Constructing Bifunctional 3D Holey and Ultrathin CoP Nanosheets for

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Energy, Environmental, and Catalysis Applications

Constructing Bifunctional 3D Holey and Ultrathin CoP Nanosheets for Efficient Overall Water Splitting Yanliu Dang, Junkai He, Tianli Wu, Linping Yu, Peter Kerns, Liaoyong Wen, Jing Ouyang, and Steven L. Suib ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08238 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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ACS Applied Materials & Interfaces

Constructing Bifunctional 3D Holey and Ultrathin CoP Nanosheets for Efficient Overall Water Splitting Yanliu Dang, † Junkai He, † Tianli Wu, § Linping Yu, ‡ Peter Kerns, ‡ Liaoyong Wen, † Jing Ouyang, ‡ and Steven L. Suib† ‡* †Institute of Materials Science, U-3136, University of Connecticut, Storrs, Connecticut 06269, United States ‡Department of Chemistry, U-3060, University of Connecticut, Storrs, Connecticut 06269, United States §

Henan Key Laboratory of Photovoltaic Materials and School of Physics & Electronics, Henan

University, Kaifeng 475004, P.R. China KEYWORDS: electrocatalysis, water splitting, all pH values, 3D holey CoP nanosheets, potentiodynamic deposition

ABSTRACT: Pursuing cost-effective water splitting catalysts is still a significant scientific challenge to produce renewable fuels and chemicals from various renewable feedstocks. Construction of controllable binder-free nanostructures with self-standing holey and ultrathin nanosheets is one of the promising approaches. Herein, by employing a combination of

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potentiodynamic mode of electrodeposition and low-temperature phosphidation, 3D holey CoP ultrathin nanosheets are fabricated on carbon cloth (PD-CoP UNSs/CC) as bifunctional catalysts. Electrochemical tests show that the PD-CoP UNSs/CC exhibits outstanding hydrogen evolution reaction performance at all pH values with overpotentials of 47, 90, and 51 mV to approach 10 mA cm-2 in acidic, neutral, and alkaline media, respectively. Meanwhile, only a low overpotential of 268 mV is required to drive 20 mA cm-2 for the oxygen evolution reaction in alkaline media. Cyclic voltammetry and impedance studies suggest the enhanced performance is mainly attributed to the unique 3D holey ultrathin nanosheets, which could increase the electrochemically active area, facilitate the release of gas bubbles from electrode surfaces, and improve effective electrolyte diffusion. This work suggests an efficient path to design and fabricate non-noble bifunctional electrocatalysts for water splitting at a large-scale.

1. Introduction Strong dependence and overuse of fossil fuels are intensifying air pollution and global warming.1 Water electrolysis makes it possible to supply clean energy through the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) via a sustainable and clean route.2–4 Due to the high cost and scarcity of Pt and Ru/Ir based compounds, earth-abundant electrocatalysts are becoming significant for HER and OER.5–7 In addition, considering that HER is favorable in acidic media while OER is advantageous in alkaline condition, it is still challenging to develop bifunctional catalysts in the same electrolyte with high efficiency for both HER and OER. Recently, transition-metal phosphides (TMPs) with good electrical conductivity have demonstrated potential as cost-effective water splitting catalysts.8–17 Among these TMPs, CoP based electrodes showed prospective as bifunctional catalysts.18 Though many binder-free


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electrodes have been reported and great progress has been achieved,19–24 a few bottlenecks about current catalysts nanostructure still exist. For example, single array possessed large thickness or diameter exposes limited active sites and hinds the contact between reactants and catalysts.25 Meanwhile, thick coatings and aggregation of electrocatalysts on electrode surface would block channels for mass and charge transfer, and decrease the number of active sites.26 Thus, rational structure design of nanoscale arrays is preferable to pursue comparable or even better electrocatalytic performances to that obtained from commercialized Pt and Ru/Ir based compounds. Ultrathin27 and holey28 nanostructures are appealing due to the high surface area and enhanced surface superhydrophilicity which makes the active sites fully accessible to reactants and facilitates the electrocatalytic performance. In this regard, integrating high-quality ultrathin and holey nanostructures on the substrate with outstanding HER and OER activities simultaneously will be very significant. Electrodeposition is a more simple and effective method for preparing tunable nanostructures on conductive substrates compared to the hydrothermal method.29 Herein, we report 3D holey CoP ultrathin nanosheets on carbon cloth (PD-CoP UNSs/CC, PD: potentiodynamic deposition) as a bifunctional electrocatalyst for HER at all pH values and OER in alkaline media. The nanoscale thickness and height (650 nm) of the CoP nanosheets with plentiful nanoholes not only facilitate electron transport and electrolyte penetration, but also improve the number of accessible active sites. Meanwhile, the intact 3D nanosheets grown on CC successfully prevent agglomeration and form ideal channels for gas release and electrolyte diffusion, ensuring the stability of catalysts. 2. Results 2.1 Material Preparation and Characterization


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The PD-CoP UNSs/CC was fabricated through a two-step process which was schematically shown in Scheme 1. The Co(OH)2 nanosheet was deposited onto the CC substrate by the potentiodynamic mode of electrodeposition. Then PD-CoP UNSs/CC was derived from the precursor by the low-temperature phosphidation process (Experimental section). X-ray diffraction (XRD) was conducted to understand the crystal structure of obtained phosphided product. Due to the small loading amount of phosphided product on CC(1.6 mg/cm-2), the X-ray diffraction pattern in Figure 1a is not intense but still shows that orthorhombic CoP (JCPDS No. 29-0497) is obtained after phosphidation. In Figure 1a, the XRD patterns located at 26.19°and 43.92°are attributed to the carbon cloth substrate, while the broad diffraction peaks at 31.6°, 36.3°, 48.1°, and 56.7° are assigned to the (011), (111), (211), and (020) facets of the orthorhombic CoP. The broad diffraction pattern indicates a small particle size of the obtained materials, which was further confirmed by transmission electron microscopy (TEM). The scanning electron microscopy (SEM) image of PD-Co(OH)2/CC shows that carbon fibers are entirely covered with uniform and ultrathin hydroxide nanosheets (Figures 1b-d). After phosphidation, the obtained PD-CoP UNSs/CC perfectly maintains the morphology with vertically grown ultrathin nanosheets on the CC and 650 nm in height (Figures 1e-g). These nanosheets form lots of open space between each other and become coarse and porous due to CoP nanoparticle formation during the phosphidation process. As a comparison, the HT-CoP NSs/CC (HT: hydrothermal reaction) was prepared by direct phosphidation of the hydrothermally synthesized precursor, morphology and structure details are shown in Figure S1. SEM images of the obtained HT-CoP NSs/CC indicates that the HT-CoP nanosheets are thick and dispersed randomly on the surface of CC due to the unmanageable hydrothermal reaction (Figure S1).29 The hydrothermal reaction takes place under high pressure and temperature to


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decompose urea and precipitate cobalt, which usually leads to excessive deposition on the substrate and forms a large dimensionality nanostructure as displayed in Figure S1. While the electrodeposition is carried out at room temperature and is controlled by set values of scan rate and number of cycles, the nanostructures of the deposited layer could be optimized and applied in electrochemical catalysis. In this work, cobalt hydroxide was first deposited on CC by decomposing nitrate when scanning in a negative direction.30 Ultrathin nanosheets are formed as shown in Figures 1b-d when the scan rate is set at 100 mV s-1. Therefore, the potentiodynamic deposition exhibits better control over the morphology of nanostructures and further affects the catalytic performance.

Scheme 1. Scheme of the synthesis of PD-CoP UNSs/CC.


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Figure 1. (a) XRD patterns of PD-CoP UNSs/CC. (b-d) SEM images of PD-Co(OH)2/CC. (e-g) SEM images of PD-CoP UNSs/CC.


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Transmission electron microscopy (TEM) was used for the structural and morphological studies of the as-prepared catalysts. Figure 2a displays an intact 3D nanostructure of the obtained PDCoP with a thick base (20 nm) and ultrathin porous nanosheets. To give more detail of the nanosheets, Figure 2b presents an enlarged TEM image. The ultrathin nanosheets are composed of interconnected nanoparticles, which is consistent with the broad X-ray diffraction pattern. As the cobalt hydroxide precursor possesses ultrathin nanosheets and undergoes shrinkage during the phosphidation process, these interconnected nanoparticles and open holes between particles are formed and are exposed to the external space. The high-resolution TEM (HR-TEM) image of the nanosheets (Figure 2c), demonstrates the lattice fringes of 0.283 and 0.254 nm which correspond to the (011) and (111) planes of the orthorhombic CoP, respectively. The selected area electron diffraction (SAED) pattern (Figure 2c, inset) shows a set of concentric rings, which can be indexed to the (011), (111), (211) and (301) facets of orthorhombic CoP, indicating the polycrystalline structure of obtained PD-CoP. Energy-dispersive X-ray (EDX) images of PDCoP nanosheets (Figure 2d) reveal a homogeneous distribution of Co and P with an atomic ratio of 1:1.01 (Figure S2) and the complete formation of CoP.


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Figure 2. (a, b) STEM images of CoP nanosheets. (c) HRTEM image and SAED pattern of PDCoP nanosheets. (d) HAADF-STEM image and selective area EDX elemental mappings of P and Co for CoP nanosheets. (e, f) XPS spectra of the PD-CoP UNSs/CC: Co 2p and P 2p.


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X-ray photoelectron spectroscopy (XPS) was utilized to characterize the surface chemistry of PD-CoP UUNSs/CC. Figures 2e and f present the high resolution XPS spectra of the Co 2p and P 2p core levels. In Figure 2e, the Co 2p3/2 region displays two main peaks at 778.3 eV and 782.1 eV and one satellite peak at 786.7 eV. In addition, the Co 2p1/2 region shows two main peaks at 793.2 eV and 798.1 eV and one satellite peak at 803.1 eV. Peaks located at 778.3 and 793.2 eV belong to the binding energies of Co species in the phosphide and peaks at 782.1 and 798.1eV correspond to oxidized Co species because of surface oxidation.31,32 The satellite peaks located at 786.7 and 803.1 eV can be ascribed to the shakeup excitation of the high-spin Co 2+ ions.33 For P 2p (Figure 2f), two peaks at 129.2 and 130.0 eV are assigned to the P 2p3/2 and P 2p1/2 in CoP, respectively, and the broad peak at 134.1 eV is related to phosphate or phosphite (POx or P-O species) due to surface oxidation.21 The binding energy of Co (778.3 eV) displays a positive shift from that of metal Co (778.1 eV), and P (129.2 eV) exhibits a negative shift from that of elemental P (130.2 eV), indicating the Co in CoP has a partial positive charge (δ+) while the P has a partial negative charge (δ-).34,35 These data imply transfer of electron density from Co to P, which is consistent with the reported charge separation of Co-P bonds due to charge transfer from Co to P.36,37 The XPS results reveal that CoP was obtained after phosphidation. Brunauer-Emmett-Teller (BET) measurements were done to determine the specific surface area and the distribution of pore sizes. However, the nanostructure grew on the substrate directly and the loading amount was low, and specific surface area information could not be obtained by BET methods. Then an electrochemical method was used to evaluate the electrochemically active surface areas (ECSA) which also correspondsto the specific surface area.28 The ECSA for PDCoP UNSs/CC and HT-CoP NSs/CC were obtained by measuring the capacitances of their double layers in a three-electrode setup.29 Figures 3a and b show the cyclic voltammetry (CV)


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results of PD-CoP UNSs/CC and HT-CoP NSs/CC electrodes in the region of 0.1-0.2 V vs. RHE at various scan rates in 0.5 M H2SO4. The double layer capacitances were calculated from the CVs by linear fitting of the current density vs. scan rate plot.12,38 As a result (Figure 3c), PD-CoP UNSs/CC has a capacitance of 63.0 mF cm-2, which is 4 times that of HT-CoP NSs/CC (15.6 mF cm-2). These results suggest PD-CoP UNSs/CC has many more active sites than HT-CoP NSs/CC. In addition to the improved active sites, the ability to release gas bubbles from the electrode surface further affects the electrocatalytic activities of gas evolution reactions.39–42 The gas bubbles trapped in the catalyst interfere with the mass and charge transfer during the electrochemical reactions and cause the ohmic resistance to increase over time.39 Figure 3d reveals the electrochemical impedance changes of PD-CoP UNSs/CC and HT-CoP NSs/CC with time of measurement. PD-CoP UNSs/CC shows a very straight curve with negligible changes, indicating the gas bubbles can be easily removed from the surface without increasing the impedance. The HT-CoP NSs/CC electrode produces a rising curve which is positively correlated with time, implying that some of the generated gas bubbles were trapped in the catalyst due to the thick nanosheets.


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Figure 3. Cyclic voltammograms for (a) PD-CoP UNSs/CC and (b) HT-CoP NSs/CC in the region of 0.1-0.2 V vs. RHE at various scan rates (20-160 mV s-1). (c) The capacitive current densities at 0.15 V vs. RHE as a function of scan rate for PD-CoP UNSs/CC, and HT-CoP NSs/CC. (d) Plots of impedance vs. time for PD-CoP UNSs/CC and HT-CoP NSs/CC electrodes. 2.2 Electrocatalytic Performances of HER and OER The electrocatalytic HER activity of PD-CoP UNSs/CC (loading mass: 1.6 mg cm-2) was first evaluated in 0.5 M H2SO4 using a three-electrode system. Resistance correction and RHE calibration (Figure S3) are utilized to exclude the effect of solution resistance and calculation errors of the reference electrode. For comparison, bare CC, HT-CoP NSs/CC (loading mass: 1.6 mg cm-2) and platinum plates were also examined. As shown in Figure 4a, the PD-CoP


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UNSs/CC requires a much lower overpotential of 47 mV to reach 10 mA cm-2 compared to bare CC (> 300 mV) and HT-CoP NSs/CC (80 mV). The electrocatalytic activity of PD-CoP UNSs/CC is obviously superior to the reported pure CoP catalysts in Table S1 and comparable to the reported doped CoP and other TMPs in Table S2. Furthermore, intrinsic electrochemical activities of catalysts were compared by the ECSA-normalized and mass-normalized polarization curves of the PD-CoP UNSs/CC and the HT-CoP NSs/CC. As presented in Figure S4, the PDCoP UNSs/CC displays slightly higher HER activity even when corrected by ECSA. To drive the current density of 10 mA mg-1, the PD-CoP UNSs/CC only needs 76 mV comparable to reported materials such as U-CoP/Ti (86 mV),43 CoP/Ti (92 mV)44 and HT-CoP NSs/CC (114 mV). To better understand the kinetic and intrinsic properties of PD-CoP UNSs/CC, Tafel slopes and exchange current density (j0) are calculated. As shown in Figures 4b and c, Pt shows a Tafel slope of 30 mV dec-1 and a j0 of 1.32 mA cm-2, which are consistent with the reported values.5,45 The PD-CoP UNSs/CC exhibits a lower Tafel slope of 46 mV dec-1 compared to HT-CoP NSs/CC (60 mV dec-1), implying faster charger transfer kinetics and a Volmer-Heyrovsky mechanism for the HER process. The highest j0 of 0.99 mA cm-2 (Figure 4c) was observed for PD-CoP UNSs/CC compared to other reported electrocatalysts listed in Table S3, indicating the highly intrinsic properties of electrode reactions. Nyquist plots (Figure 4d) show that PD-CoP UNSs/CC has a small polarization resistance of 6.7 Ω, which is much lower than that of HT-CoP NSs/CC. It indicates that the 3D holey nanostructure could enhance charge transfer, facilitating the kinetics of HER. To further evaluate the stability of PD-CoP UNSs/CC electrodes, the cyclic voltammetry (CV, Figure 4e) for 1000 cycles and galvanostatic electrolysis (Figure 4f) were carried out. The polarization curve shows little degradation after 1000 CV scans. PD-CoP UNSs/CC maintains the potential curve after 17 h, indicating remarkable stability.


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The electrocatalytic performances of PDx-CoP/CCs (x: scan rate) obtained by the potentiodynamic deposition at different scan rates and other loaded masses were studied. As shown in Figures S5a and b, PD-CoP UNSs/CC namely PD100-CoP/CC obtained at 100 mV s-1 shows the highest activity with much higher ECSA (Figure S5c), which is advantageous to improve electrocatalyst performance.46,47 Meanwhile, other loading masses of 0.3 and 1.0 mg cm-2 at the same scan rate show inferior catalytic activity (Figure S5d). When the loading mass is as high as 3.8 mg cm-2, the activity remains the same as the loading mass of 1.6 mg cm-2, which means the Heyrovsky step is the rate-determining step, and hydrogen desorption becomes the main issue for the HER process.


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Figure 4. (a) Polarization curves for bare CC, Pt, PD-CoP UNSs/CC, and HT-CoP NSs/CC in 0.5 M H2SO4. (b) Tafel plots for Pt, PD-CoP UNSs/CC, and HT-CoP NSs/CC. (c) Calculated exchange current density of the PD-CoP UNSs/CC, HT-CoP NSs/CC and Pt by applying extrapolation method to the Tafel plot. (d) Nyquist plots of PD-CoP UNSs/CC and HT-CoP


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NSs/CC at an overpotential of 50 mV. (e) and (f) Cycling stability and chronopotentiometric curve 10 mA cm-2 for PD-CoP UNSs/CC.

In addition to the excellent HER performance in an acidic solution, PD-CoP UNSs/CC could work as HER electrocatalysts at all pH values. As shown in Figure 5a, the polarization curve of PD-CoP UNSs/CC exhibits a close HER catalytic activity to state-of-the-art catalysts of Pt in 1.0 M KOH, with a low overpotential of 51 mV to drive 10 mA cm-2, which is 30 mV less than HTCoP NSs/CC. In addition, the PD-CoP UNSs/CC displayed a small Tafel slope of 53 mV dec-1 close to that of Pt of 51 mV dec-1 (Figure 5b). The HER performance of PD-CoP UNSs/CC compares favorably to its precursor PD-Co(OH)2/CC (140 mV) and the oxide of PD-Co3O4/CC (281 mV) in Figure 5c, suggesting the highly intrinsic activity of phosphides. The chronopotentiometric curve as well as the polarization curves of PD-CoP UNSs/CC in Figure 5d suggest the long-time durability of the catalyst in 1.0 M KOH for HER. Compared to acidic and alkaline media, a neutral solution is more environmentally friendly for water splitting. Our measurements show that PD-CoP UNSs/CC can approach 10 mA cm-2 at the low overpotential of 90 mV, with a Tafel slope of 101 mV dec-1 (Figure S6), which delivers the highest activity among the reported pure CoP (Table S1) for HER in neutral environments. Moreover, PD-CoP UNSs/CC shows no apparent degradation after continuous CV for 1000 cycles. Figures S7 and S8 show the post HER analysis of PD-CoP UNSs/CC at all pH values, where the PD-CoP UNSs/CC electrode maintains a similar structure and composition of as-obtained electrode. Therefore, the PD-CoP UNSs/CC demonstrates high-efficiency and stability at all pH values and a promising potential for large-scale practical application.


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Figure 5. (a) Polarization curves for bare CC, Pt, PD-CoP UNSs/CC, and HT-CoP NSs/CC 1.0 M KOH. (b) Tafel plots for Pt, PD-CoP UNSs/CC, and HT-CoP NSs/CC. (c) Polarization curves for PD-Co3O4/CC, PD-Co(OH)2/CC, and PD-CoP UNSs/CC. (d) Cycling stability and Chronopotentiometric curve at 10 mA cm-2 for PD-CoP UNSs/CC.

The outstanding HER performances of PD-CoP UNSs/CC at all pH values encourage us to apply the electrode to the more sluggish OER process in 1.0 M KOH. As shown in Figure 6a, bare CC shows poor catalytic activity for OER. PD-CoP UNSs/CC serves as the most active electrode. This material only requires an overpotential of 268 mV to drive 20 mA cm-2. This overpotential is much lower than that for IrO2 (320 mV), HT-CoP NSs/CC (306 mV) and the corresponding hydroxide precursor PD-Co(OH)2/CC (306 mV) and oxide PD-Co3O4/CC (329 mV). PD-CoP


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UNSs/CC performs the outstanding activity for OER activity among the reported pure CoP electrocatalysts (Table S1). Besides the excellent OER performance, PD-CoP UNSs/CC electrode also exhibits long-term stability for OER with only a slight shift after 1000 cycles and a negligible potential variation for 18 h (Figure 6b). The morphology of PD-CoP UNSs/CC was well preserved (Figure S9). In addition, a cell voltage of 1.56 V is needed to drive 10 mA cm-2 (Figure S10), when this material works as both electrodes in a two-electrode system. The cell voltage of PD-CoP UNSs/CC is superior to that of the reported electrocatalysts (Table S4). OER and water splitting performances of PD-CoP UNSs/CC electrodes under neutral conditions are also evaluated and shown in Figure S11.

Figure 6. (a) Polarization curves of bare CC, IrO2, PD-CoP UNSs/CC, HT-CoP NSs/CC, PDCo3O4/CC, and PD-Co(OH)2/CC for OER 1.0 M KOH. (b) Cycling stability and Chronopotentiometric curve at 10 mA cm-2 for PD-CoP UNSs/CC at 10 mA cm-2.

3. Discussion All the catalytic results above show outstanding performance of PD-CoP UNSs/CC for HER at all pH values and OER in alkaline media. The high electrocatalytic activities are closely


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associated with the electronic structure, active surface area, gas release ability, and electrolyte diffusion in these catalysts. The Tafel slopes in both acid and alkaline media indicate a Volmer-Heyrovsky mechanism for the HER process starting with the discharge step (Volmer reaction: H+(aq) + e- →Hads) to form adsorbed hydrogen atom on the active site of catalysts. This is followed by the electrochemical desorption step (Heyrovsky reaction: Hads +H+(aq) + e- → H2 (g)) to produce hydrogen. The XPS results in Figures 2e and f reveal a partial positive charge (δ+) of Co and a partial negative charge (δ-) of P in CoP, suggesting a similar catalytic mechanism as hydrogenases for enhancing HER activity.23 In the HER process, the positively charged Co (ẟ+) could serve as a hydride-acceptor in acid media, or water-acceptor in base and neutral conditions, while the negatively charged P (ẟ-) is a proton-acceptor.

28,48

The P (ẟ-, proton-acceptor) sites could facilitate the formation of

intermediates (Co-hydride) for the following hydrogen evolution when a negative potential is applied.49 Compared with metal oxides and hydroxides, the slightly charged P (ẟ-) favors hydrogen desorption and improves HER activity.28 As shown in Figure 5c, the obtained CoP exhibits much better electrochemical performance than Co(OH)2 and Co3O4. In the OER process, the Co (ẟ+) centers act as hydroxyl-acceptors, and P (ẟ-) facilitates the adsorption of hydroxyl groups on Co centers and subsequent oxygen evolution via desorption.50 Cobalt oxy/hydroxide layers are in-situ formed on the surface of CoP prior to the process of OER and act as the active sites.44,51 As shown in Figure 6a, PD-Co(OH)2 exhibits higher OER activity than HT-CoP, suggesting the ultrathin cobalt hydroxide nanosheets contribute a lot to the excellent OER performance. In this work, PD-CoP is easy to oxidize to highly active cobalt oxyhydroxide due to the ultrathin nanostructure. The 3D ultrathin nanostructure combines with vertical alignment and large open spacing and active oxyhydroxide layers on the surface to make the PD-CoP


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UNSs/CC an excellent OER catalyst. The intrinsic electronic properties of CoP lead to promising electrochemical catalysts for water splitting. The double-layer capacitances are measured to estimate ECSA, as the electrocatalytic activity of electrodes is always considered relative to the active surface area.46,47 The CVs in Figures 3a-c show that the self-standing CoP electrode obtained from potentiodynamic deposition has larger specific surface area than the hydrothermal method. The SEM images of PD-CoP UNSs/CC and HT-CoP NSs/CC (Figures 1e-g, Figure S1) suggest the abundant active sites stem mainly from the unique nanostructure of PD-CoP UNSs/CC. Compared to the thick and random dispersed sheets of HT-CoP NSs/CC, the self-supporting 3D skeleton and ultrathin nanosheets with open space help PD-CoP UNSs/CC to avoid inactivity and expose more active sites for the electrochemical reaction.28 The efficiencies of removal of gas bubbles and electrolyte diffusion are also important to the electrode’s activity.38 The confinement of produced gas bubbles on the surface of catalysts would block the active sites and hinder the efficient contact between catalyst and electrolyte.52 Beside rotating the electrode, another effective way to remove produced bubbles is enhancing the surface

superhydrophilicity

and

superaerophobicity

through

the

nanostructure

of

electrocatalysts.40–42 In this paper, the design of 3D holy nanostructure is expected to solve the gas bubble blocking phenomenon. The impedance vs. time plots (Figure 3d) confirm that the PDCoP UNSs/CC electrode maintains resistance during the gas evolution process, while the resistance increases for the HT-CoP NSs/CC electrode due to the blocking of active sites. The TEM (Figure 2) and SEM (Figures 1e-g) images of PD-CoP UNSs exhibit nanoholes among interconnected nanoparticles and open spaces between adjacent holey nanosheets that form natural channels, which effectively enhance the diffusion of electrolyte into all accessible


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catalysts, promote release of evolved gases from electrode surfaces, and facilitate ion and charge transport without blocking the active sites.40–42 The efficient removal of gas bubbles also benefits the maintenance of structure and morphology and long-term stability of electrodes. The post morphological and structural characterization in Figures S7-9, suggest that the PD-CoP UNSs/CC electrode maintains a similar structure and composition of the as-obtained electrode which contribute to the high efficiency and excellent stability for water splitting.

4. Conclusion In summary, CoP nanosheets integrated on CC are constructed from potentiodynamic electrodeposition Co(OH)2/CC precursor. Such PD-CoP UNSs/CC electrode shows 3D holey ultrathin nanostructures and exhibits outstanding catalytic performance and stability for HER and OER. When used as a cathode for HER, the PD-CoP UNSs/CC only needs a low overpotential of 47 mV to achieve 10 mA cm-2 with a small Tafel slope of 46 mV dec-1 in 0.5 M H2SO4. Furthermore, the PD-CoP UNSs/CC also displays excellent HER activity in neutral and alkaline media. A low overpotential of 268 mV is needed to approach 20 mA cm-2 for OER in alkaline media. The high efficiency and stability of PD-CoP UNSs/CC originate from synergistic effects: greatly increasing the electrochemically active area, facilitating the release of gas bubbles from electrode surfaces, and improving effective electrolyte diffusion.40–42 This work not only provides an efficient path for fabricating binder-free HER electrodes, which are prospects for renewable energy but also offers new insights into the nanostructure to better design earthabundant electrocatalysts with high efficiency and stability. 5. Experimental Section


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Materials: Co(NO3)2·6H2O, NaH2PO2, KOH, H2SO4, ethanol and acetone, Nafion solution (5 wt. %) were obtained from Sigma-Aldrich. All the chemicals are reagent-grade and used as received. Preparation of PD-Co(OH)2/CC: PD-Co(OH)2/CC was prepared as follow. In a typical synthesis, carbon cloth (CC) was cleaned by sonication sequentially in water, ethanol, and acetone for 10 min each and immersed in 0.1 M Co(NO3)2 solution for electrodeposition. Electrodeposition was carried out by potentiodynamic deposition (PD) in a conventional three-electrode system by a CHI 760E electrochemical analyzer (CH Instruments, Inc.). The carbon cloth (1 cm × 3 cm) was used as the working electrode, saturated calomel electrode (SCE) as the reference electrode and graphite rod as the counter electrode. The PD process was performed in the potential window 0 to -1.2 V vs. SCE at 100 mV s-1 scan rate with 40 cycles. After the deposition, the coated carbon cloth PD-Co(OH)2/CC rinsed first with water, ethanol and then dried at in vacuum oven at 60 °C for 2 h. Preparation of PD-Co3O4/CC: PD-Co3O4/CC was prepared by heating as-obtained PDCo(OH)2/CC at 300 °C for 120 min in static air. Preparation of PD-CoP UNSs/CC: In a typical synthesis, a piece of PD-Co(OH)2/CC and 50 mg NaH2PO2 (molar ratio: ~1:10) were placed at two porcelain boats with NaH2PO2 at the upstream side of the furnace. Subsequently, the sample was heated at 300 °C for 120 min in Argon atmosphere, and then cooled to ambient temperature under Argon. Different scan rates for electrodeposition were also investigated: 5, 20, 50, 200, 300 mV s-1, which are corresponding to PD5-CoP/CC, PD20-CoP/CC, PD50-CoP/CC, PD200-CoP/CC, PD300-CoP/CC. The loading amount of CoP onto CC is 1.6 mg cm-2, which is determined by the mass increase of pristine CC.


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Other loading masses 0.3, 1.0, 3.8 mg cm-2 were obtained using 10, 20, and 80 cycles under the same scan rate 100 mV s-1, respectively. Preparation of HT-CoP NSs/CC: HT-CoP NSs/CC was prepared with the same phosphidation process by converting HT-Co(OH)2/CC which was synthesized by hydrothermal process.53 Loading amount of PD-CoP and HT-CoP on CC were 1.6 mg/cm2. Characterization: The Powder X-ray diffraction (XRD) analysis was carried out on a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ = 1.5406 A). A beam voltage of 40 kV and a current of 44 mA were used. The morphologies of the materials were investigated using an FEI Nova SEM 450 with an accelerating voltage of 2.0 kV. Detailed information of the surface morphology, size, composition, and structure was performed on transmission electron microscopy (TEM) of Talos F200X instrument with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed in a PHI Model 590 spectrometer with multiprobes (Al Kα radiation, λ = 1486.6 eV, operated at 250 W). The energy calibration was referenced the C 1s peak at 284.6 eV during analysis. Electrochemical Measurements: Electrochemical measurements were performed using a CHI 760E electrochemical workstation using a three-electrode cell. Coated carbon cloth worked as the working electrode and a Hg/HgSO4, K2SO4(sat) reference electrode for measurements in 0.5M H2SO4, Saturated calomel electrode (SCE) for measurement in 1.0 M PBS and Hg/HgO, KOH (1.0 M) for measurements in 1.0 M KOH. All reference electrodes were calibrated against a reversible hydrogen electrode using Pt wire as the working and counter electrode in high purity hydrogen saturated electrolyte solution. The average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions


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(Supplementary Figure S5). Note that only graphite rod was used as the counter electrode for all the electrochemical measurements since Pt may dissolve in electrolytes and contaminate the working electrode. IrO2 ink was typically made by dispersing 5 mg of catalyst into 0.96 mL of water/ethanol (V/V=1:1) solvent. After adding 40 μL of 5 wt. % of Nafion solution and sonicated for at least 30 min, 300 μL of this ink was loaded onto a carbon cloth to achieve the sample loading 1.6 mg cm-2. The HER and OER activity of the catalysts was evaluated using linear sweep voltammetry (LSV) at the scan rate of 5 mV s-1 after 20 cycles of cyclic voltammetry (CV). The overpotential required to deliver current density of 10 mA cm-2 (η10) is chosen as the primary parameter for HER comparison, 20 mA cm-2 (η20) for OER comparison. Though the onset η is a good indicator of the intrinsic activity, it was not used here for comparison because of the ambiguity in determining its value. The electrode durability was tested by both CV and galvanostatic measurements. For the estimation of ECAS, a specific capacitance (Cs) value Cs = 0.040 mF cm-2 in 0.5 M H2SO4 is adopted from previous reports. The ECAS is calculated using the following equation: ECAS =CDL/Cs. CV was carried out ranging from -0.2 to 0.1 V vs. RHE for 1000 cycles at a constant scanning rate of 100 mV s-1. Moreover, polarization curves before and after the testing were collected for comparison. Electrochemical impedance spectroscopy (EIS) was carried out at potentiostatic mode with the frequency range from 0.1 to 100 kHz. Note all the LSV tests were iR compensated. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website.


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SEM images of HT-CoP NSs/CC, EDX spectrum, RHE calibrations of reference electrodes, ESCA-normalized and mass-normalized polarization curves, electrochemical characterizations of PD-CoP electrodes prepared at different conditions, post SEM and XRD characterizations of the PD-CoP UNSs/CC catalyst; table of performance comparison Table S1-S3 (PDF). AUTHOR INFORMATION Corresponding Author E-mail: [email protected] ORCID Steven L. Suib: 0000-0003-3073-311X Notes The authors declare no competing financial interests. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The TEM studies were performed using the facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA). The authors thank Dr. Frank Galasso for helpful discussions. We thank the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Biological and Geological Sciences under grant DEFG02-86ER13622.A000 for support of this research.


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ToC figure


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Constructing Bifunctional 3D Holey and Ultrathin CoP Nanosheets for

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