Interconnected Hollow Cobalt Phosphide Grown on Carbon

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Interconnected Hollow Cobalt Phosphide Grown on Carbon Nanotubes for Hydrogen Evolution Reaction Alaaldin Adam, Munzir H. Suliman, Mohammad N. Siddiqui, Zain H. Yamani, Belabbes Merzougui, and Mohammad Qamar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03427 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Materials & Interfaces

Interconnected Hollow Cobalt Phosphide Grown on Carbon Nanotubes for Hydrogen Evolution Reaction

Alaaldin Adam,†§ Munzir H. Suliman,†§ Mohammad N. Siddiqui,§ Zain H. Yamani,† Belabbes Merzougui,‡ Mohammad Qamar* †

†

Center of Excellence in Nanotechnology (CENT), §Department of Chemistry, King Fahd

University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia. Fax: (+) 966 13 860 7264 ‡

Qatar Environment & Energy Research Institute, Hamad Bin Khalifa University, Qatar Foundation, Doha 5825, Qatar.

*Corresponding author: [email protected]

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ABSTRACT – Transition metal phosphides are deemed as potential alternative to platinum for large scale and sustainable electrocatalytic hydrogen production from water. In this study, facile preparation of interconnected hollow cobalt monophosphide (CoP) supported on carbon nanotubes is demonstrated, and evaluated as a low-cost electrocatalyst for hydrogen evolution reaction (HER). Hexamethylenetetramine is used as a structure-directing agent to guide the formation of interconnected cobalt oxide, which further grows into interconnected hollow CoP. Interconnected and hollow microstructural artifacts impart benign attributes, such as enhanced specific and electrochemically active surface area, low intrinsic charge transfer resistance, high interfacial charge transfer kinetics and improved mass transport, to the electrocatalyst. As a result, as-prepared electrode exhibits remarkable electrocatalytic performance – low onset (18 mV) and overpotential (η10 = 73 mV), small Tafel slope (54.6 mVdec-1), and high turnover frequency (0.58 s-1 at η = 73 mV). In addition, the electrode shows excellent electrochemical stability. KEYWORDS: nanostructures, electrocatalysts, noble metal free, transition metal phosphide, water electrolysis, energy conversion INTRODUCTION The prospects of global energy demand and adverse climate change are driving scientists to explore sustainable and eco-friendly sources of energy. In this context, molecular hydrogen (H2) is perceived as one of the most potential future energy carriers, owing to its highest gravimetric energy density.1 In addition, use of H2 as an energy carrier is environmentally benign – produces only water as the by-product. Currently, large scale production of H2 is achieved through steam– methane reforming (CH4 + H2O ⇌ CO + 3H2). This is an energy intensive process and consumes fossil fuel and, more importantly, adds to CO2 emission through water – gas shift reaction (CO + H2O ⇌ CO2 + H2). One of the potential pathways, based on carbon-free footprints, to generate a high - flow of hydrogen is through electrocatalysis.2-5 However, the technological development of water electrolysis to achieve a high flow rate of hydrogen is essentially decelerated by the requirement of high overvoltage. Development of a catalyst that can lower the overpotential for hydrogen generation could contribute substantially in the overall electrocatalytic process. Currently, platinum (Pt) is known as a state-of-the-art electrocatalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). However, due to high cost and its 2 ACS Paragon Plus Environment

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instability in the electrochemical power systems, development of Pt-free electrode materials is central to electrochemical energy conversion devices, such as photoelectrochemical cells, electrolyzers etc. As a result, a variety of electrode materials comprising non-precious compositions, such as carbides,6-9 sulfides,10-13 nitrides,14,15 phosphides,16-20 and borides,21,22 of transition metals, have been explored. The activation potential can be substantially lowered at first hand by choosing the appropriate elemental composition of electrocatalyst. To this end, metal phosphides are considered as potential substitute of Pt due to their low-cost, high activity and excellent stability under highly acidic solution.16,17 Among metal phosphides, cobalt phosphides has received significant attention.23-30 A number of approaches are pursued to produce active electrodes comprised of CoP. This includes the preparation of CoP as nanosheet arrays on titanium mesh,31 nanowires arrays on carbon cloth,32 films on carbon cloth,33 nanocrystals on CNTs,34 nanotubes,35 porous and interconnected nanowires.36 Yet, intrinsically low electronic conductivity, deficiency of active sites and poor mass transport property remain as the major challenges in practical application of CoP.37 These limitations can be addressed by designing nanostructured catalysts endowed with favorable features. It has now been established that achieving control of the architecture of nanoscaled materials could lead to the development of new materials and systems with enhanced chemical properties. The potential for success lies in appropriate tailoring and engineering of the nanomaterials and their resulting devices for a given application. To this end, owing to high porosity, large surface area and most accessible active sites, low mass density, efficacious mass transport (diffusion of reactants and products), and fast electrical and ionic transport, hollow nanostructures hold great potential for a variety of technological applications.38 Owing to the presence of more exposed active sites per unit geometric area compared to conventional bulky particles or thin films,39,40 electrocatalysts with hollow morphology are highly favorable for electrocatalytic reactions. High surface area and open structure provide a large electrode-electrolyte contact area, or large accessible active sites for surface protons, and thus improve the electrocatalysts performance.39,40 Herein, we demonstrate a novel method to fabricate interconnected hollow CoP dispersed on carbon nanotubes (CNTs). The method is simple and involves complexation of cobalt with hexamethylenetetramine (HMT), which act as a structure-directing agent. HMT complexes with cobalt and steers the formation of interconnected cobalt oxide (CoO) nanoparticles, which 3 ACS Paragon Plus Environment


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further transforms into interconnected hollow CoP at ≥300 °C in the presence of phosphine (PH3) gas. The hollow nanospheres are predominantly interconnected, which is a desired feature from the electrical conductivity viewpoint. Performance of electrode comprising HMT-derived CoP/CNT was compared with those of Pt/C and nonhollow CoP/CNT, and durability was examined.

Fundamental

activity-regulating

features,

such

as

specific

surface

area,

electrochemically active surface area, electrical conductivity and interfacial charge transfer kinetics and turnover frequency, were investigated and correlated to the performance. EXPERIMENTAL Materials Cobalt (II) acetate tetrahydrate (99.99%), hexamethylenetetramine (HMT, ≥99%), multiwalled carbon nanotubes (>95% carbon basis), sodium hypophosphite (≥99%), and anhydrous ethanol were purchased from Sigma-Aldrich, while Nafion (5% w/w solution) was obtained from Alfa Aesar. Synthesis of cobalt phosphide/carbon nanotubes electrocatalysts (HI-CoP/CNT) A measured amount of cobalt acetate tetrahydrate (0.29 mmole) was dissolved in homogenous suspension comprising carbon nanotubes (25 mg) and anhydrous ethanol (15 mL) in a capped vial. The solution was kept under stirring for 2 h at room temperature. Then, hexamethylenetetramine (HMT) was dissolved (0.58 mmole), and the solution was kept under stirring for another 12 h to achieve a maximum complexation of cobalt with HMT. Subsequently, the solution was poured on a petri dish and the solvent was allowed to evaporate under a vigorous stirring. As the solvent evaporated, a black powder was obtained which was dried at 110 °C for overnight. Finally, desired amount of as-prepared powder and sodium hypophosphite (NaPO2H2) were placed in two separate alumina crucibles (with weight ratio of cobalt precursors:NaPO2H2; 1:20) and transferred in a tube furnace under the flow of Argon. NaPO2H2 was placed at the upstream side of the furnace. The temperature of the furnace was raised to the desired value (250, 275, 300, 350 and 400 °C) with heating rate of 5 °Cmin-1, and maintained for 2 h. After the reaction, the furnace was allowed to cool down to room temperature, and the product was collected. The collected products were characterized and used as electrocatalysts for HER reaction. For comparison, nonhollow (solid) CoP/CNT nanocomposite (denoted as CoP/CNT) was also prepared following the


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same method, but in the absence of HMT. Hollow and interconnected CoP/CNT is denoted as HI-CoP/CNT, while nonhollow (solid) CoP/CNT is denoted as CoP/CNT. Characterization Morphological and detailed microstructural attributes of the materials were discerned with the aid of transmission and high-resolution transmission electron microscope and selected area electron diffraction (TEM/HR-TEM, FEI Tecnai TF20) (SAED). Other techniques employed for characterization of the samples were: X-ray diffractometry (XRD, Rigaku MiniFlex), 1H and 13C nuclear magnetic resonance (NMR LAMBDA 500 spectrophotometer), BET surface area analyzer (Micromeritics ChemiSorb 2750) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). Electrochemical characterization Performance was evaluated in a three-electrode cell configuration connected to a potentiostat (EG&G 273A) at ambient conditions. Working electrode was prepared sonicating slurry consisting of electrocatalyst (10 mg), water and ethanol (50% V/V) and 37 µl of 1.66 % wt. Nafion® for approximately 30 min. 16 µL of sonicated solution was dropped on a pre-cleaned glassy carbon (GC) disc electrode (5.0 mm diameter), and dried under ambient conditions. The deposition steps were repeated to achieve desired catalyst loading. Saturated calomel electrode (Hg/HgCl2, SCE) and graphite rod were used as the reference and the counter electrode, respectively. The SCE potential was converted and presented against reversible hydrogen electrode (RHE). Linear sweep voltammetry was performed in a 1.0 M KOH or 0.5 M H2SO4 aqueous solution at a scan rate of 5 mVs-1. All current density was normalized to the geometric area of the glassy carbon electrode and presented after iR compensation. Electrochemical impedance spectroscopy (EIS) measurements were carried out in 0.5 M H2SO4 between the frequency range of 105 Hz and 0.01 Hz with ac amplitude of 10 mV. All the EIS data was normalized to the geometric area of the working electrode. Calculation of turnover frequency (TOF) To calculate the active sites, TOF was estimated using the following eqn 1; TOF=JA/2FN

(1)



Where, J (Acm-2) is the geometric current density recorded during the LSV measurement in 0.5 M H2SO4, A is the geometric area of the glassy carbon electrode GC (0.196 cm2), 1/2 denotes that two electrons are required to form one hydrogen molecule from water, F is the Faradic constant (96485 Cmol-1), N is the number of active sites (mol). N was determined by carrying out CV measurements between -0.2 V and +0.6 VRHE in 1.0 M phosphate buffer solution with a scan rate of 20 mVs-1. While it is difficult to assign the observed peaks to a given redox couple, N should be proportional to the integrated charge over the whole potential range. Assuming a one electron redox reaction, the upper limit of active sites was calculated as given in the following equation: N = Q/2F Where Q is voltammetric charge.

RESULTS AND DISCUSSION Powder X-ray diffractions (XRD) of HI-CoP/CNT and CoP/CNT are compared in Figure 1. Diffractions were similar for both the samples, indicating the formation of crystalline phase. Diffraction peaks centered at 2θ = 31.7 (011), 36.3 (111), 46.5 (112), 48.2 (211), 52.3 (103), 56.1 (020), and 56.7 (301) are attributed to crystalline orthorhombic cobalt monophosphide (CoP) phase (JCPDS-29-0497). The broad peak at 2θ = 26.1° corresponds to (002) plane of carbon nanotubes (CNT).41 Details of surface composition and oxidation state were collected with the aid of X-ray photoelectron spectroscopy (XPS), and results are included in Figure 1. Survey spectrum (Figure S1) verified the elemental composition of HI-CoP/CNT, which comprises carbon (C), cobalt (Co), phosphorous (P), and oxygen (O). In the spectrum shown in Figure 1, the peaks centered at 284.7 and 285.5 eV are assigned to graphitic carbon (C-C) and oxygenated carbon (C-O), respectively. The deconvoluted Co 2p XPS profile consists of several peaks. The peaks at 779.1 eV (Co 2p3/2) and 793.9 eV (Co 2p1/2) are ascribed to cobalt bonded to phosphorous in CoP, whereas spectral lines at 783.2 and 799.3 eV account for cobalt oxide or surface oxidized cobalt. In addition, there are other peaks (at 787.5 and 804.1 eV) known as “satellite” peaks, usually observed in CoP.23,31 Deconvolution of P 2p spectrum reveals two spectral signals at lower binding energy (peaks at 129.9 and 130.9 eV), corresponding to phosphorous anion bonded to cobalt in CoP, and at higher binding energy (peaks at 134.7 and 6 ACS Paragon Plus Environment

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Intensity (a.u.)

CNT 011 111

10

C 1s

211 112

HI-CoP/CNT

020 103

CoP/CNT

20

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C-C C-O

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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


Intensity (a.u.)

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CoO

-3

PO4 P-Co

Satellite 776

784

792

800

128

808

Binding energy (eV)

130

132

134

136

138

Binding energy (eV)

Figure 1. XRD patterns and XPS signatures of HI-CoP/CNT. 135.6 eV), accounting for phosphate species.23,31 The surface of CoP usually remains unstable and tend to oxidize under atmospheric environment, forming an oxide and phosphate moieties.42,43 When taken in combination, the Co 2p signal appeared at higher binding energy of 779.1 eV as compared to that of metallic cobalt (778.1 eV), whereas P 2p peak (129.9 eV) was recorded at lower binding energy as compared to that of elemental phosphorous (130.2 eV). This is ascribed to slightly positive centers on cobalt and negative on phosphorous due to electron transport between the two in the crystal of CoP.32 It has been anticipated, therefore, that HER performance originates from the CoP rather than oxidized compositions of Co or P.32 The XPS analysis of CoP/CNT was also carried out, and was found to be analogous to HI-CoP/CNT (Figure S2). Evolution in CoP microstructure with respect to growth temperature was followed by microscopic and XRD analyses. Figure 2a highlights the shape and morphology of CoP/CNT obtained without HMT. Particles were in the nanometer range (120 mA. This corroborates that as-derived HI-CoP/CNT is endowed with excellent mass transport property. Figure 4d compares the specific activity of the electrodes, estimated against BET surface area. To obtain specific activity, the current was normalized to specific surface area of the active materials instead of geometrical area of GC electrode. BET surface areas for HI-CoP/CNT and CoP/CNT were measured to be 65.3 and 56.3 m2g-1, respectively. As could be readily seen, the electrode comprising HI-CoP/CNT electrocatalyst exhibits better specific performance than that of CoP/CNT. This is attributed to the interconnected hollow features. It has been previously documented that the interconnected networks experience significantly low contact electrical resistance between adjacent particles, leading to fast electron transport across each microstructure.44 This unique structural feature of electrocatalyst is identified as key to the high HER performance.44 Figures 4e and f show the durability of electrode consisting of HI-CoP/CNT. Identical measurement conditions, applied to measure LSV, were used to assess the stability; HICoP/CNT with 40 wt% CoP, catalyst loading 1 mgcm-2, 0.5 M H2SO4. Two electrochemical modes were pursued; (1) chronopotentiometry (2) cyclic voltammetry. The former was conducted at 102 mV for 24 h, whereas cyclic voltammetry was performed between 0.2 and -0.2 VRHE with a scan rate of 100 mVs-1 for 1000 cycles. The time-dependent profile of current density recorded at 102 mV is shown in Figure 4e. A steady flow of current was recorded for 24 h. After 24 h of continuous chronopotentiometry, the used electrode was employed and the HER performance was recorded. As shown in Figure 4f, the performance of the electrode before and after durability test remains intact, attesting the excellent electrochemical stability of the electrode. Furthermore, after 1000 sweeps, the used electrode was re-polarized and the current density was quantified as a function of potential (Figure S12). As could be seen, the performance 13 ACS Paragon Plus Environment


of electrode deteriorated during cyclic voltammetry measurement, presumably due to partial transformation of CoP into CoO during cycling followed by dissolution of CoO in highly acidic media. Attempt was made to correlate the augmented performance of the HI-CoP/CNT in terms of electrochemical impedance, electrochemically active surface area (ECSA) and turnover frequency (TOF). Bulk conductivity as well as the charge transport property of as-prepared electrocatalysts was probed by electrochemical impedance spectroscopy (EIS) in 0.5 M H2SO4. Potential-dependent evolution in Nyquist plots of CoP/CNT and HI-CoP/CNT are shown in Figures 5a and b. Plots were constructed as a function of overpotential (η = 0, 10, 30, 50, 70, 90, 110 and 130 mV). The semicircular dispersion of impedance is indicative of an interfacial phenomenon operative near the electrode surface. Shorter semicircular arcs indicate faster charge transfer at the interface and vice versa. An inverse relationship between η and charge transfer resistance (Rct) was discerned – Rct decreased significantly with increasing η. This implies faster charge transfer kinetics at higher overpotential. When compared, the charge transfer kinetics of

450

0 mV 10 mV 30 mV 50 mV 70 mV 90 mV 110 mV 130 mV

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Rct CoP/CNT HI-CoP/CNT

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100

f

CoP/CNT

(56.58 mVdec-1)

75 50 25

HI-CoP/CNT

(54.60 mVdec-1)

0 -3.0

-2.5

-2.0

-1.5

-1.0

-1

Log(Rct

) (Ohm-1)

Figure 5. Nyquist plots of CoP/CNT (a) and HI-CoP/CNT (b), comparative Nyquist plots at 0 (c) and 130 mV (d), Bode plot at 130 mV (e), and Tafel slopes deduced from EIS (f).


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electrode comprising HI-CoP/CNT was more dominant at all applied potential. For instance, an enlarged view of Nyquist plots recorded at η = 0 and 130 mV for both the electrodes are presented in Figures 5c and d. In the case of HI-CoP/CNT, substantially smaller semicircle was noted at 0 mV, which suggest that the electrode is endowed with intrinsically more conductive components. This is attributed to the interconnected scaffold of CoP, as corroborated by microscopy analysis. Interconnected network of CoP could facilitate the transport of charge carriers efficiently and induce higher electrical conductivity. Usually, electrodes observing lessresistance translate into better performance. In addition to Rct, Nyquist plot recorded at 130 mV also indicates the presence of two time constants, porosity and charge transfer. Nyquist findings were substantiated by Bode Plots (Figure 5e). For HER, EIS findings are usually fitted by three types of electrical equivalent circuit models – one-time constant model,45 two-time constant parallel model46 and two time constant serial models.47 Based on Nyquist and Bode plots, the two-time constant parallel model comprising solution resistance (Rs) in series with two parallel constant phase element-resistance was used to fit the EIS data. According to this equivalent circuit model, Rs denotes a collective resistance including the resistance of wiring (Rwiring), carbon support (Rcarbon), CoP (RCop) and solution (Rsoln).48 Furthermore, charge transfer resistance, resistance of porous surface, and capacitance are symbolized by Rct, Rp and Cdl, respectively. Under acidic conditions, H2 evolution proceeds following two different routes, which involve three probable reactions (HER steps) – (Step I) Volmer reaction (adsorption), (Step II) Tafel reaction (chemical desorption) and (Step III) Heyrovsky reaction (electrochemical desorption), as shown below. Step I:

H+ (aq) + e-

Hads

Volmer reaction (adsorption)

Step II:

Hads +

H2 (g)

Tafel reaction (chemical desorption)

Hads

+ Step III: Hads + H (aq) + e

H2 (g)

Heyrovsky reaction (electrochemical desorption)

HER steps – Reactions involved in the hydrogen evolution under acidic condition. By estimating Tafel slope, the reaction mechanism could be elucidated. Calculation of the semilogarithmic values of the inverse of Rct against η results in a linear relationship with a gradient, which is attributed to the Tafel slope. Moreover, Tafel slopes estimated from EIS represent 15 ACS Paragon Plus Environment


entirely charge transport kinetics. Respective slopes for HI-CoP/CNT and CoP/CNT are shown in Figure 5f. The values are calculated to be 54.60 and 56.58 mVdec-1. Such values suggest that HER reaction occurs via Volmer-Heyrovsky mechanism, in which the electrochemical desorption of hydrogen ion and atom (Heyrovsky step) is more likely the rate-determining step. Electrochemically active surface area (ECSA) was quantified by measuring the double layer capacitance (Cdl) of cyclic voltammograms (CVs) recorded in the non-faradic region. CVs were recorded at different scan rates ranging from 5 to 150 mVs−1 (Figures 6a and c), and their corresponding plots of the current density at 0.254 VRHE vs. scan rate are shown in Figures 6b and d. The ECSA of HI-CoP/CNT and CoP/CNT were estimated to be 17.30 and 12.66 mFcm-2, respectively. Usually, higher Cdl is demonstrated as higher electrochemically active surface area. When taken in combination, both specific surface area and ECSA are more favorable for HICoP/CNT. Since electrochemical hydrogen evolution reaction is a predominantly surfacedictated process, usually a higher surface area (specific and ECSA) of electrocatalysts translate

5-150 mV 2 0 -2 0.1

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b 12.66 mF cm

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) -2

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into better performance.

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-0.2

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0.2

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0.0

0.1


Figure 6. Cyclic voltammograms (a, c) and their corresponding plots of the current density (b, d) at 0.254 VRHE vs. scan rate; a, b – CoP/CNT and c, d – HI-CoP/CNT. Cyclic voltammograms (e) recorded between -0.2 and 0.6 VRHE and their corresponding TOF profiles vs. overpotential (f). 16 ACS Paragon Plus Environment

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To further evaluate the catalytic ability of as-synthesized electrocatalysts, turnover frequency (TOF) was calculated. TOF signifies the number of reactant molecules reacted per active site in unit time. Higher TOF values denote higher catalytic ability of electrocatalyst. Since electrocatalytic performance depends on catalysts mass deposited on working electrode (catalyst loading) and usually investigated with different mass loadings, estimation of TOF would allow the comparison of catalytic performance of as-prepared electrodes with those reported in the literature. Firstly, the number of active sites was quantified by electrochemical method.2 Figure 6e compares the cyclic voltammograms of HI-CoP/CNT and CoP/CNT recorded between -0.2 V and +0.6 VRHE. Measurements were performed in neutral phosphate buffer solution (pH = 7) with a scan rate of 20 mVs-1. The number of active sites for HI-CoP/CNT and CoP/CNT was estimated to be 1.9 ×10-8 and 1.28 ×10-8 mole, respectively. The number of active sites was used to determine TOF using eqn. 1, and resulting TOF were plotted against potential (Figure 6f). At η = 73 mV (the required potential to drive a geometric current density of 10 mAcm-2), the electrode comprising HI-CoP/CNT exhibits significantly higher TOF (0.58 s-1) as compared to CoP/CNT (0.13 s-1). This is attributed to the hollow morphology, which renders more accessible functional sites for H+. CONCLUSIONS In summary, growth of interconnected hollow nanospheres of CoP on the surface of CNTs is demonstrated. Coordination between cobalt and hexamethylenetetramine was essential to obtain microstructural features. The cobalt-HMT complex decomposed to produce interconnected CoO nanoparticles, which subsequently grew into interconnected hollow CoP in the presence of PH3. The method is simple with the potential to scale-up. While interconnected CoP network seemed to impart low intrinsic resistance and high surface-charge-transfer, hollow morphology presumably amplified the mass transport property, density of accessible active sites, and specific and electrochemically active surface area. Endowed with such unique attributes, as-prepared electrode exhibited remarkable performance for HER, both in acidic and basic media. In acidic electrolyte, it required only 73 mV to drive 10 mAcm-2. Moreover, the electrode exhibited high specific activity and high turnover frequency. Explication of Tafel slope and rate-determining step indicated that the electronic structure of HI-CoP/CNT could further be modulated to facilitate the electrochemical desorption of H2 gas and achieve even higher efficiency. The electrode showed excellent stability – at least for 24 h at 102 mV. The findings offer impetus to 17 ACS Paragon Plus Environment


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engineer Pt-free electrode materials with high activity for large scale and sustainable H2 production through electrolysis. ASSOCIATED CONTENT Supporting Information. XPS survey scan spectrum of HI-CoP/CNT, XPS of CoP/CNT, FESEM and elemental mapping of HI-CoP/CNT, nitrogen adsorption-desorption isotherms, temperaturedependent evolution in XRD patterns of HI-CoP/CNT structure, 1HNMR and

13

C NMR,

polarization curves of HMT-derived CoO/CNT as a function of LSV cycles, temperaturedependent evolution in ECSA, comparative polarization curves of different electrocatalysts, polarization curve of HI-CoP/CNT in neutral medium, polarization curves obtained before and after 1000 potentiodynamic sweeps in 0.5 M H2SO4, electrical equivalent circuit model exercised to fit the electrochemical impedance (EIS) results, performance comparison of electrocatalysts (CoP) for HER. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * (M.Q.) E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors acknowledge the support provided by the Center of Excellence in Nanotechnology through NT-2018-MQ at King Fahd University of Petroleum and Minerals (KFUPM). The authors acknowledge the help of Mr. Omer Alnoor for drawing hollow and interconnected CoP nanostructures. REFERENCES 1.

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TOC Graphic


Interconnected Hollow Cobalt Phosphide Grown on Carbon

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