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Palladium Nanoparticle-Graphitic Carbon Nitride Porous Synergistic Catalyst for Hydrogen Evolution/Oxidation Reactions in Broad Range of pH and Correlating its Catalytic Activity with Measured Hydrogen Binding Energy Tanmay Bhowmik, Manas Kumar Kundu, and Sudip Barman ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02485 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016
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Palladium Nanoparticle-Graphitic Carbon Nitride Porous Synergistic Catalyst for Hydrogen Evolution/Oxidation Reactions in Broad Range of pH and Correlating its Catalytic Activity with Measured Hydrogen Binding Energy Tanmay Bhowmik , Manas Kumar Kundu and Sudip Barman ∗ School of Chemical Science, National Institute of Science Education and Research, Bhubaneswar-751005. ABSTRACT The hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in aqueous medium are the two fundamental reactions for the development of fossil energy storage and conversion devices. In the polymer electrolyte membrane fuel cell (PEMFC) carbon supported platinum (Pt/C) based catalysts are universally used in cathode and anode; but poor durability of Pt/C due degradation of catalyst in the strong oxidizing environment prevents its widespread applications. It remains a big challenge to develop a new electrocatalysts with superior activity and very high durability for HER/HOR. Here, we report synthesis of a porous Palladium nanoparticles-carbon nitride composite (Pd-CNx) for its superior activity and high durability towards HER/HOR in acidic and alkaline media. The Pd-CNx composites exhibited high catalytic activity for hydrogen evolution in acidic media with small onset potential of -12 mV and a Tafel slope of 35 mV dec-1. At a small Pd loading of 0.043 mg cm-2, this catalyst also exhibits a current density of 10 mA cm-2 at a low overpotential of -55 mV with an excellent stability. The HER activity on Pd-CNx composite is comparable to commercial Pt/C in acid media. The stability tests of this catalyst were done through large number repeated potential cycles and long term electrolysis. These confirm the exceptional durability of this catalyst, which is much better than that of Pt/C catalysts. Furthermore, this catalyst has also displayed superior HOR activity, measured by rotating disk experiment with a broad range of pH (0-14) in different buffer solution. The HER /HOR activity of porous Pd-CNx composite in different buffer solution were correlated by hydrogen binding energy (HBE) of catalyst surface. The HER/HOR activity gradually decreases with increasing the HBE as solution pH increases. The superior HER/HOR activities and very high durability at porous Pd-CNx composite are due to strong bonding between Pd and carbon (Pd-C bond), porous morphology, the synergistic interactions between Pd-NPs and carbon nitride (CNx) support.
Keywords: Palladium nanoparticle, Graphitic carbon nitride, Hydrogen evolution and Oxidation reaction, under potential deposition, Hydrogen binding energy.
Introduction The rapid depletion of fossil fuels and also various environmental problems arising for the use of fossil fuels are the driving force for developing technologies for energy conversion and its storage1. The fuel cell and electrolyzers are the most significant renewable energy conversion and storage technique respectively. Among the different type of fuel cell Proton exchange membrane fuel cells / Polymer electrolyte fuel cells (PEMFCs) are known to a clean and renewable energy resource due to their high energy conversion efficiency, high energy and power density as well as better environmental compatibility2-3. The hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in aqueous medium are the two fundamental reactions for the development of fossil energy storage and conversion devices4. The main applicability of HOR is predominantly in fuel cells where chemical energy in hydrogen converted into the electrical energy. The HER is used in electrolysers which stores the energy in chemical form by liberating the hydrogen. In fuel cell, the anodic reaction hydrogen oxidation (HOR) and cathodic reaction, hydrogen evolution (HER) were catalyzed by the noble metal Pt5-6. In practice, large scale electrochemical hydrogen production is generally
restricted by two main problems: (1) the precious metal platinum dependency and (2) lack of stability of the electrode materials under strong acidic condition of PEM cells. The research efforts are going on to find suitable HER catalyst based on more economical materials such as Pd, Ni, Mo, W and other transition metals based electrocatalysts. Nanoparticle of these metal and their inter-metallic compounds78 phosphides9-11, carbide12-13, sulphide14-15 etc, may replace the expensive Pt catalysts. In addition, enormous efforts are also devoted to find an affordable HOR catalyst which can replace precious, low durability and less CO tolerance Pt/C catalyst. One of the most effective and cheaper catalyst for anode materials of PEMFC is Pd based catalysts, since Pd based materials is about five times cheaper than Pt based materials16 and also has stronger CO tolerance17. Different groups develop noble metal Ir18, Ru19, Pd20 nanoparticles dispersing on highly conducting with high surface area commercial activated carbon (XC-72R) as a HOR catalyst. But the major drawback of carbon supported catalyst was low durability. The carbon supported catalyst is easily oxidized under cathodic potentials in strong acidic condition. Most of HOR studies on the Pd based catalysts were on bimetallic
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systems, for example Pd deposited on other foreign metal such as Ru21, Pt21, Au17, 22-23, Ir21, Re21, Ag 8. There weresome report of HOR studieson monometallic Pd supported on carbon20, WC24, WC-CNT25 etc. In order to understand apparent reaction mechanism of HOR/HER, kinetic parameters of electrochemistry such as Tafel slope with anodic and cathodic transfer coefficient (α), exchange current density (J0) at particular temperature and pressure are very important. The rate of reaction on the electrode surfaces is generally governed by the different consequent steps26 such as water dissociation step, interaction between metal surfaces and dissociated products, recombination of Habs and H2 production, and lastly OHabs desorption. The kinetic parameters such as transfer coefficient (α) and tafel slope allow us to determine the combination steps, occurred in HER/HOR and among these steps, one is the rate determining step. Conventionally and most frequently, the RDE method is used for measuring the electrochemical kinetic parameters of different single-crystalline and polycrystalline electrode. Pt metal shows highest HER/HOR activity among different monometallic system in acidic medium27. Several groups reported the HER exchange current density of Pt metal in acidic medium using RDE method is the order of 1 mA cm2 which are two order magnitude lower than the value obtained using H2 pump method4, 28. Palladium or palladium based16, 29 catalysts are reported to have much poorer HER/HOR performances in comparison with Pt-based catalysts. N.M.Markovic and his coworker reported30 220 mV/dec anodic tafel slope of PdML/Au catalyst. S. N.Pronkin and his co-workers reported20 the HOR exchange current density of Pd/C in 0.1 M HClO4 medium was found to be 0.22 mAcm-2 which was two order of magnitude lower than the measured exchange current of Pt/C using gas diffusion method. Furthermore, HOR properties of Pt metals gradually decrease with increasing the pH values. The reason behind of decreased activity and also rate controlled process for HOR in alkaline medium is not very clear. Recently J. Durst et al suggested28 that noble metals form a stronger M-H bond in basic medium compare to acidic medium and HOR activity decreases in basic media is due to formation of strong M-H bond. In addition, N.M.Markovic et al31 and M.T.M.kopper et al32 reported oxophilic effect (ability of the metals to adsorb OH group) play an important role in HER/HOR activity of metal in the basic medium. The supported metal nanostructures are widely used33 as heterogeneous catalyst in various industrial processes, organic synthesis and electrochemical processes. The size, structure, and morphology and nature of support materials of supported metal nanostructures generally determine the performances of these catalysts. The low coordination of metal nanoparticles, quantum size effect and improved metal-support interaction are probably responsible for the significant change catalytic performances of supported catalysts. The supported palladium nanoparticles exhibit superior catalytic activity for various reactions such as alcohol fuel cell34, different organic reaction34. The high catalytic CO oxidation on metal oxide supported gold nanoparticles was explained based on the synergistic effect between small gold nanoparticles and metal oxide support35. J.Sazaniya and his co-workers36 reported that, in Pt/Al2O3 composite Pt atoms are situated in the defect position of the surface with a Pt-O-Al+3 and the special location,
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binding with this co-coordinating environment modifies the electronic properties of the metal nanostructure. T.Zhang and his co-workers37demonstrated presence of Pt+δ charge particle in Pt/FeOx and superior catalytic behaviour of Pt/FeOx towards CO oxidation. In recent years carbon materials specially two dimensional graphene and commercially volcano carbon was used as a support material due to their high surface area and conductivity. But one of the major limitations for carbon supported catalyst was low electrocatalytic stability arises due to the rapid depletion of carbon. Recently Kangasniemi et al.38have also reported that the common carbon catalyst for electrocatalysis Vulcan X-72 easily oxidized under cathodic environment in polymer electrolyte fuel cell (PEFC). Several, sustainable attempt was devoted to improve tolerance and prevent oxidation of the supported materials under cathodic potentials. One of the constructive strategies was incorporation of heteroatom, example nitrogen, that coordinates different NPs easily and overcome the durability problem. Various groups reported39-40 that hetero-atom such as nitrogen based legends can easily coordinate metal nanoparticles and metal-support interaction improves tolerance as well as activity of the electrocatalysts under oxidation condition. The graphitic carbon nitride, g-C3N4 (graphene like structure) consisting repetitive s-triazine units, has become a versatile support material of metal NPs due to presence of large number active binding sites for their dispersion. The g-C3N4 and its composite has created attention due to their intriguing potential applications for various field such as photocalytic41-43, energy conversion44-46,, sensing47,48 and organic transformation49 etc. Recently, V. Di. Noto and his co-workers reported50,40 relationship between nitrogen concentration, structure and electrochemical ORR performances of PdCoNi “core-shell” gC3N4 electrocatalysts. When concentration of N in carbon nitride shell is low and cell constant of PdCoNi alloy NPs is shortest, best ORR activities were observed. Even today, finding a good support material and then developing a good electro-catalyst with superior catalytic activities and high durability remains a big challenge. Herein we report a facile synthesis of porous Pd-CNx composite and their applications in hydrogen evolution and hydrogen Oxidation reactions (HER/HOR). The porous network structure of Pd-CNx composite are formed due to reduction of PdCl2 in presence of CNx nanosheets with NABH4 and ultrasound treatment. The porous Pd-CNx composite shows superior catalytic activity towards hydrogen evolution reaction in acidic media. It exhibited an onset over potential -12mV, current density of 10 mAcm-2at small HER overpotential of 55 mV and a tafel slope of 35 mV dec-1 with excellent durability. The HER activity of porous Pd-CNx composite in acid medium is comparable with commercial Pt/C; but, in basic media HER activity of this catalyst is lower than that of Pt/C and is much better than that of commercial Pd/C. Most importantly, this Pd-CNx composite displayed very high durability in strong acidic media, which is much higher than that of commercial Pt/C. In addition, this catalyst also exhibits good HOR activity in the wide range of pH 0-14 range. The catalytic activity of HOR in different buffer solution also discussed and correlation of the catalytic HER/HOR behavior of PdCNx catalyst in different pH solution with hydrogen binding energy (HBE) was done. Our results support the hypothesis that HBE is the sole reaction descriptor for HER/HOR activity on monometallic palladium. The superior HER/HOR activities and high durability of porous Pd-CNx catalyst was
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explained based on porous structure of catalysts, strong metal carbon (Pd-C) bond, improved charge transport, and synergetic effects between CNx support and Pd-NPs.
Experimental section Preparation of g-C3N4: The synthesis of carbon nitride quantum dots (g-CNQDs) from formamide was done by microwave mediated method5152 reported by our group previously. In brief, 30 ml of formamide was heated using microwave synthesizer for 2hrs at 1800C. The resulting brown coloured solution was evaporated at 1800C in a rotary-evaporator to get bulk amount of black product. This product was washed with water, filtered and vacuum dried to get dry, solid of g-CNQDs.
presence of graphitic carbon nitride nano-sheets. Figure1 shows the powder X-ray diffraction patterns(p-XRD) of porous Pd-CNx composite and g-C3N4. The p-XRD pattern of gC3N4 and Pd-CNx composite has a diffraction peak at 2θ value of 27.20 with interlayer spacing of 3.3 Å corresponds to graphitic reflection plane(002),of carbon nitride52. In addition, five well resolved, highly intense diffraction peaks positioned at 40.100, 46.700, 68.700, 80.900, 86.700 in Pd-CNx composite correspond to the (111), (200), (220), (311), (222) reflection planes of face centered cubic palladium (JCPDS No 894897).
Preparation of porous Pd-CNx composite: Initially, 5 mg ofg-CNQDs was dispersed in 2 ml of deionised water with sonication using bath sonicator for 10 minutes forming black resultant solution. In another beaker 0.14 mole PdCl2was dissolved in 6ml de-ionised water by adding 50µL 10% HCl solution to form brownish PdCl4-2 solution. Then, PdCl4-2 solution was poured into the gCNQDs dispersion and sonicated for 10 minutes. The700 µL of 0.1 M NaBH4 solution was directly added to the mixed solution with constant sonication. Finally, a resultant black coloured reaction mixture was sonicated by probe sonicator operating at 28 kHz frequency (power 420 watt) for 180 minutes. The black solution formation was indicating the formation of metallic Pd-NPs. After ultrasound treatment a black mass was precipitated out from the solution. The black precipitate was separated by centrifugation at 10,000 rpm for 30 minutes and washed with de-ionised water twice. Finally, this solid black product was kept in vacuum over night for drying. Preparation of working electrode: The glassy carbon (GC) electrode was polished with 1.0, 0.1, and 0.05 mm alumina slurry on Buehler micro polishing cloth. The aqueous solution of the Pd-CNx composite was prepared by dispersing 0.5 mg Pd-CNx in 1 ml of de-ionized millipure water by sonication in a bath sonicator for 30 min. Pd-CNx modified GC was prepared by drop casting 10µL of the solution on GC of diameter 3 mm and drying in the ambient temperature on air. No polymer binder was used for the preparation of Pd-CNx electrode. However, Pt/C and Pd/C electrode was prepared by immobilizing the catalyst on GC electrode surface using Nafion as polymer binder. The details of Pt/C, Pd/C modified GC electrode preparation was given in ESI.
Figure 1. p-XRD of porous Pd-CNx composite (red line) and gC3N4 (black line)
This suggests the formation of Pd-NPs on CNx sheet by ultrasound mediated borohydride reduction methods. In addition, the Bragg diffraction peak positioned at 2θ = 40.10 for Pd (111) plane was used to calculate the particle size of Pd-NPs using Debye–Scherer Equation (see details in ESI). From the Debye–Scherer analysis, the average size of Pd-NPs on the porous Pd-CNx composite was found to be ~ 14nm. TEM and FESEM measurements were done to investigate the morphology of Pd-CNx catalyst and size distribution of Pd-NPs on carbon nitride. TEM and FESEM samples were prepared by evaporation of aqueous solution of Pd-CNx composite on TEM grid and Si-wafer respectively. It was reported51 by our group recently that the two dimensional thin sheets(Figure 2a) of the g-CNx are formed on evaporation induced selfassembly and condensation of g-CNQDs. Figure 2 (b-d and f) shows the representative TEM images of Pd-CNx composite showing the porous morphology of the composite. It clearly shows that porous structure is formed through interconnection of small CNx sheets containing Pd-NPs. These nanoparticles are also interconnected. The particle size of Pd-CNx catalyst was determined from Figure S1a (ESI) and Pd-NPs size distribution in Pd-CNx are shown in Figure S1b (ESI).
Results and Discussion Characterization of porous Pd-CNx composite: The Pd-CNx composite was synthesized by ultrasound mediated reduction of PdCl4-2with sodium borohydride (NaBH4) in
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Figure 2. (a) TEM image of two dimensional g-C3N4sheets. (b,c, d and f)TEM images of Pd-CNx composite showing porous morphology. (e) HR-TEM image of Pd-CNx composite. (g) EDX spectra of porous Pd-CNx composite taken from Figure 2c.
The particle size of Pd-NPs in porous Pd-CNx composite are in between 10-14 nm with average size of12 nm, which is good agreement with the particle size measured using the Debye–Scherer Equation. The HR-TEM images of Pd-CNx composite were shown in Figure 2e and Figure S1c (ESI). The lattice fringes with d spacing of 2.24 Å corresponding the (111) lattice plane of Pd-NPs are clearly seen. The EDX spectrum of porous Pd-CNxcomposite (Figure2g), taken from the Figure 2b, shows the presence of carbon, nitrogen and Pd confirming the deposition of Pd-NPs on carbon nitride sheets. The FE-SEM images of Pd-CNx composite are shown in Figure 3(a, b). In FE-SEM images of Pd-CNx composite, the porous structures of the composite are also clearly visible. It also shows that nanoparticles are self-assembled on CNx matrix and interconnected to form a porous morphology of this composite.
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The electronic state of the Pd-NPs and nature of interaction between CNx support and Pd-NPs were investigated by using X- ray Photo Electron Spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. The XPS survey scan of porous Pd-CNx composite as shown in Figure 4a, clearly indicates the presence of carbon(C), nitrogen (N), palladium (Pd) and oxygen (O) atoms. As shown in Figure S2 (ESI) the carbon (C1s) and nitrogen (N1s) XPS spectrum of g-CNx were de-convoluted to three and two main Gaussian peaks respectively. The binding energies of 285.3 eV and 286.45eV are due to the presence of (sp2) N–C=N (sp2) (carbon bonded to two nitrogen atoms) and C(–N)3 (planar trigonal carbon geometry)
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Figure 3. (a,b) FE-SEM images of porous Pd-CNx composite
respectively53, whereas the peak at 288.8 eV is attributed to COx54. The peak at 400.1 eV refers to the presence of (sp2) C–
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N1=C (sp2), nitrogen bonded to two carbon atoms whereas the second peak in this region at 401.6 eV corresponds to graphitic nitrogen55. The comparison of C1s XPS spectrum of CNx support and Pd-CNx composite was shown in Figure 4b whereas comparison of their N1s XPS is given in Figure 4c. It shows that C1s peak of Pd-CNx composite become broad and is shifted towards 2.9eV higher binding energy as compared to that of free CNx; but the position of N1s peak remains unaltered. The 3d XPS spectra of metalic palladium (Pd) always appeared as doublet (3d5/2 and 3d3/2) due to spin-orbital coupling and separated by 5.26 eV56. The 3d3/2 peaks for metallic palladium and known to appear at 335.6 eV56 (indicated as red line in Figure 4d) and electron withdrawing groups are known to increase the binding energy of Pd-NPs. The 3d XPS spectrum of porous Pd NPs supported on carbon nitride is shown in Figure 4d. It shows that the Pd 3d5/2 and 3d3/2 peaks of Pd-CNx composite appeared at 338.92 eV and 344.3 eV respectively which can be attributed to metallic palladium with higher binding energy. This binding energy shift suggests the strong interaction between Pd-NPs and CNx support. In addition, large shifts of binding energy of carbon and no change of binding energy of nitrogen are also observed when Pd-NPs are supported on CNx framework. It is also reported57-58 that binding energy of palladium is shifted higher binding energy on formation Pd-C bond. Thus, this higher binding energy of Pd in porous Pd-CNx can be attributed to the formation of the Pd-Cx. The large binding energy change of carbon and palladium confirm that there is very strong interaction between carbon and Pd in porous PdCNx composite. The Fourier transform infrared (FT-IR) spectra of carbon nitride and Pd-CNx composite have shown in Figure S3 (ESI). In the IR spectra of CNx, three bands appeared at 1213, 1317, 1408 cm-1 and these can be assigned to aromatic C=N stretch and a peak at 1600 cm-1 corresponds to C=N stretch. The intensity of these peaks decreased and two new intense peaks appeared at 1128 cm-1 and 1025cm-1 when Pd-NPs are dispersed on CNx matrix. These two new peaks are assigned to C-N single bond stretching. This appearance of new C-N single bond IR band and decrease of intensity of C=N double bond peak also suggests a strong interaction between CNx framework and Pd-NPs. The metal-carbon stretching bands for the complexes of Rh, Ir, Ga and In are reported59 to be appear in between 500 – 580 cm-1. The peak at 590 cm-1 can be assigned as Pd-C stretch. It is known60-61 that Pd has strong affinity towards C atoms and Pd-NPs are generally attached to graphene support due to their donationback donation interaction. The small electron donation from 4d orbital of Pd to 2s orbital of carbon occurs and then backdonation from 2p (C) orbital to 5s (Pd) orbital takes place. In this work, nucleation/growth of Pd occurs at carbon atoms due to the strong affinity of Pd-NPs to carbon. Our group recently reported51 the bottom up synthesis of g-carbon nitride sheets from g- carbon nitride quantum dots (g-CQDs) due to condensation of the g-CQDs and uniform growths in two dimensions. But reduction of PdCl2 in the aqueous g-CQDs solution leads to the nucleation of Pd-NPs on carbon due to strong chemical affinity of Pd towards carbon atom.
growth of carbon nitride nano-sheets containing Pd-NPs lead to formation of this porous structure of Pd-CNx composite. From p-XRD, TEM, FE-SEM, EDX, XPS, FT-IR spectra studies it can be concluded that porous morphology of PdCNx composite was formed due to the ultrasound and NaBH4 reduction of PdCl2 in presence of CNx. (a) (b)
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Figure 4. (a) XPS survey scan of porous Pd-CNx. (b,c) C 1s and N 1sXPS spectra of g-C3N4 (blue line) and porous Pd-CNx (red line) respectively. (d) Pd 3d XPS spectra of Porous Pd-CNx composite. The dash line represents exact position of 3d5/2 and 3d3/2for metallic Pd-NPs.
The amount of loading of Pd-NPs was measured with the help of ICP-OES and thermo-gravimetric analysis (TGA). The ICP-OES analysis of Pd-CNx determines 59.4 wt% Pd loading in Pd-CNx composite (Table S1). The TGA plot of CNx and Pd-CNx composite is shown in Figure S4 (ESI). The TGA curve of Pd-CNx composite is shown that catalyst contained ~ 60 wt% of Pd which is in good agreement with ICP-OES results. In addition, also CHN analysis and EDX was performed to know the composition of supported g-CNx. According to the CHN analysis N/C ratio in free g-C3N4 was ~1.152 whereas after Pd loading it was slight decreases to ~0.89 (for details see ESI Table S2).In addition, also EDX spectra of PdCNx determine the N/C is ~0.87 shown in Figure S5(ESI). The Pd wt% in the Pd-CNx catalyst calculated from different microanalysis was summarized in Table S1(ESI). Thus different microanalysis confirms the ~60 wt.% Pd loading in porous Pd-CNx composite.
The presence of Pd-NPs on CNx prevents the uniform growths of carbon nitride nano-sheets. Self-assembly and non-uniform
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Figure 5. Electrochemical behaviors of Pd-CNx in both acidic and basic electrolytes. Cyclic Voltagrams of porous Pd-CNx in comparison with GC and g-CNx in (a) 0.5M H2SO4 and (c) 0.5M KOH electrolyte. (b) CV of porous Pd-CNx composite in 0.5M H2SO4 electrolyte run from -0.08 V to +0.6V. All the CV experiment was performed with 30 mV/sce scan rate with constant Pd loading 0.043 mg/cm2
Proton adsorption-desorption behavior of Pd-CNx composite: Palladium metal62 is highly active for various electrochemical reactions occurred in galvanic cells such as cathodic hydrogen evolution reaction, small organic molecule oxidation reaction, and anodic hydrogen oxidation reaction etc. The electrochemical process involving hydrogen has created interest among the scientific community since the chemisorbed atomic H species acts as intermediate for several (HOR/HER) reactions occurring at electrode surfaces. Unlike platinum, palladium metal has much more fascination towards adsorption of H under the same electrochemical condition. The noble metal is reported to have two types of electrodeposited H intermediate during electrochemical reactions - (i) under potentials deposition hydrogen (UPD-H) which commenced at positive to reversible H+/H2 potentials and (ii) over potentials deposited hydrogen (OPD-H) which commence at potentials very close to reversible H+/H2 potentials or negative potentials to reversible H+/H2 potentials. Chemically there is no distinct difference between OPD-H and UPD-H, but in surface science UPD-H received major attention due to its surface structure dependent property. UPD-H process can be representing as–
M (hkl)+H3O++ e-↔ M (hkl) /Hθ +H2O To understand the electrochemical behavior of the prepared porous Pd-CNx composite the CV studies were performed in both acidic and alkaline medium as shown in Figure 5. The CVs of porous Pd-CNx, g-CNx modified GC electrode and GC electrode in 0.5M H2SO4 solution are shown in Figure 5a. The Pd-CNx shows strong proton adsorption-desorption peaks whereas GC and CNx/GC does not have any proton adsorption- desorption peak. A broad anodic peak at E∼1.0V corresponds to the formation of Pd-O and its corresponding cathodic peak at 0.68V for the reduction of the Pd-O.The symmetrical anodic and cathodic peaks at 0.16 and 0.23 V are assigned to UPD hydrogen adsorption (UPD-Hads) and de-
sorption (UPD-Hdes) from Pd-CNx surfaces respectively. The calculated ∆Ep for UPD-H of Pd-CNx composite was ~70mV which is close to the reversible value of 60mV.Recently, H.A.Gasteiger and his co-workers reported29 that ∆Ep value for reversible UPD-H on Pd/C surfaces was ~100mV whereas Pt/C was 50mV. Thus, porous Pd-CNx composite was more reversible and rate of H-UPD was much higher than that of Pd/C surfaces. The anodic peak at 0.09V assigned for oxidation of the Hads on surface of Pd-CNx composite. Figure 5b shows a cathodic peak at ~0.012 V assigned for OPD-H absorption peak and its corresponding asymmetric desorption peak at~0.2 V. From Figure 5b it is evident that, when potentials reach a potential negative to reversible H+/H2 potentials, the anodic desorption peak appears as broad and large peak overlaying other desorption properties such as UPD-H or anion desorption. The major issues are to separate of UPD-H from desorption peak of Hads. The palladium bulk metal or poly crystalline Pd(poly)63 are unable to separate UPD-H from other electrochemical behaviors. In literature, very few reports64 are available for Pd based catalysts where UPD-H peaks are well separated from other behaviors. For example, ultrafine NPs65 or layer Pd on Au surface66 is able to separate UPD-H from other behavior. The Pd-CNx composite can also easily separate the UPD-H from other behavior that makes it a potentials electro-catalyst. In addition, also CV of Pd-CNx in 0.5M KOH was performed and shown in Figure 5c. In alkaline medium also the catalyst can easily have separated UPDH peak from other processes. In case of Pd-CNx during HER, it can form an equilibrium between adsorbed and dissolved H with Pd lattice which lead to formation of PdHx. Electrochemical behavior has been used to investigate the relation between the phase structure and applied potentials. A.E.Russel and his co-worker67 distinguished potentials region for different Pd-H phase formation where potentials at E⩾0.05V due to βhydride formation and potentials more negative than E=0.028V identified for hyperstoichiometric phase. The H-absorption on PdCNx composite was commenced at potentials E=0.015V which corresponds to formation of hyperstoichiometric phase. The H/Pd ratio increases with an increasingly negative poten-
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tials indicating PdHx stiochmetry was very high and it may greater than one. The Habs absorption peak of Pd-CNx, close to reversible potentials and lower than 0.028 V, can be considered as hyperstoichiometric phase. Several group reported68 hyperstoichiometric phase of PdHx for Pd nanoparticle supported on zeolite or Na Y zeolite due to spillover of hydrogen on supported materials. The high stiochmetry of hydrogen for Pd-CNx composite may be attributed due to spillover over of H on supported materials and also due to porous nature of the electrode easily trap the H2 molecule in its pore and reoxidized it repetitively. The electrochemical active surface area (ECSA) was calculated from the reduction peak of Pd-O in CV.69 The ECSA of Pd-CNx composite was calculate from the Pd-O reduction peak. The ECSA of Pd-CNx calculated from reduction peak of Pd-O was 40 m2 g-1 and 51 m2 g-1 in acidic and basic medium respectively. The details of ECSA calculation was describe in FigureS6 (ESI) and ECSA value summarized in Table S3 (ESI). HER study on porous Pd-CNx composite: The electro catalytic activities of Pd-CNx/GC towards HER was investigated in N2 saturated 0.5 M H2SO4 by linear sweep voltammetry (LSV) method, performed in a standard three electrode electrochemical cell with scan rate of 10 mV/sec. The representative LSV curves of geometric current density (mA cm-2) vs applied potential for Pd-CNx catalyst was shown in Figure 6a. For comparison the catalytic activities of Pt/C, Pd/C and g-CNx modified GC electrode towards HER were also investigated as shown in Figure 6a. The g-CNx electrode was found to be very inactive towards HER as it shows very high onset-potential (-170 mV) with a very low cathodic current density. The Pt/C catalyst exhibits a very good HER performance with the onset potential near to zero. The porous Pd-CNx modified GC electrode also exhibits excellent HER activity with small onset potential of -12 mV which is much smaller than Pd/C (-41 mV) and close to the commercial Pt/C. The comparison of HER performances of Pd-CNx with Pd/C and Pt/C are summarized in Table1. For the practical purpose, it is more appropriate to compare the overpotential (η) require to achieved the current density of 10 mA cm-2 which is an important parameter for construction of a fuel cell. To acquire a current density of 10 mA cm-2, commercial Pt/C and Pd/C takeover potentials of -58 and -170 mV respectively. The porous Pd-CNx takes only -55 mV which is much lower than Pd/C and comparable with the Pt/C. In order to reach a current density of 50mAcm-2the porous Pd-CNx needed only -77 mV which is lower than that of both Pd/C (-315 mV) and Pt/C (-112mV). This clearly demonstrates that the HER kinetics on Pt-CNx is very fast like Pt/C. In order to obtained true kinetics, we have also done the HER measurement in H2 saturated 0.5 M H2SO4 with rotation speed 2000 rpm as shown in figure S7a (ESI). The onset potential of commercial Pd/C and PdCNx are 113 mV and 35 mV respectively. The onset potential of Pd-CNx was much lower than that of Pd/C and close to Pt/C in acidic medium. Tafel slope is an inherent property of the
ing step of HER. The Tafel slope can be calculated from the Tafel equation = a + b log(J) where = over potentials, b= Tafel slope, J= current density. Tafel plot of Pd-CNx, Pt/C, Pd/C were represented in Figure 6b. Commercially Pt/C catalyst shows high HER catalytic activity near zero over potentials with Tafel slope 33 mV/dec which is consistent with the previous literature value. In addition, the Pd-CNx catalyst also exhibits tafel slope of 35 mV/dec which is higher than that of commercial Pd/C (121mV/dec) and close to the Pt/C. It is well known that due to very high Hads coverage on Pt surfaces, it proceeds through the Volmer–Tafel mechanism. The low Tafel slope indicated that porous Pd-CNx composite also follow the Volmer–Tafel mechanism. Another important kinetics parameter for electrocatalytic HER measurement is exchange current density (J0) which correlates the rate of electron transfer under reversible condition. Conventionally ideal catalysts have low Tafel slope and higher exchange current density (J0). The PdCNx catalyst has exchange current densities 0.4 mAcm-2 which is better than the commercial Pd/C (0.12 mAcm-2). The specific exchange current of Pd-CNx and Pd/C was 138 mAcm-2mg-1and 35 mAcm-2mg-1 respectively. From Table 1, it is obvious that Pd-CNx exhibits higher catalytic activity than Pd/C and comparable activity with Pt/C towards HER. The degradation of carbon support for commercial Pt/C catalyst occurs due to the oxidation in strong oxidizing acidic condition that prevents the successful commercialization of PEM fuel cell. The durability of the catalysts is thus one of the most serious issue for a HER catalyst. The durability of porous Pd-CNx catalyst was investigated by using continuous LSV scans in both N2 (Figure 6c) and H2 (Figure S7b) saturated 0.5 M H2SO4 solution with a potential window 0.5 V to 0.1 V at scan speed of 10 mV s-1. The HER performances such as onset potentials and current density at any overpotential was improved after 10000 cycles of LSV scans, suggesting the superior long term stability of Pd-CNx catalyst. The comparison of time dependent current density curve under a static over potential at -60 mV of Pd/CNx with commercial Pt/C and Pd/C were also done as shown in Figure 6d. The commercial Pt/C and Pd/C shows a significant loss of current density within 10hrs of chronoamperometric measurements. In contrast, the current density of Pd-CNx catalyst shows dramatic increase from 14 mA cm-2 (at 0 hr) to 28mAcm-2 (at 24 hrs) and it remains almost constant for at least 100 hrs. In The enhanced electrocatalytic performance of Pd-CNx chronoamperometric scan is probably due to cleaning of pores of porous Pd-CNx composite facilitating the mass transport and thus current density of Pd-CNx composite is increased. It is important to mention that no polymer binder was used for the preparation of Pd-CNx electrode. For the durability test, the Pt/C and Pd/C modified GC electrodes were prepared by immobilizing of these catalysts on electrode surfaces
catalysts and it is used as significant standard for HER to determine the exact mechanism as well as the rate determin-
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(b)
(c)
(a) 0.5 M H 2SO 4
(d)
(f)
(e)
Figure 6. HER performance at various electrodes (a) LSV polarization curves (negative going) of Pd-CNx (red line), Pt/C (blue line), Pd/C (green line), and g-CNx(wine line) in 0.5M H2SO4 with scan rate 10 mV/sec.(b) The corresponding HER Tafel plot of porous Pd-CNx, Pt/C and Pd/C.(c) LSV curve on Pd-CNx surface in first cycle (red) and after 10,000 cycles(black) (scan rate 10 mV/sec). (d) Chronoamperometric responses of Pd-CNx, Pd/C and Pt/C at η = -60 mV in RHE for 48hrs. Inset: chronoamperometric response of Pd-CNx atat η = 60 mV in RHE for 100 hrs.(e) LSV curvesof Pd-CNx, Pt/C and Pd/C in 0.5M KOH solution. (f) Tafel plot of porous Pd-CNx, Pt/C and Pd/C in 0.5 M KOH. All the experiment was performed withconstant metal loading of 0.043 mg/cm2.All the LSV curves are iR corrected. The comparison of catalyst mass loading, onset-potential, exchange current density, and tafel slope of Pd-CNx with other catalysts are listed in Table S4(ESI). The mass loading of porous Pd-CNx catalyst is much lower than other previously reported catalysts.
using a polymer binder, nafion. The similar behaviour was also observed (Figure S7c (ESI)) when all these three electrodes (Pd/C, Pt/C, Pd-CNx) are prepared without any external polymer binder. The superior HER activity and excellent durability of
Pd-CNx composite in acidic medium suggest the promise for replacement of Pt/C catalyst with new polymer binder free Pd-CNxelectrocatalyst in PME fuel cell. Table 1.
Different HER parameters of porous Pd-CNx composite, Pt/C and Pd/C catalyst in acidic medium (0.5 M H2SO4)
Catalyst
Onset potential
Overpotential at 10 mAcm-2 (η10)
Overpotential at 50 mAcm-2(η50)
(mV)
Tafel slope (mV/dec) (b)
Exchange current mAcm-2 (io)
Specific exchange current(mAcm2 mg-1)
Porous Pd-CNx
-12
- 55
-87
35
0.40
138
Pd/C
-40
-170
-315
60
0.12
35
The onset-potential of Pd-CNx catalyst is -12 mV which is much lower than other non Pt group metal such as Mo, Ni, W,
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Co based catalyst and comparable with other Pt based catalyst.Also, in terms of exchange current density and tafel slope the Pd-CNx catalyst is better than any other non-noble metal catalyst and also comparable with other noble metal catalyst system. From the Table S4, it is obvious that Pd-CNx catalyst has better electrochemical hydrogen evolution activity (onset potential, durability, exchange current, catalyst loading) than that of Pt- and non Pt-based catalysts in acid media. A good HER catalyst should capable of operating efficiently in broad range of pH. To evaluate the versatility of Pd-CNx catalyst we also studied the HER activity of porous Pd-CNx in basic 0.5 M KOH medium. Figure 6e shows the LSV curve of porous Pd-CNx and commercial Pt/C, Pd/C in N2 saturated 0.5 M KOH with a scan rate 10 mV/sec. The Pt/C catalyst, as usual, exhibits best catalytic activity with very small onset potential ~ 0V (RHE). Porous Pd-CNx catalyst exhibits onset potentials (η) at -75 mV (RHE) which is smaller than commercial Pd/C catalyst (-170 mV). The HER measurement of Pd-CNx composite, Pd/C and Pt/C were also performed in H2 bubbled 0.5 M KOH solution with rotation speed 2000 rpm as shown in Figure S7d (ESI). The onset potential of Pd-CNx composite was 45 mV which is better than that of Pd/C (165 mV). Small over potentials of -180 mV and -390 mV are required for porous Pd-CNx to achieve the current density (j) of 5mAcm-2 and 30mAcm-2 respectively whereas commercial Pd/C needed of -340 mV and -592 mV to reach same current densities. To know the exact kinetics on the electrode surfaces, we also further studies the tafel analysis of Pt/C, Pd/C and porous Pd-CNx composite were shown in Figure 6f .Tafel slope value of commercial Pd/C and Pt/C are found to be 201 mV/dec and 110mV/dec in KOH solution. In comparison porous Pd-CNx composite has tafel slope value 150 mV/dec. The exchange current density is calculated similar way as describes in the case of acidic medium. The exchange current density Pd/C, and porous Pd-CNx composite is found 0.190 mAcm-2, 0.245 mAcm-2 respectively. Thus, porous Pd-CNx electrocatalysts easily outperforms the Pd/C catalyst in basic medium on the basis of onset potentials, tafel slope, and exchange current density. Thus, the HER activity of porous PdCNx catalyst was better than that of any reported Pd based catalysts in basic media.
ment was obtained include two factors (a) kinetic current density (Jk) (b) diffusion current density (Jd).The diffusion limiting current was calculatedusing following equation,
Where diffusion controlled current follow the Levich equation: Jd = 0.62nFD2/3ν-1/6C0ω1/2 = Where n is the number of electrons involved in the oxidation reaction, F is Faraday constant (96,498Cmol−1), D is the diffusion coefficient of the reactant (cm2 s−1), ν is the viscosity of the electrolyte (cm2 s−1), C0 is the H2 concentration in the solution and ω is the rotation speed (rpm). BC0 is a constant related to the concentration and diffusivity of the gas, number of the electron transfer in the reaction, kinetic velocity of the electrolyte.
(a)
(c)
(b)
(d)
HOR study on Pd-CNx surfaces in broad pH range: In PME fuel cell, the HOR is the one of the important reaction in cathode compartment. In a complete reversible PME fuel cell both the reactions (HOR and HER) occurred simultaneously. This Pd-CNx catalyst showed very good proton adsorption and superior HER activity in both acidic and basic medium; we also examined HOR activity of this Pd-CNx catalyst in different pH solution. Figure 7a shows HOR polarization (positive going) curves of Pd-CNx catalyst in H2 saturated 0.1M H2SO4 solution at different rotation speeds. All polarization curves, represented here are iR corrected. The limiting current density (JL) gradually increases with the increasing of the rotation speed from 300 rpm to 2250 rpm. The limiting current density (JL) in RDE measure
Figure 7. (a) HER/HOR polarization curve (positive going) on porous Pd-CNx in 0.1M H2SO4 solution saturated with H2 (~1atm) with different rotation speed ranging from 300 to 2250 rpm (inset: Koutech-levich plot at 0.4V in RHE). (b) HER/HOR polarization curve (positive going) on Pd-CNx composite in H2 saturated (1 atm) different selected pH electrolyte at scan rate 20mV/sec with rotating speed 1600 rpm. (c) HOR/HER polarization curve normalized to the corresponding maximum limiting current(ilim) in different selected pH electrolyte. (d) Variation of half wave potentials in whole range of pH. Pd loading: 0.043 mg/cm2.
The kinetics parameter of the HOR reaction was obtained by plotting JL-1 vs ω-1/2 at particular potentials (Koutecky-Levich plot). The K-L plot of Pd-CNx constructed at 0.4 V gives a
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straight line passing through the origin as shown in the inset of Figure 7a. This result confirms that the limiting current at 0.4 V potentials was fully controlled by diffusion of H2 mass. The BC0 value of Pd-CNx catalyst in 0.1 M H2SO4 was 0.066 mA/ (cm2disk rpm0.5) which good agreement with the calculated value 0.0678 mA/ (cm2disk rpm0.5). Therefore, the diffusion limiting current at 1600 rpm in 0.1 M H2SO4was 2.54 mA/cm2, which is matching with the experimentally measured limiting current density. Figure 7b represents the polarization curve of HOR in H2 saturated different selected pH medium with rotation speed 1600 rpm. The polarization curves in different pH solution have anodic peaks in the potential range 0.09V to 0.3V due to H desorption. The HOR/HER potentials were corrected by diffusion potentials limitation as describes by
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posite and kinetic is controlled by the H2 mass transport. When pH> 6 half wave potentials increase more rapidly suggesting a continuous decrease of the HOR activity. The kinetic parameters such as transfer coefficient (α), Tafel slope etc. and exchange current density of HOR/HER was obtained from the plot of kinetics currents vs overpotentials by fitting the Butler–Volmer equation, where Jkis the kinetics current density (mAcm-2), i0is the exchange current density (mA cm-2), αrepresents the transfer coefficient, F is Faraday’s constant 96,485 C/mol, R is the universal gas constant 8.314 J/ mol/ K and T is the temperature in Kelvin. (b)
(a)
ln where j and jL are measured and diffusion limiting current density. The diffusion corrected and iR corrected polarisation curve was shown Figure S8 (a,b) (ESI). In 0.1M H2SO4 solution the polarization curve of porous PdCNx composite has plateaus region with limiting current density 2.54 mAcm-2 which was due to limiting H2 mass transportation. The limiting plateaus current density decreases gradually with increasing pH of the solution and the limiting current density in strong basic medium, 0.5M KOH (pH-13.7) reaches to 1.52mAcm-2 as shown in Figure 7b and S9a (ESI). The H2 mass transportation gradually decreases with the increasing the pH of the solutions. Recently Y.Yan and his coworkers demonstrated70 the decrease of limiting current with the increasing the pH of the solution for polycrystalline Pt metal. The polarization curves normalised by their corresponding maximum limiting current (JL) in different selected pH solution were shown in Figure 7c and S9b(ESI). The over potential where current density half of the normalized limiting current density (ηi=0.5ilim) describes as half wave potential. With increasing pH, the half wave potentials gradually shifted towards positive potentials, from 0.02 V at pH 0.8 to 0.19 V at 13.7 pH. The half wave potentials of HOR were shifted by 0.18 V towards positive side as solution pH changes from 0.8 (0.1 M H2SO4)to 13.7 (0.1M KOH). The half wave potentials vs pH of the solution plot was shown in Figure 7d. It is observed that the half-wave potential increases slowly when pH< 6. This suggests the fast HOR kinetics on Pd-CNx com-
Figure 8.Kinetics current vs potentials plots of Porous PdCNx(square symbol) and corresponding Butler-Volmer fitting (dotted line) in (a) 0.1 M H2SO4 solution and (b)0.1M KOH medium. Inset shows the experimental polarisation curve and iR free polarization curve. Pd loading: 0.043 mg/cm2.
The kinetics current density was extracted from the positive going polarization curve by using first order Koutecky-Levich correction as
Where jkis kinetics current density (mAcm-2), j is the measured current density (mAcm-2) at 1600rpm, jlimis diffusion limiting current.
Table 2. Butler-Volmer fitting parameters of Pd-CNx both acidic and basic medium. Catalyst
Electrolyte solution
Exchange current density (i0) (mAcm-2)
Specific exchange current(mAcm2 mg-1)
Anodic transfer coefficient (α α)
Anodic Tafel slope(ba) (mV/dec)
Cathodic transfer coefficient (α α)
Cathodic Tafel slope(bc) (mV/dec)
Porous
0.1M H2SO4
0.84
280
0.62
96
0.38
155
0.1M KOH
0.037
14
0.53
112
0.5
119
Pd-CNx
Figure 8(a, b) shows the kinetics current vs potentials plots of Porous Pd-CNx(square symbol) and corresponding fitting of
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Butler-Volmer(dotted line) in 0.1MH2SO4 solution and 0.1MKOH medium respectively. In inset of Figure 8(a and b) show the experimental and iR corrected polarization curve in H2 saturated solution at rotation speed1600rpm. The HER /HOR kinetics current data was fitted by considering first αa, αc and i0as the variable parameter. In acidic medium the fitting was performed in short range (- 40 mV to +40 mV region) by considering sum of the transfer coefficient equals to one. The fitting (red dash line) curve was matching well with the experimental curve with an asymmetric value of αc = 0.62 and αa = 0.38. The exchange current density calculated from Butler-Volmer equation in acid medium is 0.84 mV. The reported exchange current of Pd based catalyst measured in acidic medium by RDE methods ranging from 0.2 mA to 0.8mA. Recently, S.N.Pronkin et al. recently reported the exchange current of Pd/C in acidic medium with RDE method was 0.22mA/cm2, which was four times less than our result. Thus HOR activity of Pd-CNx composite in acidic medium was better than that of reported Pd based catalyst. In addition, Butler-Volmer fitting of kinetic current vs potentials plot in basic medium (0.5 M KOH) was shown in Figure 8b. The best fitting was obtained with transfer coefficient αa= 0.50 and αc= 0.53. The exchange current density (i0) in basic medium was 0.037mAcm-2.The kinetic parameter Tafel slope value was calculated by
assumption, one can easily derive the relation between peak potential of UPD-Hdesin CV with the HBE of corresponding active site from following equation,
∆G (M – H) = -FEpeak Where ∆H= hydrogen binding energy, F= Faraday’s constant (96,485 C/mol), Epeak directly taken from CV’s.In order to calculate HBE of Pd-CNx composite, we have been examined the CV profile on Pd-CNx composite in different buffer medium with same electrochemical condition. The CV profiles of Pd-CNx composite in selected buffer solution is shown in Figure 9a and Figure S10 (ESI). In the CV profile cathodic peak below the potential 0.6 V in RHE consider as UPD-H region of Pd-CNx composite.
(a)
2.303RT/αF The specific exchange current density of Pd-CNx was 280 mAcm-2mg-1 and 15 mAcm-2mg-1 in acidic and basic medium respectively. The Tafel slope value of cathodic and anodic branch of HOR/HER was summarised in Table 2. J.K.Norskov and his co-workers reported71 based on their DFT calculations, α values for Tafel and Heyrovsky should be 0.64 and 0.45 respectively. For Pd-CNx catalyst in acidic medium shows best fitting of cathodic brunch with α =0.61 which is close to calculatedα value for Tafel step71. This confirm the HER on Pd-CNx catalyst in acidic medium follow the Volmer-Tafel mechanism which is good agreement with the proposed mechanism based on Tafel slope described in the previous section. Correlating HBE of Pd-CNxcomposite with UPD-Hdes peakin different buffer solution: Recently M.Auinger and his co-workers suggested72 that HOR/HER response in un-buffered solution remains unchanged on changing pH of the solution due to the local pH gradient. On the other hand M. T.M.Koper and his coworkers reported73 the change of HOR/HER on polycrystalline Pt surfaces in phosphate buffer solution with wide range of pH. Unlike un-buffered solution where kinetics doesn’t effect by pH, HER/HOR kinetics in buffer solution is fully controlled by the pH gradient of the solution. Tounderstandreason behind pH dependent HER/HOR properties of porous Pd-CNx composite, we have examined the surface properties of Pd-CNx composite, especially hydrogen binding energy (HBE) value. Several groups suggested18UPD-H desorption peak isnot influenced by the pre-adsorbed species and its position in CV easily correlated with the HBE of the corresponding active site. By considering Langmuir adsorption
(b)
(c)
Figure 9. (a) CVs of Pd-CNx composite in N2 saturated selected pH solution with scan rate 30mV/sec (b) Proton desorption and UPD-Hdes region in cathodic portion of CV swhich indicating shifting of desorption peak potential towards higher value with pH of electrolyte (c) Plot of HBE vs pH of the electrolyte. All the experiment was performed with constant Pd loading on electrode surface of 0.043 mg/cm2.
The UPD region of Pd-CNx composite has two desorption peaks. The desorption peak at lower potential (weakly bounded H) correspond to H desorption and the desorption peak at relatively higher potentials (strongly bonded H) correspond to the UPD-Hdes. Both the peaks, UPD-H and H-desorption are shifting on changing pH of the buffer solution. Figure 9b represents the anodic plot of CV in UPD-H region. From Figure 9b it is clear that the peak position gradually shifted positive
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potentials with increasing the pH of electrolyte. Figure 9c shows the HBE calculated from UPD-H peak potential vs pH.It clearly shows that gradually HBE increases with increasing the solution pH. Binding energy linearly changes with the pH with slope -16 meV. In addition, also we are studies the anion effect on the HBE in 0.1M KOH solution. Figure S11 (ESI)Shows that there is no significant change in position of UPD-H peak on addition of 0.1M KClO4 and 0.1M Na2SO4separately in 0.1 M KOH solution.This behavior suggests that HBE of Pd-CNx surface was not affected by anion adsorption. Recently, N.N.Markovic and his co-workers suggested31 that HER/HOR kinetics changes by changing acidic electrolyte to basic electrolyte and also OHads acts as intermediated in HOR reaction. The M-OHads bond energy is responsible for their change of HOR activity. On the reverse, U. Stimming and his co-workers reported74 that the intermediated, Hads and metal (M-H) bond length controlled the reaction kinetics for Pt electrode. J. Durst et al.and Y. Yan et al. hypothesized18, 28 that HOR/HER properties on noble metal (Pt, Ir) in different pH solution controlled by the HBE of the surface. Based onthe steady state CV profile of Pd-CNx catalyst and the recent literature reports, it can be concluded that the HER/HOR kinetics in different electrolyte solution is totally controlled by the pH of the electrolyte and HBE is sole descriptor for HOR/HER on monometallic palladium metal. Effect of support HER/HOR:
and
morphology
of
Pd-CNxin
The electrochemical performances porous Pd-CNx composite towards HOR/HER in acidic media is better than that of commercial Pt/C catalysts, in basic media its HOR/HER activity is slightly poorer than Pt/C, but better than that of commercial Pd/C. These superior activities of porous Pd-CNx are probably due to the combined effects of several factors. (i) The strong chemical interaction between Pd-NPs and gCNx may lead to selective growth of Pd-NPs and CNx sheets. The large exposed accessible edge of the composite mayact as active sites for HER /HOR. The intimate contact between PdNPs and g-CNx support provides good mechanical adhesion and electrical connections that may facilitate a good electron transport between CNx and Pd-NPs during cathodic polarization. The strong support-catalyst interactions in Pd-CNx composite was confirmed by XPS and FT-IR studies. In the XPS measurements, it was found that the very large carbon 1s binding energy shift of ~ 2.9eV and large BE shift of ~3.3eV for Pd metal were observed (Figure 4 b,d ). This large BE shifts for C and Pd confirm that there is strong interaction between Pd-NPs and CNx leading to formation of strong organometallic (metal-carbon) bond formation. The presence of Pd-C bond was also confirmed by the FT-IR studies (Figure S3 ESI). Thus, good electron transfers between Pd-NPs and CNx sheets occur during HER/HOR process due to strong organometallic Pd-Cx bond in Pd-CNx composite. In PEM Fuel Cell, Pt/C catalysts are normally usedin both anode and cathode compartments. The main drawback for the Pt/C catalyst is the degradation of the catalysts due to oxidation of the support in strong oxidizing environment of cathode. But, this Pd-CNx composite displayed superior durability under strong acidic media. This is probably due to strong metal-carbon
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bond which may prevent oxidation of the support and force NPs to remains bonded with CNx support. Thus, g-CNx can be an alternative support for metal nanoparticles for their electrochemical applications. (ii) The porous structure of Pd-CNx composite allows efficient mass transport through the catalyst. The reactant can easily reach to active sites such as defect, edge, etc and product can easily come out from the active through the pores. This may also be helpful in enhancing HER/HOR catalytic activities. Recently morphology dependent electrocatalytic activities towards hydrogen evolution were reported by several groups75-76. The adhesion of evolved gas on a flat electrode surface is a big problem because adhesion of gas bubbles blocks electrolyte diffusion leading to ohmic drop and decrease performance of the catalyst. The nanostructure MoS2 film was reported for much higher HER activities (overpotential, stability) than that of flat MoS2 film due to the low gas adhesion force of nanostructure MoS2 electrode75. The superior hydrogen evolution at micro and nanostructured CoS2 in comparison of flat CoS2 film was also reported76 due to enhanced release of evolved gas from the electrode surface. Based on the previous results and this work it may conclude that highly porous morphology of Pd-CNx catalyst may enhances mass transport to catalytic sites and it could significantly contribute to HER activity. (iii) The change of Fermi energy level of palladium nanoparticle may takes place due electron transfer between support and NPs resulting from the difference in chemical potential of support and metal nanoparticle. It is reported35 that enhanced activity of gold nanoparticle on metal oxide surface is due to extra negative charge on the periphery of gold due to strong interaction between gold and support. The extra positive charge in Pt- FeOx, results from electron transfer and strong metal-support interaction are responsible for superior catalytic activity towards CO reduction37. (iv)The presence of pyridinic and graphitic N-atom concentration in Pd-CNx composite improves the charge transfers kinetics due to interaction of protons with N-atoms. J.Y.Kim and his co-workers suggested77 that presence of the N-species improved the electrocatalytic activity of Cobalt doped g-C3N4 and also showed that among the different type N-functional group graphitic and pyridinic N known to play more significant role in the improvement of electrocatalytic affects by superior the charge transfer. A. Juan and coworkers reported60 that Pd atoms has strong affinity towards C-C bonds and prefer to absorb C-C bond site in compared to any other sites of graphene. Their DFT studies showed that the C-C bond near Pd absorption sites became weak due to the formation of PdC bonds. When hydrogen adsorption occurs, new Pd-H bonds were formed at expense of weakening of Pd-C and C-C bonds. It is also shown that the Pd-C bonds remain intact even after hydrogen adsorption and Pd-NPs act as bypass for hydrogen transfer to carbon supports. This spillover effect could be one of the reasons for the superior HER performances on Pd-CNx composite. The spillover effect of hydrogen is the chemisorption and dissociation of hydrogen molecule into atomic hydrogen and it subsequent migration onto the support through surface diffusion. The hydrogen molecules or adsorbed H atoms during cathodic polarization spillover from the Pd particle and diffuse into CNx surface. The spillover
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effect is increased due to the presence of large of pyrrolic/pyridinic N atom in these composite owing to the formation of N-H bond. Thereby, large amounts of hydrogen can be stored on the CNx surface from where it is subsequently released. (v) For electrochemical measurement, in general the polymer binders such as nafion or PTFE are used to immobilize the catalysts on electrode surfaces. The effective activity of the catalysts is reduced due to use of polymer binder since this polymer binder may block active sites of the catalysts as well as reduce diffusion78 and also increases the series resistance79. For the electrochemical performance of the Pd-CNx catalyst, no external binder is used for immobilization of the catalyst on electrode surfaces. Thus, better catalytic activities as well as durability of this catalyst in compare to commercial Pt/C or Pd/C is expected. Thus, based on the various reports and ours results it may be concluded that superior and stable electro catalytic activity of Pd-CNx composite towards HER/ HOR can be explained based on synergistic effects between Pd-NPs and g-CNx support resulting from the strong organometallic metal-carbon bond.
perimental data of impedance spectra is fitted with electrical equivalent circuit as shown in inset of Figure 10a, b. In equivalent circuit diagram, Rs is the series resistance, Rp is polarization resistance related to the porosity of surface, CPE1 and CPE2 were the two constant phase element (CPE) and Rct denoted overpotentials dependence charge transfer coefficient. The fitting parameter of Pd-CNx in different overpotentials was summarized in Table S5(ESI). Table S5 (ESI)shows that Rct of Pd-CNx composite gradually decreases from 175Ω at +6 mV to 34.4Ω at -12 mV with increasing overpotentials. Table S6 (ESI) describes the comparison of impedance parameter of Pd-CNx and g-CNx at -12 mV overpotentials. The Rct value of g-CNx at -12 mV was found to be 34400 Ω and it significantly decreases to 34.1Ωwhen Pd-NPs dispersed in CNx matrix. This implies that HER kinetics at Pd-CNx composite increases due to superior electron conduction through porous network resulting from strong chemical interaction between Pd-NPs and CNx matrix in Pd-CNx composite. In addition, the plot of log(R-1ct) vs overpotentials of Pd-CNx composite (Figure S12, ESI) gives rise the Tafel slope 34 mV/dec confirming the Tafel-Volmer mechanism of HER. Thus the reaction mechanism of HER was further verified with EIS spectroscopy.
Electrochemical impedance spectroscopic (EIS) study:
Conclusions:
Electrochemical impedance spectroscopy (EIS) is widely used technique to characterize the HER kinetics and to find the actual interfacial process occurs at solid-solution interfaces in the HER/HOR process. (a)
(b)
Figure 10.EIS spectroscopy studies of Porous Pd-CNx and gC3N4 (a) EIS spectra of Pd-CNx composite at different overpotential (from +6 mV to -12mV) (b) EIS spectra of g-CNx at 12mV overpotential. Inset contained the fitting circuit of impedance spectra. All the experiments were performed in 0.5M H2SO4 solution with constant Pd loading to 0.043 mg/cm2.
Figure 10a shows the Nyquist plot of EIS response of porous Pd-CNx composite at different overpotential in 0.5 M H2SO4 solution. The Nyquist plot of free CNx consist only single semicircle (Figure 10b). But the Nyquist plot of Pd-CNx consists with two quasi-semicircles indicating the porous nature of the electrode80. This confirms that, when Pd-NPs are incorporated on CNx sheets, a porous network structure of Pd-CNx composite is formed. The FE-SEM and TEM images also suggested the porous morphology of this composite. The ex-
In summary, we demonstrated an ultrasound mediated facile method for the synthesis of porous Pd-CNx composite. The superior HER catalytic behavior of Pd-CNx in both acid and alkaline medium was demonstrated. The HER activity of porous Pd-CNx catalyst is superior to commercial Pt/C catalyst in acid media whereas its activity is lower than Pt/C in basic media, but better than that of reported Pd metal based electrocatalysts. Most importantly, the durability of Pd-CNx catalyst is much higher than that of commercial Pt/C or Pd/C in acid media and current density at any overpotential remains unchanged for at least 100 hrs. In addition, hydrogen oxidation reaction at different pH (0-14) was studied using rotating disk electrode (RDE) methods. The HER/HOR activity at different pH solution was also successfully correlated with the hydrogen binding energy (HBE) of Pd-CNx composite and HBE was found to be sole descriptor for HER/HOR. The porous morphology of the catalyst, strong chemical bonding between palladium and CNx (Pd-C bond), improved charge transfer, and synergetic effects of between CNx support and Pd-NPs are responsible for the outstanding HER/HOR catalytic activities and excellent durability at porous Pd-CNx composite. The excellent HER/HOR activity of porous Pd-CNx catalyst with excellent durability, low catalysts loading is making them a promising polymer binder free electrocatalysts for fuel cell (PEMFC) and water splitting or other electrochemical devices.
AUTHOR INFORMATION Corresponding Author
Dr. Sudip barman E-mail:
[email protected]. Fax: +91 (674)2304070; Tel: +91(674)2304061
ACKNOWLEDGMENT
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SB thanks DST, Govt. of India for financial support (project number: SR/S1/PC-10/2011). We would like to thank Prof. Shikha Varma and Mr. Santosh Chowdhury (Institute of Physics, Bhubaneswar, India) for helping in XPS measurement.
†Electronic Supporting Information (ESI) available: ESI information contained details of characterization, materials, and electrochemical experiment. Additional TEM images, TGA plot, IR spectra of Pd-CNx composite, deconvulated C1s, N1s XPS spectra of g-C3N4, CVs of Pd-CNx in whole pH range and details impedance spectroscopy study are also available in ESI.
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A facile method for the formation of palladium nanoparticles-graphitic carbon nitride composite is reported. This Pd-C3N4 catalyst exhibits superior electro-catalytic activity toward hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in acidic and basic medium. The HER activity of this catalyst is comparable to commercial Pt/C; but, its durability is better than that of commercial Pt/C catalyst in acid medium. Hydrogen binding energy was found to be the sole descriptor for HER/HOR on monometallic Pd. 254x145mm (150 x 150 DPI)
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