Ball Milling-Assisted Synthesis of Ultrasmall Ruthenium Phosphide for Efﬁcient Hydrogen Evolution Reaction

: The development of scalable hydrogen production technology to produce hydrogen economically and in an environmentally friendly way is particularly important. The hydrogen evolution reaction (HER) is a clean, renewable, and potentially cost-effective pathway to produce hydrogen, but it requires the use of a favorable electrocatalyst which can generate hydrogen with minimal overpotential for practical applications. Up to now, ruthenium phosphide Ru 2 P has been considered as a high-performance electrocatalyst for the HER. However, a tedious post-treatment method as well as large consumption of solvents in conventional solution-based synthesis still limits the scalable production of Ru 2 P electrocatalysts in practical applications. In this study, we report a facile and cost-effective strategy to controllably synthesize uniform ultrasmall Ru 2 P nanoparticles embedded in carbon for highly efﬁcient HER. The key to our success lies in the use of a solid-state ball milling-assisted technique, which overcomes the drawbacks of the complicated post-treatment procedure and large solvent consumption compared with solution-based synthesis. The obtained electrocatalyst exhibits excellent Pt-like HER performance with a small overpotential of 36 mV at current density of 10 mA cm − 2 in 1 M KOH, providing new opportunities for the fabrication of highly efﬁcient HER electrocatalysts in real-world applications.


Introduction
Renewable hydrogen energy, an ideal substitute for nonrenewable fossil fuels, has attracted much attention due to its zero emissions of greenhouse gases [1,2].Therefore, the development of scalable hydrogen production technology to produce hydrogen economically and in environmentally friendly ways is particularly important.Among them, electrochemical water splitting is a new method for hydrogen production, which presents a clean, renewable, and potentially cost-effective pathway to produce hydrogen [3][4][5].As a vital step of electrochemical water splitting on the cathode, the hydrogen evolution reaction (HER) always demands the use of a favorable electrocatalyst which can generate hydrogen with minimal overpotential for practical applications [6][7][8].Up to now, precious platinum (Pt)-based catalysts have been widely acknowledged as unbeatable HER electrocatalysts due to their high current density and low Tafel slope [9][10][11].However, their widespread utility is limited by natural scarcity.Therefore, research into novel types of HER catalysts with both high activity and low price is highly important.
Recently, ruthenium (Ru)-based catalysts have gradually emerged as a new type of promising HER electrocatalysts with high intrinsic catalytic activity, which are comparable to the commercial Pt-based catalysts [12][13][14].Despite Ru being a precious transition metal belonging to the Pt group, the cost of Ru is only ca.4% of the cost of Pt per unit mass [15].Therefore, many efforts have been made to develop rationally designed Ru electrocatalysts with both high intrinsic HER performance and low cost.Inspired by the efficient metal phosphides catalysts used in the hydrodesulfurization (HDS) reaction, precious ruthenium phosphides such as RuP, RuP 2 , and Ru 2 P were well explored as high-performance electrocatalysts for the HER owing to their similarity in catalytic mechanisms [16][17][18][19][20][21].For example, Liu et al. synthesized Ru 2 P nanoparticle-decorated P/N-doped carbon nanofibers on carbon cloth involving electrochemical polymerization and subsequent metal impregnation [16].Li et al. reported the example of Ru 2 P nanoparticles supported on reduced graphene oxide nanosheets via a solution-based hydrothermal synthesis [20].The moderate Gibbs free energy of hydrogen adsorption on the Ru 2 P surface based on density function theory (DFT) calculations endows Ru 2 P nanoparticles with excellent HER electrocatalytic performance.Until now, conventional solution-based synthesis was necessary for fabricating Ru 2 P nanoparticles.However, the tedious post-treatment method as well as large consumption of solvents still limits the scalable production of Ru 2 P electrocatalysts in practical applications.Therefore, developing a novel, facile, and environmental-friendly methodology for preparing high-performance Ru 2 P electrocatalysts is of great significance.
In this study, we report a facile and cost-effective strategy to controllably synthesize uniform ultrasmall Ru 2 P nanoparticles embedded in carbon for highly efficient HER.The key to our success lies in the use of a solid-state ball milling-assisted technique, which overcomes the drawbacks of complicated post-treatment procedure and large solvent consumption compared with solution-based synthesis.To the best of our knowledge, solid-state synthesis of Ru 2 P nanoparticles has never been reported so far.More importantly, the obtained electrocatalyst exhibits excellent Pt-like HER performance with a small overpotential of 36 mV at a current density of 10 mA cm −2 in 1 M KOH, providing new opportunities for the fabrication of highly efficient HER electrocatalysts in real application.

Results and Discussion
As shown in Scheme 1, we successfully fabricated Ru 2 P nanoparticles embedded in carbon (Ru 2 P-BM-C) via a simple solid-state ball milling-assisted technique [22].To be specific, a mixture of ruthenium chloride, sodium hypophosphite, and commercial carbon black (VXC-72R, Cabot) were first milled in a planetary ball mill for 2 h.Subsequently, the resultant composite was further thermally treated under nitrogen atmosphere at 600 • C for 2 h (detailed synthetic information can be found in the experimental section).The structural features of as-prepared Ru2P-BM-C were first determined by a representative Xray diffraction (XRD) pattern.As illustrated in Figure 1, the characteristic diffraction peaks of Ru2P-BM-C can be well indexed to a typical orthorhombic phase of ruthenium phosphide (Ru2P, JCPDS No. 65-2382) [21].Meanwhile, the broad diffraction peak centered at ca. 25° is assigned to commercial Scheme 1.The synthesis route of Ru 2 P nanoparticles embedded in carbon (Ru 2 P-BM-C) via a facile ball milling-assisted method.
The structural features of as-prepared Ru 2 P-BM-C were first determined by a representative X-ray diffraction (XRD) pattern.As illustrated in Figure 1, the characteristic diffraction peaks of Ru 2 P-BM-C can be well indexed to a typical orthorhombic phase of ruthenium phosphide (Ru 2 P, JCPDS No. 65-2382) [21].Meanwhile, the broad diffraction peak centered at ca. 25 • is assigned to commercial carbon black [23].The contents of Ru and P in Ru 2 P-BM-C were determined to be 5.1 and 0.81 wt%, respectively, as determined by inductively coupled plasma-mass spectrometry (ICP-MS), indicating a nearly 1.93:1 atomic ratio for Ru/P.In addition, the bulk morphology and distribution of Ru 2 P in carbon matrix was further demonstrated by transmission electron microscopy (TEM).As shown in Figure 2a,b, a highly uniform dispersion of Ru 2 P ultrasmall nanoparticles was observed, with an average size of ca. 5 nm.In addition, high-resolution transmission electron microscopy (HRTEM) of one grain oriented along the [112] zone axis is also shown in Figure 2c.The corresponding d-spacing is measured to be 2.36 Å, which matches very well with the result from XRD (Figure 1).The structural features of as-prepared Ru2P-BM-C were first determined by a representative Xray diffraction (XRD) pattern.As illustrated in Figure 1, the characteristic diffraction peaks of Ru2P-BM-C can be well indexed to a typical orthorhombic phase of ruthenium phosphide (Ru2P, JCPDS No. 65-2382) [21].Meanwhile, the broad diffraction peak centered at ca. 25° is assigned to commercial carbon black [23].The contents of Ru and P in Ru2P-BM-C were determined to be 5.1 and 0.81 wt%, respectively, as determined by inductively coupled plasma-mass spectrometry (ICP-MS), indicating a nearly 1.93:1 atomic ratio for Ru/P.In addition, the bulk morphology and distribution of Ru2P in carbon matrix was further demonstrated by transmission electron microscopy (TEM).As shown in Figure 2a,b, a highly uniform dispersion of Ru2P ultrasmall nanoparticles was observed, with an average size of ca. 5 nm.In addition, high-resolution transmission electron microscopy (HRTEM) of one grain oriented along the [112] zone axis is also shown in Figure 2c.The corresponding d-spacing is measured to be 2.36 Å, which matches very well with the result from XRD (Figure 1).To further probe the surface chemistry of Ru2P-BM-C, X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface chemical state of Ru2P-BM-C.In the P 2p spectrum (Figure 3a), the peaks at 130.0 and 130.7 eV are attributed to Ru-P bonding.Meanwhile, the peak at 134.0 eV can be assigned to P-O signal, possibly from the oxidized P species under air [24].For the Ru 3d spectrum To further probe the surface chemistry of Ru 2 P-BM-C, X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface chemical state of Ru 2 P-BM-C.In the P 2p spectrum (Figure 3a), the peaks at 130.0 and 130.7 eV are attributed to Ru-P bonding.Meanwhile, the peak at 134.0 eV can be assigned to P-O signal, possibly from the oxidized P species under air [24].For the Ru 3d spectrum (Figure 3b), subpeaks at 280.5 and 285.0 eV are consistent with the binding energies of Ru 3d 5/2 and Ru 3d 3/2 of Ru 2 P [25,26], respectively, while the subpeak at 285.6 eV is associated with the C=O bond [27].These results are consistent with the above XRD and HRTEM analysis, further confirming the successful formation of Ru 2 P in carbon matrix.To further probe the surface chemistry of Ru2P-BM-C, X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface chemical state of Ru2P-BM-C.In the P 2p spectrum (Figure 3a), the peaks at 130.0 and 130.7 eV are attributed to Ru-P bonding.Meanwhile, the peak at 134.0 eV can be assigned to P-O signal, possibly from the oxidized P species under air [24].For the Ru 3d spectrum (Figure 3b), subpeaks at 280.5 and 285.0 eV are consistent with the binding energies of Ru 3d5/2 and Ru 3d3/2 of Ru2P [25,26], respectively, while the subpeak at 285.6 eV is associated with the C=O bond [27].These results are consistent with the above XRD and HRTEM analysis, further confirming the successful formation of Ru2P in carbon matrix.To evaluate the HER performance of Ru 2 P-BM-C, we first studied the HER activity of Ru 2 P-BM-C in N 2 -saturated 1 M KOH.The sample was loaded on a polished glassy carbon (GC) electrode using a standard three-electrode cell.An Ag/AgCl electrode and a graphite rod were utilized as the reference and counter electrodes, respectively.Linear sweep voltammetry (LSV) measurements with a sweep rate of 5 mV s −1 were performed to determine the HER activity of the sample and all polarization curves were corrected with iR-compensation.The overpotential to deliver 10 mA cm −2 current density, a metric related to solar water-splitting devices, was then carefully measured to compare the HER activity for various electrocatalysts.As illustrated in Figure 4a, Ru 2 P-BM-C presented a very small HER overpotential of 36 mV at a current density of 10 mA cm −2 in 1 M KOH, which was very close to benchmark Pt/C (31 mV@10 mA cm −2 ), and obviously outperformed many previously reported Ru-based HER electrocatalysts such as Ru 2 P nanoparticle-decorated P/N dual-doped carbon nanofibers on carbon cloth (Ru 2 P@PNC/CC-900,50 mV@10 mA cm −2 ) [16], Ru 2 P nanoparticles (54 mV@10 mA cm −2 ) [21], carbon-encapsulated ruthenium diphosphide nanoparticle (RuP 2 @NPC, 52 mV@10 mA cm −2 ) [19], RuP x nanoparticles encapsulated in uniform N,P-codoped hollow carbon nanospheres (RuP x @NPC, 74 mV@10 mA cm −2 ) [28], ruthenium incorporated cobalt phosphide nanocubes (Ru-CoP, 51 mV@10 mA cm −2 ) [29], and Ru nanodendrites composed of ultrathin fcc/hcp nanoblades (RuCu NDs, 43 mV@10 mA cm −2 ) [30], indicating the outstanding catalytic activity of Ru 2 P-BM-C in alkaline electrolyte.
To get a better understanding of HER kinetics using Ru 2 P-BM-C the corresponding Tafel plots of Ru 2 P-BM-C and commercial Pt/C were investigated in 1 M KOH.Remarkably, Ru 2 P-BM-C exhibits a low Tafel slope of 59 mV decade −1 (Figure 4b), which is comparable to the value for the Pt/C catalyst (53 mV decade −1 ).This reveals that the HER mechanism of our catalyst follows Volmer-Heyrovsky reaction where an initial proton adsorption is the rate-determining step [31].Furthermore, the HER stability of Ru 2 P-BM-C was also evaluated by cycling the Ru in Figure 5a,b, no obvious morphological change can be observed for Ru 2 P nanoparticles after 2000 cycles, further confirming the high stability of Ru 2 P-BM-C.Previous studies indicated that carbon-encapsulated electrocatalysts could significantly promote durability for HER, which may also contribute to the high stability of Ru 2 P-BM-C [32][33][34].
compare the HER activity for various electrocatalysts.As illustrated in Figure 4a, Ru2P-BM-C presented a very small HER overpotential of 36 mV at a current density of 10 mA cm −2 in 1 M KOH, which was very close to benchmark Pt/C (31 mV@10 mA cm −2 ), and obviously outperformed many previously reported Ru-based HER electrocatalysts such as Ru2P nanoparticle-decorated P/N dualdoped carbon nanofibers on carbon cloth (Ru2P@PNC/CC-900,50 mV@10 mA cm −2 ) [16], Ru2P nanoparticles (54 mV@10 mA cm −2 ) [21], carbon-encapsulated ruthenium diphosphide nanoparticle (RuP2@NPC, 52 mV@10 mA cm −2 ) [19], RuPx nanoparticles encapsulated in uniform N,P-codoped hollow carbon nanospheres (RuPx@NPC, 74 mV@10 mA cm −2 ) [28], ruthenium incorporated cobalt phosphide nanocubes (Ru-CoP, 51 mV@10 mA cm −2 ) [29], and Ru nanodendrites composed of ultrathin fcc/hcp nanoblades (RuCu NDs, 43 mV@10 mA cm −2 ) [30], indicating the outstanding catalytic activity of Ru2P-BM-C in alkaline electrolyte.To get a better understanding of HER kinetics using Ru2P-BM-C the corresponding Tafel plots of Ru2P-BM-C and commercial Pt/C were investigated in 1 M KOH.Remarkably, Ru2P-BM-C exhibits a low Tafel slope of 59 mV decade −1 (Figure 4b), which is comparable to the value for the Pt/C catalyst (53 mV decade −1 ).This reveals that the HER mechanism of our catalyst follows Volmer-Heyrovsky reaction where an initial proton adsorption is the rate-determining step [31].Furthermore, the HER stability of Ru2P-BM-C was also evaluated by cycling the Ru2P-BM-C continuously for 2000 cyclic voltammetry (CV) cycles.The resultant LSV curve after 2000 cycles exhibited almost no difference compared with the initial one (Figure 4c), indicating robust stability of Ru2P-BM-C under alkaline condition.The excellent long-term durability was also corroborated by TEM analysis.As shown in Figure 5a,b, no obvious morphological change can be observed for Ru2P nanoparticles after 2000 cycles, further confirming the high stability of Ru2P-BM-C.Previous studies indicated that carbonencapsulated electrocatalysts could significantly promote durability for HER, which may also contribute to the high stability of Ru2P-BM-C [32][33][34].3. Materials and Methods

Catalyst Preparation
In a typical synthesis, ruthenium chloride (24 mg, Aladdin, Shanghai, China) and sodium hypophosphite (500 mg, Aladdin, Shanghai, China) were mixed with commercial carbon black (100 mg, VXC-72R, Cabot, Kingchemical, Shanghai, China).The mixture was transferred to a commercially available PTFE ball mill jar (~20 mL) with twenty PTFE balls (6 mm in diameter).The reactor was placed in a high energy planetary ball mill (Changsha Deco Equipment Co., Ltd., Changsha, China, Model DECO-PBM-H-0.4L)and milled at 600 rpm for 2 h.After ball milling, the resulting product was pyrolyzed at 600 °C (heating rate: 5 K min −1 ) for 2 h under an inert N2 atmosphere.The resultant catalyst was washed and denoted as Ru2P-BM-C.

Material Characterization
X-ray diffraction (XRD) was performed on a Bruker D8 ADVANCE diffractometer (Karlsruhe, Germany) using Cu-Kα radiation with a 2θ range of 10 to 80°.The morphology of the sample was investigated by using a JEOL JEM-F200 high-resolution transmission electron microscopy (Tokyo, Japan).X-ray photoelectron spectroscopic (XPS) measurement was performed on a Thermo Fisher ESCALAB Xi+ spectrometer (Waltham, MA, USA) equipped with an Al-Kα X-ray source.The contents of Ru and P were determined by inductively coupled plasma-mass spectrometry (ICP-MS, NexION 350D, PerkinElmer, Waltham, MA, USA).

Catalyst Preparation
In a typical synthesis, ruthenium chloride (24 mg, Aladdin, Shanghai, China) and sodium hypophosphite (500 mg, Aladdin, Shanghai, China) were mixed with commercial carbon black (100 mg, VXC-72R, Cabot, Kingchemical, Shanghai, China).The mixture was transferred to a commercially available PTFE ball mill jar (~20 mL) with twenty PTFE balls (6 mm in diameter).The reactor was placed in a high energy planetary ball mill (Changsha Deco Equipment Co., Ltd., Changsha, China, Model DECO-PBM-H-0.4L)and milled at 600 rpm for 2 h.After ball milling, the resulting product was pyrolyzed at 600 • C (heating rate: 5 K min −1 ) for 2 h under an inert N 2 atmosphere.The resultant catalyst was washed and denoted as Ru 2 P-BM-C.

Material Characterization
X-ray diffraction (XRD) was performed on a Bruker D8 ADVANCE diffractometer (Karlsruhe, Germany) using Cu-Kα radiation with a 2θ range of 10 to 80 • .The morphology of the sample was investigated by using a JEOL JEM-F200 high-resolution transmission electron microscopy (Tokyo, Japan).X-ray photoelectron spectroscopic (XPS) measurement was performed on a Thermo Fisher ESCALAB Xi+ spectrometer (Waltham, MA, USA) equipped with an Al-Kα X-ray source.The contents of Ru and P were determined by inductively coupled plasma-mass spectrometry (ICP-MS, NexION 350D, PerkinElmer, Waltham, MA, USA).

Electrochemical Test
A CHI 650E electrochemical workstation (Shanghai, China) was used to measure the electrocatalytic activities towards HER.One molar KOH was utilized as the electrolyte.All experiments were conducted in a standard three-electrode electrochemical cell consisting of a glassy carbon as the working electrode substrate, a graphite rod as the counter electrode, and an Ag|AgCl (3.5 M KCl) electrode as the reference electrode.The potential measured against Ag|AgCl was further converted to the potential versus the reversible hydrogen electrode (RHE).iR-compensation was made to compensate for the voltage drop between the reference and working electrodes, which was measured by a single-point high-frequency impedance measurement.To fabricate the working electrode, 2 mg of each catalyst sample and 50 µL of 5 wt% Nafion solution (D520, Alfa Aesar, Shanghai, China) were dispersed in 1 mL of water/ethanol solution (3:1 v/v) under mild sonication for at least 40 min to form a homogeneous ink.Thereafter, 12 µL of the catalyst ink was dropped onto a mirror polished glassy carbon electrode with 3 mm in diameter and dried at ambient conditions.The loading amount of was calculated to be ~0.34mg cm −2 .The benchmark Pt/C catalyst (20 wt% Pt, JM) was also prepared with equal loading.For HER tests, the electrolyte was saturated with argon for at least 30 min and continuous cyclic voltammetry sweeps were operated to make pretreatment.Linear scan voltammetry (LSV) was performed at a scan rate of 5 mV s −1 .To prevent formation of bubbles, magnetic stirring with a stir bar with the rotation rate of 800 rpm was employed for the LSV.

Scheme 1 .
Scheme 1.The synthesis route of Ru2P nanoparticles embedded in carbon (Ru2P-BM-C) via a facile ball milling-assisted method.

Figure 3 .
Figure 3. XPS spectra of P 2p (a) and Ru 3d (b) of Ru2P-BM-C.To evaluate the HER performance of Ru2P-BM-C, we first studied the HER activity of Ru2P-BM-C in N2-saturated 1 M KOH.The sample was loaded on a polished glassy carbon (GC) electrode using a standard three-electrode cell.An Ag/AgCl electrode and a graphite rod were utilized as the reference and counter electrodes, respectively.Linear sweep voltammetry (LSV) measurements with a sweep rate of 5 mV s −1 were performed to determine the HER activity of the sample and all polarization curves were corrected with iR-compensation.The overpotential to deliver 10 mA cm−2
2 P-BM-C continuously for 2000 cyclic voltammetry (CV) cycles.The resultant LSV curve after 2000 cycles exhibited almost no difference compared with the initial one (Figure 4c), indicating robust stability of Ru 2 P-BM-C under alkaline condition.The excellent long-term durability was also corroborated by TEM analysis.As shown Catalysts 2019, 9, 240 5 of 8

Figure 4 .
Figure 4. (a) Hydrogen evolution reaction (HER) polarization curves of Ru2P-BM-C and benchmark Pt/C, respectively.(b) Tafel plots of corresponding Ru2P-BM-C and Pt/C.(c) Polarization curves of the Ru2P-BM-C catalyst decorated on a GC electrode before and after 2000 CV cycles.All tests were performed in 1 M KOH.

Figure 4 . 9 Figure 5 .
Figure 4. (a) Hydrogen evolution reaction (HER) polarization curves of Ru 2 P-BM-C and benchmark Pt/C, respectively.(b) Tafel plots of corresponding Ru 2 P-BM-C and Pt/C.(c) Polarization curves of the Ru 2 P-BM-C catalyst decorated on a GC electrode before and after 2000 CV cycles.All tests were performed in 1 M KOH.Catalysts 2019, 9, x FOR PEER REVIEW 6 of 9