Toward High-performance and Low-cost Hydrogen Evolution Reaction

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Toward High-performance and Low-cost Hydrogen Evolution Reaction Electrocatalysts: Nanostructuring Cobalt Phosphide (CoP) Particles on Carbon Fiber Paper Shu Hearn Yu, and Daniel H.C. Chua ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02755 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Materials & Interfaces

Toward High-performance and Low-cost Hydrogen Evolution Reaction Electrocatalysts: Nanostructuring Cobalt Phosphide (CoP) Particles on Carbon Fiber Paper Shu Hearn Yu, Daniel H.C. Chua.* Material Sciences and Engineering Department, National University of Singapore. Singapore 117575. Supporting Information

Abstracts In this communication, we facily fabricated nanostructured CoP particles (150 to 200 nm) on carbon fiber paper (CFP) for hydrogen evolution reaction (HER) by a simple two-step process via a green route. In the first step, crystalline Co3O4 nanocubes (150 nm - 200nm) were loaded on CFP through a hydrothermal process at low temperature (120 o C). Interestingly, crystalline Co3O4 nanocubes with a size 150 ~ 200 nm exhibited different growth mechanism in contrast to the crystalline Co3O4 nanocubes with a size < 100 nm reported earlier. In the second step, these crystalline Co3O4 nanocubes were converted to catalytically active CoP particles through CVD phosphorization (denoted as CoP/CFP-H). Remarkably, CoP/CFP-H exhibited a low Tafel slope of 49.7 mV/dec and only required -2 overpotential of 128.1, 144.4 and 190.8 mV to drive geometric current density of -10, -20 and -100 mA cm , respectively. Besides, the CoP/CFP-H also demonstrated an excellent durability in an acidic environment under 2000 sweeps at a high scan rate (100 mV/s) and a 24-hour chronopotentiometry testing. For comparison, CoP was also fabricated through an electrodeposition method, followed by CVD phosphorization (denoted as CoP/CFP-E). It was found that the latter had exhibited inferior activity compared with CoP/CFP-H. The good performances of CoP/CFP-H are essentially due to the rational designs of electrode: (i) the applications of highly HER active CoP electrocatalyst (ii) the intimate contact of nanostructured CoP on carbon fibers and (iii) the large electrochemical surface area at electrocatalyst/electrolyte interface due to the large retaining of particles features after phosphorization. Notably, the intermediate Co3O4/CFP can serve a platform to develop other cobalt-based functional materials.

Keywords: Cobalt phosphide, Hydrogen Evolution Reaction (HER), Electrocatalyst, High-performance, Low-cost

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Introduction The idea of using hydrogen as an energy carrier to diversify the heavy dependence fossil fuel and 1

hence reduce carbon emission is compelling. Today, hydrogen is produced from water splitting, biomass 2

disintegration, steam methane reformation, coal gasification and others.

Industries have long been

relying on the steam methane reformation to generate H2 for synthesizing chemical commodities, such as ammonia via the Haber-Bosch process. However, the steam methane reformation cannot be served as the sustainable solution to mitigate the energy crisis due to the emission of by-product CO2 that causes the greenhouse effect. Hydrogen produced through water splitting, either through photoelectrochemical 3

method or electrolysis method, is an attractive technique for the H2 generation. The electrocatalysts are often employed in the cathode of electrochemical devices to ensure energy efficiency. At present, the state-of-art HER electrocatalysts for the water electrolysis are noble-based materials, such as the commercial Pt/C (20 wt%) which only requires an ultra-small overpotential to reach the onset potential for HER. Nonetheless, due to the high cost and limited resources, large-scale applications of Pt electrocatalyst are not economically viable. Therefore, it is imperative to look for earth-abundant and highperformance electrocatalysts to speed up the development of hydrogen economy, preferentially through a green route. Various classes of earth-abundant transition compounds are extensively explored and identified as promising candidates for HER electrocatalysts. These include transition metal phosphides 4, 5

(FeP, Ni2P, CoP, MoP, etc.),

6

transition metal carbides (WC, TaC, Mo2C, etc.), transition metal sulfides

7

(MoS2, WS2, CoS2 etc.) and transition-metal-carbon complexes (Co-N-C)

8, 9

and their hybrids. In

particular, cobalt phosphides (e.g., CoP, Co2P) have received intensive attention among non-noble metal electrocatalyst owing to their high intrinsic HER activity, abundance and excellent stability in both acidic and alkaline media.

5, 10, 11

Recently, Juan et al. identified the Co2P (P/Co 1: 2) is catalytically less active δ+

δ-

than CoP (P/Co 1: 1) ratio phase as the phosphorous in Co -P moieties is believed as the active sites 10

for HER.

Hence, it is important to selectively phase engineer and nanostructure CoP for HER

applications. To date, the CoP can be fabricated mainly through reacting the prepared Co precursors with trioctylphosphine through hydrothermal or CVD phosphorization of the cobalt-based precursor. Unit recently, Li et al. reported a novel fabrication of CoP by pyrolyzing phytic acid cross-linked metal complex through


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calcination.

12

Due to the intrinsic semiconducting properties of cobalt phosphide and the lack of active

sites in bulk structure, two strategies are often used to maximize the electrocatalytic performance of CoP in HER: (i) nanostructuring CoP with different morphologies through chemical routes or template method, 13

mediating with some organic chemicals, such as DMF. For instance, Wang et al., 16

and Hu et al.

14

Sun et al.,

15

Ai et al.

have employed urea in hydrothermal synthesis as the structure-directing reagent to 17

promote the anisotropic growth of cobalt into the urchin-like structure or nanorod structure. Jiang et al., 18

Sun et al.

19

and Li et al.

successfully utilized metal-organic framework ZIF-67 as the template to grow

regular crystalline CoP with polyhedral structures; (ii) coupling the cobalt phosphide with highly conducting carbon-based supporter. For example, Sun et al. decorated CoP nanoparticle on carbon nanotubes;

20

Dong et al. fabricated CoP on 3D graphene aerogels;

on reduced graphene oxide sheets.

22

21

Chen et al. loaded CoP nanoparticle

Although these works successfully controlled the morphologies of

cobalt phosphides, the processes involved the employment of toxic organic chemicals that are not favorable for large-scale production (Table S1). In addition, the preparation of carbon substrates, such as graphene by modified Hummer’s method

22

or carbon nanotubes (CNTs) by CVD,

11, 20

inevitably increases

the total cost. Therefore, it is timely to develop a green and simple pathway to nanostructure CoP directly on a pristine catalyst supporter without compromising the activity. In this work, we report our efforts to nanostructure and grow CoP particles directly on carbon fiber paper (CoP/CFP) through a two-steps process via a green route without applying any toxic organic chemicals. In the first step, crystalline cobalt oxide (Co3O4) particles with size 150nm ~ 200nm were o

loaded on carbon fiber paper through a simple hydrothermal process at low temperature 120 C. Interestingly, we managed to control the size of these crystalline Co3O4 nanocubes and load it on other substrates (Figure S1). In the second step, these crystalline Co3O4 nanocubes particles were phosphorized to catalytic active cobalt phosphide (CoP) through a CVD process (CoP/CFP fabricated by a hydrothermal process, denoted as CoP/CFP-H). The schematic diagram of fabricating CoP/CFP-H is illustrated in Figure 1 (a). The CoP/CFP-H has a small Tafel slope and exhibits an excellent HER activity in 0.5 M H2SO4 with good durability. With this synthesis approach, complicated geometry and large-scale production of CoP/CFP electrode can be easily achieved (Figure S2). Notably, the Co3O4/CFP can serve as a platform to develop other cobalt-based derivatives catalysts, such as CoS2, CoSe2, and ternary



CoPS. For comparison purposes, an electrodeposition method was also employed to grow cobalt precursor on carbon fiber paper with same mass loading followed by phosphorization (CoP/CFP fabricated by an electrodeposition process, denoted as CoP/CFP-E) (Figure S3). With a different morphological feature, the CoP/CFP-E shows an inferior activity in HER. Therefore, large-scale fabrication of high-performance and low-cost HER electrocatalysts can be possibly achieved by CoP/CFP designed in our work.

Experimental section Chemicals and materials. Cobalt (II) acetate tetrahydrate (99.999% trace metals basis), Cobalt (II) sulfate heptahydrate (ReagentPlus®, > 99%), Phosphorous, red (>99.99% trace metals basis) were purchased from Alfa Aesar. All chemicals were used as received without further purification. Hydrothermal synthesis of cobalt oxide nanoparticles on carbon fiber paper. In a typical synthesis, 0.20 g of cobalt acetate tetrahydrate was dissolved in 100ml of isopropyl alcohol solution (50 Vol% : 50 Vol% = IPA: de-ionized water) and magnetically stirred for 20 minutes. A piece of carbon fiber paper (1.5 cm × 6 cm) was immersed in the transparent pink solution and transferred to an autoclave with 120 mL o volume of Teflon. The hydrothermal oven was heated from room temperature to 120 C at a slow heating o rate (2 C/min) for 15 hours. Then, the autoclave was taken out and cooled down naturally to ambient conditions. The carbon fiber paper was then rinsed carefully with de-ionized water and ethanol several o times before it was dried in a vacuum oven at 50 C overnight before further characterization. Electrodeposition of cobalt on carbon fiber paper. In a typical synthesis, 1.0 g of cobalt sulfate heptahydrate was dissolved in 150 mL of de-ionized water and magnetically stirred for 20 minutes. The obtained transparent pink solution was then transferred to a three-electrode system. The electrodeposition was performed at a fixed potential -1.0 V (vs. Ag/AgCl) with carbon fiber paper, 3.0 M Ag/AgCl electrode, and Pt foil serve as the working electrode, a reference electrode, and a counter electrode, respectively. The amount of cobalt loading can be controlled by the cut-off coulomb. After finish, the carbon fiber paper was rinsed carefully with de-ionized water and ethanol several times before it was o dried in a vacuum oven at 50 C overnight before further characterization. Phosphorization of cobalt precursor cobalt phosphite by CVD. The solid/gas-phase phosphorization of cobalt precursors was performed using a CVD. The 0.6 g of red phosphorus was put in the upstream position of the quartz tube (3~5 cm away from the heating zone) and the cobalt-loaded CFP is placed at the center of the furnace. The furnace was heated up to 500 °C for 1 hr for phosphorization with Ar gas as the carrier gas at 100 sccm before it was cooled down with the assistance of a cooling fan. Material characterizations. The morphologies of the electrode were probed by SUPRA 40 field-emission scanning electron microscopy (FE-SEM, ZEISS, Germany). The near-surface chemical information was investigated by using X-ray Photoelectron Spectroscopy (XPS, AXIS Ultra DLD) with an Al Kα X-ray


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source (1486.7 eV). Adventitious C 1s peak (284.8 eV) was used as the calibration to compensate peak shift due to charging effect. The structures of samples were characterized by thin Film XRD (Bruker D8 Advanced).The weight loadings of samples were measured by a high-precision electronic balance (BM20) before and after synthesis and divide by their respective geometric areas. Electrochemical characterization. Electrochemical measurements were conducted in a typical threeelectrode system using Autolab PGSTAT302N potentiostat/galvanostat (Metrohm), fitted with a FRA2.V10 Frequency response analyzer and a SCANGEN analog scan generator in 0.5 M H2SO4. A concentration of 3.0 M KCl Ag/AgCl electrode and non-platinised Pt rod were used as the reference electrode and counter electrode, respectively. The reported potential was iR-corrected and converted to the reversible hydrogen electrode (RHE) by using the following relationship E(RHE) = E(Ag/AgCl) + 0.21V – iR . HER polarization curve and Tafel plot: The polarization curves were recorded after initial stabilization, typically at 30th cycle by linear potential sweeps from 0 V to -0.5V (vs. RHE) at a scan rate of 5mV/s. Nyquist plot: the Nyquist plot was evaluated at -0.15V (vs. RHE) between 10 kHz and 0.05 Hz with a 5mV AC potential perturbation. Cyclic voltammogram: The cyclic voltammetry measurement was performed in a potential window between 0 and 0.3 (vs. RHE) where no faradic current flows at various scan rate (10, 20, 50, 100 mV/s). Stability Test: The stability test was performed at 100 mV/s scan rate from 0 to -0.5V (vs. RHE) for 2000 linear potential sweeps in the same three-electrode cell configuration. -2 The chronopotentiometry measurement was performed at constant current at -10 mA cm for 24 hours. The faradaic efficiency was determined by collecting the evolved gas by using an inverted measuring -2 cylinder at -100 mA cm and compared with the theoretical value.

Results and discussion The morphological features of CoP/CHP electrodes were investigated by Field-emission scanning electron microscopy (FE-SEM). It was observed that the cobalt precursors were conformally loaded on carbon fiber paper after the hydrothermal synthesis. The high-magnification SEM image further reveals that the cobalt precursors have the cubic structure with smooth surface and size ranging from 150 to 200 nm (Figure S4). Thermal phosphorization of these cubic cobalt precursors resulted in more rounded and rough morphology, with the particle features largely retained (Figure 1 b, c). The EDX mapping of CoP/CFP-H indicates the uniform elemental dispersion of cobalt and phosphorus on carbon fiber (Figure S5). Further elemental analysis of Co/P ratio was 47.12%: 52.88% close to atomic ratio 1: 1. In addition, the TEM image further reveals the nanostructured CoP particles CoP/CFP-H indeed has a more rounded shape after phosphorization (Figure 1d). The HRTEM image shows uniform lattice fringes with an interplanar spacing of 0.25 nm corresponding to (200) plane, in agreement with the XRD spectrum (Figure 1d). On the other hand, the cobalt precursor prepared through electrodeposition exhibited open-



Page 6 of 18

flake structure. Upon phosphorization, the structure altered drastically and formed large piece non-porous layer structures (Figure 1 f,g ).The EDX mapping of CoP/CHP-E is provided in supplementary document (Figure S6). This observation suggests that the nanostructured cobalt precursor prepared by the hydrothermal method is more robust for subsequent chemical conversion via heat treatment. The structural characterizations of as-prepared samples were performed by X-ray diffraction (XRD). From the XRD result, we found that cobalt oxide (Co3O4) were grown on carbon fiber paper through hydrothermal synthesis while metallic cobalt was loaded on carbon fiber paper through electrodeposition (Figure S7). In the subsequent gas/solid phosphorization process, cobalt oxide (Co3O4) and metallic cobalt were successfully converted into CoP. The most intense XRD peaks result from the o

o

o

underlying carbon fiber paper at (42.9 , 54.4 , and 77.6 ). The overlying CoP sample on CFP shows o

o

o

o

o

o

peaks at around 31.6 , 35.5 , 36.3 , 46.3 , 48.8 and 57.4 , corresponding to (011), (200), (111), (112), (211) and (103) planes of orthorhombic structure in agreement of previous works. These strong peaks well match the CoP reference (ICDD 00-029-0497), indicating the successful preparation of high-purity nanocrystalline CoP products (Figure 2 a). The near-surface chemical information was probed by X-ray photoelectron spectroscopy (XPS). Adventitious C 1s peaks (284.8 eV) was used as the reference to calibrate any peaks shifts arises from charge effect during measurement. The survey scan of XPS spectrums indicates the presence of Co, P, C and O elements (Figure S8). High-resolution scan shows the peak binding energies of Co 2p3/2 and P 2p3/2 were 778.7eV and 129.4 eV, respectively, matching the metal-phosphide CoP structure. Compares with Co(0) (778.2 eV) and P (0) (130.3 eV) from NIST XPS database,

23

the Co and P in CoP structure has positive and negative binding energy shift, respectively. δ+

This suggests that P element draws electron density from Co element in CoP compounds forming Co δ-

P moieties. On thermodynamically stable CoP (111) surface, the Co-bridge sites and P top sites are 24

believed to have HER activity based on DFT calculations.

Interestingly, in addition to the phosphorous

peak attributed to CoP, there is an addition sharp peak at 139.5 eV. This is assigned to the phosphate 3-

group (PO4 ) as the CoP is inevitably oxidized in atmosphere.

25

Interestingly, though the crystalline Co3O4 nanocubes with a size ranging from 10 nm to 100 nm have been reported extensively,

26-30

the crystalline Co3O4 nanocubes with a size larger than 150 nm is not

well known. The nucleation and growth process of crystalline Co3O4 nanocubes are briefly discussed here.


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In the first step, the cobalt acetate tetrahydrate is dissolved in an aqueous solution, forming hydrated cobalt (II) ions. These hydrated cobalt (II) ions react with the hydroxyl group from isopropyl alcohol or de30

ionized water to form cobalt (II) hydroxide at elevated temperature.

Subsequently, the cobalt (II)

hydroxide form small crystalline Co3O4 nuclei on carbon fiber through heterogeneous nucleation induced by high hydrothermal pressure. Due to the high surface energy, these crystalline Co3O4 nuclei grow quickly on carbon fiber. ∙ 4 + 2 → [ ] + 2 + 2 → + → 2 + 6 In a recent study, Ji et al. investigated the intermediate Co3O4 nanocrystals by tilted TEM and proposed that the growth mechanism of crystalline Co3O4 nanocubes (d < 100 nm) proceed through sequential "surface wrapping" mechanism.

31

In the "surface wrapping" model, the growth of Co3O4 nanocubes is

accomplished by lateral growth of one surface through the migration of adatoms. Once this surface is fully covered (wrapped), the adatoms land on another surface to ensure the isotropic growth of Co3O4 nanocubes. Here, we found that crystalline Co3O4 nanocubes with size above 150 nm exhibited different growth mechanism from high-resolution SEM. Instead of sequential growing of Co3O4 plane on each surface, the growth of nanocrystals involved a secondary nucleation on the surfaces. Once the primary plane on a Co3O4 nanocube is stabilized, the cobalt-based adatoms can land this level and create a secondary plane that spreads out laterally. Hence, the rate of growth normal to the Co3O4 nanocrystal is essentially determined by the surface nucleation and lateral growth. The multilayer growth of Co3O4 planes on the surface of crystalline Co3O4 nanocubes (d > 150 nm) is schematically illustrated below. The arrows show the advancing direction of small crystal planes (Figure 3). To investigate the catalytic activity of the as-prepared electrode toward HER, the electrochemical characterization was performed in 0.5 M H2SO4 electrolyte using a typical three-electrode system with a sweep rate 5 mV/s at room temperature. The HER polarization curves were recorded after initial th

stabilization, typically at 30

LSV measurement. As expected, the pristine CFP showed almost-zero

current density in the potential window from 0V to -40 mV, showing no HER electrocatalytic activity. The



Page 8 of 18

non-platinized Pt rod exhibited superior HER catalytic activity, which only requires only 50.5, 58.7 and -2

85.5 mV to reach a geometric current density of -10, -20 and -100 mA cm , respectively. With a mass -2

loading of only 0.30 mg cm , The CoP/CFP-H electrode only required electrode required 128.1, 144.4 -2

and 190.8 mV to reach a geometric current density of -10, -20 and -100 mA cm , respectively, showing excellent HER catalytic activity. On the other hand, by controlling the cut-off coulomb in potentiometry mode (1.0V vs. Ag/AgCl) for electrodeposition, the open-flake structure cobalt-based precursors were successfully loaded on CFP. Unlike the CoP/CFP-H, these open-flake structures in CoP/CFP-E collapsed and aggregated to bulk sheets upon conversion to CoP by heat treatment. Within the testing potential -2

regimes, CoP/CFP-E did not reach a geometric current density of -100 mA cm .The CoP/CFP-E -2

electrode needed 234.4, 269.8 and 372.6 mV to drive -10, -20 and -80 mA cm , respectively, showing an inferior activity (Figure 4 a). It is well-established that the Tafel slope reflects the intrinsic property of a catalyst. Here, we extract kinetics parameters of the electrodes by converting the HER polarization to Tafel plot (overpotential versus log | j |). Assumedly, three steps are involved in HER in acidic conditions: +

-

+

Volmer step (H3O + M + e → H2O + Had-M), Tafel step (2Had-M → H2 + M), and Heyrovsky step (H3O + -

Had-M + e → H2O + H2 + M). For a complete HER process, either Volmer-Tafel or Volmer-Heyrovsky mechanism is involved. Theoretically, Tafel slopes with 30, 40 and 120 mV/dec suggest the rate7

determining step in HER is Tafel step, Heyrovsky step, and Volmer step, respectively. After fitting the linear regions of Tafel plot (η = b log j + a), Tafel slopes of 31.7, 49.7, and 128.5 mV/dec were derived for Pt rod, CoP/CFP-H and CoP/CFP-E electrode, respectively (Figure 4 b). In agreement with previous reports, the Tafel slope of Pt rod in our work, closes to 30 mV/dec, suggesting Volmer- Tafel mechanism was involved in HER process. The Tafel slope of CoP/CFP-H electrode (49.7 mV/dec) is about 2.6 times smaller than CoP/CFP-E (128.5 mV/dec), suggesting a higher catalytic performance of CoP/CFP-H electrode. The Tafel slopes of two CoP/CFP electrodes differ much from the theoretical values of ratedetermining HER mechanisms. This phenomenon could be explained by the non-purely interfacial charge-transfer controlled process, that is, the electron transport in CoP electrocatalyst still limits the interfacial kinetic of electrolytic H2 evolution.

32

Although the Tafel slope can be theoretically be reduced

further, the durability of the electrode is often traded off. Hence, it remains challenging to fabricate high electrocatalyst with excellent durability. The HER performances of CoP/CFP-H synthesized are compared


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favorably with many other non-noble catalysts with complex fabricating processes. Notable examples include CoP decorated on reduced graphene oxide (RGO) (Tafel slope of 104~149 mV/dec, -2 22

overpotential > 200 mV to reach -10 mA cm ),

in situ coupling CoP-CNTs (Tafel slope of 52 mV/dec,

-2 11

overpotential of 139 mV to reach -10 mA cm ),

CoP nanocrystal on CNT(Tafel slope of 54 mV/dec,

-2

20

overpotential of 122 mV to reach -10 mA cm ) (Table S1).

Interestingly, although the overpotential of -2

CoP/CFP-H required to reach a geometric current density -10 mA cm is higher than self-supported CoP nanowires (Tafel slope of 51 mV/dec, overpotential of 67,100, and 204mV to drive -10, -20 and -100 mA -2

cm , respectively),

33

the CoP/CFP-H only needs 191 mV to provide a geometric current density of -100

-2

mA cm due to its low Tafel slope. Admittedly, some of these catalysts are more active than the CoP/CFP-H, their preparation processes are not environmentally friendly as some toxic chemicals are -2

-2

used (Table S1). Besides, the exchange current density of CoP/CFP-H (2.45 × 10 mA cm ) is only one -1

order lower than Pt rod (2.4 × 10

-2

mA cm ), and is significantly higher than many other non-noble

transition metal compounds, such as crystalline MoS2, WS2, whose exchange current densities are -4

-6

-2 34-36

typically in the order of 10 ~10 mA cm .

Therefore, the rational design of CoP/CFP-H electrode can

serve as one of the most catalytically active and efficient alternatives to replace Pt electrode in acidic media. Additionally, the evolved gas from the CoP/CFP-H electrode was collected and the Faradaic efficiency was determined ~ 100% (Figure S9).The high catalytic HER performances of as-synthesized CoP/CFP-H electrode is essentially attributed the rational electrode design. In the first step, the crystalline cobalt oxide (Co3O4) nanocubes were loaded on the highly conductive CFP by the hydrothermal process, ensuring intimate electrical contact and short electron transport pathway. In the second step, the anchored cobalt–based precursor was thermally phosphorized to CoP, for which highly catalytically active sites were created. Relative to the mechanical attachment by using binders, the direct coupling of nanostructured CoP and CFP is more beneficial to electron transfer. The rational design of nanostructured CoP particles on carbon fiber paper reduces electron transport pathway in the semiconducting bulk CoP. The equivalent electrical circuit was modeled at overpotential η = 100 mV for further electrochemical analysis. It was found that both CoP/CFP-H and CoP/CFP-E electrodes can be modeled by a simple Randles circuit, consisting of series resistance (Rs), constant phase elements (CPE) and



Page 10 of 18

charge-transfer resistance (Rct) as shown in the inset (Figure 4 c). The charge-transfer resistance can be calculated from the diameter of the semicircle with a small value indicating fast interfacial kinetics. After curve fitting, the charge-transfer resistance of CoP/CFP-H (6.5 Ω) was significantly smaller than CoP/CFP-E (35.8 Ω), indicating better HER catalytic activity in CoP/CFP-H electrode, in agreement with the results of HER polarization curve and Tafel plot. We further examine the electrochemical surface area of CoP/CFP electrodes by evaluating the double-layer capacitance. A cyclic voltammogram was performed at various scan rate (10, 20, 50, 100 mV/s) in a potential regime (0 ~ 0.3 V vs. RHE) where no observable Faradic reaction occurs (Figure 4 d).

The cyclic voltammogram shows various pseudo-

rectangular shapes with no Faradic current peaks, suggesting only capacitive current was measured in this potential window. To extract the double-layer capacitances of CoP/CFP electrodes, the average value of anodic and cathodic current was taken at 0.15 V (vs. RHE) for each CV curve and is plotted as a function of various scan rate. The double-layer capacitance can be obtained from the slope of the plot by the following relationship 0.5|ja-jc| = Cdlν. It was found that the electrochemical surface area of CoP/CFP-H -2

-2

(2.29 mF cm ) was approximately 3 times higher than the CoP/CFP-E (0.71 mF cm ) (Figure 4 e). Hence, the excellent HER activity of CoP/CFP-H over CoP/CFP-E could be due to good electrical contact of CoP on CFP, the short electron transport distance in electrocatalyst, and large electrocatalyst/electrolyte interface. Stability is also an essential parameter for evaluating HER electrocatalyst. Here, we examine the overall durability of CoP/CFP electrodes by a continuous linear potential sweep at fast scan rate (100 mV/s) from 0 V to - 0.5V (vs. RHE) to model the switch-on and switch-off conditions of electrolyzer or in practical applications. Before stability test, the CoP/CFP-H electrode requires the non-iR corrected -2

-2

potential of 141 and 340 mV to drive a geometric current of -10 mA cm and -100 mA cm , respectively. th

-2

-2

After 2000 LSV scans, the overpotential required to reach -10 mA cm and -100 mA cm increases to 149 mV and 353 mV. On the other hand, before stability test, CoP/CFP-E requires non-iR corrected -2

-2

overpotential 250.5 mV and 463.0 mV to reach geometric current density of -10 mA cm and -80 mA cm , respectively; after stability test, the CoP/CFP-E electrode requires 252.7 mV and 494.0 mV to drive -2

-2

geometric current density of -10 mA cm and -80 mA cm , respectively. The increase of non-iR corrected overpotential in CoP/CFP-H to drive the respective geometric current density of interest are considerably


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small after such vigorous H2 bubbling; we consider this is an indicator of good stability. Furthermore, the long-term stability test of CoP/CFP-H electrode was conducted by chronopotentiometry for 24 hours (Figure S10). The CoP/CFP-H electrode was able to maintain its activity for 24 hours at 128 mV with small potential fluctuation. Additionally, the XRD result clearly demonstrates the characteristics peak of CoP after stability test (Figure S11). Many reports suggest that the phosphorous in CoP is the catalytically active site for HER owing to the negatively charging which facilitates the absorption of the protons. To examine this postulation experimentally, we further study the influence of HER catalytic current upon introducing thiocyanate ions -

-

37

(SCN ) ions. It is well known that (SCN ) ions can poison the metal sites in acidic condition.

As shown in

-

the (Figure S12) upon the addition of 10 mM SCN ions, the current density of CoP/CFP-H decreased -2

st

-2

th

slightly from 18.5 mA cm (1 LSV) to 17.1 mA cm (10 LSV) at -0.25V , which suggests 7.6% of cobalt -

δ+

δ-

site is blocked by the SCN ions. This observation indicates that the cobalt in Co -P moieties indeed plays a less central role in HER, which could be due to the partial positive charge of cobalt site that leads δ+

δ-

to poor electrostatic affinity. However, the cooperative function of Co -P moieties is necessary for HER as the phosphorus in the moieties have optimal binding with hydrogen intermediate while the cobalt -

provides the electronic conducting property. Taking a closer look, interestingly, on introducing SCN ions, -2

the first few LSV curve shows a considerable cathodic current (−5 mA cm ) at overpotential close to zero. -

This cathodic current is attributed to the absorption of SCN ions on Co metal sites rather than the -

catalytic HER, which reduces slowly toward zero current density since the absorption of the SCN ions on Co metal sites is approaching saturation. This indirectly reminds us that the cathodic current recorded near zero potential may be attributed to some side reactions, such as absorption of impurity and chemical reduction of catalyst, other than HER. Therefore, those authors reported an ultralow onset overpotential in their work should be extra cautious. To conclude, we have successfully fabricated CoP particles (150-200 nm) on three-dimensional carbon fiber paper through a hydrothermal process via a green route followed by CVD phosphorization without using toxic organic chemicals. The high catalytic and excellent durability of CoP/CFP-H is very



promising for next-generation HER electrocatalyst. Notably, this two-step strategy can be scaled up easily and even develop other cobalt-based derivative functional materials.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: xxxxxxxx. Figures S1-S12 are the additional details on sample characterization and electrochemical characterizations. Table S1 lists out the performances of some non-noble catalyst and their synthesis methods.

Author Information *Corresponding Author: Daniel H.C. Chua. Tel: +65 6516 8933, E-mail: [email protected] (D.H.C.C.) Notes The authors declare no competing financial interest.

Acknowledgments S.H.Y and D.H.C.C., acknowledge the support from Ministry of Education and the National University of Singapore (Grant numbers: R284-000-142-112, R284-000-150-112). S.H.Y would like to acknowledge NUS Research Scholarship funding.


Page 12 of 18

Page 13 of 18 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


(a)

(b)

(c)

(d)

(e)

(f)

(g)



(a)

30

*

*

*

CoP/CFP-E CoP/CFP-H CFP

*

* *

* ICDD 00-029-0497 35

40

45

50

55

60

2 Theta

65

70

75

Intensity (a.u.)

Figure 1. (a) Schematic illustration of the synthesis procedure of CoP particles on CFP by a two-step process. (b) & (c) medium and high magnification SEM image of CoP/CFP-H, respectively. (d) & (e) medium and high magnification of TEM. The inset shows the selected area diffraction pattern (SAED). (f) & (g) medium and high magnification SEM image of CoP/CFP-E, respectively.

Intensity (a.u.)

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

Page 14 of 18

80

(b)

Co 2p 3/2

Exp data P 2p 3/2 P 2p 1/2 P-O

Co 2p 1/2

129.4

130.2

139.5

Satellite

810 800 790 780 770 760

134

132

130

128

Binding energy (eV)

Figure 2. (a) XRD of CoP/CFP electrodes; peaks mark with an asterisk “*” correspond to the CFP substrate. (b) Narrow scan XPS spectrums of CoP/CFP-H.

Figure 3. The schematic diagram of the growth mechanism of crystalline Co3O4 nanocubes with a size ranging from 150 nm to 200 nm. The arrows show the advancing direction of small crystal planes.


Current density (mA/cm 2)

10 0 -10 -20 234.1 mV 128.1 mV 50.4 mV -30 -40 -50 -60 -70 CFP -80 CoP/CFP-H -90 CoP/CFP-E -100 -110 Pt rod -120 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00

(a)

0.35

CoP/CFP-H CoP/CFP-E Pt rod

0.30 0.25

(b) 128.5 mV/dec

0.20 0.15

49.7 mV/dec

0.10 0.05

31.7 mV/dec

0.00 -1.5

Potential (V vs. RHE)

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

log (| Jgeo (mA cm-2)|)

25

ηcath = 100 mV

CoP/CFP-H CoP/CFP-E Fitting curve

(c)

Current density (mA cm -2)

-Z''(Ω Ω)

20

15

10

Rct Rs

5 CPEdl

0

0

5

10

15

20

25

30

35

100 mV/s 50 mV/s 20 mV/s 10 mV/s

1.2 1.0 0.8

0.4 0.2

Jc

0.0 -0.2 -0.4

40

0.00

0.20

10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120

(e)

Current density (mA/cm 2)

0.25

2.29 mFcm-2

0.15

0.71 mFcm-2 0.10

0.05

0

20

40

60

Ja

0.05

0.10

0.15

0.20

0.25

0.30

Potential (V vs. RHE)

CoP/CFP-H CoP/CFP-E Fitting line CoP/CFP-H Fitting line CoP/CFPE

0.30

(d)

0.6

Z' (Ω)

(Ja-Jc)/2

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


Overpotential (V)

Page 15 of 18

80

Scan rate (mV/s)

100

(f)

CoP/CFP-H initial CoP/CFP-H after 2000th CV CoP/CFP-E initial CoP/CFP-E after 2000th CV -0.5

-0.4

-0.3

-0.2

-0.1

0.0

Non-iR corrected potential (V vs. RHE)



Figure 4. (a)HER polarization curves, (b) Tafel plot of CoP/CFP electrodes and non-platinized Pt rod in 0.5 M H2SO4 at 5 mV/s. (c) Nyquist plot of CoP/CFP electrodes at overpotential 100 mV, with the equivalent electrical circuit as the inset. (d) CV scans of CoP/CFP-H between 0 and 0.3 V (vs. RHE) at various scan rates. (e) A plot of 0.5(Ja-Jc) against scan rate. (f) Non-iR corrected stability test of CoP/CFP electrodes at 100 mV/s for 2000 cycles.

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Toward High-performance and Low-cost Hydrogen Evolution Reaction

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