Few-Layer Iron Selenophosphate, FePSe3: Efficient Electrocatalyst

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Few-Layer Iron Selenophosphate, FePSe3: Efficient Electrocatalyst Towards Water Splitting and Oxygen Reduction Reactions Debdyuti Mukherjee, Muthu Austeria P, and Srinivasan Sampath ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00101 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Applied Energy Materials

Few-Layer Iron Selenophosphate, FePSe3: Efficient Electrocatalyst Towards Water Splitting and Oxygen Reduction Reactions Debdyuti Mukherjee, Muthu Austeria. P*, S. Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560012, India

KEYWORDS. Few layer ternary selenophosphate, FePSe3, Tri-functional electrocatalyst, ORR, OER, HER

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ABSTRACT

There has been a spurt of activity in using layered MPX3 (M= transition metal, X= chalcogen, S/Se/Te) compounds in various studies including catalysis and devices. In the present study, low band gap, ternary iron seleno-phosphate (FePSe3) is introduced as an excellent and highly stable tri-functional electrocatalyst for hydrogen evolution (HER), oxygen evolution (OER) and oxygen reduction reactions (ORR). It is observed that the present catalyst is useful in evolving hydrogen over a wide pH range including sea water environment. Density functional theory calculations reveal various parameters that help improve the electrocatalytic activity of the layered material. Covalency of Fe-Se bond, distortion in the crystal structure and adsorption properties are shown to be responsible for the observed high catalytic activity.


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INTRODUCTION Water splitting that involves two fundamental half cell reactions of anodic oxygen evolution (OER) and cathodic hydrogen evolution (HER) reactions has been widely accepted as promising and suitable approach to produce clean hydrogen and oxygen from aqueous media.1,2 In addition, oxygen reduction reaction (ORR) is the key electrochemical process for renewable energy conversion and storage devices such as fuel cells and metal-air batteries.3-6 However, these reactions, especially ORR and OER are strongly uphill that occur with large overpotentials7-10 due to sluggish kinetics. Hence, development of efficient electrocatalysts is highly desirable to replace the state of the art catalysts. Although the Pt group metals and Ru- or Irbased compounds are considered to be the most active for HER / ORR and OER respectively, the scarcity and high cost hinders the large scale commercial use. Exploration of active, stable, nonprecious alternate catalysts based on earth-abundant elements is very crucial in developing various conversion and storage devices. Catalysts based on transition-metal oxides, chalcogenides, phosphides etc. have been proposed as promising materials for HER / OER / ORR.1,2,11-22 Tri-functional activity that involves all the three reactions on a single catalysts is scarce.23-26 It is highly challenging and at the same time very desirable to develop efficient electrocatalysts that can catalyse all the three electrochemical processes (HER, OER and ORR), particularly using low cost, earth abundant elements. Iron-based electocatalysts are in demand due to high abundance on earth’s crust and of course economic viability. Recently, few reports have been published that deal with MPS3-type catalysts for electrocatalytic reactions mentioned above. 26-29 Electronic structure analysis leads to possible reasons behind the catalytic activities of various materials.30 Various orbitals of both metal and non-metals are known to influence


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catalytic activities depending on the redox reactions and associated interactions. Hydrogen adsorption on transition metal sites requires dz2 orbital population while ORR/OER require πtype of interactions with adsorbates like O2-, OH- and OOH- wherein dxz and dyz orbitals are involved. The frontier region consisting of all three orbitals (dz2, dxz and dyz) may lead to good electrocatalytic activity.31 In the case of non-metallic elements, it has been reported that the presence of phosphorous and sulphur improve catalytic activity towards various electrochemical reactions. As for HER, P centers have been found to enhance hydrogen desorption at high (Hads) coverages by trapping the protons and acting as base. Shi and Zhang32 explained the direct influence of P towards HER. DFT studies carried out by Wang and co-workers33 on P-terminated MoP surfaces show that the free energy (∆GH°) change associated with H adsorption changes from negative to positive values upon increasing the coverage. Earlier reports by Kibsgaard et al.34 and Tang et al.35 have revealed similar findings. In a related study,27 it is found that [P2S6]4units assist in the adsorption of H in HER activity of MPS3 - type compounds. On another perspective, when the strengths of E–H bonds (E = S, P, Se) are considered, the order is found to be S–H (363 kJ/mol) > P–H (322 kJ/mol) > Se–H (276 kJ/mol)36 that indicates Se containing catalysts would be favourable for HER. Since, ORR and OER involve multielectron transfer along with chemical steps, other factors are invoked to explain the activity of various catalysts. The presence of electronegative ligands (S2–, Se2–, and P3–) in the vicinity of catalytically active metal sites may deactivate the catalyst from coordinating with the hydroxide groups as a result of increased 3p–2p repulsion. In a different direction, Shen and co-workers37 reported on the use of bimetallic sulfide involving Co and Fe on N-doped mesoporous carbon as an efficient bifunctional catalyst for OER and ORR. A recent review by Kundu38 and et al. suggests possible ways to modify the catalytic


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properties of materials by tuning the heteroatom (S, Se, P) content and/or mixing. Apart from the above interesting results, electronic factors based on covalency between metal and non-metal39,40 and d orbital population (dz2, dxz, dyz- - rather than total d band center)31 have been shown to influence the OER/ORR catalytic activity. Favourable distortions in the crystal structures41,42 also lead to variations in the catalytic activity. In the present study, FePSe3 has been synthesized, characterized and the catalytic activities explored for HER, ORR and OER. Further, density functional theory calculations have been carried out to understand the parameters responsible for the catalytic activity. EXPERIMENTAL SECTION Materials All the chemicals and reagents used in the present study were obtained from chemical suppliers of standard sources. Elemental iron (Fe, 99.99%, Sigma, USA,), red phosphorus powder (P, 100 mesh, 99%, Alfa Aesar, USA) and selenium (Se, 99.98%, Aldrich, USA) were purchased and used as such. Analytical grade acetone was from Fisher scientific and it was distilled and used for the exfoliation of layered FePSe3. Synthesis of bulk FePSe3 Iron phosphotriselenide (FePSe3) is prepared by high temperature solid state synthesis as reported earlier for the corresponding sulphur analogue27 with slight modification. Typically, FePSe3 crystals are obtained by heating the mixture of elements (Fe, P and Se with 99.98% purity) in the required stoichiometric ratio in evacuated (~ 10-6 mbar) quartz tube at 700°C for 7 days.


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Synthesis of few-layer FePSe3 and its rGO composite Few-layer of FePSe3 was prepared by exfoliation of bulk crystals43 by direct liquid exfoliation technique. Briefly, 30 mg of the bulk crystals were dispersed in ~10 mL of acetonewater mixture (10 mL acetone and 200µL water) and sonicated for 4 h. The moderately stable colloidal dispersions were centrifuged at 3000 rpm for 15 min. to remove bulky, unexfoliated material. Very stable, clear supernatant containing large quantities of FePSe3 flakes was obtained. The concentration of the dispersion was calculated by knowing the amount settled down after centrifugation. The rGO-few layer FePSe3 composite was prepared by hydrothermal method14,27 by mixing 10 mL of FePSe3 few layer colloid (1 mg mL-1) with 10 mL of 1 mg mL-1 aqueous graphene oxide (GO, synthesis is given in SI) colloid. The mixture was sonicated for 30 min., transferred to a teflon lined autoclave and kept at 140°C for 16 h. that led to a stable dispersion. It was washed with water with resistivity 18 MΩcm and dried in vacuum at 80°C to obtain insitu rGO – FePSe3 composite. During the process of heating at 140°C and under pressure, GO got converted to rGO. Electrocatalytic studies All electrochemical measurements were carried out using a conventional three electrode system with GC (glassy carbon) coated with various samples as the working electrode, Pt and carbon rod as counter electrode and saturated calomel electrode (SCE) as reference in acidic, phosphate buffer and 3.5 wt % of NaCl media and mercury- mercuric oxide (MMO) reference electrode in alkaline medium. It should be noted that H-shaped cells were used for the measurements, where the counter electrode is kept separated from the working electrode to avoid


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any possible effect of Pt dissolution wherever Pt is used. The working electrodes were prepared by drop casting a known amount of catalyst solution onto the GC disk electrodes (3 mm diameter) to obtain 150 µg/cm2 loading. Linear sweep voltammetry (LSV) was recorded for HER and OER at a scan rate of 1 mV s

−1

at 25°C. Electrochemical impedance studies were

performed at an applied DC potential of -100 mV vs. RHE in the frequency range, from 10 mHz to 100 kHz. Before the measurements, the electrolyte was de-aerated by continuously purging with high purity N2 gas for 30 min. For ORR, prior to experiment, the electrolyte solution (0.5 M KOH) was purged with pure oxygen gas. The stability of the catalysts were determined by cyclic voltammetry (CV) carried out at a scan rate of 100 mVs-1 for 1000 cycles and by the amperometric i-t curves. All the potentials were referred to reversible hydrogen electrode (RHE). The reference electrodes were calibrated with respect to reversible hydrogen electrode (RHE), using Pt as working and counter electrodes in the respective electrolytes.44 The values obtained are, (Figure S1 in SI), Acidic medium, ERHE = ESCE + 0.282V; alkaline medium, ERHE = EMMO + 0.950V; sea water, ERHE = ESCE + 0.690V and in PBS solution, ERHE = ESCE + 0.720V. Characterization X-ray diffraction (XRD) patterns were recorded using Philips (PAN analytical) instrument (Cu-Kα radiation). Raman spectra were recorded using LabRAM (Horiba Jobin Yvon) spectrometer with an excitation wavelength of 514.5 nm and 50x long working distance objective. X-ray photoelectron spectroscopic analysis was carried out on a Kratos Axis Ultra DLD X-ray photoelectron spectrometer with monochromatic Al kα (1486.708 eV) radition. Electrical characterization was performed by van der Pauw method (4-probe configuration) using Agilent device analyzer B1500A. The morphological information was obtained from scanning electron microscopy (SEM) (Carl Zeiss ultra 55) equipped with energy dispersive X-ray


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spectroscopy (EDS). Transmission electron microscopy (TEM) images were obtained using JEOL 2100F operating at 200 kV. Electrochemical measurements were carried out using an electrochemical workstation [CH 660C, USA]. Rotating disc electrode (RDE) measurements were performed using RDE system (Pine instruments, Model AFMSRCE) by coupling with a galvanostat/potentiostat. Computational methodology The crystallographic structure of the layered FePSe3 was obtained from ICSD database (SI). The DFT calculations were carried out using CASTEP package45 (within Material Studio suite). All extended structure calculations were performed with plane wave basis set truncated at a kinetic energy of 330 eV. We used non-local corrected generalized gradient approximation based on the Perdew–Burke–Ernzerhof (PBE) formulation for optimization.46 Semiemiprical LDA+U approach is employed to understand the Fe system (3d electrons). The Vanderbilt Ultrasoft pseudo potentials and Monkhorst–Pack k-point47 mesh with separation between kpoints set at 0.08Å was employed with cutoff of 2.0e-5 eV/atom for energy and forces. The Brillouin zone of all systems was sampled with 0.015Å (5×5×2) and Monkhorst–Pack grids were used for DOS calculations. Geometry optimization was performed with optimization of ionic positions within lattice symmetry constraints to arrive at well-converged structures. The geometric- and lattice parameters are listed in Table S1 in SI. The analysis of bonding in extended structures was carried out using extended Hückel(eH)-based YAeHMOP suite of programs,48 that uses atomistic basis set (Slater type) allowing mapping of molecular orbitals of the unit cell fragments in to bands. The interaction diagrams, correlation diagrams and evaluation of mixing of MOs were carried out using CACAO package using extended Hückel method.49


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RESULTS AND DISCUSSION Bulk FePSe3 is synthesized from the corresponding elements at appropriate ratio using high temperature, solid-state reaction. The material is shiny and black in colour. The bulk material is exfoliated by liquid exfoliation method (experimental section) using acetone-water mixture. Iron selenophosphate (FePSe3) belongs to layered MPX3 (M = first row transition metal and X = S, Se) family, having rhombohedral crystal system with space group R-3 and contains three layers of FePSe3 per unit cell.50,51 The powder X-Ray diffraction pattern (Figure 1a) indicates that the as-prepared bulk FePSe3 is highly crystalline and the reflections match very well with reported JCPDF pattern (JCPDF No. – 33-0671).51 Further characterization is carried out using Raman and x-ray photoelectron spectroscopic techniques. The Raman spectrum of bulk FePSe3 (Figure 1b) shows intense peaks at 215 cm-1 and 168 cm-1 that are assigned to be due to A1g (ν1) and A1g (ν2) modes (stretching vibration of P-Se bond in P2Se6 unit). The peak at 148 cm-1 is due to the Eg (ν12) mode that does not involve any P-Se stretching component.52 The XRD pattern and Raman scattering spectra of the few layer FePSe3 (Figure S2a and S2b in SI) indicate that the material retains its composition even after exfoliation. The composite of FePSe3 with reduced graphene oxide (rGO) is prepared using hydrothermal method in order to improve the electronic conductivity of the catalyst. This is also found to retain the phase purity (Figure S2c and S2d in SI). The electrical transport properties confirm the semiconducting nature of the material with low resistivity value of 215 Ωcm at 25° C (Figure 1c). The selenium analogue exhibits appreciable conductivity as compared to the corresponding sulphur compound.53 The activation energy for conduction is determined to be 112 meV (inset of Figure 1c). The cathodoluminecence reveals that the band gap is ~1.3 eV (Figure 1d). The XPS survey spectra for both bulk and few-layer FePSe3 show the presence of Fe, P and Se in expected atomic ratio


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(Figure S3a in SI). The deconvoluted regions of Fe 2p, P 2p, Se 3p and Se 3d are given in Figure S3(b-e) (SI). It is observed that there is no shift in binding energy values observed (for the all elements, Fe 2p, P 2p, Se 3p and Se 3d) upon exfoliation by sonication in acetone, indicating that the chemical state of FePSe3 has not changed.

Figure 1. (a) X-Ray diffraction pattern (black) of as-synthesized bulk FePSe3 with corresponding JCPDF pattern (33-0671) (red), (b) Raman spectrum obtained using laser excitation wavelength of 514 nm, (c) resistivity as a function of temperature (inset shows Arrhenius plot of ln(I) vs 1000/T) and (d) cathodoluminescence (CL) of bulk of FePSe3.


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Figure 2. Scanning electron micrographs of (a) bulk FePSe3 crystals, (b) few layer FePSe3 sheets, (c) High Resolution transmission electron micrograph shows the interlayer distance of 6.62 Å and (d) HRTEM image of a layer showing different interplanar spacings (shown in the zoomed inverse FFT image). Another inset shows FFT image of the selected portion of the material. The scanning electron microscopy (Figure 2a) picture exhibits the layered nature of bulk FePSe3. The elemental mapping indicates the presence of Fe, P and Se at their expected atomic ratio, 1:1:3 (Figure S4a in SI). Fig. 2b depicts the SEM image of few-layer FePSe3 revealing ultrathin sheet-type morphology with lateral sizes of 100 nm to 200 nm. The EDS data for the exfoliated, few layer material (Figure S4b in SI) indicate the atomic ratio of Fe, P and Se as that


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of bulk material. Further, the TEM images (Figure 2c) show typical lamellar morphology of few layers with interlayer spacing of 6.62 Å.51 The HRTEM image (Figure 2d) of a single layer reveals high crystallinity of the material and the inverse FFT, shown in the inset, represents the d-spacing corresponding to different planes [1.8 Å for (119), 1.6 Å for (0 0 12) planes]. The bright field image and the HRTEM of rGO-composite is given in Figure S5 in SI. The atomic force microscopy (Figure S6 in SI) data reveals the presence of monolayer to few layer (thickness varied from 0.7 nm to 1.7 nm) material in the exfoliated material. Tri-functional electroactivity The electrocatalytic activities of the present catalysts towards OER, ORR and HER have been studied using three electrode configuration (details given in experimental section). While the OER and ORR activities have been studied in alkaline media (0.5 M KOH solution), the HER has been studied over a wide range of pH (acidic, alkaline, phosphate buffer and 3.5 % of NaCl solutions). Oxygen evolution reaction (OER) The OER activities of the pristine, few layer FePSe3 catalyst as well as its composite with rGO have been evaluated by rotating disk electrode voltammetry (RDEV) at a scan rate of 1 mVs-1 and at 1600 rpm rotation speed. The scan rate used is slow to ensure steady-state behaviour at the electrode surface, and the rotation rate is sufficiently fast to aid in the product removal and limit bubble formation on the electrode surface. Figure 3a represents the iR corrected linear sweep voltammograms on rGO-few layer FePSe3 along with its pristine counterparts (few layer FePSe3 and rGO) and the state of the art, IrO2 catalyst at 1600 rpm. The OER


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Figure 3. (a) iR corrected linear sweep voltammograms on bulk FePSe3, few layer FePSe3, rGOfew layer FePSe3, rGO and IrO2, recorded in O2-saturated 0.5 M KOH solution at a scan rate of 1 mVs-1 and at 1600 rpm. (b) Corresponding Tafel polarization plots. The current densities are obtained by normalizing OER currents with respect to the geometric area of the electrode. activity observed for the few-layer material is better than that of the bulk counterpart. The onset potential for OER occurs at ~1.59 V vs RHE (η = 360 mV) and the over potential to obtain 10 mAcm-2 current density is 430 mV (1.66 V – 1.23 V = 0.43 V) for the rGO-few layer FePSe3 catalyst which is quite comparable with the performance of several good OER catalysts.20-22,54-58 The best known catalysts for OER, IrO2 shows the onset at ~1.50 V vs RHE, indicating only 90 mV difference in onset potential between the state of the art material and rGO-few layer FePSe3. Similar studies on the pristine few layer FePSe3 show that it exhibits poor OER activity due to the semiconducting nature. Use of rGO along with FePSe3 improves the electronic conductivity thereby assisting in the catalytic activity, as discussed later (page 22). Further, the Tafel polarization plots (Figure 3b) show that the estimated Tafel slope for rGO supported few layer FePSe3 is ~115 mVdec-1, which is close to that observed on IrO2 (~ 95 mVdec-1) and comparable


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Figure 4. (a) Amperometric i-t curve for rGO-few layer FePSe3 in 0.5 M KOH medium at an over potential of 1.7 V vs RHE. High resolution XPS spectral data of (b) Fe 2p and (c) O 1s regions, recorded on the electrode after OER studies. (d) XRD pattern of rGO-few layer FePSe3 before (blue) and after (red) OER. with the values on MoS222 and other transition metal based catalysts21 (detailed comparison in Table S2 in SI). The high Tafel slope for pristine FePSe3 nanosheets indicates sluggish OER behaviour. Stability of catalysts is one of the major concerns in OER. Electrochemical stability of rGO-few layer FePSe3 during OER has been investigated by recording the amperommetric i-t curve at 1.7 V RHE, and the data shows stable currents up to ~7 h, indicating high stability of the catalyst in the electrolyte media (Figure 4a).


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The OER mechanism follows multi step electron transfer,59 where the first step involves adsorption of OH on to the catalyst surface (OH- + S → S-OH + H+ + e-, S represents the active surface site) followed by different pathways (oxide path or electrochemical oxide path) to produce oxygen. There is a strong probability of oxidation of catalysts under OER. The high resolution XPS data of the catalyst used for OER exhibits peaks at ~712 eV and ~727 eV (2p3/2 and 2p1/2), in the Fe 2p region indicating the presence of high valent Fe3+ on the surface of the catalyst after continuous OER and along with the O1s spectrum, it is confirmed that α-Fe2O3 phase60 is formed during the OER (Figure 4b and 4c). However, there is no detectable change in the XRD pattern (Figure 4d) of the material, recorded before and after OER. It is possible that only a very small, thin layer of Fe2O3 is formed on the surface of the catalyst. Fe2O3 is commonly used as catalyst for photo electrochemical water oxidation61-63 and also it is reported to exhibit reasonable activity for electrochemical OER in alkaline environment.64-66 However, it is also known to be unstable in alkaline media.67 Hence, it is likely that a small amount of Fe2O3 is present on FePSe3 possibly providing reasonably active and stable surface for OER catalysis. Oxygen reduction reaction (ORR) The catalytic activity of rGO-few layer FePSe3 has been studied towards ORR. As depicted in Figure 5a, the cyclic voltammogram shows well-defined peak at ~0.87 V vs RHE in O2-saturated electrolyte, which is absent in N2-saturated solution, indicating that the peak obtained is due to reduction of oxygen. Figure 5b depicts the rotating disk voltammetry data at different rotation rates for rGO-few layer FePSe3. The number of electrons involved and rate constant of the process are determined from the slope and the intercept of the K-L (KouteckyLevich) plot (Figure 5c) using Koutecky - Levich (K-L) equations (details in SI).


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Figure 5. (a) Voltammograms on rGO-few layer FePSe3 for ORR in presence O2-saturated and N2-Saturated 0.5 M KOH at 10 mVs-1 scan rate. The catalyst loading is 150 µgcm-2. (b) Voltammograms at different rotation rates (0 rpm – black, 100 – red, 200– blue, 400 – magenta, 600 – green, 800– navy blue, 1200– violet, 1600– purple and 2000 rpm – wine red) in O2saturated 0.5 M KOH at 5 mVs-1 scan rate. (c) Corresponding K-L plots (j-1 vs ω-0.5) (0.4 V – wine red, 0.45 V – purple, 0.5 V – violet, 0.55 V – navy blue, 0.6 V – green, 0.65 V – magenta, 0.7 V – blue, 0.75 V – red and 0.8 V – black) and (d) number of electrons (red) and rate constant (blue) at different applied potentials. The value of n (number of electron transferred) obtained for rGO - few layer FePSe3 is ~3.8 - 4 and the rate constant (k) is 1.8 × 10-2 cm s-1 at applied potential of 0.7 V vs RHE (Figure


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5d). The kinetic parameters are comparable and are better than certain 2D layered chalcogenides, reported earlier.17-19 The rate constant for MoS2/N-doped graphene68 composites and pristine MoS268 are reported to be 1.6×10-2 and 4.2×10-3 cm s-1. Pristine few-layer FePSe3 (Figure S7 in SI) however, catalyses 2 electron transfer producing H2O2 with rate constant almost one order less than the rGO composite at same potential (4.8 × 10-3 cm s-1 at 0.7 V vs RHE). The use of graphene sheets acting as “peroxide cleaner”69 has earlier been suggested. The ORR activity on bulk FePSe3 (Figure S8, in SI) suggests that the effect of number of layers is very small as reported for other 2D layered materials.70,71 The catalyst is quite tolerant towards methanol as well (Figure S9 in SI). The durability of the catalysts is good and the currents are almost retained to the extent of 90% (Figure S10, SI) over 500 cycles. The ORR activities of the present catalysts are compared with 40 wt % Pt-C and pristine rGO. As observed from Figure 6a, the difference in the E1/2 values for rGO-few layer FePSe3 and 40 wt % Pt-C is ~ 74 mV. Further, the mass transport corrected Tafel polarization plots (Figure 6b) reveals that the Tafel slope values at low current density region (Temkin adsorption region) for rGO-few layer FePSe3 and 40 wt % Pt-C are 64 mV dec-1 and 61 mV dec-1 respectively, whereas those at high current density region (Langmuir adsorption region), the slope values are 126 mV dec-1 and 123 mV dec-1 respectively. This indicates that the ORR mechanism on rGOfew layer FePSe3 is similar to that on 40 wt % Pt-C. The ORR activity is compared with pristine few layer FePSe3 and rGO, which indicates poor activity for the individual components towards ORR (Tafel slope values for these catalysts are given in Table S3, SI).


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Figure 6. (a) iR corrected linear sweep voltammograms on various catalysts (given in the figure), recorded in O2-saturated 0.5 M KOH at a scan rate of 5 mVs-1 and at 1600 rpm rotation speed. (b) Corresponding mass transport corrected Tafel polarization plots. Hydrogen evolution reaction (HER) The HER activity of the catalyst has been evaluated over wide pH range. The iR corrected linear sweep voltammograms (Figure 7a) for HER reveals that the onset potential in the case of few layer FePSe3 is ~ - 80 mV, whereas the rGO-few layer FePSe3 composite shows a value of ~ - 30 mV. The Tafel polarization plots (Figure S11a, SI) indicate Tafel slopes (b) for bulk, few layer and rGO-few layer FePSe3 to be 87, 40 and 37 mV dec-1 respectively and the exchange current densities (j0), are determined to be (4±0.5) × 10-6, (1±0.2) × 10-4, and (1.4±0.1) × 10-3 A cm-2, suggesting better HER activities for the few layer material as compared to the bulk one, which further improves in the case of rGO – few layer FePSe3. Based on the Tafel slopes, it may be inferred that the bulk FePSe3 follows a mechanistic pathway wherein Volmer step is the rate determining step (RDS), while in the case of few layer FePSe3 and rGO-few layer FePSe3,


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Figure 7. (a) iR-corrected voltammograms on bulk, few layer and rGO- few layer FePSe3 catalysts along with rGO and Pt-C, (b) TOF of the catalysts at an over potential of 0.1 V vs RHE (c) electrochemical stability of (i) rGO-few layer (blue-1st; wine red - 1000th cycle) and (ii) few layer FePSe3 (red-1st; green-1000th cycle). Electrolyte used is N2-saturated 0.5 M H2SO4 and scan rate used is 1 mV/sec. (d) Catalytic activity of FePSe3 (dotted lines) and rGO-few layer FePSe3 (solid lines) in media of different pH (red - 0.5 M KOH, green - phosphate buffer and pink - 3.5 % of NaCl solutions) towards HER. Heyrovsky step may be the RDS (detail in SI).72 The electrochemical impedance spectra (Figure S11b in SI) reveal the charge transfer resistance (RCT) values follow the order, bulk FePSe3 >


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few layer FePSe3 > rGO-few layer FePSe3 implying similar inference as obtained from the voltammograms and Tafel plots. The electrochemical parameters for HER are given in Table 1. Table 1. Comparison of electrochemical parameters obtained for bulk FePSe3, few layer FePSe3, rGO-few layer FePSe3 and 40 % Pt-C in acidic medium.

Catalyst

b / mV dec-1

J0 / A cm-2

RCT / kΩ

Bulk FePSe3

87 ± 2

(4 ± 0.5) × 10-6

43

Few layer FePSe3

40 ± 1

(1 ± 0.2) × 10-4

1.71

rGO-few layer FePSe3

37 ± 2

(1.4 ± 0.1) × 10-3

0.14

40 % Pt-C

27 ± 1

(3.5 ± 0.1) × 10-3

0.048

Metallic Fe is a poor catalyst for HER (Figure S11c in SI) and hence, it is likely that P and Se, present in [P2Se6]4- units participate favourably in the adsorption and desorption of H during HER. In the case of FePS3, the DFT calculations have revealed that S and P are the main active sites for atomic H adsorption and desorption at various coverages.27 It has been observed in the present case that the presence of both P and Se leads to better catalytic activity than that of Fe-based phosphides, sulphides and other layered selenides (detailed comparison is given in Table S5, S4 in SI). The turn-over frequency (TOF) values have been calculated for bulk, few layer and rGO-few layer FePSe3 from the overall area of the cyclic voltammograms recorded at a scan rate of 50 mVs-1 (Figure S12a in SI). The TOF values obtained are 0.092 s-1, 0.191 s-1 and 0.295 s-1 for bulk, few layer and rGO-few layer FePSe3 respectively at η = 0.1 V vs RHE (Figure


20

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7b). Relative electrochemically active surface area (ECSA) of the catalysts is determined from the slope (Cdl value) of the ½ (Ja-Jc) @ 0.15 V vs scan rate plot (Figure S12b in SI) as Cdl values are directly proportional to ECSA and the conductivity of the material. Table S6 (SI) gives the relative ECSA values for all the catalysts studied. The surface area normalized exchange current density values (J0,normalized) reveal that the activity is higher for the few layer FePSe3, indicating that the improved activity is not just due to an increase in surface area of the material. The bulk material hinders electron flow due to existence of potential barrier in between the layers, which is less in the case of corresponding few layers material.73 The amount of hydrogen gas evolved is determined by inverse burette method using an H-shaped electrochemical cell (details discussed in SI). Figure S13 in SI displays the plot of both experimentally obtained and calculated no. of moles of H2 vs time. The correlation between the experimentally determined and calculated amount of gas evolved is very good indicating ~100% faradaic efficiency. Long term stability of both few layer and rGO-few layer FePSe3 show that the electrochemical cycling for 1000 continuous cycles do not degrade the catalyst (Figure 7c) and no detectable difference in the voltammograms is observed. Amperommetric i-t curve shows similar trend as well (Figure S14a in SI). Further, characterization of the catalyst by XRD, Raman spectroscopy and SEM studies examined before and after 1000 cycles of continuous HER (Figure S14b to e in SI) reveal no structural and compositional change during the reaction. The ability of FePSe3 to evolve hydrogen in alkaline medium, phosphate buffer (pH 7) and in aqueous 3.5 % NaCl (pH 6.5) solutions is investigated using voltammetry. Very few catalysts are reported to exhibit both efficient catalytic activity as well as high stability in media of different pH values.27,74-77 The present study reveals that pristine and as well as rGO supported few layer FePSe3 efficiently evolve hydrogen over wide pH range with high stability (Figure 7d


21


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and Figure S15 in SI; Table S7 in SI). The catalytic activity for full cell water splitting device is given in the supporting information (Figure S16, SI), and it is observed that the potential range to achieve 10 mAcm-2 current for both HER and OER is 1.82 V, an over potential of 0.6 V for the over-all water splitting reaction. In all the three electrochemical reactions (OER, ORR and HER) studied, it is observed that the rGO composite exhibits better catalytic activity than the pristine counter parts of FePSe3 and rGO. The enhanced catalytic activity on rGO - few layer FePSe3 may be attributed to improved overall conductivity (Figure 8a) in presence of rGO that serves as conducting channel for facile transfer of electrons. The iR loss is minimum as observed in the voltammograms with and without iR correction (Figure S17). Apart from conductivity improvement, there is possibility of charge transfer between rGO and FePSe3 (metal d-orbital) as observed from the downfield shift of Fe 2p peak in high resolution XPS data (Figure 8b) of the composite. It is to be emphasized that there is likely to be a strong hybridization between metal d-orbitals of the catalyst (here Fe) and π-orbital of the rGO extending the d-band density of states of metal dorbitals near the Fermi level as well as the overall electrocatalytic activity as reported based on theoretical studies.78


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Figure 8. (a) Current-potential curves (2-probe measurements) for rGO supported and pristine few layer FePSe3. (b) High resolution XPS data of Fe 2p region of rGO-few layer FePSe3 composite and few layer FePSe3. The data points (○) are experimentally observed while the continuous red lines (―) are fitted curves. The deconvoluted spectra are also shown. All the three reactions have been carried out using carbon counter electrode instead of Pt foil and no change in catalytic activity is observed indicating that any contribution from Pt counter electrode is ruled out (Figure S18 in SI). Density functional theory calculations The crystal structure of bulk FePSe3 with space group R-3 in hexagonal lattice (Figure S19, Table S1 in SI) illustrates that the dimeric P2Se64- units exist in staggered conformation, and in the case of edge sharing FeSe610- units, distorted octahedral structure is observed with C3 symmetry in the molecular unit. Distorted crystal structure has been reported to play a major role in electrocatalytic activates , Yinlong Zhu41 et al. have reported that distorted crystal structure shows improved ORR catalysis in the case of Sr1−sNb0.1Co0.9O3−δ perovskite. Xu and Kitchin have reported42 the analysis of the atomic and electronic structure reactivity of four metastable oxide polymorphs of MO2 (M = Ru, Rh, Pt, Ir) revealing distortions in the MO6 octahedral geometry imparted through polymorphic structures. This is shown to cause significant redistribution of energy levels of the t2g-states. The asymmetric orbital projected DOS (spin-up and spin-down) illustrates the presence of magnetism in FePSe3 (Figure 9a). The DOS (density of states) calculations involving bulk FePSe3 show that the major contribution of the valence band consists of Se 4p and Fe 3d orbitals (Figure 9b) while phosphorus 3p orbital does not contribute to the frontier region of both


23


conduction and valance bands. Molecular orbital (MO) mapping is carried out to understand the covalent nature of Fe-Se bond in distorted FeSe6 unit. Extended Huckel method based on Win CACAO software49 is used (details in SI) to obtain the correlation diagram for FeSe6 with Oh and C3 symmetry (Figure S20 in SI). It is observed that the eg and t2g set of orbitals show bonding and antibonding interactions with 4p orbitals of selenium when the molecular unit adopts Oh symmetry. The bonding orbital (1t2g MO) possess π type interaction with Se 3p orbitals which is lower in energy than that of 1eg (σ type interaction) MO. In the distorted C3 symmetry, dx2-y2, dxy, dxz, dyz orbitals transform in to doubly degenerate e symmetry, while the dz2 orbital shows a symmetry. There is a possibility of inter mixing among all four d orbitals that shows the same symmetry. In distorted geometry, antibonding interactions are reduced in both HOMO and 15 10

DOS(electrons/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

5 0

s-up s-down p-up p-down d-up d-down total up total down

-5

-10 -15 -20 -2

-1

0

1

2

Energy(eV)

Figure 9. Density of states (DOS) for bulk FePSe3 (a) indicating the magnetic nature with asymmetric spin (spin-up and spin-down) and (b) Atom projected orbitals for individual atoms Fe, P and Se for total spin, calculated using GGA/PBE method. LUMO, in turn stabilizing the structure (Figure S21 in SI). This indirectly indicates an increase in the strength of Fe – Se bond. Further, it has been observed that bonding orbitals lie around 15


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eV showing stabilization due to lowering of symmetry (increases the mixing of Fe – d orbitals (e set) with Se 4p orbital). In C3 symmetry, hybridization of Fe 4pz and 3d orbitals leads to nodal features like dz2 and the linear combination results in two hybrid MOs that specifically points to taller dz2 in opposite directions (Figure S22a in SI). Hence, the sigma type bonding interactions of dz2 orbital is enhanced due to mixing of symmetry, and the resulting MO picture is shown in Figure 10a. Similarly, other four 3d orbitals possess matching symmetry with 4px and 4py (Figure S22b in SI) orbitals resulting in a combination with enhanced orbital coefficient (Figure 10b). The major contribution to HOMO is from dxz and dyz with minor impact from dx2-y2 and dxy due to same symmetry. Mixing of all four d orbitals lead to reorientation within Fe atom, indicating strong bonding interactions between Fe 3d and Se 4p – orbitals. This leads to three

a

c

b Figure 10. Molecular orbitals of (FeSe6)10- showing bonding interactions (green- positive and red – negative; lying around 15 eV). (a) Mixing of dz2 with Pz and (b) dxy with px orbital in Fe. (c) Projected DOS for Fe in bulk FePSe3. Low energy region of valance band showing significant p along with d orbital population (mixing of 3d and 4p orbitals).


25


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short and three long Fe - Se bonds. MO analysis indicates that the distorted FeSe6 units increase the bond strength (covalency) of Fe- Se bond. Untivich et al. have correlated the B-O bond covalency and ORR activity in the case of ABO3 perovskites.39 An increase in covalency between metal 3d orbital and non-metal 4p orbital facilitates the kinetics of O2−/OH− exchange on the transition metal surface thus enhancing the kinetics of ORR. Gang Wu et al.40 have used similar arguments to explain the enhanced bifunctional electrocatalytic activity in perovskites. Greater the covalency of transition metal cation- non-metal anion bonds, smaller would be the charge transfer gap between the two which promotes charge transfer between redox active centers and adsorbates participating in rate determining steps of OER/ORR. The DOS calculations (Figure S23 in SI) reveal Se domination at the valance band region for bulk, while Fe and Se are equally populated in the case of single layer. Considerable reduction in Fe-Se bond length is observed in the case of single layer (Table S1 in SI) with minor variation in Se-P bond length. In addition, the charge analysis indicates that the bond population increase when we go from bulk to single layer (Table S8, SI). Therefore, the change of bond length would be the next crucial factor affecting the covalency of Fe-Se with shortened bond length and enhanced bond population of Fe-Se and P-Se in single layer material. This indicates that the few-layer or single layer FePSe3 will yield better electrocatalytic performance than that of the bulk. The extended Huckel calculations for FePSe3 single layer has been carried out using YaeHMOP software.48 The projected d-orbital DOS (Figure S24 in SI) illustrates that the valance band region predominantly comprises of dxz, and dyz orbitals in the frontier. Reasonable dz2 orbital population is observed near the Fermi valance band region. This is confirmed based on


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the current molecular orbital analysis and the results emphasize the key role of distortion with dorbital population in determining the strength of adsorption. Based on the present study, it is suggested that covalency, distortions and orbital populations contribute and enhance the electro catalytic activity of FePSe3. The charge analysis reveals that when one considers the bulk to single layer, the bond population increases with shortened bond length indicating that the number of layers plays a major role in the catalytic activity. Hence, few layer FePSe3 is a very good catalyst for electrochemical reactions. CONCLUSIONS The present study reports a new and excellent tri-functional electrocatalyst for HER, ORR and OER. The layered FePSe3 is synthesized from highly earth abundant elements, Fe, P and Se. The activity improves when conducting carbon support like rGO, is used along with the ternary chalcophosphate catalyst. Further, the XPS studies reveal that there is a chemical interaction between rGO and the catalyst leading to charge transfer that results in extending the d-band density of states of metal d-orbitals near the Fermi level and subsequently enhancing the overall electrocatalytic activity. The DFT calculations reveal greater Fe-Se bond covalency, distortion in the crystal structure and favourable adsorption properties thus leading good catalytic activity. The study opens up a way to use a new series of layered phospho-chalcogenides for water splitting (HER and OER) and ORR, and it is possible to use the catalysts in fuel cells, rechargeable metal-air batteries etc. Being a low band gap layered semiconductor, the material may also be useful in field effect transistor (FET) - based devices and possibly in photocatalytic and photoelectrochemical studies as well. Some of these studies are being pursued in our laboratory.


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Supporting Information. Detailed characterization of FePSe3 and rGO-FePSe3, comparison of catalytic activities with literature etc. “This material (In in PDF) is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Web: http://ipc.iisc.ac.in/~sampath/ Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The author declares no competing financial interests. ACKNOWLEDGMENT This work was supported by DST, New Delhi, India. DM thanks CSIR for a research fellowship.. ABBREVIATIONS CV, Cyclic Voltammetry; GO, Graphene Oxide; LSV, Linear Sweep Voltammetry; RDE, Rotating Disc Electrode; rGO, Reduced Graphene Oxide; SEM, Scanning Electron Microscope; TEM, Transmission Electron Microscope; XPS, X-Ray Photoelectron Spectroscopy; XRD, XRay Diffraction.


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TOC A stable, cost effective layered trifunctional electrocatalyst based on earth abundant elements Fe, P and Se is introduced for OER, ORR and HER.

SYNOPSIS Low band gap, layered iron seleno-phosphate (FePSe3) with appreciable conductivity is introduced as an excellent and highly stable tri-functional electrocatalyst for over-all water splitting, involving hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), as well as oxygen reduction reaction (ORR; important for energy conversion reaction). The FePSe3 catalyses HER over a wide pH range and OER, ORR, in alkaline media. Synergistic effect between P and Se along with reduced graphene oxide improves the catalytic activity towards these electrochemical reactions. DFT calculations indicate the possible reasons for the high catalytic activity observed.


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Few-Layer Iron Selenophosphate, FePSe3: Efficient Electrocatalyst

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