2. Materials and Methods
All electrochemical fabrication and characterization steps were performed in a custom-built chemically inert Teflon cell with a working electrode area of 0.032 cm2
] using a BASi Epsilon (Bioanalytical Systems, Inc., West Lafayette, IN, USA) or Gamry Interface 1000 (Gamry Instruments, Warminster, PA, USA) electrochemical workstation. A standard three-electrode set up was used with an Ag/AgCl (3M NaCl) reference electrode (Bioanalytical Systems), a Pt wire counter electrode and a gold wafer substrate working electrode. The substrate was prepared from a silicon wafer plated with 1000 Å of gold over a 50 Å titanium adhesion layer (Platypus Technologies, LCC, Madison, WI, USA). All electrolytes were prepared with salts from Sigma-Aldrich (St. Louis, MO, USA), used as received, and 18 MΩ·cm water purified using a Barnstead Nanopure Infinity (APS Water Services Corp., Van Nuys, CA, USA) or Milli-Q (MilliporeSigma, Burlington, MA, USA) system.
Films were deposited through controlled potential electrolysis by stepping the potential from open circuit to −1200 mV until 100 mC had been deposited. Deposition solutions contained 0.5 M H3BO3 and 1 M Na2SO4 with NiSO4 and either CuSO4 or CoSO4 in various amounts. The total metal concentration was constant at 100 mM, of which CuSO4 (below denoted as Cu samples) or CoSO4 (below denoted as Co samples) consisted of 10, 15, 20 or 25%. The metal thin film was modified through cyclic voltammetry (CV) for 30 cycles from −200 to 1000 mV with a sweep rate of 50 mV/s in a solution containing 0.01 M K3[Fe(CN)6] and 1 M Na2SO4 in order to form the HCF as shown in Equation (2). The charge storage capacity and kinetic behavior of the HCF was characterized in a blank solution of 1 M Na2SO4 with several CV scans with sweep rates from 10 to 1000 mV/s.
Structure and composition measurements, before and after the modification and characterization steps, were performed using a TM3000 Tabletop scanning electron microscope (SEM) (Hitachi, Tokyo, Japan) with a Quantax 70 energy dispersive X-ray spectroscopy (EDS) attachment (Bruker, Madison, WI, USA). SEM images were obtained at 2500× magnification and composition measurements were extracted from EDS spectra taken at 300× magnification to determine Ni, Cu, and Co relative compositions.
Several sets of samples were fabricated, with variations to the procedure above. For one set of samples, Set A, composition and structure were analyzed both before and after the modification step using SEM and EDS. Another set of samples, Set B, was fixed in the Teflon cell throughout the deposition, modification, and characterization steps. The latter set was investigated to avoid the uncertainty of replacing the sample in the custom-built Teflon cell, and thus no pre-modification composition and structure analysis was performed with SEM and EDS measurements. For this set, the range of measurements conducted in the characterization step was extended with sweep rates from 5 mV/s to 10,000 mV/s. Finally, for a small set, Set C, only Ni was used in the deposition step. These samples provided a reference for comparison of the mixed-metal samples in Sets A and B.
From the CV data, the total charge storage capacity was calculated by integrating the anodic current density,
, as a function of potential,
is the scan rate and
is the geometric area of the working electrode. In addition, the peak current density,
, and peak potential,
, for each anodic sweep were extracted.
Average results were calculated by grouping the samples by deposition solution percentage so that trends and variations between samples could be analyzed more clearly. Weighted averages,
and weighted standard deviations,
were calculated from the individual measurements,
, and associated uncertainties,
are the weighting factors and
is the sum of the weights. Graphs of the individual results are included in the Supplementary Materials
To investigate the effect of the modification step on the metal composition, the relative amounts of the metals in the film were measured with EDS at a magnification of 300×. Figure 1
shows the composition of the samples in Set A before the modification step. The results show that the amount of Cu or Co in the film does not match the solution composition. This behavior is typical of metal deposition, particularly in the case of iron-group metals, where it is known as anomalous codeposition [26
]. The Cu samples had large variations in composition whereas the Co samples had a clearer correlation between solution percentage of metal and deposited percentage of metal.
compares the composition results for the samples in Set A before and after the modification step. Some of the Cu samples lost up to half their amount of Cu, whereas no loss of Co was observed. This is consistent with the observations that for NiCu alloys, Cu is selectively removed during electrochemical dealloying, whereas for NiCo alloys, neither metal is more stable compared to the other [31
]. Thus, for the Cu samples here, the electrochemical modification step to form the HCF (CV cycles between −200 and 1000 mV) also removes some of the Cu.
compares the solution percentage to post-modification composition for the samples in both Sets A and B. Similar trends were seen for both sets of samples with the Cu percentages being more scattered than those for Co. When comparing Set A and Set B, the nominally identical Cu samples were more consistent in the latter set when the electrochemical cell was not dismantled between the deposition and modification steps.
shows SEM images collected at 2500× magnification for a representative Co sample (a, b) and Cu sample (c, d) both before (a, c) and after (b, d) the modification step. In general, samples containing Co were smoother than those containing Cu. The Cu samples had a rough surface with dendrites, but no correlation was observed between the density of dendrites and the percentage of Cu.
Representative CV scans are shown in Figure 5
for a Co sample (a) and a Cu sample (c). For both Co and Cu samples, either one oxidation and reduction peak or two closely-spaced, overlapping peaks were observed, corresponding to the redox reaction in Equation (1). The separation between the anodic and corresponding cathodic peaks increased with increasing scan rate, which is indicative of kinetic limitations and/or resistance effects for these samples.
The dependence of the anodic current density on sweep rate is shown in Figure 5
b,d on a log-log scale. Insight into the kinetics of the reaction can be gained by performing a power-law fit,
, as shown in the figure. A value of
equal to 0.5 demonstrates a current dependent on semi-infinite linear diffusion, whereas a value of one illustrates a surface-controlled (capacitive) current [35
]. In this case, an intermediate value of
Additional kinetic analysis can be obtained by graphing the integrated charge per area as a function of scan rate, and fitting the data to the functional form described by Trasatti and coworkers,
are the capacitive and diffusional contributions to the charge respectively [36
]. From the fit parameters,
, the two contributions can be quantified at specific scan rates. Representative examples of this analysis are shown in Figure S4
, confirming that the charge storage kinetics for these samples is the result of mixed capacitive and diffusional behavior. Both an intermediate value of
in power-law fits and mixed kinetic behavior from Trasatti fits have been observed in a variety of intercalation charge storage systems [38
The integrated charge capacity of the HCF films was calculated from 20 mV/s CV data. Because the total amount of HCF formed on the deposited metal is unknown, a charge per geometric area was calculated. Figure 6
shows that integrated charge per area vs. the amount of added metal to the film. The results showed that samples fabricated from NiCu alloys yield a significantly higher charge capacity than those from NiCo alloys. There is also a positive trend in charge capacity versus added metal percentage for both types of samples. This effect is larger for the Cu samples, and therefore those samples can store more charge per substituted metal, compared to the Co samples.
In Figure 7
the peak potentials for the samples extracted from the anodic sweep of a 20 mV/s CV scan are graphed vs. the amount of added metal. The peak potentials for the Cu samples are generally larger than for the Cu samples, but there is no correlation for either alloy between the percentage of added metal and the peak potential. The averages and standard deviations of the peak potentials were calculated for each set as well as for the Co samples and Cu samples grouped together, and are listed in Table 1
. The results for the single-metal Ni samples (Set C) are also listed. There is a fair amount of variability within and between sets; however, the average peak potential for the Cu samples is slightly higher than that for either the Co samples or solely Ni samples, which are statistically similar.
The results of the kinetic analysis are shown in Figure 8
-values extracted from the power-law fits to anodic peak current density vs. scan rate are generally larger for the Cu samples than for the Co samples, and there is no observed trend between the metal concentration and the power-law exponent. The overall averages and standard deviations are listed in Table 1
. Again, there is a good deal of variability between samples. The average
-value for the Cu samples is larger than that for the Co and Ni samples.
conceptualization, A.R. and J.R.H.; funding acquisition, J.R.H.; investigation, A.R.; visualization, A.R. and J.R.H.; writing—original draft, A.R. and J.R.H.; writing—review and editing, J.R.H.
This research was funded by the U.S. National Science Foundation, RUI grant number DMR-1608327 and MRI grant number CHE-0959282.
We acknowledge support from Hope College Physics Department and the rest of the Surface Lab, including Scott Joffre.
Conflicts of Interest
The authors declare no conflict of interest.
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The dependence of the amount of deposited metal for each solution percentage for Set A. Results for the two alloys were grouped and averaged by deposition solution percentage. The line represents a 1:1 relationship. Figure S1
shows the results for the individual samples.
The dependence of the metal remaining after the modification step compared to the deposited metal for Set A. Results for the two alloys were grouped and averaged by deposition solution percentage. The line represents a 1:1 relationship. Figure S2
shows the results for the individual samples.
The dependence of the metal remaining after the modification step compared to the deposition solution percentage for Sets A and B. Results for the two alloys were grouped and averaged by deposition solution percentage. The line represents a 1:1 relationship. Figure S3
shows the results for the individual samples.
SEM images of a representative Co sample (a,b) and Cu sample (c,d) from Set A both before (a,c) and after (b,d) the modification step. The scale bar is 10 µm.
Example cyclic voltammetry (CV) scans from (a) a Co sample and (c) a Cu sample for scan rates from 10 mV/s to 1000 mV/s. The corresponding peak current density vs. graphs are shown in (b) and (d) on a log-log scale with power law fits to .
The measured anodic charge per area from the 20 mV/s CV as a function of the amount of added metal for Sets A and B. Results for the two alloys were grouped and averaged by deposition solution percentage. Figure S5
shows the results for the individual samples.
The anodic peak potential from the 20 mV/s CV as a function of the amount of added metal for Sets A and B. Results for the two alloys were grouped and averaged by deposition solution percentage. Figure S6
shows the results for the individual samples.
-value determined for a power-law fit to the anodic peak current density vs. scan rate as a function of the amount of added metal for Set A and B. Results for the two alloys were grouped and averaged by deposition solution percentage. Figure S7
shows the results for the individual samples.
Number of samples, , weighted averages, and weighted standard deviations of the peak potentials, , and power-law -values for all the samples in each set and for the Co and Cu samples grouped together.
|NiCo Set A||24||0.53||0.05||0.75||0.10|
|NiCo Set B||15||0.51||0.06||0.77||0.06|
|NiCo Sets A & B||39||0.52||0.05||0.76||0.09|
|NiCu Set A||24||0.58||0.03||0.84||0.03|
|NiCu Set B||14||0.55||0.03||0.83||0.04|
|NiCu Sets A & B||38||0.57||0.03||0.83||0.04|
|Ni Set C||4||0.537||0.024||0.713||0.012|
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