Communication pubs.acs.org/IC
Room Temperature Dissolution of Bulk Elemental Ni and Se for Solution Deposition of a NiSe2 HER Electrocatalyst Carrie L. McCarthy, Courtney A. Downes, and Richard L. Brutchey* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
energy- and capital-intensive processing methods, like physical vapor deposition, with simple solution-processing methods that are based on cheaper equipment and require less energy to operate. We developed such a solution-processing method based on the dissolution of bulk inorganic materials using a binary solvent mixture of ethylenediamine (en) and a short-chain thiol to produce molecular inks that can be subsequently deposited and recovered as high-quality thin films.13−20 In particular, for HER catalysis, we demonstrated the dissolution of Co(OH)2 and Se to form a precursor ink from which phase-pure nanostructured marcasite-type CoSe2 can be recovered.21 Herein, we demonstrate how this method can be used to prepare an ink from bulk elemental Ni and Se, which can be used to prepare highly efficient HER catalyst electrodes. Elemental Ni and gray Se were dissolved in 4:1 (v/v) en/ mercaptoethanol (merc) over the course of 48 h at 25 °C, resulting in a murky rust-colored solution (Figure S1), which upon filtering yielded a clear rusty-orange-colored ink (Figure 1a). Thermogravimetric analysis (TGA; Figure 1b) was used to show that the end of ink decomposition occurs by 350 °C. Annealing the filtered ink to 350 °C under a nitrogen atmosphere resulted in the recovery of phase-pure NiSe2, as confirmed by powder X-ray diffraction (XRD; Figure 1c). Rietveld analysis of the experimental XRD data (Figure S2 and Table S1) confirms
ABSTRACT: With hydrogen fuel becoming a more viable alternative to fossil fuels comes the need for inexpensive, low-energy hydrogen production. Here, a low-temperature direct solution-processing method is presented for the deposition of earth-abundant pyrite-type NiSe2 as an efficient hydrogen evolution reaction (HER) catalyst. Thin films of phase-pure NiSe2 are deposited from a precursor ink prepared by room-temperature dissolution of bulk elemental Ni and Se in a binary thiolamine solvent mixture. The nanostructured NiSe2 thin films demonstrate high HER catalytic activity with 100% Faradaic efficiency.
A
s we work toward a more sustainable global society, innovations in clean energy and fuel production are paramount. Fuels that produce little or no polluting byproducts when consumed are of specific importance as we aim to minimize the levels of pollution resulting from the vehicles we drive. Hydrogen fuel cells are the prime example of clean fuel utilization because they result in electricity generation with water and heat as the sole byproducts (i.e., 2H2 + O2 → 2H2O). Several vehicles utilizing fuel cells that are on the market now (e.g., Toyota Miri, Hyundai Tucson Fuel Cell, and Honda Clarity) are able to offer ranges (412−589 km/tank) and fueling times (3−5 min) comparable to those of gasoline-powered cars, achievements that affordable battery-powered electric vehicles are still working toward (charging times of several hours and ranges of 95−539 km/tank).1−3 With the technology to utilize this fuel in place, key factors governing the practicality of the mass adoption of these vehicles will be competitive fuel pricing (e.g., 270 mV), the samples with lower loadings generate higher ECSA-corrected current densities. This is characteristic of the onset of a diffusion-limited regime, resulting in decreased surface utilization at higher overpotentials for thicker films of NiSe2.33 We used electrochemical impedance spectroscopy (EIS) to further study the kinetics of the three samples (Nyquist plots; Figure S8), from which we observed a general trend of decreased charge-transfer resistance (Rct) with increased NiSe2 loading at each applied potential. Interestingly, as the applied potential is increased, the relative difference in the Rct values for the different loadings decreased (i.e., 30−60 Ω at η = 250 mV compared to 1.2−1.7 Ω at η = 450 mV; Table S2). This is likely due to diffusion limitations at higher overpotentials for the thicker eightlayered sample, in agreement with the ECSA-corrected LSV data. Rct decreased with increased potential, as expected when increasing the external driving force for the HER reaction. In addition to the solution resistance (Rs) and Rct components, the equivalent circuit model used to fit the EIS data (Figure S9) includes an overpotential-independent component (R1-CPE1) that has previously been associated with the porosity of the electrode or the contact between the electrode surface and catalyst layer.34−37 This component contributes minimal resistance to the systems (0.2−0.5 Ω) and does not appear to trend with the catalyst loading. Bulk elemental Ni and Se were readily dissolved in an en/merc solvent mixture that was solution-processed to yield phase-pure NiSe2 films that facilitated HER catalysis with 100% Faradaic efficiency and low η10 mA/cm2 values. The onset potential, exchange current density, Tafel slope, and specific HER electrocatalytic activity of the films proved to be comparable to reports for similar transition metal dichalcogenide electrocatalysts, which require multiple synthetic steps utilizing more energy-intensive conditions. This work demonstrates how the thiol-amine solvent system is a valuable tool in the development of inexpensive solution-processing methods for a variety of alternative earth-abundant catalysts. Furthermore, such innovations in material processing bring powerful contributions toward achieving low-cost hydrogen fuel for fuel-cell vehicles and other applications.
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circuit model used to model EIS data, and table of relevant electrocatalytic values (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Twitter: http://twitter.com/ brutcheygroup. ORCID
Richard L. Brutchey: 0000-0002-7781-5596 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) under Grant DMR-1506189. C.L.M. thanks the NSF for a graduate research fellowship. We thank P. Cottingham for assistance with material characterization.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01594. Experimental details, Rietveld analysis of the XRD data, SEM−EDS data, XPS survey scan, CV data and corresponding derivation of Cdl, LSV and Tafel plots for various NiSe2 loadings, EIS Nyquist plots, equivalent C
DOI: 10.1021/acs.inorgchem.7b01594 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.7b01594 Inorg. Chem. XXXX, XXX, XXX−XXX
