Research Article Cite This: ACS Catal. 2017, 7, 8605-8612
pubs.acs.org/acscatalysis
Understanding Variability in the Hydrogen Evolution Activity of a Cobalt Anthracenetetrathiolate Coordination Polymer Courtney A. Downes and Smaranda C. Marinescu* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
ABSTRACT: Here, a cobalt dithiolene coordination polymer (CP) based on 9,10-dimethyl-2,3,6,7-anthracenetetrathiolate was synthesized via an interfacial reaction and was electrochemically characterized on glassy carbon (GCE) and graphite (GR) electrodes. Double-layer capacitance measurements, electrochemical impedance spectroscopy studies, and Tafel analyses were used to understand the role of electrochemically accessible active sites, electron and charge transfer, and electrical integration between the catalyst and the support in the resultant electrocatalytic hydrogen evolving activity. Overpotentials to achieve 10 mA/cm2 ranging from 445 to 571 mV and from 388 to 527 mV for GCE|CP and GR|CP, respectively, were observed. Changes in the double-layer capacitance, which is related to electrochemically active surface area, and charge transfer resistance were determined to be the critical factors in the observed enhancement in catalytic activity, whereas bulk catalyst loading, which had been previously used to describe the hydrogen evolution reaction performance of CPs, was not the optimal indicator of catalytic activity. KEYWORDS: electrocatalysis, cobalt dithiolene, coordination polymer, surface immobilization, hydrogen evolution, solar energy conversion
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dithiolene was an efficient HER catalyst.19,20 These investigations were expanded to include replacement of the cobalt center with other earth-abundant metals such as iron and nickel12 and modifications to the ligand backbone via replacement of sulfur with selenium generating diselenolatebased CPs.13 Electrochemical studies of metal dithiolate and diselenolate CPs in acidic aqueous media revealed these materials were active HER catalysts that could operate over a wide range of overpotentials depending on the catalyst loading. The cobalt dithiolene CPs based on benzenehexathiolate,14 triphenylene2,3,6,7,10,11-hexathiolate,14 and benzene-1,2,4,5-tetrathiolate11 exhibited overpotentials of 340, 530, and 560 mV to reach 10 mA/cm2 of HER activity at catalyst loadings of 7.0 × 10−7, 11 × 10−7, and 5.5 × 10−7 moles of Co/cm2, respectively. For the cobalt CP based on benzene-1,2,4,5-tetraselenolate, a range of overpotentials were measured (602−343 mV) as the catalyst loading was increased from 3.7 × 10−7 to 9.2 × 10−7 moles of Co/cm2.13 However, it was observed that very small changes in catalyst loading could lead to drastic reductions in the overpotential, and even samples with the same catalyst loading exhibited different overpotentials. The aim of this investigation is to identify the origins of the large variance in achievable overpotentials for dithiolate- and diselenolate-based CPs as bulk catalyst loading, which we have previously used to explain
INTRODUCTION Sustainable production of hydrogen from water splitting (2H2O → 2H2 + O2) has emerged as a promising pathway for the storage and conversion of renewable energy resources. Solar energy, in particular, is an attractive alternative to fossil fuels because of its unparalleled abundance. The ability to store solar energy in H2, a clean and carbon-neutral energy carrier, as a method to mitigate its intermittent nature, is vital for the implementation of a sustainable alternative to the fossil fueldominated economy.1 The hydrogen evolution reaction (HER, i.e., 2H+ + 2e− → H2) requires the development of inexpensive catalysts capable of achieving high current densities at low overpotentials.2,3 The replacement of Pt-based catalysts with earth-abundant materials is necessary for large-scale application of H2-based technologies.2,3 The immobilization of molecular catalytic units through adsorption,4−6 covalent attachment,7−9 or noncovalent interactions10−16 has emerged as a useful strategy for combining the attractive attributes of both homogeneous and heterogeneous electrocatalysis in an effort to replace noble metals in solar-tofuel converting devices.17,18 Heterogenizing molecular catalysts improves the stability and durability of these systems while maintaining well-defined catalytic units. Recent work in our laboratory has utilized dinucleating11−13 and trinucleating14 ligand scaffolds to synthesize metal coordination polymers (CPs) through an interfacial reaction, allowing for the immobilization of catalytic units via CPs. Initial investigations focused on CPs with a cobalt dithiolene catalytic unit11,14 because it had been well-established that cobalt benzene-1,2© XXXX American Chemical Society
Received: August 31, 2017 Revised: October 19, 2017
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DOI: 10.1021/acscatal.7b02977 ACS Catal. 2017, 7, 8605−8612
Research Article
ACS Catalysis
To investigate the electrocatalytic hydrogen evolution of 1, we performed detailed electrochemical measurements in a three-electrode configuration with a N2-saturated pH 1.3 H2SO4 electrolyte, where all of the potentials here are referenced to the reversible hydrogen electrode (RHE). Initial electrochemical testing was performed using glassy carbon electrodes (GCEs) modified with 1, denoted GCE-1−GCE-4. Polarization curves of GCE-1−GCE-4 are shown in Figure 1a.
differences in the HER activity of these systems, may not be the dominating factor in their resultant overpotential. Here, a novel cobalt dithiolene coordination polymer 1, based on 9,10-dimethyl-2,3,6,7-anthracenetetrathiolate was synthesized and electrochemically characterized on both glassy carbon and graphite electrodes to identify the origins of the large variability in the overpotential. Numerous glassy carbon and graphite electrodes modified with 1 were characterized by capacitance measurements, electrochemical impedance spectroscopy studies, and Tafel analyses. The role of electrochemically accessible active sites, electron and charge transfer, and electrical integration between the catalyst and the support were evaluated and used to develop strategies for optimizing the HER activity of dithiolate- and diselenolate-based coordination polymers.
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RESULTS AND DISCUSSION Cobalt coordination polymer 1 was synthesized using a liquid− liquid interfacial reaction modified from the reported synthetic procedure for the cobalt benzene-1,2,4,5-tetraselenolate CP.13 9,10-Dimethyl-2,3,6,7-anthracenetetra(thioacetate) (ATTAc4) was prepared according to a reported procedure,21 and the acetyl protecting group was removed in the presence of a base, such as NaOH, at 65 °C to form sodium ATT. An acetonitrile/ ethyl acetate solution of [Co(MeCN)6][BF4]2 was gently layered on top of the aqueous solution of sodium ATT. The organic solvents were allowed to evaporate over several hours, leaving a film at the gas−liquid interface, which was deposited on the support of interest. The Fourier transform infrared (FTIR) spectrum (Figure S1) of 1 shows the disappearance of the strong CO stretch of the acetyl protecting group around ∼1700 cm−1 indicating successful deprotection prior to the formation of 1.
Figure 1. (a) Polarization curves for GCE-1 (red), GCE-2 (green), GCE-3 (blue), and GCE-4 (purple) with a scan rate of 100 mV/s. (b) Current density difference (Δj = ja − jc) at 0.15 V versus RHE plotted against the scan rate (squares). The Cdl values are estimated through linear fitting of the plots (dashed lines). All measurements were performed in N2-saturated pH 1.3 H2SO4 solutions at room temperature.
All polarization curves were compensated for ohmic drop (see the Supporting Information for details). To compare the HER activities exhibited by different 1-modified GCEs, the overpotential (η) needed to reach a current density of 10 mA/cm2 was evaluated.22 The measured overpotentials ranged from 445 to 571 mV (Table 1). Table 1. Overview of 1-Modified Glassy Carbon Electrodes
Top-down scanning electron microscopy images of 1 (Figure S2) show the filmlike nature that results from the deposition method presented here. Morphological differences on the edges of the cracked film may influence the electrochemical behavior, which is further discussed below. X-ray photoelectron spectroscopy (XPS) studies of 1 reveal the presence of cobalt, sulfur, and sodium (Figure S3). Deconvolution of the cobalt 2p region reveals the presence of four peaks at binding energies (BEs) of 778.7, 781.7, 793.9, and 796.6 eV with the lower-BE peaks corresponding to the 2p3/2 levels and the higher-BE peaks corresponding to the 2p1/2 levels (Figure S3a). Analogous cobalt features have been observed for the previously reported cobalt dithiolene coordination polymers11,14 and surface-adsorbed cobalt dithiolene molecular catalysts.4 Additional features at BEs of 1071.6, 227.2, and 162.8 eV correspond to Na 1s, S 2s, and S 2p, respectively (Figure S3).
sample
Cdl (mF/cm2)
Rct (Ω)
η10 mA/cm2 (mV)
GCE-1 GCE-2 GCE-3 GCE-4
8.60 − 5.81 5.77
714.6 853.3 1061 1365
445 461 504 571
To explore the origin of the large range of overpotentials for 1, the double-layer capacitance (Cdl), which can be used to estimate the electrochemically active surface area (ECSA), was measured via cyclic voltammetry (Figures S4−S6). The current response in the potential window of 0.1−0.2 V versus RHE at different scan rates (20−150 mV/s) should be due only to double-layer charging and discharging. The capacitance can be calculated from the scan rate dependence of the charging current density at 0.15 V versus RHE, where the slope of the 8606
DOI: 10.1021/acscatal.7b02977 ACS Catal. 2017, 7, 8605−8612
Research Article
ACS Catalysis plot of Δj versus scan rate is twice the Cdl. The measured Cdl values (Figure 1b and Table 1) revealed that GCE-1, which operates at the lowest overpotential, has the largest accessible surface area. An increased Cdl corresponds to a reduction in the overpotential to reach 10 mA/cm2 (Figure S7). However, such small changes in Cdl from 5.77 to 5.81 mF/cm2 for GCE-4 and GCE-3 did not alone explain the 67 mV reduction in the overpotential. To further probe the variance in overpotentials, electrochemical impedance spectroscopy (EIS) was performed to investigate the HER electrode kinetics and interfacial properties. Nyquist plots of GCE-1−GCE-4 recorded at −0.57 V versus RHE are shown in Figure 2. Only one semicircle was
inadequate physical contact is observed as 1 is delaminated from the surface during electrochemical testing. The poor contact results in large and variable Rs and Rct values. The GCEs used in these experiments have an active area of 0.07065 cm2; however, the total surface area is 0.32 cm2 with the active glassy carbon area surrounded by a nonconductive support. The deposition method presented here, immersing the electrode face down through the film at the gas−liquid interface, offers little control over how the catalyst is dispersed over the conductive and nonconductive regions of the GCE. This lack of control over the area of deposition and the conductive nature of metal dithiolene coordination polymers27,28 could result in variance in the measured resistance values. With these considerations, further electrochemical testing of 1 was performed on bulk graphite electrodes. Polarization curves of six graphite electrodes modified with 1, denoted GR-1−GR-6, are presented in Figure 3a. As with the
Figure 2. Nyquist plots (markers) with respective fits (solid lines) measured at −0.57 V versus RHE for GCE-1 (red), GCE-2 (green), GCE-3 (blue), and GCE-4 (purple). All measurements were performed in N2-saturated pH 1.3 H2SO4 solutions at room temperature.
observed in each Nyquist plot, suggesting the catalytic reaction was characterized by one time constant. The absence of Warburg impedance indicated that mass transport was rapid enough so the reaction was kinetically controlled. Therefore, the equivalent circuit based on one time constant, 1T, was used to fit the EIS data (Figure S8). Rs is attributed to the uncompensated solution resistance; Rct is the charge transfer resistance related to the kinetics of electrocatalysis, and CPE is a constant phase element that represents the double-layer capacitance under HER conditions. To obtain a satisfactory simulation of the experimental EIS data, it was necessary to use CPE rather than a capacitor in the equivalent circuit. The use of a CPE is required to explain depressed semicircles because of uneven charging of the double layer due to microscopic surface roughness and inhomogeneity.23−26 The extracted parameters for the fits to this equivalent circuit for the four 1-modified GCEs are summarized in Tables S1−S4. The Rct values at −0.57 V versus RHE for GCE-1−GCE-4 were 714.6, 853.3, 1061.0, and 1365.0 Ω, respectively (Table 1). The lower value of Rct corresponds to a faster reaction rate and a higher catalytic activity for HER (Figure S9) as observed for the most active sample, GCE-1, which exhibits the lowest Rct, 714.6 Ω, and overpotential, 445 mV. The observed Rct and Rs (ranging from 57 to 70 Ω) values for 1 on GCE are much larger than desired for promotion of fast electron transfer and high HER activity. The large Rs values may indicate poor electrical integration of the catalyst and the conductive glassy carbon support. Additionally, Rs values were observed to fluctuate significantly as a function of applied overpotential, which is not expected. These discrepancies may result from poor physical contact between 1 and GCE and the use of an electrode with an ill-defined active surface area. The
Figure 3. (a) Polarization curves for GR-1 (red), GR-2 (orange), GR-3 (yellow), GR-4 (green), GR-5 (blue), and GR-6 (purple). The scan rate was 100 mV/s. (b) Current density difference (Δj = ja − jc) at 0.15 V versus RHE plotted against the scan rate (squares). The Cdl values are estimated through linear fitting of the plots (dashed lines). All measurements were performed in N2-saturated pH 1.3 H2SO4 solutions at room temperature.
glassy carbon electrodes, a wide range of overpotentials to reach 10 mA/cm2 (527−388 mV) were achieved for 1-modified GR. Further exploration of the origins of this wide range of overpotentials revealed differences in the double-layer capacitance values (Figure 3b, Figures S10−S14, and Table 2). The overpotential necessary to reach 10 mA/cm2 of HER activity decreased with an increase in Cdl (Figure S15). The double-layer capacitance measurements are related to the electrochemically accessible surface area and do not always trend with the bulk catalyst loading.29 To examine the relationship between Cdl and bulk catalyst loading, 1 was removed from the surface of the graphite electrodes through washing following electrochemical testing and the cobalt concentration was measured via ICP-MS. Measuring the catalyst loading following electrochemical testing and through 8607
DOI: 10.1021/acscatal.7b02977 ACS Catal. 2017, 7, 8605−8612
Research Article
ACS Catalysis
moles of Co/cm2 led to Cdl values of 3.63 and 6.34 mF/cm2, respectively. However, for GR-5 and GR-4, the increase in catalyst loading from 1.17 × 10−7 to 2.49 × 10−7 moles of Co/ cm2 resulted in a Cdl change of only 1.21 to 1.25 mF/cm2. Therefore, while Cdl generally increases with catalyst loading, the magnitude of the increase in Cdl is not correlated to the magnitude of the increase in bulk catalyst loading as other factors such as morphology influence the number of accessible active sites. Conversion of the current density in the polarization curves (Figure 3a) to current per moles of cobalt (Figure S18) revealed the catalytic current trends similarly when analyzed as a function of geometric surface area or cobalt concentration. Double-layer capacitance and bulk catalyst loading measurements were not performed for GR-1. EIS measurements were performed over a range of overpotentials (570−170 mV), and the corresponding Nyquist and Bode plots for GR-2 and GR-4−GR-6 can be found in Figures S19−S26. The modified graphite electrodes exhibit comparable EIS responses, indicating similar mechanisms for hydrogen evolution. The appearance of a depressed semicircle at high frequencies (Figure S27), in addition to the lowfrequency semicircle, required the use of an equivalent circuit with two time constants as opposed to the 1T model used for the GCE data. To fit these data, the two-time constant parallel (2TP) and two-time constant serial (2TS) models were employed (Figure S8).23,24,30 For the 2TP model, both time constants change with overpotential and are related to the kinetics of HER. The two time constants (R1-CPE1 and R2-CPE2) are attributed to the adsorption of hydrogen on the electrode surface23,24,30,31 and the charge transfer kinetics. The 2TS model has been used to describe the HER response on porous electrodes, with only one time constant at low frequencies (Rct-CPE2), related to charge transfer kinetics, changing as a function of overpotential. The second time constant (R1-CPE1) at high frequencies is
Table 2. Overview of 1-Modified Graphite Electrodes sample
Cdl (mF/cm2)
bulk loading (×10−7 moles of Co/cm2)
Rct (Ω)
η10 mA/cm2 (mV)
GR-1 GR-2 GR-3 GR-4 GR-5 GR-6
− 6.34 3.63 1.25 1.21 0.94
− 5.78 4.81 2.49 1.17 2.49
13.05 26.32 86.27 191.2 235.7 270.6
388 404 432 486 507 527
collection of the catalyst via washing may lead to lower than expected cobalt concentrations. Dithiolate-based coordination polymers have been shown to delaminate from the electrode surface during electrochemical characterization,11,14 and the method of collection (washing) does not guarantee complete removal of the CP from the electrode. In Table 2, the ICP-MS-measured cobalt concentrations are displayed. Overall, Cdl generally increased with bulk catalyst loading (Figure S16a), resulting in lower overpotentials to achieve 10 mA/cm2 at higher concentrations of 1 (Figure S16b). The current density at −0.4 and −0.5 V versus RHE also increased with bulk catalyst loading and electrochemically accessible surface area (Figure S17). Measured catalyst loadings for GR-4 and GR-6 were both 2.49 × 10−7 moles of Co/cm2, but the Cdl values were 1.25 and 0.94 mF/cm2, respectively, indicating electrochemically accessible surface areas can differ even for samples with the same bulk loading. These differences may result from the varying morphologies observed for 1 at the edges of the deposited film (Figure S2). This example highlights the importantance of measuring the electrochemically accesible surface area as the bulk catalyst did not adequately explain the low observed activity for GR-6. Additionally, small changes in bulk catalyst loading did not always result in small changes in Cdl values. For GR-3 and GR2, increases in catalyst loading from 4.81 × 10−7 to 5.78 × 10−7
Figure 4. EIS spectra measured at −0.57 V versus RHE presented as (a) Nyquist plots (markers) with respective fits (solid lines) and (b and c) Bode plots for GR-1 (red), GR-2 (orange), GR-3 (yellow), GR-4 (green), GR-5 (blue), and GR-6 (purple). (d) Tafel analysis of GR-2 (orange), GR-3 (yellow), and GR-5 (blue). Exchange current densities of 10−6.35, 10−6.1, and 10−7.01 A/cm2 were obtained for GR-2, GR-3, and GR-5, respectively. All measurements were performed in N2-saturated pH 1.3 H2SO4 solutions at room temperature. 8608
DOI: 10.1021/acscatal.7b02977 ACS Catal. 2017, 7, 8605−8612
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ACS Catalysis
mechanistic pathways with the Tafel slope being dependent on the rate-limiting step. In acidic electrolytes, three reaction steps are possible for HER: the Volmer (discharge) reaction (H3O+ + e− → Hads + H2O; b = 120 mV/decade), the Heyrovsky (ion + atom) reaction (H3O+ + e− + cat-H → cat + H2 + H2O; b = 40 mV/decade), and the Tafel (combination) reaction (cat-H + cat-H → 2cat + H2; b = 30 mV/decade).41 The linear portions of the Tafel plots were fitted to the Tafel equation, η = b log j + a (where η is the overpotential, a the Tafel constant, b the Tafel slope, and j the current density), yielding Tafel slopes of 130, 134, and 144 mV/decade for GR-2, GR-3, and GR-5, respectively. The decrease in the Tafel slope was correlated with a reduction in Rct measured by EIS and overpotential to reach 10 mA/cm2 (Table 1), resulting in improved HER kinetics and electron transport through the material.42 The large Tafel slopes for the 1-modified GR suggest a rate-limiting Volmer-like step, which has been previously observed for related surface-immobilized metal dithiolene catalysts.6,11,12,14 Theoretical studies43 of cobalt dithiolene complexes have proposed that reduction and thiolate protonation are the first steps for HER, which corresponds to the Volmer discharge reaction. The decrease in Tafel slope was also accompanied by an increase in Cdl (Table 1), with GR-2 exhibiting the largest electrochemically active surface area of the electrodes evaluated by Tafel analysis. The reduction in the Tafel slope suggests a smaller barrier for the discharge reaction (catalyst reduction and thiolate protonation), which may be directly attributed to the increase in the electrochemically accessible catalytic units necessary for reduction and protonation. To assess the stability and efficiency of 1, controlled potential electrolysis and chronoamperometry experiments were performed in pH 1.3 solutions. Shorter duration electrolysis experiments performed at −0.72 V versus RHE revealed continuous charge build-up over 1 h with little loss of activity, and Faradaic efficiencies of 97% were achieved for 1 (Figure S32). Longer electrolysis experiments performed at −0.72 V versus RHE on graphite (Figure S33) and glassy carbon (Figure S34a) electrodes also demonstrated stable HER with oscillations in the generated current arising from removal of adsorbed H2 from the electrode surface. Cyclic voltammetry experiments following electrolysis for 7 and 12 h demonstrated no significant change in the electrochemical behavior (Figure S34b). X-ray photoelectron spectroscopy analysis after electrolysis for 1 (Figure S35) and 12 h (Figure S36) at −0.72 V versus RHE in pH 1.3 solutions revealed Co, Na, and S features analogous to the ones observed before, indicating that CP 1 is stable during electrochemical testing.
independent of overpotential and has been assigned to several phenomena such as the porosity of the electrode23,30,32−34 or the interfacial resistance resulting from electron transfer between the catalyst and the electrode.35−37 The 2TP and the 2TS models were used to fit the experimental EIS data; however, the Bode plots indicated the depressed semicircle observed at high frequencies was independent of overpotential, indicating the 2TS model was suitable (Figures S20, S22, S24, and S26). The values from this fit are summarized in Tables S5−S10. Rs and R1 values were unaffected by changes in overpotential (Figure S28), and the Rs values were significantly reduced (10− 12 Ω) in comparison to the Rs values recorded on the GCE (57−70 Ω), indicating enhanced electrical integration between the catalyst and electrode. The Rct values decreased with an increased overpotential (Figure S29), corresponding to a faster reaction rate. Additionally, as the overpotential was increased, the Bode plots for the 1-modified GR displayed a reduction in the intensity of the lower-frequency semicircle, resulting from an increase in the reaction rate, and a shift of the lowerfrequency peak to higher frequencies, indicating a shorter reaction time constant (Figures S20, S22, S24, and S26). Comparison of the EIS response for all six graphite-modified electrodes at −0.57 V versus RHE in Figure 4a−c clearly demonstrates that a lower Rct is related to improved HER performance. Rct values of 13.05, 26.32, 86.27, 191.2, 235.7, and 270.6 Ω were found for GR-1−GR-6, respectively. These Rct values are much lower than those observed on the GCE, indicating much faster electron transfer and HER kinetics. The electrode with the lowest Rct, GR-1, exhibits the smallest η10 mA/cm2 of 388 mV. A linear relationship between Rct and η10 mA/cm2 was observed (Figure S30), confirming a lower Rct is necessary for high HER activity. The Bode plots (Figure 4b,c) also indicated an increased reaction rate as the intensity of the lower-frequency semicircle was reduced and shifted to higher frequencies as the HER performance was improved from GR-6 (η10 mA/cm2 = 527 mV) to GR-1 (η10 mA/cm2 = 388 mV). As we have previously discussed, increasing the Cdl values of the 1-modified GR led to a decreased overpotential, which is expected for a larger number of electrochemically accessible active sites. However, determining the Cdl using cyclic voltammetry experiments measured in the nonfaradaic region can result in an underestimation or overestimation of the catalytically active area. Phenomena such as surface coordination and intercalation of ions and potential-dependent conductivities can result in the measurement of capacitance that is not related to the electrochemically accessible surface area of the catalyst.23,26,38−40 Therefore, the double-layer capacitance under HER conditions (Cdl*) is extrapolated from the EIS data to provide a second measure of the accessible surface area to confirm the results obtained from the CV experiments. Cdl* values from EIS experiments performed at lower overpotentials revealed larger surface areas for the more active electrodes (Figure S31), which is analogous to the trend observed for the Cdl values measured during the CV experiments. For the less active samples, the Cdl* values were constant with overpotential. However, for the more active samples, Cdl* decreased with an increase in overpotential because of surface adsorption of hydrogen occluding the catalyst surface (Figure S31). This is clearly seen as the Cdl* for GR-2 decreased at larger overpotentials. The Tafel plots for GR-2, GR-3, and GR-5 are displayed in Figure 4d. Tafel analysis can be used to determine different
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CONCLUSIONS In summary, we report the synthesis of a cobalt dithiolene coordination polymer based on 9,10-dimethyl-2,3,6,7-anthracenetetrathiolate via an interfacial reaction. The electrochemical hydrogen evolution ability of 1 was investigated on glassy carbon and graphite electrodes. Previous reports on related coordination polymers revealed large variances in overpotential with small changes in catalyst loading. Through analysis of the double-layer capacitance and electrochemical impedance spectroscopy of 1 on glassy carbon and graphite, it was revealed that electrochemically accessible surface area, rather than bulk catalyst loading, and charge transfer resistance were better indicators of changes in overpotential. EIS measurements also demonstrated large and variable Rs values (57−70 Ω) for 1modified GCE, suggesting poor electrical integration between 8609
DOI: 10.1021/acscatal.7b02977 ACS Catal. 2017, 7, 8605−8612
Research Article
ACS Catalysis the substrate and the catalyst. Graphite proved to be a more suitable electrode material, resulting in stable and low (∼10 Ω) Rs values indicating better electrical integration. For 1-modified GR systems, overpotentials ranging from 388 to 527 mV were achieved, and this variance in overpotential correlated well with changes in Cdl and Rct. Investigating the fundamental electrochemical characteristics of the cobalt anthracenetetrathiolate CP has provided insight into the critical factors that affect HER activity. This understanding enables the design of coordination polymers with improved catalytic performance and furthers the viability of CPs as a method for the immobilization of molecular catalysts for solar-to-fuel converting devices.
liquid interface. Following deposition, the substrate was washed with water and methanol. Electrochemical Methods. Electrochemistry experiments were performed using a VersaSTAT 3 potentiostat. A platinum wire used for the electrochemical studies was purchased from Alfa Aesar. The electrochemical experiments were performed in a three-electrode configuration electrochemical cell under an inert atmosphere using glassy carbon electrodes (GCE, 0.07065 cm2 surface area) or graphite rods (GR, 0.32 cm2 surface area) as the working electrode. Graphite rods were purchased from Graphite Machining, Inc. (grade NAC-500 purified,
