New and Efficient Electrocatalyst for Hydrogen Production from Water

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A New and Efficient Electrocatalyst for Hydrogen Production from Water Splitting: Inexpensive, Robust Metallic Glassy Ribbons Based on Iron and Cobalt Fabao Zhang, Jili Wu, Wei Jiang, Qingzhuo Hu, and Bo Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09222 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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A New and Efficient Electrocatalyst for Hydrogen Production from Water Splitting: Inexpensive, Robust Metallic Glassy Ribbons Based on Iron and Cobalt Fabao Zhang, Jili Wu, Wei Jiang, Qingzhuo Hu, and Bo Zhang* Institute of Amorphous Matter Science, School of Materials Science and Engineering & Anhui Provincial Key Lab of Functional Materials and Devices, Hefei University of Technology, Hefei 230009, China Supporting Information Placeholder large-scale melt-spinning process rather than wet-chemical methods.23 By this synthetic method, the massive and uniform MG ribbons can be obtained. Inspired by the desirable properties of MGs, including excellent mechanical properties, low production costs and good corrosion resistance, the present work designed a new MG based on the transition metals Fe and Co with the aim of obtaining an HER catalyst. This MG has a nominal composition of Fe40Co40P13C7 MG and is synthesized in a ribbon state by conventional melt-spinning. The results presented herein demonstrate that this MG is an excellent HER catalyst with high electrocatalytic activity and long-term stability in a typical acid solution. Based on its superior mechanical properties and low mass production costs, the present Fe40Co40P13C7 MG could quite possibly represents a new family of HER catalytic materials having commercial applications.

ABSTRACT: Efficient, stable electrocatalysts are required to promote the hydrogen evolution reaction (HER) to obtain hydrogen as a clean, sustainable fuel via water splitting. In the present work, ribbons of the metallic glass Fe40Co40P13C7 were produced using a conventional melt-spinning technique and assessed as electrocatalysts for HER. In 0.5 M H2SO4, these ribbons generated an overpotential of 118 mV at a current density of 10 mA cm−2. This overpotential remained essentially constant over 20 h under these conditions. Based on the excellent properties, these glassy ribbons represent a new type of highly active, robust HER catalyst suitable for practical applications. Keywords: electrocatalysts, HER, ribbon, highly-active, earthabundant

Because hydrogen is both clean-burning and renewable, it is considered to represent an ideal energy carrier as an alternative to fossil fuels.1-3 The electrochemical generation of hydrogen via water splitting is an efficient means of obtaining this fuel, as water is both abundant and carbon-free.4 However, the hydrogen evolution reaction (HER), representing a half reaction in the water splitting process, requires efficient electrocatalysts to improve the efficiency of the process.5 At present, the state-of-the-art HER electrocatalysts are all based on noble metals, such as Pt, even though the high price and scarcity of these materials seriously hinders the adoption of water splitting as a hydrogen generation process.6 As a result of the above, the development of HER electrocatalysts with efficiencies comparable to those of noble metal-based catalysts but with reduced fabrication costs and increased stability has been of significant interest in both the academic and industrial communities. In recent years, a variety of earth-abundant transition metals and their nitrides,7 borides,8 carbides,9,10 sulfides,6,11-12 and phosphides,13-16 have been synthesized and have shown promise as replacements for noble metal-based catalysts. Simultaneously, the synthesis or modification of transition metal HER catalysts on the nanoscale has been used to improve their performance.17,18 Amorphous materials, such as metallic glasses (MGs, also known as glassy metals or amorphous alloys), have been suggested as potential catalysts owing to their significant stability in corrosive media as well as the high catalytic activity that results from their metastable properties.19,20 Recently, both Pd40Ni10Cu30P2021 and Fe40Ni40P2022 MGs have been found to exhibit superior efficiency as HER catalysts, and Fe-based nanomaterials have previously been shown to be cost-effective and to offer good anticorrosion characteristics.17 MGs are typically synthesized via a

Figure 1. (a) Schematic diagram showing the main idea of the melt-spinning technique for preparation of glassy ribbons and the optical images of (b) the as-prepared MG ribbons, (c) a handwrapped ribbon sample around a pencil in a diameter of 10 mm, (d) a ribbon sample being bent by fingers and (e) a ribbon sample released from being bent 180 degrees by hand, indicating that it has significant plasticity against bending fracture. A schematic diagram of the melt-spinning process used to synthesize Fe40Co40P13C7 MG is shown in Figure. 1a. This same method is widely utilized in the production of Fe-based MGs.23 The as-prepared amorphous ribbons have a thickness of approximately 30 µm and a width of approximately 2 mm, as shown in

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Figures. 1b-e. These ribbons can be easily wrapped or bent into different shapes and exhibited the same exceptional mechanical properties, such as high tensile strength, as previously reported Fe-based MG ribbons.23 Room temperature tensile stress–strain curves indicate that the maximum strength of this material was 1245 ± 100 MPa (Figure S1, Supporting Information), demonstrating the excellent mechanical properties of these ribbons. The X-ray diffraction (XRD) data for this material in the asprepared state are shown in Figure 2a. This pattern exhibits a broad peak at a 2θ of approximately 44°. The ribbons are also assessed by differential scanning calorimetry (DSC) at a heating rate of 20 K min-1 and exhibited an obvious glass transition temperature (Tg) (Figure S2). Both the XRD and DSC results indicate that the as-prepared samples are composed of a fully amorphous phase.23 This amorphous state is further demonstrated by high resolution transmission electron microscopy (HR-TEM) images (Figure 2b) that show a typical amorphous appearance down to the nanometer scale without any evidence of crystalline particles.

work is ca. 46 mV dec-1, which is larger than 31 mV dec-1 of pure Pt plate but is comparable to many earth-abundant nanomaterials that usually synthesized by wet chemical methods (Table S1). Its comparable Tafel slope value demonstrates the more rapid kinetics of the MG and thus its good HER activity. The Tafel slope derived for the MG in this work is also lower than that of the catalyst materials noted above (Table S1). This result suggests that the present MG ribbons are slightly better HER catalysts than CoP/NTs, which has a reported Tafel slope of 60 mV dec-1.14 It is worth noting that the Tafel slope of the MG is even comparable to that reported for the noble metal-based Pd40Ni10Cu30P20 MG (58 mV dec−1).21 Detailed cyclic voltammograms were obtained from the amorphous ribbons in a non-faradic region at various scan rates from 100 to 500 mV s−1, as shown in Figure. S5. The electrochemical double layer capacitance (Cdl) of this material is determined to be 7.3 mF cm-2, which implies that the MG affords a larger electrochemically active surface area (ECSA) compared to amorphous nanosheets.16 Together, these results demonstrate that the as-prepared MG shows promise as a highly active catalyst.

Figure 2. XRD diffraction pattern (a) of the MG ribbon in the asprepared state and after the HER testing for 20 hours in the acid solution; (b) HRTEM results of the as-prepared MG sample, scale bars: 5 nm and 2 nm-1 (inset). The electrochemical activity of this MG was investigated in H2saturated 0.5 M H2SO4 using a three-electrode apparatus. The effect of solution resistance was compensated for by referencing all the potentials in this study to a reversible hydrogen electrode using the equation E (jRs corrected) = E – jRs, where j is the current density and Rs is the uncompensated electrolyte resistance determined via electrochemical impedance spectroscopy (Figure S4). The compensated linear sweep voltammetry (LSV) results obtained from a ribbon sample are presented in Figure. 3a. The cathodic current densities of the MG ribbons were 5, 10, 20 and 120 mA cm−2 at overpotentials of 103, 118, 132 and 166 mV, respectively (Figure. 3a, black curve). For comparison purposes, Figure. 3a also displays the HER activity of pure Pt, which is known to be a highly active electrocatalyst for the HER.24,25 At the same current density of 10 mA cm−2 (Table S1), the present MG produced lower over-potentials than those previously reported for the good HER catalysts CoP nanoparticles deposited on reduced graphene oxide sheets (CoP/RGO, 240 mV),13 CoP nanotubes (CoP/NTs, 130 mV),14 Ni5P4 films (140 mV),15 Cobaltembedded nitrogen-rich carbon nanotubes (Co-NRCNTs, 260 mV),26 NiCoFe layered double hydroxides supported on carbon fiber cloth (NiCoFe ALDHs/CFC, 200 mV)27 and CuCo2O4 nanowires (CuCoO-NWs, 140 mV).28 The overpotential value obtained from our sample is very close to that reported for a commercial Pt/C catalyst (108 mV),21 but superior to that generated by a Fe40Ni40P20 MG (193 mV).22 The Tafel slopes derived from the linear regions of the LSV polarization curves of the MG ribbon and Pt can be well fit using the Tafel equation (ղ = b log j + a, where b is the Tafel slope and j is the current density), as shown in Figure. 3b. The Tafel slope of the MG sample in the current

Figure 3. (a) LSV curves of the Fe40Co40P13C7 MG, after 2000 cycles testing, and Pt in H2-saturated 0.5 M H2SO4 solution. (b) Corresponding Tafel plots for the MG and Pt electrodes.

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Figure 4. j-t curves of the Fe40Co40P13C7 MG ribbon at a constant potential of -0.49 V (the corresponding current density is about 10 mA cm-2) and -0.53 V (the corresponding current density is about 100 mA cm-2) in H2-saturated 0.5 M H2SO4 solution. The stability of an HER catalyst is another important consideration. To assess stability, a current density versus time (j-t) plot was generated for the MG ribbons at a constant potential of -0.49 V (the corresponding current density is about 10 mA cm-2) and 0.53 V (the corresponding current density is about 100 mA cm-2) in 0.5 M H2SO4 over 20 h as shown in Figure 4. The ribbons show generally stable catalytic activity, indicating the superior durability of this material. The structure of the as-tested sample after 20 h is assessed by XRD and the resulting pattern is provided in Figure 2a. These data indicate that the glassy state of the as-prepared sample is retained, with no evidence of crystalline diffraction peaks, after testing. The LSV curve of the sample after 2000 cycles testing is provided in Figure 3a. The negligible difference in the electrocatalytic activity after 2000 cycles offers further evidence that the MG ribbons have excellent electrochemical stability. For comparison, a graphite rod was also used as the counter electrode for the current HER (Figure S13), it clearly indicates that there is no obvious difference between using Pt foil and graphite rod as counter electrodes. The chemical states of the constituent elements of the asprepared and tested samples were examined by high-resolution Xray photoelectron spectroscopy (XPS). The Fe 2p and Co 2p XPS spectra of these specimens are shown in Figure. 5. Peak deconvolution of the Co 2p 3/2 shows two main peaks at approximately 778.3 and 780.9 eV and two satellites at approximately 783.7 and 787.4 eV, respectively, while the Co 2p 1/2 shows two main peaks at approximately 793.2 and 796.9 eV and two satellites at approximately 790.6 and 802.9 eV (Figure. 5a). After the 20 h HER test, the two elemental Co peaks of the Co 2p 3/2 at approximately 778.3 eV and the Co 2p 1/2 at approximately 793.2 eV disappear (Figure. 5c).29 Similarly, two peaks resulting from elemental Fe (at 707.0 and 720.1 eV, Figure. 5b and d) disappear after the 20 h trial.27 The XPS results for C and P are presented in Figure S6. In this case, there are no obvious changes in chemical states within the experimental resolution. Therefore, the XPS results also support the high stability of the MG sample, with only minimal reduction of elemental Fe and Co.

provides chemical homogeneity on the disordered surface. To examine this theory, corresponding crystallized samples of the MG ribbon were also tested, with the results shown in Figures S7 and S8. It is evident that the electrochemical activity of the crystallized samples is greatly reduced, and their corrosion-resistance become poor because the corrosion potential is negatively increased (Figure. S8). Therefore, the good HER performance of the MG ribbon is likely to be associated with the disordered arrangement of the amorphous structure of this material. However, there are many high-performing crystalline HER catalysts (Table S1), including Co/Fe-based oxides,17,27 phosphides,13-16 and carbides.9,10 Considering the multicomponent composition of Fe40Co40P13C7 MG, it is very likely that the good HER performance of the present MG is due to a combination of the metastable state, a local chemical environment similar to that of a crystalline catalyst.22 On the one hand, the MG ribbons with the intrinsic chemical heterogeneity of amorphous structure display a wider distribution of free energy of adsorbed hydrogen (∆GH), which causes more hydrogen ions to be absorbed and provide abundant types of active sites.21 On the other hand, the synergistic effect of element Fe, Co, P, and C, can provide better electrocatalytic.17 Therefore, the present work might help provide a useful strategy for the design of high performance electrochemical catalysts based on MG alloys containing typical crystalline catalyst components or compositions. In summary, we designed and fabricated Fe40Co40P13C7 MG ribbons through a conventional industrial melt-spinning technique. These glassy ribbons exhibit a good HER activity very close to that of a standard commercial Pt/C, but are much less expensive owing to the simple preparation process and the use of earthabundant raw materials. These properties demonstrate that this glassy alloy has the potential for industrial applications. This work also suggests an effective approach to identifying new electrochemical catalysts based on metallic glass alloys.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.×××××××. Details on the fabrication process, their characterization and electrochemical measurements; supplementary experimental results (PDF)

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the national natural science foundation of China (No. 51322103 and 51571079), China MOST 973 Program (No. 2015CB856800), National Key Research and Development Project (No.2016YFB0300500) and the fundamental research funds for the central universities (No. JZ2016HGPB0671). This research was also supported by the China Postdoctoral Science Foundation (No.2016M602001).

Figure 5. High-resolution XPS spectra of Co 2P (a, c) and Fe 2p (b, d) for the as-prepared and 20h tested MG ribbons The HER activity of previously synthesized glassy nanomaterials has been ascribed to the metastable glassy state,19,20 which

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