Hydrogen evolution reaction measurements of dealloyed porous NiCu
© Koboski et al.; licensee Springer. 2013
Received: 5 October 2013
Accepted: 3 December 2013
Published: 17 December 2013
Porous metals are of interest for their high surface area and potential for enhanced catalytic behavior. Electrodeposited NiCu thin films with a range of compositions were electrochemically dealloyed to selectively remove the Cu component. The film structure, composition, and reactivity of these samples were characterized both before and after the dealloying step using scanning electron microscopy, energy-dispersive spectroscopy, and electrochemical measurements. The catalytic behavior of the dealloyed porous Ni samples towards the hydrogen evolution reaction was measured and compared to that of the as-deposited samples. The dealloyed samples were generally more reactive than their as-deposited counterparts at low overpotentials, making the dealloying procedure a promising area of exploration for improved hydrogen evolution catalysts.
KeywordsDealloying Alloy NiCu Ni Cu Hydrogen evolution reaction
Nanoporous metal structures are of significant interest for a wide variety of applications due to their low density, high surface area, enhanced optical properties, and improved catalytic behavior . Electrochemical dealloying of a metallic alloy has been used to produce a number of different nanoporous metals, including nickel [2–4], gold [5–12], copper [8, 13, 14], silver [8, 15], iron , platinum , and palladium .
In most cases, during dealloying, the less noble (more thermodynamically active) component is selectively oxidized from the alloy, while the remaining material may rearrange to form an interconnected network of pores [19, 20]. However, Searson and coworkers recently showed that the more noble component of an alloy can be selectively removed if more thermodynamically active component is kinetically stabilized. In particular, the nickel component of a NiCu alloy was passivated in the electrolyte chosen for the dealloying procedure, allowing copper to be electrochemically removed . This demonstration, which has also been shown in other electrolytes [22, 23], opens up a wider range of alloy combinations that can be electrochemically dealloyed to produce nanoporous materials.
Searson and coworkers used the results of NiCu dealloying to identify an interesting core/shell structure in the originally deposited alloy . This structure was subsequently confirmed by spatially resolved composition measurements , and the kinetics of the deposition process that facilitates its formation was studied . By combining this core/shell structure with deposition into nanoporous templates and selective dealloying, the fabrication of nickel nanotubes is possible [24, 25, 27].
The magnetic behavior of these dealloyed NiCu samples have been characterized [21, 24, 28]. Modifications have also been made to the nanoporous structure for specific intended applications. For example, they have been used as templates for the deposition of oxide materials to fabricate pseudocapacitors with high specific capacitance [29–34], for the deposition of silicon to fabricate high-capacity current collectors for battery applications , and for the deposition of silver for surface-enhanced Raman spectroscopy applications . Small amounts of metallic palladium have been deposited on nanoporous nickel substrates, and the resulting catalytic activity towards methanol and ethanol oxidation was characterized .
Here we characterize the catalytic activity of dealloyed NiCu samples towards the hydrogen evolution reaction (HER). Efficient and cost-effective production of hydrogen is an important area of research for renewable and environmentally friendly energy technology. Nickel and nickel alloys show the potential to be lower-cost options for electrocatalysis of hydrogen production compared to other precious metals such as platinum [38–43]. Porous Ni films showing enhanced activity towards the HER have been produced by leaching of Zn and Al from NiZn [2, 44–47] and NiAl [48–52] alloys respectively. However, the HER reactivity of porous Ni films produced from selective removal of Cu from NiCu has not yet been explored.
In this work, NiCu thin films with varying compositions were electrodeposited, and the copper was selectively removed via electrochemical dealloying. The structure, composition, and reactivity of the samples were characterized both before and after the dealloying step using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electrochemical measurements.
Deposition and dealloying
The gold wafers on which the NiCu was deposited were cleaved from a silicon wafer plated with 1,000 Å of gold over a 50 Å titanium adhesion layer (Platypus Technologies, LLC, Madison, WI, USA). The electrochemical measurements were completed using a BAS Epsilon Electrochemical Workstation (Bioanalytical Systems, Inc., West Lafayette, IN, USA) and a custom-built Teflon cell  with a defined working electrode area of 0.032 cm2, a platinum wire (Alfa Aesar, Ward Hill, MA, USA) counter electrode, and an Ag/AgCl (3 M NaCl) reference electrode (Bioanalytical Systems, Inc., West Lafayette, IN, USA). All potentials are reported with respect to the Ag/AgCl reference electrode. The electrolyte solutions were made using water that had been purified through successive reverse osmosis, deionization, and UV purification stages. All chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received. All experiments were carried out at room temperature.
The films were deposited from 0.5 M H3BO3 and 1 M Na2SO4 solutions with varying NiSO4 and CuSO4 concentrations (the sum of which was held constant at 0.11 M). The potential of the working electrode was stepped from open circuit to -1,200 mV until a total 50 mC of charge had been deposited. The dealloying step was performed in a 1 M Na2SO4 solution using linear sweep voltammetry (LSV). The potential was swept from 0mV to between 2,100 and 2,400mV at a scan rate of 5mV/s.
Characterization of the composition, structure, and reactivity of all the samples was performed before and after the dealloying step. Electrochemical capacitance measurements were carried out in a 1 M Na2SO4 solution using cyclic voltammetry (CV). The potential was cycled from -250 to 0 mV back to -250 mV at scan rates from 25 to 400 mV/s. The average current for the forward and reverse scans was graphed vs. the scan rate to extract the observed capacitance, a measure of the effective area of the sample.
Measurement of the HER was performed in 1 M NaOH. The sample was first pretreated by the application of a constant current of 50 μ A for 5 min. Then, the HER measurement was completed by sweeping the potential from -1,400 to -1,200 mV at a scan rate of 5 mV/s. The potential vs. Ag/AgCl was converted to overpotential based on the standard electrode potential of the HER and the pH of the electrolyte , and the current density was calculated with respect to the geometric area of the sample . The current vs. overpotential data were fit to the Tafel equation to obtain the Tafel slope and exchange current density for the measured HER .
SEM and EDS measurements were carried out using a TM3000 Tabletop SEM (Hitachi, Tokyo, Japan) with a Quantax 70 EDS attachment (Bruker, Madison, WI, USA). Images were taken over a variety of field view sizes from ×60 to ×30,000 magnification. Composition measurements were extracted from EDS spectra taken at ×250 magnification, and Quantax 70 software was used to extract Ni and Cu compositions from the spectra.
Results and discussion
Deposition and dealloying results
For all the samples studied, the capacitance either stayed statistically the same or increased, suggesting that the dealloying procedure either did not change the effective surface area of the sample or caused it to increase. For the samples with between 3% and 15% Cu removed, the capacitance ratio decreases as the amount of copper removed increases. This observation is consistent with the SEM images in Figures 3 and 4. The samples with larger initial copper content tended to have rougher initial topography, such as that in Figure 3e, and thus had higher initial capacitance measurements. In addition, those samples tended to have larger pits seen in the post-dealloy topography, such as in Figure 3f, which increased the measured capacitance only modestly. For the samples with smaller amounts of copper removed, there is more variation in the resulting capacitance ratio. The largest increases in capacitance occurred for samples with a moderate initial copper content combined with a small amount of copper removal, resulting in numerous small pits in the post-dealloy topography. The largest capacitance ratio observed for these samples implies a factor of 3 increase in surface area after dealloying.
Hydrogen evolution reaction measurements
For the as-deposited samples, the Tafel slopes tend to be around 100 to 125 mV/dec. In contrast, the Tafel slopes for the dealloyed samples are generally higher, most above 175 mV/dec. One possible reason for these larger Tafel slopes is a decrease in effective area available for reaction at higher overpotentials due to larger gas evolution rates. This effect may be increased by the more porous nature of the dealloyed samples, allowing gas bubbles to be trapped more easily. To confirm this hypothesis, additional measurements of the effective surface areas at different applied potentials during HER conditions are needed.
The exchange current densities for the as-deposited samples were generally lower than those for the dealloyed samples. The increase in exchange current density for the samples after dealloying is more pronounced (over an order of magnitude) for the samples with larger initial Cu content. This increase cannot be explained purely by an increase in effective surface area. The measured capacitances generally increased by a factor of 2 to 3 after dealloying (Figure 5), so the additional increase in reactivity must be due to structural and compositional changes in the thin films.
Electrodeposition and electrochemical dealloying of NiCu thin films were used to fabricate porous samples. The hydrogen evolution reactivity of electrodeposited NiCu samples was measured before and after some of the Cu was selectively removed. The dealloyed samples are generally more reactive at lower overpotentials, but less reactive at higher overpotentials. The increase in reactivity for the dealloyed samples, as measured by the exchange current density, cannot be explained only by an increase in effective surface area. Thus, some of the reactivity increase must be due to the changes in composition and structure of the samples from the dealloying procedure. The decrease in reactivity at higher overpotentials is hypothesized to be the result of trapped hydrogen bubbles decreasing the effective surface area of the samples. Further experiments are ongoing in our laboratory to investigate the effective surface area of as-deposited and dealloyed samples as a function of potential. The dealloying procedure used here is a promising method for the fabrication of effective catalysts for HER, particularly for use at low overpotentials.
This material is based upon work supported by the National Science Foundation under grants no. RUI-DMR-1104725, REU-PHY/DMR-1004811, ARI-PHY-0963317, and MRI-CHE-0959282.
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