Making Ag Present Pt-like Activity for Hydrogen Evolution Reaction

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Making Ag Present Pt-like Activity for Hydrogen Evolution Reaction Jing-Fang Huang, and Yi-Ching Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Sustainable Chemistry & Engineering

Making Ag Present Pt-like Activity for Hydrogen Evolution Reaction Jing-Fang Huang* and Yi-Ching Wu Department of Chemistry, National Chung Hsing University, 145 Xingda Road, Taichung 402, Taiwan, R.O.C. *corresponding author email: [email protected]

Silver (Ag) has a high electrical and thermal conductivity, low cost, and is relatively abundant in the earth compared to other Pt-group metals. Unlike Pt-related noble metals, the poor hydrogen evolution reaction (HER) activity of Ag in acidic environments makes it rarely considered as a potential HER electrocatalyst. We demonstrate a spongy-like highly nanoporous Ag foam obtained using a simple Ag surface treatment through the repetitively electrochemical anodic formation of AgBr/cathodic reduction back to Ag exhibited Pt-like high HER activities. The spongy-like nanoporous Ag foam effectively increases the number of surface active sites and lowers the Gibbs free energy of adsorbed atomic hydrogen (Had) on catalysts (∆GM-H) from multiple Ag grains constructing nanoarchitecture branches.

KEYWORDS: Hydrogen Energy; Silver; Electrocatalysis; Hydrogen Evolution Reaction; Nanoporous

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INTRODUCTION Hydrogen evolution reaction (HER) is a very important process that is one of the most extensively studied processes in electrochemistry.1-2 HERs have attracted renewed attention because hydrogen is considered a prospective energy carrier in the future.3-5 It is well known that Pt and Pt-group materials are the most widely used electrocatalysts for the electrocatalytic and photocatalytic production of H2 from water using HER. Pt-group materials are highly active (low overpotential) and stable under the often harsh operational conditions used in electrolyzers, but their use is limited for practical applications on a global scale owing to their high price and low abundance.6-15 The famous “volcano plot”, which plots the HER activity (the exchange current density) for different catalysts in acids as a function of the Gibbs free energy of adsorbed atomic hydrogen (Had) on catalysts (∆GM-H), with the Pt family at the top, can serve as a guide to determine alternative catalysts, and indicates that the promising catalyst may lie at the top of the curve, binding Had intermediates neither too weakly nor too strongly (Figure S1).10, 16 Several new earth-abundant HER catalysts have been identified, including low-cost transition metal sulfides, e.g., MoS2 and WS2, and other related materials.10, 17-21 For MoS2, the value of ∆GM-H lies just below those of the Pt group metals, indicating the potential of MoS2-based materials as an alternative to Pt for the HER.10 Ag with relatively good physical properties, e.g., high electrical and thermal conductivity, is low cost and relative abundant on the earth when compared to other Pt-group metals, i.e., Pt, Pd, Ir, and Au.22-23 Unlike Pt-related noble metals, the poor HER activity of Ag makes it rarely considered as a potential HER electrocatalyst, especially in acidic environments, although some studies have demonstrated that Ag or its alloys show good HER activity in alkaline conditions.24-


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Based on the prediction of the volcano plot (Figure S1), we can easily understand why Ag

materials show weaker activities in the HER owing to their positive ∆GM-H. Three possible reaction steps is used to explain the HER process on various electrocatalysts in acidic media:27 (1) The first Volmer step is an electrochemical hydrogen adsorption: H3O+ + e- + M → MHad + H2O, which is followed by either the (2) Heyrovsky step (electrochemical desorption): MHad + e+ H+ → M + H2 or (3) Tafel step (chemical desorption) 2MHad → 2M + H2, where M contributes an empty metal active site and MHad represents an Had intermediate on the metal active site. The extent of the HER reaction is limited both by the number of available surface sites, which directly affects the HER kinetics, and the suitable ∆GM-H, which is related to the stabilization of the product of the electron transfer, and further causes a positive shift in the H reduction potential (or decreased HER overpotential).28 These two key factors will be determined based on the electrode material and its form.24, 28 In this study, a simple multiple scan cyclic voltammetry (MSCV) procedure was used to treat an Ag surface in various sodium halide aqueous solutions. We reported that the spongy-like highly porous Ag foam with nanoarchitecture branches that are constructed by Ag grains was directly grown on the Ag surface by the repetitively anodic formation of AgBr/cathodic reduction back to Ag in a 0.5 M NaBr aqueous solution. The highly porous Ag foam provides a high-density nanosized grain boundary that is found in the interconnected Ag particles. We also demonstrated that the unique microstructure significantly enhanced the exchange current density in the HER, and also caused the HER onset potential to positively shift to be very close to that on a Pt substrate. The highly porous Ag foam exhibited high durability in terms of the HER, high stability, and corrosion resistance in acidic conditions owing to the intrinsic stability of Ag and the three-dimensional (3D) nanostructured active sites that are directly grown on Ag substrates.


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EXPEIMENTAL SECTION Chemicals. Deionized water (specific resistivity = 18.2 MΩ cm) was used to prepare all of the solutions. All chemicals were of analytical grade unless otherwise stated. 95–98% H2SO4 (Aldrich), 99.0% NaCl (JT-Baker), 99.0% NaBr (JT-Baker), and 99.0% NaI (JT-Baker) were used as received. Preparation of MSCV redox-treated silver electrodes. The electrochemical experiments were performed using a CHI 760C potentiostat/galvanostat and a three-electrode electrochemical cell. To prevent Cl- and Pt interference from the typical Ag/AgCl reference electrode and Pt counter electrode, Hg/HgSO4 (0.5 M H2SO4) was used as a reference electrode, and a graphite rod or a silver wire was used as a counter electrode. The potentials were measured against the reference electrode and converted to the reversible hydrogen electrode (RHE) reference scale by ERHE = E

Hg/HgSO4

+ 0.68 + 0.059pHelectrolyte. All the potentials were reported versus RHE unless

otherwise noted. The polished silver wire electrode was cleaned by immersion in 70% ethanol, rinsed with deionized water, and dried before being used as the working electrode. The Ag wire working electrode was treated using the multiple scan cyclic voltammetry (MSCV) procedure in a controlled potential range, 0.4 V, in 0.5 M NaClaq (0.05 V–0.45 V), NaBraq (-0.1 V–0.3 V), and NaIaq (-0.3 V–0.1 V), respectively. The resulting Agxn (“x = Cl, Br, and I” indicates that Ag is treated in NaClaq, NaBraq, and NaIaq, respectively; “n = 800, 400, 200, 100, and 50” refers to the number of treating potential cycles.), leaving the surface with a nanoporous structure, was then electrochemically cleaned using a CV between 0.5 V and -0.4 V versus RHE, with a scan rate of 0.1 V/s in an Ar-purged 0.5 M H2SO4 until reproducible CVs were obtained. The microstructure of post-treated Agxn was examined by the electrochemical characterization and image analysis.


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The image analysis of post-treated Agxn was performed using a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) and a JEOL JIB-4601F (a Schottky-type SEM observation system incorporates with a high-power focused-ion beam (FIB) column). The FIB was used for fine milling and performing 3D structure analysis with automatic cross-section milling, observation, and analysis at fixed intervals. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) were used for surface-element analysis of the asprepared

Agxn.

The

XPS

data

were

acquired

using

the

ULVAC-PHI,

PHI5000

VersaProbe/scanning ESCA microprobe. For X-ray diffraction (XRD) measurements (Cu Kα radiation), an Ag foil (10 × 10 mm) was used as a working electrode for the preparation of XRD Agxn samples. Electrochemical characterization of hydrogen evolution reaction (HER). The working electrodes, include glassy carbon (GC) and pre- and post-treated Ag wire, were electrochemically cleaned using a cycling potential between 0.5 V and -0.4 V versus RHE with a scan rate of 0.1 V s-1 for 20 times in an Ar-purged 0.5 M H2SO4aq. The electrocatalytic activity towards HER was examined using CV at a scan rate of 0.05 V s-1 and linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 in an Ar-purged 0.5 M H2SO4aq at 25 °C. To quantify the HER activity, LSV scans were re-plotted as potential versus log(current density (j)) to obtain Tafel plots. From the least-square fits of the linear regions of the Tafel plots to the Tafel equation, the kinetic exchange current density (jo) and Tafel slope were determined, and are provided in Table S1 for Agxn electrodes. Electrochemical impedance spectroscopy (EIS) was conducted at a -0.2 V bias voltage in the frequency range of 1–105 Hz with a single modulated AC potential of 5 mV in an Ar-purged 0.5 M H2SO4aq. Experimental EIS data were analyzed and fitted using the Zview software.


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AgBr800 Electrocatalyst Durability Treatments. The durability of the AgBr800 electrocatalyst was investigated for various treatments, including the accelerated durability test (ADT, potential scanning) and potentiostating electrolysis treatment (potential holding) at ambient temperature: ADT − The ADT was conducted using CV from 0.5 to -0.4 V vs. RHE at a scan rate 0.05 V s-1 for 5000 cycles in a 0.5 M H2SO4aq at room temperature. The CVs before and after cycling are recorded under quasi-equilibrium conditions at a scan rate of 0.05 V s-1. Potentiostating electrolysis treatment (potential fixing) − For the potentiostating electrolysis treatment, AgBr800 was performed by extended electrolysis at fixed potentials in a 0.5 M H2SO4aq for over 60 h. RESULTS AND DISCUSSION The typical cyclic voltammograms (CVs) for a polycrystalline Ag wire are recorded in 0.5 M NaX (X = Cl, Br, and I) aqueous solutions (NaXaq) with a scan rate of 0.05 V s-1 at 25 °C (Figure 1a). In all NaXaq, these voltammograms present similar features involving a sharply increasing anodic current that corresponds to the formation of AgX on the positive potential scanning, a hysteresis loop resulting in the reversely negative potential scanning, and following a cathodic peak that is related to the electroreduction of the AgX to Ag. The hysteresis loop can be attributed to the inhibition of Ag anodic oxidation by adsorbed X-; the X- adsorbed Ag is oxidized at a more positive potential than the fresh Ag surface. Once the X- adsorbed Ag is oxidized,

the

bulk

fresh


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a

120

j (mA cm )

NaI NaBr NaCl

60

-2

0

b

120 -2

j (mAcm )

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


-60

60 0 -60 -120 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2

-120 0.6

E (V vs. RHE)

0.4

0.2

0.0 -0.2 -0.4 -0.6 -0.8

E (V vs. RHE)

c d

Figure 1. (a) The CVs for a polycrystalline Ag wire were recorded in 0.5 M NaXaq (X = Cl, Br, and I) with a scanrate of 0.05 Vs-1 at 25 oC. The inset is a MSCV of Ag example recorded in 0.5 M NaBraq with a scanrate of 0.05 Vs-1 at 25 oC. (b) Top-view SEM image of an AgBr800 wire, Scale bar, 5 µm. (c) The corresponding higher magnification SEM image. Scale bar, 1 µm. (d) The corresponding cross-sectional SEM image. Ag surface is oxidizable at less positive potentials.29-30 For the same concentration of NaXaq, the onset potential of Ag oxidation in NaClaq is more positive than those in NaBraq and NaIaq. This shift in the anodic onset potential is related to trends in the AgX complex formation constant (or standard potential of the corresponding reactions).31 It also means that the stability of AgX is in the order: AgI > AgBr > AgCl. The trend of the AgX stability also determines the order of the


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peak potential corresponding to the electroreduction of AgX to Ag (from positive to negative is AgCl, AgBr, and AgI).32 Here, we treated the Ag surface in different NaXaq using MSCV for a fixed potential range of 0.4 V, including an anodic overpotential range of 0.1 V for Ag oxidation, and a potential range of 0.3 V to finish the electroreduction of AgX to Ag with a scan rate of 0.05 V s-1 at 25 °C (inset in Figure 1a). During a complete potential scanning cycle, it is expected that the oxidative or corrosive AgX microstructure was formed during anodic potential scanning, and then reduced to the Ag element form in the reversed cathodic potential scanning. The micromorphology of the as-treated Ag surface (the term Agxn, “x = Cl, Br, and I” indicates that Ag is treated in NaClaq, NaBraq, and NaIaq, respectively; “n = 800, 400, 200, and 100” is the number of treating cycles.) was examined using scanning electron microscopy (SEM). AgI800 shows a rough irregular surface (Figure S2). The morphology comprises the results of the only anodic treating Ag reported in the literature.33 Interestingly, nanoporous Ag networks were formed and completely covered on the Ag surface as a result of similar treatment in NaBraq and NaClaq (Figures 1 and S2). In particular, for AgBr800, Figures. 1b–d show the spongy-like highly porous Ag foam with a thickness of ~5.0 µm evaluated from the cross-sectional image of AgBr800 (Figure 1d) owing to a coarsening of the interconnected silver particles, growth of sintering necks, and a reduction in size of the void spaces. Based on the unique nanostructure of AgBr800 in the SEM results, a potential seed-growth process for the formation of the nanoporous networks is proposed in that the pre-electroxidative AgX species serves as not only the seeds but also as Ag sources for the growth of the Ag nanoporous network through the electroreduction of AgX. It has been reported that a porous AgX film is formed at the electrode surface during the anodic corrosion

of

Ag.34-35

The

adsorbed


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Scheme 1. Possible seed-growth process for the formation of the nanoporous Ag networks. AgX species were formed at the submonolayer level, and they then diffused out of the electrode as a complex ion, and there was then the nucleation and growth of a 3D AgX phase during the electrodissolution of Ag. The immediate electroreduction of as-formed AgX to Ag during reversed cathodic potential scanning was introduced here. During the electroreduction of the AgX process, the growth rate of Ag determines the final microstructure of the Ag nanoporous networks (Scheme 1). In the detailed microstructure, the main difference between AgBr800 (Figures 1 and S2) and AgCl800 (Figure S2) is that the branches of AgBr800 are constructed by several nanosized grains (50–100 nm), whereas the branches of AgCl800 are relatively dense and thick. It is possible that AgCl seeds are more easily reducible than AgBr seeds, and further causes the higher Ag growth rate from the reduction of AgCl to form a dense and thick ligament size. The study of HER serves to provide a simplified model to determine electrode performances. The mechanism or electrocatalytic performance of HER has been known to behave differently depending on the electrode material. Figure 2 shows the CVs for HER and the corresponding Tafel plots measured on different electrode materials, including Pt, glassy carbon (GC), pristine Ag, and Agx800 in 0.5 M H2SO4aq. Surprisingly, the poor HER activity of Ag was significantly


9


12 9

GC pristine Ag Pt AgBr800

6

AgI800

j / mA cm

-2

a

AgCl800

3 0 -3 0.6

b

0.5

Overpotential / V

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

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0.4

0.4

0.2

0.0 -0.2 -0.4 -0.6 -0.8

E / V vs. RHE Pt Pristine Ag AgBr800 AgCl800 AgI800

0.3 0.2 0.1 0.0 0.1

1

10

j / mA cm

-2

100

Figure 2. (a) CVs of various electrode materials for HER. (b) The corresponding Tafel polarization curves of various electrode materials for HER. improved close to Pt after the MSCV treatment in NaBraq. The HER onset potential of the pristine Ag (-0.42 V) shifted to more positive potentials (i.e., a reduced overpotential) with the increase in the number of treating cycles in NaBraq (Figure S3). In Figure 2a, the HER onset potential of the AgBr800 (-0.05 V) appears to be very close to that of a polycrystalline Pt electrode (-0.02 V), which is an indication of significantly increased HER activity. However, the extent of the changes in the HER onset potential on AgI800 (-0.40 V) and AgCl800 (-0.25 V) is much less than on AgBr800. The overpotentials of Pt and AgBr800 required to drive cathodic current densities of 10 mA/cm2 are 30 and 108 mV, respectively. The overpotential of AgBr800 is lower than the most active for electrocatalysts based on non-noble materials.6 In addition to a rapidly increasing cathodic reduction current for HER, the anodic wave for hydrogen oxidation was also observed


10

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on AgBr800, implying that the high hydrogen evolution efficiency is similar to that of Pt. Only pure Ag without Pt or halide contamination was detectable on the AgBr800 surface by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) (Figures S4 and S5). The structure of Agx800 was further analyzed by X-ray diffraction (XRD) (Figure S6). The XRD pattern of Ag foil was adopted for comparison. In both patterns, typical characteristic Ag diffraction peaks at 2θ angles of 38.2, 44.5, 64.4, 77.4, and 81.7, which correspond to the (111), (200), (220), (311), and (222) crystal planes can be detected, further confirming that Agx800 existed in metal form, and as a polycrystalline fcc structure with no such preferential crystallographic orientation (Figure S6). This result implies that the unique microstructure, in particular for a large number of nanosized grains comprising nanoporous branches of AgBr800, may be a key reason for the significant improvement in the HER activity. To further evaluate the catalytic activities of the different electrodes, the Tafel slope and the exchange-current density, j0 (Figure 2B and Table S1) were determined by fitting j-E data to the Tafel equation.28 The Tafel slope is used to distinguish between different mechanistic pathways in HER, and j0 indicates the most inherent measure of activity. For Pt, the slope of the Tafel plot was ∼30 mV/dec, with a j0 of 2.1 mA cm-2. Both values are consistent with the known Pt HER performance.1,

28

Interestingly, for the pristine Ag and Agx800, the Tafel slopes of 100–119 mV/dec indicate that the Volmer or proton discharge step is the rate-deter-mining step (rds) for these kind of materials.28, 36 This means that the MSCV treatment could not change the intrinsic rds for HER on Ag, even though the HER onset potential presents a notable positive shift on AgBr800. However, the j0 of AgBr800 showed an extraordinary enhancement to 1.2 mA cm-2, which is approximately 4615 times higher than that of the pristine Ag (2.6 × 10-4 mA cm-2), and 182 and


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545 times higher than that of AgCl800 (6.6 × 10-3 mA cm-2) and Agl800 (2.2 × 10-3 mA cm-2), respectively. The j0 is correlated with the rate of electron transfer (or the reaction rate)

500 AgBr800 AgCl800

400

AgI800

2

300

Pristine Ag 2.0 2

200

-Z" / Ω cm

-Z" / Ω cm

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

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100

1.5 1.0 0.5 0.0 0

0

1

2

2

3

Z' / Ω cm

0

50 100 150 200 250 300 350 2

Z' / Ω cm

Figure 3. Nyquist plots of pristine Ag and Agx800 (x = Br, Cl, and I) recorded at -0.2 V bias voltage. Symbols are the experimental points and solid lines are the modelled data. The insets are the zoomed part of the Nyquist plot at high frequencies and the one-time constant electrical equivalent circuit used to describe the EIS response of HER. during HER. Based on the rds, Volmer reaction, it is believed that the ultra-high reaction rate positively correlates with the obvious increase in the number of active sites from the unique microstructure of AgBr800, and also implies that a large number of grain boundaries that are specifically found in the interconnected Ag particles are potential “hot spots” (or edge sites) for Had, which is the key intermediate for HER. Figure 3 shows Nyquist plots and the corresponding fitted data of the pristine Ag and Agx800 recorded by electrochemical impedance spectroscopy (EIS) at -0.2 V vs. RHE. The values for each electronic element related to the interfacial properties of catalysts are summarized in Table


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S2. Rs (0.17 Ω cm2) indicates the electrolyte resistance. Rct evaluated from the semicircle of the plots indicates the faradaic charge-transfer resistance for HER on related electrocatalysts.

a

50 Initial After 5000 cycles

j / mAcm

-2

40 30 20 10 0 0.4

-2

b j / mAcm

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


0.2

0.0

-0.2

-0.4

E / V vs.RHE

40 30

η = 0.18 V

20

η = 0.15 V η = 0.12 V

10 0

0

20

40

60

Time / h

Figure 4. (a) Stability tests of AgBr800 catalyst through the CV scanning: the polarization curves before and after 5000 potential cycles are displayed. (b) Time dependence of cathodic current density during electrolysis over 60 h at fixed overpotentials of 0.12, 0.15, and 0.18 V, respectively. The pristine Ag gives a large value of Rct (1743.6 Ω cm2), indicating that the ultra-high faradaic charge-transfer resistance is the limiting factor for HER on the pristine Ag. After the MSCV


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treatment in NaBraq, Rct gradually decreases with the treating cycles (Figures 3 and S7), and the optimal AgBr800 provides the lowest resistance values of 2.7 Ω cm2, which is 34 and 205 times lower than that of AgCl800 (91.9 Ω cm2) and Agl800 (552.8 Ω cm2), respectively. The ultra-low impedance for AgBr800 is related to the increase in the number of surface active sites and the reduced ∆GM-H during the H+ discharge reaction step, resulting in superior HER kinetics. To assess its durability and stability in HER process, AgBr800 has been continuously potential scan cycled 5000 times from 0.2 V to -0.4 V vs. RHE with a scan rate of 0.05 V/s. The polarization curves for the first and 5000th cycle show almost identical behaviors without any obvious change (Figure 4). The intrinsic stability of Ag in acidic media and Ag grain active sites grow well on the Ag substrate decide the highly stable HER performance. Furthermore, the catalyst is surveyed by electrolysis at fixed potentials over long periods in mimicking the practical HER operation. At a lower overpotential of 0.12 V, the catalyst current density (∼10 mA cm-2) remains stable for electrolysis over 60 h. At higher overpotentials, 0.15 V and 0.18 V, causing high HER current densities (∼18 and ∼30 mA cm-2), a small current density loss (

Making Ag Present Pt-like Activity for Hydrogen Evolution Reaction

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