Hydrogen Evolution Reaction in Alkaline Media: Alpha- or Beta-Nickel

Subscriber access provided by READING UNIV

Letter

Hydrogen Evolution Reaction in Alkaline Media: Alphaor Beta-Nickel Hydroxide on the Surface of Platinum? Xiaowen Yu, Jun Zhao, Li-Rong Zheng, Yue Tong, Miao Zhang, Guochuang Xu, Chun Li, Jing Ma, and Gaoquan Shi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01103 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Energy Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 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

ACS Energy Letters

Hydrogen Evolution Reaction in Alkaline Media: Alpha- or Beta-Nickel Hydroxide on the Surface of Platinum? Xiaowen Yu,a‡ Jun Zhao,b‡ Li-Rong Zheng,c Yue Tong,a Miao Zhang,a Guochuang Xu,a Chun Li,a Jing Ma,b* and Gaoquan Shia* a

Department of Chemistry, MOE Key Laboratory of Bioorganic Phosphorus Chemistry &

Chemical Biology, Tsinghua University, Beijing 100084, P. R. China. b

School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R.

China. c

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Science, Beijing 100049, P. R. China. *[email protected]; [email protected]

ACS Paragon Plus Environment

1

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

Page 2 of 21

ABSTRACT Reducing the energy consumption of hydrogen evolution reaction (HER) at platinum (Pt) electrode is important for hydrogen economy. Herein, we report the loading of alpha- or beta-nickel hydroxide (α- or β-Ni(OH)2) nanostructures on the surface of Pt electrode to improve its catalytic activity and stability for HER in alkaline electrolytes. Both experimental and theoretical studies reveal that β-Ni(OH)2 is a better co-catalyst of Pt than α-Ni(OH)2 for promoting HER, attributing to the higher water dissociation ability of β-Ni(OH)2, as well as the stronger interactions between β-Ni(OH)2 and Pt electrode. Particularly, the overpotential of HER in 0.1 M KOH at 10 mA cm–2 is decreased from 278 mV at Pt electrode to 92 mV at βNi(OH)2/Pt electrode, and the Tafel slope decreased from 62 to 42 mV dec–1, correspondingly. The performance of β-Ni(OH)2/Pt catalytic electrode surpasses most of the previously reported electrodes for the same purpose.

TOC GRAPHICS


2

Page 3 of 21 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

ACS Energy Letters

Electrochemical catalysis of hydrogen evolution reaction (HER) is an important process of producing molecular hydrogen.1–4 Platinum (Pt) is considered to be the best electrocatalyst for HER because of its strong ability for the adsorption and recombination of reactive hydrogen intermediates.5 Nevertheless, the catalytic activity of a Pt electrode for HER in an alkaline medium is much weaker than that in an acidic electrolyte.6–9 This is mainly due to the low efficiency of water dissociation on the surface of Pt in an alkaline medium (H2O + Pt + e– → PtH + OH–), resulting in high overpotential and sluggish kinetics of HER; thus, production of molecular hydrogen in alkaline media needs high energy consumptions.10, 11 Incorporation nickel hydroxide (Ni(OH)2) with Pt can synergistically catalyse HER in alkaline media. In this system, Ni(OH)2 provides active sites for cleaving H-OH bonds, and Pt facilitates the combination of generated hydrogen intermediates into H2 molecules.12–15 On the other hand, Ni(OH)2 is a promising electrode material with a variety of applications.16 It has two well-defined polymorphs: α-Ni(OH)2 and β-Ni(OH)2; they are different in crystal structures and lattice parameters, thereby in their electrocatalytic activities.16 The discrepant activities of α- and βNi(OH)2 in supercapacitors and fuel cells have already been extensively investigated.17–19 However, the difference in their electrocatalytic activities for HER in alkaline media, especially in the cases of incorporating them with Pt electrodes, has been ignored for many years. Herein, we systematically studied the interactions between α- or β-Ni(OH)2 and water molecules (or Pt substrate) to reveal its role as a co-catalyst of Pt during HER. The Pt electrode modified with α- or β-Ni(OH)2 nanostructures (α- or β-Ni(OH)2/Pt electrode) exhibited higher catalytic activities for HER than that of pristine Pt electrode in alkaline media. Particularly, the β-Ni(OH)2/Pt electrode showed a much lower overpotential and smaller Tafel slope of HER compared with those of α-Ni(OH)2/Pt electrode. The superior performance of β-Ni(OH)2/Pt


3


Page 4 of 21

electrode was experimentally and theoretically studied to be attributed to the strong hydrogen bonding interactions between β-Ni(OH)2 and water molecules, which greatly promotes the dissociation of water at Pt electrode. In addition, the strong electron transfer between β-Ni(OH)2 and Pt substrate makes the Pt atoms near β-Ni(OH)2 clusters being more active for adsorbing hydrogen intermediates. α- and β-Ni(OH)2 are assemblies of layered nickel oxide (NiO2, Figure 1a, b). The main difference between them is their intercalated ions and thereby their interlamellar spacings.16 The interlayers of α-Ni(OH)2 are intercalated by water molecules and anions, making it has a larger interlamellar distance (0.70–0.90 nm) than that of β-Ni(OH)2 (0.46–0.48 nm).20, 21 Owing to the existence of various intercalated ions, the NiO2 layers in an α-Ni(OH)2 lattice tend to form a helical layered structure (Figure 1a). By contrast, β-Ni(OH)2 consists of only stacked NiO2 layers, and each NiO2 layer has a hexagonal arrangement of nickel atoms with octahedral coordination of oxygen atoms (Figure 1b).16 α- and β-Ni(OH)2 nanostructures were synthesized via a hydrothermal process according to the literature.17 They were loaded on the surfaces of Pt rotating disk electrodes (RDE) and denoted as α- and β-Ni(OH)2/Pt electrodes. The morphologies and structures of α- and βNi(OH)2 powders have been carefully characterized and discussed (Figure S1 to S3). The mass loading of Ni(OH)2 on the surface of a Pt electrode has a strong influence on the HER performance of a modified Pt electrode (Figure S4); a relatively small mass loading (13 µg cm–2) of Ni(OH)2 facilitates the formation of an electrode with high catalytic activity. This is probably due to that a large amount of Ni(OH)2 clusters adsorbed on the surface of Pt restrict its contact with electrolyte, decrease the total Ni(OH)2/Pt perimeter length available for the reaction to occur, and thus suppressing the HER current. The coverage of α- or β-Ni(OH)2 on the surface of


4

Page 5 of 21 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

ACS Energy Letters

Pt electrode with a mass loading of 13 µg cm–2 was quantitatively analysed to be 40% – 45% (Figure S5). Except for specific description, the Ni(OH)2/Pt electrodes studied hereafter all have the optimized mass loading of Ni(OH)2 (13 µg cm–2).

Figure 1. (a, b) Crystal structures of α-Ni(OH)2 (a) and β-Ni(OH)2 (b). (c) LSV curves of Pt, αNi(OH)2/Pt, and β-Ni(OH)2/Pt electrodes for HER in 0.1 M KOH. (d) Tafel plots corresponding to panel c. (e) LSV curves of β-Ni(OH)2/Pt electrodes for HER in 0.1 M KOH, 1.0 M KOH, and 0.1 M PBS, respectively; Pt electrode for HER in 0.1 M HClO4 was tested for comparison; The LSV curves were all collected at a scan rate of 10 mV s–1 with iR-compensation, the rotating rate of electrodes is 1600 r.p.m. (f) Stability test of Pt, α-Ni(OH)2/Pt, or β-Ni(OH)2/Pt electrode in 0.1 M KOH at a constant potential of –0.09 V vs. RHE.


5


Page 6 of 21

The HER performance of α- or β-Ni(OH)2/Pt electrode was tested by taking out linear sweep voltammetry (LSV) curve in 0.1 M KOH (pH 13.0) at room temperature. A Pt electrode was also studied for comparison. The freshly polished Pt electrode exhibited an overpotential of 278 mV at 10 mA cm–2 for HER (Figure 1c), and a relatively slow kinetics with a Tafel slope of 62 mV dec–1 (Figure 1d). This performance is comparable to that of widely reported pure Pt electrode, such as Pt(111) electrode, which performed an overpotential of about 280 mV at 10 mA cm–2.12 However, under the same condition, the α-Ni(OH)2/Pt electrode showed a much lower overpotential (124 mV at 10 mA cm–2) and a smaller Tafel slope (48 mV dec–1) than those of Pt electrode (Figure 1c and 1d). The catalytic performance of α-Ni(OH)2/Pt electrode is comparable to those of reported Ni(OH)2/Pt (111) and Pt nanowires/single-layer-Ni(OH)2 (Pt NWs/SL-Ni(OH)2) electrodes in the same electrolyte (Table S1).12, 22 Actually, these previously reported Ni(OH)2 materials prepared by electrodeposition or chemical exfoliation were mainly composed of α-Ni(OH)2. Interestingly, our β-Ni(OH)2/Pt electrode displayed a much stronger catalytic activity for HER than that of α-Ni(OH)2/Pt electrode, further lowering the overpotential to 92 mV (at 10 mA cm–2) with an extremely low Tafel slope of 42 mV dec–1. The performance of β-Ni(OH)2/Pt electrode even surpasses that of the best Pt3Ni frames/Ni(OH)2/C electrode under the same condition (Table S1).23 The HER in alkaline media undergoes two steps that described as follows.24, 25 H2O + M + e– → M-Hads + OH–

(Volmer)

H2O + M-Hads + e– → H2 + M + OH– (Heyrovsky) or


6

Page 7 of 21 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

ACS Energy Letters

2M-Hads → 2M + H2

(Tafel)

where, M refers to the surface of Pt electrode; and M-Hads depicts hydrogen intermediates adsorbed on the electrode surface. The onset-potential of HER is determined by Volmer step. On the basis of the observations described above, it is reasonable to conclude that the Ni(OH)2 on the surface of a Pt electrode greatly enhanced the water dissociation at Volmer step, especially in the case of using β-Ni(OH)2. Tafel slope can be used to deduce the rate determining step in HER. From Butler-Volmer kinetics, it was deduced that the Tafel slope is 118, 39, or 30 mV dec–1 when the Volmer, Heyrovsky, or Tafel step is the rate determining step, respectively.9, 26 All these electrodes exhibited Tafel slopes in the range of 42 to 62 mV dec–1, near to the ideal value for Heyrovsky step (39 mV dec–1), revealing that the Heyrovsky step is the rate limiting step for HER on these electrodes. The lower Tafel slope at β-Ni(OH)2/Pt electrode than those at Pt and αNi(OH)2/Pt electrodes also indicated the higher efficiency of water dissociation induced by βNi(OH)2. In order to further clarify the effects of Ni(OH)2 on HER by excluding the influence of Pt substrate, an Au RDE was used to replace Pt RDE because of its poor ability of adsorbing hydrogen intermediates.12, 13 As expected, the bare Au RDE behaved poorly as catalytic electrode for HER in 0.1 M KOH electrolyte, delivering weak current densities within the testing potential range (Figure S6). After loading α- or β-Ni(OH)2 on the surface of Au, the HER performance of Au RDE was greatly improved. The current densities of α- and β-Ni(OH)2/Au electrodes at –0.5 V (vs. RHE) were 7 and 12 times stronger than that of bare Au electrode. These results confirm that Ni(OH)2 indeed participates in HER via enhancing water dissociations, even on the surface of Au electrode with weak electrocatalytic activity. In addition, the HER performance of α- or βNi(OH)2 on the surface of inert electrode (e.g., glassy carbon electrode, GC) was also tested


7


Page 8 of 21

under the same condition (Figure S6). Both α- and β-Ni(OH)2/GC electrodes exhibited poor activity for HER compared with the cases when Ni(OH)2 on active metal substrates (e.g., Ni(OH)2/Au and Ni(OH)2/Pt), indicating that Pt is the main active species in Ni(OH)2/Pt system for HER. However, the β-Ni(OH)2 always exhibited higher ability for dissociation of water molecules than α-Ni(OH)2, regardless on the surface of GC, Au, or Pt electrode. The HER performances of β-Ni(OH)2/Pt electrode in other electrolytes were also studied (Figure 1e). In 1.0 M KOH (pH 13.7), the HER at β-Ni(OH)2/Pt electrode showed a slightly higher overpotential of 108 mV at 10 mA cm–2 and a smaller Tafel slope of 39 mV dec–1 than those in 0.1 M KOH (Figure S7), which is caused by the increased pH value and ion concentration of the electrolyte. This performance surpasses that of the previously reported best Pt NWs/SL-Ni(OH)2 electrode under the same condition (Table S1).22 In 0.1 M phosphorus buffer solution (PBS, pH 7.0), the HER at β-Ni(OH)2/Pt electrode exhibited an overpotential of 105 mV (at 10 mA cm–2) with a Tafel slope of 51 mV dec–1 (Figure S8). In all the systems described above, β-Ni(OH)2/Pt electrode performed much better than α-Ni(OH)2/Pt electrode. The β-Ni(OH)2/Pt electrode reduced the overpotential (at 10 mA cm–2) by 29 and 21 mV relative to those of α-Ni(OH)2/Pt electrode in 1.0 M KOH and 0.1 M PBS, respectively. These results are in good agreement with that collected in 0.1 M KOH (reduced the overpotential by 32 mV). Notably, the onset-potential of β-Ni(OH)2/Pt electrode in 0.1 M KOH or 0.1 M PBS is comparable to or even surpasses that of Pt electrode in an acidic electrolyte (0.1 M HClO4, Figure 1e). More importantly, the β-Ni(OH)2/Pt electrode narrowed the difference in overpotential (at 5 mA cm–2) between acid and alkaline/neutral environment to only 20 mV. Considering the different morphologies of α- and β-Ni(OH)2 nanostructures, the current densities in LSV curves were normalized by the Brunauer-Emmett-Teller (BET) surface areas of


8

Page 9 of 21 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

ACS Energy Letters

the nanostructures (Figure S9 and S10). The resulted specific activities of β-Ni(OH)2/Pt electrode in various electrolytes are much higher than those of α-Ni(OH)2/Pt electrode, decreasing the overpotential for 82 ~ 107 mV (at –0.5 mA cm–2BETNi) and increasing the current density by 6 ~ 9 factors (at –0.10 V vs. RHE). These results indicate that the discrepant activities of α- and βNi(OH)2 on the surface of Pt electrodes are mainly attributed to their different chemical structures rather than their geometrical difference. Besides, the α-Ni(OH)2/Pt and β-Ni(OH)2/Pt electrodes were tested on the roating disk elctrodes and the generated H2 gases can be easily released from the surface of samples, thus the differences in their H2 release efficiency can be ignored. Pt has a poor stability in both acidic and alkaline media. This is mainly due to that Pt has a weak capacity of desorbing OH– ions from its surface, thus largely reducing the possibility of adsorbing new hydrogen intermediates.12 After adsorption of Ni(OH)2 clusters on its surface, the stability of Pt electrode was greatly improved (Figure 1f). For example, a Pt electrode showed a sharp decrease of HER current density in 0.1 M KOH within the first 1,000 s, and finally decreased by 30% of its initial current after 5,000 s. By contrast, the α- and β-Ni(OH)2/Pt electrodes retained 80% and 96% of their initial current densities of HER even after 10,000s. In order to perform a long-time stability test of β-Ni(OH)2/Pt electrode, β-Ni(OH)2 nanoplates were loaded on a piece of Pt foil. This β-Ni(OH)2/Pt foil electrode maintained about 82% of its initial current after 25 h, accompanying with negligible morphological and structural changes (Figure S11 and S12). The high stability of this β-Ni(OH)2/Pt electrode is possibly due to the strong adhesion interaction of β-Ni(OH)2 on Pt substrate.27 The valence bond structures of the two polymorphs of Ni(OH)2 on the surfaces of Pt foils were analysed by X-ray photoelectron spectroscopy (XPS) to further explain the difference in


9


Page 10 of 21

their catalytic performances. In the Ni 2p XPS spectrum of α-Ni(OH)2/Pt surface (Figure 2a), the peaks located at 855.7 and 873.5 eV are assigned to Ni 2p3/2 and Ni 2p1/2, and the peaks at 861.6 and 879.9 eV are the satellite peaks of the same species.28, 29 These four peaks in the spectrum of β-Ni(OH)2/Pt are all negatively shifted for 0.3–0.4 eV compared with those in the spectrum of αNi(OH)2/Pt, indicating the relatively lower valence state of Ni in β-Ni(OH)2, and this state of Ni may be close to that of ideal NiOH, which is calculated to be highly desired for stabilizing and activating the Pt substrate in alkaline HER.27 In Figure 2b, the O 1s XPS spectrum of βNi(OH)2/Pt shows a single peak at 530.6 eV, which is associated with a hydrated phase of nickel (Ni-O-H).30 By contrast, the O 1s peak of α-Ni(OH)2/Pt exhibits a 0.3 eV positive shift (530.9 eV), in consistent with the results of its Ni 2p spectra. Another peak at 532.5 eV in the O 1s spectrum of α-Ni(OH)2/Pt is attributed to NO3– ions, which was originated from the Ni(NO3)2 precursors and mainly intercalated into the interlayer of α-Ni(OH)2 lattice.30 The lower binding energy of Ni-OH bonds in β-Ni(OH)2 indicates its longer Ni-O bonds than those of α-Ni(OH)2, making the former bonds have weaker bonding energies but stronger hydrogen bonding interactions with HER reactants (water molecules in alkaline media). It is worth noting that the interlamellar water molecules in α-Ni(OH)2 lattice cannot act as the reactants of HER, because they preferentially interact with lamellar hydroxyl groups via hydrogen bonding, being unmovable. Thus, the reactants from alkaline electrolyte adsorbed on the outmost layer of αNi(OH)2 lattice rather than penetrated into its interlamellar layers. Furthermore, the relatively strong Ni-O bonds in α-Ni(OH)2 lattice result in weakening of the hydrogen bonding interactions between lamellar hydroxyl and reactants (O-H…H-O-H), which slows down the Volmer step of HER. The above observations can reasonably explain the larger onset-potential of α-Ni(OH)2/Pt electrode than that at β-Ni(OH)2/Pt electrode.


10

Page 11 of 21 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

ACS Energy Letters

Figure 2. (a, b) Ni 2p (a) and O 1s (b) XPS spectra of α- or β-Ni(OH)2/Pt electrode. (c) Ni Kedge XANES spectra of Ni foil, α- or β-Ni(OH)2 powders, and α- or β-Ni(OH)2/Pt electrode. (d) Magnitude of k3-weighted Fourier transforms and fitting results of the Ni K-edge XANES spectra for α- and β-Ni(OH)2/Pt electrodes.

The local atomic structures of Ni atoms in the two polymorphs of Ni(OH)2 powders and Ni(OH)2/Pt electrodes were studied by X-ray absorption near-edge structure (XANES) spectroscopy (Figure 2c). The Ni K-edge XANES spectra of α- and β-Ni(OH)2/Pt electrodes are similar to those of α- and β-Ni(OH)2 powders, indicating the structures of hydroxides kept intact during modification of Pt electrodes. However, both the pre-edge and main absorption edge of Ni in the spectrum of α- or β-Ni(OH)2/Pt electrode are shifted to lower energies, accompanying with the decrease in absorption intensity than those in the spectrum of the corresponding hydroxide powder (Figure 2c and Figure S13). This phenomenon is caused by the electron transfer from Pt substrate to the hydroxide, and these electron-deficient (or hole accumulated) Pt atoms would


11


Page 12 of 21

promote the activity of nearby Pt atoms to adsorb hydrogen intermediates (Hads).31 Particularly, the energy shift of the absorption edge (1.2 eV) for β-Ni(OH)2 or β-Ni(OH)2/Pt is much larger than that for α-Ni(OH)2 or α-Ni(OH)2/Pt (0.4 eV), indicating the stronger interaction between βNi(OH)2 and Pt substrate. Furthermore, the k-space spectra of α- and β-Ni(OH)2/Pt electrodes show similar oscillations (Figure S14), indicating their similar coordination environments surrounding Ni atoms. There are two well-resolved peaks in the R-space spectra shown in Figure 2d. The first peak is attributed to the first Ni-O shell and the second peak is assigned to the first Ni-Ni shell.32,

33

The fitted results indicated that β-Ni(OH)2 has a coordination of Ni-O at a

distance of 2.06 Å and Ni-Ni distance at 3.14 Å, slightly larger than those of α-Ni(OH)2 (Ni-O at 2.03 Å and Ni-Ni at 3.09 Å, Table S2). These results are in consistent with the crystal lattices of Ni(OH)2 polymorphs.32 The Ni-Ni bonds in the coordination shell of α-Ni(OH)2 are 0.05 Å shorter than those in β-Ni(OH)2. This bond contraction is resulted from the hydrogen bonding between the hydroxyl groups of α-Ni(OH)2 and its interlamellar water.32 In addition, the larger Debye-Waller factors (see Table S2 for parameter σ2) of α-Ni(OH)2 also reflect that the Ni centres in α-Ni(OH)2 has a more structural distortion than that in β-Ni(OH)2, in accordance with the proposed turbostratic stacking structure of α-Ni(OH)2. As confirmed by the LSV curves normalized by the active surface area of Pt (Figure S15), the discrepant interactions of two Ni(OH)2 polymorphs with Pt surfaces are responsible for the different activities of Pt atoms near these Ni(OH)2 clusters. The structural change of β-Ni(OH)2 on the surface of Pt electrode during HER was monitored by using in-situ Raman spectroscopy (Figure 3a and 3b). Thanks to the surfaceenhanced Raman effect of Ni(OH)2 on Pt surface (Figure S16), the slight change of bonds in βNi(OH)2 lattice during HER is able to be captured. It should be noted here that we collected the


12

Page 13 of 21 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

ACS Energy Letters

spectra only at potentials ≥ 0 V (vs. RHE), because the gas bubbles generated at the potentials below 0 V (vs. RHE) would partially block the irradiation of laser beam. The Raman bands located at 313 and 448 cm–1 are assigned to the E-type vibration of Ni-OH (γNi-OH) and A1g-type stretch of Ni-O (νNi-O), respectively (Figure 3a).34,

35

The sharp single peak at 3580 cm–1 in

Figure 3b is attributed to the symmetric stretching of the hydroxyl groups (νOH) in β-Ni(OH)2.30, 34

The broad band from 3100 to 3600 cm–1 is related to the O-H stretching of water.20, 35 During

the cathodic polarization process, the γNi-OH and νNi-O bands of β-Ni(OH)2 kept their positions, reflecting the valence state of β-Ni(OH)2 was intact. However, the intensities of these bonds weakened gradually during cathodic polarization, probably due to that water molecules in alkaline media were driven to adsorb on β-Ni(OH)2 lattice (Figure 3a). Furthermore, the intensity ratio (Iγ/ν) of γNi-OH and νNi-O increased from 0.62 (at initial state) to 1.06 (at 0 V vs. RHE) upon lowering the applied potential. This is mainly due to that the strong hydrogen bonding interaction between water and surface hydroxyl groups of β-Ni(OH)2 at low potentials heavily limited the stretching of Ni-O bonds. During the same process, the 3580 cm–1 sharp band and the broad water bands gradually weakened (Figure 3b), implying that cathodic polarization restricted the O-H stretching of water molecules and β-Ni(OH)2 lattice. However, the stretching bands of Ni-O and O-H bonds can be quickly recovered after unloading the bias and removing the bubbles from the electrode (Figure S17). These results indicated that the interactions between water molecules and β-Ni(OH)2 lattice were reversible, and the chemical states of β-Ni(OH)2 kept intact during HER.


13


Page 14 of 21

Figure 3. In-situ Raman tests of β-Ni(OH)2/Pt electrode at low (a) and high (b) wavenumbers in 0.1 M KOH electrolyte.

To further understand the excellent HER performances of Ni(OH)2/Pt electrodes, the free energy of the Volmer and Heyrovsky steps and the local density of states (LDOS) for α- and βNi(OH)2/Pt electrodes were theoretically analysed by density functional theory (DFT) calculations (Figure 4 and Figure S18). As shown in Figure 4a, the adsorption of water molecules onto β-Ni(OH)2/Pt electrode is energetically more favourable than that onto αNi(OH)2/Pt electrode, with a more negative adsorption energy (–0.85 eV). This is attributed to the stronger hydrogen bonding interaction between water molecule and β-Ni(OH)2. By contrast, for α-Ni(OH)2/Pt, the intra-layer water molecules saturated part of the hydroxyl groups in αNi(OH)2 by forming hydrogen bonds (H-O-H…O-H), thus largely limiting the adsorption of reactive water molecules on the surface of α-Ni(OH)2. The free energy diagram in Figure 4b illustrates that the chemisorption energies (Volmer: –0.20 eV; Heyrovsky: 0.78 eV) of βNi(OH)2/Pt electrode for each reaction is lower than that of α-Ni(OH)2/Pt electrode (Volmer: – 0.56 eV; Heyrovsky: 1.14 eV), indicating much facile Volmer and Heyrovsky steps at βNi(OH)2/Pt electrode. The charge difference isosurface of α- or β-Ni(OH)2/Pt electrode (inset in


14

Page 15 of 21 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

ACS Energy Letters

Figure 4b and Figure S18b) reveals the remarkable charge transfers between α- or β-Ni(OH)2 clusters and Pt substrate, in accordance with the XANES results. The edges of α- or β-Ni(OH)2 clusters can provide a large amount of active sites to accelerate water dissociation and improve the adsorption of Hads on Pt sites, being consistent well with the experimental results. Particularly, the significant peaks in the distribution of the LDOS for β-Ni(OH)2/Pt electrode confirm that β-Ni(OH)2/Pt electrode is more active than α-Ni(OH)2/Pt electrode (Figure 4c). This is mainly due to that the high edging catalytic activities of β-Ni(OH)2 and its strong interaction with Pt substrate largely promoted the activity of Pt for HER.

Figure 4. (a) Free energies of H2O adsorption, H adsorption and H2 evolution on α- or βNi(OH)2/Pt electrode; for clarity, the adsorbed H atom for H2O molecule in the top site on Pt surface are colored in green. (b) Adsorption free energy diagram for the Volmer and Heyrovsky steps. (c) Local density of states (LDOS) of α- or β-Ni(OH)2/Pt electrode.


15


Page 16 of 21

On the basis of experimental results and DFT calculations, the mechanism of HER at Ni(OH)2/Pt electrode in alkaline media can be proposed as follows (Figure 5). This reaction involves two steps. (1) Volmer step: the edges of Ni(OH)2 lattice adsorbs water molecules via hydrogen bonding with its surface lamellar hydroxyl groups (O-H…H-O-H). Successively, the adsorbed water molecules dissociate into Hads species and OH– ions after getting electrons from the electrode during cathodic polarization. Hads species preferentially combine with the Pt atoms near Ni(OH)2 lattice because of Pt’s strong ability of adsorbing H atoms and the OH– ions finally desorb from the surface of Ni(OH)2. (2) Heyrovsky step: M-Hads species combine with H atoms from another water molecule to form H2 molecules, and the resulting H2 gas release from Pt surface; meanwhile, the dissociated OH– ions also desorb from the surface of Ni(OH)2. The facile desorption of OH– ions from the surface of Ni(OH)2 at both Volmer and Heyrovsky steps greatly promotes the HER kinetics at Ni(OH)2/Pt electrodes, thus largely decreases the onsetpotential and Tafel slope at Ni(OH)2/Pt electrode relative to that at Pt electrode. β-Ni(OH)2 has stronger interaction with water molecule and Pt substrate than α-Ni(OH)2, providing βNi(OH)2/Pt electrode with more favourable sites for adsorbing water molecules and hydrogen intermediates, as well as for desorbing OH– ions.

Figure 5. Schematic diagrams of Ni(OH)2/Pt electrode (only a β-Ni(OH)2 sheet is shown as an example) for Volmer and Heyrovsky steps during HER.


16

Page 17 of 21 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

ACS Energy Letters

In conclusion, α- or β-Ni(OH)2 modified Pt electrode was prepared for alkaline water electrolysis. The HER performances of α- and β-Ni(OH)2/Pt electrodes are much superior to that of pristine Pt electrode. Among them, β-Ni(OH)2/Pt electrode exhibited stronger activity, higher kinetics, and better stability for HER compared with those of α-Ni(OH)2/Pt electrode. Structural characterizations and DFT simulations indicate that β-Ni(OH)2 has stronger interactions with water molecules and Pt substrate than those of α-Ni(OH)2, thus providing β-Ni(OH)2/Pt electrode with an outstanding catalytic performance for HER. This is the first work to experimentally and theoretically reveal the discrepant activities of α- and β-Ni(OH)2 for HER, indicating β-Ni(OH)2 is an excellent co-catalyst of Pt for alkaline water electrolysis. This work uncovers design principles of HER catalysts and holds promising for practically propelling hydrogen economies.

ASSOCIATED CONTENT Supporting Information. Experimental details, characterizations, theoretical calculations, additional tables and figures are presented in Supporting Information. AUTHOR INFORMATION ‡

These authors contributed equally to this work.

NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT


17


Page 18 of 21

This work was supported by National Key R&D Program of China (2016YFA0200200) and the National Natural Science Foundation of China (51433005, 21674056). The authors thank the 1W1B station for XAFS measurements in Beijing Synchrotron Radiation Facility (BSRF). REFERENCES (1) Gandia, L. M.; Oroz, R.; Ursua, A.; Sanchis, P.; Dieguez, P. M. Renewable Hydrogen Production: Performance of an Alkaline Water Electrolyzer Working under Emulated Wind Conditions. Energy Fuels 2007, 21, 1699-1706. (2) Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332337. (3) Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2017, 2, 1937-1938. (4) Lv, Z.; Tahir, M.; Lang, X.; Yuan, G.; Pan, L.; Zhang, X.; Zou, J.-J. Well-Dispersed Molybdenum Nitrides on a Nitrogen-Doped Carbon Matrix for Highly Efficient Hydrogen Evolution in Alkaline Media. J. Mater. Chem. A 2017, 5, 20932-20937. (5) Markovic, N. M.; Sarraf, S. T.; Gasteiger, H. A.; Ross, P. N. Hydrogen Electrochemistry on Platinum Low-Index Single-Crystal Surfaces in Alkaline Solution. J. Chem. Soc. Faraday Trans. 1996, 92, 3719-3725. (6) Sheng, W.; Gasteiger, H. A.; Shao-Horn, Y. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes. J. Electrochem. Soc. 2010, 157, B1529-B1536. (7) Zeng, K.; Zhang, D. Recent Progress in Alkaline Water Electrolysis for Hydrogen Production and Applications. Prog. Energy Combust. Sci. 2010, 36, 307-326. (8) Rheinländer, P.; Henning, S.; Herranz, J.; Gasteiger, H. A. Comparing Hydrogen Oxidation and Evolution Reaction Kinetics on Polycrystalline Platinum in 0.1 M and 1 M KOH. ECS Trans. 2012, 50, 2163-2174. (9) Conway, B. E.; Tilak, B. V. Interfacial Processes Involving Electrocatalytic Evolution and Oxidation of H2, and the Role of Chemisorbed H. Electrochim. Acta 2002, 47, 3571-3594.


18

Page 19 of 21 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

ACS Energy Letters

(10) Durst, J.; Siebel, A.; Simon, C.; Hasche, F.; Herranz, J.; Gasteiger, H. A. New Insights into the Electrochemical Hydrogen Oxidation and Evolution Reaction Mechanism. Energy Environ. Sci. 2014, 7, 2255-2260. (11) Sheng, W.; Myint, M.; Chen, J. G.; Yan, Y. Correlating the Hydrogen Evolution Reaction Activity in Alkaline Electrolytes with the Hydrogen Binding Energy on Monometallic Surfaces. Energy Environ. Sci. 2013, 6, 1509-1512. (12) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.-C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces. Science 2011, 334, 1256-1260. (13) Danilovic, N.; Subbaraman, R.; Strmcnik, D.; Chang, K.-C.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts. Angew. Chem. Int. Ed. 2012, 51, 12495-12498. (14) Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550-557. (15) Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Energy and Fuels from Electrochemical Interfaces. Nat. Mater. 2017, 16, 57-69. (16) Hall, D. S.; Lockwood, D. J.; Bock, C.; MacDougall, B. R. Nickel Hydroxides and Related Materials: A Review of their Structures, Synthesis and Properties. Proc. R. Soc. A 2015, 471, 20140792. (17) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured Alpha-Nickel-Hydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077-7084. (18) Yeo, B. S.; Bell, A. T. In Situ Raman Study of Nickel Oxide and Gold-Supported Nickel Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Phys. Chem. C 2012, 116, 8394-8400. (19) Patil, U. M.; Gurav, K. V.; Fulari, V. J.; Lokhande, C. D.; Joo, O. S. Characterization of Honeycomb-Like "Beta-Ni(OH)2" Thin Films Synthesized by Chemical Bath Deposition Method and their Supercapacitor Application. J. Power Sources 2009, 188, 338-342.


19


Page 20 of 21

(20) Hall, D. S.; Lockwood, D. J.; Poirier, S.; Bock, C.; MacDougall, B. R. Applications of In Situ Raman Spectroscopy for Identifying Nickel Hydroxide Materials and Surface Layers during Chemical Aging. ACS Appl. Mater. Interfaces 2014, 6, 3141-3149. (21) Wang, H.; Gao, J.; Li, Z.; Ge, Y.; Kan, K.; Shi, K. One-Step Synthesis of Hierarchical Alpha-Ni(OH)2 Flowerlike Architectures and their Gas Sensing Properties for NOx at Room Temperature. CrystEngComm 2012, 14, 6843-6852. (22) Yin, H.; Zhao, S.; Zhao, K.; Muqsit, A.; Tang, H.; Chang, L.; Zhao, H.; Gao, Y.; Tang, Z. Ultrathin Platinum Nanowires Grown on Single-Layered Nickel Hydroxide with High Hydrogen Evolution Activity. Nat. Commun. 2015, 6, 6430. (23) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with ThreeDimensional Electrocatalytic Surfaces. Science 2014, 343, 1339-1343. (24) Birry, L.; Lasia, A. Studies of the Hydrogen Evolution Reaction on Raney NickelMolybdenum Electrodes. J. Appl. Electrochem. 2004, 34, 735-749. (25) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory. Angew. Chem. Int. Ed. 2015, 54, 52-65. (26) Mahmood, N.; Yao, Y.; Zhang, J.-W.; Pan, L.; Zhang, X.; Zou, J.-J. Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions. Adv. Sci. 2017, 1700464. (27) Zeng, Z.; Chang, K.-C.; Kubal, J.; Markovic, N. M.; Greeley, J. Stabilization of Ultrathin (Hydroxy)oxide Films on Transition Metal Substrates for Electrochemical Energy Conversion. Nat. Energy 2017, 2, 17070. (28) Biesinger, M. C.; Payne, B. P.; Lau, L. W. M.; Gerson, A.; Smart, R. S. C. X-ray Photoelectron Spectroscopic Chemical State Quantification of Mixed Nickel Metal, Oxide and Hydroxide Systems. Surf. Interface Anal. 2009, 41, 324-332. (29) Yu, X.; Yang, P.; Chen, S.; Zhang, M.; Shi, G. NiFe Alloy Protected Silicon Photoanode for Efficient Water Splitting. Adv. Energy Mater. 2017, 7, 1601805. (30) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni-Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2013, 135, 12329-12337.


20

Page 21 of 21 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

ACS Energy Letters

(31) Liu, R.; Wang, Y.; Liu, D.; Zou, Y.; Wang, S. Water-Plasma-Enabled Exfoliation of Ultrathin Layered Double Hydroxide Nanosheets with Multivacancies for Water Oxidation. Adv. Mater. 2017, 29, 1701546. (32) Pandya, K. I.; Ogrady, W. E.; Corrigan, D. A.; McBreen, J.; Hoffman, R. W. Extended Xray Absorption Fine Structure Investigations of Nickel Hydroxkies. J. Phys. Chem. 1990, 94, 21-26. (33) Zhao, Y.; Jia, X.; Chen, G.; Shang, L.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; O'Hare, D.; Zhang, T. Ultrafine NiO Nanosheets Stabilized by TiO2 from Monolayer NiTiLDH Precursors: An Active Water Oxidation Electrocatalyst. J. Am. Chem. Soc. 2016, 138, 6517-6524. (34) Bernard, M. C.; Keddam, M.; Takenouti, H.; Bernard, P.; Senyarich, S. Electrochemical Behavior of Quasi-Spherical Beta-Ni(OH)2 Particles Studied by Raman Spectroscopy. J. Electrochem. Soc. 1996, 143, 2447-2451. (35) Kostecki, R.; McLarnon, F. Electrochemical and In Situ Raman Spectroscopic Characterization of Nickel Hydroxide Electrodes. J. Electrochem. Soc. 1997, 144, 485-493.


21

Hydrogen Evolution Reaction in Alkaline Media: Alpha- or Beta-Nickel

Recommend Documents