Accurate Coulometric Quantification of Hydrogen Absorption in

Methods/Protocols Cite This: Chem. Mater. 2018, 30, 3963−3970

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Accurate Coulometric Quantification of Hydrogen Absorption in Palladium Nanoparticles and Thin Films Rebecca S. Sherbo,†,⊥ Marta Moreno-Gonzalez,†,⊥ Noah J. J. Johnson,† David J. Dvorak,‡ David K. Fork,§ and Curtis P. Berlinguette*,†,‡,∥ †

Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada Stewart Blusson Quantum Matter Institute, The University of British Columbia, 2355 East Mall, Vancouver, British Columbia V6T 1Z4, Canada § Google LLC, 1600 Amphitheatre Parkway, Mountain View, California 94043, United States ∥ Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6Y 1Z3, Canada

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‡

S Supporting Information *

ABSTRACT: We report here an electrochemical method for precise and accurate quantification of hydrogen absorption in palladium materials. We demonstrate that conventional chronocoulometry overreports adsorbed hydrogen due to charge from the accompanying hydrogen oxidation reaction (HOR). We designed and built a bespoke electrochemical flow cell that mitigates the concurrent HOR reaction and consequently provides improved accuracy and reproducibility relative to other existing electrochemical techniques. The efficacy of this technique is demonstrated experimentally for a series of palladium sample types: a 100 nm electron-beam deposited thin film, a 20 μm electrodeposited palladium film, a casting of 21 nm edge-length cubic nanoparticles, and a casting of 27 nm edge-length octahedral nanoparticles. We contend that this method is the most effective for measuring hydrogen uptake in different palladium samples.

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INTRODUCTION Palladium is one of few metals that reversibly absorbs large amounts of hydrogen into the lattice. The octahedral holes in the face-centered cubic (fcc) lattice of palladium are a suitable host for a single hydrogen atom, and these hydrogen atoms can diffuse throughout palladium to saturate the material with absorbed hydrogen.1 This property is useful for a range of applications, including reversible room temperature hydrogen storage,2−4 hydride batteries,5 hydrogen sensing,6−8 and catalysis.9−14 Nanoparticulate samples2−4,6−8,10,11,13−16 and thin films5,17 represent a large fraction of these studies because they provide a larger relative surface area for absorption than bulk palladium. However, the accurate quantification of hydrogen in these types of samples is technically challenging. Hydrogen absorption in palladium occurs spontaneously in the presence of gaseous dihydrogen and when a reductive electrochemical potential is applied to a palladium electrode immersed in aqueous media (Figure 1).18 Surface-adsorbed hydrogen atoms, derived from the dissociation of dihydrogen gas or the reduction of protons, transition readily from the surface to the bulk. The measurement of hydrogen in palladium is made complicated by the fact that hydrogen desorbs spontaneously if the applied pressure or potential is removed. A larger metal surface area relative to bulk palladium enables more hydrogen to be absorbed but also provides more © 2018 American Chemical Society

Figure 1. Electrochemical reactions at a palladium electrode in aqueous media. (a) Reductive conditions reduce protons to adsorbed hydrogen atoms that can either absorb into the palladium lattice (i.e., absorption) or combine and proceed through the HER. (b) Oxidative conditions drive the conversion of absorbed hydrogen into protons (i.e., desorption) or the HOR. This study seeks to suppress HOR for more accurate measurement of absorption and desorption.

pathways for desorption. These dynamics complicate the quantification of hydrogen absorption, which is already a nontrivial matter given that hydrogen is a light element difficult to detect by mass and X-rays. The measurement of hydrogen in bulk palladium can be performed effectively by measuring the differences in resistance Received: March 30, 2018 Revised: May 28, 2018 Published: June 14, 2018 3963

DOI: 10.1021/acs.chemmater.8b01324 Chem. Mater. 2018, 30, 3963−3970

Chemistry of Materials

Methods/Protocols

before and after hydrogen absorption,19 gravimetric determination,20 tracking the change in lattice constant of palladium by X-ray diffraction21−23 and measuring the charge associated with the hydrogen absorption event coulometrically.20,24−27 However, these methods do not necessarily extend to nanoparticulate systems or thin films where in situ measurements on small sample quantities are desired. The in situ measurement of palladium resistance, which increases as hydrogen is absorbed into the lattice, requires calibration against a direct measure of hydrogen absorption, but pressure−composition isotherms are not readily available for samples other than bulk palladium. Lower resistance ratios have been reported for thin palladium films,28−30 but it is unknown whether these samples absorb less hydrogen or if other factors convolute the measurement. Moreover, it is not known if this measurement works under electrochemical conditions. Gravimetry is an ex situ method that is incompatible with the small sample sizes common to thin films and nanoparticles unless an extremely sensitive ultramicrobalance or a quartz crystal microbalance (QCM) is employed.31 The expansion of the palladium lattice during hydrogen absorption can be measured in situ with an X-ray diffractometer but only with a specialized high-pressure sample stage.32 It has recently been shown that in situ transmission electron microscopy (TEM),33 X-ray absorption spectroscopy (XAS),34,35 scanning transmission electron microscopy (STEM),36 and diffractive imaging37 can report on local hydrogen absorption, but these methods are not conducive to the study of large sample sets, do not necessarily quantify a hydrogen absorption value, and are often incompatible with electrochemical cells. Coulometry thus emerges as a powerful method for measuring hydrogen absorption in situ on a wide variety of small-volume samples of thin films or nanoparticles without calibration or complex instrumentation. This technique involves applying a constant potential to absorb hydrogen into a palladium electrode, and the corresponding current can be measured as a function of time to determine the amount of charge associated with absorbed hydrogen. However, caution must be used with this method because electrochemistry is a nonselective technique: electrons can be associated with any number of processes rather than the hydrogen absorption event of interest. For example, a negative bias in 1 M H2SO4 drives the hydrogen evolution reaction (HER; 2H+ + 2e− → H2) at the palladium surface at a similar potential as hydrogen absorption into the palladium (H+ + e− → Habs; Figure 1a). The desorption event (Habs → H+ + e−) during the anodic scan is therefore typically used to quantify the amount of hydrogen absorbed because, in principle, there is no H2 available for the competing hydrogen oxidation reaction (HOR; H2 → 2H+ + 2e−; Figure 1b). However, this situation requires that H2 not be available for electrochemistry at the electrode surface. Hydrogen bubbles at the electrode can also provide a high local concentration of H2 that can mediate the absorption of hydrogen while desorption is occurring, further complicating the measurement. Stirring the solution, bubbling argon gas through the electrolyte or using a rotating ring-disk electrode will reduce the amount of dissolved hydrogen in solution but will not ensure complete bubble removal from the surface.38,39 We report here an electrochemical flow cell (Figure 2) for the precise and accurate coulometric quantification of hydrogen absorption by palladium films and nanoparticles. The flow of electrolyte directed at the palladium surface serves to displace hydrogen bubbles from the surface of the electrode and prevent

Figure 2. (a) 3D schematic representation of the designed flow cell: a palladium sample supported on glass is sealed into the back of the cell with an O-ring. The electrolyte inlet is angled 45° to direct the electrolyte toward the sample surface to remove H2 bubbles; the electrolyte outlet is directed straight out of the cell, and ports for the reference (RE) and counter electrode (CE) are included at the top of the cell. (b) Image of the flow cell with a palladium sample at the back.

continued absorption during the desorption measurement. The continual renewal of electrolyte into an open reservoir maintains low concentrations of dissolved hydrogen in solution. We demonstrate that this cell precludes the HOR process from occurring, thus enabling the hydrogen absorption value to be measured selectively. We demonstrate the effectiveness of this cell using thin film electron-beam deposited palladium, thickfilm electrodeposited palladium, and two types of nanoparticle geometries.

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EXPERIMENTAL SECTION Materials. PdCl2 (99.9%) was purchased from Strem Chemicals. Kapton (300 HN) substrates were purchased from American Durafilm. Sodium tetrachloropalladate (II) (>99.99%), polyvinylpyrrolidine (MW 55 000), cetyltrimethylammonium chloride solution (25 wt % in water, CTAC), ascorbic acid (≥99%), citric acid (≥99.5%), potassium bromide (≥99%), TraceCERT certified high purity palladium reference (1000 mg/L Pd in 5% HCl), hydrochloric acid (37%, ACS reagent grade), and sulfuric acid (95−98%, ACS reagent grade) were purchased from Sigma-Aldrich. Nitric acid (70%) was purchased from Fisher Scientific. Ag/AgCl reference electrodes were purchased from BASi (RE6). Pt wire counter electrodes were purchased from CH Instruments. All reagents were used as received unless explicitly stated below. Materials Preparation. Electron-Beam Deposited Pt and Pd. Substrates were prepared by sonicating a Kapton HN film with a thickness of 0.003″ for 1 min in each of DI water, IPA, acetone, IPA, and DI water and then blow dried with N2 gas. This was followed by 1 min of O2 plasma stripping at 100 W. For Pt films, electron beam evaporation in a Kurt J. Lesker PVD75 system was used to deposit thin films of Cr, Au, and Pt at room temperature. The base pressure was 2 × 10−6 Torr, and the deposition rates were 0.8 A s−1, 1.4 A s−1, and 0.7−1.3 A s−1 for Cr, Au, and Pt, respectively. Film thicknesses were 10, 20, and 100 nm as measured by the integrated quartz crystal microbalance (QCM) thickness monitor. For Pd films, electron beam evaporation in a Kurt J. Lesker PVD75 system was used to deposit thin films of Cr, Au, and Pd at room temperature. The base pressure was 2 × 10−6 Torr, and the deposition rates were 0.4, 1.5, and 0.7 A s−1 for Cr, Au, and Pd, respectively. Film thicknesses were 10, 20, and 100 nm as measured by the integrated QCM thickness monitor. Electrodeposited Pd. Substrates were prepared by sonicating a Kapton HN film with a thickness of 0.003″ for 1 min in each of DI water, isopropanol (IPA), acetone, IPA, and DI water and 3964



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were not corrected for uncompensated resistance, which was typically on the order of 3−5 Ω for Pdfilm and Pdedep and 12−20 Ω for Pdcube and Pdocta. Electrochemically active surface areas (ECSA) were determined for each sample using the same electrolyte as the absorption experiments but performing CVs at varied scan rates in the potential region with no Faradaic current and plotting the current at a given potential vs scan rate. The capacitance was determined by the slope and divided by the geometric surface area to give normalized ECSA values. CVs examining absorption were performed at scan rates that were fast enough to produce a sufficient signal beyond the baseline while sufficiently slow to resolve individual peaks. Absorption and desorption potentials for absorption experiments were chosen based on absorption peaks in cyclic voltammograms and confirmed by measuring absorption at higher potentials and ensuring absorption had reached a plateau. Hydrogen Absorption Quantification. All H:Pd absorption values were determined by converting the oxidative (desorption) charge to moles using Faraday’s constant. The moles of hydrogen absorbed were divided by the moles of palladium in the sample. Moles of palladium in Pdfilm were determined by the mass of palladium deposited during electron beam deposition as determined by QCM. Moles of palladium in Pdedep were determined by measuring the mass of the substrate and sample before and after deposition. Moles of palladium in Pdcube and Pdocta were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements performed on a Varian Model 725ES optical emission spectrometer. Pdcube and Pdocta nanoparticles were dissolved off the gold surface in conc HNO3 (70%) and diluted in ∼2 wt % HNO3 solution for analysis. Calibration standards for ICP-OES were prepared from TraceCERT certified high purity palladium reference (1000 mg/L Pd in 5% HCl).

then blow dried with N2 gas. This was followed by 1 min of O2 plasma stripping at 100 W. Electron beam evaporation in a Kurt J. Lesker PVD75 system was used to deposit thin films of Cr and Au at room temperature. The base pressure was 2 × 10−6 Torr, and the deposition rates were 1.0 and 1.4 A s−1 for Cr and Au, respectively. The film thicknesses were 10 and 50 nm as measured by the integrated QCM thickness monitor. The substrate was then immersed in a 15.9 mM PdCl2 1 M HCl electrolyte, and a potential of −0.1 V vs Ag/AgCl was applied for 5500 s. Approximately 10 mg of Pd material was deposited as determined by mass measurements. Palladium Cubic Nanoparticle Synthesis. Pd nanocubes were prepared by combining polyvinylpyrrolidone (105 mg, 118 mM), ascorbic acid (60 mg, 42.5 mM), and potassium bromide (600 mg, 630 mM) in 8 mL of water in a glass vial (20 mL) and heated to 80 °C for 10 min. Three milliliters of Na2PdCl4 solution (57 mg, 64.5 mM) was added, and the solution was stirred and maintained at 80 °C for 3 h. The solution was then cooled to room temperature and centrifuged at 12 000 rpm for 30 min. The nanoparticle pellet was dispersed in 5 mL of ethanol. Palladium Octahedral Nanoparticle Synthesis. Pd nanooctahedra were prepared by combining citric acid (60 mg, 78 mM) and ascorbic acid (62 mg, 88 mM) in 4 mL of water in a glass vial (20 mL). A CTAC solution (4 mL, 200 mM) was added to the mixture and heated to 100 °C for 20 min. Three milliliters of an aqueous K2PdCl4 solution (22 mg, 68 mM) was added while stirring vigorously. Continued heating at 100 °C for 3 h was applied, and then the solution was cooled to room temperature. The mixture was centrifuged at 12 000 rpm for 30 min, and the nanoparticle pellet was dispersed in 5 mL of ethanol. Nanoparticle Casting. Dispersed nanoparticles (octahedra and cubes) were drop cast onto a Kapton substrate with a 10 nm Cr adhesion layer and a 50 nm Au conductive layer and heated at 100 °C for 1 h. Before electrochemical use, the samples were exposed to an ozone-producing UV light source (Atlantic Ultraviolet G18T5VH/U; λmax = 185 and 254 nm). The samples were treated for 15 h at ∼1 cm from the source. The UV treatment was performed to remove organic ligands from the nanoparticles40 and ensure good electrical contact between the substrate/nanoparticles and the nanoparticles with each other. Physical Methods. Scanning electron microscopy (SEM) was performed on palladium samples using an FEI Helios NanoLab 650 dual beam scanning electron microscope. Images had an accelerating voltage of 1 kV and a current of 50 pA. Transmission electron microscopy (TEM) images were obtained on a FEI Tecnai Osiris (scanning) transmission electron microscope operating at 200 kV. Samples were prepared by drop casting a dilute dispersion of nanoparticles on a 400 mesh Formvar carbon film coated Cu grid and airdried before imaging. Electrochemistry. All absorption experiments were performed electrochemically with a Metrohm Autolab PGSTAT302N or PGSTAT204N. Palladium samples (Pdfilm, Pdedep, Pdcube, and Pdocta) on Kapton substrates were attached to a glass substrate with Kapton tape (for adhesion and surface area constraint) and compressed into the working electrode position in the flow cell. Pt wire was used as a counter electrode, and Ag/AgCl was used as a reference electrode. One molar H2SO4 electrolyte was used for all electrochemical absorption experiments unless otherwise specified. Potentials

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RESULTS AND DISCUSSION The electrochemical flow cell we developed is depicted in Figure 2. A palladium sample deposited on Kapton with a glass backing is compressed with an O-ring to seal the cell on one face, and a glass slide is utilized as a transparent seal on the opposing face. Barbs on either side of the cell are connected to Tygon tubing and act as inlet and outlet for a flowing electrolyte solution. Flow from the inlet barb is targeted directly at the sample face to remove gas bubbles at the sample surface. The electrolyte is pumped through the cell at adjustable flow rates of 50−500 mL min−1, while the total volume of the electrolyte inside the cell is 8 mL. The cell is completed by two 5 mm diameter round ports on the top face for the inclusion of reference (Ag/AgCl) and counter (Pt wire) electrodes through O-ring seals (Figure S1). The exclusive use of Teflon for the cell body and Viton elastomer O-rings was required to minimize metallic impurities interfering with measurements. To validate the apparatus and experimental design, we conducted proof-of-principle experiments with a platinum thinfilm electrode. Platinum does not absorb hydrogen under ambient conditions, enabling both HER and HOR to be examined without the interference of absorption events. Figure 3 shows cyclic voltammograms (CVs) collected for a platinum thin film in the flow cell without any external movement of electrolyte (denoted as static), and under a constant flow of electrolyte (denoted as flow). (These same reaction conditions are utilized for all experiments denoted static and flow in this manuscript, and all potentials are referenced to Ag/AgCl.) A 3965



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determined by electron microscopy. The Pdfilm sample shows small grain sizes of ∼50 nm and a reasonably flat surface, while a much more complex surface morphology is observed for the Pdedep sample. The Pdcube and Pdocta samples are nearly uniform in size and geometry, but the Pdcube samples are slightly smaller than the Pdocta samples and thus Pdcube samples have a slightly larger surface area for a given mass of palladium. To compare the surface-area-to-bulk ratios of each sample, we developed an empirical ratio that compares the electrochemically active surface areas (ECSAs, Figure S3) to the masses of each sample (Table S1). This effective surface area-to-bulk metric follows the trend Pdcube > Pdocta > Pdfilm > Pdedep, and the extent to which hydrogen absorption creates a measurement error is expected to follow this trend because the HOR interference reaction is a surface phenomenon. The coulometric stripping experiment used for all sample types follows the same protocol shown in Figure 5a for Pdfilm. A

Figure 3. Cyclic voltammograms of a 100-nm Pt film (1.77 cm2) recorded at 100 mV s−1 in 1 M HCl electrolyte under static and flow conditions. HOR is observed at −0.2 V vs Ag/AgCl only when the electrolyte is under static conditions.

cathodic sweep of the films under both static and flow conditions revealed nearly superimposable electrochemical behavior, as both experiments indicated sharp increases in current flow at −0.2 V due to HER. When the scan direction is reversed, however, a peak at −0.2 V corresponding to HOR is observed under static conditions but not in the case where the electrolyte is under flow. The absence of HOR at the electrode when immersed in a flowing electrolyte validates our hypothesis that dissolved H2 and H2 gas bubbles are removed and are unable to be oxidized at the surface in the time frame of the experiment. The effect of flow can also be seen visually by noting the formation of H2 gas bubbles on the surface under static conditions and the removal of those bubbles when flow conditions are applied. We then set out to leverage our ability to suppress HOR under flow with four different palladium samples deposited on Kapton: a 100 nm electron-beam deposited thin film (Pdfilm), a 20 μm electrodeposited palladium film (Pdedep), a casting of 21 nm edge-length cubic nanoparticles (Pdcube), and a casting of 27 nm edge-length octahedral nanoparticles (Pdocta) (Figure S2). These samples were selected so that differences in morphologies and surface area-to-bulk ratios, which affect the extent of absorption, adsorption and HOR, could be tested. Figure 4 shows the morphologies of the four samples

Figure 5. (a) Chronoamperometry measurements recorded for Pdfilm at a reductive potential (absorption) and oxidative potential (desorption) under static and flow conditions, which demonstrate the differences in oxidative current when flowing electrolyte is used in the system. (b) The integration of current as a function of time during sequential reduction and oxidation of Pdfilm shows that charge continues to increase during oxidation under static conditions. This result clearly shows the amount of hydrogen absorbed can be overestimated. (Absorption is performed at −0.4 V vs Ag/AgCl for 60 s; desorption is performed at 0.35 V vs Ag/AgCl for 60 s, and integration is reset between the reductive and oxidative experiments.)

constant reductive potential is applied to absorb hydrogen into the palladium. Hydrogen absorption is equilibrated before a constant oxidative potential is applied to desorb the hydrogen from the palladium. Figure 5b shows both absorption and desorption charge as a function of time from the integration of the current in Figure 5a. The maximum amount of charge passed during the desorption process is then used to calculate the moles of hydrogen absorbed by the palladium film. The reductive current and charge are both higher for flow conditions relative to static conditions due to improved mass transport of reactants when the electrolyte is constantly refreshed at the electrode surface. Under oxidative conditions, the hydrogen oxidation reaction, present only for the static conditions, leads to higher current and charge relative to flow conditions. Under both static and flow conditions, there is a rapid increase in the desorption charge for ∼10 s (Figure 5b, RHS). Charge continues to increase under static conditions due to oxidation of dihydrogen at the electrode surface, while the charge slowly decreases under flow conditions, possibly due to a slow reductive process. Hydrogen absorption in Pdfilm was quantified as a molar ratio of H:Pd at different absorption potentials by calculating the ratio of moles of hydrogen desorbed (obtained from desorption

Figure 4. Electron beam imaging of (a) 100 nm electron beam deposited palladium thin films (Pdfilm) by SEM, (b) 20 μm electrodeposited palladium thick films (Pdedep) by SEM, (c) 21 nm edge length palladium nanocubes (Pdcube) by TEM, and (d) 27 nm edge length palladium nanooctahedra (Pdocta) by TEM. 3966



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charge) to the moles of palladium in the film (obtained from electron-beam deposition QCM measurements). A CV of Pdfilm was recorded (Figure 6a) to determine at what potentials

some hydrogen may desorb before the measurement is recorded and thus materials with very high absorption may be slightly underreported with our apparatus. We extended this study to thicker electrodeposited palladium films (Pdedep) to demonstrate the flexibility of the technique for different palladium sample types. The same coulometric methodology was used (see Figure 5) with longer time measurements to account for the significant increase in film thicknesses. The molar ratios of H:Pd were calculated for these samples by using coulometry measurements to determine the moles of hydrogen and mass measurements of the sample before and after electrodeposition to determine the moles of palladium. Figure 7 shows H:Pd ratios with standard deviations

Figure 6. (a) CV of Pdfilm at 10 mV s−1 in 1 M H2SO4 electrolyte under flow conditions, showing the potentials at which hydrogen absorption, hydrogen evolution, and hydrogen desorption occur, and (b) quantification of hydrogen absorption in Pdfilm at varied applied reductive potentials in both flow (blue) and static (orange) conditions. Each absorption potential is applied for 45 s, and desorption is performed at 0.35 V vs Ag/AgCl for 60 s.

hydrogen absorption and desorption occur. Figure 6b shows the calculated H:Pd ratios at 50 mV potential increments under flow and static conditions. The increase in H:Pd at more reductive potentials tracks closely with the increase in reductive current in the CV. At −0.4 V, a sufficient potential for hydrogen absorption, a plateau in absorption is observed under flow conditions. Under static conditions, H:Pd ratios continue to increase at more reductive potentials that correspond to more hydrogen evolution in the CV. At potentials negative of −0.4 V, the H:Pd ratio is over one, which is unlikely in defect-free palladium, where all octahedral holes are occupied at a ratio of one. A similar experiment was performed to measure H:Pd ratios with varied time at a constant potential of −0.4 V vs Ag/ AgCl (Figure S4) to demonstrate that hydrogen absorption is at equilibrium under flow conditions by ∼45 s. These data demonstrate the importance of using flowing electrolyte for accurate absorption measurements. Double-layer capacitance is a possible source of error in the charge measurement. This charge stored at the double layer is inherent to any surface in an electrochemical cell and is increased with higher surface area to samples. We applied the coulometric stripping technique at 0 V where no Faradaic process occurred to ensure that double layer capacitance did not convolute the absorption measurements. We made this measurement on Pdfilm because of its larger surface-area-to-bulk ratio where a surface process would have more impact. The charge at 0 V corresponded to a subtraction of H:Pd = 0.0002, well within the error of the measurement. On this basis, we were able to rule out this effect, impacting our quantification of hydrogen uptake in the Pdfilm and Pdedep samples. An additional concern with this technique was the possible spontaneous desorption of absorbed H atoms (when the reductive potential is removed) as H2 gas instead of oxidation to H+. To mitigate this effect, we altered the interval times between the application of reductive and oxidative potentials (Figure S5). The difference between a 10 ms interval time (H:Pd = 0.54 ± 0.02) and a 0.2 ms interval time (H:Pd = 0.65 ± 0.03) was significant, and we therefore applied the 0.2 ms interval time for all measurements. This interval time is at the limit of our instrumentation. We therefore acknowledge that

Figure 7. Quantification of hydrogen absorption for Pdfilm and Pdedep under both flow (blue) and static (orange) conditions. Absorption for Pdfilm is performed at a constant potential of −0.5 V vs Ag/AgCl for 60 s, and desorption is performed at 0.25 V vs Ag/AgCl for 60 s. Absorption for Pdedep is performed at −0.3 V vs Ag/AgCl for 2000 s, and desorption is performed at 0.25 V vs Ag/AgCl for 1200 s. Error bars represent standard deviation in the measurements for three or more samples.

for Pdedep and Pdfilm under flow and static conditions, with more pronounced differences for Pdfilm than Pdedep, as predicted by the higher surface area to bulk ratio of Pdfilm (Table S1). The importance of the flow cell for Pdedep samples is still notable when examining the standard deviations of the absorption measurements: the standard deviations are ∼10% of the measurement values under static conditions for both Pdedep and Pdfilm, while flow lowers the standard deviations to ∼2%. The flow cell therefore positively affects both the accuracy and precision of absorption quantification. Accordingly, properly measured H:Pd under flow conditions for both Pdfilm and Pdedep are 0.65 ± 0.03 and 0.64 ± 0.01, respectively. Finally, we examined the absorption properties of palladium nanoparticles with both cubic and octahedral geometries (Pdcube and Pdocta). The extremely high surface area-to-bulk ratio of nanoparticles makes the hydrogen absorption measurement particularly challenging due to significant surface adsorption (Hads). In bulk and thin film palladium, the adsorption process typically accounts for a negligible quantity of charge, whereas this charge can be significant for nanoparticulate samples. The surface adsorption process is evident in the cyclic voltammogram of Pdcube (Figure 8a) at −0.075 V, a feature absent in thin film samples (c.f. Figure 6a). As the adsorption and absorption features are distinct electrochemical processes, it is possible to subtract the integrated adsorption charge from total charge to obtain a more accurate measurement of hydrogen absorption. Figure 8b shows the desorption charge at various applied reductive potentials in both the adsorption and absorption regions. A plateau is observed between −0.05 and −0.15 V (dotted line) that corresponds to the charge of Hads, and another plateau is 3967



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Coulometry under standard static conditions cannot be used to accurately quantify absorption for these high surface area materials, a point clearly demonstrated by the ∼40% standard deviation calculated for Pdcube. The H:Pd ratio determined for Pdocta is statistically significantly higher than that of Pdcube. This difference could be attributed to a difference in size, shape, or the exposed facet ((111) for Pdocta vs (100) for Pdcube).41,42 Further absorption studies controlling these variables with this coulometric method may better elucidate the mechanism of hydrogen absorption in Pd nanoparticles.

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CONCLUSIONS We demonstrated that an electrochemical flow cell can be used to remove hydrogen from the electrode and electrolyte. Flowing electrolyte prevents H2 gas adsorption and consequent oxidation of this H2 during a coulometric measurement. The use of this flow cell was demonstrated for four different types of palladium samples (thick electrodeposited films, thin electronbeam deposited films, and cubic and octahedral nanoparticles), indicating that the use of selective flow coulometry can be used for absorption measurements in samples with differing morphologies and surface area-to-bulk ratios. In cases where large surface areas may impact absorption quantification, selecting the correct potential and subtracting out other processes can improve the accuracy of this technique even further. Future work will aim to consolidate the absorption data for bulk nanoparticle samples with studies performed on individual nanoparticles to get a clearer mechanistic picture of hydrogen absorption in palladium. We will also apply this technique to other metals so as to demonstrate its overall applicability in the absorption of hydrogen in materials.

Figure 8. (a) Cyclic voltammetry under static conditions of Pdcube at 5 mV s−1 scan rate in 1 M H2SO4 electrolyte showing peaks corresponding to hydrogen adsorption (Hads) and hydrogen absorption (Habs) and (b) desorption charge from coulometric stripping at varied reductive potentials for Pdcube, demonstrating a plateau corresponding to Hads between −0.05 and −0.15 V vs Ag/ AgCl and a plateau corresponding to Habs between −0.4 and −0.5 V vs Ag/AgCl.

observed between −0.4 and −0.5 V (dotted line) that corresponds to the total charge (Hads + Habs). The charge for Habs is determined by subtracting Hads charge from the total charge. The charge for double layer capacitance of nanoparticles can also be significant; however, because this charge is convoluted with adsorption charge, it is also subtracted to leave only Habs charge. In the case of Pdocta, the adsorption feature is not as prominent in the CV because the surface area is comparably lower (Table S1, Figure S6a). We were still able to use coulometric measurements to distinguish an adsorption plateau between −0.1 and −0.15 V and therefore take the adsorption charge into account to obtain a reliable absorption measurement (Figure S6b). This method can be used for any type of nanoparticle, even those for which surface features are not easily resolvable in scanning methods such as cyclic voltammetry. To quantify absorption in the nanoparticle samples, the adsorption charge was subtracted from absorption charge to deconvolute the processes and quantify only absorbed hydrogen (Figure 9). A molar ratio was determined by using hydrogen absorption charge and moles of palladium in the sample, as determined by ICP-OES measurements on dissolved nanoparticles after absorption measurements (Figure S7). H:Pd values obtained for Pdcube and Pdocta under electrolyte flow conditions were 0.56 ± 0.06 and 0.85 ± 0.06, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01324. Additional photographs of the setup, nanoparticle characterization, electrochemical characterization, and ICP-OES quantification, including Figures S1−S7 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rebecca S. Sherbo: 0000-0003-2376-6335 Noah J. J. Johnson: 0000-0002-0721-3186 Curtis P. Berlinguette: 0000-0001-6875-849X Author Contributions ⊥

R.S.S. and M.M.-G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Canadian Natural Science and Engineering Council (RGPIN 337345-13), Canadian Foundation for Innovation (229288), Canadian Institute for Advanced Research (BSE-BERL-162173), Canada Research Chairs, and Google LLC for financial support. This research was undertaken thanks in part to funding from the Canada First Research

Figure 9. Quantification of hydrogen absorption for Pdcube and Pdocta under flow (blue) and static (orange) conditions. Absorption is performed at −0.6 V vs AgCl for 60 s, and desorption is performed at 0.25 V vs Ag/AgCl for 60 s. Adsorption charge is subtracted for the H:Pd measurement under both flow and static conditions. Error bars represent standard deviation in the measurements for four samples. 3968



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Excellence Fund, Quantum Materials and Future Technologies Program. We thank Des Lovrity and Pritesh Padhiar in Mechanical Engineering Services for machining of the flow cell. We also thank Dr. Gethin Owen in the Centre for HighThroughput Phenogenomics for assistance with scanning electron microscope imaging and Dr. Maureen Soon in the Pacific Centre for Isotopic and Geochemical Research (PCIGR) for assistance with ICP-OES analysis. This work also made use of the 4D LABORATORIES shared facilities supported by the Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF), Western Economic Diversification Canada (WD), and Simon Fraser University (SFU).

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Accurate Coulometric Quantification of Hydrogen Absorption in

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