Nickel Foam as an Efficient Bifunctional

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Plasma Hydrogenated TiO2/Nickel Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting Yong Yan, Xing Cheng, Wanwan Zhang, Ge Chen, Hongyi Li, Alexander Konkin, Zaicheng Sun, Shaorui Sun, Dong Wang, and Peter Schaaf ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04496 • Publication Date (Web): 22 Nov 2018 Downloaded from http://pubs.acs.org on November 25, 2018

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ACS Sustainable Chemistry & Engineering

Plasma Hydrogenated TiO2/Nickel Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting Yong Yan,a Xing Cheng,a Wanwan Zhang,a Ge Chen,a*Hongyi Li,d Alexander Konkin,c Zaicheng, Sun,a Shaorui Sun,a* Dong Wang,b* Peter Schaafb [a] Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental & Energy Engineering, Beijing University of Technology, Pingle yuan 100, 100124 Beijing, P. R. China Email: [email protected] and [email protected] [b] Chair Materials for Electronics, Institute of Materials Engineering and Institute of Micro- and Nanotechnologies MarcoNano®, TU Ilmenau, Gustav-Kirchhoff-Str. 5, 98693 Ilmenau, Germany Email: [email protected] [c] Center for Micro- and Nanotechnologies MacroNano®, TU Ilmenau, GustavKirchhoff-Str. 7, 98693 Ilmenau, Germany [d] Key Laboratory of Advanced Functional Materials, School of Materials and Engineering, Beijing University of Technology, Pingle yuan 100, 100124 Beijing, P. R. China

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ABSTRACT Electrochemical water splitting is one of the most efficient technologies for hydrogen production, and fabrication of low-cost, robust, and high active electrocatalysts to replace the noble metal-based materials is one of the key issues. By using H2 plasma treatment, the TiO2/nickel foam composite is converted to be an efficient bifunctional electrocatalysts towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in the alkaline electrolyte. The investigation reveals the existence of abundant oxygen vacancies of TiO2, which might lead to the dramatic improvement of the electrical conductivity and faster charge transfer rate; also, density function theory (DFT) calculations suggest the oxygen vacancies activate surrounding surface lattice oxygen to induce the favorable reactive-intermediate adsorption energy of TiO2 for hydrogen evolution; and adjusts the strength of the chemical bonds between the TiO2 surface and reactive-intermediates to more favorable values, inducing the lower energy barrier for oxygen evolution. The finding confers a unique function to TiO2 that is different from its widely accepted role as an electrocatalytically inert semiconductor material, suggesting the H2 plasma treated TiO2/nickel foam could be bifunctional electrocatalyst for overall water splitting.

KEYWORDS Titanium dioxides, Plasma treatment, Defects, Electrocatalysis, Water splitting


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INTRODUCTION Electrochemical water splitting, composed of the two half reactions of the hydrogen (H2) and oxygen (O2) evolution reactions (HER and OER), is widely considered an attractive technology, since it provides an effective route to the production of renewable energy.1-3 Generally, platinum (Pt) is believed to be the most active and stable catalysts for the HER; and iridium (Ir) or ruthenium oxide (RuOx) is regarded as the most active and stable catalysts for the OER. However, the high cost and scarcity of these noble metals hinder the scale-up deployment.4-7 Thus, many research efforts have been devoted to the search for the non-noble metal catalysts.7 In recent years, transition metal (Fe, Co, Ni, Mo)-based materials, such as oxides, sulfides, carbides, phosphides, perovskites, binary alloys, or ternary composites, have been identified as promising HER and OER catalysts under alkaline conditions.3, 8-16 Nevertheless, these materials often need complex preparation procedures, and some catalysts containing medium cost elements (Co, Mo, etc).17 Owing to the advantages of low-cost, nontoxicity, chemical stability, and unique photonic and electrical properties, titanium dioxides (TiO2) has been widely used in many areas such as photovoltaic, secondary batteries, and sensors;18 unfortunately, TiO2 is treated as an electrocatalytically inert material, because of its low electrical conductivity and poor surface reactivity. Hence, in the view of practical application, converting this typical low-cost and easily synthesized semiconductor material into HER and OER catalysts is highly favorable, but a great challenge.



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Recently, it has been shown that oxygen vacancies may induce a substantial change in the electronic structure of metal oxides, which might strongly impact on the adsorption energy of the reactive-intermediates of catalytic reactions on their surface.19 Through a plasma-engraving strategy, the oxygen vacancies can be created on Co3O4 surface with more Co2+ formed; and the proper control of Co2+/Co3+ ratio in Co3O4 could lead to modifications on its electronic and thus catalytic properties.20 Creating oxygen vacancies on pyramidal nanofacets of cobalt (II) oxide nanorods can effectively tune its atomic structure, leading to the superior catalytic activity and durability towards oxygen reduction/evolution reactions.21 And the chemical and electronic property of NiO nanorods is successfully optimized through oxygen vacancy engineering; the oxygen vacancies on the surface of NiO NRs remarkably enhance

their

electronic

conduction

and

promote

HER

reaction

kinetics

simultaneously.22 Interestingly, nano-structured TiO2 exposed with the (001) facets, self-doped by oxygen vacancies, exhibits a competitive oxygen reduction activity and excellent durability.23 Besides, this material also exhibits a high catalytic activity for electrochemical reduction of nitrobenzene.24 More importantly, the rutile TiO2 single crystals can be activated toward the HER in alkaline media through a thermal reduction under vacuum conditions, oxygen vacancies and low coordinated Ti ions is believed to impact the HER activity of the reduced TiO2.25 In our previous work, we have demonstrated the H2 plasma treated TiO2 possessed a large amount of oxygen vacancies, and exhibited improved interface lithium storage, photocatalytic activity of CO2 reduction, and photothermal therapy performance.26-28 Besides, some recent 4 ACS Paragon Plus Environment

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studies indicated that plasma treatment is one of the efficient technologies in the enhancement of the activity of electrocatalysts,29-34 and the strategy about creating the defects or dopants in the surface and subsurface of electrocatalyts is highly favorable for many different electrochemical reactions.35-38 Thus, it’s reasonable to assume that the H2 plasma treated TiO2 will modulate the surface atomic and electronic structure, and enhance the electrochemical performance of TiO2 towards HER and OER. In this study, TiO2 is coated on nickel foam (Ni foam) by using a modified sol-gel method, named TiO2/Ni foam; subsequently, the TiO2/Ni foam is treated by H2 plasma, named H2 plasma (HP)-TiO2/Ni foam. Compared with the TiO2/Ni foam, the HP-TiO2/Ni foam display much enhanced activities toward both HER and OER in the alkaline media, suggesting it could be bifunctional electrocatalyst for overall water splitting. The result also suggest the plasma engrave could be an efficient pathway to tune the electronic structure of metal oxides, leading to the enhancement of its catalytic activity.

EXPERIMENTAL SECTION Materials and synthesis: Titanium (IV) butoxide (TBOT) and Pluronic@F-127 (P2443) were purchased from Sigma-Aldrich Co., and used as Ti source and soft template, respectively. 3.4 g of TBOT, 2.5 g of F-127, and 2 mL of HCl (37%) were added into in 10.3 mL ethanol with high-speed stirring (1600 r/min) at room temperature for 2 h, and a clear Ti source sol was obtained. The sheets of Ni foam (Changsha Liyuan Newmaterial Co. Porosity: 95%) with size of 2 × 5 cm were 5 ACS Paragon Plus Environment


washed by 1.0 M HCl aqueous solution, acetone, and isopropanol under ultrasonication for 15 min, respectively, and then were dried with N2 gas. To prepare the TiO2/Ni foam, 0.5 mL Ti source sol was drop casted on the sheet of Ni foam, and then transferred into quartz crucible and heated at 500 °C in box furnace for 3 h (heating rate 2 °C min-1) with air-blown. The TiO2/Ni foam was obtained. To prepare the HP-TiO2/Ni foam, the TiO2/Ni foam (loading mass of 1.22 ~ 1.54 mg cm-2) was transferred into a chamber for high-power density H2 plasma treatment, and there an instrument of inductively coupled plasma (Plasmalab 100 ICP-CVD, Oxford Instruments) was used. The H2 plasma treatment was performed at 300 °C for 30 min. The ICP power was 3000 W, the chamber pressure was 25.8 ~ 27.1 mTorr, and the H2 flow rate was 50 sccm. After this treatment, HP-TiO2/Ni foam (loading mass of 1.16 ~ 1.41 mg cm-2) was obtained. TiO2 thin film for the electric resistivity measurement was deposited using a PicoSun SUNALETM R-150 atomic layer deposition (ALD) system (PicoSun, Finland) according to the following procedure. The reaction temperature was 90°C, and titanium tetrachloride (TiCl4) and H2O were chosen as the precursors of Ti and O, respectively. TiCl4 was pulsed for 0.1 s and purged for 10 s, followed by a 0.1 s pulse and 10 s purge of H2O. This procedure was repeated for 400 times according to the growth rate of ~0.5 Å per cycle, and the amorphous TiO2 thin film with the thickness of 20 nm was obtained. After annealing at 500 °C in box furnace for 3 h (heating rate 2 °C∙min-1) with air-blown, the crystallized TiO2-p thin film (anatase) was prepared. Further, theHP-TiO2 thin film was obtained after H2 plasma treatment. 6 ACS Paragon Plus Environment

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Characterization: X-ray diffraction (XRD) pattern of the samples were recorded on a SIEMENS/BRUKER D5000 X-ray diffractometer using Cu-Kα radiation at 40 kV and 40 mA, with the samples being scanned from 2θ =10° - 80° at a rate of 0.02 °s-1. Scanning electron microscopy (SEM) images were taken with a Hitachi S-4800 instrument. Transmission electron microscopy (TEM) images were taken using a Tecnai F20 microscope on powder samples deposited onto a copper microgrid coated with carbon at an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) analysis was performed by a spectrometer (Kratos Axis Ultra XPS) with monochromatized Al-Kα radiation and an energy resolution of 0.48 eV. Cathodoluminescence (CL) spectra were acquired by using GatanMono3+ system with an accelerating voltage of 15 kV at T = 80 K. Electron paramagnetic resonance (EPR) spectra were recorded at 77K using a Bruker BioSpin CW X-band (9.5 GHz) spectrometer ELEXYS E500. Oxygen-temperature programmed desorption (O2-TPD) measurements were conducted in a U-shaped quartz reactor, and the desorption signal of oxygen was recorded with an online INFICON IPC400 quadrupole mass spectrometer. The electrical resistivity of TiO2 layer was measured by using the hall measurement station (Agilent). 500 nm Al contact pads with distance of 1.5 mm were firstly deposited on TiO2 and HP-TiO2 thin film by electron beam evaporation (a mask was used to obtain the contact pads). TiO2 layer without H2 plasma treatment was not measurable due to the excess resistivity; while the HP-TiO2 layer could be well measured because of the highly enhanced electrical conductivity.



Electrochemical measurements: The electrocatalytic activity of the TiO2/Ni foam and HP-TiO2/Ni foam for hydrogen evolution (HER) and oxygen evolution (OER) reaction were evaluated in a three-electrode configuration connected to an electrochemical workstation (VMP3, Bio Logics, France). Ni foams were investigated as control sample under the similar conditions. TiO2/Ni foam and HP-TiO2/Ni foam electrodes were used as the working electrodes, a saturated silver chloride (Ag/AgCl/KCl (sat’d)) electrode as the reference electrode, and a graphite rod as the counter electrode, 1.0 M KOH as electrolyte. For the conversion of the obtained potentials (vs. Ag/AgCl/KCl (sat’d)) to a reversible hydrogen electrode (RHE), a value of (0.197 + 0.059pH) V was added. Firstly, cyclic voltammetry (CV) measurement was performed ten cycles in the voltage range of 0.1 ~ 0.5 V vs RHE at a scan rate of 10 mV s-1 as electrode activation process. Subsequently, linear sweep voltammetry (LSV) at a scan rate of 2 mV s-1 and chronopotentiometric (CP) analysis (potential-time characteristics) were recorded under the steady current density of 10 mA cm-2. The method of calculation onset potential in our study is to draw tangents in non-faradaic zone (horizontal baseline) and faradaic zone (rising current line) in LSV curve, abscissa of point where these two tangents intersect gives the onset potential value. The bi-functional reversibility of HP-TiO2/Ni foam electrode was investigated by the potential-time (E-t) trace at a constant current density alternating between +10 mA cm-2 and -10 mA cm-2 every 1800 s. Electrochemical impedance spectroscopy (EIS) analysis were taken using a 20 mV amplitude alternating current (AC) signal over a frequency range from 200 kHz to 10 mHz. The catalytic performance of 8 ACS Paragon Plus Environment

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TiO2/Ni foam or HP-TiO2/Ni foam electrodes for overall water splitting were performed using a two-electrode cell, in which TiO2/Ni foam or HP-TiO2/Ni foam were used as both working and counter electrode in the anode and cathode of cell, respectively. To assess the Faradic efficiency, the formed H2 and O2 were collected by a closed container, and their amounts were analyzed by gas chromatography (GC) from the CP measurements of HER and OER. And then calculate the theoretical amount of H2 and O2 with I-t curve by applying the Faraday law. Theoretical calculation: All the calculations were performed using first-principles density function theory (DFT) with the VASP package. The projected-augmented wave (PAW) method was applied to treat the ion-electron interactions. The exchangecorrelation energy of electrons was calculated in the generalized gradient approximation (GGA) scheme with the PBE functional parameterization. The energy cut-off was 400 eV, and the energy criterion of the self-consistent convergence was placed as at 0.0001 eV/atom. The TiO2-(101) surface is constructed with six layers and 2×2 two dimensional supercell, and the vacuum layer is more than 19.0 Å. The k point sampling in the first Brillouin zone is set as 3×3×1. The ratio of Ti4+:Ti3+ is determined as 3:1 from the XPS results, and then one sixth oxygen atoms are removed from TiO2 to construct the structure of modified HP-TiO2. More details about the HER and OER calculation are described in the Supporting Information.



RESULTS AND DISCUSSION The scanning electron microscopy (SEM) images of HP-TiO2/Ni foam demonstrated that a TiO2 layer with a thickness of 1.1 ± 0.3 μm uniformly covered the skeleton of Ni foam with the formation of a conformal coating; and the magnified image shows that the TiO2 layer was composed of numerous tiny nanoparticles (Figure S1 and S2). X-ray diffraction (XRD) patterns of HP-TiO2 exhibited the typical characteristic peaks of the anatase phase (Figure 1a). The irregular-shaped nanoparticles aggregated with each other are also observed in the transmission electron microscopy (TEM) images of HP-TiO2, and their size distribution is in the range of 9 to 16 nm (Figure 1b and d). The high resolution (HR)-TEM images show that HP-TiO2 is well crystallized; particularly, there is no obvious disordered layer on the surface of nanocrystals (Figure 1c and e), which is different from the other literatures about reduced/hydrogenated TiO2 samples.39-41 The explanation might be caused by the different titanium precursors and plasma hydrogenated treatment.42 X-ray photoelectron spectroscopy (XPS) analysis is used to investigate the surface species of the both HP-TiO2/Ni foam and TiO2/Ni foam (Figure S3). The Ti 2p core level spectrum of TiO2/Ni foam shows two primary peaks at ~458.9 and ~464.7 eV, corresponding to the Ti 2p3/2 and Ti 2p1/2 peaks of the Ti4+ species (upper panel in Figure 2a).40,

43, 44

For HP-TiO2/Ni foam, these Ti4+ peaks negatively shift to ~458.3

and ~464.5 eV; in addition, two extra shoulder peaks at ~456.2 and ~461.8 eV are also observed, which can be assigned to the Ti 2p3/2 and Ti 2p1/2 peaks of the Ti3+ species (lower panel in Figure 2a).44,

45

Peak-fitting of the spectrum reveals the


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content of the Ti3+ species on the surface of HP-TiO2 layer reaches to a level of 27.3% (Figure S4). The results indicate that a large amount of oxygen vacancies are formed in the HP-TiO2 sample.44, 46 It should be noted that no Ni species were observed in both HP-TiO2/Ni foam and TiO2/Ni foam samples, indicating the Ni foam were fully covered by TiO2 layer. (a)

XRD TiO2

Intensity

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


HP-TiO2

10

20

30

40

50

60

70

80

2 theta /degree

Figure 1. (a) XRD patterns, TEM, and high resolution (HR)-TEM images of (b, c) TiO2 and (d, e) HP-TiO2.

Electron paramagnetic resonance (EPR) is used to further investigate the defects in the TiO2. As indicated in Figure 2b, no obvious signal was observed for pristine TiO2, revealing the few defects in its crystal lattice. In contrast, HP-TiO2 yielded two 11 ACS Paragon Plus Environment


signals: the strong feature with an average g value (gav) of ~1.935 can be ascribed to the presence of a large amount of Ti3+ centers in the surface or subsurface lattice sites;47, 48 the shoulder peak at a g-value of ~2.003 correspond to the O species (O2- or O-) derived from the interaction between adsorbed O2 (from air) and top layer Ti3+ sites.44,

49

Besides, O2-temperature programmed desorption (TPD) analysis

demonstrates that HP-TiO2 displays the enhanced desorption peaks in the ranges of 50 - 165 and 200 - 275 °C, which can be assigned to the removal of oxygen adsorbed species (O2- and O-) located at surface defects, further manifesting its increased surface and subsurface Ti3+ species (Figure 2c).50 To obtain insight into the defect states of HP-TiO2 in terms of the electronic structure, the light emission properties of the samples are studied by cathodoluminescence (CL) spectroscopy (Figure 2d). HPTiO2 shows a broader band with much higher intensity compared with pristine TiO2, signifying the formation of more and various intra-gap states accompanying the abundant oxygen vacancies as electron donors (Figure S5).51 It is believed that inducing a high concentration of Ti3+ defects could effectively reduce the resistivity of the TiO2 materials. Hence, the electrical conductivity of TiO2 thin films (20 nm, grown on the quartz glass substrate) before and after H2 plasma treatment were measured using the standard four-point probe method. The HP-TiO2 thin film exhibits a surprisingly high conductivity with the value of 72.9 S cm-1 (Figure S6a); whereas the conductivity of the TiO2 is not even measurable due to the limited range (≤ 10-8 S∙cm-1). This means that the resistivity of HP-TiO2 decreases by at least ~10 orders of magnitude in comparison with that of TiO2 owing to the presence of numerous defects. 12 ACS Paragon Plus Environment

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Figure 2. (a) XPS Ti 2p core-level spectra of the TiO2/Ni foam (upper panel) and HPTiO2/Ni foam (lower panel). (b) EPR spectra of TiO2 and HP-TiO2 recorded at 77 K. (c) Comparison of the chemical O2 adsorption properties of TiO2 and HP-TiO2 by TPD (upper panel), and the difference spectrum obtained by subtracting the spectrum of the TiO2 from that of the HP-TiO2 (lower panel). (d) CL spectra of TiO2 and HPTiO2. The electrocatalytic performance of HP-TiO2/Ni foam for the HER was investigated in 1.0 M KOH solution. For comparison, the TiO2/Ni foam and pure Ni foam was tested under similar conditions. Figure 3a shows the linear sweep voltammetry (LSV) curves of these samples. HP-TiO2/Ni foam exhibited a small onset potential (η) of ~30 mV; for achieving the current densities of 10, 20, and 50 mA cm-2, this electrode required overpotentials of 133, 186, and 279 mV, respectively. As a non-Pt catalyst, the activity of HP-TiO2/Ni foam is comparable to the commonly used molybdenum13 ACS Paragon Plus Environment


based sulfides, carbides, and transition metal dichalcogenides (lower panel of Figure S7and Table S1). In contrast, the catalytic performance of TiO2/Ni and pure Ni foam is poor, which both display larger onset potentials, and much lower current densities at the same overpotentials.

Figure 3. Electrocatalytic performance of HP-TiO2/Ni foam for the HER in 1.0 M KOH: (a) LSV curves of TiO2/Ni foam, HP-TiO2/Ni foam, and Ni foam. (b) Tafel plots and related data of TiO2/Ni foam, HP-TiO2/Ni foam, and Ni foam. (c) Nyquist plots of TiO2/Ni foam and HP-TiO2/Ni foam at an overpotential of 150 mV. (d) Chronopotentiometric (CP) measurement of the HER long-term stability of TiO2/Ni foam and HP-TiO2/Ni foam. The reaction kinetics of the HER for these catalysts were interpreted by the corresponding Tafel plots (Figure 3b). HP-TiO2/Ni foam exhibited a Tafel slope of ~122 mV dec-1; this value is smaller than that of pure Ni foam (135 mV dec-1) and TiO2/Ni foam (131 mV dec-1), and consistent with the recent reports on alkaline HER 14 ACS Paragon Plus Environment

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catalysis.52, 53 In addition, the exchange current densities (J0) of these samples were obtained from the intercept of the linear region of the Tafel plots. It is observed that J0 of the HP-TiO2/Ni foam electrode is up to 2038 μA cm-2 that is ~45 times greater than TiO2/Ni foam electrode, which explains the greatly enhanced HER activity. To further understand the differences of the surface reactivity of TiO2-p and -d, the relative ECSA of the TiO2 electrodes were estimated from the double layer capacitance (Cdl) by cyclic voltammetry (CV) measurements. As shown in Figure S6b, the Cdl of the TiO2/Ni foam and HP-TiO2/Ni foam electrodes is 0.188 and 1.809 mF∙cm-2, respectively; the much larger ECSA of HP-TiO2/Ni foam indicates its highly increased number of active sites for the redox process. Electrochemical impedance spectroscopy (EIS) analysis is performed to investigate the ionic and ohmic resistances of the TiO2 electrodes in HER catalysis (Figure 3c). Both TiO2/Ni foam and HP-TiO2/Ni foam show two semicircles in the Nyquist plots at an overpotential of 150 mV; the large semicircle in the high-frequency range can be attributed to the charge-transfer resistance (Rct), whilst the small semicircle in the low-frequency range is related to the impendence of the H* adsorption onto the catalysts (Rad).54, 55 For the HP-TiO2/Ni foam electrode, Rct and Rad are ~6.9 and ~3.6 Ω, respectively, which is lower than the values for TiO2/Ni foam (Rct ~11.2 Ω; Rad ~4.4Ω), revealing the improved charge transfer and H* adsorption energy of HPTiO2/Ni foam. The durability of the HP-TiO2/Ni foam catalyst is evaluated by chronopotentiometric (CP) measurements under the cathodic current density of 10 mA cm-2 (Figure 3d). A degradation from 133 to 183 mV appeared at the initial stage 15 ACS Paragon Plus Environment


(0-300 s) of this testing. Then, the overpotential of 183 mV remained almost unchanged for 7200 s, implying an excellent durability of HP-TiO2/Ni foam catalyst for HER. The CP curve of TiO2/Ni foam is also stable, while the overpotential is much larger, and reaches to 303 mV. The OER activity of TiO2/Ni foam and HP-TiO2/Ni foam are also assessed in 1.0 M KOH solution, using Ni foam as the control sample. Figure 4a shows the LSV curves of these samples. The HP-TiO2/Ni foam electrode could generate the anodic current density of 10, 20, and 50 mA cm-2 at the overpotentials of 309, 348, and 424 mV, respectively. This performance is comparable to many other non-noble metal OER catalysts (upper panel of Figure S7 and Table S2). The broad peak observed at 1.37 V might result from the annihilation of small amount of top layer Ti3+ defects on the surface of HP-TiO2 nanocrystals by electrochemical oxidation, which is reversible under cathodic currents (Figure S8). For TiO2/Ni foam and pure Ni foam, the catalytic activity is much worse, current densities of less than 7 mA cm−2 are obtained, although the overpotential is above 500 mV. Moreover, HP-TiO2/Ni foam possesses a lower Tafel slope and a larger J0 compared with those of TiO2/Ni foam and pure Ni foam (Figure 4b), implying faster reaction kinetics on its surface. Similar to the HER catalytic process, the Nyquist plots of the TiO2/Ni foam and HP-TiO2/Ni foam electrodes for OER exhibit two semicircles at an overpotential of 390 mV (Figure 4c), which correspond to the electrochemical reaction resistance derived from the chargetransfer (Rct, high-frequency semicircle) and reactive-intermediate (OH*, O*, and OOH*) adsorption (Rad, low-frequency semicircle), respectively.56 It is observed that 16 ACS Paragon Plus Environment

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HP-TiO2/Ni foam has a smaller Rct (~8.1 Ω) and Rad (~2.0 Ω) relative to TiO2/Ni foam (Rct ~10.1 Ω; Rad ~6.4 Ω), suggesting the improved electronic and ionic conductance in the electrolyte. CP measurement reveals that the overpotential of the HP-TiO2/Ni foam electrode is generally stable over a period of 7200 s, indicating its favorable durability for OER (Figure S9a); however, a minor degradation from 387 to 426 mV appear at the initial stage (0-400 s) of this testing. For comparison, the overpotential of the TiO2/Ni foam electrode was higher than 510 mV approaching the stable OER current density of 10 mA cm-2, and this value increased to 564 mV after 7200 s (Figure S9b). To further investigate the bi-functional reversibility of HP-TiO2/Ni foam, the positive and negative potentials were alternatively applied to this material, in which the current density was periodically switched between +10 and -10 mA cm-2 for the OER and HER, respectively (Figure 4d). The potential-time (E-t) trace demonstrates that the HP-TiO2/Ni foam electrode has an instant response for the interconversion between OER and HER catalysis. The potential gap between the OER and HER was firmly maintained at 1.81 V for 4 hours. Moreover, it is demonstrated that the faradaic efficiencies of both HER and OER on HP-TiO2/Ni foam electrodes are nearly ~100% with the 1:2 molar ratio of O2 and H2, indicating the current obtained from electrolysis are almost fully derived from the O2 and H2 evolutions, the side reaction is rare.



Figure 4. Electrocatalytic performance of HP-TiO2/Ni foam for the OER in 1.0 M KOH: (a) LSV curves of TiO2/Ni foam, HP-TiO2/Ni foam, and Ni foam. (b) Tafel plots and related data of TiO2/Ni foam, HP-TiO2/Ni foam, and Ni foam. (c) Nyquist plots of TiO2/Ni foam and HP-TiO2/Ni foam at an overpotential of 390 mV. Electrocatalytic performance of HP-TiO2/Ni foam as both HER and OER bifunctional catalyst for overall water splitting in 1.0 M KOH: (d) The potential-time (E-t) trace at a constant current density alternating between +10 mA cm-2 and -10 mA cm-2 every 1800 s. (e) LSV curves of TiO2/Ni foam and HP-TiO2/Ni foam. (f) CP measurements of the long-term stability of TiO2/Ni foam and HP-TiO2/Ni foam under a current density of 10 mA cm-2.


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In considering the high and stable activities of HP-TiO2/Ni foam electrode toward HER as well as OER in alkaline solution, this material is used as bifunctional electrocatalyst for both the anode and cathode in an electrolyzer with 1.0 M KOH toward overall water splitting. The LSV curve of the HP-TiO2/Ni foam electrolyzer shows that the obvious catalytic current was formed when the bias potential was larger than ~1.55 V, and the current density of 10, 20, and 50 mA cm-2 could be achieved at the bias potential of 1.71, 1.80, and 1.96 V, respectively. In contrast, the current density of the TiO2/Ni foam electrolyzer are approximately 1 order of magnitude lower than HP-TiO2/Ni foam electrolyzer (Figure 4e). More importantly, the durability of the HP-TiO2/Ni foam electrolyzer is excellent, the bias potential of 1.81 V was basically constant under the steady current density of 10 mA cm-2, with only a slight increase (~0.03 V) after 7200 s. In contrast, the bias potential of the TiO2/Ni foam electrolyzer is much larger (2.11 V), despite with the similar stability (Figure 4f). Density functional theory (DFT) calculations are performed to further understand the related mechanism. The O-2s states, whose energy level is stable for slightly different chemical environments, are set at the same energy value (-16.4 eV) to compare the DOS of TiO2 and HP-TiO2 (Figure S10a). For HP-TiO2, a defect energy state appears and connects with the conduction band minimum, which is mainly attributed to Ti-3d states (Ti3+ species, Figure S10b). The band gap almost disappears (Fermi level lies inside the conduction band), confirming its high electrical conductivity.



Figure 5. (a) HER free-energy diagram for different H* absorption sites of the TiO2 and HP-TiO2 surfaces. Schematic diagram of the catalytic activity zone of the (b) HPTiO2 and (c) TiO2 surface for the HER. For HER catalysis, the relative free energy of H* absorption on the surface of catalysts is the key parameter to evaluate their activity.57 On the surface of TiO2, the ΔGH* values of on O2- and Ti4+ sites are about -0.88 and 4.98 eV, respectively (Figure 5a). For HP-TiO2, the surface defects on the top layer are kept in hydrogen evolution, and Ti4+ and Ti3+ are co-existed on the top layer. Therefore, the possible sites of HPTiO2 for absorbing H* include O (I) (the oxygen around Ti4+), O (II) (the oxygen around Ti3+), Ti3+, and Ti4+ sites, and the relative free energies are 0.12, 0.48, 0.49 and 4.33 eV, respectively (Figure 5a). It is believed that the Ti4+ sites on both the TiO2 and HP-TiO2 surfaces do not contribute to the catalytic activity of HER due to the 20 ACS Paragon Plus Environment

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quite high positive ΔGH* value. On the other hand, if H* is absorbed at the O2- site of the TiO2 surface, H-1s and O-2pz (the dangling bond) will induce the formation of a strong chemical bond, which may result in a large absorption energy and unfavorable negative ΔGH* value. Similarly, H* is assumed to be absorbed at the Ti3+ or O (II) site of the HP-TiO2 surface, the defect electronic states around Ti3+ overly weakens the chemical bond between H* and the surface of the catalysts, which also lead to an undesirable positive ΔGH* value. Conversely, when H* is absorbed at the O (I) sites of the HP-TiO2 surface that is away from Ti3+ and slightly affected by the defect states, the chemical bonds have an appropriate strength, with a highly favourable ΔGH* value (0.12 eV, close to zero). As indicated in Figure 5b, the zones of HP-TiO2 surface around Ti4+ sites (except the Ti4+ sites themselves) possess the high activity to HER; while the zones around Ti3+ sites have a relatively low activity. For OER catalysis, it is proposed that the defects on the top layer of HP-TiO2 are oxidized and occupied by the oxygen atoms in the initial stage of the OER process for the purpose of simplifying model; while the large amount of subsurface Ti3+ is maintained (the constructed surface is shown in Figure S11). During the OER process, the various reactive-intermediates, OH*, O*, and OOH*, are formed and absorbed on the surface of the catalysts. In alkaline media, the OER catalysis could be generally considered as the following four steps: 56 Step 1: M (catalysts) + OH- → M-OH* + eStep 2: M-OH* + OH- → M-O* + H2O (aq) + eStep 3: M-O* + OH- → M-OOH* + e21 ACS Paragon Plus Environment


Step 4: M-OOH* + OH- → M + O2 + H2O (aq) + e-

Figure 6. (a) The absorption energies of the three intermediates of OH*, O*, and OOH* on the surface of TiO2 and HP-TiO2. (b) The ΔG values of the four OER element steps on the surface of TiO2 and HP-TiO2. Hence, the absorption energies of the intermediates determining the ΔG of each element step were analyzed, and the geometry structures of the intermediates absorbed on the surface are presented in Figure S12. As shown in Figure 6a, the absorption energies of the three intermediates on the surface of TiO2 are obviously larger than those on the HP-TiO2 surface. For TiO2, the ΔG values of the four element steps of the OER (the potential is zero voltage, i.e., U = 0 V) are 2.60, 3.06, -0.04, and -1.0 eV, respectively (Figure 6b), and The large positive values of the first and second steps are attributed to the small absorption energies of OH* and O*, respectively, which means the free energies of the reaction systems are dramatically increased for these two steps; the second step of OER with ΔG = 3.06 eV would induce the large overpotential of TiO2 electrode. For HP-TiO2, the relative larger absorption energies of the intermediates adjust the ΔG of the four element steps to more favourable values, 0.49, 1.63, 2.23, and 0.31 eV, respectively (Figure 5b). The largest ΔG value, 22 ACS Paragon Plus Environment

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2.23 eV, is pushed to the third step, which is significantly smaller than 3.06 eV at the second step of TiO2. These results clarify the much lower overpotential of HP-TiO2, and its substantially enhanced activity in OER catalysis compared with TiO2. Based on the above mentioned calculations, we can observe that the electronic defect states of HP-TiO2 strongly impacts on the strength of the chemical bonds between the TiO2 surface and reactive-intermediates, which adjusts the reaction free energy for each element reaction step in both the OER and HER processes to a more proper value, and thus, triggers the high electrocatalytic activity of HP-TiO2 for overall water splitting.

CONCLUSIONS The TiO2/Ni foam composite is converted to be an active electrocatalysts towards both HER and OER in the alkaline electrolyte through using H2 plasma treatment. It is revealed that a high concentration of surface oxygen vacancies endows the TiO2 material with favourable H*, OH*, O*, and OOH* intermediate adsorption energy, which enable the substantially improved dynamics of HER and OER on its surface. Hence, TiO2 is transformed into a relatively high active bifunctional electrocatalyst for overall water splitting; the work confers a unique function to TiO2 that is different from its widely accepted role as an electrocatalytically inert semiconductor material. The work not only confers a unique function to TiO2 that is different from its widely accepted role as an electrocatalytically inert semiconductor material but also provides



a plasma engrave method to tune the electronic structure of TiO2 which lead to the enhancement of its catalytic activity.

ASSOCIATED CONTENT Supporting Information. Additional SEM, XPS, CL, detailed description, electrocatalytic performance comparison, and theoretical calculation structural model of TiO2 samples. This materials is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Prof. Dr. Ge Chen

[email protected]

Prof. Dr. Shaorui Sun [email protected] Dr. Dong Wang

[email protected]

ACKNOWLEDGMENT The authors are grateful to Mr. Stefan Hanitsch and Dr. Yang Xu from Ilmenau University of Technology for their help with sample preparation. This work was supported by National Natural Science Foundation of China (NSFC 11475012, U1607110), Deutsche Forschungsgemeinschaft (DFG, Grant SCHA 632/20-1), Natural Science Youth Fund of Jiangsu Province (Grant No. BK20170828), Beijing Municipal Commission of Education Foundation (KZ201610005002), Beijing


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Table of contents (TOC):

By introducing the abundance of surface defects (Ti3+ species/oxygen vacancies), TiO2, a typical low-cost, earth-abundant, but electrocatalytically inert semiconductor material, is triggered to be a promising electrocatalyst for both HER and OER in the same alkaline media.

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Nickel Foam as an Efficient Bifunctional

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