Increasing Gas Bubble Escape Rate for Water Splitting with

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Increasing Gas Bubble Escape Rate for Water Splitting with Non-woven Stainless Steel Fabrics Ling Wang, Xiaolei Huang, Songshan Jiang, Meng Li, Kai Zhang, Ying Yan, Huiping Zhang, and Jun Min Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12895 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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ACS Applied Materials & Interfaces

Increasing Gas Bubble Escape Rate for Water Splitting with Nonwoven Stainless Steel Fabrics Ling Wang†, Xiaolei Huang†*, Songshan Jiang‡, Meng Li&, Kai Zhang§, Ying Yan‡, Huiping Zhang‡, and Jun Min Xue†* †

Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, 9 Engineer-

ing Drive 1, Singapore 117576, Singapore. ‡

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China.

&

School of Power Engineering, Chongqing University, Chongqing 400044, PR China.

§

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049,

PR

China.

ABSTRACT: Water electrolysis has been considered as one of the most efficient approaches to produce renewable energy. While, efficient removal of gas bubbles during the process is still challenging, which is proved to be critical and can further promote electrocatalytic water splitting. Herein, a novel strategy is developed to increase gas bubble escape rate for water splitting, by using NWSSF (nonwoven stainless steel fabrics) as conductive substrate decorated with flake-like FeNi LDH nanostructures. The as-prepared FeNi LDH@NWSSF electrode shows a much faster escape rate of gas bubbles as compared to other commonly used 3D porous catalytic electrodes, and the maximum dragging force for a bubble releasing between NWSSF channels is only one seventh of the dragging force within NF channels. As a result, it exhibits excellent electrocatalytic performance for both OER and HER, with low overpotentials of 210 and 110 mV at the current density of 10 mA cm-2 in 1 M KOH for OER and HER, respectively. There is almost no current drop after long-time durability test. In addition, its performance for full water splitting is superior to those previously reported catalysts, with a voltage of 1.56 V at current density of 10 mA cm-2.

KEYWORDS: Water splitting; Non-woven stainless steel fabric; Ni-Fe LDH; Low bubble dragging force; Efficient bubble release

1. INTRODUCTION Development of efficient catalytic electrode to reduce the overpotential for water splitting has been pursued for more than three decades.1-3 In general, a catalytic electrode for water splitting consists of active catalytic materials and a conductive substrate on which the active catalytic materials are deposited. Currently, most of the research efforts have been focused on designing and synthesizing new catalytic materials by increasing their intrinsic activity of each active site or the number of active sites, thus enhancing the efficiency and selectivity for hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) involved.4 Little attention has been paid on the design of the conductive substrate, which actually is very important for the water splitting performance,5, 6 in particular, when the research work is transferred to real industry use. For an efficient conductive substrate for water splitting, large specific surface area, good conductivity and excellent chemical/electrical/mechanical stability are the most needed characteristics.7-10 In general, 3-dimensional (3D) porous structures are commonly adopted for substrate design because they can offer expanded contact area between catalysts and electrolyte than that of planar substrate. Moreover, the porous structure could reduce ionic diffusion path, facilitate ionic transport to the interior parts of the substrate, and hence im-

prove the electrochemical surface area (ECSA).11, 12 Despite that the 3D porous substrates offer favorable electrochemical surface area, the concern of gas bubble releasing behavior appears to arise due to their complex internal pore structures.13, 14 During the water splitting process, numerous gas bubbles will be generated and get trapped into the porous structures. If not released promptly, these trapped gas bubbles will inhibit effective contact between electrolyte and substrates. This will lead to the generation of “dead corners” at the substrates, resulting in sudden ohmic drop and impeded mass transfer, and as a consequence, the catalytic performance of the catalytic electrode will be largely affected.4, 15 Therefore, driving off the as-formed gas bubbles promptly from the electrodes would be an important requirement in the design of 3D porous substrates for water splitting. Herein, a new type of 3D porous substrate, nonwoven stainless steel fabrics (NWSSF), is fabricated for water splitting with increased gas bubble escape rate. In this fabric-like substrate structure, stainless steel fibers are bonded together through heat treatment, as so called “nonwoven” process, making sure of the interconnected pore channels of the substrate. The perimeter of pore channels and fiber diameter are fully optimized in order to reduce the gas bubble dragging force and thus facilitate the escape of the gas bubbles from the fabrics. Each stainless steel fiber is decorated with flake-like

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iron nickel layered double hydroxides (FeNi LDH), which act as active catalysts for water splitting. These FeNi LDH nanostructures can also promote the removal of small bubbles efficiently from the substrate surface, via lowering the contact area between the bubbles and the substrate surface. This new 3D porous catalytic electrode (FeNi LDH@NWSSF) showed a much faster escape rate of gas bubbles as compared to other commonly used 3D porous catalytic electrodes. As a resultt, FeNi LDH@NWSSF exhibited remarkable catalytic activities for both OER and HER, which are reflected in its low overpotentials of 210 mV (OER) and 110 mV (HER) at 10 mA cm-2 and long term stability. It was also demonstrated that FeNi LDH@NWSSF possessed outstanding activity for full cell water splitting, with an overpotential of 1.56 V to drive a current density of 10 mA cm-2. 2. EXPERIMENTAL SECTION 2.1 Fabrication of non-woven stainless steel fabrics (NWSSF). Non-woven stainless steel fabrics were fabricated by wet lay-up papermaking process and followed by a heat treatment process. First, cellulose and stainless steel fibers (23 mm in length and 6.5 µm in diameter) with a mass ratio of 1:6 were added into 3 L of water and stirred vigorously to form a uniform suspension. A paper-like precursor was then formed by filtering the suspension via wet lay-up process, pressing at 300 kPa for 1 h and drying at 110 °C for 12 h. After that, the paper-like precursor was heat treated at 1050 °C for 30 min under N2 atmosphere to remove the cellulose and form a NWSSF substrate with 3-dimensional network structure, and the thickness of the substrate was about 1-2 mm. The other two different mass ratios of cellulose and stainless steel fibers were also prepared for comparison, named as NWSSFC1 with the mass ratio of 0.5:6.5, and NWSSF-C2 with the mass ratio of 2:5. 2.2 Preparation of iron nickel layered double hydroxides on NWSSF (FeNi LDH@NWSSF). Nanostructured FeNi LDH@NWSSF was synthesized through a urea-assisted hydrothermal method on NWSSF. In a typical procedure, Fe(NO3)3·9H2O (19 mg), Ni(NO3)2·6H2O (274 mg), urea (99 mg) were dissolved in 50 mL of deionized water and stirred to form a clear solution. The aqueous solution and SNWSSF (placed vertically) were transferred to a 100 mL Teflon-lined stainless steel autoclave, which was sealed and maintained at 150 °C for 8 h. The as-prepared FeNi LDH@NWSSF was obtained by rinsing with deionized water, ethanol and ovendried. The weight of FeNi double hydroxides was measured by weighing the NWSSF before and after hydrothermal process, a loading amount of 0.875 mg cm-2 was achieved. For comparison, the FeNi LDH loaded onto nickel foam (NF) and stainless steel foam (SSF) were prepared following the same procedure with the same Fe/Ni ratio, and they were named as FeNi LDH@NF and FeNi LDH@SSF, respectively. The FeNi LDH@NWSSF-C1 and FeNi LDH@NWSSF-C2 were also prepared. FeNi layered double hydroxides powder was collected from the bottom of autoclave, washed and dried for characterization use. 2.3 Characterization. Microscopic investigation was taken using a Zeiss Supra 40 field-emission scanning electron microscopy. Chemical compositions of the electrocatalysts were checked by X-ray photoelectron spectroscopy (XPS) using an Axis Ultra DLD X-ray photoelectron spectrophotometer

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equipped with an Al K X-ray source (1486.69 eV). Analysis of crystal phase was conducted by powder X-ray diffraction (XRD), using Cu Kradiation (40 kV, 40 mA). Conductivity test was carried out on FLUKE 45 dual display multimeter. 2.4 Pore network modelling. A 3D model of the pore network of NWSSF was established by X-ray CT (Computed Tomography) scanner Xradia 520 Versa, the sample was approximately 2 mm in diameter and 2 mm in height. The network formed by interconnected pores was then digitalized at a given resolution. 2.5 Electrochemical Measurement. All the electrochemical performance measurements were conducted via a VMP3 electrochemical workstation (Bio-logic Inc) and carried out at room temperature (25 ±1 °C) using 1 M KOH as the electrolyte. The FeNi LDH@NWSSF, FeNi LDH@NF and FeNi LDH@SSF could be used directly as working electrodes (1 × 1 cm). For electrocatalytic HER and OER measurements, Hg/HgO (1 M KOH) was used as the reference electrode. Graphite plate and Pt were used as the counter electrodes for HER and OER, respectively. The scan rate for linear sweep voltammetry (LSV) was kept at 5 mV s-1. Chronopotentiometry was carried out under a current density of 10 mA cm-2 for 18 h. All potentials had been calibrated with respect to the reversible hydrogen electrode (RHE) using equation: Evs.RHE = Evs.Hg/HgO + 0.059 × pH + 0.098, where Evs.Hg/HgO was the potential measured against the Hg/HgO reference electrode. The potentials were corrected by 85% iR-compensation. For overall water splitting measurements, the FeNi LDH@NWSSF were used as electrodes for both HER and OER, and LSV was measured from 1 to 2 V. The electrochemical double-layer capacitance (Cdl) of different electrodes were measured to estimate effective electrochemical surface area (ECSA) via cyclic vlotammograms scan in no Faradic potential range from -0.2 to -0.1 V vs. Hg/HgO at different scan rate from 10 to 100 mV s-1. The current density at -0.15 V vs. Hg/HgO was plotted against scan rate, and the slope of linear fit curve is the Cdl. Bubble evolution process was recorded with the assistance of Photron FASTCAM Mini AX100 operated at 10000 fps (frames per second) in 896 × 488 resolution. 3. RESULTS AND DISCUSSION 3.1 Bubble Release Behavior in 3D porous structures In general, the gas bubble release process during water splitting can be divided into two steps. In the first step, small bubbles are preferentially generated at the active catalyst sites of the substrate surface. The removal rate of these small bubbles from the substrate surface is mainly determined by its surface property. In general, hydrophilic surfaces are useful for fast removal of small gas bubbles.1 Besides, it was reported that surface modification is an efficient approach for bubble removal by lowering the contact area between the gas bubbles and electrode surfaces, such as constructing nano-needles, nano-flake, or nano-cones.15-18 Once the small bubbles are removed from the substrate surface, in the second step, they will merge into large bubbles and get trapped into the porous structure during their transportation from the substrate surface into electrolyte. The escape rate of these large bubbles from the porous substrate is very critical for the catalytic performance of the electrode.19-21 The longer these gas bubbles get trapped in the porous structure, the larger the catalytic performance is affected. In principle, these large gas bubbles, which are driven by the buoyancy force, will move upwards to escape from


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the porous structure spontaneously. However, the escape of these large gas bubbles will be largely jeopardized by the dragging force imposed from the pore structures. Thus, how to reduce the dragging force and thus increasing the gas bubble escape rate become important for the design of the porous substrate for water splitting. To the best of our knowledge, so far there is no clear understanding of the correlation between the gas bubble dragging force and pore structure. Therefore, it has been a big challenge to design a 3D porous substrate with optimum gas bubble escape rate. In this work, for the first time, it is proposed that such dragging force is mainly determined by the perimeter of pore channel and fiber diameter. This provides a new guidance for 3D porous substrate design for water splitting.

Figure 1. (a-c) Schematic demonstration of oxygen/hydrogen gas bubbles’ releasing process from a ring-shaped pore channel. There are minimum and maximum force between gas bubbles and fiber at stage 1 and stage 2, respectively. The bubble escape resistance is zero at stage 3 when θ > 90°. (d-f) Cross-section view of (a-c). A schematic demonstration of oxygen/hydrogen gas bubble releasing behavior from the pore channels of a porous structure was illustrated in Figure 1, by taking a ring-shaped pore channel as an example. As shown in Figure 1a and 1d, when a gas bubble just gets contact with the ring fiber, there is no dragging force from the pore channel. When the gas bubble is further moving upwards and passing through the channel (Figure 1 (b) and (e)), a dragging force is imposed from the ring fiber which is derived from the surface tension of the electrolyte. The maximum dragging force (Stage 2 in Figure 1b) could be estimated as follows: !

2 2 ∗ "

# (1)

$! % !&

Where γelectrolyte is the surface tension of electrolyte, L is the pore channel diameter on the electrode, and R is radius of fibers on the electrode, C is the circumference of pore channel (π ). More calculation details are available in Supporting Information. Therefore, according to equation (1), the maximum dragging force that a gas bubble should overcome is proportional to perimeter of the pore channel and the fiber diameter of the porous substrate.

3.2 Fabrication and Structure of the FeNi LDH@NWSSF Electrode Based on the bubble releasing behavior from the pore channels as discussed above, an optimized 3D porous substrate, nonwoven stainless steel fabrics (NWSSF), was fabricated. Flake-like iron nickel layered double hydroxides (FeNi LDH) nanostructure were grown onto NWSSF to act as active catalysts, as well as to promote the removal of small bubbles form the electrode surfaces. 22 A typical fabrication procedure of FeNi LDH@NWSSF was schematically shown in Figure 2a together with the scanning electron microscope (SEM), X-ray CT (Computed Tomography) characterizations of the growth of FeNi double hydroxides and the porous structure (Figure 2b-e). As shown in Figure 2a, there were three steps in fabricating FeNi LDH@NWSSF: i) stainless steel fibers and the binder cellulose were mixed together to fabricate precursor via wet lay-up papermaking process; ii) calcination process to remove the binder and bond all fibers together to form a 3D interconnected pore structure; and iii) FeNi LDH were grown onto NWSSF substrates via hydrothermal method. The diameter of the used stainless steel fibers was 6.5 µm, which was the finest stainless steel fiber commercially available. The amount ratio between the cellulose and fibers were fully optimized to reach the smallest pore channel structure, under the restriction that all fibers should be bonded together well. The fiber diameter and pore channel perimeter were two critical parameters for gas bubble escape dragging force. Figure 2b showed the SEM image of the mixture between cellulose and fibers. After heat treatment, as shown in Figure 2c, all fibers were bonded together to form an interconnected porous structure, of which the perimeters of most pore channels were below 100 µm. As a comparison, the commercial nickel foam (NF) displayed a much larger pore channel perimeter of 300 to 600 µm. The statistics of the pore channel perimeter distributions for both NWSSF and NF were provided in Figure S1. The SEM images of NWSSF-C1 and NWSSF-C2 before and after calcination were available in Figure S2. To further characterize the interconnected pore structure of NWSSF, X-ray CT was performed with Xradia 520 Versa (Figure 2d, Movie S1 (Supporting Information)). The obtained CT results revealed the uniform pore structure of NWSSF with good connectivity, and this interconnected porous structure would turn out to be crucial to water electrolysis. Through a hydrothermal process, FeNi LDH was grown uniformly onto NWSSF surface (Figure 2e). The obtained 3D catalytic electrode (FeNi LDH@NWSSF) could serve for both HER and OER during water splitting, as illustrated in Figure 2f. Two controlled samples, FeNi LDH@NF (nickel form) and FeNi LDH@SSF (stainless steel foam) were also prepared, as shown in Figure S3. The electrical conductivity of the as-prepared FeNi LDH@NWSSF (27,000 S m-1) was comparable to FeNi LDH@NF (30,000 S m-1) and FeNi LDH@SSF (15,000 S m-1). The crystalline structure and chemical composition of FeNi LDH@NWSSF were further determined by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Figure 3a shows an XRD pattern of the product collected at the bottom of autoclave after hydrothermal process. The diffraction peaks at 2θ ≈ 13°, 19°, 33°, 38°, 52°, 59°, and 63° correspond to the (003), (006), (009), (015), (101), (110) and (113) planes, which could be indexed as a rhombohedral structure, and are consistent with those of well-known hydrotalcite-like LDH materials, indicating FeNi LDH was successfully synthesized.12, 23-26 The XPS results were calibrated by C 1s peak (284.6 eV), and the wide-scan XPS spectrum of FeNi LDH@NWSSF was shown in Figure S4. With the EDX result together in Figure S5, the catalytic electrode contained a range of elements Fe, Ni, Mo, Cr, N, P, C and O, which are consistent with that of reported 316L stainless steel.25, 27 The O 1s XPS spectra of FeNi LDH@NWSSF (Figure 3b) could be well fitted with two types of bonding peaks,



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the peak at lower binding energy of 530.5 eV was characteristic of M-O-M bonding and the other peak at the higher binding energy of 531.5 eV was assigned to M-OH bonding. Figure 3c shows the XPS core-level spectra of Fe 2p, which features with two peaks at about 712.3 eV (Fe 2p3/2) and 724.7 eV (Fe 2p1/2). The ratio of Fe 2p3/2 to Fe 2p1/2 is about 2:1, and the greater peak of Fe 2p3/2 than that of Fe 2p1/2 resulted from spin-orbit (j-j) coupling. The XPS spectrum of the Fe 2p could be reasonably well resolved to show

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the characteristic peaks of Fe3+ valence state in FeNi LDH@NWSSF. The energy separation between the envelope (712.3 eV) and the satellite (720.3 eV) was 8 eV, and this value is consistent with Fe3+ oxidation states and is different to the value of Fe2+ (5 eV).28-30 In the Ni 2p narrow scan XPS spectrum (Figure 3d), the peaks at 855.5 and 873.1 eV correspond to the Ni 2p3/2 and Ni 2p1/2 species, respectively, and both peaks could be fitted with satellite peaks located at about 6 eV

Figure 2. (a) Schematic diagram of the synthesis of FeNi LDH@NWSSF. (b) SEM image of NWSSF before calcination. (c) SEM of NWSSF after calcination. (d) X-ray CT result of NWSSF after calcination. (e) SEM of FeNi LDH@NWSSF. (f) Schematic drawing of FeNi LDH@NWSSF as bifunctional electrocatalysts for water splitting, serving as both HER and OER electrodes. higher than the main peaks. Based on the XRD and XPS data, FeNi LDH was successfully prepared on NWSSF. Electrochemical surface area (ECSA) of electrocatalysts plays an important role in their electrocatalytic performance, and the ECSA could be evaluated by electrochemical double-layer capacitance (Cdl).31 As shown in Figure S6, the FeNi LDH@NWSSF delivered an average Cdl of 1.75 mF cm-2, which was comparable to that of FeNi LDH@NF (1.51 mF cm-2) and FeNi LDH@SSF (1.32 mF cm-2). The ECSA of FeNi LDH@NWSSF-C1 and FeNi LDH@NWSSFC2 were available in Figure S7.

Figure 3. (a) XRD of FeNi LDH collected at the bottom of autoclave after hydrothermal process. XPS spectra of (b) O 1s (c) Fe 2p and (d) Ni 2p for FeNi LDH@NWSSF. 3.3 Electrocatalytic performance of FeNi LDH@NWSSF

The electrocatalytic activity of FeNi LDH@NWSSF towards OER was evaluated under alkaline media (1 M KOH) using a three-electrode system, in which the FeNi LDH@NWSSF was directly used as the working electrode. For comparison, the activity of FeNi LDH@NF and FeNi LDH@SSF were also studied. Polarization curves were measured from linear sweep voltammetry (LSV) with a scan rate of 5 mV s-1 (Figure 4a). It was clearly seen that FeNi LDH@NWSSF performed the best, showing higher current density at lower overpotential compared to other two electrodes. The oxidation peak at around 1.43 V was attributed to the conversion of Ni(II) to Ni(III), which were believed to be the active sites for OER in alkaline media.32-34 There was almost no onset overpotential for FeNi LDH@NWSSF electrode, and the overpotential was as low as 210 mV at the current density of 10 mA cm-2. Also, it presented a much higher current density with the increase in overpotentials, indicating its superior OER activity. For instance, the current density of FeNi LDH@NWSSF reached 100 mA cm-2 at 1.51 V vs. RHE, which was greater than that of FeNi LDH@NF (33 mA cm-2) and FeNi LDH@SSF (20 mA cm-2) at the same applied potential, even higher or comparable to other OER electrocatalysts (Table S1, Supporting Information). The corresponding Tafel slope of FeNi LDH@NWSSF (56 mV dec-1) was lower than FeNi LDH@NF (67 mV dec-1) and FeNi LDH@SSF (68 mV dec-1), demonstrating superior kinetic activity of FeNi LDH@NWSSF for OER (Figure 4b). Stability of the electrocatalysts was also a critical parameter since a practical water-splitting device should ideally last for several years. To access the stability of FeNi LDH@NWSSF for OER, chronopotentiometric test was conducted over 18 hours at a current density of 10 mA cm-2. As displayed in the Figure 4c, the voltage remained around 1.44 V vs. RHE at a current density of 10 mA cm2 . The OER polarization curves of pure NWSSF, NF and SSF substrates were available in Figure S8 (a). According the results, these substrates displayed poor activity for OER. The EIS spectra of FeNi LDH@NWSSF, FeNi LDH@NF and FeNi LDH@SSF was available in Figure S9, and FeNi LDH@NWSSF displayed the lowest charge transfer resistance (Rct) of 0.30 Ohm, compared


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to that of FeNi LDH@NF (0.65 Ohm) and FeNi LDH@SSF (1.07 Ohm).

of pore channel and the fibre diameter of the electrode. The pore perimeter distribution of NWSSF and NF were displayed in Figure 5i-j, respectively. As shown in the figures, 70% pores in NWSSF were 30-100 µm, while most pores in NF were within the range of 200-700 µm. The average pore perimeter of NWSSF was 59.5 µm, but the average pore perimeter of NF was about 444.5 µm as shown in Figure S1 c-d. In addition, the diameter of the stainless steel fibre was 6.5 µm, but the diameter of nickel foam fibre was about 50 m. Based on equation 1, the maximum dragging force (Fmax) for a bubble releasing inside FeNi LDH@NWSSF electrode was about one seventh as compared to that of FeNi LDH@NF electrode. More calculation details were available in Supporting Information.

Figure 4. (a) Polarization curves and (b) Tafel plots of FeNi LDH@NWSSF, FeNi LDH@NF and FeNi LDH@SSF for OER at a scan rate of 5 mV s-1. The inset of (a) represents the LSV curves in the potential range of 1.30 to 1.55 V vs. RHE. (c) Time dependence of catalytic current density during electrolysis for FeNi LDH@NWSSF at a current density of 10 mA cm-2. Besides the lower overpotential and Tafel slope for OER, FeNi LDH@NWSSF electrode also showed faster bubble escape rate than that of FeNi LDH@NF electrode (Movie S2-3), which was considered to be critical for improving the performance of electrodes, and could lead to a faster current increasing and more stable working states even under high overpotentials.15 Typical images of bubble evolution on FeNi LDH@NWSSF and FeNi LDH@NF electrodes with high-speed, microscale visualization were shown in Figure 5(a-h). Figure 5a showed that the gas bubbles were hung at the edge of FeNi LDH@NWSSF electrode at one moment, indicating its superaerophobic surface property. In aqueous condition, the superaerophobic surface is able to trap a continuous water film thus leading to low adhesive force of between gas bubbles and electrode surface, facilitating the fast removal of small bubbles form the electrode surface.35, 36 What’s more, FeNi LDH@NWSSF catalytic electrode showed a very fast gas bubble escape rate from the pore channels. As shown in Figure 5 (b-d), the as-formed oxygen bubbles could be totally released from the porous structure within 0.025 s. As a comparison, FeNi LDH@NF electrode showed a much slower gas bubble escape process during the water splitting. As shown in Figure 5 e, the gas bubbles at FeNi LDH@NF electrode (FeNi LDH grown on commercial nickel foam) were adhered firmly onto the electrode surface, indicating its non-superaerophobic surface property. Accompanied with the growth and coalescence, some of the oxygen bubbles finally got trapped inside the pore structure. It took ~ 1s to release these bubbles from the pore structure, which was much slower than that of FeNi LDH@NWSSF electrode. The fast bubble release through FeNi LDH@NWSSF electrode was ascribed to its small pore perimeter and small fibre diameter. As shown in equation 1, the maximum dragging force that a bubble should overcome during escape was proportional to the perimeter

Figure 5. (a) Shapes of gas bubbles on the bottom of FeNi LDH@NWSSF (contact angle is about 169.7°, indicating its superaerophobicity). (b)-(d) In situ observations of oxygen evolution reaction on FeNi LDH@NWSSF. (e) Photograph of gas bubbles on FeNi LDH@NF. The generated oxygen bubbles were trapped onto the electrode. (f)-(h) In situ observations of oxygen evolution reaction on FeNi LDH@NF. The pore perimeter distribution of (i) FeNi LDH@NWSSF and (j) FeNi LDH@NF. Scale bars: 100 µm. The HER activities of FeNi LDH@NWSSF and other control samples were also investigated in 1 M KOH with a scan rate of 5 mV s-1. Figure 6a showed the LSV curves on the RHE scale. As shown in the figure, FeNi LDH@NWSSF presented a small onset overpotential of 65 mV versus the reversible hydrogen electrode (RHE) and a current density of 10 mA cm-2 at an overpotential of 110 mV, whereas the FeNi LDH@NF and FeNi LDH@SSF required about 165 and 290 mV of overpotential to reach the same current density. A further increase of overpotentials led to a rapid rise in current density (100 mA cm-2 at an overpotential of 260 mV). The HER performance of FeNi LDH@NWSSF were excellent compared to other reported HER electrocatalysts, such as FeNi layered double hydroxide (LDH)/nickel foam(NF),37 Cu3P/NF,21 carbon paper(CP)@NiP,38 FeP/reduced graphite oxide (rGO),39 etc. (Table S1) The reaction kinetics of the electrocatalysts were also evaluated by Tafel slope (Figure 6b). The measured Tafel slope of FeNi LDH@NWSSF was 113 mV dec-1, lower than that of FeNi LDH@NF (125 mV dec-1) and FeNi LDH@SSF (132 mV dec-1), indicating more rapid kinetic activity of FeNi LDH@NWSSF. The HER stability of FeNi LDH@NWSSF was also investigated, as displayed in Figure 6c, and the chronopotentiometric curve did not show any potential



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fluctuation at the current density of 10 mA cm-2. The HER polarization curves of pure NWSSF, NF and SSF substrates were available in Figure S8 (b), and they did not contribute too much to the performance. To further investigate the bifunctional performance of FeNi LDH@NWSSF for water splitting, a two-electrode configuration was assembled directly by applying the FeNi LDH@NWSSF as both anode and cathode in 1 M KOH. At a low potential of 1.56 V, this electrolyzer exhibited a current density of 10 mA cm-2 (Figure 6e), which was superior to those reported catalysts. (Table S1, Supporting Information). Figure 6d showed the combination of HER and OER polarization curves, and the potential difference between HER and OER at the current density of 10 mA cm-2 is 1.55 V. Besides, the FeNi LDH@NWSSF electrocatalyst presented excellent long-term stability for overall water splitting, the potential hardly increased within 18 hours test at a constant current density of 10 mA cm-2, as shown in Figure S10.

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compared to other commonly used 3D porous catalytic electrodes during the process of water splitting. With the small pore channels and thin stainless steel fibres, FeNi LDH@NWSSF electrode showed a much reduced dragging force between the gas bubbles and the electrode surface, which was around one seventh of the dragging force of the nickel foam. The catalytic activity of FeNi LDH@NWSSF could be dramatically promoted in comparison to other commercial 3D substrates. The obtained FeNi LDH@NWSSF exhibited low overpotential of 210 and 110 mV at current density of 10 mA cm-2 for OER and HER, respectively. In addition, its performance for full water splitting is superior to those of most reported catalysts, with a voltage of 1.56 V at current of 10 mA cm-2. This work opens up the possibility of designing efficient bubble escape systems for water splitting.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. Calculation of dragging force; SEM images of FeNi LDH on NF and SSF; Wide scan XPS of Ni-Fe LDH@PSSF; EDX of NWSSF; ECSA of FeNi LDH@NWSSF, FeNi LDH@NF and FeNi LDH@SSF; Comparison of electrocatalytic performances of reported electrocatalysts; CT results of NWSSF; Bubble release behavior within LDH@NWSSF and FeNi LDH@NF.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X. Huang). * E-mail: [email protected] (J. Xue). ORCID Ling Wang: 0000-0003-4018-1742 Meng Li: 0000-0003-0087-3082 Notes

The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Figure 6. (a) Polarization curves and (b) Tafel plots of FeNi LDH@NWSSF, FeNi LDH@NF and FeNi LDH@SSF for HER in 1M KOH at a scan rate of 5 mV s-1. The inset of (a) represents the LSV curves in the potential range of 0 to -0.30 V vs. RHE. (c) Time dependence of catalytic current density during electrolysis over FeNi LDH@NWSSF electrode at a current density of 10 mA cm-2 (d) Polarization curve recorded in a three-electrode configuration in a wide potential window (-0.5 V to 1.7 V versus RHE) showing the bifunctionality of FeNi LDH@NWSSF toward both HER and OER. The inset represents the combination of HER and OER polarization curves within the current density between -40 mA cm-2 and 40 mA cm-2. (e) Polarization curve recorded at a scan rate of 5 mV s-1 in a two-electrode configuration. 4. CONCLUSIONS In summary, we have fabricated a new type of NWSSF (nonwoven stainless steel fabrics) conductive substrate loaded with flake-like FeNi LDH nanostructure for water splitting in alkaline media. It was found that the as-prepared FeNi LDH@NWSSF electrode showed a much faster escape rate of gas bubbles as

ACKNOWLEDGMENT We gratefully acknowledge the financial support provided by Singapore MOE Tier 1 Funding R-284-000-162-114, Singapore NRF–CRP16–2015–01, National University of Singapore Strategic Fund R261509001646 & R261509001733, National Key Research and Development Program of China (Grant No. 2016YFA0400900), the National Natural Science Foundation of China (Grant No. 11535015, U1632110).

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Increasing Gas Bubble Escape Rate for Water Splitting with

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