Self-Assembly-Induced Mosslike Fe2O3 and FeP on Electro-oxidized

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Self-Assembly-Induced Mosslike Fe2O3 and FeP on Electrooxidized Carbon Paper for Low-Voltage-Driven Hydrogen Production plus Hydrazine Degradation Ying Wang, Zhimin Chen, Hao Wu, Fei Xiao, Erping Cao, Shichao Du, Yiqun Wu, and Zhiyu Ren ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04274 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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ACS Sustainable Chemistry & Engineering

Self-Assembly-Induced Mosslike Fe2O3 and FeP on Electrooxidized Carbon Paper for Low-Voltage-Driven Hydrogen Production plus Hydrazine Degradation Ying Wang,† Zhimin Chen,*,† Hao Wu,† Fei Xiao,† Erping Cao,† Shichao Du,† Yiqun Wu,†,‡ and Zhiyu Ren*,† Wang, Ying; Heilongjiang University, School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080. Chen, Zhimin; Heilongjiang University, Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education of China), School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080. Wu, Hao; Heilongjiang University, School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080. Xiao, Fei; Heilongjiang University, School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080. Cao, Erping; Heilongjiang University, School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080. Du, Shichao; Heilongjiang University, School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080. Wu, Yiqun; 1. Heilongjiang University, Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education of China), School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080; 2. Chinese Academy of Sciences, Shanghai Institute of Optics and Fine Mechanics, 390# Qinghe Road., Jiading District, Shanghai, P.R. China, 201800. Ren, Zhiyu; Heilongjiang University, Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education of China), School of Chemistry and Materials Science, 74# Xuefu Road., Nangang District, Harbin, P.R. China, 150080.

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Self-Assembly-Induced Mosslike Fe2O3 and FeP on Electrooxidized Carbon Paper for Low-VoltageDriven Hydrogen Production plus Hydrazine Degradation Ying Wang,† Zhimin Chen,*,† Hao Wu,† Fei Xiao,† Erping Cao,† Shichao Du,† Yiqun Wu,†,‡ and Zhiyu Ren*,† † Key

Laboratory of Functional Inorganic Material Chemistry (Ministry of Education of China),

School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, People’s Republic of China. ‡ Shanghai

Institutes of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai

201800, People’s Republic of China. *Corresponding Authors E-mail: [email protected]; [email protected]

ABSTRACT

Energy-saving electrolytic hydrogen production is the precondition for implementing largescale hydrogen energy exploitation. Replacing the sluggish water oxidation reaction with thermodynamically more favorable pollutant electrooxidation (degradation) is a very promising approach to combine energy-efficient hydrogen production and sewage treatment. Herein, a homologous asymmetrical two-electrode configuration, made up of self-assembly-induced mosslike Fe2O3 and FeP on electro-oxidized carbon paper (ECP), was used to electrolyze the


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integrated hydrazine oxidation-hydrogen evolution system. The free-standing Fe2O3/ECP electrode was fabricated by pyrolyzing iron phthalocyanine/ECP self-assembly in air and serves as an anodic reaction catalyst. Subsequent phosphidation of the Fe2O3/ECP leads to the formation of the FeP/ECP catalyst for hydrogen evolution reaction. The crosslinked mosslike Fe2O3 and FeP nanoparticles densely covered on the surface of ECP provide richly exposed catalytic sites and the well-distributed stacking holes among nanoparticles offer the expedited electrolyte/gas transmission path. Coupled with hydrazine oxidation, the Fe2O3/ECPǁFeP/ECP configuration presents a superior cell voltage of only 0.93 V at 10 mA cm-2 for synchronous hydrogen production, which is substantially lower than that of traditional overall water splitting system. Besides the low-voltage-driven energy efficiency, the catalyst electrodes also afford excellent run-to-run reproducibility and stability, as well as good batch-to-batch repeatability. Such a win-win coupling strategy offers the prospect of synchronously achieving energy-saving hydrogen production and the green conversion (or degradation) of hydrazine in wastewater.

KEYWORDS: hydrogen production, hydrazine degradation, energy-saving electrolysis, ironbased electrocatalyst, π-π stacking self-sssembly, free-standing electrode

Introduction With the increasingly serious energy crisis and environmental pollution problems, it is imperative to develop clean energy sources (ie solar, wind, tidal, hydrogen, etc.) so as to slash the large consumption of fossil fuels and satisfy the massive energy demand.1,2 Among them, hydrogen energy with eco-friendly, recyclable, high energy density and high calorific value can


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be converted from other intermittent energies.3-5 Hydrogen (H2) production from electrochemical splitting water provides a trusty and sustainable method for this purpose.6 The oxygen evolution reaction (OER), relative to hydrogen evolution reaction (HER),7 is a sluggish four-electron oxidation process,8 which generally forces a sizeable overpotential requirement for practical water splitting even when OER is promoted by comparatively higher activity, noble metal-including catalysts.9 In order to abate the thermodynamic cell voltage for low-voltage-driven energy-saving hydrogen production, a candidate strategy is to replace the anodic OER with other more readily oxidation (ie oxidation of methanol, ethanol, urea, hydrazine, etc.) 10-16 It may provide the best of both worlds by combining the energy-saving electrolytic hydrogen generation with the green conversion (or degradation) of chemicals.17-19 As an important chemical, hydrazine (N2H4) is widely applied to various fields, including agriculture, pharmacy, fuel cell, chemical industries, and catalysis.20 However, owing to its strong toxicity and corrosion, trace levels of N2H4 can lead to the damage of liver, lung, kidney, and central nervous system. What is more serious is that long-term exposed to the environment of hydrazine will result in death.21,22 It follows that the treatment of residue hydrazine in wastewater is necessary.23 The standard oxidizing potential of hydrazine is about -0.33 V versus reversible hydrogen electrode (vs. RHE) far lower than that of OER (1.23 V vs. RHE).24-26 It may easily be conceived that, combining the HER with the oxidation of hydrazine not only could meet the demand for the low-voltage-driven electrolytic hydrogen generation, but also could achieve the clean conversion of hydrazine in wastewater into nitrogen and water. Recently, X. Sun and Xia et al. have demonstrated that the replacement of OER with hydrazine oxidation reaction can minimise the energy loss of HER.27,28


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Noble metal catalysts, e.g. Pt-based materials, can effectively motivate HER and hydrazine oxidation.29,30 However, their large-scale industrial applications are limited by the expensive cost and scarce reserve.31,32 Taking into account the practical use of industry, it is imperative to develop the earth-abundant and cost-effective substitutes for noble metals as electrocatalysts.33 The current prevailing electrolytic systems often adopt incompatible integration of the bifunctional catalyst, leading to the inferior performance. Relatively, constructing the homologous asymmetrical electrocatalyst system, which would enable complementary catalyst pairs with reactivity matching and reduced cost, may be an effective strategy and is scarcely reported. Fe-based compounds have attracted wide attention, not only due to the fact that they are the richest and cheapest sources, but also due to the abundant and adjustable hybridized orbital of iron.35,36 Also, it has been demonstrated that iron phosphides and iron oxides is one of the best candidates for HER and electrochemical oxidation, respectively.36-38 For the practical applications, furthermore, the largearea, uniform, and stable free-standing electrode is another essential factor. Generally, the powder catalysts should be loaded on current collectors using binder.39 Regrettably, the additional binder not only covers some active sites, raises the series resistance, but also impedes diffusion of electrolytes and reactants, resulting in the decrease of catalytic activity. In addition, the post-load process is cockamamie, and the catalysts easily fall off, which leads to the poor reproducibility and stability.40 By contrast, the free-standing electrodes prepared by the direct growth or assembly, which make catalysts anchored on the conductive substrates, can avoid these disadvantages.41 With above motivation, herein, a homologous asymmetric two-electrode configuration was constructed to realize low-voltage-driven hydrogen production plus hydrazine degradation, in which crosslinked mosslike Fe2O3 and FeP nanoparticles densely covered on electro-oxidated carbon papers (ECPs) were worked as the free-standing electrodes for hydrazine oxidation and


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HER, respectively. The crosslinked Fe2O3 and FeP nanoparticles is derived from decomposition of iron phthalocyanine (FePc), which uniformly self-assembles on a surface of ECP via π-π stacking interactions. The unique self-assembly-induced mosslike structure with a great deal of crosslinked small-sized nanoparticles and stacking holes gives the free-standing electrodes superior electrocatalytic activity. Moreover, due to the traditional OER replaced by the hydrazine oxidation reaction, only a cell voltage as low as 0.93 V is required to generate hydrogen with a current density of 10 mA cm-2. Experimental Preparation of ECPs. ECPs were readily prepared by a simply electrooxidation of carbon paper (CP) in a mixed strong acid solution according to the previously described method.42 Prior to electrochemical oxidation, the CP (TGP-H-060, Toray Industries. Inc.) was cut into rectangular pieces (1 cm × 2 cm) and washed successively with acetone, ethanol and deionized water by ultrasonication for 0.5 h at a time. The electrochemical oxidation of CP was carried out on a CHI 760D electrochemical workstation through a chronoamperometry technique in a conventional three-electrode system (platinum plate as counter electrode and saturated calomel electrode (SCE) as reference electrode). The reaction was performed under a constant voltage of 3 V in an electrolyte of mixed sulfuric acid (98%) and nitric acid (65%) (v:v = 1:1) with different electrooxidation time (time of 5, 10, 15, 20 and 30 mins for the synthesis of ECP-5, ECP-10, ECP15, ECP-20 and ECP-30, respectively). After the electrooxidation process, ECPs were washed with deionized water and ethanol several times. Preparation of the FePc/ECP precursor. The FePc/ECP precursor was prepared by using a solution self-assembly method based on π-π stacking interactions between FePc and ECP. In a


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typical procedure, 90 mg of FePc powders, synthesized according to previous reports,43,44 were first dispersed in 5 mL of deionized water, and the suspension was maintained in a strong ultrasonic bath for 4 h at 313 K. Next, the dispersion was mixed with a piece of ECP (1 cm × 2 cm), and permitted to self-assemble under vigorous ultrasonication for further 6 h. Afterwards, 0.5 ml of hydrazine solution (80 wt% in water) was slowly added into the above mixture, and oscillated on a shaking table for 12 h at 313 K. Finally, the resultant FePc/ECP precursor was picked out, washed with water, and dried in oven. Preparation of the free-standing Fe2O3/ECP and FeP/ECP electrodes. The free-standing Fe2O3/ECP electrode was prepared by a simple pyrolysis of FePc/ECP precursor in air. Briefly, a piece of FePc/ECP precursor in porcelain boat was placed inside a muffle furnace and calcined in air at 673 K for 1 h with a heating rate of 2 K min-1. After cooling to room temperature, the desired free-standing Fe2O3/ECP electrode was obtained. For comparison, the precursor was also heattreated at 573 K, 623 K, 723K, and 773 K, respectively. Subsequent phosphidation of the obtained Fe2O3/ECP leads to the formation of the FeP/ECP electrode. In the typical procedure, a piece of the obtained Fe2O3/ECP electrode was placed at the downstream side of a porcelain boat; meanwhile, 0.2 g of anhydrous NaH2PO2 was placed at the upstream side of the same porcelain boat. Subsequently, the porcelain boat was carefully put at the center of the quartz tube furnace, and heated to 573 K for 1 h with a heating rate of 2 K min-1 in N2 atmosphere. After cooling to room temperature, the free-standing FeP/ECP electrode was washed by deionized water, followed by drying under vacuum. As a control, the Fe2O3/CP and FeP/CP electrodes were also obtained using the same method except for the replacement of ECP with CP. For comparison, Pt/C (20 wt% Pt on Vulcan XC-72) dispersion was drop-coated onto ECP with the loading of 0.07 mg/cm2.


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Characterization. X-ray diffraction (XRD) patterns were recorded on a Bruker/D8 Advance diffractometer using Cu kα radiation (λ = 1.5406 Å) in the 2θ range of 5° - 80°. Nitrogen adsorption-desorption isotherms were collected on a Micromeritics Tristar II 3020 nitrogen adsorption apparatus. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area. Scanning electron microscopy (SEM) images were gained via utilizing a Hitachi S-4800 field emission scanning electron microscopy with an operating voltage of 5.0 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos AXIS Ultra DLD spectrometer fitted with a monochromatic Al Kα x-ray source. The evolved hydrogen (H2) was confirmed and quantified by a Shimadzu GC-2010 Plus gas chromatography equipped with a Carbxen TM 1010 PLOT column and a thermal conductivity detector. Electrochemical measurement. A computer-controlled CHI 760D electrochemical workstation (CH Instruments, Shanghai, China) was used for all electrochemical measurements at room temperature (298 ± 1 K). A standard three-electrode system was made up of the free-standing electrodes, a graphite rod, and a SCE as the working, auxiliary, and reference electrodes, respectively, and the electrolyte was 1.0 M aqueous KOH solution with and without 100 mM hydrazine hydrate. Before measuring, each free-standing electrode was activated by continuous cyclic voltammetry (CV) scanning at a scan rate of 50 mV s-1 until a stable CV curve was obtained. The electrocatalytic activities were evaluated by recording polarization curves using linear sweep voltammetry (LSV) with iR compensation at a scan rate of 2 mV s−1. The long-term durability tests towards hydrazine oxidation and HER were assessed by chronopentionmetry and chronoamperometry, respectively. The electrochemically active surface area (ECSA) was estimated from the electrochemical double-layer capacitance (Cdl), which is calculated from the CV curves measured in a non-Faradaic region at different scan rates (20 mV s-1-180 mV s-1).


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Electrochemical Impedance Spectroscopy (EIS) measurements were performed at open circuit potential with a frequency range from 0.01 to 100 kHz. All potentials measured were referenced to the RHE by using eq 1. E (vs. RHE) = E (vs. SCE) + 0.242 + 0.0591pH.

(1)

Results and discussion

Scheme 1. Schematic illustration of synthetic process for the free-standing Fe2O3/ECP and FeP/ECP electrodes. Preparation and characterization of the free-standing Fe2O3/ECP and FeP/ECP electrodes. The detailed synthetic process of the free-standing Fe2O3/ECP and FeP/ECP electrodes is shown in Scheme 1. Firstly, the pristine CP was electro-oxidized in a mixed strong acid solution to enhance the surface area. It is clear from Figure S1 that, after being electro-oxidized for 15 mins, ECP-15 still presents graphitic carbon structure (JCPDS card No. 41-1487), except that the diffraction peak becomes weaker and broader relative to CP due to partial damage of graphite


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structure.45 As expected, numerous rougher wrinkles appear on the surface of ECP-15, and the accessible BET surface area of ECP-15 quintuples that of CP, which is conducive to more FePc self-assembly on it (Figure S2 and S3). Secondly, FePc with 18π-electron conjugated planar structure was successfully assembled on the surface of ECP-15 via π-π stacking interactions to form FePc/ECP-15 precursor, as evidenced by the presence of characteristic diffraction peaks of FePc (JCPDS card No. 37-1845) in the XRD pattern of FePc/ECP-15 precursor (Figure S1).46 Finally, the FePc/ECP-15 precursor was converted into the free-standing electrodes by directly oxidization in air at 673 K to form Fe2O3/ECP-15, and the succedent low temperature phosphating to obtain corresponding FeP/ECP-15 (see the details in the Experimental Section). These are consistent with the consequence from the XRD patterns (Figure 1), in which the characteristic diffraction peaks assigned to hematite (JCPDS card No. 89-0597) and iron phosphide (JCPDS card No. 39-0809) emerge in the XRD patterns of the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes, respectively.

Figure 1. XRD patterns of the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes.


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Figure 2 illustrates the typical SEM images of the free-standing Fe2O3/ECP-15 and FeP/ECP15 electrodes. Obviously, thousands of crosslinked mosslike Fe2O3 nanoparticles derived from FePc uniformly adhering to the surface of ECP-15. The formation of such well-distributed mosslike Fe2O3 densely covered on ECP can be ascribed to the close and uniform self-assembly between FePc and ECP via π-π stacking interactions and the inhibition of FePc’s external ring on the undesirable grain growth of nanoparticles during the heat treatment.47 Also, there are plenty of holes caused by the accumulation of nanoparticles on the surface of the free-standing Fe2O3/ECP15 electrode, which is in favour of the mass transport and gas release during the electrochemical reaction. After low temperature phosphating, no conspicuous difference in the morphology between the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes can be observed, except for the looser surface of FeP/ECP-15 owing to the Kirkendall effect (Figure 2C and 2D).48 Compared with Fe2O3/CP and FeP/CP (Figure S4), which exhibits nanoparticles covering disorderedly on the surface of CP and the size of particles being large and small, the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes covered by homogeneous mosslike structure with the thousands of nanoparticles have the larger specific surface area, more exposed active sites to facilitate the diffusion of electrolytes and reactants, resulting in improving the electrocatalytic performance.


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Figure 2. The typical SEM images of the free-standing Fe2O3/ECP-15 electrode (A and B) and FeP/ECP-15 electrode (C and D). To explore the chemical composition of the free-standing electrodes, XPS were performed.49 The survey spectra (Figure 3A) show the presence of C, O and Fe elements in both the freestanding Fe2O3/ECP-15 and FeP/ECP-15 electrodes. And only the difference peaks assigned to P 2p and P 2s appear in that of the free-standing FeP/ECP-15 electrode, which further illustrate the successful transformation from Fe2O3 to FeP by low temperature phosphating. The high-resolution Fe 2p core-level spectra for the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes can be deconvoluted into four peaks (Figure 3B). In the Fe 2p XPS spectrum of the free-standing Fe2O3/ECP-15 electrode, the main binding energies of Fe 2p3/2 and Fe 2p1/2 appear at 711.3 eV and 724.8 eV, and two shape-up statellite peaks are located in 718.5 eV and 732.5 eV, which is indexed to the oxidized state of FeIII. The peaks of 707.3 eV and 720.2 eV corresponds to the binding energies of Fe 2p3/2 and Fe 2p1/2 in Fe-P, respectively; while, the peaks at 711.8 eV and 728.2 eV are ascribed to the partial Fe oxidation state due to the air exposure.50 Comparing with metallic Fe


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(707.0 eV), there is an obvious positive shift of Fe 2p3/2, indicating the partial electron transfer from Fe to P in the free-standing FeP/ECP-15 electrode.51,52 In the high-resolution P 2p spectrum (Figure 3C), the P 2p3/2 and 2p1/2 are located in 129.6 eV and 130.4 eV, respectively; while, the binding energy of 133.7 eV is attributed to the formation of P-O band. Accordingly, a negatively shift of P 2p3/2 binding energy can also be observed relative to that of pure P (130.2 eV).53,54 The electron diffusion between Fe and P is propitious to the adsorption/desorption of reactants or resultants during electrocatalytic process.55 Herein, the positive Fe and negative P centers acts as hydride- and proton-acceptor sites in electrocatalytic process, respectively.56 The high-reduction O 1s spectrum of the free-standing Fe2O3/ECP-15 electrode is consisted of two characteristic peaks that can be assigned to lattice oxygen (530.1 eV) and hydroxyl oxygen (531.6 eV) (Figure 3D); while, the two characteristic peaks for the free-standing FeP/ECP-15 electrode are attributed together to P-O that tallied with P 2p XPS result.


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Figure 3. (A) XPS spectra of the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes. (B-D) The high-resolution P 2p, Fe 2p, and O 1s XPS spectra of the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes, respectively. Electrocatalytic activity of the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes. The electrocatalytic activity of the free-standing Fe2O3/ECP-15 and reference electrodes (e.g., Fe2O3/CP, ECP-15, and CP) towards hydrazine oxidation reaction was investigated in 1.0 M KOH with 100 mM hydrazine in a typical three-electrode electrochemical configuration. The polarization curves after iR compensation of them are shown in Figure 4A. Among the four freestanding electrodes, the applied potential of the free-standing Fe2O3/ECP-15 electrode is 664 mV vs. RHE to drive the current density of 10 mA cm-2, much lower than that of Fe2O3/CP (723 mV) and ECP-15 (878 mV) electrodes; while the pristine CP shows rarely any activity, suggesting an advantageous capability of the free-standing Fe2O3/ECP-15 electrode in hydrazine oxidation reaction. Certainly, the oxidative potential of Fe2O3/ECP-15 electrode towards hydrazine is affected badly by its concentration in the system. As shown in Figure S5, the onset potential of hydrazine oxidation gradually decreases with the increase of the hydrazine concentration, until the onset potential reaches 0.61 V (vs. RHE) at 100 mM hydrazine. The larger onset potentials of hydrazine oxidation at the concentration of hydrazine below 100 mM are mainly due to the limit of the mass transfer, and even so, they are still far below that of the traditional OER. An anode potential of 0.90 V can be saved to reach 10 mA cm-2 (1.56 V vs. RHE for OER) (Figure S6). The electrocatalytic performance of the as-obtained electrodes towards hydrazine oxidation was further studied by corresponding Tafel plots (Figure 4B). The straight section of Tafel plots accords with Tafel equation using eq 2


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η = blog|j|+ a,

(2)

herein, η is the applied potential, j is the current density, b is the Tafel slope, and a is the constant).57 The Tafel slope of the free-standing Fe2O3/ECP-15 electrode is about 179.2 mV dec-1, much lower than that of the free-standing Fe2O3/CP (433.5 mV dec-1), ECP-15 (506.7 mV dec-1), and CP (844.9 mV dec-1) electrodes, implying its’ faster catalytic kinetics for hydrazine oxidation reaction.

Figure 4. (A) LSV curves and (B) Tafel plots of Fe2O3/ECP-15, Fe2O3/CP, ECP-15, and CP electrodes in 1.0 M KOH with 100 mM hydrazine at a scan rate of 2 mV s-1 for hydrazine oxidation; (C) Nyquist plots of Fe2O3/ECP-15 and Fe2O3/CP electrodes; (D) chrono-potentiometric curve of Fe2O3/ECP-15 electrode with constant current density of 10 mA cm-2.


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The efficient activity of the free-standing Fe2O3/ECP-15 electrode towards hydrazine oxidation depends largely on the unique mosslike structure, the thousands of crosslinked small-sized nanoparticles and homogeneous porosity structure, which is conducive to exposing active sites and the transport of reactants and electrolyte. As the hard evidence, the ECSA was estimated by measuring the Cdl.58 In comparison, the Cdl of the free-standing Fe2O3/ECP-15 electrode can be calculated to be about 45.5 mF cm-2, which is closely 40 times higher than that of the free-standing Fe2O3/CP electrode (1.21 mF cm-2) (Figure S7). It indicates that Fe2O3/ECP-15 electrode with a great deal of uniform and small-sized nanoparticles can offer more active sites for hydrazine oxidation reaction. In addition, the EIS was measured to further illustrate the hydrazine oxidation kinetics occurring at the electrode/electrolyte interface.59 As shown in Figure 4C, only an obvious semicircle located in the high frequency region can be observed, which is assigned to the chargetransfer resistance (Rct).60 The semicircle radius of the free-standing Fe2O3/ECP-15 electrode is distinctly smaller than that of Fe2O3/CP, ECP-15, and CP electrodes, indicative of a smaller Rct between hydrazine and Fe2O3/ECP-15 electrode (as shown in Table S1). The fast charge-transfer can promote the reaction kinetic during the hydrazine oxidation process. It mainly attributes to mosslike Fe2O3 crosslinked nanoparticles uniformly loaded on the surface of ECP-15, which also produces many tiny pores to expedite the mass transport and gas generated release. Another critical parameter to investigate the electrocatalyst activity is the reproducibility and stability. It is evident that, from the chronopotentiometric curve of the free-standing Fe2O3/ECP-15 electrode in 1.0 M KOH with 100 mM hydrazine (Figure 4D), a potential about 0.70 V is demanded to drive the current density of 10 mA cm-2 at the beginning; while, the potential increases gradually with the consumption of hydrazine. When the electrolyte is refreshed three times after 14 h, the oxidation


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potential rapidly declines to 0.70 V again, indicating that the free-standing Fe2O3/ECP-15 electrode shows a satisfactory reproducibility and stability.

Figure 5. (A) LSV curves and (B) Tafel plots of FeP/ECP-15, FeP/CP, ECP-15, CP, and 20% Pt/C electrodes in 1.0 M KOH with a scan rate of 2 mV s-1 for HER, the inset in (A) is enlargement; (C) Nyquist plots of FeP/ECP-15 and FeP/CP electrodes; (D) LSV curves of FeP/ECP-15 electrode before and after 3000 cycles between 0 and -0.3 V (vs. RHE) at a scan rate of 100 mV s-1, the inset in (D) is the time-dependent current density curve for 24 h at -0.2 V (vs. RHE). The electrocatalytic activity of FeP/ECP-15 and the related reference electrodes towards HER was also assessed via similar method as described previously. As shown in the polarization curves after iR compensation (Figure 5A), the commercially available 20% Pt/C catalysts emerges famous


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HER performance with minimum overpotential. Among the four free-standing electrodes, the FeP/ECP-15 electrode shows the smallest onset potential of -77 mV vs. RHE and the overpotential of 138 mV vs. RHE to drive current density of 10 mA cm-2 (onset potential of -208 mV and -328 mV for FeP/CP and ECP-15; overpotential of 265 mV and 367 mV for FeP/CP and ECP-15 at 10 mA cm-2, respectively). And, for the pristine CP, almost no HER activity can be observed. In addition, there is a rapid increase of the current densities as the potential becomes more negative, suggesting an advantageous capability of the free-standing FeP/ECP-15 electrode in accelerating HER. The Tafel plots of different electrodes were also investigated as shown in Figure 5B. The free-standing FeP/ECP-15 electrode exhibits a small Tafel slope of 63.9 mV dec-1 in the low current density region, revealing that the rate-limiting step of HER is electrochemical desorption and that the HER process happens through a Volmer-Heyrovsky mechanism.61 For comparing, the Tafel slope of 20% Pt/C is 62.9 mV dec-1, which is analogical with the reported value in an alkaline condition; while the Tafel slopes of the free-standing FeP/CP electrode (66.9 mV dec-1), ECP-15 (139.3 mV dec-1), and CP (756.8 mV dec-1), are bigger than that of the free-standing FeP/ECP-15 electrode under the same condition. In addition, the value of exchange current density (j0) of the free-standing FeP/ECP-15 electrode is calculated to be 3.16 × 10-2 mA cm-2 by an extrapolation method using the Tafel plots (Figure S8),62 which is almost 30 times greater than that of the freestanding FeP/CP electrode (1.41 × 10-3 mA cm-2). Simultaneously, a summary of the HER overpotential and Tafel slope with the reported Fe-based electrocatalysts in alkaline condition is shown in Table S2. As can we directly see that the free-standing FeP/ECP-15 electrode is one of the best HER electrocatalysts in an alkaline condition. The Nyquist plots of the free-standing FeP/ECP-15 and FeP/CP electrodes are shown in Figure 5C. The semicircle of the free-standing FeP/ECP-15 electrode is clearly smaller than that of the


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free-standing FeP/CP, suggesting that the FeP/ECP-15 electrode has the lowest charge-transfer resistance (Table S1) at the catalyst/electrolyte interface, and superior charge transport kinetics.63 It is defined that the free-standing FeP/ECP-15 electrode is one of the best HER electrocatalysts. To explore the durability of the free-standing FeP/ECP-15 electrode, the time-dependent current density curve was measured under a static overpotential of 200 mV for 24 h in 1.0 M KOH (inset of Figure 5D). At the continuous voltage with ceaselessly releasing of H2 bubbles, the current density of the free-standing FeP/ECP-15 electrode still maintains at 91.2 %, suggesting its high long-term stability. Moreover, the electrochemical stability of the free-standing FeP/ECP-15 electrode was further testified by the polarization curves before and after 3000 cycles CV (Figure 5D). After 3000 cycles, comparing with the first circle test, indiscernible activity decline can be observed. Apparently, the electro-oxidation of CP is essential to make nanoparticles cover on the surface of ECP uniformly. Therefore, the influence of the electro-oxidation time of CP on electrocatalytic activity was also monitored. From the relevant polarization curves (Figure S9), it can be seen that, the electrocatalytic activity of the free-standing electrodes increases firstly and then deceases; and the Fe2O3/ECP-15 and FeP/ECP-15 electrode emerges the optimum hydrazine oxidation and HER activity, respectively. The electro-oxidation can make the surface of CP rougher, yielding large ECSA to stimulate electrochemical reaction, as shown in Figure S10 and S11. It should be stressed, however, that, after the electro-oxidation for 14 mins, the conductibility of ECP distinctly decreases as evidenced by the sharp declination of current (Figure S12), probably because most of C-C bonds on ECP surface is destroyed during the long and strong oxidation.64 Moreover, the electrocatalytic performance of electrodes also is affected by the concentration of the FePc


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dispersion and calcination temperature in air (Figure S13 and S14). The optimum FePc concentration and calcination temperature is 16 mg/mL and 673 K, respectively.

Figure 6. (A) LSV curves of the homologous asymmetric Fe2O3/ECP-15ǁFeP/ECP-15 electrolysis system in 1.0 M KOH with and without 100 mM hydrazine, respectively, the inset in (A) is photograph of a two-electrode configuration, (B) The amount of hydrogen theoretically calculated and experimentally measured vs. time for the asymmetric Fe2O3/ECP-15ǁFeP/ECP-15 system at a current density of 40 mA cm-2 for 4000 seconds in 1.0 M KOH with 100 mM hydrazine. Considering the outstanding activity of the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes toward hydrazine oxidation and HER, respectively, a homologous asymmetric twoelectrode electrolysis configuration was constructed to demonstrate the low-voltage-driven hydrogen generation plus hydrazine degradation in alkaline electrolyte, in which the free-standing Fe2O3/ECP-15 and FeP/ECP-15 electrodes works for anode and cathode, respectively. Compared with the cell voltage of 1.83 V at the current density of 10 mA cm-2 in hydrazine-free electrolyte, the asymmetric Fe2O3/ECP-15ǁFeP/ECP-15 system just demands that of 0.93 V in 1.0 M KOH with 100 mM hydrazine (Figure 6A), proving that the ingenious collocation can reduce about 900 mV for electrolytic hydrogen generation in hydrazine-containing electrolyte at 10 mA cm-2. Based on above result, the Fe2O3/ECP-15ǁFeP/ECP-15 system in hydrazine-containing electrolyte owns


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much better energy conversion efficiency than that in hydrazine-free electrolyte. The Faraday efficiency of hydrogen generation was calculated to compare the amount of experimentally quantified hydrogen with theoretically calculated hydrogen in the two-electrode configuration (Figure 6B). As can we see that the Faraday efficiency of hydrogen generation for the homologous asymmetric Fe2O3/ECP-15ǁFeP/ECP-15 system is closed to 100 % in hydrazine-containing electrolyte. Meanwhile, the stability of that also was tested by the time-dependent current density under a static cell voltage of 1.0 V using the same pair of electrodes, and the electrolyte solution was refreshed every five hours for the compensation of hydrazine consumption (Figure S15). It is apparent that the electrolysis current just seems to bounce back after each electrolyte refreshing during the course of over 20 h, suggesting its superior stability in the long-term electrochemical process. The remarkable energy-saving effect and excellent stability of the homologous asymmetric Fe2O3/ECP-15ǁFeP/ECP-15 couple can be comparable and even superior to most previously reported low-voltage-driven electrolysis system (Table S3). Furthermore, in view of the practical application, the repeatability of the asymmetric electrolytic system prepared by different batches of Fe2O3/ECP-15 and FeP/ECP-15 electrodes was also evaluated by LSV experiments. Apparently, benefiting from the stable electrocatalytic acitivity of Fe2O3/ECP-15-Rx and FeP/ECP-15-Rx electrodes, the cell voltages of the two-electrode system at a current density of 10 mA cm-2 firmly hold at about 0.93 V from batch to batch (Figure S16). Conclusions In conclusion, the homologous free-standing Fe2O3/ECP and FeP/ECP electrodes with crosslinked nanoparticles and well-distributed porous structure were successfully prepared via the


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self-assembly of FePc and ECP, pyrolysis in air, and followed phosphidation. The formation of such unique mosslike structure attributes to the intimate self-assembly between FePc and ECP, and the inhibitory action of external organic ring on the aggregation and growth of nanoparticles during the thermal treatment process. Benefiting from the abundant exposed active sites and the unimpeded path for reactants/gas transmission, the resulting asymmetric Fe2O3/ECPǁFeP/ECP system features a remarkably small cell voltage of 0.93 V to reach the current density of 10 mA cm-2 for synchronous hydrogen generation and hydrazine degradation, which is much lower than that of conventional overall water splitting. Meanwhile, the ingenious collocation also presents excellent reproducibility and stability for the practical application. This work opens new opportunities for the energy-efficient and eco-friendly electrolytic hydrogen generation coupling with hydrazine-rich wastewater purification. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Contrast sample’s structural and compositional characterizations, and electrocatalytic hydrazine oxidation/HER performance comparison. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z. M. Chen) *E-mail: [email protected] (Z. Y. Ren) ACKNOWLEDGMENT The authors gratefully acknowledge the support of this research by the National Natural


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TOC/Abstract Graphic (For Table of Contents Use Only)

An asymmetric electrolysis system constructed by self-assembly-induced homologous mosslike Fe2O3 and FeP on electro-oxidized carbon paper was exploited to achieve low-voltage-driven hydrogen production and hydrazine degradation in wastewater.


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Self-Assembly-Induced Mosslike Fe2O3 and FeP on Electro-oxidized

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