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High-Sulfur-Vacancy Amorphous Molybdenum Sulfide as a High Current Electrocatalyst in Hydrogen Evolution

Ang-Yu Lu, Xiulin Yang, Chien-Chih Tseng, Shixiong Min, Shi-Hsin Lin, Chang-Lung Hsu, Henan Li, Hicham Idriss, Jer-Lai Kuo, Kuo-Wei Huang, and Lain-Jong Li*

stable, or higher efficiency replacement for Pt at high current density regimes shall promote the use of hydrogen energy. Various economical materials such as Mo2C,[7] MoB,[8] TiS2,

[9] TiCN,[10,11] FeP,[12] NiP,[13] CoP,[14,15] and CoSe2

[16] have been proposed as alternative HER catalysts. The crystalline transi-tion metal dichalcogenide (TMD) such as MoS2

[17–21] is one promising candidate to replace Pt for HER and the active sites have been identified at the edges of the crystals.[22–24] To further enhance the catalytic activity, one approach has successfully activated the WS2 basal planes by converting its crystalline phase from 2H to 1T.[25] Another strategy to create sulfur vacancies on crystalline MoS2 is using Ar plasma to modify its basal planes.[26] However, the structure of MoS2 can be easily damaged by Ar plasma due to the strong sput-tering effect.[27] Amorphous MoSx has been recognized as a cheap and efficient HER catalyst recently.[28–30] Nevertheless, their electrochemical characteristics are still far behind the practical requirements for high current density operation.

In this report, we use remote hydrogen plasma to create sulfur vacancies on amorphous molybdenum sulfide, which leads to significant enhancement in their HER performance. The hydrogen plasma treatment of a-MoSx not only increases the active site density but also changes the surface energy of the catalysts, which results in inhibiting the bubble trapping on catalytic surfaces at high current densities. Because the commonly used linear sweep voltammetry (LSV) method is not suitable at high current regimes, we use the galvanostatic measurement method to assess the voltage contributed by the intrinsic activities (Vact) and mass transfer diffusion resistance (Vtrans), respectively. The measurements in PEM-based elec-trolyzer also prove that the a-MoSx exhibits superior perfor-mance than Pt at high current densities. The hydrogen plasma treated a-MoSx shows higher stability in time-dependence test compared with Pt catalyst which is notorious for the poisoning from impurities or CO.[31,32] The exploration of such economic, highly efficient, and stable catalysts for high current HER makes the hydrogen production more practical. The calculation in Figure S1 (Supporting Information) con-cludes that a high HER working current density is necessary to produce enough hydrogen in a reasonably short time for home H2-fueling station.

We prepared pristine a-MoSx on carbon cloth (CC) by the thermolysis of ammonium thiomolybdate ((NH4)2MoS4) DOI: 10.1002/smll.201602107

Hydrogen Evolution

A.-Y. Lu, Dr. X. Yang, C.-C. Tseng, Prof. S. Min, Dr. H. Li, Prof. K.-W. Huang, Prof. L.-J. LiPhysical Sciences and Engineering DivisionKing Abdullah University of Science and Technology (KAUST)Thuwal 23955-6900, Kingdom of Saudi ArabiaE-mail: lance.li@kaust.edu.sa

Prof. S.-H. Lin, Dr. J.-L. KuoInstitute of Atomic and Molecular SciencesAcademia SinicaTaipei 10617, Taiwan

Prof. S.-H. LinDepartment of Materials and Optoelectronic ScienceNational Sun Yat-sen UniversityKaohsiung 80424, Taiwan

Dr. C.-L. HsuResearch Center for Applied SciencesAcademia SinicaTaipei 10617, Taiwan

Dr. H. IdrissSABIC Corporate Research and Development (CRD) (KAUST)Thuwal 23955-6900, Kingdom of Saudi Arabia

Large-scale energy harvesting from potentially unlimited energy sources such as wind and solar power is an urgent issue. Hydrogen is a clean and high-density energy carrier for replacing petroleum fuels to relieve issues associated with global warming. A minimum potential of 1.229 eV called reversible potential exists for water electrolysis[1–3] and an extra potential (overpotential) is needed to activate the reac-tion depending on the electrode materials. Water electrolysis in alkaline electrolytes is a matured technology for hydrogen production[1] but its working current density is typically lim-ited to 0.2–0.4 A cm−2. Instead, water electrolysis in acidic electrolytes (proton exchange membrane, PEM) is a high cur-rent density (0.6–2.0 A cm−2) alternative.[4–6] In corrosive and acidic environment, the catalyst requires expensive materials such as IrO2 or RuO2 anodes for oxygen evolution reaction (OER) and Pt or Ru cathodes for hydrogen evolution reac-tion (HER). Although Pt is very costly, it is still widely used as the HER catalyst owing to its high efficiency at all cur-rent densities. Hence, any breakthrough in lower cost, more

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at 150 °C in an H2/Ar environment.[28] Figure 1a schemati-cally illustrates the setup of the remote H2 plasma for hydro-philic treatment and creating defects on pristine a-MoSx. The photos in Figure 1b show the contact measurement and contact behavior of water droplets to the catalysts, where the surface of pristine a-MoSx on carbon cloth is hydrophobic (137°) but it becomes hydrophilic (7°) after H2 plasma treat-ment. The transmission electron microscopy (TEM) images and X-ray diffraction (XRD) results in Figure S2 (Sup-porting Information) prove that the obtained materials are amorphous. Meanwhile, the energy filtered elemental map-ping in Figure S3 (Supporting Information) shows that the amorphous structure exhibits homogeneous elementary distribution.

To evaluate the atomic ratio of a-MoSx, X-ray photoelec-tron spectroscopy (XPS) is adopted to characterize the sur-face bonding structures and estimate the amount of sulfur vacancies. Figure 1 displays the binding energies of Mo and S in the pristine and H2 plasma treated a-MoSx. Since the Mo and S binding energies of interest are partially over-lapped, several fitting criteria are used to resolve these peaks (detailed in the Supporting Information). The pristine a-MoSx exhibits two characteristic peaks at 232.6 and 229.5 eV, attrib-uted to Mo 3d3/2 and Mo 3d5/2 binding energies for MoIV species.[33] A minor contribution of MoV and MoVI is also present.[34] The S 2p1/2 and S 2p3/2 doublets at (163.5 eV; 162.3 eV) and (164.6 eV; 163.4 eV) are assigned to divalent sulfide ions (S2−) and single-valent sulfide ions (S2

2−), respec-tively.[34,35] After H2 plasma treatment, the peak intensity of Mo does not present pronounced change, but the intensity of S 2s shows an obvious decrease. To further estimate the content of sulfur, we compared the normalized S 2p spectra by the Mo intensities in Figure 1d. The intensity of S2− and S2

2− is significantly reduced after plasma treatment, sug-gesting that the active hydrogen atoms/ions effectively react with S atoms and create more S-vacancies in the structure.

The stoichiometric formula for the pristine and H2 plasma treated a-MoSx is determined as MoS3.1 and MoS1.7, respec-tively, indicating that S-vacancies were generated during plasma treatment. The increase in hydrophilicity revealed in Figure 1b is due to the presence of hydrophilic groups (OH−) as revealed by the XPS spectrum in Figure S4 (Supporting Information). The plasma-treated a-MoSx is likely adsorbed with oxygen-containing species such as O2 and H2O when the sample was exposed to the air.

To reveal the catalytic properties of pristine- and sulfur- deficient MoS2, we performed first-principles calculations of the hydrogen-adsorption free energy difference (ΔG) for the HER reaction on the pristine basal plane and the sulfur vacancy sites. Figure 2b summarizes the ΔG for HER of the MoS2 lattice (4 × 4) with 0 (MoS2), 1 (MoS1.9375), 2 (MoS1.875), and 4 (MoS1.75) sulfur vacancies. The hydrogen atom can only be attached on top of the S in defect-free MoS2; however, the ΔG (2.06 eV) is found to be too large and thus the basal plane would be inert. If one S-vacancy is created, H can be attached to the defect site and the HER is activated as the ΔG (−0.07 eV) is close to the ideal ΔG (0 eV). Notably, the ΔG of four S-vacancy model (MoS1.75), with similar chemical composition to the a-MoSx with hydrogen plasma treatment (MoS1.7), is −0.30 eV. With the increasing defect number, the hydrogen adsorption energy becomes stronger and thus ΔG moves further away from zero, causing lower HER effi-ciency. The conclusion drawn from our results is consistent with previous reports,[26,36] where a dangling bond state lies slightly below the Fermi level when a defect point is cre-ated on MoS2, and this state was found to be responsible for hydrogen adsorption on the sulfur vacancies. Note that the simulation on how the S-vacancies on MoS2 basal plane involve on HER reaction may provide insights for the amor-phous MoSx systems.

Figure 3a shows the typical HER polarization curves for MoS3.1 and MoS1.7 catalysts on CC (loading amount

small 2016, DOI: 10.1002/smll.201602107

Figure 1. a) Schematic illustration for the experimental setup of inductively coupled plasma. b) The contact measurement and the photo for contact behaviors of water droplets to pristine and H2 plasma treated a-MoSx. c,d) XPS fitting for the a-MoSx catalysts before and after H2 plasma treatment. c) The pristine a-MoSx exhibits two characteristic peaks at 232.6 and 229.5 eV, attributed to the Mo 3d3/2 and Mo 3d5/2 binding energies for MoIV. The minor portion of MoV (233.6 eV; 230.4 eV) and MoVI (235.7 eV; 232.6 eV) are also presented. d) The S 2p1/2 and S 2p3/2 doublets at (163.5 eV; 162.3 eV) and (164.6 eV; 163.4 eV) are assigned to divalent sulfide ions (S2−) and single-valent sulfide ions (S2

2−), respectively.

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Figure 2. a) The atomic models for proton bonded on MoS2 lattice (4 × 4) with 0 and 2 defects. b) First-principles calculation of hydrogen binding energy for crystalline MoS2 and that with sulfur vacancies. The energy level with different defective level MoS2 plots the Gibbs free energy (ΔG) for the HER reaction (2H+ + 2e− → H2).

Figure 3. HER characteristics for Pt foil, Pt/CC, MoS3.1, and MoS1.7 in 0.5 m H2SO4 solutions. The polarization curves of the catalysts by linear sweep voltammetry method at a) low current density and b) high current density measurement with a scan rate of 1 mV s−1. c) Corresponding Tafel plots for these catalysts, where the slopes at low current regions allow the assignment of HER mechanism (Table S5, Supporting Information). At high current densities the slopes increase and significantly vary with catalysts. d) The electrical double layer capacitance (EDLC) measurement on MoS3.1 and MoS1.7. e,f) The time-dependent performance of Pt foil, Pt/CC, and MoS1.7 for HER stability tests. e) 100 mA and f) 1 A as the starting operated current density at the selected overpotential.

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1 mg cm−2) measured from 0 to 500 mA cm−2 with a scan rate 1 mV s−1, where a platinum mesh is used as the anode. Two standard Pt catalysts, a flat Pt foil (25 μm thick), and the CC deposited with Pt particles (Pt/CC; Pt loading is 1 mg per geometrical area; see the Experimental Section) are also included for comparison. The measured current is normal-ized by the geometrical area of cathodes and the potential is referenced to the reversible hydrogen electrode (RHE) after uncompensated resistance correction (iRs correction, see the Experimental Section). The overpotential required to reach 10 mA cm−2 (η10) is 206 and 143 mV for MoS3.1 and MoS1.7, suggesting that the H2 plasma treatment improves the cata-lytic activity of a-MoSx. However, it is still much larger than 52 mV for Pt foil and 22 mV for Pt/CC. Surprisingly, the HER performance for MoS3.1 and MoS1.7 becomes compa-rable to or better than platinum catalysts with the increasing current density (Figure 3b). The Tafel plot in Figure 3c shows that Pt and Pt/CC exhibit almost ideal values (close to 30 mV dec−1), where the rate determination step is identi-fied as Tafel reaction. The Tafel slope for MoS3.1 and MoS1.7 is 84.2 and 39.5 mV dec−1, respectively, and the rate determina-tion step is governed by the Heyrovsky step. To estimate the effective electrochemical surface area (ESA), we measure the electrochemical double-layer capacitances (Cdl) using the CV method[12,37] (detailed in Figure S6, Supporting Informa-tion) as summarized in Figure 3d. The capacitance of a-MoSx is increased by 37% after hydrogen plasma treatment from 143 mF for MoS3.1 to 196 mF for MoS1.7. Meanwhile, we esti-mate the active site number of a-MoSx by using Cu under-potential deposition method (UPD) (detailed in Figure S7, Supporting Information). In a typical UPD method, the cov-erage density of the monolayer copper on the catalytic sur-face is assumed to be the same as the density of HER active sites. This method has been used to estimate the active site density for Pt,[38] WS2,

[25] and CoP.[14] The active site density of MoS1.7 is estimated to be 1.31 × 1018 and it is higher than MoS3.1 (1.19 × 1018). These results are consistent with the per-formance enhancement in HER of a-MoSx after H2 plasma treatment. Notably, we find that the polarization curves in Figure 3b show a fluctuated signal at high current regimes especially for Pt catalysts. The Tafel slope of Pt and Pt/CC drastically increases to 764 and 598 mV dec−1 while MoS1.7 still maintains a slope at 112 mV dec−1 in Figure 3c, indicating that other factors come into play at a high current density. Since the internal resistance dominates the LSV curves at high current densities due to the large inaccuracy of internal resistance calibration, we perform the galvanostatic full cell measurements to further investigate this phenomenon.

Another very critical factor for HER is the catalytic sta-bility. Figure 3e,f reveals the HER stability for the MoS1.7, Pt foil, and Pt/CC operated at around 100 mA cm−2 and 1 A cm−2, respectively. Both Pt foil and Pt/CC exhibit obvious clear HER efficiency degradation within few hours. The degradation for Pt is due to the known poisoning phenomenon from impurities and CO in solutions.[31,32] This was not the case for MoS1.7 indi-cating its excellent stability even at a high current density.

To evaluate the high current HER performance for var-ious catalysts, Figure S8 (Supporting Information) illustrates a schematic for the proposed measurement setup using the

galvanostatic method. An Ag/AgCl reference electrode is added in between a cathode and an anode (Pt mesh) to monitor the potential of HER and OER simultaneously. For galvanostatic measurement, we recorded the voltage differ-ences between the cathode and anode, the HER and OER potentials (vs Ag/AgCl) are then converted to the voltages versus RHE,[39,40] presented as VHER and VOER, respec-tively, in the following discussions. The reversible voltage is 1.229 V (Vrev at 25 °C)[2,41] based on thermodynamic calcu-lations. Figure 4a shows the applied total voltage (Vtotal) as a function of measured cathodic current density, where the Vtotal is the sum of VOER, VHER, and Vrev. In our study, a large surface commercial Pt mesh (4 cm2) is used as the anode and thus the obtained VOER is almost identical for the four different cathodes. The voltage consumed by the cathode is known as VHER = Vohm + Vact + Vtrans.

[4,41,42] From the overall voltage consumption, we observe that the cell with MoS1.7 cathode is higher than those with Pt cathode at low current densities, whereas it becomes lower at high current densities (>1 A cm−2). The following analysis reveals that the excellent HER performance for MoS1.7 is due to its low Vtrans.

The Vohm at different HER current (i) is expressed by the Ohmic equation Vohm = i × Rs, where Rs is the uncompensated resistance including solution resistance and electrical circuit resistance. The obtained Vohm values are almost the same for all catalysts plotted as the dashed lines in Figure 4b. Thus, we can eliminate the factors contributed from the measurement system.

The Vact is the activation voltage of HER which can be expressed by the Butler–Volmer equation[43,44] (Equation (1)), and the solid curves in Figure 4b plot the Vact as a function of current density j[42,44]

sin2act

1

0V RT h

jjαν=

−

(1)

where j0 is the cathode exchange current density (A cm−2), ν is the stoichiometric coefficient of electrons in cathode reaction, and α is the charge transfer coefficient. The j0 for various cathodes is obtained from the fitted Tafel curves at Tafelian regimes in Figure 3c. The fitting results are detailed in Table S4 (Supporting Information). The charge transfer coefficient (α) for Pt foil and Pt/CC is defined as 2 (Tafel mechanism). For a-MoSx based catalysts following the Hey-rovsky mechanism, the charge transfer coefficient can be cal-culated by using equation α = 1 + β, where β is the symmetry factor as discussed in Table S5 (Supporting Information). The symmetry factor can be extracted from the Tafel slope differ-ence between electrochemical impedance spectroscopy (EIS) measurement (AC method) and linear sweep voltammetry (LSV) measurement (DC method) (detailed in Figures S9 and S10, Supporting Information). The charge transfer coeffi-cients of MoS3.1 and MoS1.7 are estimated as 1.486. Figure 4b compares the activation overvoltage, where the results sug-gest that the intrinsic electrochemical property for Pt cata-lysts is better than MoSx as expected.

The Vtrans is plotted in Figure 4c using the equation Vtrans = VHER − Vact − Vohm. As stated before, the Vtrans is a good gauge for mass transfer caused by H2 bubbles. At a high HER current density, the Vtrans for MoS1.7 is at least

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0.2 V lower than those of the other catalysts. The origin for the outstanding HER performance of MoS1.7 is its low Vtrans. The results are in-line with the observation of H2-bubbles formation on catalysts. The video clips in the Supporting Information demonstrate that at a high HER current den-sity the actual bubbles formed are small and the process of releasing H2 bubbles from MoS1.7 is fast due to the presence of hydrophilic functional groups while large bubbles obvi-ously stick on Pt surfaces, resulting in lower electrolysis effi-ciency.[45,46] We conclude from the measurements that the H2-bubble transport is a performance-limiting factor for Pt at high current densities.

We have also performed the HER measurements for various catalysts in a PEM-based electrolyzer, with the setup is illustrated in Figure 5a. The polarization curves for MoS1.7 and Pt/CC catalysts measured from 0 to 1 A cm−2 with the scan rate of 2.5 mA s−1 are shown in Figure 5b, and where the RuO2 deposited on CC is used as the cathode. The Pt/CC shows an excellent catalytic activity at a low current den-sity (<200 mA cm−2). However, when the current density is higher than 0.2 A cm−2, the required voltage for hydrolysis is higher than the cell with MoS1.7. At 1 A cm−2 the applied voltage for MoS1.7 is 2.76 V, superior to that of Pt/CC catalyst (3.05 V). To eliminate the factor of Ohmic loss in the elec-trolysis system, the resistances (Rs) were extracted from the EIS measurements in Figure 5c. It is noted that the results in PEM-based delectrolyzer are consistent with the con-clusion drawn from galvanostatic measurements that the

MoS1.7 catalyst is a promising material to replace Pt for large-scale application.

As a proof-of-concept demonstration for the solar pow-ered electrolysis, we use two commercially available 20 W panels to perform the high current electrolysis on a 1 cm2 sized cathode and a large Pt foil as anode to avoid the limi-tation of oxygen evolution reaction (detailed in Figure S11, Supporting Information). In Figure S12 (Supporting Informa-tion), the current density for Pt foil, Pt/CC, and MoS1.7 is 1.45, 1.49, and 1.97 A cm−2, and the actual H2 production rate is 12.34, 12.54, and 16.8 mL min−1, respectively. The result again proves that the MoS1.7 is an excellent candidate to replace Pt cathodes for actual hydrogen production.

In summary, we report a method to create sulfur vacancies on a-MoSx by using hydrogen plasma to enhance the catalytic and surface properties. It imposes mild surface modification and efficiently change the surface chemical bonding struc-tures as evidenced by the XPS measurements. Meanwhile, plasma treatment is a commercial technique in semicon-ductor industries, suggesting that hydrogen plamsa method can be utilized for large-scale application. The hydrogen plasma is a scalable way to create defects but without dam-aging the structure. The overpotential (η10) of a-MoSx is decreased from 206 to 143 mV after plasma treatment due to the increase in active site density. This work also reports the methodology of combining potentiostatic, Galvanostatic methods, and EIS to quantify the overvoltage consumed by activation, ionic transport/current collector, and H2 transport.

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Figure 4. Galvanostatic measurements for Pt foil, Pt/CC, MoS3.1, and MoS1.7 catalysts in water splitting. a) The reconstruction of the I–V curve divided to the reversible voltage, OER voltages (VOER), and HER voltages (VHER). b) The polarization curve of HER with iR compensation (dashed curve) and the simulation of activation overvoltage for HER by Bulter–Volmer equation (line curve). c) The voltages of mass transport (Vtrans) as a function of HER current density. The Vtrans is dominated by the gas bubble blocking which is reducing the surface coverage of electrode.

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This approach is valuable for exploring the catalytic behav-iors of various electrocatalytic systems at high working cur-rent densities. The measurements in PEM-based electrolyzer also prove that the a-MoSx exhibits superior performance than Pt. We believe the hydrophilic amorphous molybdenum sulfide is a cheap alternative, which exhibits superior catalytic and stability performance than Pt at high current densities.

Experimental Section

Thermolysis to Form Pristine a-MoSx Catalyst: The MoSx cata-lysts were synthesized by drop-casting 45 μL of ammonium thio-molybdate solution (5 wt% of (NH4)2MoS4 in dimethylformamide (DMF)) onto a commercial CC (1 cm × 1.5 cm). The carbon cloth coated with (NH4)2MoS4 was dried on a hot plate at 100 °C for 10 min. Afterward, the samples were annealed in a CVD furnace with H2/Ar flow (500 Torr; H2:Ar = 20:80) at 150 °C for 1 h to evap-orate DMF solvent and form the pristine a-MoSx (MoS3.1) catalysts. The loading amount of a-MoSx on CC is around 1 mg cm−2, which is comparable with the 1 mg cm−2 used for Pt on CC.

Hydrogen Plasma Treatment: The hydrophilic treatment on a-MoSx/CC catalysts was performed by using an inductively cou-pled plasma (ICP) system with a commercial 13.56 MHz RF source. The samples were placed in a 1 in. quartz tube at a distance of 10 cm from the center of coil at the downstream side. The chamber was vacuumed to the base pressure of 3 mTorr. The active hydrogen atoms/ions were generated in a quartz tube by flowing 20 sccm high purity hydrogen (99.999%) gas with power of 50 W and pressure of 100 mTorr at room temperature for 30 min. After the hydrogen plasma treatment, the samples were exposed to the atmosphere condition.

Preparation of Pt/CC Catalysts: The Pt/CC used was prepared by electrochemical deposition of Pt on cleaned carbon cloth from

the K2PtCl6 solution at 1 mA. The Pt loading was controlled by the deposition time and the Pt loading amount for the reference sample was controlled at 1 mg per unit geometrical area.

Electrochemical Measurements: The electrochemical measure-ments were performed in a PGSTAT 302N Autolab Potentiostat/Galvanostat (Metrohm) at room temperature. High surface area Pt foil and Ag/AgCl (in saturated KCl) electrodes were used as the counter and reference electrodes, respectively. The HER activity was studied by measuring polarization curve with LSV potentio-stat method at 1 mV s−1 in 0.5 m H2SO4 electrolyte solution. The electrolyte solution was purged with high purity Ar for 30 min prior to the measurements. The stability tests were examined by using chronoamperometry method to record the current changes with time at a fixed overvoltage. The EIS was performed using the FRA Impedance Potentiostat modules at the voltage of OCP to 500 mV. The amplitude of ac signal was 10 mV. Impedance was measured in the range of 104–0.01 Hz with 10 point dec−1.

iR Correction: The voltage consumption was induced during the potentiostat measurement due to an uncompensated resistance (Rs) between the reference electrode and the working electrode (cath-odes). This uncompensated resistance includes solution resistance and electrical circuit resistance. Through EIS measurements and fit-ting with suitable models as described in the Supporting Informa-tion and Figure S9 (Supporting Information), an accurate value of Rs can be determined at high frequency region (104–103 Hz), where the behavior of the capacitances is like an effective short circuit. The voltage consumption by Ohmic loss can be described by Ohmic’s law

V i ROhm s= ⋅ (2)The level of uncompensated resistances in the measured

system is around 1.3–1.6 Ω.Galvanostatic Measurement: The electrochemical activities of

cathodes in water splitting were evaluated using a power supply and two high impedance voltage meters in a 0.5 m H2SO4 solution

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Figure 5. The electrochemical performance for MoS1.7 and Pt/CC catalysts in a PEM electrolyzer. a) Schematic illustration for the PEM electrolyzer for water splitting. b) The polarization curves for the MoS1.7 and Pt/CC catalysts measured from 0 to 1 A cm−2. c) EIS measurements for the electrolyzer on the MoS1.7 and Pt/CC catalysts.

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at 25 °C. The power supply (PGSTAT 302N Autolab, Metrohm) was used to apply a full cell voltage between the anode and cathode and record the current using a two-terminal linear sweep voltam-metry galvanostatic method at a scan rate of 2.5 mA s−1. High sur-face area Pt foil and Ag/AgCl (in a saturated KCl solution) were used as the anode and reference electrodes, respectively. The open-circuit voltages (OCV) of cathodes and anodes were monitored in a VPM3, Bio-logic with separate channels every second during the measurements. Measured voltages in V versus saturated Ag/AgCl were converted to RHE. The solution resistance between the cathode and reference was measured by using a module in PGSTAT 302N (Autolab, Metrohm).

PEM Electrolyzer: The electrolysis tests were performed in a home-made PEM electrolyzer cell flowing with 0.5 m H2SO4 solution as the electrolyte at 45 °C. The electrolyzer is composed of a proton exchange membrane, two catalysts coated carbon clothes, and two titanium current collectors. Nafion117 membrane (175 μm) was used as the proton exchange membrane. The Nafion117 mem-brane was treated in 0.5 m H2SO4 for 1 h at 80 °C to activate the membrane and then rinsed by deionized water before use. Ruthe-nium oxide nanoparticles (RuO2) were used as the anode catalyst. The catalyst was synthesized by annealing ruthenium chloride solution (20 mg RuCl3 in 500 μL isopropanol) at 350 °C for 5 h on carbon paper (CP) with the loading amount of 2 mg cm−2. Two cur-rent collectors were made of titanium with flow channels to avoid corrosion in an acid environment. The power supply (PGSTAT 302N Autolab, Metrohm) was used on the electrolyzer to measure the polarization curves and EIS. The polarization curves were meas-ured using the LSV method with a scan rate of 2.5 mA s−1.

RHE Calibration: The calibration of reference electrode (satu-rated Ag/AgCl) was measured in 0.5 m sulfuric acid with saturated hydrogen gas.[16,47,48] Two Pt wires were used as working and counter electrodes. The cyclic voltammetry curve was scanned in the range between 0 and −0.25 V with 5 mV s−1 scan rate in Figure S13 (Supporting Information). The voltages at zero current position are the equilibrium voltage of hydrogen adsorption (for-ward) and desorption (backward) on platinum electrodes. The average of two equilibrium voltages is the experimental voltage of RHE.

Characterizations: The field-emission scanning electron micro-scope (FESEM, FEI Quanta 600) was used to observe the surface morphology of the catalysts. The nanoscale crystal structure was revealed by a transmission electron microscopy (FEI Titan ST, operated at 300 KV). The crystalline structure of the samples was analyzed by XRD (Bruker D8 Discover diffractometer, using Cu Kα radiation, λ = 1.540598 Å). XPS studies were carried out in a Kratos Axis Ultra DLD spectrometer equipped with a monochro-matic Al Kα X-ray source (hν = 1486.6 eV) operating at 150 W, a multichannel plate, and delay line detector under a vacuum of 1 × 10−9 mbar. The survey and high-resolution spectra were col-lected at fixed analyzer pass energies of 160 and 20 eV, respec-tively. Binding energies were referenced to the C 1s peak (set at 284.4 eV) of the sp2 hybridized (CC) carbon from the sample.

Density Functional Theory Calculations: Density functional theory calculations were performed using Vienna ab initio simula-tion package (VASP).[49] Perdew–Burke–Ernzerhof (PBE) functional was adopted with DFT-D2 van der Waals correction.[50] The Gibbs free energy difference[51] for 4 × 4 supercells was calculated, with energy cutoff of 460 eV and k-point mesh 4 × 4 × 1. A vacuum

spacing of 15 Å was used to avoid the artificial interlayer coupling for periodic boundary calculations. The crystal structures were relaxed with Hellman–Feymann force smaller than 0.01 eV Å−1.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

A.-Y.L. and X.Y. contributed equally to this work. L.J.L. thanks the support from KAUST, SABIC (Saudi Arabia), Academia Sinica, MOST (Taiwan) under Grant Nos. NSC102-2119-M-009-002-MY3 and AOARD FA23861510001 (USA).

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Received: June 26, 2016Revised: July 25, 2016Published online:

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