Hydrogen generation from catalytic glucose oxidation by Fe-based electrocatalysts

Accepted Manuscript Hydrogen generation from catalytic glucose oxidation by Febased electrocatalysts

Puyu Du, Jingjing Zhang, Yanhui Liu, Minghua Huang PII: DOI: Reference:

S1388-2481(17)30226-6 doi: 10.1016/j.elecom.2017.08.013 ELECOM 6018

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

3 July 2017 15 August 2017 15 August 2017

Please cite this article as: Puyu Du, Jingjing Zhang, Yanhui Liu, Minghua Huang , Hydrogen generation from catalytic glucose oxidation by Fe-based electrocatalysts, Electrochemistry Communications (2017), doi: 10.1016/j.elecom.2017.08.013

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ACCEPTED MANUSCRIPT Hydrogen generation from catalytic glucose oxidation by Fe-based electrocatalysts Puyu Du, Jingjing Zhang, Yanhui Liu and Minghua Huang* Institute of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, China. E-mail: [email protected]

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Abstract

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Iron phosphide films (Fe2P) grown in situ on stainless steel mesh (SSM) exhibit excellent electrocatalytic performance toward the glucose oxidation reaction (GOR)

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with robust durability. During GOR, the Fe2P could be further transformed into the oxidized Fe species with high catalytic activity. The integrated two-electrode glucose

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electrolytic cell utilizing Fe2P/SSM and Pt/C exhibited a cell voltage 300 mV lower than water splitting alone, indicating an efficient pathway for H2 production. These

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features suggest that the replacement of the sluggish oxygen evolution reaction (OER) with the thermodynamically more favourable GOR in the Pt/C||Fe2P/SSM

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configuration is an attractive alternative for electrolytic H2 generation.

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Keywords：Fe-based electrocatalyst；Stainless steel mesh；Glucose oxidation； H2 generation；Water splitting

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1. Introduction

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Electrochemical water splitting is one of the most attractive routes to H2 production for clean and renewable energy [1-4], however, the high overpotential for this reaction has severely constrained wider application [5,6]. Recently, a variety of efficient catalysts, such as CoOx-CoSe, CoP and MoS2, have been developed to improve the efficiency of the water splitting process [7-11]. Despite this recent progress, the high potential of OER remains the major bottleneck to the wider application of water splitting techniques on an industrial scale [12-15]. Current research is therefore focused on the challenge of finding desirable new pathways to decrease the overpotential.

ACCEPTED MANUSCRIPT One promising strategy which has been developed recently to obtain energy-saving electrolytic H2 generation has been through replacement of the OER with oxidation of more readily oxidizable species, such as urea [16], hydrazine [3], alcohols [17] and 5-hydroxymethylfurfural [18]. This attractive approach has not, thus far, been applied to the GOR. Compared with the above

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oxidizable species, glucose is considered to be an attractive sustainable source due to their low cost, non-toxicity and renewability [19]. H2 production through

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GOR may offer other benefits, such as the purification of glucose-rich wastewater [20] and the formation of valuable by-products, such as

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gluconolactone, which are widely used by the food and pharmaceutical industries [21, 22].

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Herein we investigate the feasibility of iron phosphide (Fe2P) grown in situ on stainless steel mesh (SSM) as catalysts for GOR. Fe2P growth was

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readily achieved by the simple and direct phosphidation of commercially available SSM, which was selected as it exhibits good durability and high

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conductivity [23,24]. This in situ growth makes it possible to directly obtain

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Fe-based catalysts from the SSM and thus avoid the detachment of the catalysts during the vigorous gas evolution process.

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2. Experimental

2.1. Preparation of Fe2P/SSM electrodes

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A piece of SSM (Fe 96%) was cleaned by ultrasonication in 5 M HCl and water. After drying, SSM and NaH2PO2 (1:10 mass ratio) were placed at two separate sides of a porcelain boat with NaH2PO2 at the upstream side. The porcelain boat was then placed into a quartz tube furnace and heated at 250℃ for 6 h under N2. After being cooled to room temperature under N2, the Fe2P/SSM was washed with water. These synthetic conditions were optimized to maximize the catalytic activities of the Fe 2P-based electrocatalysts. The catalytic activities are affected by the synthetic time and temperature, whereas the ratio of Fe and P is not the key factor.

ACCEPTED MANUSCRIPT 2.2. Preparation of Pt/C, RuO2 and IrO2 loaded electrodes 5 mg of commercial 20 wt % Pt/C (>90 m2/g, <3.5 nm, Johnson Matthey), RuO2 (99.9%, 45-70 m2/g, ≤74 μm, Aldrich) or IrO2 (99.9%, 45-66 m2/g, ≤ 5 μm, Macklin) and 0.1 mL of Nafion solution (5 wt%, Aldrich) were ultrasonically dispersed in 5 mL water/isopropanol (4:1 v/v) mixed solution.

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Then, 1.5 mg cm-2 of Pt/C, RuO2 or IrO2 was loaded on a SSM. 2.3. Characterization

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X-ray diffraction (XRD) analysis was recorded by BrukerD8ADVANCE. Scanning electron microscopy (SEM) image was obtained by Hitachi S-4800.

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X-ray photoelectron spectra (XPS) data was obtained by an ESCALAB 250Xi. 2.4. Electrochemical measurements

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Electrochemical measurements were performed with a three-electrode setup in 10 M KOH solution. Pt mesh and Hg/HgO electrode were chosen as

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the counter and reference electrodes, respectively. Linear sweep voltammetry (LSV) was used to evaluate the electrochemical activity of the Fe2P/SSM with

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iR-95% compensation. Others electrochemical measurements were tested

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without compensating iR drop. All potentials measured were calibrated to reversible hydrogen electrode (RHE). The hydrogen production and Faradaic efficiency were measured through water-gas displacing method. The

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electrochemically active surface area (ECSA) is equal to the ratio of double layer capacitance (Cdl) and specific capacitance (Cs), where Cs = 0.04 mF cm-2

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is adopted from the previous reports [25,26].

3. Results and discussion As shown in the inset of Fig. 1A, the colour of the SSM changed from silvery white to dark grey after the phosphidation treatment. Fig. 1A shows XRD pattern for the Fe2P scratched from SSM. It displays four well-defined diffraction peaks at 40.3, 44.2, 47.3, 52.9 and 54.1 o, which are assigned to (111), (201), (210), (002) and (300) crystal planes of Fe 2P phase (JCPDS No.65-1990), respectively [27]. SEM examinations reveal that the skeleton of

ACCEPTED MANUSCRIPT the pristine SSM substrate is maintained in the Fe2P/SSM (Fig. 1B). In contrast to the smooth surface of the original SSM (inset of Fig. 1C), many Fe2P nanorods are buried into the SSM with a transverse diameter around 30 nm (Fig. 1C, D), indicating that the SSM surface is roughened. The high resolution Fe 2p spectrum in XPS measurement is deconvoluted

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into three sub-peaks at binding energies of 707.0, 710.9 and 719.9 eV, which can be associated to Fe2P and oxidized Fe species from the SSM, respectively

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[25,28]. Similarly, the XPS spectrum of P 2p (Fig. 2C) shows two peaks located at 129.1 and 129.9 eV, representative of P 2p 3/2 and P 2p 1/2 in the

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Fe2P, respectively [28]. The large P peak signal at 133.7 eV can be attributed to oxidized species which probably originate from surface oxidation caused by

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exposure to air [29]. This result supports the successful formation of Fe2P by direct phosphidation of the SSM.

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Our primary interest for the Fe2P/SSM is catalytic GOR activity, the major competition to this process is the OER. Fig. 3A displays LSV curves of the

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Fe2P/SSM in 10 M KOH with and without glucose present. In the absence of

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glucose, the Fe2P/SSM exhibits catalytic OER activity with an onset potential of 1.43 V vs. RHE, beyond which a dramatic increase in anodic current with vigorous O2 evolution was observed. With the introduction of glucose, a rapid

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current density rise is observed at 1.33 V vs. RHE, which shifts 100 mV more negative than that without glucose. Such increase can therefore be attributed to

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the GOR. Significantly, when compared to the OER process, no gas bubbles are observed on the anode during the GOR, indicating GOR is the prioritized reaction. These results suggest that GOR is thermodynamically more favourable than OER on the Fe2P/SSM. The onset GOR potential for Fe2P/SSM is lower than those recently reported for catalysts, such as NiCoP/Ti [30], Ni NPs/TiOxNy [31] amongst others [19,32,33], which implies that Fe2P/SSM may be a more suitable catalyst for GOR. In addition, the Fe2P/SSM exhibited significantly better GOR activity than that of the state-of-the-art RuO2/SSM, although slightly lower than that of

ACCEPTED MANUSCRIPT another benchmark IrO2/SSM (Fig. 3B). The Tafel plot was also used to investigate the favourable kinetics of GOR (the inset of Fig. 3B), which is demonstrated by the fact that the Tafel slope of Fe 2P/SSM (71 mV dec-1) is smaller than that for RuO2/SSM (91 mV dec-1) and IrO2/SSM (90 mV dec-1). The relative ECSA can be estimated for Fe2P/SSM and bare SSM by measuring the Cdl due to the proportional relationship [25]. The Cdl value of the

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Fe2P/SSM and bare SSM is about 0.38 mF cm-2 and 0.16 mF cm-2, respectively

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(Fig. 3C-E). According to the Cdl, the ECSA of Fe2P/SSM is found to be ca. 9.5 cm2, 2.4-times higher than that of bare SSM (4.0 cm2), indicating higher surface

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roughness of the modified SSM, and a larger number of available catalytic active sites for GOR [25]. To evaluate the durability of the Fe2P/SSM for GOR,

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the time-dependent potential curve was recorded under a constant current density of 10 mA cm-2 (Fig. 3F). The potential to afford such current density is

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ca. 1.35 V and no significant decay is observed for at least 24 h, indicating high stability for GOR in the concentrated alkaline solution.

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SEM was employed to evaluate the physical stability of Fe 2P/SSM. The

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inset of Fig. 3F shows SEM image for the Fe2P/SSM after 24 h glucose electrolysis, revealing the presence of the cracks relative to the fresh sample (Fig. 1B). However, no detachment or dissolution of the film from the SSM

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was observed despite the apparent wear on the electrode. The analysis of this post-GOR electrode by XPS measurement showed partial oxidation of Fe

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species and the formation of phosphate species (Fig. 2C, D) [34-36]. It has been proposed that oxidized Fe species act as highly active catalysts for the electrochemical oxidation reaction [37,38], while the existence of phosphates possibly provides further synergistic ligands with the oxidized Fe species to enhance the catalytic activities due to the affinity between P atoms and water molecules [39]. Glucose electrolysis using a two-electrode setup has been utilized for H2 production in practical alkaline electrolysers. To estimate the formed Fe2P/SSM for such applications, a test cell was constructed by assembling Fe2P/SSM as

ACCEPTED MANUSCRIPT the anode for GOR and Pt/C as a benchmark cathode for H2 generation. A current density of 10 mA cm-2 can be obtained with a cell voltage of 1.52 V from the Pt/C||Fe2P/SSM test cell without glucose. With the addition of glucose the required voltage drops to 1.22 V for the same current density (Fig. 4B), which suggests that the GOR could efficiently replace OER to enhance H2

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production. The actual volume of hydrogen gas produced was measured quantitatively via water-gas displacing method. The hydrogen yield in

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GOR-assisted water splitting was larger than that evolved from water splitting (Fig. 4C), whilst, both in the absence and presence of glucose the produced H 2

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is in agreement with the amount expected from a 100% Faradaic efficiency in the electrochemical process [40]. The stability of the Pt/C||Fe2P/SSM

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electrolytic cell was assessed at 10 mA cm -2 with glucose and showed no

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obvious decay for the cell voltage for at least 24 h (Fig. 4D).

4. Conclusion

A simple one-step method was successfully used to fabricate the

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Fe2P/SSM, which exhibits outstanding catalytic activity for the GOR process

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with favorable kinetics and robust durability. Compared with conventional water splitting, the cell voltage is significantly reduced by ca. 300 mV for the

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integrated two-electrode glucose electrolytic cell with Fe2P/SSM and Pt/C, indicating a more energy-efficient H2 generation. The studied system may

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significantly facilitate the widespread deployment of electrolytic H2 production on industrial scale, while the growth of catalytic material from a secure substrate introduces a robust methodology for the development of novel catalysts.

5. Acknowledgements This work was supported by the National Natural Science Foundation of China (21405145). We also thank for help from Dr. John B. Henry (University of Wolverhampton, UK).

Notes and references

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Fig. 1 (A) XRD pattern of Fe2P with the corresponding standard patterns. Inset of (A)

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Inset of (C) SEM images of bare SSM.

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optical images of bare SSM and Fe2P/SSM. (B, C, D) SEM images of Fe2P/SSM.

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Fig. 2 (A, B) XPS spectra of Fe 2p and P 2p, (C, D) XPS spectra of Fe 2p and P

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2p after 24 h stability test.

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Fig. 3 (A) LSV plots for Fe2P/SSM in 10 M KOH with and without 0.5 M

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glucose. (B) LSV plots for RuO2/SSM, Fe2P/SSM and IrO2/SSM in 10 M KOH with 0.5 M glucose. Inset: Tafel plots of RuO2/SSM, Fe2P/SSM and IrO2/SSM. CVs of Fe2P/SSM (C) and bare SSM (D) at different scan rates. (E) Capacitive

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currents at 0.45 V (vs. RHE) versus scan rate for Fe2P/SSM and bare SSM. (F)The time-dependent potential curve of Fe2P/SSM at 10 mA cm-2. Inset: SEM

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image after 24 h stability test.

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Fig. 4 (A) LSV curves, (B) Comparison of the cell voltages to achieve current densities (10, 20, 50, 100 mA cm-2) and (C) The hydrogen yield of theoretically

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calculated and measured at 1.9 V for the Pt/C||Fe2P/SSM couple in 10 M KOH with and without 0.5 M glucose. (D) The time-dependent potential curve of the

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Pt/C||Fe2P/SSM couple with constant current density of 10 mA cm-2.

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Graphical Abstract

A promising strategy has been developed to get energy-saving electrolytic H2 generation at the integrated two-electrode system employing Fe2P/SSM and Pt/C by replacing OER with

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thermodynamically more favorable oxidation of glucose.

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Highlights

1. Fe2P is in situ grown on SSM substrate via direct phosphorization method. 2. Fe2P/SSM was developed as an efficient electrocatalyst toward GOR.

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3. GOR and HER has been coupled to get enhanced H2 generation.

4. Fe2P/SSM||Pt/C electrolyzer is assembled to efficiently catalyze GOR and HER.

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5. Fe2P/SSM||Pt/C only needs a cell voltage of 1.22 V at 10 mA cm-2 to produce H2.