CoP Hybrid

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Pt-like Hydrogen Evolution Electrocatalysis on PANI/CoP Hybrid Nanowires by Weakening the Shackles of Hydrogen Ions on the Surfaces of Catalysts Jin-Xian Feng, Si-Yao Tong, Yexiang Tong, and Gao-Ren Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12968 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Journal of the American Chemical Society

Pt-like Hydrogen Evolution Electrocatalysis on PANI/CoP Hybrid Nanowires by Weakening the Shackles of Hydrogen Ions on the Surfaces of Catalysts Jin-Xian Feng,† Si-Yao Tong,‡ Ye-Xiang Tong,† and Gao-Ren Li†* †

MOE Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chemistry & Energy

Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China ‡

Zhixin High School, Guangzhou 510080, China

E-mail: [email protected]

ABSTRACT The search for high active, stable, and cost-efficient hydrogen evolution reaction (HER) electrocatalysts for water electrolysis has attracted great interest. The coordinated water molecules in the hydronium ions will obviously reduce the positive charge density of H+ and hamper H+ to receive electrons from the cathode, leading to large overpotential of HER on non-precious metal catalysts. Here we realize Pt-like hydrogen evolution electrocatalysis on PANI nanodots (NDs)-decorated CoP hybrid nanowires (HNWs) supported on carbon fibers (CFs) (PANI/CoP HNWs-CFs) as PANI can effectively capture H+ from hydronium ions to form protonated amine groups that have higher positive charge density than those of hydronium ions and can be electro-reduced easily. The PANI/CoP HNWs-CFs as low-cost electrocatalysts show excellent catalytic performance towards HER in acidic solution, such as super high catalytic activity, small Tafel slope and superior stability.

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INTRODUCTION Hydrogen gas (H2) has attracted great interest because it can provide an efficient energy system that is free from environmental issues related to the combustion of coal, oil, and natural gas.1-3 Water electrolysis is an environmentally friendly method to produce high-purity hydrogen. However, the effective electrocatalysts, especially Pt-based catalysts, are usually required for water electrolysis for hydrogen evolution reaction (HER) at low overpotential.4-6 The large-scale application of Pt will be greatly hindered owing to the scarcity and high cost. To replace high-cost Pt-based materials, varieties of non-precious electrocatalysts such as transition-metal carbides, selenides, phosphides and chalcogenides, have been widely studied.7-13 However, for various kinds of non-precious electrocatalysts, it still remains difficult to achieve the catalytic activity and stability matching those of Pt-group electrocatalysts.14-15 The typical process of HER on most of non-precious electrocatalysts in acidic solution can be divided into the following two radical steps (1-2):16-17 M + H+ads+ e → M-H

(1)

M-H + H+ + e → M + H2

(2)

In principle, the HER in acidic solution almost has no hamper because the protons can receive electrons easily from the cathode due to its high density of positive charge. But unfortunately, the hydrogen ions (H+) in aqueous media always coordinate with water molecules to form hydronium ions (usually denoted as H13O6+) with strong hydrogen bond connections because the positively charged H+ trends to bind with O atoms of water.18 The coordinated water molecules in hydronium ions will obviously reduce the positive charge density of H+ and will hamper H+ to receive electrons from the cathode,19-20 leading to large overpotential of HER on non-precious metal catalysts. Therefore, if the coordinated water molecules in hydronium ions can be eliminated, the H+ on the surfaces of catalysts will receive electrons easily and HER will be obviously promoted. Polyaniline (PANI) has abundant lone electrons on N atoms, which is able to capture H+ from hydronium ions by amine groups. So the existence form of H+ ions in aqueous media can be transformed from hydronium ions to the protonated amine groups by PANI.21 As we all know, the protonated amine groups 2 Plus Environment ACS Paragon

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will eliminate the negative influence of hydronium ions caused by the binding water molecules, and accordingly the H+ will receives electrons easily.22-25 Based on the above considations, here we fabricated PANI nanodots (NDs)-decorated CoP hybrid nanowires (HNWs) supported on carbon fibers (CFs) (PANI/ CoP HNWs-CFs) as a state-of-art non-precious metal electrocatalyst for HER in acidic media. The hybrid structures show following advantages: (i) PANI with lone electron pairs on N atoms will capture H+ from hydronium ions, and thus will eliminate the effect of coordinate water molecules around H+ to benefit HER; (ii) the hierarchical structure of PANI/CoP HNWs-CFs will favor the mass transportation/diffusion during HER; (iii) The electronic interactions between PANI and CoP in PANI/CoP HNWs-CFs can achieve high conductivity and will accelerate electron transfer for HER. The experimental results demonstrate that the PANI/CoP HNWs-CFs electrocatalysts exhibit Pt-like electrocatalytic activity and high stability towards HER with low onset potential and small Tafel slope.

EXPERIMENTAL SECTION Materials synthesis The chemical reagents utilized in this study all were analytical grade, and they were utilized directly without any purification. Electrodeposition was carried out by galvanostatic electrolysis in a twoelectrode cell. The counter electrode was a graphite electrode (1.8 cm2, spectral grade), and the working electrode was a carbon fibers (CFs) (2 cm×0.5 cm) that was purchased from Phychemi Company in Hong Kong. The fabrication procedures of the typical PANI/CoP HNWs-CFs are given as following. Different types of PANI/CoP HNWs-CFs with different PANI/CoP mass ratio can be obtained by controlling the concentration of Co(NO3)2 in step (1) and the concentration of aniline solution in step (3). (1) Co(OH)2 NWs-CFs was electrodeposited on CFs in 10 mL solution of 0.05 M Co(NO3)2 + 0.25 M urea solution at 95 °C for 10 h. (2) CoP NWs-CFs was fabricated by low-temperature phosphorization using PH3, and the experimental details are provided as following: Co(OH)2 NWs-CFs and NaH2PO2 were put at separate positions in a porcelain boat with NaH2PO2 at the upstream side of the furnace. The porcelain boat was heated to 300 °C for 1.5 h under a nitrogen atmosphere and then was cooled to room temperature. The 3 Plus Environment ACS Paragon


temperature increasing rate is 3 oC/min. (Caution! the produced PH3 is considered as being corrosive and flammable. The gas tightness of the furnace must be at high level and concentrated CuSO4 solution was utilized as a tail gas absorber). (3) PANI NDs were decorated on CoP NWs-CFs via chemical polymerization. A piece of CoP NWs-CFs was firstly immersed into 10 mL aqueous solution of 0.01 M aniline at 298 K, then 10 µL 0.20 M K2S2O8 solution were added in to the above system with stirring for 30 s to make sure the K2S2O8 mix with the solution uniformly. After 20 min polymerization, the sample was washed by de-ionized water for three times, and was dried at 70 oC to remove excess moisture. Structural and electrochemical characterizations (1) The powder X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku D/Max 2550 Xray diffractometer with Cu Kα radiation (λ = 1.5418 Å). (2) The X-ray photoelectron spectroscopy (XPS) spectra were measured on an XPS, ESCA Lab250 X-ray photoelectron spectrometer. All XPS spectra were corrected using C 1s line at 284.6 eV, and curve fitting and background subtraction were accomplished. (3) The transmission electron microscope (TEM) images of samples were measured with a TEM, JEM2010HR and high resolution TEM (HRTEM, 200 kV or 300 KV). (4) The determination of the contents of Co and P by using ICP (iCAP 6500 Duo, Thermo Fisher) and the contents of C and N by using element analysis (Elememtar, type: Vario EL cube). (5) The scanning electron microscope (SEM) images of samples were measured with Zeiss Sigma field emission SEM (FE-SEM, JSM-6330F). (6) The electrocatalytic properties of PANI/CoP HNWs-CFs were studied in a three-electrode system using a CHI 760D (Shanghai, China) instrument. A saturated calomel electrode (SCE) and a carbon rod were utilized as the reference electrode and counter electrode, respectively. The potential, measured against a SCE electrode, was converted to the potential versus the reversible hydrogen electrode (RHE) according to E vs. RHE = E vs. SCE + E0 vs. SCE + 0.059 pH. The working electrode was directly immersed in the electrolyte. For HER experiments, the linear sweep voltammograms (LSVs) and cyclic voltammograms (CVs) were measured at a scan rate of 5 mV/s in the solution of 0.5 M H2SO4. For 4 Plus Environment ACS Paragon

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comparative studies, the electrocatalytic activities of PANI/CoP HNWs-CFs, CoP NWs-CFs, PANI NDs-CFs and CFs were also measured under the similar conditions. All the electrochemical data were exhibited without iR-drop compensation. The studies of the existence state of H+ on the surface of electrocatalyst The existence state of H+ on the different electrocatalysts was studied by solid-state magic angle spinning nuclear magnetic resonance (1H MAS NMR) measurement, which was carried out on a Bruker Avance III spectrometer at a magnetic field of 9.4 T corresponding to Larmor frequency of 400.1 MHz. The liquid phase samples were tested on 1H nuclear magnetic resonance Bruker Avance III spectrometer at a magnetic field of 9.4 T corresponding to Larmor frequency of 400.1 MHz, and the chemical shifts were calibrated by CD3SO. The amount of H+ captured by PANI is determined as following: The PANI is consisted by C6H5NH monounits, in which the -NH- is the site for H+ capturing. Each C6H5NH monounit captures one H+. In this study, 10 mL solution of 0.001 M (pH=11) NaOH was employed as H+ neutralizer. The sample of 10 cm2 geometric surface area was utilized, and the pH detector (PHS-4CT, Shanghai, China) was utilized to test pH value of NaOH solution after neutralization. The amount of H+ captured by PANI (per cm2) can be calculated via Equation 3: n(H+) (mol/cm2)=[0.001-10-(14-pH)]×0.01/10

(3)

As mentioned above, PANI chains can be considered as a series of C6H5NH monounits. The molar number of C6H5NH monounit can be calculated via Equation 4: n(C6H5NH monounits)(mol)=m(PANI)/Mr(unit)

(4)

The rate of the -NH- protonated by H+ (R) per cm2 can be calculated via Equation 5: R=[n(H+)×Mr(unit)]/m(PANI)

(5)

where Mr(unit) is the relative molecule mass of a monounit of PANI (Mr(unit)=91.4679). n(H+) can be obtained via Equation S1 and m(PANI) is the PANI loading mass (g) per cm2. Density Functional Theory (DFT) Calculations The computational modeling of the reactants, intermediates and products and reaction process involved 5 Plus Environment ACS Paragon


in HER was performed by DFT with the Gaussian 09W programe, B3LYP method, LanL2DZ basis set. All of the structures were fully optimized and are relaxed to the ground state, and spin-polarization was considered in all calculations. The convergence of energy was set to 1×10-8 and the potentials were set to 0 V. The CoP models were constructed based on the (211) crystal facet of CoP (JCPDS No. 29-0497). The Gibbs free energy change (∆G) of each reaction step is calculated via Equation 6: ∆G = Etot(b) – E tot(a) + ∆EZPE -T∆S

(6)

where Etot(b) is the energy of the given unit cell with intermediate of the latter state, Etot(a) is the energy of the intermediates of the previous state, ∆EZPE is the difference corresponding to the zero point energy change between the intermediates of the previous state and the latter state, ∆S is entropy change between the intermediates of the previous state and the latter state.

RESULTS AND DISCUSSION The PANI/CoP HNWs-CFs electrocatalysts were fabricated as illustrated in Scheme S1. CFs were used as a support and current collector, and their scanning electron microscope (SEM) image is shown in Figure S1, which shows the CFs are consisted of networks. Cobalt hydroxide [Co(OH)2] nanowires (NWs) are firstly coated on the surface of CFs to form Co(OH)2 NWs-CFs, and their SEM and TEM images are shown in Figure S2a-b. Then CoP NWs-CFs were fabricated by heat-treatment of Co(OH)2 NWs-CFs in PH3 atmosphere at 300 oC for 1 h. SEM and TEM images of CoP NWs are shown in Figure S3, which shows the uniform coating of CoP NWs on CFs. The diameters of CoP NWs are ~50 nm. Finally, PANI NDs are further decorated on the surfaces of CoP NWs-CFs to form PANI/CoP HNWs-CFs via chemical polymerization as illustrated in Figure 1a. X-ray diffraction (XRD) pattern of PANI/CoP HNWs-CFs is shown in Figure 1b, and the diffraction peaks of CoP are observed at 31.6º, 33.8º, 36.3º, 37.4º, 48.5º and 57.1º, which can be indexed to (011), (002), (111), (102), (211) and (301) planes of CoP (JCPDS No. 290497), respectively. No peak is seen for PANI. A broad peak at 20~30º is seen for PANI/CoP HNWs-CFs and PANI NDs-CFs, indicating PANI NDs are amorphous. SEM images of PANI/CoP HNWs-CFs with different magnifications are shown in Figure 1c-d, which shows nanowire array structure of PANI/CoP HNWs-CFs and the diameters of nanowires are ~50 nm. Compared with SEM image of CoP NW in 6 Plus Environment ACS Paragon

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Figure 1e, Figure 1f shows that the PANI NDs of ~20 nm are successfully decorated on the surface of CoP HNW to form PANI/CoP HNW. To further demonstrate the hierarchical nanowire-nanodots structure of PANI/CoP HNWs, TEM image of PANI/CoP HNWs is shown in Figure 1g, which shows the diameters of PANI/CoP HNW are ~50 nm and PANI NDs of ~20 nm are decorated on the surface of CoP NWs. The selected area electron diffraction (SAED) of PANI/CoP HNWs is shown in inset in Figure 1g, which shows that CoP NWs own a single crystalline structure and the characteristic diffraction points of CoP, such as (102), (211), (011) and (111), are clearly observed. In order to identify PANI NDs and CoP NWs, STEM-EDS element mappings of Co, P and C were measured, and the results are shown in Figure 1h-j, which shows the distributions of CoP and PANI in the HNW (Co-K and P-K comes from CoP, and C-K comes from PANI). When the mappings of Co-K and C-K are superimposition, the result is shown in Figure 2h, which shows that the PANI aggregates on CoP NWs as NDs with diameter of ~20 nm. The atomic ratio of Co and P in the NW area is ~1.12:1, indicating Co and P serve as CoP. The atomic ratio of C and N in the nanodot area is ~6.15:1 and is close to the atom ratio of C/N in PANI monounit, indicating the existence of PANI. High-resolution TEM (HRTEM) image of a typical boundary area of PANI NDs and CoP NWs is shown in Figure 1k, which shows that the interplanar spacings are determined to be 0.283 and 0.190 nm, which are identical with (011) and (211) lattice spacings of CoP, respectively. The above results of SAED and HRTEM images are well coincident with those of XRD of PANI/CoP HNWs-CFs. Fourier transform infrared spectroscopy (FT-IR) of PANI/CoP HNWs-CFs was further measured to study the existence of PANI, and the results are shown in Figure S4a. The characteristic peaks in FT-IR spectrum such as N=Q=N (Q=quinone) ring vibrations at 1140 and 1188 cm-1, para-substitute C-N peaks at 1279 and 1346 cm-1, benzene rings vibration at 1394 and 1429 cm-1, C-C peak at 1492 cm-1, C=C peak at 1572 cm-1, C-H at 3024 cm-1 and para-substituted N-H peaks at 3249 and 3180 cm-1 are clearly seen, proving the existence of PANI.26-27 X-ray photoelectron spectroscopy (XPS) spectra were employed to study the chemical states of CoP and PANI. The peak at 778.63 eV of Co 2p3/2 and that at 793.50 eV of Co 2p1/2 in Figure S4b are close to the binding energies of Co element of CoP, and the peak at 782.24 eV for Co 2p3/2 and that at 798.32 eV for Co 2p1/2 can be assigned to Co element of CoPOx, which can be 7 Plus Environment ACS Paragon


attributed to surface oxidation of CoP because of the unavoidable air.28-29 For PANI/CoP HNWs-CFs, the chi square value (χ2) of Co 2p, P 2p and N 1s is 0.03722, 0.03829, and 0.03536, respectively, and the ratio of Co oxide and Co peaks is about 5.34:1. As the XPS spectrum is a kind of surface-sensitive technique, so the signal of CoPOx peaks on the surface of sample is more obvious than that of CoP located in the inner. But this does not mean that CoPOx is more than CoP in the sample. From XRD pattern in Figure 1b, which shows that the CoP mainly exist in the PANI/CoP HNWs-CFs, and almost no CoPOx appears, indicating that the content of CoPOx is very low. The peak of P 2p1/2 at 129.40 eV and that at 130.26 eV can be attributed to P 2p3/2 and P 2p1/2 of P element in CoP, respectively, as shown in Figure S4c. The peak at 134.28 eV in Figure S4c can be assigned to P element of CoPOx arising from the superficial oxidation of CoP.29 The peaks at 399.76, 400.47 and 402.40 eV in N 1s spectra of PANI/CoP HNWs-CFs can be assigned to –N=, -NH- and N+ groups of PANI, respectively,30-31 as shown in Figure S4d. So XPS results well prove the co-existence of CoP and PANI in PANI/CoP HNWs-CFs. To further characterize the interactions between CoP and PANI, XPS can be utilized to determine the electronic structures of CoP and PANI in PANI/CoP HNWs-CFs so that the interactions can be clarified. XPS spectra of CoP NWs-CFs, PANI NDs-CFs and PANI/CoP HNWs-CFs are shown in Figure 2a-d. The binding energies of Co 2p3/2 and Co 2p1/2 of CoP in PANI/CoP HNWs-CFs are negatively shifted compared with those of CoP NWs-CFs (~0.32 eV shift for Co 2p3/2 and ~0.48 eV shift for Co 2p1/2) as shown in Figure 2a-b. The binding energies of P 2p3/2 (129.42 eV) and P 2p1/2 (130.27 eV) of PANI/CoP HNWsCFs are also negatively shifted compared with those of CoP NWs-CFs (~0.52 eV shift for P 2p3/2 and ~0.48 eV shift for P 2p1/2) as shown in Figure 2c. The spectrum of N 1s of PANI/CoP HNWs-CFs in Figure 2d shows that the –N=, -NH- and –N+- characteristic peaks all exhibit positive shifts of ~0.33, ~0.30 and ~1.07 eV, respectively, compared with those of PANI NDs-CFs. The above results suggest that the CoP shows negative charge while the N in PANI shows positive charge, implying a partial electrons are transferred from PANI to CoP through the electronic interactions.31-32 Such effects suggest the combination of PANI and CoP is tightly so that it will be beneficial for electron transport during HER. The HER catalytic activity of PANI/CoP HNWs-CFs electrocatalysts was studied in acid solution (0.5 8 Plus Environment ACS Paragon

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M H2SO4, pH=1) and the results were shown in Figure 3a-f. CoP NWs-CFs and PANI NDs-CFs were also fabricated for comparative studies and their SEM images are shown in Figure S3 and S5, respectively. The polarization curves of PANI/CoP HNWs-CFs with different mass ratios of PANI/CoP are shown in Figure S6a, which shows that the electrocatalytic activity increases firstly and then decreases with the mass ratio of PANI/CoP increasing from 0.05 to 0.25. When the mass ratio of PANI/CoP is 0.15, the PANI/CoP HNWs show the highest electrocatalytic activity. Here the most superior activity of PANI/CoP HNWs-CFs with mass ratio of PANI/CoP of 0.15 can be attributed to the highest electrochemically active surface area (ECSA). For PANI/CoP HNWs-CFs with the different mass ratios of PANI/CoP, the ECSA values are different. As we all know, higher ECSA will provide more acitive sites and higher catalytic activity. The ECSA of PANI/CoP can be estimated by measuring the capacitance of double layer at the solid-liquid interface. The capacitance can be obtained from the charging current density differences ja-jc plotted against the scan rates of PANI/CoP HNWs-CFs as shown in Figure S6b, which shows PANI/CoP HNWs-CFs with mass ratio of PANI/CoP of 0.15 owns the highest ECSA. So the highest ESCA leads to the most superior catalytic activity of PANI/CoP HNWs-CFs with mass ratio of 0.15. The loading mass of PANI/CoP HNWs in all the experiments was kept 0.80 mg cm-2, which shows the highest electrocatalytic activity as shown in Figure S7. For the polarization curves as shown in Figure 3a, the onset potential of PANI/CoP HNWs-CFs is only ~15 mV, which is much lower than those of CoP NWs-CFs (60 mV), PANI NDs-CFs (150 mV) and CFs (No activity), and the PANI/CoP HNWs-CFs provides much higher current densities than CoP NWs-CFs, PANI NDs-CFs and CFs at the same potentials. PANI/CoP HNWs-CFs also gives much higher mass current densities than CoP NWs-CFs and PANI NDs-CFs at the same potential as shown in Figure S8, indicating PANI/CoP HNWs-CFs owns much higher mass catalytic activity than CoP NWs-CFs and PANI NDs-CFs. Meanwhile, it is worth noticing that the current density of PANI/CoP HNWs-CFs is higher than Pt metal electrode at the same potential as shown in Figure 3b (Here Pt metal electrode is a pure polycrystalline). When compared with 20 wt% Pt/C catalysts, PANI/CoP HNWs-CFs shows much higher current density in relative high potential interval (>66 mV) although it shows slightly lower current density in low potential interval, further indicating its ultra-high electrocatalytic activity. 9 Plus Environment ACS Paragon


The lower electrocatalytic activity of PANI/CoP HNWs-CFs than 20 wt% Pt/C catalysts at low potential interval can be explained as following: the current density at low potential region is small, indicating the rate of HER is slow. The HER at low potential is mainly controlled by the thermodynamic behavior and the dynamics behavior is not a key factor. However, for PANI/CoP HNWs-CFs, the role of PANI is to promote the dynamics behavior because it can effectively capture H+ from hydronium ions to form the protonated amine groups that can be easily transported on the surface of CoP. Therefore, at low potential the role of PANI is not obvious for the enhancement of catalytic activity of CoP, and the catalysis of PANI/CoP HNWs-CFs is mainly shown in CoP, leading to lower catalytic activity of PANI/CoP HNWsCFs than that of commercial Pt/C. The Tafel slope of PANI/CoP HNWs-CFs in the overpotential interval between 0.02~0.20 V is 34.5 mV/dec, which is smaller than those of CoP NWs-CFs (41.6 mV/dec), PANI NDs-CFs (142.1 mV/dec) and CFs (169.5 mV/dec) in the same overpotential interval as shown in Figure 3c. It is worth noticing that the Tafel slope of PANI/CoP HNWs-CFs (46.3 mV/dec) is much smaller than that of CoP NWs-CFs (111.8 mV/dec) at higher current density (10~40 mA cm-2) as shown in Figure S9. The above results suggest that the kinetic radical step of HER process on PANI/CoP HNWs-CFs and CoP NWs-CFs were not sluggish at the low current density. However, at higher current density, the kinetic radical step on CoP NWs-CFs is sluggish while that on PANI/CoP HNWs-CFs still keeps fast.34 Therefore, PANI/CoP HNWs-CFs shows much enhanced kinetic radical step for HER. In addition, Nyquist plots shown in Figure 3d indicate that the electronic transport and mass transport resistances of PANI/CoP HNWs-CFs are much smaller than those of CoP NWs-CFs. The electrochemically active surface areas (ECSAs) of PANI/CoP HNWs-CFs, CoP NWs-CFs and PANI NDs-CFs were determined by measuring the double layer capacitance,35 and Figure S10 shows that the ECSA of PANI/CoP HNWs-CFs is larger than those of CoP NWs-CFs and PANI NDs-CFs. So the enhanced electrocatalytic activity of PANI/CoP HNWs-CFs compared with those of CoP NWs-CFs and PANI NDs-CFs can be attributed to small electronic/mass transfer resistances and large ECSAs. The electrocatalytic activity of PANI/CoP HNWs-CFs is also much higher than those of many other HER electrocatalysts in acid media as shown in Table S1. 10 Plus Environment ACS Paragon

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To further evaluate the stability of electrocatalysts, the chronoamperometry measurements of HER on PANI/CoP HNWs-CFs were carried out for a long time at the overpotentials of 50, 75, 100 and 120 mV for 30 h as shown in Figure 3e. The current density of PANI/CoP HNWs-CFs shows negligible decrease during all the overpotentials, suggesting that the electrocatalytic performance of PANI/CoP HNWs-CFs is highly stable. The surface morphology of PANI/CoP HNWs-CFs was maintained very well after the constant reaction of 30 h at 120 mV as shown in Figure S11. The FT-IR spectra in Figure S12 indicate that the main characteristic peaks still exist after constant reaction of 30 h at 120 mV. XRD patterns of PANI/CoP HNWs-CFs before and after HER are almost the same as shown in Figure S13. Both FT-IR and XRD results indicate that PANI/CoP HNWs-CFs has high structural stability. In addition, it is worth noticing that the current density of PANI/CoP HNWs-CFs is much higher than Pt metal and 20 wt% Pt/C catalysts at overpotential of 100 mV for 30 h as shown in Figure 3f, suggesting that the PANI/CoP HNWs-CFs has the potential to replace Pt-based electrocatalysts for HER in acid media. Most importantly, the stability of PANI/CoP HNWs-CFs is higher or comparable with many other advanced electrocatalysts in acid media as shown in Table S2. Here Pt-like hydrogen evolution electrocatalysis on PANI/CoP HNWs-CFs is successfully realized by weakening the shackles of H+ on the surfaces of electrocatalysts. As we all know, the H+ ions in acid solution are surrounded by coordinated water molecules and exist as hydronium ions. The hydronium ion is usually denoted as H13O6+, and it is relative difficult to be electro-reduced compared with the protonated amine. The existence of hydronium ions in acid solution is demonstrated as following. Here 1H nuclear magnetic resonance (1H NMR) was employed to study the chemical state of hydrogen in water and 0.5 M H2SO4 solution as shown in Figure 4a. As we know, the less electron density outside of H nuclear, the higher chemical shift of H will appear.36 As for pure water, a peak of 1H NMR signal appears at ~4.45 ppm. The solution of 0.5 M H2SO4 shows three 1H NMR signals at ~3.61, ~4.52 and ~8.27 ppm, which come from H13O6+, water and HSO4-, respectively.37 Compared with that of pure water (4.45 ppm), 1

H NMR signal of water in solution of 0.5 M H2SO4 shows ~0.06 ppm positive shift. Compared with that

of the concentrated H2SO4 (9.47 ppm), 1H NMR signal of H+ ions in hydronium ions in 0.5 M H2SO4 11 Plus Environment ACS Paragon


solution (8.27 ppm) shows obvious negative shift. The above results indicate that the formation of hydrogen bonds between O atoms of water and H+ ions from sulfuric acid, suggesting that the H+ in the solution of 0.5 M H2SO4 are surrounded by coordinated water molecules and serve as hydronium ions. Therefore, the weakening of the shackles of H+ in acid solution will be important for the further enhancement of HER catalytic activity. Besides enhancing the conductivity, here PANI as a cocatalyst can obviously weaken the shackles of H+ ions on the surfaces of catalysts by transforming hydronium ions to the protonated amine. So HER catalytic activity of PANI/CoP HNWs-CFs can be obviously enhanced when the hydronium ions are transformed to the protonated amine. To prove that PANI has the ability of capturing H+ from hydronium ions for the enhanced catalytic activity, CoP NWs, PANI NDs and PANI/CoP HNWs were immersed in 0.5 M H2SO4 solution for 2 h and then were leached to remove the excess acid (denoted as CoP NWs-acid, PANI NDs-acid, PANI/CoP HNWs-acid, respectively). The above samples before and after immersing in 0.5 M H2SO4 solution were tested by 1H NMR with magnetic angle spinning (MAS) (1H NMR MAS) technique, and the results were shown in Figure 4b. Except for CoP NWs and CoP NWs-acid, all the samples show the peaks at the chemical shifts of 6.88~7.08 ppm that can be attributed to the signal of H atoms in benzene ring of PANI. The broad peak at 8.82 ppm for PANI NDs and that at 8.85 ppm for PANI/CoP HNWs can be attributed to the amine group (-NH-) in PANI.38 For PANI NDs-acid and PANI/CoP HNWs-acid, the peak below 9 ppm disappears, and a new peak at 9.39 ppm for PANI NDs-acid and that at 9.31 ppm for PANI/CoP HNWs-acid are clearly observed. These new peaks correspond to H signals of the protonated amine groups (-NH2+-) in PANI.39 Such 1H NMR MAS spectrum proves that the PANI can effectively capture H+ from hydronium ion by its amine groups. No obvious H signal is detected for CoP-acid, suggesting that the CoP cannot capture the H+ from the solution. As shown in Figure 4a-b, the chemical shift of -NH2+- is much close to that of the concentrated H2SO4, suggesting the H+ in -NH2+- has similar electron density outside of H nuclear to that in the concentrated H2SO4. The high positive charge density of -NH2+- will obviously promote the reduction of H+. To further determine adsorption state of the captured H+ on the various electrocatalysts, Fourier transform infrared spectroscopy with the attenuated total reflection tech12 Plus Environment ACS Paragon

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nique (FT-IR ATR) was employed, and the results are shown in Figure 4c. For CoP, no peak is detected. For PANI NDs-acid and PANI/CoP HNWs-acid samples, despite of characteristic peaks like C-C, C=C and benzene rings vibrations of PANI, some more evidences of the captured H+ are listed as following: i) For CoP NWs-acid, the broad -OH characteristic peak at ~3300 cm-1 suggests the existence of hydrogen network consisted by H+ and water molecules in 0.5 M H2SO4, but the -OH characteristic peak moves to ~3450 cm-1 and becomes narrow for PANI/CoP HNWs-acid, suggesting the hydrogen network is weakened by PANI/CoP HNWs;40 ii) For PANI NDs-acid and PANI/CoP HNWs-acid, the obvious peaks at 1100~ 1150 cm-1 appear compared with PANI NDs and PANI/CoP HNWs, and the double peaks at 1200~1300 cm-1 for PANI NDs-acid and PANI/CoP HNWs-acid are detected, suggesting the existence of the positively charged para-substituted C-N bonds (C-N+)41; iii) Compared with N-H (3248 and 3182 cm-1) vibrations of PANI NDs and PANI/CoP HNWs, the –NH2+– vibration apprears at 3223 cm-1 for PANI NDs-acid and PANI/CoP HNWs-acid, indicating the existence of the protonated amine.42 So the above FT-IR ATR results further confirm that PANI can efficiently capture H+ via the formation of the protonated amine. The formation of the protonated amine groups will improve the concentration of H+ on the surface of catalysts and thus will be beneficial for HER. To determine the concentration of H+ on the surfaces of different electrocatalysts, herein CoP NWs-CFs, PANI NDs-CFs and PANI/CoP HNWs-CFs were treated in 0.5 M H2SO4 solution and then were immersed in 0.001 M NaOH solution (pH=11) to neutralize the captured H+, so the amount of the captured H+ can be calculated by means of measuring pH change of the solution after neutralization (The relative method was detailedly described in the supporting information). Figure 4d shows that the pH value of 0.001 M NaOH solution after neutralization by the acid-treated PANI/CoP HNWs-CFs, PANI NDs-CFs and CoP NWs-CFs is 7.43, 7.65 and 10.72, respectively, which corresponds to the amount of the captured H+ is 0.9997 µmol cm-2, 0.9996 µmol cm-2 and 0.4752 µmol cm-2 for PANI/CoP HNWs-CFs, PANI NDs-CFs and CoP NWs-CFs, respectively. The above results show that the PANI/CoP HNWs-CFs and PANI NDs-CFs both have higher H+ concentration than CoP NWs-CFs, suggesting that the PANI can efficiently capture H+ and thus will obviously enhance H+ concentration on the surfaces of electrocatalysts to promote HER. 13 Plus Environment ACS Paragon


The electron affinity of H+ is an important factor for HER. To predict the electron affinity of H+ in the different states, the density functional theory (DFT) calculation was performed, and the results are shown in Figure 5a. The key paragraphs of the protonated amine group and hydronium ion (denoted as H13O6+) are inserted in Figure 5a. The simplified models of H13O6+, CoP-H13O6+, PANI-H+ and CoP/PANI-H+ are shown in Figure S14-15. DFT calculations indicate that the individual free H+ has the highest positive charge density (1 unit). For H13O6+, the positive charge density of H+ is only ~0.48 unit. For CoP-H13O6+, the positive charge density of H+ is ~0.51 unit, which is lower than that of H+ in PANI-H+ (0.57 unit) and that of H+ in CoP/PANI-H+ (0.62 unit). The above results suggest that the H+ captured by PANI has stronger electron affinity than that of hydronium ion, suggesting that the protonated amine groups will be easier to receive electron from the cathode. The H+ transfer from the protonated amine groups to CoP catalyst is also crucial in the HER catalysis because the low transfer barrier can make the captured H+ take part in the following HER radical steps easily. Herein, DFT calculation was employed to study free energy changes (∆G) of H+ transfer from PANIH+ to CoP and from H13O6+ to CoP, and the results are shown in Figure 5b. The relative calculation models were shown in Figure S16. Compared with CoP-H13O6+ (0.625 eV), CoP/PANI-H+ owns smaller ∆G of H+ transfer pathway (0.531 eV), indicating that the H+ in PANI-H+ easily transfers to CoP compared with that in hydronium ions. Therefore, PANI can promote H+ transfer to CoP, and accordingly HER performance can be effectively enhanced. Additionally, the free energy change of H adsorption (∆GH*) calculated by DFT is shown in Figure 5c according to the simplified models in Figure S17-18. Compared with those of individual CoP (-0.155 eV) and PANI (1.168 eV), the ∆GH* of hybrid PANI/CoP (-0.087 eV) is more close to 0 eV, which will lead to the ideal H adsorption on the surface of catalyst.43-45 The above results suggest that PANI/CoP hybrids can optimize ∆GH* to improve HER performance. The promotion mechanism of HER on PANI/CoP HNWs by transforming hydronium ions to the protonated amine gruops is illustrated in Figure 5d. To further prove the role of protonated amine species in HER, the LSV curves of PANI/CoP and CoP HNWs in solutions of 1.0 M phosphate buffered saline (PBS) (pH=7) and 0.5 M H2SO4 for HER are compared as shown in Figure S19. For 1.0 M PBS media (pH=7), the HER catalytic activities of 14 Plus Environment ACS Paragon

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PANI/CoP HNWs-CFs and CoP NWs-CFs almost are the same (see blue curves) as no protonated amine specie is formed. However, for 0.5 M H2SO4 solution, the HER catalytic activity of PANI/CoP HNWsCFs is much higher than that of CoP NWs-CFs (see red curves). Therefore, the protonated amine group is a key factor for the enhanced catalytic activity of PANI/CoP HNWs-CFs.

CONCLUSIONS We realize Pt-like hydrogen evolution electrocatalysis on PANI/CoP hybrid nanowires by weakening the shackles of hydrogen ions on the surfaces of catalysts. PANI can effectively capture H+ from hydronium ions to form protonated amine groups that have higher positive charge density than those of hydronium ions. This will be beneficial for HER because the H+ in the protonated amine groups has stronger electron affinity than that in hydronium ion and is easier to receive electrons from the cathode. Furthermore, the H+ transfers from the protonated amine group to CoP is faster than that of hydronium ion, and PANI/CoP hybrid owns ideal H adsorption on the surface of catalyst. The above advantages endow the PANI/CoP HNWs-CFs with the significantly improved catalytic activity and durability even better or comparable with Pt-based electrocatalysts. Here the concept of weakening the shackles of hydrogen ions to improve HER performance can be extended to other reactions and suggests a possible principle for the design of efficient electrocatalyst for other electrochemical reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.******. Details of the characterizations and electrochemical data (PDF)

AUTHOR INFORMATION Corresponding Author 15 Plus Environment ACS Paragon


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[email protected] Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS This work was supported by National Basic Research Program of China (2015CB932304 and 2016YFA0202603), NSFC (91645104), Science and Technology Program of Guangzhou (201704030019), Natural Science Foundation of Guangdong Province (2016A010104004, 2017A010103007), Guangdong Science and Technology Innovation Leading Talent Fund (2016TX03N187), Fundamental Research Fund for the Central Universities (16lgjc67), and High School Talent Program of China.

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b

PANI/CoP HNWs-CFs PANI NDs-CFs

20

*

* **

(301)

*

* PANI * CoP (211)

Amorphous PANI (011) (002) (111) (102)

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.

Page 19 of 24

*

*

25

30

35

40 45 50 2θ/degree

55

60

65

Figure 1. (a) Schematic illustration of the microstructure of PANI/CoP HNWs-CFs; (b) XRD patterns of PANI NDs-CFs and PANI/CoP HNWs-CFs; (c-d) SEM images of PANI/CoP HNWs-CFs with different magnifications; (e) SEM image of a typical CoP NW and (f) SEM image of a typical PANI/CoP HNW; (g) TEM image of PANI/CoP HNWs (inset: SAED pattern of CoP NW); (h) STEM image of PANI/CoP HNW; (i-j) Elemental mappings of C+Co and P in PANI/CoP HNWs; (k) HRTEM image of PANI NDs/CoP HNWs border.



Co 2p3/2

Co in CoPOx Co in CoP 778.63

b

Co in CoPOx

Sat.

Co 2p1/2

Co in CoP 793.50

Sat.

PANI/CoP HNWs-CFs ∆=0.32 eV

778.95

Intensity/a.u.

Intensity/a.u.

a

PANI/CoP HNWs-CFs ∆=0.48 eV 793.98

CoP NWs-CFs

CoP NWs-CFs

778

c

780

782 784 786 788 Binding energy/eV

129.42 129.94 P 2p1/2 130.27 130.75 ∆=0.52 eV

128

129

792

792 794 796 798 800 802 804 806 808 Binding energy/eV

d

∆=0.30 eV

-NH399.76

P in surface CoPOx

Intensity/a.u.

P 2p3/2

790

P 2p

PANI/CoP HNWs-CFs P in CoP

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

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∆=0.48 eV

CoP NWs-CFs 130 131 132 133 134 Binding energy/eV

-N= 400.47

N 1s +

-N -

402.40 PANI/CoP HNWs-CFs

∆=0.33 eV

400.17 399.43

∆=1.07 eV

401.53

PANI NDs-CFs

398

399

400 401 402 403 Binding energy/eV

404

Figure 2. XPS spectra of (a) Co 2p3/2; (b) Co 2p1/2; (c) P 2p of CoP NWs-CFs and PANI/CoP HNWs- CFs; (d) XPS spectra of N 1s of PANI NDs-CFs and PANI/CoP HNWs-CFs.


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0

b0

-20

Current density/mA cm

PANI NDs-CFs

-40 -60 CoP NWs-CFs

PANI/CoP HNWs-CFs

-80

-100

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

PANI/CoP HNWs-CFs 20% Pt/C Pt metal

-20

-2

CFs

Current density/mA cm-2

a

-40 -60 -80

-100 -0.20

-0.15

Potential/V vs. RHE PANI/CoP HNWs-CFs PANI NDs-CFs CoP NWs-CFs CFs

0.2

0.1

-0.10

-0.05

0.00

Potential/V vs. RHE

d35

PANI/CoP HNWs-CFs

PANI NDs-CFs

30 25

142.1 mV/dec -Z''/Ohm

c0.3 Overpotential/V

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


20

169.5 mV/dec

15

41.6 mV/dec

CoP NWs-CFs

10 5

34.5 mV/dec 0.0

-0.9 -0.6 -0.3 0.0 0.3 -2 0.6 Log(j/mA.cm )

0.9

1.2

0

0

5

10

15 20 Z'/Ohm

25

30

Figure 3. (a-b) Polarization curves of PANI/CoP HNWs-CFs, PANI NDs-CFs, CoP NWs-CFs, CFs, Pt metal (Pt loading is 4.5 mg/cm2) and 20 wt% Pt/C (Pt loading is 0.012 mg/cm2); (c) Tafel plots and (d) Nyquist plots of PANI/CoP HNWsCFs, PANI NDs-CFs, CoP NWs-CFs and CFs; (e) Chronoamperometry measurements of PANI/CoP HNWs-CFs under the various overpotentials; (f) Chronoamperometry measurements of PANI/CoP HNWs-CFs, Pt metal and Pt/C catalysts for 30 h HER at the overpotential of 100 mV.



B

H2SO4 (0.5 M)

H in 80000 concentrated H2SO4

H2SO4 (Concentrated)

Intensity/a.u.

H2O

0.07 ppm H2O

9.47 60000

4.45 4.52

-

40000

H in HSO4 ions of 0.5 M H2SO4

+

H in hydronium ions of 0.5 M H2SO4

3.61

b

4.5

PANI/CoP HNWs-acid

-OH -NH+ 2

C=C PANI NDs-acid -OH with H bond

PANI/CoP HNWs

6.89

H signal of -NH8.82 8.85

3

4

5

6

7 8 δ/ppm

d11

Benzene rings vibrations

C-C H2O

9.39

6.84

PANI NDs-acid

2

0 7.8 8.4 9.0 9.6 10.2

Chemical shift/ppm

c

7.08

CoP NWs-acid CoP NWs

9

10 11 12 10.72

+

C-N

CoP NWs-acid

10

pH value

4.0

+

H signal of -NH2 9.31

20000

Potonated N=Q=N

3.5

7.01

PANI NDs

8.27

3.0

H signal of benzene ring PANI/CoP HNWs-acid

Intensity/a.u.

a

Absorbance/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

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9

N-H 8

PANI/CoP HNWs

7.43 PANI NDs

CoP NWs

7

3600 3400 3200 3000 1800 1600-1 1400 1200 1000 Wavenumber/cm

PANI/CoP HNWs-CFs

7.65

PANI NDs -CFs

CoP NWs -CFs

Figure 4. (a) 1H NMR spectra of 0.5 M H2SO4, the concentrated H2SO4, and water; (b) 1H NMR spectra of PANI NDs,

CoP NWs and PANI/CoP HNWs before and after 0.5 M H2SO4 treatment; (c) FT-IR ATR spectra of PANI NDs-CFs, CoP NWs-CFs and PANI/CoP HNWs-CFs after immersing in 0.5 M H2SO4 solution; (d) pH value of 10 mL 0.001 M NaOH solution after immersing PANI NDs-CFs, CoP NWs-CFs and PANI/CoP HNWs-CFs that all have been treated in 0.5 M H2SO4 (Inset: The procedures of measuring the H+ captured on various electrocatalysts).


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a

b

1.00

+

0.6 0.5

H from CoP/PANI-H to CoP surface

(A): Protonated amine group

+ +

H13O6

0.2

PANI-H

+

0.4

+

0.51 +

0.48

0.62

CoP/PANI-H

0.57

CoP-H13O6

0.6

(B): H13O6+

+

Different types of H

c 1.2 1.1

+

+

0.4

0.531

+

CoP-H13O6

CoP-H + 6H2O +

CoP/PANI-H

+

CoP-H /PANI

0.3 0.2

+

Different types of H transfer pathways

d

PANI/CoP CoP

∆ G/eV

+

0.8

Bare H

Charge density on H

+

H from CoP-H13O6 to CoP surface

0.625

+

1.0

1.168

PANI 1.0 ∆ GH*/eV

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


0.9 0.0

-0.087

-0.1 -0.2

M

-0.155 M-H

M+1/2 H2

Figure 5. (a) Charge distributions on the different types of H+ [Inset: model of a protonated amine group in PANI (A)

and H13O6+ (B)]; (b) Free energy change of different kinds of H+ transfer pathway; (c) Free energy profile of H adsorption on the different catalysts; (d) Scheme of HER improvement by capturing the H+ in hydronium ions to form the protonated amine groups on PANI/CoP HNWs.



TOC

-2

0

Current density/mA cm

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

-10

PANI/CoP HNWs-CFs 20 wt% Pt/C Pt metal

-20 -30 -40 -50 -0.4

-0.3

-0.2

-0.1

0.0

Potential/V vs. RHE


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CoP Hybrid

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