NiO nanorod arrays for efficient alkaline

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Designing hybrid NiP2/NiO nanorod arrays for efficient alkaline hydrogen evolution Mengying Wu, Pengfei Da, Tong Zhang, Jing Mao, Hui Liu, and Tao Ling ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02691 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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ACS Applied Materials & Interfaces

Designing hybrid NiP2/NiO nanorod arrays for efficient alkaline hydrogen evolution Meng-Ying Wu,† Peng-Fei Da,† Tong Zhang,† Jing Mao,*,† Hui Liu,*,† and Tao Ling,*,† †

Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, School of

Materials Science and Engineering, Tianjin University, Tianjin 300072, China. E-mail: [email protected]; [email protected]; [email protected]

ABSTRACT: Transition-metal phosphides (TMPs) have lately drawn intensive attentions due to their noble metal-free properties and high catalytic activities for hydrogen evolution reaction (HER). The current research mainly focuses on the development of TMPs toward HER in acidic solutions, however, less efforts have been directed to specifically design TMPs for alkaline HER. Here, we design a new bi-functional metal phosphide-oxide catalyst to facilitate the overall multistep HER process in alkaline environments. In this newly catalytic system, oxygen vacancy rich NiO provides abundant active sites for dissociation of water, and the negatively charged P species in NiP2 facilitate adsorption of hydrogen intermediates. The resulting hybrid NiP2/NiO NRs show excellent alkaline HER catalytic activity and stability. Our work demonstrates that it is highly promising to engineer multiple components in hybrid catalytic systems to enhance the overall reaction kinetics, thus achieve improvements in catalytic performance.

KEYWORDS: Electrocatalysis; alkaline HER; nanorod arrays; hybrid catalysts; metal phosphide-oxide catalyst

INTRODUCTION As a renewable energy resource, hydrogen has been extensively investigated as a promising alternative to the fossil fuels.1,2 Compared with the traditional steam reforming of natural gas, the production of hydrogen in an alkaline electrolyzer is more viable for industrial utilization.3,4 The practical application of this technology, however, is often hampered by the slow kinetics of the hydrogen evolution reaction (HER) in a basic environment.3,5-7 Therefore, it is critical to develop highly efficient and cost-effective catalysts to realize hydrogen production through alkaline water splitting.8-15 1

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Recently, transition-metal phosphides (TMPs, such as FeP, CoP, Ni2P, Cu3P, and MoP) have drawn considerable attention owing to their hydrogenases-like catalytic mechanism and high activity toward HER.16-38 Intensive efforts have been undertaken to improve the catalytic activities of TMPs. For example, hybridizing TMPs with the conductive substrates to enhance their charge transfer.24 Constructing self-supported binder-free three-dimensional architectures, such as nanosheets,20,21 nanoflakes,39 and nanowire (nanorod) arrays,17,28,29 et al., to provide direct electron pathway and facilitate the electrolyte penetration and diffusion of ionic species. Additionally, doping metal to modify the electronic structures of TMPs to improve their electrocatalytic performance.19,40 Although significant advances have been made, there is still large room to further improve the activity of TMPs, and make them highly competitive with the noble-metal based catalysts and useful for industrial applications. Moreover, to date, both experimental and theoretical investigations mainly focus on the development of TMPs based electrocatalysts toward HER in acidic solutions,19,27,28 however less efforts have been directed to specifically design TMPs for alkaline HER. For HER in alkaline electrolyte, the generally accepted reaction mechanism is via either Volmer-Heyrovsky or Volmer-Tafel pathway (Volmer: * + H2O + e-⇌H* + OH-; Heyrovsky: H* + H2O + e-⇌H2 + OH-; Tafel: 2H*⇌H2, where H* represents a hydrogen atom chemically adsorbed on an active site).3,6,7 Due to the slow reaction kinetics of Volmer step, a significantly lower catalytic activity has been experimentally observed and theoretically verified on electrocatalysts (e.g. platinum) in alkaline media compared to those in acidic media.3,6,7 In the past decades, a lot of scientific research has revealed that rational design of hybrid nanostructures is a possible way to enhance the overall HER process in alkaline media.5-7,41-43 Subbaraman et al. fabricated a hybrid Ni(OH)2/Pt catalyst to promote HER in alkaline electrolyte.7 Subsequently, Dai et al. developed nanosized NiO/Ni heterostructures as active HER hybrid electrocatalysts in alkaline solutions.5 However, there have not yet been reports about TMP-based hybrid nanostructures using as efficiently alkaline HER electrocatalysts so far. In the present work, we managed to fabricate hybrid NiP2/NiO nanorod arrays to facilitate the overall multi-step HER pathway in alkaline environments: oxygen (O) vacancy rich NiO to promote the dissociation of water, and NiP2 to adjust adsorption/desorption of the hydrogen intermediates to form H2 molecules. Our experimental results demonstrate that the hybrid NiP2/NiO nanorods (NRs) exhibit significantly enhanced electrocatalytic performance relative to the state-of-the-art NiP2 NRs, displaying an overpotential of ~131 mV to generate a HER current density of 10 mA cm-2, comparable with the most efficient alkaline HER electrocatalysts, to the best of our knowledge. Moreover, benefiting from the self-supported electrode 2


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structure, the in-situ fabricated hybrid NiP2/NiO NRs exhibit excellent durability. It is highly expected that rational designing hybrid nanostructures in-situ on conductive substrate will provide new routes to practically engineer TMP-based electrocatalysts.

EXPERIMENTAL SECTION Synthesis of NiO NRs on CFP substrate. The schematic illustration for the synthesis of hybrid NiP2/NiO nanorod arrays is shown in Fig. 1. Firstly, NiO NRs (Fig. S1) were synthesized on carbon fiber paper (CFP) using the cation exchange method in gas phase44-46 using ZnO NRs as the sacrificial template.47,48 The experimental details are similar with our previous work.49 Synthesis of hybrid NiP2/NiO NRs on CFP substrate. Hybrid NiP2/NiO NRs were obtained by partial phosphorization. In detail, NaH2PO2•H2O and NiO NRs were placed 15 cm upstream and 5 cm downstream, respectively, from the tube center. After outgassed, the tube furnace was heated at 360, 380 or 400 oC for 90 min under N2 atmosphere to obtain hybrid NiP2/NiO NRs with varied composition ratio of NiP2 and NiO. Pure NiP2 NRs were phosphorized at 450 oC for 2 h. Commercial Pt/C loaded on CFP was used as a comparison with the mass loading of ~0.2 mg cm-2. To compare the HER performance of pure Pt catalysts, Pt/NiP2 NRs and Pt/NiP2/NiO NRs, Pt catalysts with identical mass were deposited onto NiP2 NRs, NiP2/NiO NRs, and CFP substrate by magnetron sputtering. Then, they were annealed in Ar gas at 400 oC to make the Pt nanoparticles crystallized.

Materials characterization. The morphologies of the catalysts were analyzed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL 2100). Powder X-ray diffraction (XRD) spectra were obtained on using a Bruker D8 advance XRD. X-ray photoelectron spectrometer (XPS, ThermoFisher Scientific) was used for the analysis of the composition of the as-synthesized samples. The loading masses of electrocatalysts were measured by inductively coupled plasma mass spectrometry (ICP-MS, Perkin-Elmer, NexION 300Q).

Electrochemical characterization. A typical three-electrode system with an electrolyte of 1 M KOH was used for electrochemical measurements. The working electrode is the catalysts grown on CFP, the reference electrode is a saturated calomel electrode (SCE), and the counter electrode is a graphite rod. Linear sweep voltammetry (LSV) was 3



recorded with a scan rate of 5 mV s 1. All potentials were quoted with respect to the reversible hydrogen −

electrode (RHE) through RHE calibration (Fig. S2).

RESULTS AND DISCUSSION The hybrid NiP2/NiO nanorods were grown on conductive substrate by a cation exchange route combined with subsequent partial phosphorization reaction (Fig. 1). Firstly, NiO NRs were fabricated via cation exchange method (Fig. 2a). This specific method favours creating vacancies on the surface of exchanged product (Fig. S3), since vacancies control the communication between the parent and product crystals.50 Fig. 2b displays the O 1s spectra of as-synthesized NiO NRs, where the peak II is assigned to oxygen defect sites. As seen, the larger peak II area of NiO NRs compared with commercial NiO powder (hereafter referred to as ‘reference NiO’) suggests plenty of O-vacancies are located on as-synthesized NiO NRs. Further evidence comes from an observation of a noticeable peak shift towards low binding energy in Ni 2p spectrum of NiO NRs compared with reference NiO (Fig. 2c), indicating electron transfer from O-vacancies to Ni. Moreover, the existence of abundant O-vacancies in NiO NRs was further confirmed by the slight expansion of crystal lattice (peak shifts toward lower diffraction angles in Fig. 2d). Taken together, these results demonstrate the creation of abundant O-vacancies on the surface of NiO NRs.

Figure 1. Schematic illustration of the synthesis of hybrid NiP2/NiO nanorod arrays on CFP.

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Figure 2. Characterization of NiO NRs. (a) SEM image. (b), (c) and (d) O 1s, Ni 2p XPS spectra and XRD patterns of NiO NRs and commercial NiO powder (reference NiO), respectively.

Afterwards, the surface of NiO NRs was partially phosphorized to NiP2 to form hybrid NiP2/NiO NRs. The molar composition ratio of NiP2 and NiO in the final hybrid NRs can be well controlled by tuning the phosphorization temperature. XRD patterns of the hybrid NiP2/NiO NRs prepared at different phosphorization temperatures are shown in Fig. 3a, wherein NiO is employed as reference. As seen, with the increase of the phosphorization temperature, the peak intensities of NiP2 increase, while those of NiO decrease. When phosphorized at 450 oC, the NiO NRs were completely transformed into NiP2 NRs. The molar ratios of NiP2 and NiO in hybrid NRs were further estimated by energy dispersive spectra (EDS, Table S1). Specifically, phosphorization at 360, 380 and 400 oC yields the NiP2:NiO ratio of 1:4, 1:2 and 3:2, respectively. Hereafter, hybrid NiP2/NiO NRs with NiP2:NiO ratio of x:y was referred to as x:y NiP2/NiO NRs. As-fabricated hybrid NiP2/NiO NRs well preserve the one-dimensional structure of original NiO NRs, 5



and no significant change in morphology was observed for hybrid NiP2/NiO NRs with varied NiP2:NiO composition ratios (Fig. S4). Characterizations of 1:2 NiP2/NiO NRs are shown in Fig. 3b-3f. The SEM images (Fig. 3b and Fig. 3c) present that the carbon fibres are homogeneously covered by densely packed and vertically aligned nanorods with the diameter of approximately 100 nm, which contact directly with the conductive carbon fibre substrate. The TEM image (Fig. 3d) demonstrates that uniform nanoparticles (NPs) with the average sizes of 3 nm are distributed on the surface of the nanorod. In Fig. 3e, the well-resolved lattice fringes with inter-planar distances of 0.20 nm well match the (220) plane of NiP2 (JCPDS No. 21-0590). The corresponding EDS mapping reveals that the signal of Ni is uniformly distributed over the entire nanorod, while those of O and P are dominantly located in the inner and outer area of the nanorod (Fig. 3f). These collective results demonstrate that hybrid NiP2/NiO NRs with well-controlled microstructures are successfully fabricated directly on CFP substrate, and the NiP2 NPs are dominantly distributed on the surface of NiO NRs.

Figure 3. Characterization of hybrid NiP2/NiO NRs. (a) XRD spectra of hybrid NiP2/NiO NRs, NiO NRs and NiP2 NRs. The standard PDF peaks of NiO (JCPDS No.47-1049) and NiP2 (JCPDS No.21-0590) are given for reference. (b) and (c) SEM images of 1:2 NiP2/NiO NRs. (d) High magnification TEM image of a single NiP2/NiO NR, with the inset showing the morphology of this nanorod. (e) HRTEM image of an individual NiP2 NP. (f) EDS elemental mappings of an individual hybrid NiP2/NiO NR. 6


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The local chemical environment of hybrid NiP2/NiO NRs are further investigated by XPS. Fig. 4 presents the typical P 2p XPS spectra of hybrid NiP2/NiO NRs and NiP2. The peaks centred at around 129, 130, and 134 eV are corresponding to 2p3/2, 2p1/2 signals of P and P-O, respectively.21 A shift of these three peaks toward the low binding energy direction is evident in NiP2/NiO NRs with respective to pure NiP2 NPs. This result demonstrates the NiP2 and NiO components in the hybrid system are chemically coupled; electrons are accumulated in P atoms in both NiP2 (2p3/2 and 2p1/2) and surface oxidized P (P-O peak) of the hybrid NiP2/NiO NRs. Reasonably, large amounts of O-vacancies in NiO NRs (Fig. 2 and Fig. S5) can act as electron donors, transferring electrons to the P p band. As will be discussed later, this chemically coupled hybrid system is preferable for catalysis.

Figure 4. XPS spectra of P 2p peaks for hybrid NiP2/NiO NRs and pure NiP2 NRs.

Hybrid NiP2/NiO NRs grown on CFP with different NiP2/NiO composition ratios were directly employed as the working electrodes for alkaline HER in 1 M KOH (Fig. S6), and their activity was compared with NiP2 NRs, NiO NRs (Fig. S7) and the state-of-art 20 wt% Pt/C catalysts. Polarization curves were recorded from the LSV measurements at a sweeping rate of 5 mV s−1 (Fig. 5a). And the overpotentials of various NiP2/NiO NRs and NiP2 NRs to yield a current density of 10 mA cm-2 are summarized in Fig. 5b. As shown in Fig. 5a and Fig. 5b, the 1:4 NiP2/NiO NRs afford almost identical HER performance as the NiP2 NRs, while the 1:2 NiP2/NiO NRs display obviously higher electrocatalytic HER activity than the NiP2 NRs. Impressively, the 1:2 NiP2/NiO NRs show an overpotential of ~131 mV to generate a current density of 10 mA cm-2, better than those of the NiP2 NRs (164 mV), the reported active metal phosphides (e.g. CoP,28 209 mV), and ternary phosphides (e.g. NiCoP,19 209 mV). Moreover, as-fabricated 1: 2 NiP2/NiO NRs are among 7



the most efficient alkaline HER electrocatalysts (Table S2). It should be noted that the worse performance of 3:2 NiP2/NiO NRs compared with NiP2 NRs mainly arises from the relatively poor electronic conductivity of NiO compared with NiP2, as evidenced by the electrochemical impedance (EIS) characterizations (Fig. S8).

Figure 5. Alkaline HER performance of hybrid NiP2/NiO NRs. (a) Polarization curves of hybrid NiP2/NiO NRs with varied NiP2:NiO composition ratios, NiP2 NRs and Pt/C. The measurements are recorded in 1 M KOH solution at a sweep rate of 5 mV s−1. All curves are iR-corrected. (b) Comparison of overpotential (within our available ±5 % experimental error) to generate a current density of 10 mA cm-2 for hybrid NiP2/NiO NRs and NiP2 NRs. (c) Long term durability of 1:2 NiP2/NiO NRs at overpotential of 131 mV. (d) Corresponding Tafel plots of the LSV curves in (a).

Besides the high HER activity in alkaline, the hybrid NiP2/NiO NRs also exhibit long-term durability (Fig. 5c and Figs. S9-S12). As displayed in Fig. 5c and Fig. S11, the 1:2 NiP2/NiO NRs maintain more than 90 % of initial HER current after 20 h continuous testing. This excellent durability of NiP2/NiO NRs was further evidenced by the XRD and SEM characterizations that the structure and morphology of NiP2/NiO NRs are intact after stability test (Figs. S9, S10, and S12). Such excellent durability of origin from directly 8


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growth of the hybrid NiP2/NiO NRs on conductive substrates, thus avoid peeling issues, which usually encountered by particulate catalysts during the production of large amounts gas . To discern the catalytically alkaline HER kinetics of hybrid NiP2/NiO NRs and NiP2 NRs, Tafel plots were examined (Fig. 5d). Notably, at low overpotentials, Tafel slope of about 120, 40, and 30 mV dec−1 indicates the HER pathway is rate-controlled by Volmer, Heyrovsky, and Tafel reactions, respectively.51 For pure NiP2 NRs and 3:2 NiP2/NiO NRs, the Tafel slope values are both 130 mV decade−1, which is in good agreement with the value of reported metal phosphides (e.g. NiCoP,19 124 mV decade-1), suggesting that the rate of alkaline HER on NiP2 surface is limited by Volmer step. In contrast, for 1:4 NiP2/NiO NRs and 1:2 NiP2/NiO NRs with more NiO component, the Tafel slope values are significantly decreased to around 93 mV decade−1. This demonstrates their promoted kinetics of Volmer reaction, which depends on the energy barrier of water dissociation to form adsorbed hydrogen intermediates.6,7 Therefore, the enhanced kinetics of Volmer step for 1:4 NiP2/NiO NRs and 1:2 NiP2/NiO NRs should be attributed to facilitated water dissociation on these two hybrid NiP2/NiO NRs. To verify above hypothesis, a comparison of the HER performances of pure Pt catalysts, Pt/NiP2 NRs, and Pt/1:2 NiP2/NiO NRs was conducted. Pt catalysts with the identical mass loading were sputtered onto the CFP substrate (Fig. S13) and 1:2 NiP2/NiO NRs. The HER performance of Pt/NiP2 NRs and Pt/1:2 NiP2/NiO NRs were recorded in 1 M KOH, while the HER performance of pure Pt catalysts was tested both in 1 M KOH and 0.5 M H2SO4. It should be noted that although Pt is usually regarded as the best catalyst for converting H* to H2 (with the optimal H* adsorption energy), water dissociation is inefficient on Pt surface.6,7 Therefore, the HER activity of Pt in 1 M KOH is significantly lower than that in 0.5 M H2SO4 as expected (Fig. 6). The comparable activities of Pt/NiP2 NRs and Pt indicate that water dissociation is also energetically unfavorable on the surface of NiP2 NRs, similar to that on the Pt surface. In sharp contrast, when NiP2 is coupled with NiO, the HER activity of Pt/1:2 NiP2/NiO NRs is remarkably enhanced as compared with that of Pt and Pt/NiP2 NRs in 1 M KOH. These results undoubtedly demonstrate the facile water dissociation on NiO, supported by greatly reduced Tafel slope of hybrid NiP2/NiO NRs with more NiO component in Fig. 5d (1:2 and 1:4 NiP2/NiO NRs). This finding is supported by previous works that O-vacancies present on the surface of metal oxides can provide active sites for OH-H bond cleaving during Volmer step.7

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Figure 6. Comparison of the HER activities of Pt/1:2 NiP2/NiO NRs, Pt/NiP2 NRs and pure Pt catalysts.

Besides the aforementioned facile water dissociation, the hybrid NiP2/NiO NRs are also more favorable for hydrogen intermediates adsorption compared with pure NiP2 NRs. Previous reports demonstrate the characteristic of “ligand effect” of P to metallic atom for bonding to hydrogen intermediates.19,52 However, the binding of hydrogen intermediates on P sites is slightly stronger than optimum (with hydrogen adsorption free energy equals to zero),25 which is disadvantageous to hydrogen intermediates desorption and subsequent H2 production. Our above XPS analysis indicates that the P species in NiP2 is more negatively charged after chemically coupled with NiO, which will weaken the binding of the hydrogen intermediates on P sites, thus leads to higher activity of hybrid NiP2/NiO NRs. This fact is in good agreement with recent report that the more negatively charged P species in Ni0.51Co0.49P are extremely beneficial for collection of protons.21 Overall, above results unambiguously demonstrate that NiO and NiP2 together enhance the overall alkaline HER mechanism in the new bi-functional catalyst system. The optimal hybrid metal phosphide/oxide system can be achieved by tailoring the active sites for both efficient water dissociation on NiO and subsequent adsorption/recombination of the reactive hydrogen intermediates on the adjacent NiP2 (Fig. 7).

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Figure 7. Schematic illustration of (a) designing and (b) tailoring the active sites in hybrid metal oxide/phosphide system.

CONCLUSIONS In summary, we design a new bi-functional NiP2/NiO NRs catalyst to facilitate the overall multistep HER process in alkaline conditions. We demonstrate that the O-vacancy rich NiO provide abundant active sites for dissociation of water, and the negatively charged P species in NiP2 component facilitate adsorption of hydrogen intermediates. As a result, the hybrid NiP2/NiO NRs show excellent HER performance with a low overpotential, a small Tafel slope and good durability. The further performance of hybrid catalysts can be anticipated by increasing the loading mass and improving the electronic conductivity of metal oxides. It is highly expected that this rational designed metal phosphide/oxide hybrid nanostructure in-situ on conductive substrate will provide new routes to practically engineer TMP-based HER electrocatalysts for industrial applications.

ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: . Calibration of the reference saturated calomel electrode, illustration diagram of cation exchange process, SEM, XPS and electrochemical characterization of hybrid NiP2/NiO NRs, photograph of the home-made electrochemical testing cell, characterization of Pt catalysts on CFP, quantitative EDS analysis of hybrid NiP2/NiO NRs, and summary of the recently reported highly active HER catalysts in alkaline solution.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]; [email protected]

ACKNOWLEDGMENTS This work was supported by the National Science Fund for Excellent Young Scholars (51722103), the Natural Science Foundation of China (51571149, 21576202, and 51502199), the Natural Science Foundation of Tianjin city (15JCYBJC18200).

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