Sulfur-doped di-cobalt phosphide outperforming precious metals as a

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Sulfur-doped di-cobalt phosphide outperforming precious metals as a bifunctional electrocatalyst for alkaline water electrolysis Mohsin Ali Raza Anjum, Mahesh Datt Bhatt, Min Hee Lee, and Jae Sung Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03908 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Chemistry of Materials

Sulfur-doped di-cobalt phosphide outperforming precious metals as a bifunctional electrocatalyst for alkaline water electrolysis Mohsin Ali Raza Anjum, †,‡ Mahesh Datt Bhatt, † Min Hee Lee† and Jae Sung Lee*†

†School

of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), 50 UNIST-gil, Ulsan, 44919 Republic of Korea.

‡Chemistry

Division, Directorate of Science, Pakistan Institute of Nuclear Science and

Technology (PINSTECH), P.O. Nilore, Islamabad, 45650 Pakistan.

Abstract

Metallic di-cobalt phosphide (Co2P) is doped with electronegative sulfur (S:Co2P) by using an economical and eco-friendly thiourea-phosphate assisted strategy. Density functional theory (DFT) calculation in conjunction with XPS reveals that S-doping

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decreases the electron density near the Fermi level to reduce the metallic nature of Co2P. Thus a more positive charge is induced onto Co to balance between hydride (Coδ+ Hδ-) and proton (S/Pδ- Hδ+) acceptors. As a result, it increases the number of active Co2+ sites as well as the turnover frequency of

a single site. The hybrid

electrodes obtained by loading S:Co2P nanoparticles on N-doped carbon cloth or nickel foam (NF) exhibit outstanding activity and stability of hydrogen and oxygen evolution reactions in alkaline electrolytes outperforming conventional, precious metal-based Pt/C and IrO2 catalysts and most of other state-of-the-art non-precious metal electrocatalysts reported so far. An alkaline electrolyzer with S:Co2P@NF as both cathode and anode produces a stable current densities of 100 mA/cm2 at 1.782 V, which is superior to IrO2Pt/C electrolyzer (1.823 V).

The electrolysis of water by using electricity generated from a renewable energy source is a sustainable way to secure hydrogen fuels to drive fuel cell vehicles.1-4 A significant challenge in the electrochemical water splitting is to replace the most active incumbent precious-metal-based electrocatalysts (Pt and IrO2/RuO2) with equally


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efficient non-precious metals for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively.5-7 A variety of cheaper earth-abundant materials have been explored for HER6,

8, 9

and OER.10-13 However, designing a practical

electrolyzer by pairing HER and OER active catalysts is not an easy task because of their mismatched stability and activity in electrolytes of widely different pH ranges. Therefore, there is a dire need to develop a highly active, stable and inexpensive bifunctional electrocatalyst performing well for both HER and OER in the same pH electrolyte. Recently, cobalt-based compounds like phosphates, chalcogenides, oxides/hydroxides and phosphides have been extensively explored due to their exceptional bifunctional water splitting catalytic activity and electrochemical stability in acidic and alkaline solutions.11,

14, 15

In addition, uniform grafting of Co-based

nanomaterials onto stable electrode materials such as nickel foam (NF) or carbon cloth (CC) reduces large overpotentials and enhances stability.16-18 Nanostructured mono cobalt phosphide (CoP) has been extensively studied last decade due to its high performance for both HER and OER in a wide pH range.19-21 However, metallic di-cobalt phosphide (Co2P) has not been explored that much 3 ACS Paragon Plus Environment


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because current synthetic methods cannot control the phase purity between CoP and Co2P.22 A few successful examples of Co2P synthesis employ solution-phase reactions involving highly toxic organic phosphines i.e. tri-phenylphosphine, tri-n-octylphosphine or tri-n-octylphosphine oxide as a P source in a high boiling solvent (e.g. oleylamine) in inert atmosphere23-26 or direct PH3 gas,27, different Co/P stoichiometries.22,

29

28

which produce a mixture of phases with

According to previous experimental and theoretical

studies, the metal to P ratio in transition metal phosphides plays an important role in water splitting as P acts as a proton acceptor and the metal acts as a hydride acceptor.30-32 Increasing the P content or metal-phosphorus bonds on the surface results in improved HER activity.24 Therefore, due to its low Co-P or more metal (Co) densities, Co2P requires more overpotentials as compared to CoP to generate the same current density.24 However, doping of more electronegative heteroatoms (N, S or O) in the metal-rich phosphides can improve their intrinsic HER/OER activity.27, 33, 34 Recently, similar approaches have been exercised to regulate HER/OER kinetics of Co2P by incorporating oxygen or surface oxidation of metallic cobalt phosphide27,

28

and sulfur

doping into mono cobalt phosphide (S:CoP) reported by our group.34 Due to lack of 4 ACS Paragon Plus Environment

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phase controllability, high toxicity and expensive precursors of currently available methods for phosphides synthesis, it is highly desirable to develop eco-friendly and economic synthetic strategy to grow Co2P nanocrystals for stable HER/OER electrodes with controlled phase/shape and electronic properties. Here we report a phase-pure Co2P with its electronic properties regulated by incorporating more electronegative sulfur into its structure (S:Co2P) by using our previously developed economical and eco-friendly urea-phosphate-assisted strategy applied for the synthesis of S:CoP.34 The dopant S induces more positive charge on metallic cobalt to create a balance between hydride (Coδ+ Hδ-) and proton (S/Pδ- Hδ+) acceptors requierd for the efficient catalysis of HER/OER as in CoP. Sulfur doping also increases the number of active Co2+ sites as well as the intrinsic activity per site or turnover frequency (TOF). The S:Co2P exhibits comparable HER and OER performances to those of S:CoP reported by our group.34 The density functional theory (DFT) study in conjunction with X-ray photoelectron spectroscopy (XPS) further confirms the decrease in density of states (DOS) near Fermi level due to S-doping, which reduces metallic character of Co2P. The S:Co2P nanoparticles (NPs) grafted on 5 ACS Paragon Plus Environment


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N-doped carbon cloth (NCC) or nickel foam (NF) as bifunctional OER/HER electrodes provide a current density of 100 mA/cm2 at a low potential of 1.782 V in an alkaline electrolyzer, which is superior to an IrO2-Pt/C electrolyzer (1.823 V) of the conventional precious metal-based electrodes as well as most of state-of-the-art, non-precious metal bifunctional electrocatalysts reported so far. The unique merits of our synthetic approach are two-folds; avoidance of the use of expensive and toxic organic phosphines in high boiling organic solvents or PH3, and easy control of the phase/shape of transition metal phosphides.

RESULTS AND DISCUSSION

Synthesis of sulfur-doped metallic di-cobalt phosphide nanoparticles. Ultrasmall-sized, sulfur-doped metallic di-cobalt phosphide (S:Co2P) NPs are synthesized by using indigenously formulated thiourea-phosphate, which liberates H2S gas and ureaphosphate in aqueous H3PO4 solution upon thermal decomposition.33,

34

When

Co(NO3)2·6H2O salt was mixed with an optimized amount of thiourea-phosphate, cobalt


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ions (Co2+) react with H2S and urea-phosphate under ion-assisted solvothermal condition to form a reduced S-incorporated Co-phosphate precursor, which is reduced in H2 at an optimized temperature of 600 oC to yield S:Co2P NPs. In order to grow these NPs directly on N-doped graphene (NG) or NCC, graphene oxide or oxidized carbon cloth was directly added into the starting reaction solution to obtain the S-doped Cophosphate@N-doped support precursor followed by reduction in H2 gas. The synthetic procedure is illustrated in Scheme 1 (see Experimental Section for detail). To prepare S:Co2P@NF, a nickel foam was dip-coated with a Co-phosphate precursor. Simple urea was used to prepare undoped Co2P NPs according to otherwise the same method.

Scheme 1. Schematic procedure to synthesize S:Co2P nanoparticles on a conductive support.



Structural characterization before and after HER/OER. The powder X-ray diffraction (XRD) reveals that a pure phase of orthorhombic Co2P nanocrystals (space group Pnma, ICDD No.: 01-089-3032) with an average size of ~18 nm have been successfully formed after H2 reduction at 600 °C as shown in Figure 1a. No significant change in bulk crystal structure of Co2P is detected in XRD pattern even after S-doping. The similar XRD behavior is observed for pure Co2P (Figure S1 of Supporting Information, SI), S:Co2P/NG, and S:Co2P/NCC except the (002) graphitic peak for NG and NCCsupported samples. Scanning electron microscopy (SEM) images (Figure 1b, c) of the as-synthesized S:Co2P and S:Co2P/NCC reveal closely interconnected 50-60 nm grainsized nanocrystals. The similar morphologies are also observed by SEM images of S:Co2P@NG, S:Co2P@NF (Figure S2), and S:Co2P/NCC (Figure S3). BET anaylysis was conducted for both Co2P and S:Co2P NPs, interstingly, the S:Co2P NPs have 2.02 times more mesoporous area than bare Co2P (43.1 m2/g) as shown in Figure S4.

XPS was conducted in order to understand the oxidation states of S:Co2P/NCC electrode before and after 20 h-long HER or OER. Due to higher electronegativity of S


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(2.44) than those of Co (1.70) and P (2.04), S-doping induces more positive charge on Coδ+ (0

Sulfur-doped di-cobalt phosphide outperforming precious metals as a

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