Fabrication of Robust Hydrogen Evolution Reaction Electrocatalyst Using Ag₂Se by Vacuum Evaporation

Abstract

Much research has been done on reliable and low-cost electrocatalysts for hydrogen generation by water splitting. In this study, we synthesized thin films of silver selenide (Ag₂Se) using a simple thermal evaporation route and demonstrated their electrocatalytic hydrogen evolution reaction (HER) activity. The Ag₂Se catalysts show improved electrochemical surface area and good HER electrocatalytic behavior (367 mV overpotential @ 10 mA·cm⁻², exchange current density: ~1.02 × 10⁻³ mA·cm⁻², and Tafel slope: 53 mV·dec⁻¹) in an acidic medium). The reliability was checked in 0.5 M sulfuric acid over 20 h. Our first-principles calculations show the optimal energy of hydrogen adsorption, which is consistent with experimental results. The works could be further extended for finding a new catalyst by associating the selenide, sulfide or telluride-based materials without complex catalyst synthesis procedures.

1. Introduction

Due to the environmental problems caused by the huge amount of fossil fuels, it is imperative to advance renewable and environmentally-friendly energy sources. Hydrogen generation from water splitting has been considered as a replacement for traditional fossil fuels. Electrochemical water splitting from various catalysts is an efficient way to generate hydrogen [1]. For decades, Pt and noble metals have been used as catalyst materials for hydrogen generation [2,3,4,5,6,7]. From an economic point of view, high prices of those noble metals are a fundamental issue. Therefore, exploring robust, cost-effective, earth-rich, and non-noble metal catalysts to replace Pt is very crucial for hydrogen generation from electrochemical water splitting. Until now, many researchers have made great efforts to search for efficient electrocatalysts, using earth-abundant materials such as metals (Mo, Co, W, Ni, Fe, etc.) along with selenides, sulfides, carbides, phosphides and hybrids [6,7,8]. However, most of the catalysts presented much poor catalytic performance compared to Pt. Besides, many catalysts are synthesized using a complicated preparation procedure to enhance the catalytic properties which, in turn, increases costs.

Since silver (Ag) has the highest electrical conductivity and is rather richer than Pt [9,10], Ag could be considered as a potential candidate. However, the biggest issue of pristine Ag is poor hydrogen evolution reaction (HER) activity, especially in acidic environments. Even though Ag exhibits poor electrocatalytic activity, recently the Ag and S combination (Ag₂S) exhibited promising, good HER properties [11,12,13,14]. The synergetic chemical coupling behavior between Ag and S elements are assumed to provide the enrichment in the HER activity. Ag₂S possesses a bandgap of 1.1–2.1 eV with the excellent electrical conductivity, and also theoretically proves its ability for HER reaction [12,13,14]. A few reports for Ag₂S or Ag₂S/Ag have been presented [13,15]. Ren et al. [15] observed good HER performance of a porous Ag₂S/CuS electrocatalyst (200 mV overpotential @ 10 mA·cm⁻², Tafel slope: 75 mV·dec⁻¹). Basu et al. [13] designed Ag₂S/Ag catalyst (199 mV overpotential, Tafel slope: 102 mV·dec⁻¹). Being inspired by excellent electrical properties of Ag₂S, we combined Ag and Se to implement Ag₂Se as catalysts. To the best of our knowledge, this is the first report of Ag₂Se as the electrocatalyst for HER. To overcome previously mentioned obstacles, in this paper, we prepared thin films of Ag₂Se using a single-step process (vacuum evaporation). Ag₂Se films with various thicknesses were deposited onto Au/Si in vacuum at room temperature. Ag₂Se catalysts show good HER properties (367 mV overpotential @ 10 mA·cm⁻², exchange current density: ~1.02 × 10⁻³ mA·cm⁻², and Tafel slope: 53 mV·dec⁻¹).

2. Materials and Methods

2.1. Material Synthesis

Highly pure Ag₂Se powder (99.99%) was procured from Sigma Aldrich (St. Louis, MO, USA). Initially, Au (10–20 nm)/Si substrates were cleaned by deionized (DI) water, methanol, acetone, and isopropyl alcohol. The substrates were loaded on the sample holder and the evaporation chamber was vacuumed by diffusion pump. Ag₂Se thin films (100–250 nm) were coated at a rate of 1.0 Å·s⁻¹. The rate of deposition and film thickness were monitored by a quartz crystal thickness monitor during the film deposition.

2.2. Electrochemical Measurements

Electrochemical properties were analyzed using Biologic SP-300 potentiostat. To estimate the HER electrochemical performance for different electro-catalysts, LSV polarization analyses were employed in three-electrode electrochemical cells, in which the silver electrode (Ag/AgCl) used as the reference electrode, the graphite rod as the counterpart electrode, and the Ag₂Se as the working electrode in 0.5 M H₂SO₄ solution. The scan rate was at 10 mV/s. The CV measurements were conducted between +0.1 and −0.4 V vs. Ag/AgCl. LSV curves were recorded using Ag/AgCl, then potentials were changed into the reversible hydrogen electrode (RHE) scale with the support of subsequent calculation: E (sat. Ag/AgCl) + E° (sat. Ag/AgCl) + 0.059 pH. Besides, the working electrode was performed for 20 h to obtain stable polarization curves. EIS studies were performed in the frequencies from 1 Hz to 1 MHz (10 mV). All the polarization measurements were carried out with iR correction using ohmic loss.

2.3. Characterizations

Physical morphology and structure of catalysts were studied with scanning electron microscopy (SEM, HITACHI S-4700, Tokyo, Japan). X-ray diffraction (Rigaku, Tokyo, Japan) with Cu-Kα radiation was used to check the film crystallinity. The Raman spectral results were collected using an Ar laser (512 nm) (Renishaw inVia RE04, Gloucestershire, United Kingdom) with a scan speed of 30 s and 1 µm spot size. X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe (25 W Al Kα), Kanagawa, Japan) was used for the chemical compositions.

3. Results

Figure 1 illustrates the schematic diagram of thermally evaporated Ag₂Se films with various thicknesses (100, 150, and 200 nm), from the bulk polycrystalline Ag₂Se source. Raman scattering analysis was performed for Ag₂Se thin films, and their spectra are shown in Figure 2a. The Raman spectra show a rather broad peak @ 232 cm⁻¹, which is the characteristics of Ag₂Se [16,17]. Further, structural characterizations were employed by XRD. From the XRD spectra (Figure 2b), the polycrystalline nature (120), (021), (201), (121), (211), (221), (002), (301), (040), (321), (241), (312), (431), and (033) diffraction lines are observed, which corresponds to the primitive lattice structure of orthorhombic Ag₂Se. The peak intensities are varied due to different thickness of Ag₂Se films. The observed peaks clearly follow the standard Ag₂Se nanocrystal with an orthorhombic system (JCPDS: 89-2591).

SEM examinations were carried out to reveal the surface structure and homogeneity of Ag₂Se thin films. As shown in Figure 3, the evaporated Ag₂Se films have uniform morphologies over the surfaces of the film. The smooth surface is found in a 100 nm-thick Ag₂Se film (labeled as Ag₂Se-100) due to low film thickness, as shown in Figure 3a,b. For 150 nm-thick-Ag₂Se (labeled as Ag₂Se-150) in Figure 3c,d, the film surface is covered with an irregular spatial type of grains. The different sizes of grain clusters are exhibited by agglomerations which tend to form larger size grains due to increased film thickness. For the 200 nm-thick Ag₂Se film (labeled as Ag₂Se-200) in Figure 3e-f, the surface of the film consists of different sizes of grain domains with different shapes, possessing pinholes, and hillocks which may affect the conductivity of the film. The existence of Ag and Se stoichiometric ratio, and atomic homogeneity (Ag₂Se-200) were proved by energy-dispersive X-ray spectroscopy (EDS) (Figure S1, and Figure S2 of Supplementary Materials).

The chemical compositions of Ag₂Se were further investigated by XPS analysis. XPS spectra are shown for Ag₂Se-200 (Figure 4). For Ag 3d orbitals, as shown in Figure 4a, binding energies at 368 and 374 eV are ascribed to the spin-orbit doublet Ag 3d_5/2 and Ag 3d_3/2, respectively [18]. The energy peaks observed at 53.88 and 54.78 eV are assigned to the Se 2p_1/2 and Se 2p_3/2 orbital divalent selenide (Se²⁻), respectively, as shown in Figure 4b. The calculated stoichiometric ratio is 2:1 and observed XPS results are coordinated with the literature for Ag₂Se crystal [18]. The survey spectrum is given to verify all the elements in the Ag₂Se film (Figure S3).

The catalytic activity of the Ag₂Se and Pt electrode for the HER was examined using a three-electrode system in 0.5 M H₂SO₄ electrolyte solution (linear sweep voltammetry (LSV), 10 mV/s scan rate). As expected, the commercial Pt wire exhibited very efficient HER activity (54 mV overpotential @ 10 mA·cm⁻²). Ag₂Se-200 showed 367 mV overpotential @ 10 mA·cm⁻², which is 15 and 23 mV lower potential than that of Ag₂Se-100 and Ag₂Se-150 (

Table 1. HER parameters for Ag₂Se, Ag, and Pt.

Sample	Overpotential (mV vs. RHE) at 10 mA·cm−2	Tafel Slope (mV·dec−1)	Exchange Current Density (j0, mA·cm−2)
Pt	54	31	9.86 × 10−1
Ag2Se-200	367	53	1.02 × 10−3
Ag2Se-150	382	51	5.12 × 10−4
Ag2Se-100	390	55	6.45 × 10−4
Ag	588	87	1.31 × 10−5

). HER electrocatalytic activities revealed that Ag₂Se thickness plays a vital role because of the variation of active edges and morphological properties with deposition time. For Ag₂Se-250 sample, the lower catalytic activity may be due to the surface continuity of the film (Figure S4). Thick films could disturb electrolyte penetration and diminish the role of inherent Ag₂Se active edges thereby decrease the electrocatalytic activity. A pure 200 nm-thick Ag film (labeled as Ag) exhibited 588 mV at 10 mA·cm⁻², as shown in Figure 5a. In earlier studies, HER properties were demonstrated for the various layer materials including, bulk and their exfoliated layers of TMD materials such as high-quality monolayer MoS_2(1−x)Se_2x (η_{10 mA·cm}⁻² = 273–300 mV) [19], S-doped MoSe_2−x nanotubes and MoSe₂ nanocaterpillars (η_{2 mA·cm}⁻² = 95–318 mV) [20], tungsten selenide thin films deposited on tungsten foils (η_{10 mA·cm}⁻² = 350 mV) [21], electrodeposition of amorphous cobalt–cobalt oxide/cobalt selenide (CoO_x–CoSe) composite film on a 3-dimensional porous Ni foam (NF) (η_{10 mA·cm}⁻² = 300–380 mV) [22], and electrodeposited cobalt selenide nanostructures (η_{10 mA·cm}⁻² = 472–481 mV) [23].

The extracted Tafel slope values are 53, 51, 55 and 87 mV·dec⁻¹ for Ag₂Se-200, Ag₂Se-150, Ag₂Se-100, and pure Ag, respectively (Figure 5b). We believe that increasing the deposit time would promote the surface-active edge sites and increase the catalytic activity of the electrode. In addition, Ag₂Se-150 and Ag₂Se-200 catalysts offer lower Tafel slopes than previous selenide-based HER catalysts, such as CoS₂-WS₂ electrocatalysts (66.0–79.6 mV·dec⁻¹) [24], cobalt selenide/NiFe layered on exfoliated graphene foil (68–236 mV·dec⁻¹) [25], hybrid structure of MoS₂-WS₂ (72 mV·dec⁻¹) [26], NiSe nanofiber (64 mV·dec⁻¹) [27], MoSe₂/graphene hybrid nanostructures (67 mV·dec⁻¹) [28], WSe₂ and WS_2(1−x)Se_2x (99 and 105 mV·dec⁻¹) [29], hybrid structure of MoS₂/NbSe₂ nanobelts (79.5 mV·dec⁻¹) [30], WS_2(1−x)Se_2x nanoribbons (68 mV·dec⁻¹) [31], WS₂ nanotube and phosphorous doped WS_2(1−x)P_2x nanoribbon (NR) (83 and 71 mV·dec⁻¹) [32].

HER kinetics can be processed via a Volmer-reaction (discharge step, Equation (1)) tailed by either an Heyrovsky reaction (ion-atom step, Equation (2)), or combination process (Tafel-reaction, Equation (3)) [33,34].

$$H_3{O}^{+}+e^{-}\to H_{ads}+H_{2}O$$

(1)

$$H_{ads}+H_3{O}^{+}+e^{-}\to H_2+H_{2}O$$

(2)

$$H_{ads}+H_{ads}\to H_2$$

(3)

The Tafel slope of 51 mV·dec⁻¹ for Ag₂Se-150 signifies that HER occurs through the Volmer–Heyrovsky kinetics, where a rapid protons discharge (Equation (1)) is tailed by a Heyrovsky reaction (Equation (2)) or combination reaction step (Equation (3)) [35,36]. Exchange current density (J₀), another HER characteristic assessment of electrocatalysts, was obtained by extrapolation of Tafel plot towards cathodic current (Table 1). The reported values of J₀ for ternary selenide- or sulfide-based catalysts for HER are presented in Supplementary Materials. Cyclic voltammetry (CV) was measured for the double layer capacitances (C_dl) and the assessment of electrochemically effective surface area (ECSA), assuming that those quantities are directly proportionate [11,37]. The C_dl of Ag₂Se samples are measured by CV at non-faradaic potential regions (Supplementary Figure S5), and is extracted at a given potential (0.24 V vs. RHE) from the differences between cathodic and anodic current density values (∆j = j_a − j_c) against the various scan rates (20–100 mV·s⁻¹). Ag₂Se-150 displays 55.9 μF·cm⁻² of C_dl value and it is greater than that of the Ag₂Se-100 (41.1 μF·cm⁻²) and Ag₂Se-200 (50.5 μF·cm⁻²), indicating the propagation of more active edges in the catalyst, resulting in the enhanced catalytic activity. From these capacitance values, the obtained ECSA values are 2.34 cm² (Ag₂Se-200), 3.19 cm² (Ag₂Se-150), 2.88 cm² (Ag₂Se-100) [38]. The results indicate that the Ag₂Se-150 has the largest ECSA among the three samples.

For an efficient HER catalyst, cycling stability is also important. To probe the stability of Ag₂Se in an acidic medium, an incessant HER cycling test at a static overpotential was conducted. The robustness of Ag₂Se-200 was further proved by employing continuous hydrogen evolution in 0.5 M H₂SO₄ for 20 h, and then, the polarization curve was recorded between −0.6 and 0.2 V vs. RHE. The polarization curves of the three electrodes even after the 20 h successive HER operation, were changed little from their initial curves, as shown in Figure 6a–c. For the chronoamperometric (j-t) profile, a static overpotential of 367 mV (Ag₂Se-200) was used for 20 h, as shown in Figure 6d. The current density exhibits a robust stability performance in a continuous HER operation (over 20 h). Electrochemical impedance spectroscopy (EIS) was carried out to examine the junction reactions and electrocatalyst kinetics of HER process over the frequency range between 1 Hz and 1 MHz. Nyquist plots (Figure 6e) reveal the charge-transfer resistance (R_ct) values at 16.6 Ω (Ag₂Se-100), 22.1 Ω (Ag₂Se-150), and 30.2 Ω (Ag₂Se-200) for Ag₂Se catalyst. The small series resistance (4.1–4.5 Ω) for all catalysts confirms the significance of the preparation of high conductive Au substrate, which allows comfort and operative electrical integration that diminishes ohmic losses.

XPS spectra taken after the 20 h continuous HER operation revealed the slight alteration in the Se binding energy spectrum intensity due to the Se active facets contribution to produce and sustain HER properties in Ag₂Se-200 film (Figure S6), supporting the long-time HER robustness of Ag₂Se. An SEM image taken after the 20 h stability test is shown in Figure S7.

To validate the capability of Ag₂Se [39,40] acts as a catalyst for decomposing water molecules, we performed ab initio total-energy calculations based on the density functional theory (DFT) [41] within the generalized gradient approximation [42] for the exchange-correlation functional. The wave functions were extended using a plane-wave basis set [43,44], and the cutoff energy was 600 eV. Various model structures were considered: A water molecule and a hydroxyl (-OH) group were adsorbed on a 2 nm-thick slab of Ag₂Se. We used the nudged elastic band (NEB) method to calculate the activation energy barrier. Six replicas were chosen, including the initial and final configurations. For a reaction path of decomposition, as shown in Figure 7, we considered several steps (stage A, stage Bi (i = 1, 2, 3), and stage C). At stage A, an H₂O molecule is located 3.5 Å from the Ag₂Se surface. Stages B₁, B₂, and B₃ are intermediate configurations, and stage C is the final configuration in which a hydroxyl (-OH) group is adsorbed onto the Ag₂Se surface. At stage B₁, the H₂O molecule is bonded to a Selenium atom in Ag₂Se. At stage B₂, the HO-H bond of a water molecule (H₂O) becomes dissociated. At stage B₃, the separated H atom hops to an adjacent Se atom and is bonded. We calculated the total energy of each system and found that the reaction energy (E_r) is −2.88 eV. Here, E_r is defined as: Er = E [OH at Ag₂Se] + 1/2 E [H₂] − E [H₂O] − E [Ag₂Se].

The activation energy barrier is calculated to be 0.77 eV. The activation energy is dependent on the path with initial and final configurations, but we can expect that the actual energy barrier could be less than 1 eV because we did experiments at room temperature. We note that dissociation of HO-H bond of a water molecule (H₂O) requires 5.03 ± 0.2 eV [45] in the absence of the catalyst. The geometrical information about the absorption process is presented in the Supplementary Figure S8. For the binding of the hydroxyl group, the bond length is slightly longer than the well-known covalent bond lengths [46] of Se and O atoms because the O atom also has weak interaction with a neighbor Ag atom. The covalent radius of Se is 1.2 Å, and that of O is 0.66 Å.

4. Conclusions

Ag₂Se thin films with various thicknesses (100–200 nm) were deposited by a simple thermal evaporation route. The electrochemical surface area values assessment revealed the mechanism of HER electrocatalytic properties in Ag₂Se thin films. The Ag₂Se catalysts showed good HER electrocatalytic performances in an acidic medium (367 mV overpotential at 10 mA·cm⁻², 53 mV·dec⁻¹ Tafel slope, and ~1.02 × 10⁻³ mA·cm⁻² exchange current density). Using first-principles calculations, we investigated the water splitting mechanism for the Ag₂Se catalysts. The activation energy barrier was estimated to be 0.77 eV. From the observed results, we believe that Ag₂Se-based catalyst will pave a novel pathway to improve the favorable use as the heterogeneous catalysis for catalytic electrochemical reactions and hydrogen-driven solar energy.