Electrochemical Formation of Amorphous Molybdenum

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Electrochemical Formation of Amorphous Molybdenum Phosphosulfide for Enabling the Hydrogen Evolution Reaction in Alkaline and Acidic Media Ummul K Sultana, and Anthony Peter O'Mullane ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00489 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Electrochemical Formation of Amorphous Molybdenum Phosphosulfide for Enabling the Hydrogen Evolution Reaction in Alkaline and Acidic Media Ummul K. Sultana† and Anthony P. O’Mullane*,† School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), Brisbane, QLD 4001, Australia

KEYWORDS: Molybdenum phosphosulfide, hydrogen evolution reaction, water splitting, amorphous materials, electrocatalysis, electrodeposition

ABSTRACT: The formation of a non-precious metal catalyst that is active for the hydrogen evolution reaction (HER) over a wide pH range is of particular interest. In this work a mixed anion electrocatalyst, namely amorphous molybdenum phosphosulfide films containing oxygen were produced via a simple electrochemical process allowing the composition and morphology of the film to be controlled. The catalyst shows a homogeneous distribution of Mo, S, P and O with a high number of active sites due to its amorphous state. (NH4)2MoS4 was used as a molybdenum and sulfur source while NaPO2H2 was used as the phosphorus source. Thiourea was also investigated as an additional sulfur source during the electrodeposition process. The electrocatalyst was active for the HER in acidic media as expected, but significantly showed excellent performance in alkaline media where Tafel slope values of 36 mV dec-1 and 122 mV dec-1 were recorded respectively. In addition the electrocatalyst demonstrated long term stability in both media for several hours. The materials were characterized using scanning electron microscopy (SEM), high resolution transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and their physicochemical properties correlated to their electrocatalytic activity for the HER.

INTRODUCTION Enabling intermittent renewable energy to be stored is paramount for realising the large scale adoption of wind and solar energy and ultimately relinquishing our reliance on fossil fuels. Storing energy chemically as a fuel such as hydrogen is attractive as it is scaleable and can in principle be utilized domestically or integrated into the grid at a larger scale. Although hydrogen can be produced in many ways, doing so in an environmentally friendly manner is of great importance. Therefore, research into the feasibility of the hydrogen economy as first postulated by Bockris1 has gained significant momentum. Hydrogen is particularly attractive as it can be generated using renewable energy sources, stored via liquefaction or in other carriers such as ammonia and utilized in fuel cells to regenerate clean electricity. Therefore electrochemical water splitting has received a significant amount of attention and can be considered as one of the most sustainable methods of producing hydrogen.2 The electrochemical generation of hydrogen from water is highly effective at precious metals such as Pt, Rh and Ir,3 but the large scale adoption of water electrolysis will be impeded by the scarcity and cost of these catalyst materials. Therefore there is a large ongoing research effort to replace precious metals with more earth abundant alternatives. However, it is quite challenging to develop such materials that are active and able to sustain prolonged periods

periods of electrolysis, and therefore is of particular interest to the research community as well as industry.4-7 Many earth abundant materials have been investigated, including transition metal oxides and hydroxides,8-9 chalcogenides,10-11 carbides,12 and phosphides.6, 13 In particular MoS2 has been investigated as it was one of the first non-precious metal based materials that rivalled Pt for its activity for the hydrogen evolution reaction (HER) in acidic conditions where edge sites were postulated to be the active site.14-15 Researchers have since focused on the morphological and physical improvement of this catalyst by studying strained-exfoliation,16 porous structure formation17 quantum dots,18 clusters,19 vertically aligned films,20 amorphous films21 and nanoflowers.22 Molybdenum based phosphides have also been identified as active catalysts for the HER with low overpotentials.23-25 However it has been reported that poor interparticle electron transport due to the lack of conductivity in MoS2 nanostructured materials is a limiting factor for the HER.26 Therefore doping MoS2 with a variety of other elements has also been investigated for improving the electronic and catalytic performance of this material. Xie et al. doped oxygen into ultrathin MoS2 nanosheets which increased the density of active sites as well as introduced essential conductivity for better HER activity.27 Zhang et al introduced Cl into amorphous MoSx films to further enhance the defect density and perturb the electronic structure of the molybdenum sulfide which resulted in better HER perfor-

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mance than the non-doped material. Deng et al. reported that single Pt metal atom doping also increases the activity of S atoms in the basal plane of MoS2.28 Increased interlayer spacing was also found to be a good approach for enhanced HER activity since it increases the active sites as well as modifies the electronic structure which optimizes the free energy of hydrogen absorption.29-30 Jaramillo et al. took a nonmetal doping approach to enhance the activity of MoP film by introducing sulfur into the structure where it showed excellent performance.24 Ye et al. investigated the mechanism of this mixed anion catalyst by optimizing the S and P contents in a phosphor-sulfide system.14 They showed that P could be used for modifying the electron transport properties, hence changing the overall HER performance. For synthesizing these sulfide or phosphide based HER catalysts most researchers have adopted a two-step preparation method along with harsh post treatments like sufidation,24 or high temperature annealing.14 The applicability of metal phosphsulfides to the HER in acidic electrolyte has also been demonstrated at cobalt phosphosulfide31 and iron phosphosulfide32 showing the effectiveness of mixed anion catalysts for this reaction. However, to date there has been no report on using an electrochemical approach to fabricate a mixed anion electrocatalyst as many researchers have focused on developing electrocatalysts with two metal centres rather than two anion centres. In addition the majority of MoS2 based materials are crystalline, only active in acidic solution and generally degrade rapidly under conditions of high pH. Developing a HER electrocatalyst that is also effective in alkaline electrolyte is highly attractive as many earth abundant catalysts are already available for the other half of the water splitting reaction used in alkaline electrolysers, namely the oxygen evolution reaction.33-34 Therefore, metal phosphides are of interest given their activity in electrolytes of high pH.6 In principle combining the chemical properties of molybdenum sulfide and molybdenum phosphide should result in a catalyst that is active over a wide pH range. Here, we use an electrochemical approach to electrodeposit amorphous and homogeneous molybdenum phosphosulfide films in a single step. This electrocatalyst demonstrates excellent HER activity under both alkaline and acidic conditions while also being highly stable for longer periods of electrolysis. The major advantage of this method is that it can be performed easily in a single step which does not require posttreatment, is scalable and also does not require an expensive set up which can be undertaken under ambient conditions.

EXPERIMENTAL SECTION Chemicals. Ammonium tetrathiomolybdate, thiourea, sodium hypophosphite, sodium acetate and sodium hydroxide (SigmaAldrich) were used as received without any further purification. Sulfuric acid (96%) was purchased from EMD chemicals. For each experiment and other washing purposes Milli-Q water (resistivity of 18.2 MΩ cm) from a Millipore Milli-Q system was used. Analytical grade chemicals and Milli-Q water were used for preparing all electrolyte solutions. Electrodeposition of the MoSx based film. All the experiments were conducted using a conventional three electrode system. While depositing on glassy carbon (GC), the working electrode was glassy carbon (area was 0.071 cm2, Bioanalyti-

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cal Systems, Inc.), platinum (Bioanalytical Systems, Inc.) was the counter electrode and Ag|AgCl (3 M KCl, Bioanalytical Systems, Inc.) was the reference electrode. Some electrodeposition experiments were also performed on indium doped tin oxide (ITO) films (Delta Technologies 4 - 10 Ω/sq) and were cut into 1 cm × 1 cm pieces purely for electron microscopy imaging. All HER experiments were performed with the materials on a GC electrode. For the experiments undertaken with Pt a Pt disk electrode (5 mm diameter, Pine instruments) was used. All electrochemical experiments were conducted using a BioLogic VSP workstation operated by EC-lab software (version 10.44). Before every experiment the exposed area of the GC working electrode was wet polished with 0.3 µm-sized alumina powder on a micro cloth and was rinsed with Milli-Q water. Before performing any electrochemical experiment the solutions were purged with nitrogen gas for 10 min. The pH of the electrolyte solutions were measured by a Metrohm 826 pH meter. For the electrodeposition experiments the electrolyte consisted of the species outlined in Table 1 where sodium acetate was used as the supporting electrolyte. The pH of the electrolytes was 7.1. Electrodeposition was carried out by repetitive cyclic voltammetry where the cycle number was varied and optimized to give the best HER performance. After each deposition the deposited films were washed with Milli-Q water and dried with pure nitrogen gas to remove any electrolyte solution from the surface. A schematic showing the experimental setup is shown in Scheme 1.

Scheme 1: Electrochemical approach to synthesizing MoSxPy materials. For the electrocatalytic studies, namely the HER performed using linear sweep voltammetry or constant current electrolysis at 10 mA cm-2, the electrolyte was 1 M NaOH. A carbon counter electrode (inert graphite rod 1 mm diameter, Johnson Matthey Ultra ‘‘F’’ purity grade) was also used for the HER studies where no difference was seen compared to the use of a Pt counter electrode under both linear sweep voltammetry (Figure S1) and constant current electrolysis conditions (Figure S2). Electrochemical Impedance Spectroscopy (EIS) was carried out over a frequency range of 10 mHz to 200 KHz with an amplitude of 5 mV. The potential chosen for the EIS measurements was based on the potential that was required to facilitate the HER at -3 mA cm-2. For all the electrochemical data the potential was converted to the RHE scale via the fol-

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lowing formula, ERHE = EAg/AgCl + 0.059 × pH + 0.197 V. The current density reported in this work was normalized to the geometric surface area of the underlying electrode. The composition of different deposition electrolytes and corresponding sample identifications are listed in Table 1. Surface characterization. X-ray photoelectron spectroscopy data were collected using an Omicron Multiscan Lab Ultra-high Vacuum Scanning Tunnelling Microscope (UHVSTM) where a 125 mm hemispherical electron energy analyser was incorporated. Non-monochromatic Mg Kα (1253.6 eV) Xray source (DAR 400, Omicron Nanotechnology) was used for XPS experiments and the incident angle was 65° to the surface of the sample. The analyser passed energy of 50eV with steps of 0.5eV and the dwell time was 200 ms. High-resolution scans with a narrow region for Mo 3p, S 2p, P 2p, C 1s and O 1s were taken at 20 eV pass energy, 0.1 eV steps and with a 200 ms dwell time. Besides a wide scan of low binding energy region was performed from 250eV to 0eV swept at high resolution. The base pressure in the analysis chamber was 1.0 × 10-9 torrs and the pressure was 1.0 × 10-8 torrs during the measurement. Atomic compositions of the surface were calculated using the CasaXPS version 2.3.15 software and a linear baseline with Kratos library Relative Sensitivity Factors (RSFs). EDX was performed on a FEI Quanta 200 Environmental SEM at an operating voltage of 25 kV. Samples were 100 nm thick when prepared by this electrodeposition method. X-ray diffraction (XRD) patterns of the sample were collected using a Philips PANalytical X’pert Pro diffractometer. CuKα radiation (λ= 1.5418 Å) and a fixed power source (40 kV and 40 mA) were used. Images were captured using a FEI Quanta 200 Environmental SEM at an operating voltage of 5 kV. HRTEM images were taken using a JEOL 2100 instrument at 200 KV. A high-sensitivity silicon drift X-ray detector for more accurate compositional analysis and a Gatan Orius SC1000 CCD camera is equipped for better image acquisition Table 1: Sample identification and corresponding electrolyte compositions. Sample ID

Chemicals and concentration in molarity (NH4)2MoS4

TU

NaPO2H2

NaOAc

S1

0.005

-

-

0.1

S2

0.005

0.005

-

0.1

S3

0.005

-

0.5

0.1

S4

0.005

0.005

0.5

0.1

S5

0.005

0.005

0.05

0.1

S6

0.005

0.005

1.0

0.1

at -0.25V with a preceding shoulder and can be attributed to the cathodic deposition of molybdenum sulfide films via,35 [MoS4]2- +2H2O + 2e- → MoS2 +2HS- + 2OH-

(1)

Upon the addition of 5 mM thiourea (S2) as an additional sulfur source, which was used to change the composition of the film, the cathodic peak shifted to a slightly more negative potential and increased in intensity. Thiourea has been employed previously to electrochemically generate other metal sulfide films such as CoSx whereby S2- can be produced via,3637

CS(NH2)2 + 2OH- →

S2- +(OC(NH2)2 +H2O (2)

which clearly perturbs the cyclic voltammetric response compared to sample S1. For the electrolyte containing 5 mM (NH4)2MoS4 and 0.5 M of hypophosphite as a phosphorous source (S3) a cathodic peak was observed at 0 V followed by a broad response from -0.1 to -0.4 V where the latter is due to molybdenum sulfide formation. The peak at 0 V is attributed to the formation of molybdenum phosphide and is consistent with metal phosphide formation such as NiP where the P source is from hypophosphite. It has been determined that PH3 is generated during the reduction process which reacts with metal cation species, in this case any Mo cations, to generate metal phosphide.38 CoP films have also been prepared by cathodic electrodeposition using the same phosphorous source.39 When both thiourea and sodium hypophosphite were introduced into the solution (S4) both cathodic processes were significantly enhanced as well as the appearance of more defined anodic processes where the latter is consistent with metal phosphide oxidation.38 In previous work repetitive potential cycling was shown to be an effective method for producing high quality films that were active for the HER as reported by Merki40 and Vrubel.35 It is also used as an approach to generate nanostructured electrode surfaces with high surface area and increased number of active sites.41-42 Therefore this protocol was undertaken here for all samples and an example of the deposition of sample S4 that includes the additional sulfur and phosphorous sources is shown in Figure 1b. Upon repetitive potential cycling, the magnitude of the cathodic and anodic processes increase indicating the growth of a film on the electrode surface that is accessible to the electrolyte and is conducting, thereby allowing continuous growth to occur. The presence of sulfur and phosphorous in sample S4 was confirmed by XPS and EDS mapping during TEM as discussed in detail later. This shows that an electrochemical approach can be used to create a mixed anion metal catalyst in only one step.

RESULTS AND DISCUSSION The electrochemical deposition of MoSx films can be undertaken via a repetitive cycling protocol. Figure 1 shows the cyclic voltammetric response recorded at a GC electrode in four different solutions. When the electrolyte contained only 5 mM (NH4)2MoS4 (S1), a distinct cathodic peak was observed



Figure 1. Cyclic voltammograms recorded at a GC electrode at 50 mV s-1 (a) in 5 mM (NH4)2MoS4 without (S1) or with 5 mM of Thiourea (S2), with 0.5 M NaPO2H2 and 0.1 M of C2H3NaO2 (S3) and the mixture of all components (S4). (b) 1st, 5th, 10th, 20th, 30th and 50th deposition cycles recorded for sample S4 at a scan rate of 50 mV s-1. The HER was then studied for each sample initially in 0.5 M H2SO4 to identify the best composition and identify whether the incorporation of phosphorous was beneficial to the reaction. Acidic conditions were chosen as MoSx based materials show high activity and long term stability in this electrolyte.5, 27, 35, 43 Figure 2a shows polarization curves recorded for samples S1, S2, S3 and S4 where sample S4 shows the lowest onset potential and highest current density over the potential range investigated. A current density of 10 mA cm-2 was achieved at an overpotential of only 200 mV while a current density of 100 mA/cm2 could be reached at an overpotential of 275 mV. A low Tafel slope of 36 mV dec-1 was found (Figure 2b) which is lower than the samples that did not contain phosphorous. This value is comparable to that achieved at Pt (30 mV dec-1) and indicates an efficient electrocatalyst. When compared to a Pt electrode, the performance in terms of onset potential and the current density attained at low overpotentials is lower (Figure S3) in both acidic and alkaline media. However, in alkaline conditions sample S4 performs better than Pt at higher current densities (> 50 mA cm-2). It has been established that the activity of Pt for the HER in alkaline electrolyte is much weaker than in acidic medium.44 This is due to the low efficiency of water dissociation on the surface of Pt in alkaline medium. Therefore the data obtained here indicates that the molybdenum phosphosulfide material may facilitate this reaction step at higher current densities compared to Pt. It should be noted that the number of deposition cycles was optimised for the best performance and was found to be 50 cycles which resulted in films with a thickness of ca. 140 nm. The HER can be described via the following processes whereby initially there is a discharge step, Volmer: H3O+ + e- → Hads + H2O

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The Tafel slope value of 36 mV dec-1 indicates that the reaction pathway may proceed via the Heyrovsky or Tafel steps being rate limiting.45 Given that the presence of phosphorous enhances the HER activity of MoSx, the concentration of the phosphorous source in the electrolyte was varied to find the optimum composition (samples S4, S5 and S6). In this case there was no significant difference in performance when the hypophosphite concentration was varied from 0.05 to 1.0 M. However it should be noted that when 0.5 M hypophosphite is used in the absence of the additional sulfur source from thiourea (sample S3) then the activity was diminished. Therefore the presence of thiourea in the electrolyte is also critical to good HER performance. An important consideration for these materials is their long term stability under electrolysis conditions. Sample S4 was tested at 10 mA cm-2 for 20 h (Figure 2c) where it can be seen that the overpotential only increased marginally in that time frame. Linear sweep voltammetry performed after this test showed only a slight decrease in the current density. Although MoSx based materials have been extensively reported for the HER, it is almost exclusively in acidic conditions. There are very few reports of MoSx materials being active and stable in alkaline media. Generally hybrid systems need to be used such as Ni-P/MoSx,46 Co-Mo-Sx chalcogels,47 CoS doped β-Co(OH)2@MoS2+x48, MoS2 grown on graphene/Ni foam,49 where the presence of the second metal phosphide or sulfide is key to HER activity in alkaline media. Although metal phosphides are known to be active in these conditions previous reports on MoSx catalysts containing phosphorous have only reported data for the HER in acidic media. However recently, Li et. al.43 reported that crystalline MoS2 films produced by chemical vapour deposition could be activated into more effective HER catalysts via potential cycling (8000 cycles) in acidic solution or treatment in acidic solutions of trifluoromethanesulfonimide. This resulted in proton intercalation between MoS2 that increased the activity of the active sites but also resulted in activity in solutions of pH 12. The intercalated protons were found to be stable and not neutralized as the OH- ions were determined to be too large to penetrate the MoS2 layers. The presence of the protons was postulated to induce a p-doping effect and change the electronic nature of the active site as well as increase the electrical conductance of MoS2. In this work we achieve comparable HER performance (Figure 2d) to Li’s material in terms of onset potential and current density which does not require any activation or chemical pretreatment. The performance is significantly better than MoS2 grown on a Ni foam support with graphene where a potential of -0.6 V vs RHE was needed to provide a current density of ca. 4 mA cm-2.49

(3)

which can be followed by an electrochemical process (Heyrovsky step) or recombination (Tafel step) to generate hydrogen gas.45 Heyrovsky: Hads + H3O+ + e- → H2 + H2O Tafel: Hads + Hads → H2

(4)

(5)


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Figure 2. Linear sweep voltammograms (LSV) recorded at a scan rate of 5 mV s-1 in 0.5 M H2SO4 (a) for samples S1-S6, (b) corresponding Tafel plots for samples S1-S4, (c) sample S4 after 20 hours of electrolysis at a current density of 10mA/cm2, (d) LSVs of sample S1 and sample S4 before and after 20 hours of electrolysis at 10 mA/cm2 in 1 M NaOH and Nyquist plot for samples S2-S4 recorded in (e) 0.5 M H2SO4 and (f) 1 M of NaOH. In comparison to sample S1 that does not contain phosphorous the activity is significantly enhanced demonstrating the importance of phosphorous to the activity in alkaline solution. The catalyst is also stable over 20 h of electrolysis at 10 mA cm-2 with even a slight decrease in the overpotential with time. Linear sweep voltammetry performed after the electrolysis experiment showed an improvement in activity which was not observed to as significant an extent in acidic solution (Figure 2d). The origin of this phenomenon requires further investigation but the effect has also been reported for CoSx materials which were held at a constant potential in neutral pH for several hours to improve HER activity.50 This enhanced stability is excellent when compared to samples S1 and S2 which both show a dramatic decrease in HER activity after only one cycle (Figure S4). A Tafel slope of 122 mV dec-1 (Figure S5) indicates a change in the HER mechanism compared to the reaction in 0.5 M H2SO4 and is now consistent with the Volmer step being the rate determining step. A comparison of sample S4 with previously reported MoSx and MoP materials for the HER as well as the limited reports of metal phosphosulfides activity in acidic solution is provided in Table S1. It is clear from this data that the molybdenum phosphosulfide generated here is highly advantageous compared with those materials as

it demonstrates activity and importantly stability over a very wide pH range. Illustrated in Figure 3a is a TEM image of sample S4 which was prepared via sonication of the film into a solution to allow for imaging in the microscope. Also shown in Figure 3b is the EDX elemental mapping which shows a homogeneous distribution of Mo, S and P as expected but also the presence of oxygen. The incorporation of oxygen is interesting as it has been reported that oxygen doped MoS2 films is also an excellent material for the HER.51 The presence of oxygen introduces disorder into MoS2 which not only creates more active sites but also increases electric conductivity. The chemical composition of the samples was then investigated via X-ray photoelectron spectroscopy (XPS) to probe the surface structure that influences the electrochemical activity (Figure 3c-f and Figure S6 and S7) and is summarized in Table 2. The atomic ratio of Mo : S : P was found to be 1 : 1.56 : 0.58 for sample S4 which strongly indicates that P replaces some of the sulfur atoms in a MoS2 material. EDX analysis using the TEM for this sample gave a ratio of Mo : S : P of 1 : 1.44 : 0.47. For samples S1 and S2 the composition is closely related to MoS2 and the S content only increase marginally when thiourea is present in the electrolyte (S2). This is also reflected in the comparable HER activity (Figure 2a). A clear difference in the P concentration can be seen for samples S3 and S4 even though the amount of hypophosphite was kept constant in the electrolyte. Therefore there appears to be a significant interplay between thiourea and hypophosphite in solution which allows for more P incorporation at the expense of sulfide formation which is also reflected in the increase in current seen for the electrodeposition process (Figure 1a). At present it is unclear why this is the case and is the subject of further study. However it is clear that the incorporation of P into the MoSx material is highly beneficial for the HER in both acidic and alkaline media (Figure 2a and d).

Figure 3. (a-b) Elemental mapping and (c-f) X-ray photoelectron spectroscopy (XPS) spectra of Mo 3d, P 2p, S 2p and O 1s of the as deposited sample S4 from a solution containing 0.005M of (NH4)2MoS4, 0.005 M of Thiourea, 0.5 M of NaPO2H2 and 0.1M of NaOAc. The chemical state of sample S4 was further probed with high resolution XPS. When the Mo 3d region was deconvoluted (Figure 3c) a dominant doublet is evident at 229.4 eV (Mo 3d5/2) and 232.8 eV (Mo 3d3/2) which is consistent with 2HMoS2.26, 52 The doublet with peaks at 228.2 eV and 231.4 eV



can be attributed to a Moδ+ species (0 < δ

Electrochemical Formation of Amorphous Molybdenum

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