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Article
Effect of Chromium Doping on Electrochemical Water Oxidation Activity by Co CrO spinel catalysts 3-x
x
4
Chia-Cheng Lin, and Charles C. L. McCrory ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02170 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016
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ACS Catalysis
Effect of Chromium Doping on Electrochemical Water Oxidation Activity by Co3-xCrxO4 spinel catalysts Chia-Cheng Lin, Charles C. L. McCrory* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States ABSTRACT: We have synthesized a series of Co3-xCrxO4 (0 < x < 2) catalysts via thermal decomposition and explored their activity for the oxygen evolution reaction (OER). By changing the Cr content, we were able to modify the overall OER activity. Among the most active compositions investigated is Co2.25Cr0.75O4, which achieves 10 mA cm-2 current density per geometric area and a mass activity of 10.5 A/g at 0.35 V overpotential. We hypothesize that this enhanced OER activity is due to the increase of the adsorption energy of the intermediates (*O, *OH and *OOH) resulting from the increased electrophilicity of Co2+ sites in the material after the introduction of the electron deficient Cr. This systematic doping strategy enables the investigation of the correlation between the doping and OER activity, and allows for the rational design of future catalysts with higher activity and efficiency. Keywords:
electrocatalyst,
oxygen
INTRODUCTION Electrochemical water splitting has attracted intense attention for its potential application as an energy storage strategy for intermittent energy sources such as solar and wind.1-14 However, the lack of existing catalysts to facilitate the efficient four-electron oxidation of water in the oxygen evolution reaction (OER) hinders the development of water-splitting technologies.15,16 Significant effort has been devoted to develop cost-effective OER electrocatalysts that operate with high efficiency and stability at low overpotentials.17-19 IrO2 and RuO2 are often considered as reference materials for OER due to their high geometric OER activity in alkaline conditions,20-24 but their scarcity,25 instability in alkaline media,26-28 and low specific activity compared to other transition metal oxide catalysts hinder the practical utility. There is a need for the discovery of new earth-abundant materials with activity rivaling that of expensive, noble-metal catalysts. As an alternative to noble-metal catalysts, numerous materials comprised of earth-abundant metals have been explored as OER catalysts.23,24,29-34 In particular, Co3O434-38 and electrodeposited CoOx films23,39-42 have been extensively studied for OER and show reasonable activity and stability in alkaline solution. Doping additional transition metals into the Co3O4 lattice, in some cases, has led to increased OER activity. For instance, NiCo2O4,43-45 and ZnxCo3-xO4,46 and CoFeCrO447 all show enhanced activity for OER compared to Co3O4. The increased activity of MxCo3-xO4 materials (M = Ni, Zn, Fe, Cr, etc.) compared to Co3O4 is likely due to the modulation of the adsorption energy of surface intermediates in the oxygen evolution reaction. Theoretical studies have suggested that the adsorption energies of the intermediates (*O, *OH and *OOH) dictate the OER activity.48-52 Therefore, the stabilization of adsorption energies of surface intermediates through the doping of electropositive hetero metals in the Co3O4 lattice should increase the OER activity of the resulting material.53 Other doping studies have shown that doping the Co3O4 spinel with metals like Mn lead to decreased OER activity,54 where-
evolution
reaction,
water
splitting
as doping with Ni,55,56 Cu,56 or Li56 leads to increased OER activity.56 In this report, we investigate the activity of a series of spinel Co3-xCrxO4 catalysts for OER. Specifically, we report the correlation between the substitution of electropositive Cr into the Co3O4 framework and the OER activity. By systematically doping increasing amounts of Cr into the Co3O4 lattice, we discovered that Co2.25Cr0.75O4 is a particularly active catalyst, achieving a current density of 10 mA cm-2 at an overpotential of only η ~ 0.35 V, a specific activity per BET surface area at 0.35 V overpotential of , , ~ 0.01 mA cm-2BET, and a mass activity of , ~ 10.5 A/g. Investigations of this material’s stability and Faradaic efficiency are also reported.
EXPERIMENTAL Materials. Cobalt(II) acetate tetrahydrate (Co(OAc)2·4H2O, 99.995%), sodium hydroxide (NaOH, BioUltra), ferrocenecarboxylic acid (97%), sodium phosphate monobasic dihydrate (NaH2PO4·2H2O, ACS grade) and 5 wt % Nafion 117 solution (in a mixture of lower aliphatic alcohols and water) were purchased from Sigma Aldrich. Hydrochloric acid (HCl, ACS grade) and oxalic acid dihydrate (H2C2O4·2H2O, ACS grade) were purchased from Acros Organics. Chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O, 99%) was purchased from Strem Chemicals Inc. Ethanol (EtOH, 200 proof) was purchased from Decon Labs. Ultrapure water (18.2 M Ω cm resistivity) was purified with a Thermo Scientific Barnstead Nanopure water purification system. Nitrogen (N2) was boiloff gas from a liquid nitrogen source. Oxygen (O2, ultra high purity, 99.993%) was purchased from Cryogenic Gases. All chemicals were used as received without any further purification. Mixed metal oxide synthesis. Co3-xCrxO4 (x = 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2) nanoparticles were synthesized according to a modified literature procedure involving the thermal decomposition of cobalt-chromium oxalate gel precur-
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sors.57 The gel precursors were prepared by co-precipitation of cobalt (Co2+) and chromium (Cr3+) with oxalic acid in EtOH. Taking the preparation of Co2CrO4 for example, cobalt acetate (0.01 mol, 2.491 g) and chromium nitrate (0.005 mol, 2.001 g) were first dissolved in 40 mL of EtOH, and stirred at 50 °C for 30 min. A separately prepared oxalic acid ethanolic solution (1.8 M, 10 mL) was added dropwise into the previous solution. After reaction at 50 °C for 2 h, the solvent was removed by heating under reduced pressure. Heating the ground gel precursor to 400 °C under air for 2 h yielded Co2CrO4 nanoparticles. Characterization. Powder X-ray diffraction (PXRD) data were recorded with a Bruker D8 Advance Powder X-ray Diffractometer with Cu Kα radiation source (40 kV, 44 mA). Nitrogen physisorption was measured on a Micromeritics ASAP 2020 surface area and porosimetry system. Samples were degassed at 110 °C under vacuum overnight before analysis. Surface area was calculated with the Brunauer-EmmettTeller (BET) method in the relative pressure range of 0.005 to 0.25 of adsorption data. Metal content analysis was conducted with an ICP-OES spectrometer (Perkin-Elmer Optima 2000 DV with Winlab software). All ICP-OES measurements were conducted in at least 3 independent experiments, and reported values are averages of these experiments. X-ray photoelectron (XPS) spectra were acquired on a Kratos Axis Ultra XPS with a monochromatic Al x-ray source operating at 8 mA and 14 kV. XPS data analysis was processed using CasaXPS version 2.3.17 (Casa Software Ltd). Scanning electron microscope (SEM) images were collected with a JEOL-7800FLV. Working electrode preparation. Glassy carbon disks (⌀ = 5 mm, t = 4 mm, 0.196 cm2 surface area, Sigradur G, HTW Hochtemperatur-Werkstoff GmbH) were used as working electrodes. The disks were lapped with silicon carbide abrasive papers (CarbiMet 2, 600/P1200, Buehler), followed by sequential polishing with diamond abrasive slurries (MetaDi Supreme, Buehler) in an order of 9 µm, 6 µm, 3 µm, 1 µm, and 0.1 µm diameter particle based slurries (1 min polishing each) on synthetic nap based polishing pads (MD Floc, Struers). The lapping and polishing were performed using a LaboSystem (LaboPol-5 and LaboForce-1, Struers) with 5 psi of applied pressure per disk, 8 rpm of the head speed and 200 rpm of the platen speed. The polished disks were sonicated in a dilute nitric acid solution (10 wt %) for 10 min, followed by sonicated in acetone and D.I. water and blow-dryng with N2. The Co3-xCrxO4 catalysts were drop casted onto the glassy carbon electrodes from catalyst inks by following a reported protocol.34 Briefly, catalyst inks were made of 80 mg of the catalysts, 3.8 mL of water, 1.0 mL of isopropanol, and 40 µL of 5 % Nafion 117 solution. The inks were sonicated for 10 min immediately prior to drop casting onto the glassy carbon disks. Unless otherwise noted, 10 µL of the inks (calc. 165 µg of the catalyst) were applied onto the disks, resulting in a catalyst loading of 0.84 mg/cm2. The disks were then dried in an oven at 60 °C for 10 min. Electrochemical analysis. Electrochemical measurements were conducted with a Bio-Logic SP200 or a SP300 potentiostat/galvanostat with a built-in EIS analyzer. The modified glassy carbon working electrodes were mounted into a Pine Instrument Company E6-series ChangeDisk RDE assembly and affixed to an MSR rotator. Reference electrodes (commercial silver chloride electrodes, CH Instruments) were externally referenced to a solution of ferrocenecarboxylic acid in 0.2
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M phosphate buffer at pH 7 (0.284 V vs SCE, 0.329 V vs Ag/AgCl)58 prior to each set of experiments, and the auxiliary electrodes were carbon rods (99.999 %, Strem Chemicals). Data were recorded using the BioLogic EC-Lab software package. All electrochemical measurements were conducted in customized H-shaped two-compartment cells while the working electrodes were mounted in an RDE assembly. Typically, the main chamber held the working and reference electrodes in ca. 120 mL of 1 M NaOH solution, while the second chamber held the auxiliary electrode in ca. 18 mL of 1 M NaOH solution. Prior to each set of measurements, the electrolyte solution was sparged with O2 for at least 30 min, and the solution was continuously sparged with O2 during rotating disk electrode voltammetry (RDEV), chronoamperometry (CA), and chronopotentiometry (CP) measurements. Note that each electrochemical measurement was conducted at least three times, and reported values are averages of these runs. Prior to each set of experiments, the uncompensated solution resistance (Ru) was measured with a high-frequency single-point impedance measurement at 100 kHz with a 20 mV amplitude about the open-circuit potential (OCP), and RDEV and CA measurements were corrected for IR drop at 85 % through positive feedback using the Bio-Logic ECLab software. CP measurements were manually corrected for IR drop. Our typical electrochemical setup resulted in Ru = ~10 Ω in 1 M NaOH. Faradaic efficiency. Dissolved O2 was quantified using a Unisense Microsensor Monometer equipped with an Ox-500 oxygen probe. The Ox-500 probe was calibrated with a 3-point calibration in N2-sparged (0 %), air-saturated (20.8 % O2) and O2-sparged (100 %) solutions. The dissolved O2 concentration in O2-saturated solution at 20 °C was determined from a linear interpolation of solubility data reported at 15 and 25 °C: [O2]saturated = 0.83 mM in 1 M NaOH.59,60 Faradaic efficiency measurements were conducted in an H-shaped twocompartment cell as previously reported.23 The first compartment was airtight and contained a glassy carbon working electrode, a Ag/AgCl reference electrode and an Ox-500 oxygen probe with a total volume of 71.8 mL. The other compartment accommodated a carbon rod counter electrode, and the two compartments were separated by a Nafion 117 membrane (t = 0.007 in.). Both compartments were filled with 1 M NaOH solution and the first compartment was completely filled with no noticeable headspace. The solution was air-saturated prior to use, and the concentration of dissolved O2 was monitored for 10 min at open circuit potential (OCP), followed by a controlled current electrolysis, where the current density was held at 10 mA/cm2 for 20 min passing a total charge of 2.35 C. The concentration of the dissolved O2 was monitored during this time, and the total amount of O2 produced was determined by taking the difference between the measured equilibrated O2 concentration (5 min after the end of the electrolysis) minus the background O2 (at OCP). The Faradaic efficiency was calculated by taking the increase of the dissolved O2 amount divided by the theoretical value calculated from the total charge passed. Conductivity measurement. The conductivity measurements were performed on a Rucker & Kolls model 260 room temperature probe station equipped with a Miller Design P-10 micropositioner, a computer with custom LabView software, National Instruments BNC-2110 Connector and DL Instruments 1211 Current Preamplifier. The scan range is from 0.25 V to -0.25 V with a scan rate of 10 mV/sec and 1 mV intervals.
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The pellet samples were prepared by compression with a hydraulic press die at an applied pressure of 10,000 psi.
RESULTS AND DISCUSSION Synthesis and characterization of Co3-xCrxO4 nanoparticles. The Co3-xCrxO4 electrocatalysts were synthesized by a two-step gelation-thermal decomposition method, which employs thermal decomposition of pre-made cobalt(II)chromium(III) oxalate gel precursors at 400 °C in air for 2 h. As shown in Figure 1, the PXRD pattern of the pristine Co3O4 matches well with that of the spinel Co3O4 reference. The absence of the diffraction peaks of other phases, i.e. CoO and Cr2O3, in the range examined suggests the synthesized materials are phase pure with negligible impurity.
OES). The nominal metal compositions are in good agreement with the experimental results, which further suggests that the material preparation via the two-step glass gel-thermal decomposition method yields the desired product with nearnegligible variation. The metal composition, BET surface area, estimated grain size and lattice parameter data are summarized in Table 1.
Figure 2. Lattice parameter dependence on Cr content across the Co3-xCrxO4 series. The black circles are the lattice parameters determined from the XRD measurements, the black dash line is the linear fitting for the experimental data. (y = 8.1 + 0.1 x, r2 = 0.98), and the red line is the expected dependence calculated from Vegard’s law.
Figure 1. PXRD data for the Co3-xCrxO4 series. The dash line is a guide to the eye to illustrate the shift in the (311) peak as a function of increasing Cr content. The PXRD patterns of the doped samples shift towards low angle region with increasing Cr content, implying an increase in d-spacing. These peak shifts, especially for the peak (311), become more pronounced as more Cr is incorporated, while no peaks of Cr2O3 phase are present even when 67 % of the Co was replaced by Cr. The absence of the peaks from the impurity phases shows that the Cr doping by the two-step method within the range examined in this work does not lead to any undesired phase segregation. The lattice parameters (a) were calculated according to the (311) peak position for comparison of the lattice dimension expansion (d-spacing increase) as a function of Cr content. As depicted in Figure 2, a increases monotonically from calc. 8.081 Å (Co3O4) to calc. 8.285 Å (CoCr2O4) with increasing Cr content, a 2.5 % expansion between these two extremes. The measured increase in lattice parameter as a function of increasing Cr content matches well with the dependence as calculated from Vegard’s law. Table 1 summarizes the specific surface area data of the catalysts obtained by nitrogen physisorption analysis (BET). The sample surface areas are in the range between 50 and 100 m2/g which are similar with that of Co3O4. Metal composition of each material was determined by elemental analysis (ICP-
Electrocatalytic Activity for OER. The electrocatalytic activity of each material was measured using previously reported protocols.11,12,14 Loadings of 0.165 mg of each catalyst were deposited onto glassy carbon electrodes in a Nafion binder via drop casting, resulting in coverages of 0.84 mg/cm2geo. The activity was then measured using a combination of RDEVs, CA, and CP measurements at an electrode rotation rate of 1600 rpm in O2-sparged 1 M NaOH as previously described.23,34 The primary figures of merit used in this study are the overpotential required to achieve a current denisty of 10 mA cm-2geo per geometric area (ηj=10 mA/cm2,geo), the current density per geometric area at 0.35 V overpotential (, , ), and the specific activity per BET surface area at 0.35 V overpotential (, , ), and the mass activity defined as current density per mass loading , , ). Representative RDEVs for OER by three Co3-xCrxO4 catalysts are shown in Figure 3, and all catalysts investigated in Figure S1. The RDEV of the pristine Co3O4 consists of three features: a non-Faradaic region associated with capacitive charging (0.1 V < η < ~0.2 V), an anodic peak (η ~ 0.27 V), and a rapid onset of increasing current associated with water oxidation (η > 0.35 V). The anodic peak prior to water oxidation is assigned to the surface redox couple Co3+/4+ (CoOOH ⇌ CoO2).61 As more Cr is incorporated, this anodic peak becomes increasingly less prominent, and eventually becomes seemingly insignificant at Cr content of 25% in Co2.25Cr0.75O4. This loss of the anodic Co3+/4+ peak suggests that Cr substitutes into the lattice as Cr3+ and replaces Co3+ sites. In
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Co2.25Cr0.75O4, we expect that nearly 37.5% of the Co3+ in the Co3O4 parent material has been replaced by Cr3+.
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pendent OER experiments. In undoped Co3O4, ηj=10 mA/cm2,geo = 0.42 ± 0.01 V. As more Cr is incorporated into the material,
Table 1. Metal contents, lattice parameters and BET surface area of Co3-xCrxO4 series. Grain size (nm)b
SBET (m2/g)
8.081
16.68
41.54 ± 0.94
8.4
8.106
14.04
51.72 ± 0.72
16.7
16.2
8.124
9.85
85.58 ± 0.92
Co2.25Cr.75O4
25.0
24.1
8.135
9.76
74.17 ± 0.92
Co2CrO4
33.3
32.9
8.203
8.96
40.11 ± 0.90
Co1.75Cr1.25O4
41.7
40.8
8.209
8.28
50.90 ± 0.81
Co1.50Cr1.50O4
50.0
50.2
8.233
13.69
45.96 ± 0.74
Co1.25Cr1.75O4
58.3
58.3
8.267
9.26
79.83 ± 0.46
CoCr2O4
66.7
65.9
8.285
7.07
104.67 ± 0.48
a b
Catalyst
calc. Cr at %a
Cr at % (ICP-OES) Lattice parameter (Å)b
Co3O4
0.0
-
Co2.75Cr.25O4
8.3
Co2.50Cr.50O4
Cr at % = nCr / (nCo + nCr) × 100 % Calculated from XRD (311) peak
There is also a positive shift in the Co oxidation potential with increasing Cr content (Figure S1). This is indicative of a changing electronic environment surrounding the Co sites that makes oxidation of Co sites more difficult, presumably due to incorporation of the more electropositive Cr. A similar trend has been reported for Fe-doped electrodeposited Co1-xFexOOH systems.62 Moreover, there is an observed increase in the measured pseudo-capacitive charging current between ~0.1 – 0.3 V with increasing Cr content up to Co2CrO4. At higher Cr concentrations than Co2CrO4, this pseudo-capacitive charging current decreases again. One possible explanation for this may be that Cr-doping changes the concentration of CoOOH phases in the material, thus changing the film capacitance.
Figure 3. Representative RDEVs of Co3O4 (black), Co2.75Cr0.25O4 (blue) and Co2.25Cr0.75O4 (green) in O2-sparged 1 M NaOH. Scan rate = 10 mV/sec; rotator speed 1600 rpm. Co2.75Cr0.25O4 and Co2.25Cr0.75O4 exhibit an increase in OER activity compared with Co3O4 as seen in Figure 3. The activity of the Co3-xCrxO4 as a function of Cr content is further summarized in Figure 4 and Table 2. Note that all activity measurements reported here are averaged from at least 3 inde-
the magnitude overpotential for OER initially decreases to a minimum of ηj=10 mA/cm2,geo = 0.35 ± 0.01 V for Co2.25Cr0.75O4. However, at Cr content with x > 0.75, the magnitude overpotential begins to increase again, eventually reaching 0.40 ± 0.03 V for CoCr2O4. Similarly, OER activity as defined by the current density per geometric area at 0.35 V overpotential also increases with the addition of Cr to maximum of , , = 8.84 ± 2.69 mA/cm2 for catalysts with x = 0.75, and then decreases at higher Cr contents. Note that the same catalyst loading was used for each Co3-xCrxO4 catalyst investigated, the , , is directly proportional to , , , and so follows an identical trend. In order to exclude surface area variation as the possible cause for the enhanced activity, the specific activity was compared by normalizing the current density with the BET surface area of the catalyst loaded on the electrode. The BETnormalized specific activity follows a similar general trend to that of the other activity metrics, suggesting that the activity differences are likely due to intrinsic changes in the per-site activity rather than due to variances in the surface area. Based on our measurements, Co2.5Cr0.5O4, Co2.25Cr0.75O4, and Co2CrO4 show the highest overall activity of the compositions investigated. All three materials operate with comparable overpotentials at 10 mA cm-2 and specific activities per BET surface area, although Co2.25Cr0.75O4 operates with slightly higher activity per geometric area and mass activity. The electrocatalytic activity of the Co2.25Cr0.75O4 material compares favorably to OER activity benchmarks, operating with higher or comparable mass activity and/or specific activity to that reported for the NiFeOx activity benchmark,63 IrO2,34,64 and RuO2.34,64,65 Comparisons of the OER activity of Co2.25Cr0.75O4 to the NiFeOx benchmark, IrO2, RuO2, and a few other recently reported highly-active OER catalysts is shown in Table S2. Tafel slopes for OER were determined for each material to provide qualitative mechanistic insight. Although determining explicit reaction mechanisms from Tafel slopes alone for OER
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is non-trivial due to the multi-step mechanistic complexity of the reaction,8,66 a comparison of Tafel slopes may provide a
qualitative insight as to whether two materials might operate with similar or different catalytic mechanisms.67
Overpotentials were averaged from at least 3 different RDEV scans and chronoamperometry measurements at j = 10 mA/cm2.
Table 2. Relevant OER parameters for Co3-xCrxO4 catalysts in 1 M NaOH Catalyst
b c
" / , (V)
a !
, , (mA/cm2)
b
, , b (µA/cm2BET )
, c (A/g)
Tafel slope (mV/dec)
Co3O4
0.42 ± 0.01
0.42 ± 0.11
1.20 ± 0.31
0.50 ± 0.13
52 ± 3
Co2.75Cr0.25O4
0.38 ± 0.01
3.53 ± 1.76
8.04 ± 4.02
4.19 ± 2.20
61 ± 6
Co2.5Cr0.5O4
0.36 ± 0.01
5.84 ± 1.87
8.05 ± 2 .57
6.95 ± 2.22
61 ± 4
Co2.25Cr0.75O4
0.35 ± 0.01
8.84 ± 2.69
14.06 ± 4.28
10.52 ± 3.20
60 ± 3
Co2CrO4
0.37 ± 0.01
4.01 ± 1.16
11.78 ± 3.41
4.77 ± 1.38
56 ± 5
Co1.75Cr1.25O4
0.38 ± 0.01
4.55 ± 0.59
10.55 ± 1.36
5.41 ± 0.70
66 ± 1
Co1.5Cr1.5O4
0.39 ± 0.01
4.16 ± 0.96
10.68 ± 2.47
4.95 ± 1.14
51 ± 4
Co1.25Cr1.75O4
0.38 ± 0.01
3.70 ±1.19
5.47 ± 1.76
4.41 ± 1.42
70 ± 6
CoCr2O4
0.40 ± 0.03
2.45 ± 0.64
2.77 ± 0.72
2.92 ± 0.76
87 ± 16
Current densities were averaged from at least 3 different RDEVscans and chronopotentiometry measurements atη= 0.350 V. Calculated based on the as-deposited 0.84 mg cm-2 catalyst loading.
Tafel plots of each catalyst were constructed from a combination of steady state chronoamperometry and chronopotentiometry measurements at 1600 rpm rotation rate in O2-saturated solution and are shown in Figure S2. Tafel slopes were determined from a regression analysis of the curves over two decades in current density centered at the region around 10 mA/cm2 in Figure S2, and are summarized in Table 2. The measured Tafel slope for OER by Co3O4 is near 50 mV/dec, which is consistent with most previous reports that show Tafel slopes near 50 mV/dec44,56,68-70 or 60 mV/dec34,55,7072 for this material depending on preparation procedure, the nature and concentration of the electrolyte, and other measurement conditions. Most specimens, including Co2.25Cr0.75O4, also evolve O2 with Tafel slopes around 50-60 mV/dec. This suggests that these materials might evolve O2 via a similar mechanism to Co3O4. However, Co1.25Cr1.75O4 and CoCr2O4 both operate with larger Tafel slopes than other materials, perhaps suggesting a different OER pathway is taken when a large fraction of the Co3+ is replaced by Cr3+. Dependence of OER Activity on Catalyst Loading. For the highly-active Co2.25Cr0.75O4 system, additional experiments were conducted to determine the effect of catalyst loading on OER activity. Plots of , , and , , as a function of catalyst loading are shown in Figure 5 and the corresponding data is summarized in Table S1. The overpotential needed to achieve 10 mA cm-2geo current density decreases as the loading increases. Likewise, the current density per geometric area at η = 0.35 V increases linearly with increased catalyst loading. These results confirm the catalytic OER is first order in catalyst loading as expected by most postulated OER mechanisms.22,48,50-52,73,74
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Figure 4. Comparative plots of Co3-xCrxO4 catalyst OER activities. (a) Overpotential at current density of 10 mA/cm2geo (blue circles), (b) current per geometric area and (c) current density per BET surface area at η = 0.35 V (green triangles). In contrast the specific current density slightly decreases with increased catalyst loading, and the Tafel slope remains roughly constant within error (Table S1). One would expect both of these values to remain constant independent of catalyst loading—in a process first order in catalyst, the mechanism at each active site should be unperturbed regardless of the number of catalyst active sites. The small decrease of specific current density with increasing catalyst loading could be due several effects. First, inefficient charge transport to exterior active sites may lower the activity per site at higher catalyst and binder loadings. For example, in a recent study investigating TiO2 as an OER catalyst, the sample thickness was shown to affect the efficiency of the charge carrier tunneling.75 In addition, more catalyst/binder deposited on the surface might also cause larger agglomeration that both decrease the number of exposed active sites and/or hinder mass transfer of locally formed O2 that could leads to the creation of a diffusion barrier, and eventually results in slightly deteriorated activities due to inefficient product removal. Figure S4 shows representative SEM images of the Co2.25Cr0.75O4 particles on a glassy carbon electrode (0.84 mg catalyst/cm2geo), Co2.25Cr0.75O4/Nafion composite on a glassy carbon electrode (0.84 mg catalyst/cm2geo) and solely Nafion binder on a glassy carbon. The observed agglomeration of nanoparticles in Figure S4(a) is consistent with the fact that bare particles tend to aggregate to minimize the surface energy when no ligands or spacers are present. Although aggregated as clusters, it is still evident that the dimension of the individual metal oxide particles is around 20 nm which is in agreement with the grain size estimation from PXRD peak width (Table 1). The SEM image of the metal oxide/Nafion composite in Figure S4(b) shows the particle assemblies coated with a thin layer of coating of Nafion on top, similar to what has been reported previously for nanoparticles dropcasted by a similar method.34 This suggests that the Nafion binder acts as a thin layer of coating on top of the metal oxides rather than the thick overcoat observed when solely Nafion film is deposited on a glassy carbon electrode (Figure S5 (c)). The aggregation of the particles is inevitable when utilizing drop casting as the deposition method, and it might account for the variation of the specific current density calculation. Finally, the low specific activity at higher loadings may be due to a loss of catalyst particles from the surfaces due to poor adherence. During rotating disk electrode voltammetry measurements at 1600 rpm rotation rate, we observed a loss of catalytic activity at high catalyst loadings (≥1.26 mg/cm2), and a film failure was observed for the high loading sample (Figure S6 (b)) while applied potential larger than +0.38 V. We attribute this to detachment/delamination of the Nafion-catalyst composite, perhaps due to internal pressure from trapped locally-evolved O2 inside void spaces within the film. Because this loss of catalyst occurs on a relatively short time scale, and because we normalize , , area to the BET surface area of catalyst initially loaded onto the surface, this catalyst detachment may artificially lower the reported specific activity at high coverage. Catalyst Stability and Faradaic Efficiency for Co2.25Cr0.75O4. Additional experiments were conducted to
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determine the stability and Faradaic efficiency for OER by the highly-active catalyst composition Co2.25Cr0.75O4. The Faradaic efficiency measurement of the Co2.25Cr0.75O4 catalyst is shown in Figure S3, with the average efficiency of 0.93 ± 0.03. This measurement confirms the O2 was primarily produced during the experiments.
Figure 5. Comparative plots of Co3-xCrxO4 catalyst OER activities with different loadings. The unity equals the loading of 0.84 mg/cm2. (a) Overpotential at current density of 10 mA/cm2geo (blue circles), (b) current density per geometric surface area at η = 0.35 (red squares) and (c) current density per BET surface area at η = 0.35 (green triangles). The red dash line is a linear fit to the data in (b), and is provided as a guide to the eye. Catalyst stability was measured using two different metrics: 24-h constant current measurements at 10 mA cm-2geo and potential cycling measurements between η = 0 to 0.4 V. In the controlled current stability study (Figure S8 (a)), the overpotential starts at η ~ 0.35 V and remains relatively constant over the course of 10-15 h of constant polarization. After 10-15 h, there is an abrupt increase in the operating overpotential to that of the glassy carbon background.23,24 This dramatic loss of activity may be due to film delamination similar to that observed at short time in the high loading experiment (Figure S6 (b-c)). Over long times, there may be increasing internal pressure due to trapped O2 in void spaces in the film, leading to a gradual weakening of the film and eventual film delamination. The representative SEM images after the long term stability measurement (Figures 6 (c, d), S7) reveal a residual binder
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framework with foam like structure that was not observed before the stability test (Figure 6 (a, b)). Potential cycling stability measurements (Figure S8 (b)) were used to provide additional insight into the catalyst stability. Cycling measurements were performed by scanning the potential rapidly between η = 0 to 0.4 V and back to 0 V at a scan rate of 50 mV/s. After each set of 50 repetitions, chronopotentiometry (at j = 10 mA/cm2geo) measurements were taken to determine the operating overpotential at 10 mA/cm2geo. The operating overpotential increased by 0.02 V after the first two sets of cycling, and then stabilized at ηj=10 mA/cm2,geo ~ 0.38 V until the end of the measurement (10,000 cycles, ~28h).
Figure 6. Representative SEM images of Co2.25Cr0.75O4/Nafion/GC before (a, b) and after (c, d) controlled current stability measurement. XPS was used to further investigate the catalyst change before and after cycling runs. Figures S9 show the representative high resolution XPS core scans in the Co 2p, Cr 2p, O 1s, and C 1s regions before and after the cycling stability experiments. The two peaks centered at 780.5 eV and 790.4 eV are assigned to Co 2p3/2 and Co 2p1/2, respectively. The satellite regions near 790 eV and 805 eV reveal similar Co2+/Co3+ states that have been observed in pure Co3O4.76,77 For Cr 2p spectrum before the cycling measurement, the peaks at 585.9 eV and 576.6 eV are attributed to the Cr 2p1/2 and Cr 2p3/2, respectively. This feature resembles that of Cr2O3 samples suggesting the Cr3+ oxidation state.76 Moreover, the Cr 2p3/2 peak has a pronounced shoulder near 575 eV suggesting the presence of a Cr(OH)3-like environment.76 The peak area of the shoulder at 575 eV initially accounts for 16.4% of the total Cr 2p3/2, but decreases to 12.6% after the cycling stability test. This change suggests that chemical environment becomes more oxygenrich after the oxidative polarization of OER. The O 1s spectrum shows that both oxygen in hydroxyl groups (or defect sites in the structure) and oxygen in lattice are present.77 The Co to Cr ratio before the cycling is 3.28 ± 0.27 (based on quantitative analysis of the Co 2p3/2 and Cr 2p3/2 peaks), while the ratio is 2.92 ± 0.33 after the cycling. This suggests that there is not a significant composition change after the 10,000 cycle stability measurement, i.e. no appreciable preferential corrosion/dissolution occurred. However, the diminishing
shoulder peak of O 1s (531.5 eV), which is attributed to oxide defect sites or hydroxyl groups,77 suggests the loss of defect sites and surface oxyhydroxyl (MOOH) groups. This may suggest a certain degree of surface rearrangement which may cause the loss of the OER activity, and thus increased the overpotential. Discussion of Trends in Catalysts’ Activities. The nature of the active sites for OER in Co3O4 (Co2+Td and Co3+Oh) remain an item of scientific discussion.52,78,79 A study based on in situ Raman spectroscopy suggests the oxidized product of Co3O4, i.e. CoOOH, is the active species of OER.80 Another recent study using in operando X-ray absorption spectroscopy indicates the Co2+Td would lead to the formation of CoOOH species under alkaline reaction conditions.81 The observation of different surface Co intermediate species via rapid scan FTIR spectroscopy shows different mechanistic paths for photocatalytic OER.82 Another theoretical calculation report suggests the active sites in Co3O4 are overpotential and plane (facet) dependent.83 Reports also suggest that the adsorption energy difference of the intermediates, i.e. *O, *OH and *OOH, on the surface will be the major factor affecting the OER activity.52,84-88 Regardless of the actual geometrical active sites, the surface CoOOH species is generally considered to be a key intermediate in the OER mechanism. In the proposed mechanism for photocatalytic OER,82 two different major pathways are proposed in the electrochemical OER: a slow route involving only a single Co site, and a fast route that in which dual Co sites participate. A computational study also suggest two similar pathways for electrochemical OER.83 The reaction at the single Co sites starts with the nucleophilic addition of a water molecule, followed by proton transfer to a neighboring -OH to form an O-O bond leading to the formation of η–OOH. The hydroperoxyl (-OOH) group is then oxidized by losing a proton and electron, and eventually releases O2. In the dual-Co site counterparts, the nucleophilic addition of a water molecule to bridged oxygen (η3–oxo), followed by a proton transfer to an adjacent Co forms a µ3-OOH. A further oxidation of the hydroperoxyl results in O-O bond formation followed by the release of the O2. Based on the above proposed mechanisms and intermediate energetics of the two different reaction pathways, the following is our hypothesis of the possible factors affecting the OER activity as a function of Cr incorporation. The incorporation of Cr3+ will modify the d-band of adjacent Co, and the more electron-deficient Cr3+ will withdraw electron density away from Co. In consequence, the adjacent Co atoms become more electrophilic and make the nucleophilic addition of water molecules more favorable. This may contribute to some of the increase in activity as a function of increasing Cr doping. However, the introduction of Cr3+ also increases the lattice parameter, i.e. the distance between adjacent Co atoms. This may lead to the hindrance of the OER via the dual-Co site pathways by preventing the formation of either the µ-superoxo at two Co centers or the bridging oxygen (η3–oxo) required for the proton transfer to adjacent Co species. The addition of Cr3+ also dilutes the surface density of Co3+, and the loss of the possible active sites for the dual-Co route may also account for the deteriorated activity observed at high Cr loadings. Note that it is also possible that a dual Co-Cr site may be the active site for OER (rather than a dualCo site). Such a mechanism would also be consistent with our observed trends in OER activity with increasing Cr doping.
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It has been shown that the incorporation of Ni atoms can reduce the overpotential of the surface active β-CoOOH phase.52,89 The Fe doping into NiOOH can stabilize the OER active γ-Ni1−xFexOOH species.85 For OER catalysts, higher conductivity will lead to less potential drops (IR drop) and thus have higher OER activity.22,75,86,90,91 Previously, incorporation of Cr into Co3O4 films prepared via chemical vapor deposition (CVD) reveals a monotonic increase of conductivity to 0.096 Cr/Co, equivalent to Co2.74Cr0.26O4.92 Moreover, a recent report shows an enhancement of OER activity for CoCr double layer oxyhydroxide catalysts, and this enchancement is attributed to increased electrical conductivity upon Cr3+ incorporation as well a space expansion between the oyxhydroxide layers that facilitates the rapid transport of electrons and reactants.93 The electrical conductivity measurements (Table S3) show that all the Co3-xCrxO4 specimens have comparable resistance as prepared in dried, pressed pellets. This may suggest that the electrical conductivity difference might not be the main factor contributing to the improved or deteriorated OER activity. However, note that the ex-situ resistance measurements of the Co3-xCrxO4 are insulating, which should prevent electrocatalysis. The fact that electrocatalysis is observed in these materials is likely due to the emergence of a surface conductive phase during electrocatalytic operation, presumably a highlyconductive CoOOH as has been previously reported in electrodeposited films.62,63 Thus, even though our conductivity measurements of the as-prepared materials suggest that the electrical conductivity difference might not be the main factor contributing to the improved or deteriorated OER activity, we cannot definitively conclude this is true under electrocatalytic operating conditions.
Conclusion We have successfully prepared series of Co3-xCrxO4 OER catalysts. By systematically modifying the Cr content in the materials, we were control the lattice parameters of the specimens and modulate the materials’ overall OER activity. In particular, the OER activity initially increases with increasing Cr content, reaching a maximum OER activity with Co2.25Cr0.75O4. At higher Cr loadings with x > 1, there is a decrease in the OER activity. We attribute the trends in OER activity with increasing Cr content to 1) the tensile strain induced change of the intermediate energetic and 2) the electron withdrawing Cr making the Co more electrophilic, and favoring the nucleophilic addition of water Additional analysis of the Co2.25Cr0.75O4 species show that it is a highly-active OER catalyst, evolving O2 with a current density of 10 mA cm-2 at an overpotential of only ηj=10 mA/cm2,geo ~ 0.35 V with near unity Faradaic efficiency, with a specific activity of , , ~ 0.01 mA/cm2BET, and a mass activity of , , = 10.5 A/g. Moreover, it shows good long term stability as evidenced by only minor loss of catalytic activity after 10,000 potential cycles, although there is a film delamination after ~10-15 h when a constant 10 mA cm-2geo current density is applied. This film delamination may be due to increasing internal pressure due to trapped O2 in void spaces in the film.
AUTHOR INFORMATION Corresponding Author *
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ASSOCIATED CONTENT Supporting Information
The comparative table of the OER activities of other nanopaticulate catalysts is included in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENT This work was supported by start-up funding from the University of Michigan Department of Chemistry. We thank Levi Thompson for the use of his lab’s gas adsorption instrument and Wei-Chung Wen for his assistance with gas adsorption measurements. We also thank Che-Hung Liu for his assistance with electrical conductivity measurements. We thank Kwan Yee Leung for her assistance with SEM imaging. REFERENCES (1) Gratzel, M. Acc. Chem. Res. 1981, 14, 376−384. (2) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141−145. (3) Turner, J. A. Science 2004, 305, 972−974. (4) Lewis, N. S.; Nocera, D. G. P Natl. Acad. Sci. USA 2006, 103, 15729−15735. (5) Crabtree, G. W.; Dresselhaus, M. S. MRS Bull. 2008, 33, 421−428. (6) Gray, H. B. Nat. Chem. 2009, 1, 7−7. (7) Lewis, N. S. Science 2007, 315, 798−801. (8) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110, 6446−6473. (9) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Chem. Rev. 2010, 110, 6474−6502. (10) Turner, J. Nat. Mater. 2008, 7, 770−771. (11) Khaselev, O.; Turner, J. A. Science 1998, 280, 425−427. (12) Turner, J. A. Science 1999, 285, 1493−1493. (13) Chen, Z. B.; Jaramillo, T. F.; Deutsch, T. G.; KleimanShwarsctein, A.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M.; McFarland, E. W.; Domen, K.; Miller, E. L.; Turner, J. A.; Dinh, H. N. J. Mater. Res. 2010, 25, 3−16. (14) Turner, J.; Sverdrup, G.; Mann, M. K.; Maness, P. C.; Kroposki, B.; Ghirardi, M.; Evans, R. J.; Blake, D. Int. J. Energy Res. 2008, 32, 379−407. (15) Conway, B. E.; Liu, T. C. Langmuir 1990, 6, 268−276. (16) Cherevko, S.; Zeradjanin, A. R.; Keeley, G. P.; Mayrhofer, K. J. J. J. Electrochem. Soc. 2014, 161, H822−H830. (17) Frydendal, R.; Paoli, E. A.; Knudsen, B. P.; Wickman, B.; Malacrida, P.; Stephens, I. E. L.; Chorkendorff, I. Chemelectrochem 2014, 1, 2075−2081. (18) Seabold, J. A.; Choi, K. S. J. Am. Chem. Soc. 2012, 134, 2186−2192. (19) Kim, T. W.; Choi, K. S. Science 2014, 343, 990−994. (20) Feng, Z. X.; Hong, W. T.; Fong, D. D.; Lee, Y. L.; Yacoby, Y.; Morgan, D.; Shao-Horn, Y. Acc. Chem. Res. 2016, 49, 966−973. (21) Wang, J. H.; Cui, W.; Liu, Q.; Xing, Z. C.; Asiri, A. M.; Sun, X. P. Adv. Mater. 2016, 28, 215−230. (22) Trasatti, S. Electrochim. Acta 1984, 29, 1503−1512. (23) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2015, 137, 4347−4357. (24) McCrory, C. C. L.; Jung, S. H.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977−16987.
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ACS Catalysis
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