Mechanistic Insight of Enhanced Hydrogen Evolution Reaction Activity

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Mechanistic Insight of Enhanced Hydrogen Evolution Reaction Activity of Ultra-Thin h-BN Modified Pt Electrodes Anku Guha, Thazhe Veettil Vineesh, Archana Sekar, Sreekanth Narayanaru, Mihir Ranjan Sahoo, Saroj Kumar Nayak, Sudip Chakraborty, and Tharangattu N. Narayanan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00938 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Mechanistic Insight of Enhanced Hydrogen Evolution Reaction Activity of Ultra-Thin h-BN Modified Pt Electrodes Anku Guha,1# Thazhe Veettil Vineesh,1# Archana Sekar,1# Sreekanth Narayanaru,1# Mihir Sahoo,2 Saroj Nayak,2 Sudip Chakraborty,3,* and Tharangattu N. Narayanan1,* 1

Tata Institute of Fundamental Research - Hyderabad, Sy. No. 36/P, Gopanapally Village, Serilingampally Mandal, Ranga Reddy District, Hyderabad 500107, India. * Corresponding Author: E-Mail: [email protected] or [email protected] (T. N. N.)

2

School of Basic Sciences, Indian Institute of Technology, Bhubaneswar, Odisha 751013 India.

3

Materials Theory Division, Uppsala University, Box-516, Uppsala, SE-75120, Sweden. * Corresponding Author: E-Mail: [email protected] (S.C.) #

Equally Contributing Authors

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Abstract: Enhancing the intrinsic activity of a benchmarked electrocatalyst such as platinum (Pt) is highly intriguing from fundamental as well as applied aspect. In this work, hydrogen evolution reaction (HER) activity of Pt electrodes, benchmarked HER catalysts, modified with ultra-thin sheets of hexagonal boron nitride (h-BN) is studied in acidic medium (Pt/h-BN), and augmented HER performance, in terms of over potential at 10 mAcm-2 current density (10 mV lower than Pt nanoparticles) and lower Tafel slope (29±1 mV/decade), of Pt/h-BN system is demonstrated. The effects of h-BN surface modification of bulk Pt as well as Pt nanoparticles are studied, and the origin of such an enhanced HER activity is probed using density functional theory based calculations. The HER charge transfer resistance of h-BN modified Pt is found to be drastically reduced, and this enhances the charge transfer kinetics of the Pt/h-BN system due to the synergistic interaction between h-BN and Pt. Enormous reduction in the hydrogen adsorption energy on h-BN monolayers is also found when they are placed over Pt electrode (-2.51 eV (hBN) to -0.25 eV (h-BN over Pt)). Corrosion preventive atomic layers such as h-BN protected Pt electrodes having better performance than Pt electrodes open possibilities of benchmarked catalysts by simple modification of surface via atomic layers.

Keywords: Hydrogen Evolution Reaction; Heterogeneous Catalysis; DFT Calculations; Platinum; hexagonal Boron Nitride.


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1. Introduction Fossil-free paths for producing fuels and chemicals are receiving global attention, and one of the viable resources towards this goal is electrochemical conversion of molecules in the atmosphere such as water, carbon dioxide, nitrogen etc. into high value products such as hydrogen (H2), hydrocarbons, ammonia etc.1. Designing novel electro-catalysts, and tuning their intrinsic activity and number of active sites are highly essential for making these energy technologies to be viable for the future1. One of the important electrochemical reactions in this regard is hydrogen evolution reaction (HER), and noticeable efforts were undertaken towards the development of non-precious metal catalysts for this reaction in acidic environment2-3,4. Hydrogen evolution from the cathodic compartment of water electrolyser is the greener route of producing H25, and which is also found to be economically scalable with a cost of ~$5-$10/kg (depending on the plant design)1. High gravimetric energy density (~142 MJ/kg) of compressed H2 (liquid H2 or compressed (150 bar H2 (g)) also relies its potential as a futuristic fuel6. Regarding the catalysing materials of this reaction, platinum (Pt) lies in the top of volcano plot with almost negligible overpotential and thermo-neutral (H2 adsorption free energy ∆GH~0 eV)7. The lack of world-wide availability of Pt and cost make bottlenecks in the commercialization of HER technology8. This has surged towards the alternative materials for HER, and rendered limited success in terms of competing Pt in performance9,10. Further, Pt dissolution in the harsh acidic environment in a long run process is another challenge in this cathodic process11. One of the ways is to protect Pt with porous materials such as high surface area carbon, which will eventually help to protect Pt as well as decrease the amount of Pt (in weight), has led to the development of benchmarked HER catalysts such as Pt/C (20 wt%/80 wt%)12. But carbon corrosion (carbon in Pt/C) in proton exchange fuel cell is another prevalent hindrance in its commercial applicability, and development of alternatives for Pt/C, where its performance should be on par with Pt (Pt/C) without deteriorating the catalytic performance is highly essential13. Various attempts have been made in the past towards this goal and wrapping Pt with other carbon structures such as nanotubes, graphene etc. is yet another technique adopted but with limited success 14,15. Wrapping of metallic surfaces with hexagonal boron nitride (h-BN or h-BN/BN/hBN)) is found to be an efficient method to avoid metallic corrosion, and recent


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theoretical studies show that h-BN on certain metals like gold can modify the inherent catalytic activity of the metal due to the band structure modulations16. Such modification induced enhancement in the oxygen reduction reaction (ORR), catalytic efficiency of gold surface was initially demonstrated by Uosaki et al. In their work, Density-functional theory (DFT) based calculations show that the band gap of an h-BN monolayer is 4.6 eV while a slight protrusion of the unoccupied BN states toward the Fermi level is observed when BN is supported on Au (111) due to the BN−Au interaction. Further, it is also proven that, a theoretically predicted metastable configuration of O2 on h-BN/Au(111) is possible and this can aid as precursors for ORR, and free energy landscapes for ORR on hBN/Au(111) show that O2 to H2O2 is possible at this electrode. The experiment shows that overpotential is drastically reduced in h-BN/Au electrode in comparison to bare gold (but still not competing with Pt), and this phenomenon is not seen in other electrodes such as glassy carbon (GC). The same group has also shown the enhanced HER performance of inert gold electrode while modifying with boron nitride nanosheets (BNNS) (still the performance is lower than that of Pt), but while modifying Pt with BNNS reduced its HER activity17. The DFT studies show that edge atoms in BN provide energetically favored sites for adsorbed hydrogen, i.e., the intermediate state of HER. But this DFT study was not extended to Pt (to prove why Pt in the experiment was not showing an enhanced activity while modifying with h-BN) though h-BN monolayer can also readily bind to the surface of various transition metals such as Pt due to the mixing of the dz2 metal orbitals with the N-pz and B-pz orbitals of h-BN18. In this study, a novel Pt/h-BN electrode having activity higher than Pt is demonstrated for HER catalysis. Here, a detailed experimental and theoretical (DFT based) study is conducted on two different types of Pt/h-BN systems towards their HER. Initially, platinum electrode (3 mm diameter working electrode, it is named as ‘bulk Pt’) is modified with shear exfoliated h-BN (to make the h-BN as a few layered system) and coated on the electrode in different weight proportions. Second system is the Pt nanoparticles of 2-5 nm diameter decorated on exfoliated hBN sheets. This Pt/h-BN nanoparticle system is drop casted over GC electrode towards HER performance studies. The cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance studies (EIS) are conducted to prove the augmented performance of the h-BN modified Pt electrodes towards HER. Further, large amount of h-BN containing Pt system is found to be diminishing in performance (less than even Pt) and this observation is in 4 ACS Paragon Plus Environment

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corroboration with Uosaki et al. observation on Pt modified BNNS. A DFT based HER reaction co-ordinates study is conducted to unravel the origin of augmented HER performance of Pt/h-BN system. Further, as we have seen in our previous reports, these substrate induced band structure modification mediated enhanced catalytic activities of atomic layers is limited to a few layered systems 10. 2. Results and Discussions 2.1 Pt/h-BN Analyses Shear exfoliated h-BN and different compositions of Pt nanoparticles decorated h-BN sheets (named as Pt/h-BN (2:1), (1:1), (1:4) etc.) are prepared using the method detailed in experimental section. Figure 1A and 1B show the TEM and HR-TEM images of Pt/h-BN (2:1). The TEM images show that Pt nanoparticles are decorated on h-BN sheets (only on h-BN sheets, h-BN sheets are having thickness ~ 4-5 nm, as inferred from atomic force microscopy (results are not shown19)) and the HR-TEM shows the crystalline planes of Pt. TEM and HR-TEM also indicate the presence of ultra-small sized Pt nanoparticles (~ 5 nm, figure S1 (B1-B4 and figure S2)) with almost uniform size distribution. More TEM and HR-TEM images of different magnifications are provided in the supporting information, figure S2, indicating the presence of Pt nanoparticles only on h-BN nanosheets. Figure 1C is the X-ray diffraction pattern (XRD) of the Pt/h-BN (2:1). The shear exfoliated h-BN has broad peaks at angles (2θ θ) of 26.3, 41.8, 55 and 76 corresponding to (002), (100), (001) and (110) planes of h-BN, respectively (XRD pattern of h-BN alone is shown in the inset). The diffraction peaks at 40, 46, 68, and 81 can be indexed to (111), (200), (220) and (311) planes of the Pt, respectively (JCPDS card 4-802). The broadening of Pt diffraction peaks indicates the nanometer size of the Pt particles, which is in corroboration with the TEM analyses (figure 1A and 1B). The energy dispersive spectrum (EDS, attached to a scanning EM) of Pt/h-BN (2:1) is shown in the inset of figure 1A, showing the presence of Pt, boron, and nitrogen (a small fraction of oxygen possibly from the adsorbed one). In the Raman spectrum of Pt/h-BN (2:1) (figure 1D), the characteristic peak of h-BN at 1347 cm1

is visible instead of 1366 cm-1. This phenomenon suggests that there is an weak interaction

between Pt nanoparticle and h-BN which is furthure supported by FT-IR spectra shown in figure S9 in supporting information section.


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Figure 1: (A) Low resolution TEM image of Pt/h-BN (2:1), (inset energy dispersive spectrum of Pt/h-BN (2:1)), (B) HR-TEM image of Pt/h-BN(2:1), (C) XRD pattern of Pt/h-BN and sheared exfoliated h-BN(inset), (D) Raman spectrum of Pt/h-BN (2:1).

2.2 Electrochemical Studies A systematic study using CV is a convenient and efficient method to estimate the amount of Pt (active surface) on an electrode20. Figure 2A depicts the CV of bulk Pt electrode21 (diameter: 3 mm) in 0.5 M H2SO4 solution with a scan rate of 200 mV/s (in CVs, the current densities calculated using geometrical surface area are mentioned as Current Densitygeo and those with electrochemical surface area (ECSA) as Current Density

ECSA).

The different phenomena

occurring at Pt electrode in different potentials are marked in the figure. The green shaded area 6 ACS Paragon Plus Environment

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represents the under potential deposition (HUPD region) of Pt (H2 desorption)22. This bulk Pt electrode (same exposed geometrical area) while modified with shear exfoliated h-BN (scanning electron microscope images of bulk Pt and bulk Pt modified h-BN (electrode prepared as explained in the experimental section) are shown in figure S2 B and C) has lower exposed Pt area as shown in figure 2B (and supporting information figure S3, 50 mV/s). Further, the same effect has observed in the CVs of different amount of Pt nanoparticles loaded h-BN composites too (figure 2C). A systematic variation in CVs of Pt/h-BN NP system is given in supporting information, figure S4, where their CVs are compared with Pt NP system. Hence the CV based studies show that while modifying the Pt with h-BN (either in bulk Pt or NP system), the active exposed Pt surface area is drastically reduced. It is to be noted that the amount of catalyst loaded (by weight) kept constant in all the cases, and this is an important parameter in catalysis since the mass transfer and charge transfer can be limited by the amount of catalyst.1


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Figure 2: (A) CV of bulk Pt electrode showing different Pt phenomena happening in Pt electrode (current density is calculated using geometrical area in all cases) in a scan rate of 200 mV/s, (B) comparison in CVs of bulk Pt electrode and shear exfoliated h-BN modified bulk Pt electrode (scan rate 50 mV/s), (C) comparison in CVs of Pt nanoparticle and Pt nanoparticle decorated sheared exfoliated h-BN (50 mV/s).

The LSV measurements are conducted in a voltage window of 0.1 to -0.2 V vs. RHE with a scan rate of 10 mV/s in 0.5M H2SO4 solution for unravelling the HER activities of different electrodes (figure 3A and 3B). Bulk Pt electrode shows an HER onset potential -0.07V vs. RHE and an over potential at 10 mAcm-2 of -0.128 V, which is in tune with the earlier reports23. Bulk Pt electrode modified with h-BN shows almost the same HER onset potential while the over 8 ACS Paragon Plus Environment

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potential at 10 mAcm-2 is found to be -0.118 V. This reduction in the over potential is quite significant and shows the enhanced charge transfer from Pt/h-BN system while modifying with the h-BN. Pt NP shows the HER reaction onset potential of -0.02V vs. RHE which is also similar to the reported values of Pt24 The over potential is (at 10 mA cm-2) found to be -0.1 V. Different amounts of Pt/h-BN systems have different current densities at hydrogen adsorption region though the potential (for hydrogen adsorption) remains the same in all cases. Different extend of current densities indicate the different extend of adsorption. Pt/h-BN (2:1) has almost same current density as that of Pt NP in this region and further reduction of hydrogen also starts at the same onset potential. But the over potential at the 10 mAcm-2 current density of (2:1) sample is found to be -0.09 V, which is 10 mV lower than that of Pt NP. This indicates the higher charge transfer rate in Pt/h-BN (2:1) than in Pt NP, though the exposed Pt is less as inferred from the CV studies. But, while increasing the h-BN amount decreased the charge transfer rate, and as it can be seen from the Pt/h-BN (1:4) sample that the onset potential of the HER is -0.05 V vs. RHE and the over potential is found to be -0.127 V at 10mAcm-2. It is to be noted that the modification of Pt with larger amount of h-BN (1:4) has a much lower activity than (2:1) Pt/hBN system. It is to be noted that the shear exfoliated h-BN has no significant activity towards HER, as shown in figure S5, when it is deposited over glassy carbon (GC) electrode. Further, the activities of Pt/h-BN (both bulk and nanoparticle systems) are tested with graphite counter electrode too (to check the Pt dissolution issue from counter electrode), and the results given in figure S6 show that the current densities are same as that of with Pt counter electrode.


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Figure 3: (A) LSV of bulk Pt electrode and shear exfoliated h-BN modified bulk Pt electrode, (B) LSV of different Pt/h-BN composites comparing with Pt nanoparticle, (C) Nyquist plots of different Pt/h-BN composites(corresponding Randal’s circuit (inset)).

The enhanced charge transfer properties of h-BN modified Pt system are further studied using EIS studies (10 mHz to 7 MHz) and the representative Nyquist plots are given in the figure 3C. The EIS of different Pt/h-BN composites are conducted in 0.5M H2SO4 with an input sine wave having amplitude of -0.02V vs. RHE. The charge transfer resistance (Rct or RCT) values are calculated by fitting the Nyquist plots with the Randel's circuit. The corresponding Randal’s circuit is also given in the inset (figure 3C). The Rct values of Pt NP, Pt/h-BN (2:1), (1:1), and (1:4) are found to be 33Ω, 18Ω, 67Ω and 113Ω respectively. The lowest Rct of Pt/h-BN (2:1) is in corroboration with the low overpotential of this sample in LSV measurements. This indicates 10 ACS Paragon Plus Environment

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that Pt modifying with optimum amount of h-BN can drastically modify its inherent activity towards HER. Further, the performance of Pt/h-BN (2:1) is compared with that of benchmarked commercial catalyst Pt/C (Alfa Aesar, 35489) and is given in figure 4A and 4B. The current densities in these cases are calculated using ECSA. The details of the ECSA calculations are given in supporting information (figure S7). The CVs clearly show the presence of lower exposed Pt area in Pt/h-BN (2:1) (ECSA is 0.1 cm2 for Pt/h-BN (2:1) where it is 0.18 cm2 for Pt/C). The over potential at 10 mAcm-2 current densitiy is found to be 74.4 mV and 70.3 mV respectively for Pt/C and Pt/h-BN (2:1). The LSVs calculated using geometrical area are shown in figure S8, and both are having the same current densities and onset potentials, though the Pt exposure is lower in Pt/h-BN (2:1). The LSVs performed using rotating disc electrode (mass transfer controlled) at 1600 rotation per minute (rpm) are also showing (figure S8B) the same trend with higher current densities (higher current densities due to low diffusion layer). Hence the Pt/h-BN (2:1) has better performance both in terms of ECSA and gemoetrical surface area. Tafel slope analyses are also conducted to further quantify the enhanced kinetics of charge transfer in Pt/h-BN systems. The results are shown in figure 4B.

Figure 4: (A) The LSVs (scan rate 50 mV/s) and (B)Tafel slope analyses for Pt/h-BN (2:1) and Pt/C (20:80, commercial catalyst). The current densities in this plots are calculated using ECSA. The fitted regions of the Tafel slope calculations in figure 4B are marked with dotted overlays.


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The Tafel slope analysis shows that Pt/C has the slope of 34±1 mV/decade and this value is in tune with reports,20 with exchange current density (jo) 0.47±0.01 X 10-4 Acm-2. The Pt/h-BN (2:1) shows a Tafel slope of 29±1 mV/decade and an exchange current density (jo) of 0.43±0.05 X 10-4 Acm-2. This (lower Tafel slope with similar jo values) indeed shows the facile HER charge transfer kinetics of Pt/h-BN (2:1) in comparison to the benchmarked Pt/C, though the exposed Pt area is lower in Pt/h-BN (2:1). The molecular origin of h-BN in enhancing the catalytic activity of Pt is not yet clear from these experiments. Since the enhancement in the activity (though onset potential for HER is same in bare Pt (bulk or NP) and Pt/h-BN (2:1)) is seen both in bulk Pt and Pt NP systems, it is apparent that this cannot be attributed to a finite size effect from the Pt system. Definitely h-BN edges play a significant role in catalytic reaction, as reported previously 16,25. But it is seen that enhancing the insulating h-BN amount will not help to augment the activity of Pt. Hence to understand the mechanism behind the enhanced HER activity of optimized Pt/h-BN system, detailed DFT based calculations are performed.

2.3 DFT Calculations The schematic of the theoretically constructed Pt/h-BN system is shown in figure 5 (details in experimental section). h-BN is an insulator and its optical spectrum (from DFT studies, for details, see experimental section) is shown in figure S10, where it is showing a bandgap of 6 eV. The h-BN modified Pt optical spectrum shows that it resembles to that of a metallic (figure 5B), indicating a drastic reduction in the bandgap. The HER activity of a given surface is determined through the free energy of adsorbed hydrogen at equilibrium condition of the reaction. The adsorption free energy corresponding to HER mechanism is defined as ∆GH*= Eads + ∆ZPE – T∆SH (H* signifies hydrogen adsorbed on the surface), where Eads signifies the adsorption energy of atomic hydrogen, while ∆ZPE is the change in zero-point energy (ZPE) of hydrogen in the adsorbed state and gas phase and T∆S represents the entropy difference of hydrogen in adsorbed and gas state. The value of ∆ZPE can vary from 0.0 to 0.04 eV, whereas the third term gives rise to correction value of 0.2 eV under the experimental conditions with the assumption that the vibrational entropy is very small in the adsorbed state26-28. The adsorption entropy of atomic hydrogen is ∆SH ≈ 1/2∆SoH, where ∆SoH denotes the entropy of H2 in the gas 12 ACS Paragon Plus Environment

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phase which is approximately 0.4 eV. Therefore, the adsorption free energy corresponding to HER mechanism can be expressed as ∆GH*=∆Eads + 0.24 eV, where for an ideal HER catalyst the value of ∆GH* should be around zero. Based on this principle, we have determined the hydrogen adsorption energy on h-BN and Pt modified h-BN sheet. As mentioned in the computational section, different possible adsorption sites like at the top, bridge and hollow on both h-BN and Pt modified h-BN sheet are explored. The adsorption energy can be determined by employing the following equation: EXads = Esystem+X -Esystem - 1/2 EX

.1.

where Esystem+X (X is adsorbate (hydrogen(H) in this case)) is the total energy of the surface (in this case h-BN sheet and Pt modified h-BN sheet) with adsorbed hydrogen, Esystem is the total energy of the surface and EX is the total energy of the isolated molecule (Hydrogen in this case) in the gas phase. The hydrogen adsorption energy on h-BN sheet is -2.75 eV which has been changed to -0.49 eV with the inclusion of Pt on the surface. This leads to the adsorption free energy on h-BN sheet as -2.51 eV, whereas the value increases upon inclusion of Pt on hBN sheet up to -0.25 eV. All these values have been depicted in the reaction coordinate (figure 5 C) along with the corresponding values of graphene (a well researched 2D material, where it can also be used to protect Pt though it's also made of carbon and hence issues related to carbon poisoning in Pt/C can occur here too) and Pt. The ∆G value of 0.09 eV, in case of pristine Pt, as referenced in figure 5C is the ideal case so far for HER mechanism, however the expensiveness of using pristine Pt (along with other issues such as changing the activity of the Pt surface drastically upon cycling) is the bottleneck from the application perspective, which was one of the prime motivations in this work. It is worth to note that, the study on the Pt/graphene layered stacking system shows that Pt forms clusters in place of thin film type configuration on graphene, and this is in tune with earlier report29. The reason of this lower ∆G is the bonding between Pt and h-BN sheet. If one looks at the minimum energy configuration of h-BN-Pt, covalent bonding has been observed. To probe the experimental evidence for covalent bonding between Pt and h-BN, fourier transform – infrared spectroscopy (FT-IR) is conducted on Pt/h-BN (2:1), Pt NP and shear exfoliated h-BN, and the results are shown in figure S9. B-N in-plane and out of plane stretching modes are observed in h-BN and Pt/h-BN at 1370 and 817 cm-1, respectively30. But, red shifted shoulder peaks (marked in arrows) are observed in both stretching vibrations (B-N in plane and out of 13 ACS Paragon Plus Environment

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plane), possibly due to the interaction between metal (Pt) and h-BN, which are partially shifting the B-N stretching to lower energies (but there is a majority of unaffected B-N).31 The h-BN interactions with various transition metals including Pt are reported previously, and B-N bond softening and corrugations on h-BN are the pronounced effects of these interactions.28 Similar results are obtained from micro-Raman analyses of shear exfoliated h-BN and Pt/h-BN (2:1) too (figure 1D). The shear exfoliated h-BN has the E2g B-N vibration mode at 1366 cm-1, while this has been broadened and shifted to lower frequencies (higher wavenumbers) in Pt/h-BN (2:1) indicating the possibilities of bond softening due to the interactions with metal (Pt)

30,31

. But,

detailed experiments including the work function analyses are needed to probe this interaction, and these studies are in progress. Present theoretical study shows that h-BN is acting as a connecting bridge between adsorbate hydrogen and Pt. There is a substantial charge transfer between Pt and h-BN, which indirectly bonding with hydrogen (it is shown in figure S11). Further, the condition of thermo-neutral (∆G~ 0 eV) for hydrogen adsorption-desorption energy is the trade-mark of any good HER catalyst. There is a substantial decrease in ∆G value from -2.51 eV to -0.25 eV with a difference of 2.26 eV is leading to the HER enhancement of h-BN sheet modified with Pt and this trend interestingly has been observed in the experimental scenario as well on h-BN sheet with the inclusion of Pt on the surface. This leads to the inference that both h-BN and Pt in the composite contribute to the HER, which is not the case with pristine h-BN, and hence this observed enhancement in HER activities of Pt/hBN composite system is a synergistic effect.


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Figure 5: (A) Side and bird's eye perspective of Pt modified h-BN, (B) optical spectrum of Pt modified h-BN, (C) free energy diagrams for HER on Pt modified hBN, graphene, Pt, and Pt/hBN (h-BN-Pt composite). At last, the stability tests for both bulk Pt and bulk Pt modified with shear exfoliated hBN are conducted by cycling the electrodes in 0.5 M H2SO4 (within a potential window of -0.16 15 ACS Paragon Plus Environment

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V to 0.24 V vs. RHE in a scan rate of 50 mV/s and the electrolyte is stirred using a magnetic stirrer/bead set up in a constant rate to avoid the bubble formation), and the results are given in the supporting information figure S12. The h-BN modified Pt electrode shows higher current density and lower onset potential (even after 590 cycles) than bulk Pt electrode, though the activities of both the systems are reduced after cycling. The decrease in the activity of bulk Pt in concentrated H2SO4 medium under CV conditions in the above mentioned potential window can be due to sulfur deposition, as reported by Umeda et al32. But detailed surface studies are needed to probe into the exact mechanism. A careful coverage of the Pt electrode with ultra-thin uniform h-BN monolayer might further protect the initial activity of Pt/h-BN and such efforts are in progress.

3. Conclusion It is established that the intrinsic HER activity of benchmarked Pt can be enhanced by making suitable composite or surface modification with ultra-thin layers of inactive h-BN. The surface modification of bulk Pt electrode with ultra-thin sheets of h-BN and formation of Pt nanoparticles - h-BN composite system having optimum amount of Pt and h-BN (2:1) are resulted in to enhanced HER activity, in terms of lower over potential at 10 mAcm-2 current density (10 mV lower than Pt nanoparticle) and lower Tafel slope (29±1 mV/decade), than the benchmarked Pt/C(34±1 mV/decade) catalyst. Exchange current densities of Pt/C and Pt/h-BN are found to be 0.47±0.01 X 10-4 Acm-2 and 0.43±0.05 X 10-4 Acm-2, respectively. The enhanced activity of Pt/h-BN system is proven to be due to the enhanced the charge transfer kinetics of Pt/h-BN system where the adsorption free energy modification of h-BN placed on a Pt electrode is found to be taking place. Lowest charge transfer resistance is observed for Pt/h-BN (2:1) while doing the impedance analyses, which is in tune with the LSV studies, indicating the augmented charge transfer kinetics of Pt modified h-BN electrode. The edge states of BN can enhance the adsorption of hydrogen since the hydrogen adsorption of BN nanotubes is proven in the past. The drastic change in the hydrogen adsorption free energy of single layer h-BN on Pt is proven in the present study where it is changed from -2.51 eV to -0.25 eV from bare h-BN to Pt/h-BN. In a nutshell, an interaction between Pt and ultra-thin h-BN in Pt/h-BN composite systems is established and the enhanced HER activity of these systems is due to a synergistic effect of both Pt and hBN. Hence this study opens new avenues in designing novel, economically feasible 16 ACS Paragon Plus Environment

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benchmarked catalysts by simple surface modification using corrosion resistant atomic thin layers, and it is also proven here that intrinsic activity of even the best catalyst known for a reaction can be enhanced by simple surface modification. Further, combining the experimental data with DFT calculations is found to be giving good reactivity-selectivity descriptors33.

4. Experimental Section Synthesis of exfoliated h-BN Bulk h-BN crystals (Sigma Aldrich) are mixed with dimethyl formamide (DMF, 0.5 g in 250 mL) and this mixture was homogenized using a high shear mixer, It is found that 5000 rpm shear rate for 3 hours (at room temperature) leads to the formation of layered sheets from the bulk crystals (transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images are given in the supporting information figure S1 (A1-A4), showing the exfoliated few layered nature of h-BN sheets (a few of them are mono layered and many are a few layered). The sheets are found to be dispersible in DMF and are further characterized using various techniques. This is named in the manuscript as shear exfoliated h-BN.

Synthesis of Pt Nanoparticles decorated h-BN sheets Pt nanoparticles decorated exfoliated h-BN sheets are prepared by chemical reduction of potassium hexachloroplatinate (K2PtCl6) in ethylene glycol water mixture containing exfoliated h-BN. In this protocol, 20 mg of shear exfoliated h-BN, 100mg of K2PtCl6 (in which 40mg Pt is present), 10 mL of DI water are added into 40mL ethylene glycol in a 100 mL glass container. The mixture is first ultrasonicated for uniform dispersion and then the reduction reaction is carried out under constant magnetic stirring at 150˚C for 3 hours. The product, Pt/h-BN, from this protocol is named hereafter (2:1) composite, which is then filtered using vacuum filtration chamber and washed with distilled water several times. The resulting product is dried and used for further electrochemical studies. For comparative study, other compositions of Pt nanoparticles on h-BN surface (1:1 and 1:4) are also synthesized by varying the K2PtCl6 content, and bare Pt nanoparticles (Pt NP or h-BN free Pt nanoparticles) are also prepared by the same procedure (without h-BN) by reducing 100 mg of K2PtCl6 in water ethylene glycol (1:4) mixture 17 ACS Paragon Plus Environment

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by keeping the mixture at 150 oC for 3 hours. The TEM and HR-TEM images of these Pt NP are given in the figure S1 (B1-B4).

Electrochemical Measurements The electrochemical studies for different Pt/h-BN composites are performed using a threeelectrode electrochemical system using Saturated Calomel Electrode (SCE) as reference electrode and platinum or graphite rod as counter electrode. Prior to surface coating, the surface of GCE working electrode is polished in sequence with 0.3 and 0.05 µm alumina powders on a polishing cloth and rinsed with deionized water. 2 mg sample is dissolved in 1 ml of water: ethanol (3:1) mixture which also consists of 50µL of Nafion solution (5wt %) and the mixture is sonicated for 1 hour. 3µL suspension from the as-prepared samples is subsequently drop-casted on the surface of the 3mm GCE (net loading of 0.086 mg/cm2) and dried in air for LSV, EIS, and CV studies. All the electrochemical experiments are performed at room temperature. The same ink preparation method is followed for Pt bulk electrode (3 mm diameter) modification using hBN too.

Computational Methodology: In order to model the experimental condition of Pt nanoparticle decorated on h-BN nanosheet to envisage the enhancement HER, a systematic electronic structure calculation is performed based on DFT framework as implemented in Vienna Ab-initio Simulation Package (VASP)34,35 program. The adsorption free energy (∆G) of the adsorbed hydrogen on pristine hBN sheet and Pt modified h-BN sheet are calculated to account for the Pt-h-BN composite on the HER mechanism. In order to achieve less lattice mismatch, we have considered bigger supercells for both h-BN and Pt surface, which contains 81 B, 81 N and 192 Pt atoms. We have also considered a substantial vacuum of 15 Å, for both the pristine surfaces and the composite. Different configurations of Pt-h-BN composite are investigated in order to find the minimum energy configuration of the complete system. The optical response of both pristine h-BN sheet and Pt-h-BN composite are also determined separately to see the effect of Pt decoration on h-BN sheet. Generalized gradient approximation (GGA) type Perdew-Burke-Ernzerhof (PBE) exchange correlation functional is used throughout the calculations36. The full ionic relaxation of 18 ACS Paragon Plus Environment

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the individual systems and their composite is ensured until the corresponding Hellman-Feynman forces are smaller than 0.001eV/Å. We have considered 3x3x1 Gamma k-mesh while optimizing h-BN, Pt and Pt-h-BN systems. In order to take account dispersive forces between the layers, we have considered van der Waals dispersion of the type DFT-D3 as formulated by Grimme. The minimum most energy configurations of the individual systems and Pt-h-BN composite are subsequently used to determine their optical response as well the adsorption free energies of the adsorbed hydrogen state on them. For determining the optical absorption spectra to see the response of the individual and composite systems in the visible range, double the number of occupied states is considered in order to have adequate number of unoccupied states for the allowed transitions between valance and conduction bands. Based on Fermi’s Golden Rule to determine the oscillator strength of allowed transitions, the optical absorption spectra is calculated, which eventually the imaginary part of the frequency dependent dielectric function. The adsorption of hydrogen on top of both h-BN sheet and Pt-h-BN composite are considered with all possible adsorption sites along with the variation of surface-adsorbate distance to the trade-off between chemisorption and physisorption. It is worth to mention that the consideration of free energy of hydrogen adsorption at constant potential could also be a activity trend indicator 37.

Acknowledgements Authors thank Tata Institute of Fundamental Research, Hyderabad, India for the financial support towards this research. PRACE facility is ackonwledged for the provided computing time. Supporting Information. TEM and HR-TEM images; LSVs; ECSA Calculations; Scan rate; FT-IR specra; Absorption Spectra; Charge distribution for Pt/hBN This material is available free of charge via the Internet at http://pubs.acs.org.

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Mechanistic Insight of Enhanced Hydrogen Evolution Reaction Activity

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