Natural multi-channeled wood frameworks for electrocatalytic hydrogen evolution

Electrochimica Acta xxx (xxxx) xxx

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Natural multi-channeled wood frameworks for electrocatalytic hydrogen evolution Bin Hui*, Kewei Zhang, Yanzhi Xia**, Chengfeng Zhou State Key Laboratory of Bio-Fibers and Eco-Textiles, Shandong Collaborative Innovation Center of Marine Biobased Fiber and Ecological Textile, Institute of Marine Biobased Materials, School of Materials Science and Engineering, Qingdao University, Qingdao, 266071, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 September 2019 Received in revised form 16 October 2019 Accepted 10 November 2019 Available online xxx

Developing stable and efficient nonprecious electrocatalysts for hydrogen evolution reaction (HER) is desirable but remains challenging. In this work, a natural wood-based 3D framework was designed to load an amorphous NiP alloy using a simple electroless plating method. The NiP alloy anchored in Poplar wood exhibited the highest HER activity in alkaline solution, with a low overpotential of 83 mV at a current density of 10 mA cm
2 and a small Tafel slope of 73.2 mV dec
1 compared to the other channeled woods (Faxinus mandshurica wood and Larch wood). This self-supported monolithic NiP/Poplar wood sustained a high current density of ~1200 mA cm
2 for 36 h. These superior electrocatalytic performances greatly depend on both the unique multi-channeled wood structures and the firm interfacial combination between a wood and amorphous NiP alloy. This work offers a new avenue for the design and optimization of transition metal phosphides and natural multi-channeled wood for high electrocatalytic activity and superior cycling durability towards hydrogen generation. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Natural wood channels Amorphous NiP alloy Alkaline Hydrogen evolution reaction

1. Introduction Hydrogen, as a carbon free fuel with high energy density (120e142 MJ kg
1), has been put forward as an idea energy carrier to replace traditional fossil fuel [1e3]. A facile, practical, and sustainable way to produce high-purity molecular hydrogen is via water electrolysis using electricity. Among kinds of electrocatalysts for hydrogen evolution reaction (HER), Pt and Pt-based alloys are the state-of-the-art those which show small Tafel slope and low overpotential [4,5]. Unfortunately, the high cost and low abundance of such noble metals limit their large-scale commercial applications. Exploring the low-cost, stable, and high-performance electrocatalysts have been widely investigated. Among them, transition metal phosphides (TMPs) such as FeP [6], MoP [7], CoP [8], and NiP [9] have received intense interest because of their analogous mimics of active sites in hydrogenases and good conductivity. Furthermore, the transition metals with amorphous structures can remarkably enhance the catalytic properties due to high number of active sites and short-range order of amorphous materials [10,11]. To engineer the electrode architecture is thought to be another

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Hui), [email protected] (Y. Xia).

effective strategy to significantly improve the catalytic activity and stability of transition metal electrocatalysts [12e14], in particular at a high current density (eg., 1000 mA cm
2) required for practical industrial water electrolysis [15,16]. In situ depositing active catalysts on an open and porous substrate to construct self-supported monolithic electrodes without using binders can decrease interface resistance, increase structural stability, and expose more active sites, which is promising for mass production [17,18]. Towards this end, numerous efforts have been devoted to growing TMPs catalysts on a three-dimensional (3D) porous substrate via using various methods towards efficient HER [2,19,20]. However, many reported TMPs catalysts were prepared by using poisonous reagents and harmful gases, processing at high temperature and/or high pressure, or using special instruments. This hinders the largescale application. Exploring simple and efficient methods to grow TMPs need to be addressed. In another aspect, the most common 3D substrates used in the present are Ni, Cu, or Fe foams, which can improve HER performance to a certain extent, but active materials are inclined to peel out from substrates at a high current density [21], and the substrate has low corrosion resistance in strong acid or alkaline solutions [22]. Therefore, it is urgent but still tremendously challenging to develop stable and biodegradable 3D substrates for loading electrocatalysts. Wood, as a naturally abundant, biodegradable, and green

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structural biomass, not only has excellent mechanical properties, but also exists many aligned vertical channels [23,24]. The unique aligned channels in wood are used for pumping ions, water, and other ingredients through wood trunk to satisfy the demand for photosynthesis. Different woods show a variety of channels owing to their types [25]. For instance, softwoods normally have a homogeneous channel structure. Hardwoods typically possess a higher density and a dense structure compared to softwoods. Interestingly, the hierarchical structures which include wood vessels, fibers, rays, and some cells in various scales are characteristic for hardwoods. This unique structure can generally play an important role in reaction process involved in multiphase contact [26]. In addition, the wood surface with numerous hydroxyl groups and rough sites provide a profound basis for firmly grabbing metals or their compounds [27]. So far, Hu and co-workers firstly reported a porous wood carbon by a heat shock method which was used for electrolyte permeation and hydrogen gas removal during HER. Due to the channeled structure, the derivative wood exhibited a low overpotential and a small Tafel slope in acid solution [28]. However, alkaline water electrolysis is greatly important because it is one of the most widely used technologies in industry. The HER mechanism for the channeled wood skeleton in alkaline solution needs to be explored and basic laws of electrode kinetics should be verified. Herein, inspired by the multi-channeled structures of natural wood materials, we develop a highly active NiP catalyst anchored in wood framework by using rapid and scalable electroless plating for highly efficient HER. Benefiting from the unique 3D self-supported structure, the obtained NiP/Poplar wood exhibited superactivity and striking durability towards electrocatalytic hydrogen evolution in 1 M KOH solution. The as-prepared NiP/Poplar wood only required the overpotential of 83 mV to reach 10 mA cm
2, had the low Tafel slope of 73.2 mV dec
1, and also had excellent stability, maintaining almost unchanged for 36 h at a current density of ~1200 mA cm
2. This work gives a fresh impetus to the development of wood-based electrocatalysts with high-efficient catalytic performance and long-term cycling stability at a high current density. 2. Experimental 2.1. Sample preparation Experimental chemicals used in the synthesis are all analytical grades and without further purification. The pristine woods (Poplar wood, Faxinus mandshurica wood, and Larch wood) obtained from Harbin were cut into samples with a size of 1.5 cm  1.0 cm  0.1 cm, and ultrasonically treated by distilled water, acetone, and ethanol, respectively. The Ni foam was obtained from Hefei Kejing Material Technology Co., Ltd., and treated in 1 M HCl solution for 20 min before use. NiSO4$6H2O, C6H5Na3O7$2H2O, NaH2PO2$H2O, NH3$H2O, NaOH, KOH, PdCl2, HCl, NaBH4, C2H5OH, and CH3COCH3 were purchased from China Pharmaceutical Group Co., Ltd. 2.2. The synthesis of NiP/Poplar wood electrode material Prior to electroless plating, the pristine wood was immersed in aqueous PdCl2 solution (0.2 g L
1 PdCl2, 15 mL 37% HCl) for 8 min at room temperature, and then washed with distilled water to remove free Pd2þ. The Pd2þ was then reduced to Pd0 by soaking the wood in aqueous NaBH4 solution (5 g L
1 NaBH4, 2 g L
1 NaOH) at room temperature. Finally, the NiP alloy grew in the activated wood via electroless plating in precursor solution for 20e120 min at 60  C. The composition of plating solution is given in Table 1. The Ni and P with different ratios deposited on Poplar wood, NiP/Faxinus

Table 1 Composition of plated NiP alloy. Chemicals

Content (g L
1)

NiSO4$6H2O NaH2PO2$H2O C6H5Na3O7$2H2O NH3$H2O

15e30 20e50 30e80 30e60

mandshurica wood, NiP/Larch wood, and NiP/Ni foam were prepared as mentioned above. 2.3. Characterization The field emission scanning electron microscopy (FESEM) images were obtained on electron microscope (Quanta 250 FEG, FEI, USA). The elemental composition of the resulting material was analyzed by energy dispersive X-ray spectrometer (EDS) attached to the FESEM instrument. X-ray diffraction (XRD) was carried out with DX2700 operating at 40 kV and 30 mA equipped with Cu Ka radiation (l ¼ 1.5418 Å). The high-resolution transmission electron microscopy (HRTEM) analysis was conducted using a JEM-2100F electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was carried out with ESCALab 250 electron spectrometer (Thermo Scientific Corporation) to characterize the elemental composition and chemical states of samples. The electronic conductivity for the sample was evaluated using high temperature four point test system (HRMS800, Partulab, China). The wettability was evaluated by testing contact angle using a contact angle analyzer (Theta), and 3D mapping images and surface roughness were obtained through an accessory equipped with the Theta. 2.4. Electrochemical measurements All the electrochemical experiments were carried out by a CHI 750 E electrochemistry workstation in a standard three-electrode system. The NiP/wood or Ni foam samples (1.0 cm  1.0 cm) were directly used as the working electrode. An Ag/AgCl electrode with 1 M KOH filling solution was used as the reference electrode. A graphite rod was used as the counter electrode in order to prevent the interference of platinum foil as the counter electrode. 1 M KOH solution was used as electrolyte. Linear scan voltammetry (LSV) was carried out at 5 mV s
1 for polarization curves. The potential measured in this study was converted to reversible hydrogen electrode (RHE) according to Eq. (1).

 E ðRHEÞ ¼ E

 Ag þ 0:197 þ 0:059  pHeiR AgCl

(1)

All the presented curves were the steady-state ones after several cycles. All the polarization curves were iR-corrected. The durability test was determined by chronoamperometry. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 0.01e100000 Hz. The double layer capacitance (Cdl) was conducted with cyclic voltammograms (CVs) scanning from
0.016 to 0.184 V vs. RHE with different scan rates from 20 to 120 mV s
1. 3. Results and discussion The schematic illustration for the preparation process of the NiP/wood is described in Fig. 1. The wood was cut perpendicular to its growth direction to obtain the three-dimensional (3D) ordered porous framework (Fig. 1a and b). The NiP alloy was grown in micro-channels through a simple electroless plating method to

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Fig. 1. Schematic illustration for the preparation process. (a) Tree. (b) Pristine woods with various channeled structures: Poplar wood, Faxinus mandshurica wood, and Larch wood, respectively. (c) SEM image of the as-prepared NiP alloy in wood channels and (d) the corresponding magnified SEM image of (c) (the inset in (d) shows electronic conductivity). Schematic representation showing (e) the pristine wood, (f) the Pd0/wood, and (g) the NiP/wood, respectively (Step I representing the activation process and Step II representing the plating process).

endow active sites of catalysts, showing excellent electronic conductivity of 59.17 S cm
1 (Fig. 1c and d). In detail, after pristine wood was treated by the palladium ion (Pd2þ) and then palladium (Pd0) produced by the reduction of NaBH4 solution, the NiP alloy electrocatalysts in wood channels were rapidly self-assembled in the precursor solutions containing nickel and phosphorous salts. The specific operation conditions for the NiP catalysts are shown in experimental section. The involved reactions could be described as following Eqs. (2) and (3). Under appropriate pH and temperature, the Ni2þ and H2PO
2 in plating solution created Ni and P, respectively, which were together embedded in wood channels. 2

2þ 2H2 PO
2 þ 4OH þ Ni /2HPO3 þ 2H2 O þ Ni þ H2

(2)

þ
2H2 PO
2 þ 6H þ 4H /4H2 O þ 2P þ 3H2

(3)

It should be noted that nonmetallic materials need to be activated before electroless plating due to a lack of catalytic nuclei for initiating the autocatalytic process. Pristine wood is a kind of typical nonmetallic material. Hence, activation treatment is necessary. For the initial activation prior to electroless plating, it is illustrated in Step I process, which represents from pristine wood (Fig. 1e) to Pd0/wood (Fig. 1f). It can be verified in Fig. S1 (Supporting Information) that Pd2þ had been reduced to Pd0 through the change of binding energy of Pd 3d5/2 from 337.29 eV to 335.51 eV [29]. Chemical composition analysis in Fig. S1c (Supporting Information) shows that the atomic percentage of Pd0 activator takes up mere 1.23%, but it is enough to trigger the following plated process. In particular, the Pd0 particles had no entrance into the lattice of the NiP alloy because the activation was completed before plating process. After electroless plating, the asprepared NiP alloy could hold tightly to the wood surface owing to plenty of hydroxyl groups and much roughness from pristine wood. The firm interface of NiP/wood can bear a high current density, and not be easily destroyed by a strong acid or alkaline electrolyte. Meanwhile, the self-supported wood channels (Fig. 1g) also play a vital role during HER process. The open and hierarchical 3D wood structure promotes rapid permeation of electrolyte, and the generated hydrogen gas on catalysts’ surface quickly releases from micro-channels without hindering the pathway of mass transfer. Fig. 2a and b shows the top-view SEM images of pristine Poplar wood, where multi-channels with 40e80 mm were surrounded by lots of small channels with 5e20 mm. After the treatment of

electroless plating, Polar wood structures remained merely changed and the inner surfaces of pores were covered by a compact and uniform NiP film (Fig. 2c and d). Fig. 2e shows the crosssectional view of treated Poplar wood by NiP alloy, demonstrating the existence of multi-channeled structures in different scale. The top-view SEM images of the NiP/Faxinus mandshurica wood and the NiP/Larch wood are also presented. For NiP/Faxinus mandshurica wood (Figs. S2a and S2b, Supporting Information), its big channel and small channel were about 121 mm and 12 mm, respectively. On account of Larch wood belonging to softwood, its channel was relatively homogeneous with an average size of 39 mm (Figs. S4a and S4b, Supporting Information). The different channeled structures from various woods may perform a different function during hydrogen evolution reaction. Elemental mapping images of NiP alloys anchored in Poplar wood (Fig. 2f), Faxinus mandshurica wood (Figs. S2c and S2d, Supporting Information), and Larch wood (Figs. S4c and S4d, Supporting Information) revealed a homogenous distribution of Ni and P elements. EDS spectra of the NiP alloy/Poplar wood in Fig. 2h confirmed that most Ni and little P elements existed, compared to pristine Poplar wood only composed of C and O elements (Fig. 2g). Elemental composition from EDS analysis further indicated that the atomic ratio of Ni to P was close to 16 : 1 (Fig. 2i). The EDS spectra of NiP/Faxinus mandshurica wood and NiP/Larch wood are shown in Fig. S3 (Supporting Information) and Fig. S5 (Supporting Information), respectively, which displayed a similar result to NiP/Larch. In order to further determine surface compositions and phase structures of the NiP/wood, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) analyses were carried out, respectively. A typical XPS wide spectrum of the surface of the NiP/Poplar wood is shown in Fig. 3a, which revealed that Ni, P, C, and O elements were detected. In the XPS high-resolution scan of Ni 2p (Fig. 3b), two peaks with binding energies of 874.52 eV and 856.19 eV corresponded to Ni2þ 2p1/2 and Ni2þ 2p3/2, respectively, which also had two related satellite peaks. The other two peaks located at 870.31 eV (2p1/2) and 852.97 eV (2p3/2) indicated the presence of metallic Ni, showing that part of Ni2þ in plating solution had been reduced to Ni0 via electroless plating [10]. For the P 2p spectra in Fig. 3c, the peak at 133.20 eV corresponded to the phosphate species (PeO), and the two peaks at 130.63 eV and 129.70 eV were assigned to the P 2p1/2 and P 2p3/2 of phosphides, respectively [30,31]. Furthermore, the P with the binding energy of 129.70 eV carries a large amount of negative charge, which can

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Fig. 2. Morphologies and compositions characterizations for the pristine Poplar wood and the NiP/Poplar wood. SEM images of pristine Poplar wood: (a) top view, (b) magnified image of (a). SEM images of the NiP/Poplar wood: (c) top view, (d) magnified image of (c), and (e) aligned micro-channels from cross section. The insets in (a) and (c) showing the pristine Poplar wood and the NiP/Poplar wood samples with a size of 1.5 cm  1.0 cm, respectively. (f) Corresponding elemental mapping images of C, O, Ni, and P in NiP/Poplar wood. EDS spectra of (g) the pristine Poplar wood and (h) the NiP/Poplar wood, respectively. (i) Atomic percentage of C, O, Ni, and P in the pristine Poplar wood and NiP/Poplar wood, respectively.

capture positively charged protons for hydrogen evolution during electrocatalysis [32]. For the O 1s in Fig. 3d, the XPS spectra exhibited the two peaks that were tentatively assigned to OeP (533.51 eV) and OH
(531.99 eV). The presence of O results from the H2PO
2 resides and the oxidized Ni at the surface because of its exposure to atmosphere. The phase structures of the pristine wood and the as-prepared NiP/wood electrocatalyst were further examined by XRD. As seen from Fig. 3e, the pristine woods (Poplar wood, Faxinus mandshurica wood, and Larch wood) had typical cellulose peaks in 2q ¼ 16.2 and 22.3 , corresponding to (101) and (002) facets, respectively. After the treatment of electroless plating, the cellulose peaks almost disappeared, and there appeared a steamed bread peak in 2q ¼ 45.2 , which was attributed to Ni (111) facet, indicating that the phase structure was amorphous. The HRTEM image of the NiP alloy scraped off from three kinds of wood surfaces further confirmed that co-deposited P was embedded in the Ni lattice, as shown in Fig. 3f, Fig. S6 (Supporting Information), and Fig. S7 (Supporting Information), respectively. In comparison to crystalline NiP, the amorphous NiP which lacks long-range order has short-range atomic arrangements and structural disorder. The

structural disorder and defect sites provide more efficient reaction centers and thus greatly enhance the hydrogen generation, although it might be seriously harmful for electronic and photonic properties [10]. The formation of the NiP/wood interface and its effect on HER are illustrated in Fig. 4. For the pristine Poplar wood in Fig. 4a, its initial water contact angle (CA) was 95 and the CA value was quickly reduced to 23 in 10 s (Fig. 4b). This result indicated the good wettability of pristine Poplar wood, which was ascribed to the existence of plenty of hydrophilic hydroxyl groups and porous structure of wood (Fig. 4c,e). In addition, the wood possesses rough surface (Fig. 4d), which can be evaluated in root mean square (RMS) values [33]. Calculated from the 3D mapping image with high “mountains” and low “valleys” (Fig. 4e), the RMS value of crosssectional wood surface reached 2.2, which was indicative of much roughness. The good wettability and rough surface of pristine Poplar wood show great advantage to enhancing the interfacial combination with metals or alloys. As shown in Fig. 4f, the NiP/ wood exhibited good compatibility of as-formed interface, which was beneficial for the stability of the NiP/wood when it was at a

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Fig. 3. (a) Survey XPS spectrum of NiP/Poplar wood. High-resolution XPS spectra of (b) Ni 2p, (c) P 2p, and (d) O 1s of NiP/Poplar wood. (e) XRD patterns of (i) pristine Poplar wood, (ii) NiP/Poplar wood, (iii) pristine Faxinus mandshurica wood, (iv) NiP/Faxinus mandshurica wood, (v) pristine Larch wood, and (vi) NiP/Larch wood. (h) HRTEM image of the NiP nanoparticles from NiP/Poplar wood (inset is the corresponding TEM image at low magnification).

large current density or in strong alkaline (or acid) solutions. Fig. 4g shows the mechanism of the interface on HER in alkaline medium. The unique composition and structure of wood play an important role in the dissociation of H2O molecules and desorption of H2 bubbles during HER process. Detailedly, the hydrophilic wood and hydrophobic NiP alloy in a state of imbalance effectively promote H2O to rapidly immigrate to the electrocatalytic surface for dispersing H2 bubbles, and thus enormously enhance HER performance. The electrocatalytic HER performances of NiP/Poplar wood, NiP/ Faxinus mandshurica wood, and NiP/Larch wood (the ratio of Ni to P, 16 : 1) were tested by using a typical three-electrode system at a scan rate of 5 mV s
1 in 1 M KOH solution. Fig. 5a shows the iRcorrected linear sweep voltammetry (LSV) curves, which imply that the NiP/Poplar wood has a lowest overpotential. The overpotential of NiP/Poplar wood reached only 83 mV at a current density of 10 mA cm
2, and the corresponding overpotentials were 150 mV

for NiP/Faxinus mandshurica wood and 105 mV for NiP/Larch wood at the same current density. This result indicated that the appropriate wood channels had a great effect on the improvement of HER performance. The LSV curves for Poplar samples with different ratios of Ni to P (20 : 1 and 9 : 1) were also presented in Fig. S8 (Supporting Information). The Tafel slops of the corresponding LSV curves were calculated. As seen from Fig. 5b, the NiP/Poplar wood shows the smaller Tafel slope (73.2 mV dec
1) compared to that of NiP/Faxinus mandshurica wood (114.5 mV dec
1) and NiP/ Larch wood (81.2 mV dec
1), indicating a faster HER rate. The Tafel value of the NiP/Poplar wood also suggested that the corresponding HER followed a Volmer-Heyrovsky mechanism, as shown in Eqs. (4) and (5).

H2 O þ e
¼ Hads þ OH


(4)

H2 O þ e
þ Hads ¼ H2 þ OH


(5)

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Fig. 4. (a) Photograph of a pristine Poplar wood cut perpendicular to its growth direction. (b) Water contact angles of 95 (in initial time) and 23 (in 10 s) for pristine Poplar wood, displaying the superior hydrophilic property. (c) Schematic illustration of the NiP alloy recombined with pristine wood. (d) Naturally rough wood surface in cross section of pristine wood. (e) Composition of cellulose chains and structure of the 3D mapping image with high “mountains” and low “valleys”. (f) The excellent joint of the NiP/Poplar wood interface (inset shows the water contact angle). (g) The diagram of the mechanism of interface on HER in alkaline medium.

To gain insights into the intrinsic activity of as-prepared catalysts, the double-layer capacitances (Cdl) were tested to estimate the electrocatalytic active surface areas (ECSA). Fig. 5d shows that the NiP/Poplar wood had the highest Cdl of 24.1 mF cm
2 among all the samples from the calculation of CV curves in Fig. 5c, and Figs. S9 and S10 (Supporting Information), indicating the highest catalytically active surface area, which was mainly attributed to the structure of wood channels and more exposed active sites [34]. In addition, the HER kinetic process of samples were investigated via electrochemical impedance spectroscopy (EIS). As depicted in Fig. 5e, in contrast with the NiP/Faxinus mandshurica wood and the NiP/Larch wood, the NiP/Poplar wood had the smallest chargetransfer under the same overpotential, which further demonstrated the especial 3D channeled structures from some kind wood improved charge-transfer capability. The HER kinetic process of the catalyst was further investigated by EIS experiments at various bias during hydrogen evolution. The Nyquist and Bode plots of the assynthesized NiP/Poplar wood are shown in Fig. 5f and g, respectively. It was observed that the diameter of the semicircles of the catalyst decreased gradually upon the application of a more negative bias, suggesting that a lower charge transfer resistance at negative bias leads to more efficient hydrogen evolution. From the Bode phase diagram in Fig. 5g, one semicircle was observed, and the semicircle became smaller and overlapped towards high

frequency. At the same time, overall impedance became very low at a negative bias due to vigorous H2 evolution. Thus, an equivalent electrical circuit was used to described the HER kinetic process of the NiP/Poplar wood catalyst (inset in Fig. 5e). In order to clearly explain the effect of wood channel structure on HER kinetic process, the diagram for channels change trends in softwood (Larch wood) and hardwoods (Faxinus mandshurica wood and Poplar wood) is shown, and the mechanism for mass transport during HER in wood channels is proposed. Notably, apart from the wood channels, micropores and mesopores can also help hydrogen gas release and electrolyte permeation during HER process, but various woods have amazingly similar microporous and mesoporous structures [25]. The micropores and mesopores from pristine wood and NiP/wood are shown in Fig. S11 (Supporting Information). Herein, we focus on this research of effects of different large-scale channeled structures of wood on HER performances. It is seen in Fig. 6a that the channeled structure of Larch wood is homogeneous. The Faxinus mandshurica wood and Poplar wood have big channels and small channels, as shown in Fig. 6b and c, respectively. Based on the calculation from SEM images (Fig. 2; Figs. S2 and S4, Supporting Information), the size ratio of a big pore and a small pore was 5 : 1, 10 : 1, and 1 : 1 for Poplar wood, Faxinus mandshurica wood, and Larch wood, respectively. The hierarchical structure for hardwood has generally vital role in the penetration of

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Fig. 5. Electrochemical performance of as-prepared NiP/woods in 1 M KOH. (a) iR-corrected HER polarization curves, (b) corresponding Tafel plots, (c) CV curves of the NiP alloy in Poplar wood at different scan rates, (d) electrochemical double-layer capacitance (Cdl), (e) electrochemical impedance spectra that show the Nyquist plots of NiP alloys in Poplar wood, Faxinus mandshurica wood, and Larch wood on the same overpotential of
1.15 V vs. Ag/AgCl electrode, respectively (inset is the equivalent electrical circuit used to model the HER kinetics process. Rs and Rct were the electrolyte resistance and charge-transfer resistance of working electrode, respectively. CPEdl resulted from the constant phase element of working electrode), (f) Nyquist plots and (g) Bode plots of the NiP alloy in Poplar wood on application of different bias (
0.70 to
1.08 V vs. Ag/AgCl electrode), (h) Timedependent current density curve of the NiP/Poplar wood under static current density of ~1200 mA cm
2 for 36 h (without iR correction) and the inset shows the fluctuation of current density resulted from accumulation and release of hydrogen bubbles in 180 s.

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Fig. 6. The change trends of channels in (a) Larch wood, (b) Faxinus mandshurica wood, and (c) Poplar wood. (d) The proposed mechanism for mass transport during HER in channeled structures.

electrolyte and release of gas [25]. Especially for Poplar wood, its numerous small channels with short boundary end shorten the distance of electron transport and prevent the H2 molecules growing into a big bubble. When the bubble diameter reaches the diameter of wood channel, H2 bubble quickly blasts. In the interface, the acquired blasting force disperses numerous OH
generated from Volmer step and Heyrovsky step in alkaline medium, and thus efficiently avoids the poisoning of active sites. Therefore, the channeled structure from Poplar wood can greatly enhance the efficiency for HER. The chronoamperometry test for NiP/Poplar wood was carried out to investigate long-term stability under a designated overpotential. It is seen in Fig. 5h that the large current density of ~1200 mA cm
2 still retains stable for 36 h without observable degradation, confirming superior cycling durability. As observed in the inset in Fig. 5h, the frequency of fluctuation of current density for sample was tremendously fast, which was indicative of a fast kinetic process. The chronoamperometry test for Poplar sample with the ratio of Ni to P (9 : 1) is also shown in Fig. S12 (Supporting Information), exhibiting good HER stability. Also, obvious amorphous and morphology changes were not detected from XRD pattern (Fig. S13, Supporting Information) and SEM image (Fig. S14, Supporting Information) of the NiP/Poplar wood after durability test. This superior cycling durability at a high current density in 1 M KOH is rare in previous reports, which stems from the stable combination of channeled wood and NiP alloy. On one hand, the wood surface has plenty of hydrophilic hydroxyl groups. On the other hand, the natural grooves structures from wood are existed, and thus numerous sites can be anchored by a metal alloy. As a result, the as-prepared NiP/wood electrocatalyst keeps extremely stable even at high current density for a long time. Additionally, for the sake of further demonstrating the merit of channeled wood as prominent carrier, we carried out the experiment on the same NiP alloy depositing on Ni foam and tested

corresponding electrochemical performances in 1 M KOH solution, as seen in Fig. S15 in the Supporting Information. Compared with that of NiP/wood, the NiP/Ni foam had a higher overpotential of 236 mV at a current density of 10 mA cm
2 and a larger Tafel slope of 178.5 mV dec
1, indicating a weak HER performance. The electrocatalytic active surface area of the sample from CV curves was estimated to only 5.3 mF cm
2 (Figs. S16 and S17, Supporting Information). The chronoamperometry test for the NiP/Ni foam showed that the current density fluctuated heavily, indicative of a poor long-term stability. The SEM images of the sample after a long-term test displayed that part of NiP alloy came off from Ni foam (Figs. S18a and S18b, Supporting Information), and the mapping images (Fig. S18c, Supporting Information) and EDS analysis (Figs. S19 and S20, Supporting Information) also testified the point by qualitative or quantitative analyses. Therefore, it is concluded that the natural multi-channeled wood, as an electrocatalyst carrier for HER, has the unusual combination ability with inorganic metals and tremendously enhances the kinetic process during HER. Notably, the channeled wood presented in this study is abundant on land and easy to biodegrade and recycle. This is of great significance for future applications in practice. 4. Conclusions In summary, an amorphous NiP alloy was deposited in wood channels via a simple yet effective electroless plating method. The wood framework with numerous aligned micro-channels can facilitate electrolyte permeation and hydrogen bubble release. These unique wood channels played a key role during HER in alkaline solution. As a result, the freestanding NiP/Poplar wood showed the high electrocatalytic activity toward hydrogen evolution, in comparison with that of NiP/Faxinus mandshurica wood and NiP/Larch wood, respectively. The NiP/Poplar wood possessed a small Tafel slope of 73.2 mV dec
1 and a low overpotential of

Please cite this article as: B. Hui et al., Natural multi-channeled wood frameworks for electrocatalytic hydrogen evolution, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135274

B. Hui et al. / Electrochimica Acta xxx (xxxx) xxx

83 mV at a current density of 10 mA cm
2. As a novel HER electrocatalyst, the NiP/Poplar wood displayed a superior stability even at a high current density of ~1200 mA cm
2. These outstanding HER performances are decided by the unique channeled structures of Poplar wood and the as-formed stable interface of NiP/wood. This work provides a reliable way to improve the interfacial stability and HER performance of electrocatalysts in alkaline medium by using a multi-channeled wood instead of nickel, copper, or iron foams.

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Declaration of competing interest The authors declare no conflict of interest. Acknowledgements All authors commented on the final manuscript. This work was supported by the Postdoctoral Science Foundation of China (No. 2018M632626), the Shandong Provincial Natural Science Foundation (No. ZR2019BC007, ZR2017BEM045), the National Natural Science Foundation of China (No. 51973099), and the Taishan Scholar Program of Shandong Province.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135274.

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Please cite this article as: B. Hui et al., Natural multi-channeled wood frameworks for electrocatalytic hydrogen evolution, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135274