Three-Dimensional Au(NiMo)/Ti Catalysts for Efficient Hydrogen Evolution Reaction

In this study, NiMo catalysts that have different metal loadings in the range of ca. 28–106 µg cm−2 were electrodeposited on the Ti substrate followed by their decoration with a very low amount of Au-crystallites in the range of ca. 1–5 µg cm−2 using the galvanic displacement method. The catalytic performance for hydrogen evolution reaction (HER) was evaluated on the NiMo/Ti and Au(NiMo)/Ti catalysts in an alkaline medium. It was found that among the investigated NiMo/Ti and Au(NiMo)/Ti catalysts, the Au(NiMo)/Ti-3 catalyst with the Au loading of 5.2 µg cm−2 gives the lowest overpotential of 252 mV for the HER to reach a current density of 10 mA·cm−2. The current densities for HER increase ca. 1.1–2.7 and ca. 1.1–2.2 times on the NiMo/Ti and Au(NiMo)/Ti catalysts, respectively, at −0.424 V, with an increase in temperature from 25 °C to 75 °C.


Introduction
Although fossil fuels, such as coal, oil, and natural gas, are the main energy sources and are widely used to meet energy needs, the increasing emissions of pollutants, carbon dioxide (CO 2 ), and other greenhouse gases require the development of sustainable technologies to meet ever-increasing energy needs. Among various candidates to fulfill energy demands, hydrogen (H 2 ) can be a potential substitute fuel for effective energy production and storage. H 2 is a clean, economical renewable energy source and an excellent energy storage medium with excellent energy conversion efficiency, higher gravimetric energy density than gasoline (120 vs. 44 MJ kg −1 ), eco-friendliness, and zero carbon dioxide emission [1][2][3][4]. H 2 , an important chemical feedstock widely used in petroleum refining and ammonia synthesis, is industrially produced via coal gasification and steam reforming reaction under harsh conditions, resulting in the emission of greenhouse gases and micro-pollutants [5,6]. Among the various available methods, electrocatalytic water splitting is one of the most promising alternatives for H 2 production that gained intense research interest in the last decades, as electricity-driven water splitting generates green H 2 .
It is well-known that H 2 production by electrocatalytic water splitting in alkaline media is limited by the sluggish hydrogen evolution reaction (HER) kinetics and enormous electricity consumption. The HER mechanism of electrocatalytic water splitting includes three main steps, i.e., Volmer, Heyrovsky, and Tafel reactions, as shown below, in alkaline media [7]: Volmer reaction: H 2 O + e − → H* + OH − Heyrovsky reaction: H* + H 2 O+ e−→ H 2 + OH − Tafel reaction: H* + H* → H 2 Overall reaction:

Fabrication of Catalysts
The catalysts were prepared by a facile, two-step process that involves electrodeposition of Ni 2+ and Mo 6+ ion on the surface of the Ti electrode, followed by a spontaneous Au displacement from the Au (III)-containing solution. Before the deposition of the NiMo catalysts, the Ti plates were pretreated in diluted H 2 SO 4 (1:1 vol) at 70 • C for 3 s. NiMo catalysts were electroplated on the Ti surface (1 × 1 cm) from a bath containing 0.03 M Na 2 MoO 4 along with 0.1, 0.2, and 1.0 M NiSO 4 in an acidic condition (1.5 M H 2 SO 4 and 1 M HCl). The chronopotentiometry was used for the NiMo coatings deposition on the Ti surface. The plating of coatings was carried out at the current of 0.1 mA and 1 mA for 3 min at each current. The Au crystallites were deposited on the prepared NiMo/Ti electrodes by their immersion into 1 mM HAuCl 4 + 0.1 M HCl solution for 10 s. After plating, the samples were taken out, thoroughly rinsed with deionized water, and air-dried at room temperature.

Characterization of Catalysts
The morphology and composition of the prepared catalysts were investigated by scanning electron microscopy (SEM) TM 4000 Plus (HITACHI, Tokyo, Japan).
XRD patterns of pure Ti sheet, Ni-Mo/Ti, and Au-Ni-Mo/Ti catalysts were measured using an X-ray diffractometer D2 PHASER (Bruker, Karlsruhe, Germany). The measurements were conducted in the 2θ range of 10-90 • .
The metal loadings were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis. The ICP-OES spectra were recorded using an Optima 7000DV spectrometer (Perkin Elmer, Waltham, MA, USA) at wavelengths of λ Ni 231.604 nm, λ Mo 202.031 nm, and λ Au 267.595 nm.

Electrochemical Measurements
A conventional three-electrode electrochemical cell was used for electrochemical measurements. The fabricated NiMo/Ti and Au(NiMo)/Ti catalysts were employed as working electrodes, a Pt sheet was used as a counter electrode, and a calomel electrode was used as a reference. All potentials in this work were converted to the reversible hydrogen electrode (RHE) scale using the following Equation (5): Current densities were calculated using the electrodes' geometric area of 2 cm 2 . Linear sweep voltammograms were recorded in a 1 M NaOH solution and always deaerated by argon (Ar) for 20 min prior to measurements. HER polarization curves were recorded from the open circuit potential (OCP) to −0.42 V (vs. RHE) at a polarization rate of 10 mV·s −1 . Polarization curves were recorded at several temperatures from 25 to 75 • C, and temperatures were set with a water jacket connected to a LAUDA Alpha RA 8 thermostat. Stability was studied by recording chronoamperometry (CA) curves for HER at a potential of −0.22 V (vs. RHE) for half an hour. All electrochemical measurements were performed with a Metrohm Autolab potentiostat (PGSTAT302, Utrecht, The Netherlands) using the Electrochemical Software (Nova 2.1.4).

Results
In this study, we investigated the electrocatalytic activity of prepared 3D binary NiMo/Ti and ternary Au(NiMo)/Ti catalysts for HER. These catalysts were deposited on the Ti surface (1 × 1 cm) using an electrochemical bath. The optimal conditions for different 3D binary catalyst depositions were determined and are given in Table 1. The electrochemical deposition was carried out by applying the constant current of 0.1 mA and 1 mA for 3 min at each current. The Au crystallites were deposited on the prepared NiMo/Ti electrodes by their immersion into 1 mM HAuCl 4 + 0.1 M HCl solution for 10 s at room temperature. The morphology and composition of the prepared catalysts were studied by SEM. Figure 1 shows SEM images of the prepared different compositions 3D NiMo/Ti and Au(NiMo)/Ti catalysts. The low magnification SEM image of the 3D NiMo/Ti-3 catalyst (Figure 1c) proves the formation of many cedar leaf-like Ni-Mo alloy structures in a large area, in which leaf-like Ni-Mo alloy is uniformly dispersed on the Ti foil. These cedar leaf-like structures still retain much space among the leaves, forming a porous morphology that can be expected to facilitate electrolyte penetration. Many nanoparticles can be seen in Figure 1c, and they irregularly stack together, forming a cedar leaf-like structure. When Au crystallites were deposited on the prepared NiMo/Ti-3 electrode by being immersed into 1 mM HAuCl 4 + 0.1 M HCl solution for 10 s, the porous leaf-like alloy structure was immediately covered with a tiny globular surface ( Figure 1f). The mass of the elements (metal loadings) on the Ti substrate surface was determined by ICP-OES analysis ( Table 2). It can be seen that the formed 3D binary NiMo/Ti catalysts contained ca. 82.8-93.7 wt.% of Ni, whereas those 3D ternaries Au(NiMo)/Ti catalysts possessed 76.6-87.9 wt.% of Ni. The total metal loadings (µg metal cm −2 ) in the prepared catalysts are quite different and vary from 23.9 up to 106.2 µg metal cm −2 . It should be noted that Ni and Mo amounts increase in the NiMo coatings by increasing the Ni 2+ concentration in the plating solution, whereas the Mo amount was kept the same. Calculated Mo:Ni and Au:NiMo mass weight ratios are given in Table 2.
As seen from the data in Table 2, mass weight ratios Mo:Ni for NiMo/Ti increase due to the rise of Ni 2+ concentration in the plating solution. A similar phenomenon is observed in the case of Au crystallite-modified NiMo/Ti catalysts. Mass ratios Mo:Ni also increase with the increase in the Ni 2+ concentration in the plating solution (Table 2). Moreover, Au loadings in the AuNiMo/Ti-1, AuNiMo/Ti-2, and AuNiMo/Ti-3 catalysts increased while the deposition times of Au crystallites were the same-10 s. The mass ratio Au:NiMo slightly decreases. The increased Ni amount in the catalysts allows for achieving a higher Au loading in the ones. Figure 2 shows XRD patterns for a pure Ti sheet (lower curve) and NiMo/Ti-3 and Au(NiMo)/Ti-3 catalysts (upper curves). Symbols indicate the positions of the XRD peaks of Ti (ICDD card no 00-044-1294).  Table 2. (c) The inset represents a photo of a cedar leaf.
As seen from the data in Table 2, mass weight ratios Mo:Ni for NiMo/Ti increase due to the rise of Ni 2+ concentration in the plating solution. A similar phenomenon is observed in the case of Au crystallite-modified NiMo/Ti catalysts. Mass ratios Mo:Ni also increase with the increase in the Ni 2+ concentration in the plating solution (Table 2). Moreover, Au loadings in the AuNiMo/Ti-1, AuNiMo/Ti-2, and AuNiMo/Ti-3 catalysts increased while the deposition times of Au crystallites were the same-10 s. The mass ratio Au:NiMo slightly decreases. The increased Ni amount in the catalysts allows for achieving a higher Au loading in the ones. Figure 2 shows XRD patterns for a pure Ti sheet (lower curve) and NiMo/Ti-3 and Au(NiMo)/Ti-3 catalysts (upper curves). Symbols indicate the positions of the XRD peaks of Ti (ICDD card no 00-044-1294).   Table 3). Overpotentials (vs. RHE) to reach the current density of 10 mA·cm −2 (η 10 ) were found in a gradual increasing order for both Au(NiMo)/Ti and NiMo/Ti catalysts as follows: Au(NiMo)/Ti-3 (−252 mV) < Au(NiMo)/Ti-2 (−298 mV) < Au(NiMo)/Ti-1 (−308 mV) NiMo/Ti-3 (−288 mV) < NiMo/Ti-2 (−344 mV) < NiMo/Ti-1 (−349 mV). It was determined that mass weight ratios Mo:Ni for NiMo/Ti catalysts increase due to the increased Ni 2+ concentration in the plating solution (Table 2). A higher massweight ratio Mo:Ni induces a more pronounced activity of the NiMo/Ti-3 catalyst for HER. Additionally, the increased amount of Ni in the NiMo/Ti catalysts allows for achieving higher Au loading in the Au(NiMo)/Ti catalysts. This is the main factor that influences the lowering overpotential at Au(NiMo)/Ti catalysts compared with NiMo/Ti catalysts. The higher activity of Au crystallite-modified NiMo/Ti catalysts may be related with the synergetic effect of Au, Ni, and Mo [35].
HER polarization curves were then further used for constructing the Tafel plots and calculating the Tafel slope. Tafel slope values of 99.6, 100.5, and 130.4 mV·dec −1 were found for HER at NiMo/Ti-1, NiMo/Ti-2, and NiMo/Ti-3 catalysts, respectively. For those 3D ternary Au(NiMo)/Ti catalysts, Tafel slope values of 143.8, 98.7, and 131.2 mV·dec −1 were determined at Au(NiMo)/Ti-1, Au(NiMo)/Ti-2, and Au(NiMo)/Ti-3 catalysts, respectively (Figure 4a',b', Table 3). The determination of the Tafel slope explores the HER kinetics by measuring the increase in current density with the increase in overpotential, whereas the exchange current density (j 0 ) reflects the electrode's intrinsic activity for HER. The exchange current density (j 0 ) was calculated for HER at all six catalysts by extrapolating the Tafel plots, η vs. log j. Thus, the j 0 value of 0.144, 0.011, 0.076, 0.075, 0.006, and 0.005 mA·cm −2 were calculated for Au(NiMo)/Ti-3, Au(NiMo)/Ti-2, Au(NiMo)/Ti-1, NiMo/Ti-3, NiMo/Ti-2, and NiMo/Ti-1 catalysts, respectively (Table 3). It is worth noting that the j 0 value determined for HER at the Au(NiMo)/Ti-3 coating was ca. 2-28 times higher than that determined for the rest of the studied catalysts.  It was determined that mass weight ratios Mo:Ni for NiMo/Ti catalysts increase due to the increased Ni 2+ concentration in the plating solution (Table 2). A higher mass-weight ratio Mo:Ni induces a more pronounced activity of the NiMo/Ti-3 catalyst for HER. Additionally, the increased amount of Ni in the NiMo/Ti catalysts allows for achieving higher Au loading in the Au(NiMo)/Ti catalysts. This is the main factor that influences the lowering overpotential at Au(NiMo)/Ti catalysts compared with NiMo/Ti catalysts. The higher activity of Au crystallite-modified NiMo/Ti catalysts may be related with the syn-   HER polarization curves were then further used for constructing the Tafel plot calculating the Tafel slope. Tafel slope values of 99.6, 100.5, and 130.4 mV·dec −1 were f for HER at NiMo/Ti-1, NiMo/Ti-2, and NiMo/Ti-3 catalysts, respectively. For those 3 nary Au(NiMo)/Ti catalysts, Tafel slope values of 143.8, 98.7, and 131.2 mV·dec −1 determined at Au(NiMo)/Ti-1, Au(NiMo)/Ti-2, and Au(NiMo)/Ti-3 catalysts, respec (Figure 4a',b', Table 3). The determination of the Tafel slope explores the HER kinet measuring the increase in current density with the increase in overpotential, where exchange current density (j0) reflects the electrode's intrinsic activity for HER. Th change current density (j0) was calculated for HER at all six catalysts by extrapolatin Tafel plots, η vs. log j. Thus, the j0 value of 0.144, 0.011, 0.076, 0.075, 0.006, and mA·cm −2 were calculated for Au(NiMo)/Ti-3, Au(NiMo)/Ti-2, Au(NiMo)/Ti-1, NiMo NiMo/Ti-2, and NiMo/Ti-1 catalysts, respectively (Table 3). It is worth noting that value determined for HER at the Au(NiMo)/Ti-3 coating was ca. 2-28 times higher that determined for the rest of the studied catalysts.
Another crucial criterion for an advanced electrode material is its electroche stability. Chronoamperometric measurements with all six catalysts were carried ou Another crucial criterion for an advanced electrode material is its electrochemical stability. Chronoamperometric measurements with all six catalysts were carried out in 1 M NaOH at −0.22 V for 2 h. Initially, in the first 50-200 s, a decrease in current density was observed for all investigated catalysts. However, after approximately 500 s, the current densities settled down and remained apparently parallel throughout the experiments. CA results confirm the result of LSV analysis in terms of the ternary Au(NiMo)/Ti-3 catalyst, giving the highest current density during HER (−10.36 mA·cm −2 at 50 s) ( Figure 5). A more than 2.5 times lower current density was obtained with Au(NiMo)/Ti-2 (−3.81 mA·cm −2 ) and ca. 5 times lower with Au(NiMo)/Ti-1 catalysts (−2.02 mA·cm −2 ). In the case of the binary NiMo/Ti-3 catalyst, a comparatively lower current density was recorded (−6.13 mA·cm −2 at 50 s) along with a 3-5 times lower value for NiMo/Ti-2 (−2.06 mA·cm −2 ) and NiMo/Ti-1 (−1.23 mA·cm −2 ) catalysts. results confirm the result of LSV analysis in terms of the ternary Au(NiMo)/Ti-3 cataly giving the highest current density during HER (−10.36 mA·cm −2 at 50 s) ( Figure 5). A mo than 2.5 times lower current density was obtained with Au(NiMo)/Ti-2 (−3.81 mA·cm and ca. 5 times lower with Au(NiMo)/Ti-1 catalysts (−2.02 mA·cm −2 ). In the case of t binary NiMo/Ti-3 catalyst, a comparatively lower current density was recorded (−6. mA·cm −2 at 50 s) along with a 3-5 times lower value for NiMo/Ti-2 (−2.06 mA·cm −2 ) an NiMo/Ti-1 (−1.23 mA·cm −2 ) catalysts. A comparison of HER parameters generated using herein-tested NiMo/Ti an Au(NiMo)/Ti catalysts in an alkaline medium with some electrodes reported in the liter ture is given in Table 4.  A comparison of HER parameters generated using herein-tested NiMo/Ti and Au(NiMo)/Ti catalysts in an alkaline medium with some electrodes reported in the literature is given in Table 4.  MCFs-mesoporous composite films, HNW-hybrid nanowire, WC-tungsten carbide, TM-Ti mesh, and NR/NF-nanorods/Ni foam. ** Overpotential at 10 mA·cm −2 .

Conclusions
In summary, NiMo and Au(NiMo) catalysts supported on a titanium surface were studied as electrocatalysts for HER in an alkaline medium. NiMo/Ti catalysts with different total metal loadings in the range of ca. 28-106 µg cm −2 were prepared using a simple and low-cost metal electrodeposition method. The decoration of the prepared NiMo/Ti catalysts with a small amount of Au-crystallites in the range of ca. 1-5 µg cm −2 was carried out using the galvanic displacement method.
It was determined that, among the investigated catalysts, the Au(NiMo)/Ti-3 catalyst with the Au loading of 5.2 µg cm −2 exhibits the highest current density, as well as exchange current density during HER in a 1 M NaOH solution. Moreover, the Au(NiMo)/Ti-3 catalyst also displays excellent HER performance with an overpotential of 252 mV at a current density of 10 mA·cm −2 .