel hydroxide, where the oxidation of highly disordered αNiII(OH)2 yields the higher-valent phase γ-Ni(O)(OH) with a nickel oxidation state of approximately +3.7.22,23 The Bode cycle captures the distinctive redox chemistry for the β and α phases of nickel,14 although the assignments for β-Ni(OH)2 are often still used to describe the electrochemistry of amorphous phases when the behavior of α-NiII(OH)2 may be more relevant. Nevertheless, a unifying feature of both descriptions is that the first oxidation process yields a species considered to be © XXXX American Chemical Society

Received: April 8, 2016 Revised: July 22, 2016

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solutions: 0.5 M K3PO4 (labeled Ni-PO4), 0.5 M K2CO3 (Ni-CO3), 1 M KOH (Ni-OH), or a mixture of 0.5 M K3PO4 and 0.5 M K2CO3 (Ni-cond). All electrolyte solutions used for conditioning were at pH 14. Electrochemistry. Electrochemical data was recorded with a CHI660E or CHI920C potentiostat using a platinum wire counter electrode, Ag/AgCl reference electrode (SSCE), and the appropriate working electrode. The reference electrode was calibrated against RHE by bubbling H2 gas over a Pt electrode in 1 M KOH. Potentials were not corrected for uncompensated resistance, which was typically on the order of 10 Ohms. CVs were acquired at a scan rate (v) of 1 mV s−1 unless otherwise indicated. Physical Methods. UV−visible spectroscopy was performed using a PerkinElmer Lambda 35 UV−visible spectrophotometer with a solid sample holder accessory over a wavelength range of 190−1100 nm. Xray diffraction experiments were performed with a Bruker D8 Advance diffractometer. A Cu Kα source was used to collect between 2θ angles of 5° and 90° with a 0.04° step size and 1.2 s/step. Scanning electron microscopy was carried out on Ni-fresh samples using an FEI Helios NanoLab 650 dual beam scanning electron microscope. Images had a horizontal field width of 2.98 μm with an accelerating voltage of 3 kV and a current of 50 pA. Ni-fresh samples used for SEM measurements were deposited on either silicon wafers that were coated with a 10 nm iridium film or glassy carbon plate. Ir films were deposited with a Leica EM MED 020 coater, and the thickness was monitored by an EM QSQ 100 Quartz crystal microbalance. Cross-sectional SEM micrographs were acquired on a series of films prepared from 0.42, 0.28, and 0.13 M solutions in isopropanol to generate a calibration curve describing the relationship between solution concentration and the resultant film thickness. The reliability of the calibration curve was tested by measuring the thickness of a NiOx film that was prepared on a glassy carbon plate using a 0.2 M solution of nickel 2-ethylhexanoate in hexanes. Measurement of Diffusion Coefficients. Diffusion coefficients were measured by (i) fitting cyclic voltammetric behavior to the Randles−Sevcik equation and (ii) by application of the potentiostatic intermittent titration technique (PITT). Although the small interlayer spacing in crystalline β-Ni(OH)2 may exclude OH−, and thus H+ may be the primary diffusing species for highly ordered films, we assume OH− to be the primary diffusing species in this study of highly disordered films in strongly alkaline media (i.e., there are 14 orders of magnitude difference between [H+] and [OH−]). The electrochemical response for solid electrode coatings is dependent on the time domain probed by the experimental protocol (e.g., t for chronoamperometry, RT/Fv for CV) relative to the characteristic diffusion time of the diffusing species (τ) as defined in eq 1

Figure 1. Possible reaction mechanism for disordered NiOx. Two electron transfer steps (Eo′1 and Eo′2) are linked by a chemical reaction that generates an O−O bond. The kinetics of bond formation (kapp,1) induce changes in voltammetric behavior; the kinetics of O2 release (kapp,2) can be experimentally measured, and a chemical process exists to return an electrochemically reduced electrode to the initial state (kaging) (vide infra).

This manuscript serves to address how experimental parameters, particularly diffusion of OH− through the film and CV scan rates, can impact the dynamic anodic behavior. We also disclose here a preconditioning process that yields predictable steady-state behavior and overcomes irreproducible voltammetric issues common to these films.17 Our ability to test different electrodes that display reproducible behavior enabled us to investigate how electrolyte solution pH and composition, film thickness, and scan rate affect the dynamic response.

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EXPERIMENTAL SECTION

Materials. Ni(II) 2-ethylhexanoate (78% w/w in 2-ethylhexanoic acid) was purchased from Strem Chemicals. Ni(NO3)2·6H2O (99.9985%) was purchased from Alfa-Aesar, and KOH, K2CO3, and KH2PO4 were purchased from Fisher Scientific. TEC7 grade conductive fluorine-doped tin oxide-coated glass (FTO) was purchased from Hartford Glass. Electrolyte Solution Preparations. Iron impurities were removed from all aqueous solutions following a previously described method.19 Ni(NO3)2·6H2O (5.01 g, 0.0172 mol) was dissolved in a minimal amount of water. A 50 mL aliquot of 30% KOH was then added to precipitate Ni(OH)2. One-half of the precipitate collected by centrifugation was added to a Nalgene bottle containing 250 mL of 30% KOH with the other half added to a Nalgene bottle containing 250 mL of 0.5 M K2CO3 and 0.5 M K3PO4 electrolyte. This procedure was repeated for the buffered 0.5 M K2CO3 and 0.5 M K3PO4 electrolytes and the unbuffered 1 M KOH solution. Electrolyte solutions were prepared in the pH 10−14 range by adding the appropriate volume of 30% KOH to 0.5 M K2CO3 (pKa2 = 10.3), 0.5 M K3PO4 (pKa3 = 12.7), or both. Fresh electrolyte solution was used for every pH value. The pH of each solution was recorded with a Fisher Scientific Accumet AB150 pH meter. Ni-Fresh Preparation. Thin films of nickel(II) 2-ethylhexanoate were spin-coated at 3000 rpm onto a conductive substrate (FTO or Au) from a solution of the metal complex in hexanes or isopropanol. The concentration of the precursor solution was varied to control film thickness. Coated electrodes were irradiated with UV light (Atlantic Ultraviolet GPH436T5/VH/HO/4PSE lamp, 185/254 nm) until vibrational modes associated with the organic ligands were no longer observed by FT-IR spectroscopy. Films used for spectroelectrochemical experiments were annealed at 200 °C for 1 h. Electrochemical Conditioning. In cases where Ni-fresh was stabilized by preconditioning, the protocol consisted of (i) potential cycling between 1.0 and 1.7 V vs RHE at 5 mV s−1 for 25 cycles, (ii) 20 cycles of alternating 300 s potential steps between 1.5 and 1.0 V, (iii) 5 cycles of alternating potential steps between 1.5 V (1800 s) and 1.0 V (180 s), and (iv) application of 0.85 V for 1 h. A series of films were subjected to this conditioning procedure in different electrolyte

τ=

L2 DOH

(1)

where L is the diffusion length, and DOH is the diffusion coefficient. Assuming 1-dimensional diffusion (i.e., a uniformly deposited film), L was taken as the thickness of the film. The Randles−Sevcik equation describes a linear relationship between jp and v1/2 for cyclic voltammetric redox peaks observed under semi-infinite diffusion conditions (eq 2).

jp =

1/2 0.446z 3/2F 3/2DOH ΔCOHν1/2

R1/2T1/2

(2)

The terms in eq 2 are defined here following a previous analysis of the intercalation of Li+ in graphite:28 z is the electrons transferred per diffusing species (zOH = 1), and ΔCOH is the change in concentration due to the electrochemical reaction (approximated as CNi). In the “long-time domain”, when t > τ, finite-space diffusion conditions exist. Chronoamperometric behavior in this time domain is described by eq 329,30 B

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Chemistry of Materials ⎛ π 2D ⎞ ⎛ 2ΔQDOH ⎞ OH ⎟ − t⎜ ⎟ ln(i) = ln⎜ 2 2 ⎠ ⎝ L ⎝ 4L ⎠

t>τ (3)

where ΔQ is the amount of charge passed during the potential step. The linear slope of a plot of ln(i) vs t when t > τ can therefore be used to measure DOH. Finite-space diffusion conditions in cyclic voltammetry are characterized by a linear relationship between peak current density (jp) and v.31,32 In the “short-time domain” when t < τ, semi-infinite diffusion conditions are assumed to exist. Chronoamperometric behavior under such conditions is described by eq 4.

it 1/2 =

1/2 ΔQDOH

π 1/2L

t99% oxygen content present as hydroxide (OH−).35 Electrochemical Conditioning Protocol. Cyclic voltammograms recorded on as-prepared NiOx (Ni-fresh) on fluorine:tin oxide (FTO) substrates are characterized by a broad anodic peak at ∼1.40 V relative to RHE and a cathodic peak positioned at ∼1.25 V; a rapid rise in current at potentials >1.5 V is diagnostic of rapid OER catalysis. A gradual increase in anodic and cathodic peak currents and a slight cathodic shift in the redox peak locations were measured over 25 cycles, where the first five cycles are shown in Figure 2a. The electrochemical response was slightly different for each electrode despite our best efforts to hold all experimental parameters at parity. We therefore developed a conditioning protocol where each electrode was subjected to a succession of CVs, potential-step experiments, and electrochemical reductions to stabilize the electrochemical response (see Experimental Section). The conditioned films (Ni-cond) each displayed consistent behavior after being subjected to this conditioning protocol in a buffered solution containing K3PO4 and K2CO3 (Figure 2b). The first oxidative scan for each of Ni-cond yielded a sharper anodic peak (Ep,a1), whereas subsequent oxidative scans were characterized by a cathodically shifted anodic signal with a peak (Ep,a2) that is lower in intensity relative to that of Ep,a1. The Ep,c signal remains static during these cycles so long as no iron impurities were present (Figure 2b). The Ep,a2 and Ep,c signals did not change during subsequent voltage cycling, indicating

that steady-state behavior is achieved immediately following the first oxidation scan. The ratios of integrated charge (Qp,a1/Qp,c) passing through Ep,a1 and Ep,c for a series of films were determined to range from 1.7 to 2.0 for each of the films (Table 1). Table 1. Relative Integrated Charge Comprising Ep,a1 (Qp,a1) and Ep,c (Qp,c) for the First CV Measured on Ni-cond electrodea

Qp,a1b/mC cm−2

Qp,cb/mC cm−2

Qp,a1/Qp,c

a b c d e

67 63 52 76 14

34 38 28 40 8

2.0 1.7 1.9 1.9 1.8

a Individual CVs for electrodes a−c are shown in Figure 2. bCalculated by integration of cyclic voltammetric peaks at 1 mV s−1.

Role of Electrolyte Anion During Electrode Conditioning. The impact of each anion present in the mixed-buffer electrolyte solution (e.g., PO43−, CO32−, and OH−) was deconvoluted by preconditioning a series of NiOx films in pH 14 electrolyte solutions (i.e., each solution contains 1 M KOH) that contained 0.5 M K3PO4, 0.5 M K2CO3, or only 1 M KOH; these films are denoted Ni-PO4, Ni-CO3, and Ni-OH, respectively. Dynamic electrochemical behavior was measured under each of these conditions (Figure 3), but the ratio of Qp,a1/Qp,c followed the trend Ni-PO4 > Ni-CO3 > Ni-OH (Table 2). Further, examination of the first derivative of the cyclic voltammograms shown in Figure 3 (Figure S3) reveal a dependence of shape and location of the redox peaks on the preconditioning protocol. Ni-PO4 and Ni-CO3, which both exhibit Qp,a1/Qp,c > 1.5, show comparable peak shapes and locations, whereas Ni-OH shows a more complex shape and anodically shifted peaks. Film Thickness. Cross-sectional SEM revealed a linear correlation between NiOx film thickness and the concentration of the nickel(II) 2-ethylhexanoate in isopropanol-based precursor solutions (Figures S5 and S6). CVs acquired on identically prepared films on FTO revealed that Qp,c (measured C

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Qp,a1/Qp,c ratio of 1.7 was measured for an exceptionally thin NiOx film (16 nm) where charge-trapping is expected to be nominal (Table 2, Figure S4).24,25 Approximation of Hydroxide Diffusion Coefficients (DOH). The diffusion of OH− (DOH) was quantified for NiOx films conditioned in the presence of different anions using the measured values for film thickness and concentration (Figure 4). CVs acquired for each of the films yielded a linear Figure 3. CVs recorded on Ni-PO4, Ni-CO3, and Ni-OH at 1 mV s−1 in pH 14 electrolyte solutions. The oxidative charge passed during initial oxidation (Qp,a1) was sensitive to the anion present during the preconditioning protocol; the cathodic charge passed (Qp,c) remained constant. The potential was swept between 1.05 and 1.75 V vs RHE for all CVs shown; the window is truncated to highlight the redox peaks.

Table 2. Anodic and Cathodic Charge Passed For Films Conditioned in Indicated Electrolyte Solutions samplea

thickness/nmb

Qp,a1c/mC cm−2

Qp,cc/mC cm−2

Qp,a1/Qp,c

Ni-cond Ni-CO3 Ni-CO3 Ni-cond Ni-OH Ni-CO3 Ni-PO4

16 39 50 143 150 180 225

3.5 9.3 13 29 25 39 55

2.1 5.2 6.7 19 20 24 30

1.7 1.8 1.9 1.5 1.3 1.6 1.8

Figure 4. A correlation is observed between Qp,a1/Qp,c and the experimentally measured diffusion coefficient for hydroxide (DOH) through NiOx films. Qp,a1/Qp,c values were determined by integration of the appropriate redox peaks in cyclic voltammograms acquired at 1 mV s−1 for freshly prepared NiOx films and films that had been preconditioned in the presence of PO43−, CO32−, and OH−. Diffusion coefficients are the average value measured using the Randles−Sevcik equation and the potentiostatic intermittent titration technique. A full list of values is provided in the Supporting Information.

a

CVs for entries 1−3 are shown in Figure S4; entry 4 is shown in Figure 2, and entries 5−7 are shown in Figure 3. bCalculated from Qp,c values, see below. cCalculated by integration of cyclic voltammetric peaks at 1 mV s−1.

correlation between peak current density and √v and distinct curvature when peak current density was plotted against v, consistent with semi-infinite diffusion conditions (Figure S7). With appropriate selection of scan rates, application of the Randles−Sevcik eq (eq 2) to variable scan-rate CV data has been shown to be capable of calculating diffusion coefficients in porous heterogeneous films from such linear correlations.28,32 Recognizing the shortcomings of this model (e.g., assumption of fast reversible kinetics; neglecting electron diffusion through film), chronoamperometric data was analyzed using the potentiostatic intermittent titration technique (PITT) to provide a secondary measurement and support the validity of DOH values measured by CV. DOH values measured through PITT were comparable to those calculated using the Randles− Sevcik equation in both the long- (Figure S8) and short-time domains (Figure S9). Application of eq 1 indicates a characteristic diffusion time on the order of ∼2−3 s for the films examined here. Scan rates above 8−12 mV s−1 would therefore be expected to exhibit semi-infinite diffusion conditions, which is in agreement with voltammetric results. On this basis, we feel the relative DOH values can report on the relative porosities of the films. The DOH values measured by each of the experimental techniques were observed to increase for Ni-OH < Ni-CO3 < Ni-PO4. This behavior mirrors that of the Qp,a1/Qp,c ratios, which were also observed to increase for Ni-OH < Ni-CO3 < Ni-PO4. Notably, DOH values measured for the anodic peaks were consistently larger than those measured for the cathodic peaks, a feature that is likely responsible for the comparably sharper nature of the anodic peaks in each of the conditioned NiOx films. Charge-Trapping of Oxidized Nickel. UV−visible spectra were acquired on Ni-OH, Ni-CO3, and Ni-PO4 films with systematically varied thicknesses to determine the extent

at 1 mV s−1) was also linearly correlated to film thickness. A separate NiOx film, deposited on glassy carbon from a 0.2-M solution of nickel 2-ethylhexanoate in hexanes, was measured to be ∼175 nm by scanning electron microscopy (Figure S1), which is in good agreement with past measurements.33 Comparison of this film with the calibration curves in Figure S6 reveal that, although solvent has an impact on film thickness, the linear relationship between Qp,c and film thickness continues to provide a reasonable approximation of film thickness. Considering the stable and highly reproducible behavior of the cathodic redox process in NiOx films, Qp,c was used to determine the thickness of all films used. The concentration of nickel atoms within the film (CNi) can be described with eq 5 C Ni =

Q p,c ALn p,cF

(5)

where Qp,c is the integrated charge under the cathodic peak, A is the geometric electrode surface, and np,c is the number of electrons transferred per nickel atom (previously measured to be ∼1.6).22−24 Recognizing that the slope of Figure S6b provides an experimental measure of L·A/Qp,c, CNi can be approximated as 7.5 × 10−3 mol cm−3 for the films examined here. A series of conditioned films with estimated thicknesses of 16−225 nm were evaluated to resolve how film thickness affects the dynamic electrochemical behavior. The ratio of Qp,a1/Qp,c was measured to be at least 1.3 for each of the conditioned films and was generally closer to 2 (Table 2). Importantly, a D

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Chemistry of Materials to which charge trapping24,25 is responsible for the dynamic oxidative behavior (Figures S10−S12). Spectra were acquired before and after an anodic sweep from 1.0 to 1.6 V vs RHE and then again after a subsequent cathodic sweep. In agreement with previous literature, a linear correlation between charge passed and absorbance was observed (Figure S13). The linear correlation enables a direct comparison of the ratio of change in absorbance upon oxidation to that upon reduction (Aox/Ared) with the electrochemically measured Qp,a1/Qp,c ratios. Comparison of these measures for Ni-OH (Figure S10), Ni-CO3 (Figure S11), and Ni-PO4 (Figure S12) indicated that, although Aox/Ared and Qp,a1/Qp,c were nearly equivalent in the thinnest films (∼20 nm), Qp,a1/Qp,c became increasingly larger as film thickness increased. Scan-Rate Dependence. A series of CVs were acquired at pH 14 on a single Ni-cond sample while varying the scan rate from 5 to 50 mV s−1. The Qp,a1/Qp,c ratios obtained for this series of experiments increase with progressively slower scan rates (Figure 5). The ratios approach a value of 2 at slower scan rates and trend toward a value of 1 as the scan rate increases.

The passage of excess anodic charge during initial oxidation has been observed and ascribed to portions of the film becoming trapped in the oxidized state.24,25 A recent report found that Qp,a1/Qp,c increases from approximately 1 to 3 when the film thickness increased on gold substrates.25 This same study showed the opposite correlation when using a modified fabrication procedure with Qp,a1/Qp,c increasing from 1 to 2 with decreasing film thickness, indicating a sensitivity of the electrochemical behavior to fabrication and handling conditions. Spectrophotometrically measured Aox/Ared ratios for our photodeposited films were comparable to the Qp,a1/Qp,c ratios for the thinnest of films examined (∼20 nm thick). However, Aox/Ared decreased while Qpa,1/Qp,c increased with increasing film thickness. This trend cannot be fully ascribed as merely a charge-trapping mechanism, which would be expected to produce an opposite trend.25 This electrochemical sensitivity to fabrication protocol prompted us to establish how film porosity affects the behavior. We tested this by measuring DOH for films conditioned in different electrolyte solution compositions and found that DOH increased for Ni-OH, Ni-CO3, and Ni-PO4, respectively. Importantly, there exists a linear correlation between DOH and Qp,a1/Qp,c (Figure 4). The incorporation of anions between the nickel layers of Ni(OH)2 is known, with larger anions generally forcing larger interlayer spacing.36,40 The amorphous nature of the films under investigation here preclude the use of powder XRD to correlate interlayer spacing to anion identities, but it seems reasonable to assume that the electrolyte anions are playing a similar structural role that induces an increase in DOH. We also note that CO32− is expected to be present to a certain extent in the studies here given the samples were not studied under an inert atmosphere.36 These collective results highlight the important role that film fabrication and testing conditions have in observing the dynamic electrochemical response. With a better handle on how each of these experimental factors affect the electrochemical response, we set out to link this data to our previously proposed mechanism where four charges are removed from each nickel site (Figure 1). Our proposal assigned the four electrons that comprise Ep,a1 to two 2-electron oxidation steps (Eo,1 and Eo,2) linked by an O−O bond forming step (kapp,1) to form a high-valent nickel film, “Ni IV -OO”. Because cleavage of the O−O bond in [NiII(OOH)]− is a slow chemical step, subsequent voltammetric sweeps effectively measure the interconversion of [NiII(OOH)]− and NiIV(OO) (Eo′2), and thus, Ep,a2 is presented as a 2-electron process. The voltammetric behavior of the first scan can be regenerated by first electrochemically reducing the electrode and then letting it rest at open circuit for >20 min to enable the chemical step (kaging) to furnish [NiIIOH]−. A series of digital simulations were performed to generate a preliminary model detailing the expected voltammetric behavior for the proposed reaction mechanism (Supporting Information). A pseudo-first order approximation was employed for the simulations to gain perspective on the effect of OH− availability on the observed behavior. The activity of OH − (aOH) was assumed to be constant during the electrochemical experiment, and the O−O bond forming reaction was assumed to be irreversible (i.e., kf,1 ≫ kb,1). The bimolecular O−O bond forming reaction (eq 6) was therefore modeled as a pseudo-first order chemical reaction (eq 7; kapp,1 = kf,1·aOH), where the reaction rate was dependent on the surface

Figure 5. (a) A series of CVs acquired on Ni-cond exhibited a decrease in the magnitude of excess anodic charge, Qp,a1/Qp,c, as the voltammetric scan rate increased. (b) A series of CVs acquired at 1 mV s−1 while varying electrolyte solution pH revealed a decrease in Qp,a1/ Qp,c as electrolyte solution pH decreased.

pH Dependence. CVs were acquired on a single Ni-cond sample under varied pH conditions, and the results were analyzed to elucidate the effect of OH− concentration on the dynamic electrochemical behavior. The results acquired at 1 mV s−1 indicate a clear trend that increased concentrations of OH− induce larger Qp,a1/Qp,c values (Figure 5).

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DISCUSSION It is a challenge to obtain reproducible electrochemical behavior for nickel films due to structural differences, phase transitions, impurities, electrolyte solution composition, and variable behavior during voltage cycling.13−15,17−19,21,22,24,25,36−39 We report here an electrochemical conditioning protocol that yields consistent and reproducible voltammetric behavior for independently prepared photodeposited NiOx films (Figure 2). Each of the conditioned films was characterized by dynamic electrochemical behavior, a Qp,a1/Qp,c ratio of at least 1.3, and stable Ep,a2 and Ep,c values during successive cycling. The Qp,a1/ Qp,c ratio was measured to be ∼2 when conditioned in K2CO3 and/or K3PO4 (i.e., Ni-cond, Ni-PO4, Ni-CO3) and 1.3 when conditioned in 1 M KOH (Ni-OH). The dynamic response for each of these films could be reproduced by letting the electrochemically reduced films rest at open-circuit potential for >20 min.23 We attribute this regenerative behavior to the chemical displacement of OOH− by OH− (kaging in Figure 1). E

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Chemistry of Materials concentration of the reactant, NiIVO (ΓNiIVO), and not the product, [NiIIOOH]− (ΓNiIIOOH). −dΓNi IVO = k f,1aOH ΓNi IV O − k b,1ΓNi IIOOH dt

(6)

−dΓNi IVO = kapp,1ΓNi IVO dt

(7)

inducing a 0.61 unit decrease in log(λ1). Although this preliminary model predicts the correct trends in the changes of Qp,a1/Qp,c to [OH−] and v, a more rigorous model is clearly required for reliable extraction of kinetic rate constants; further refinements of the modeling are underway. The simulated model still holds value, however, as it suggests that any factors that affect the kinetics of O−O bond formation within the nickel hydroxide films will influence the observed voltammetric behavior. Factors that alter [OH−] within the film, including experimental factors such as solution pH and scan rate (Figures 5 and 6), or material-dependent properties such as ionic diffusion rates within the film (Figure 4), would therefore be expected to alter the observed voltammetric behavior, as is experimentally observed here.

Simulated CVs suggest that the system transitions through three unique kinetic regimes. The first electron transfer, Eo,1, may be variably expressed as a single, fully reversible redox process where the metal-peroxide species is not formed, a quasireversible redox process where a mixture of metal oxyl and peroxide exist, or a completely irreversible redox process that yields an abundant metal-peroxide species. The use of a dimensionless kinetic parameter (λ1)41 helps to highlight the kinetic regime in which the system exists is entirely dependent on the relative rate of the O−O bond forming reaction and the scan rate (eq 8)

λ1 =

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SUMMARY An electrochemical conditioning protocol is reported here that yields reproducible voltammetric behavior for different NiOx anodes. This procedure is broadly applicable to films produced by different methods and offers the opportunity for more standardized testing of nickel-based electrodes. These conditioned films, Ni-cond, after being left to rest at open-circuit for 20 min, each yielded dynamic oxidative behavior with Qp,a1/ Qp,c values between 1.3 and 2.0. The anodic peak decreased in size and shifted cathodically during successive cycling; the cathodic peak remained unchanged. We have interpreted this dynamic electrochemical behavior within the framework of our recently proposed mechanism for the redox chemistry of NiOx, wherein the initial 4-electron oxidation process is bisected by irreversible O−O bond formation, trapping the film in a subsequent 2-electron cycle (Eo′2). Electrochemical characterization of NiOx films under various solution compositions revealed the diffusion coefficients for OH− within the films to be sensitive to counter-anions present in the electrolyte, where larger anions induce larger DOH values. A correlation between the measured DOH values and Qp,a1/Qp,c values indicates that the rate at which OH− is delivered to the active site governs the extent of dynamic anodic response. This conclusion is supported by digital CV simulations that show a clear dependence of Qp,a1/Qp,c values to both the rate of O−O bond formation and scan rates. These collective results provide a framework through which the variability of behavior of nickel hydroxide films can be interpreted.

RTkapp,1

(8) Fν where R, T, and F are the universal gas constant, temperature, and Faraday constant, respectively. Calculation of the charge under the initial anodic peak (Qsim,a1) and the cathodic peak (Qsim,c) in the simulated CVs provides a lens through which the experimental results can be analyzed (Figure 6). A semilog plot of Qsim,a1/Qsim,c against

Figure 6. (a) A comparison of simulated CVs acquired with all parameters, except scan rate, held constant. Simulations shown were performed using kapp,1 of 3.7 × 10−3 s−1, as estimated from the experimental data. The Qsim,a/Qsim,c ratios for the first and fifth cycles are provided in the figure. (b) Analysis of simulated CVs indicates that Qp,a1/Qp,c can vary from 1.0 to 2.0 with a value of 2.0 observed only at sufficiently high λ1 values. Experimentally obtained Qp,a1/Qp,c1 values acquired at different electrolyte pH values (red circles) and variable scan rates (blue triangles) are shown.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge on the ACS Publications Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.chemmater.6b01420. Additional cyclic voltammograms, scanning electron micrographs, X-ray diffractograms, UV−visible spectra, diffusion coefficient measurements, a film thickness calibration curve, and a description of electronic simulations (PDF)

log(λ1) yields a sigmoidal plot where the reversible regime is characterized by Qsim,a1/Qsim,c values of 1.0, the irreversible regime by 2.0, and the quasi-reversible regime by a steady change from 1.0 to 2.0. A comparison of this model with experimental data estimates a kapp,1 of 3.7 × 10−3 s−1 under our standard experimental conditions (using an average Qp,a1/Qp,c of 1.9 from Table 1). Experimental CVs were acquired at different [OH−] and ν to test the sensitivity of Qp,a1/Qp,c to λ1. CVs recorded on a single Ni-cond film revealed a decrease in Qp,a1/Qp,c from 1.8 to 1.4 as the pH was reduced from 14.0 to 10.2 (Figure 6b), corresponding to a 0.5 unit shift in log(λ1). The Qp,a1/Qp,c values extracted from CVs recorded at constant pH show that a 10-fold increase in v came closer to the predicted value,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.chemmater.6b01420 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

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ACKNOWLEDGMENTS University of British Columbia (UBC) start-up funds are recognized for their financial support. This research used facilities funded by UBC and the Canadian Foundation for Innovation.

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DOI: 10.1021/acs.chemmater.6b01420 Chem. Mater. XXXX, XXX, XXX−XXX