Anion and Cation Modulation in Metal Compounds for Bifunctional

ACS2GO © 2017. ← → → ←. loading. To add this web app to the home screen open the browser option menu and tap on Add to homescreen...
0 downloads 0 Views 4MB Size
Download PDF
Anion and Cation Modulation in Metal Compounds for Bifunctional Overall Water Splitting Jingjing Duan,† Sheng Chen,† Anthony Vasileff,† and Shi Zhang Qiao*,†,‡ †

School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

‡

S Supporting Information *

ABSTRACT: As substitutes for precious cathodic Pt/C and anodic IrO2 in electrolytic water splitting cells, a bifunctional catalyst electrode (Fe- and O-doped Co2P grown on nickel foam) has been fabricated by manipulating the cations and anions of metal compounds. The modified catalyst electrode exhibits both superior HER and OER performances with high activity, favorable kinetics, and outstanding durability. The overall ability toward water splitting is especially extraordinary, requiring a small overpotential of 333.5 mV to gain a 10 mA cm−2 current density. A study on the electrocatalytic mechanism reveals that the atomic modulation between cation and anion plays an important role in optimizing the electrocatalytic activity, which greatly expands the active sites in the electrocatalyst. Further, the three-dimensional conductive porous network is highly advantageous for the exposure of active species, the transport of bubble products, and the transfer of electrons and charges, which substantially boosts reaction kinetics and structure stability. KEYWORDS: bifunctional catalyst electrodes, overall water splitting, heterogeneous catalysis, cation doping, anion doping ater electrolysis (2H2O → 2H2 + O2) is considered to be a secure and sustainable technology which can resolve the looming energy and environmental crisis because it can generate clean H2 energy with zero environmental emissions.1,2 However, commercial Pt/C electrocatalysts, used for the cathodic hydrogen evolution reaction (HER; H2O → H2), and benchmark IrO2 electrocatalyst, used for the anodic oxygen evolution reaction (OER; H2O → O2), are high-cost and low in abundance, which greatly impedes the development of water electrolysis technology.3,4 Currently, only ∼4% of H2 is generated by water electrolysis.1 Therefore, there is still huge scope to achieve the commercial success of a sustainable H2 economy.1 Electrocatalysts for the HER and OER have been actively pursued recently to replace Pt/C and IrO2, respectively. Such examples include metal sulfides5,6 and phosphides7,8 for the HER, and metal oxides9 and hydroxides10 for the OER. Cathodic HER during water electrolysis in alkaline conditions usually requires a higher overpotential when compared with acidic conditions. This translates to higher energy input, which is due to the extra energy barrier originating from the additional water association step.5,6,11 Although nonprecious metals (such as Raney Ni) have been

W

© 2016 American Chemical Society

explored in alkaline electrolytes as HER electrocatalysts, their activity is not sufficient for the large-scale production of H2.12−14 Therefore, it is pressing to develop HER catalysts that are highly active and stable in alkaline electrolytes. Recently, metal phosphides have shown great potential toward the HER in alkaline electrolytes, which is related to their metallic conductivity and the “ensemble effect”. The negatively charged P acts as a proton acceptor and weakens the metal (positively charged, hydride acceptor as M−H) bond strength to promote hydrogen desorption (unlike pure metals which adsorb hydrogen too strongly, causing desorption of H2 to be kinetically sluggish).15,16 However, the surface oxidation of metal phosphides resulting from the under-coordinated HER active P sites has proven to be disadvantageous for the HER because it can compromise HER performance by blocking protons from reaching active sites and reducing network conductivity.17 The design of bifunctional electrocatalysts for overall water splitting requires rich active sites for not only cathodic HER but also anodic OER, which remains a great Received: June 28, 2016 Accepted: September 13, 2016 Published: September 13, 2016 8738

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745

Article

www.acsnano.org

Article

ACS Nano challenge.18,19 Metal oxides, especially cobalt oxides, represent a class of conventional and low-cost OER catalysts which are abundant in nature, diverse, and, most critically, inherently active toward the OER.20 Consequently, the surface oxidation of metal phosphides is advantageous for the OER process,21,22 and therefore, oxygen-doped metal phosphides could be an potential candidate as a bifunctional water splitting electrocatalyst. Mixed metal phosphides have exhibited increased HER activity in comparison to single metal phosphides because the mixed metals can push the hydrogen adsorption free energy to an intermediate value.23 Meanwhile, mixed metal oxides (Fe, Co, Ni) outperform their single metal oxide/hydroxide counterparts in OER catalysis, possibly correlating to the orbital occupation of surface metal cations, the electron transfer between the different metals, and their oxophilicity (e.g., M− OH bond strength), which is believed to facilitate the OER initialization.9,24−26 Therefore, cation doping is considered to be effective in modulating the electrocatalytic activity at the atomic level, which has been commonly investigated.27 However, non-metal-anion doping in metal compounds has been rarely reported but is expected to improve the intrinsic activity of electrocatalysts further because of the significant tuning of the electronic structure.17,28−32 For instance, the d electronic structure of Mo in MoS2(1−x)Se2x was tuned by changing the Se:S ratio in the anions to achieve higher HER activity, which is related to the bonding energy with reactive adsorbates.29 When S in CoS2 was substituted by P, the antibonding orbitals were depleted because of P’s fewer valence electrons, which strengthens the metal−ligand bonding and thus the stability; also, the hydrogen adsorption free energy can be tuned to more thermoneutral by changing S:P ratios, leading to enhanced HER activity while preserving the pyrite structure.28 Moreover, O incorporation in MoS2 can promote the hybridization between Mo d-orbital and S p-orbital, leading to a much smaller band gap, better conductivity compared to that of the pristine 2H-MoS2, and easy electrocatalysis.32 Herein, we have developed a bifunctional catalyst electrode (Fe- and O-doped Co2P−CoFePO) toward overall water splitting using metal hydroxide grown on nickel foam (NF) as the template. The doping percentages of cations and anions were tuned by simply adjusting the amount of Fe (Fe(NO3)3) and P (triphenylphosphine, TPP) sources in order to optimize the HER and OER activity. The manipulated catalyst electrode has displayed outstanding water electrolysis performance, requiring low overpotentials of 87.5 mV for the HER, 274.5 mV for the OER, and 333.5 mV for the overall water splitting to achieve a 10 mA cm−2 current density. To the best of our knowledge, this is one of the earliest works investigating Odoping in metal phosphides for use as a bifunctional water splitting electrocatalyst, which provides a direction for exploring potential candidates to replace precious metals in the field of electrocatalysis.32,33 Further, utilization of NF as a conductive substrate for the electrocatalyst affords higher accessibility to water molecules and mechanical robustness because of its three-dimensional (3D) porous structure and facilitates transfer of electrons and charges.34,35

Figure 1. (a) Scheme of the synthesis process of CoFePO; (b) FESEM image of CoFeOH; (c) FESEM image, (d) TEM image, and (e) SEM EDS spectrum of CoFePO.

°C for 3 h in N2. The CoFeOH nanowire clusters are tippointed with a diameter of ∼20 nm and length of ∼200 nm (scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) in Figure 1b and Figures S1 and S2). CoFePO has a similar morphology except that the intact CoFeOH nanowires were decomposed into ultrasmall but interconnected nanoparticles (several nanometers), which means CoFePO is actually composed of nanoparticles arranged as nanowires (Figures 1c,d and S3a,b). Comparing the SEM EDS spectra and mapping images of CoFeOH and CoFePO (Figures 1e and S3c), it is confirmed that phosphorus is added after annealing in the presence of TPP. The above results were verified by the transformation from cobalt hydroxide (CoOH) to cobalt oxide (CoO, annealing CoOH in N2 without TPP, Figure S4) and oxygendoped cobalt phosphide (CoPO, annealing CoOH in N2 with TPP, Figure S5). Even though P and Fe can be observed by the SEM EDS in CoFePO, whether there is a homogeneous anion- and cationdoped phase or heterogeneous multiphase is not clear. Therefore, we further conducted TEM electron energy loss spectroscopy (EELS) mapping on as-obtained CoFePO, as shown in Figures 2a1−a4 and S6. The homogeneous distribution of Co, Fe, P, and O elements is highly dependent on the selected spatial structure of CoFePO, which suggests the uniform chemical compositions throughout CoFePO.29,37 Importantly, this indicates there is only one-phase metal compound formed in CoFePO, which can be further verified by the XRD patterns (Figure 2b). Except for XRD peaks that originated from the NF substrate, XRD patterns of CoO sample can be attributed to CoO phase (JPCDS No. 48-1719), while those of CoPO and CoFePO are very close to Co2P (JPCDS No. 54-0413) in the absence of CoO. When we take a closer look at the dominant (111) peak, it up-shifts relative to the

RESULTS AND DISCUSSION As displayed in Figure 1a, cobalt iron hydroxide was first grown on NF (denoted as CoFeOH) through a chemical bath deposition process at 85 °C for 24 h,36 and then CoFePO was obtained by annealing CoFeOH in the presence of TPP at 600 8739

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745

Article

ACS Nano

Figure 2. (a1−a4) TEM EELS mapping of Co, Fe, P, and O elements obtained from the selected area of Figure S6. (b) XRD patterns of asprepared materials; inset is the enlarged XRD patterns, and the red lines are from the standard XRD peaks of Co2P (JPCDS No. 54-0413).

Figure 3. Polarization curves of (a) HER and (b) OER; insets are the polarization curves under 10 mA cm−2. (c) Turnover frequency values of HER at 300 mV and OER at 400 mV. (d) Chronoamperometric response of CoFePO tested for 100 h HER and OER process. (e) Polarization curves tested in a homemade two-electrode system. (f) Chronoamperometric response of CoFePO−CoFePO; inset is the polarization curves before and after the 100 h stability testing. All the testing was conducted in 1 M KOH electrolyte.

standard Co2P (2θ = 40.83° with a d value of 2.208 Å) with CoPO (2θ = 40.90° with a d value of 2.205 Å) and CoFePO (2θ = 40.96° with a d value of 2.200 Å) after P- and/or Fedoping. This is related to the shrinkage of the Co2P unit cell with O substituting some of the P atoms and Fe substituting some of Co atoms.29,31 Therefore, seldom separated phases of

CoO, pure Co2P, or Fe2P were formed in CoFePO, but the formation mechanism is different from what was expected. During the formation of CoFe phosphides, the O from the precursor was simultaneously doped into it, not P-doped into CoFe oxides. Raman spectra exhibit the same results, with Raman shifts of CoPO and CoFePO close to but different from 8740

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745

Article

ACS Nano

Figure 4. XPS spectra of as-prepared CoFePO, CoPO, and CoO materials: (a) Co 2p, (b) O 1s, (c) P 2p, (d) Fe 2p, and (e) survey scan of CoFePO.

that of Co2P (Figure S7).38 As a result, the cation and anion modulation can be achieved by adjusting the metal salt precursors and annealing treatment with the P source, respectively. To examine the water splitting performance of as-obtained catalysts, both cathodic HER and anodic OER processes were each evaluated in a three-electrode system (without iR correction for all the data). In Figure 3a, the CoFePO displays remarkable HER activity, with a low overpotential of 87.5 mV to achieve 10 mA cm−2 current density (defined as η10‑HER), which is very close to 56.5 mV of commercial Pt/C and much smaller than 165.5 mV of CoPO without Fe, 191.5 mV of CoO without Fe and P, and 358.5 mV of the NF substrate. The HER kinetics were revealed using Tafel plots (Figure S8). CoFePO exhibits favorable HER kinetics with a small Tafel slope (38.1 mV dec−1) close to that of Pt/C (30.5 mV dec−1) and smaller than those of CoPO (63.4 mV dec−1), CoO (88.4 mV dec−1), and NF (128.1 mV dec−1), which was measured in the low

overpotential range.39−41 Accordingly, a Volmer−Heyrovsky mechanism was determined for CoFePO with the Heyrovsky step (electrochemical desorption step) as the rate-determining step for HER.11 Therefore, with the highest HER activity and most favorable kinetics, the as-obtained CoFePO achieves the best HER performance among all the studied non-noble samples and is one of the best among reported catalysts in basic electrolyte.16,39,42 The electrochemical impedance spectra (EIS) testing was conducted to evaluate the electron and charge transfer ability during the electrochemical process.6 Surprisingly, the system resistance (Rs, tested at 1.023 mV vs RHE) of CoFePO (1.42 Ω cm2) is higher than that of CoPO (0.82 Ω cm2), implying a lower conductivity of CoFePO compared to that of CoPO (Figure S9a). The charge transfer resistance (Rct, measured at −0.227 V vs RHE where HER occurs) of CoFePO (0.81 Ω cm2) is also larger than that of CoPO (0.57Ω cm2), suggesting that CoFePO has a lower charge transfer ability than CoPO 8741

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745

Article

ACS Nano

Figure 5. (a) HER and (b) OER polarization curves of CoFePO materials with differing amounts of P source (0.325, 0.75, 1, 1.25, and 1.5). (c) Overpotentials to achieve a current density of 10 mA cm−2.

CoFePO still displays a larger TOF (6.8 s−1, Figure 3c) toward the OER tested at 400 mV than CoPO (1.2 s−1) and CoO (0.3 s−1).43 Consequently, the CoFePO with cation and anion doping at the atomic level exhibits not only superior HER activity but also outstanding OER activity, both of which are among the best for reported materials.16,20,39,42,44 Of equal importance to high activity, a proof-of-concept electrocatalyst also requires durability and stability during water splitting. According to the separate stability tests of CoFePO for both the HER and OER (Figure 3d), over 90% of the initial current density was retained after 100 h of the HER process, while 85% remained after 100 h of the OER process. Therefore, CoFePO is a robust electrode for both HER and OER.10,16 The electrochemical parameters are summarized in Tables S1 and S2 for the HER and OER, respectively. The overall water splitting activity was tested in a homemade two-electrode system (Figure 3e). The overpotential to obtain a current density of 10 mA cm−2 (η10) using the CoFePO couple electrode was found to be as low as 333.5 mV, which outperformed the state-of-the-art Pt/C−IrO2 counterparts (491.5 mV), CoPO−CoPO (445.5 mV), CoO−CoO (500.5 mV), and NF−NF (659.5 mV). Under the electrolysis process, bubbles were observed on the surface of the electrodes (see inset in Figure 3e and the supporting video). The electrodes retained 74% of the initial current density after 100 h of operation (Figure 3f). The η10 obtained using a two-electrode system is slightly smaller but close to that of the three-electrode system (362 mV, adding η10‑HER and η10‑OER), maybe because of the different testing configurations.41,45 This demonstrates that the water splitting performance of CoFePO is outstanding in comparison to that of recently reported catalysts and even benchmark catalysts (Table S3). To gain insight into the origin of the extraordinary water splitting ability, X-ray photoelectron spectroscopy (XPS) was collected to investigate the composition and element valence status before and after anion and cation doping (Figure 4 and

(Figure S9b). Therefore, it leaves the question as to why CoFePO with a lower electron and charge transfer ability than CoPO shows higher HER activity. We sought to determine the electrochemical active surface area (ECSA), which represents the amount of active sites in electrocatalysts, to solve this mystery (Figure S10).23 Divided by the average specific capacitance (40 μF cm−2) of reported materials, the calculated ECSAs are 1145.1, 860.6, 457.6, and 72.6 for CoFePO, CoPO, CoO, and NF, respectively. The ECSA was improved by 1.88fold from CoO to CoPO, possibly due to the annealing process that introduces nanopores and macropores in and between the nanowires (see Figures S4b,c and S5b,c; the roughness was clearly enhanced, and the nanowires become incomplete). After Fe-doping, the ECSA was further improved by 1.33-fold from CoPO to CoFePO, possibly because of the much smaller nanoparticles of the latter (from tens of nanometers to several nanometers). The HER turnover frequency (TOF) at an overpotential of 300 mV was calculated (Figure 3c), and the TOF of CoFePO (16.87 s−1) was greatly improved after cation doping (CoPO; 2.62 s−1) and anion doping (CoO; 0.82 s−1).23,43 CoFePO also displays high OER activity, requiring an overpotential of 274.5 mV to achieve 10 mA cm−2 current density (η10‑OER, obtained by subtracting 1.23 V vs RHE from 1.5045 V, Figure 3b), which is smaller than those of the benchmark IrO2 (351.5 mV), CoPO (304.5 mV), CoO (346.5 mV), and NF (431.5 mV). Additionally, CoFePO has favorable OER kinetics with a small Tafel slope (51.7 mV dec−1, Figure S11), which is smaller than those of IrO2 (81.8 mV dec−1), CoPO (60.7 mV dec−1), CoO (80.7 mV dec−1), and NF (120.0 mV dec−1). The η10‑OER and Tafel slope of CoFePO material are smaller than those of many reported materials, indicating its extraordinary OER activity.20,44 Similarly with HER, the Rct of CoFePO during OER is higher than that of CoPO, indicating that the charge transfer ability of CoFePO is worse than that of CoPO (Figure S12). However, with a much higher ECSA, 8742

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745

Article

ACS Nano

CONCLUSIONS In summary, we have developed a highly efficient bifunctional catalyst electrode for overall water splitting through the cation and anion manipulation in metal compounds. The obtained CoFePO catalyst electrode displays outstanding HER and OER performance with low overpotentials, robust operability, and stability. Significantly, its optimized overall water splitting activity with η10 as low as 335.5 mV even outperforms the stateof-the-art Pt/C−IrO2 counterparts (491.5 mV), which was achieved by modulating the anion and cation percentages in metal compounds by expanding the amount of HER active sites. This provides a pathway for the design and synthesis of the electrocatalyst, which is believed to broaden the horizon of materials chemistry and electrocatalysis.

Figure S13 and Table S4). The main peak of Co 2p3/2 binding energy (BE) down-shifts from 780.0 eV of CoO to 778.8 eV of CoPO, together with the up-shifting of O 1s BE from 529.7 to 532.1 eV, suggesting the newly formed Co−P and O−P bonds (Figure 4a,b).23,46 By comparing the survey scans of CoO and CoPO (Figure S13), new peaks emerge which can be assigned to P 2s and P 2p, with a P/O ratio of 39.5 atom %. The deconvolution of the high-resolution P 2p XPS spectrum of CoPO (Figure 4c) displays three peaks at 129.6 (2p3/2), 130.6 (2p1/2), and 133.3 eV (PO43−). The lower BE of P 2p3/2 in CoPO compared to that of P0 (130.2 eV) suggests strong bonding is present between metal and P (Co−P, Figure 4c), indicating the formation of Co2P with oxygen-doping, which is consistent with the XRD results.23,46,47 Moreover, the effect of cation Fe-doping was also investigated, and a new Fe 2p peak was observed by comparing the XPS survey scans of CoFePO and CoPO (Figures 4d,e and S13b), with a Fe/Co ratio of 15.8 atom %.23 The BE values of Co 2p and O 1s in CoFePO located at 779.0 and 532.3 eV are close to those of CoPO, also suggesting the presence of P as P− metal and P−O bonding. However, the Co 2p BE in CoFePO still increases a little bit in comparison to that of CoPO (778.8 eV), probably because of the presence of Fe in the lattice. The O 1s BE displays the same trend by increasing from 532.1 to 532.3 eV. The deconvolution of P 2p in CoFePO shows peaks at 129.8 (2p3/2), 130.7 (2p1/2), and 133.9 eV (PO43−), which are also marginally higher than those in CoPO, and with a P/O ratio of 42.3 atom %. Importantly, the deconvolution of the high-resolution Fe 2p spectrum can be assigned to Fe 2p3/2 (711.3 eV), Fe 2p3/2 satellite (715.9 eV), Fe 2p1/2 (724.4 eV), and Fe 2p1/2 satellite (730.2 eV).23 According to the BE shifts of Co 2p, O 1s, P 2p, and the newly appearing Fe 2p peak, it can be proposed that Fe was also successfully doped into CoFePO. The element percentage measured using XPS is summarized in Table S5. Further, an electrochemical study was conducted by adjusting the amounts of Fe and P source to examine the optimal percentages of cation and anion in metal compounds. For reliable comparison, we held constant the amount of P source (TPP) in the synthesis of CoFePO catalysts (such as CoFePO-0.325, CoFePO-0.75, CoFePO-1, CoFePO-1.25, and CoFePO-1.5). The HER and OER polarization curves are displayed in Figure 5a,b, and η10‑HER, η10‑OER, and η10 (calculated by adding η10‑HER and η10‑OER) are compared in Figure 5c. The η10 promoted by CoFePO-0.325 is 487 mV, which decreases to 484 mV for CoFePO-0.75 and then 362 mV for CoFePO-1. By further increasing the TPP amount, η10 increases to 437 mV for CoFePO-1.25 and then 520 mV for CoFePO-1.5. Therefore, the optimum anion doping percentage with a P/O ratio of 42.3 atom % is obtained in CoFePO-1, whereby moderate binding of the intermediates and products with the catalyst surface is achieved, combining a preferential amount of HER and OER active sites.15 Moreover, a detailed electrocatalytic HER and OER investigation on the cation doping effect suggests the best Fe-doping amount for electrolytic water splitting is a Fe/Co ratio of 15.8 atom % (Figure S14). This might be related to a near-optimal reaction free energy obtained at this Fe/Co ratio or possibly the orbital occupation and electron transfer between mixed metal cations.9,23,24 Therefore, it can be concluded that the atomic modulation (both anion and cation) plays a critical role in the optimization of water splitting activity.

EXPERIMENTAL SECTION Materials Synthesis. NF was cleaned by sonication in acetone, ethanol, and Milli-Q water for 30 min successively before being used as substrates. In a typical synthesis procedure, 291 mg of Co(NO3)2· 6H2O and 40.4 mg of Fe(NO3)3·9H2O were dissolved in 4 mL of Milli-Q water. A solution of urea was then made by dissolving 792 mg of urea into 1.2 mL of water. The above two solutions were mixed in a glass vessel, and the clean NF (1 cm × 3 cm) was then immersed in the resultant solution. This reaction system was then kept at 85 °C for 24 h. After 24 h, the as-obtained CoFeOH was washed with water three times and dried at 60 °C. To synthesize CoFePO, the dried CoFeOH was mixed with 2.103 g of TPP and heated at 600 °C for 3 h in a N2 atmosphere with a heating rate of 2 °C min−1. To prepare the sample without Fe−CoPO, only 291 mg of Co(NO3)2·6H2O was dissolved in 4 mL of H2O and then mixed with the same urea solution as above (1.2 mL of H2O and 720 mg of urea). The subsequent procedure followed is similar to that for CoFePO. Characterization. The morphologies of samples were characterized using TEM (Tecnai G2 Spirit) and SEM (QUANTA 450). EDS was measured on a QUANTA 450 SEM. Before SEM testing, the as-prepared samples were coated with Pt. The EELS mapping was conducted on the Phillips CM200 TEM. XRD patterns were recorded on a Philips 1130 X-ray diffractometer (40 kV, 25 mA, Cu Kα radiation, λ = 1.5418 Å). Raman spectra were obtained on a WiTEC alpha 300R Raman microscope with a 532 nm solid laser as an excitation source. XPS was performed in an ultrahigh vacuum apparatus built by SPECS (Berlin, Germany) at 200 W using a nonmonochromatic X-ray source for Mg. The valence band XPS was performed using an Al anode. The XPS spectra were referenced to the C 1s peak (285 eV). Electrochemical Testing. HER and OER analyses were performed in a three-electrode glass cell on a CHI 650 workstation (Pine Research Instruments, USA) using a standard Ag/AgCl/KCl (4 M) electrode as the reference electrode, a graphite rod as the counter electrode, and the prepared materials as the working electrodes. The overall water splitting tests were conducted in a two-electrode glass cell which was assembled in the lab using the prepared materials both as the cathode and the anode. The loading mass of metals for the CoFePO sample was 2.187 mg cm−2; therefore, the same amount of Pt and IrO2 was coated on NF for comparison. Specifically, 10.935 mg of Pt/C or 2.55 mg of IrO2 was dissolved in 1 mL of ethanol, and the obtained suspension was coated on NF (1 cm × 1 cm) and dried under ambient conditions. Fifty microliters of 1% nafion was then pipetted onto the dried NF. In addition, 20 microliters of Pt/C powder suspension (2 mg mL−1) was cast on a rotating disk electrode (0.196 cm2) to compare with the sample on NF. The polarization curves were collected at a scan rate of 1 mV s−1. EIS spectra were recorded under an AC voltage amplitude of 5 mV with frequencies from 1 Hz to 1 × 105 Hz. Rs data were obtained in the high-frequency zone. All measurements were carried out in N2saturated 1 M KOH to eliminate dissolved O2. All potentials were calibrated to RHE by adding a value of (0.197 + 0.059 × pH) V. 8743

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745

Article

ACS Nano Turnover Frequency. The TOF values were calculated by assuming that every metal atom was involved in the catalysis (lower limits):23,43

TOF =

Hydrogen Evolution Catalyst Electrodes. Adv. Mater. 2015, 27, 4234− 4241. (7) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. P. Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 53, 6710−6714. (8) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. P. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0−14. J. Am. Chem. Soc. 2014, 136, 7587−7590. (9) Yang, Y.; Fei, H.; Ruan, G.; Xiang, C.; Tour, J. M. Efficient Electrocatalytic Oxygen Evolution on Amorphous Nickel-Cobalt Binary Oxide Nanoporous Layers. ACS Nano 2014, 8, 9518−9523. (10) Lu, X.; Zhao, C. Electrodeposition of Hierarchically Structured Three-Dimensional Nickel-Iron Electrodes for Efficient Oxygen Evolution at High Current Densities. Nat. Commun. 2015, 6, 6616. (11) Duan, J. J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@N-Graphene Films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931−940. (12) Marceta Kaninski, M. P.; Miulovic, S. M.; Tasic, G. S.; Maksic, A. D.; Nikolic, V. M. A Study on the Co-W Activated Ni Electrodes for the Hydrogen Production from Alkaline Water Electrolysis-Energy Saving. Int. J. Hydrogen Energy 2011, 36, 5227−5235. (13) McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. Ni-Mo Nanopowders for Efficient Electrochemical Hydrogen Evolution. ACS Catal. 2013, 3, 166−169. (14) Ledendecker, M.; Clavel, G.; Antonietti, M.; Shalom, M. Highly Porous Materials as Tunable Electrocatalysts for the Hydrogen and Oxygen Evolution Reaction. Adv. Funct. Mater. 2015, 25, 393−399. (15) Liu, P.; Rodriguez, J. A. Catalysts for Hydrogen Evolution from the [NiFe] Hydrogenase to the Ni2P(001) Surface: The Importance of Ensemble Effect. J. Am. Chem. Soc. 2005, 127, 14871−14878. (16) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. (17) Kibsgaard, J.; Jaramillo, T. F. Molybdenum Phosphosulfide: An Active, Acid-Stable, Earth-Abundant Catalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 14433−14437. (18) Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional Non-Noble Metal Oxide Nanoparticle Electrocatalysts through Lithium-Induced Conversion for Overall Water Splitting. Nat. Commun. 2015, 6, 7261. (19) Andersen, M.; Medford, A. J.; Nørskov, J. K.; Reuter, K. Analyzing the Case for Bifunctional Catalysis. Angew. Chem., Int. Ed. 2016, 55, 5210−5214. (20) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (21) Chen, G. F.; Ma, T. Y.; Liu, Z. Q.; Li, N.; Su, Y. Z.; Davey, K.; Qiao, S. Z. Efficient and Stable Bifunctional Electrocatalysts Ni/NixMy (M = P, S) for Overall Water Splitting. Adv. Funct. Mater. 2016, 26, 3314−3323. (22) Jiao, L.; Zhou, Y. X.; Jiang, H. L. Metal-Organic FrameworkBased CoP/Reduced Graphene Oxide: High-Performance Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Sci. 2016, 7, 1690− 1695. (23) Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Nørskov, J. K.; Abild-Pedersen, F.; Jaramillo, T. F. Designing an Improved Transition Metal Phosphide Catalyst for Hydrogen Evolution Using Experimental and Theoretical Trends. Energy Environ. Sci. 2015, 8, 3022−3029. (24) Fominykh, K.; Chernev, P.; Zaharieva, I.; Sicklinger, J.; Stefanic, G.; Doblinger, M.; Muller, A.; Pokharel, A.; Bocklein, S.; Scheu, C.; et al. Iron-Doped Nickel Oxide Nanocrystals as Highly Efficient Electrocatalysts for Alkaline Water Splitting. ACS Nano 2015, 9, 5180−5188. (25) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The

j 4×F×n

where j is the current density (A cm−2), F is Faraday’s constant (96485.3 C mol−1), and n is moles of electrocatalysts (mol cm−2). ECSA. Electrochemical capacitance measurements were used to determine the active surface area of each catalyst. To measure the electrochemical capacitance, the potential was swept between 0.823 and 1.023 V vs RHE at different scan rates (20, 40, 60, 80, 100, 120, 140, 160, 200, 250, and 300 mV s−1), where no faradaic process occurs. The capacitive currents were measured at 0.923 V vs RHE, plotted as a function of scan rate, and then a linear fit was used to determine the specific capacitance. The specific capacitance can be converted into an ECSA using the specific capacitance value for a flat standard with 1 cm2 of real surface area. The specific capacitance for a flat surface is generally found to be in the range of 20−60 μF cm−2; therefore, an average value of 40 μF cm−2 was used in the calculation. Mass Activity. The values of mass activity (A g−1) were calculated using the following equation:43

mass activity (A g −1) =

j M

where M is the electrocatalyst loading amount (mg cm−2) and j is the measured current density (mA cm−2).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04252. Additional figures and tables (PDF) Supporting video (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Australian Research Council (ARC) through the Discovery Project program (DP130104459, DP140104062, and DP160100927). REFERENCES (1) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972−974. (2) Cobo, S.; Heidkamp, J.; Jacques, P. A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; et al. A Janus Cobalt-Based Catalytic Material for Electro-Splitting of Water. Nat. Mater. 2012, 11, 802−807. (3) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I. B.; Nørskov, J. K. Computational High-Throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2006, 5, 909−913. (4) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (5) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. J. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296− 7299. (6) Duan, J. J.; Chen, S.; Chambers, B. A.; Andersson, G. G.; Qiao, S. Z. 3D WS2 Nanolayers@Heteroatom-Doped Graphene Films as 8744

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745

Article

ACS Nano Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136, 6744−6753. (26) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt-Iron (Oxy)Hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638−3648. (27) Li, J. Y.; Yan, M.; Zhou, X.; Huang, Z. Q.; Xia, Z.; Chang, C. R.; Ma, Y.; Qu, Y. Mechanistic Insights on Ternary Ni2‑xCoxP for Hydrogen Evolution and Their Hybrids with Graphene as Highly Efficient and Robust Catalysts for Overall Water Splitting. Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601420. (28) Caban-Acevedo, M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H. C.; Tsai, M. L.; He, J. H.; Jin, S. Efficient Hydrogen Evolution Catalysis Using Ternary Pyrite-Type Cobalt Phosphosulphide. Nat. Mater. 2015, 14, 1245−1251. (29) Gong, Q.; Cheng, L.; Liu, C.; Zhang, M.; Feng, Q.; Ye, H.; Zeng, M.; Xie, L.; Liu, Z.; Li, Y. Ultrathin MoS2(1‑x)Se2x alloy Nanoflakes for Electrocatalytic Hydrogen Evolution Reaction. ACS Catal. 2015, 5, 2213−2219. (30) Zhao, Y.; Kamiya, K.; Hashimoto, K.; Nakanishi, S. In Situ CO2Emission Assisted Synthesis of Molybdenum Carbonitride Nanomaterial as Hydrogen Evolution Electrocatalyst. J. Am. Chem. Soc. 2015, 137, 110−113. (31) Zhuo, J.; Cabán-Acevedo, M.; Liang, H.; Samad, L.; Ding, Q.; Fu, Y.; Li, M.; Jin, S. High-Performance Electrocatalysis for Hydrogen Evolution Reaction Using Se-Doped Pyrite-Phase Nickel Diphosphide Nanostructures. ACS Catal. 2015, 5, 6355−6361. (32) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering in Oxygen-Incorporated MoS2 Ultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013, 135, 17881−17888. (33) Fan, X.; Luo, C.; Lamb, J.; Zhu, Y.; Xu, K.; Wang, C. PEDOT Encapsulated FeOF Nanorod Cathodes for High Energy Lithium-Ion Batteries. Nano Lett. 2015, 15, 7650−7656. (34) Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X. P. A Cost-Effective 3D Hydrogen Evolution Cathode with High Catalytic Activity: FeP Nanowire Array as the Active Phase. Angew. Chem., Int. Ed. 2014, 53, 12855−12859. (35) Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-Index Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14023−14026. (36) Chen, S.; Duan, J. J.; Bian, P. J.; Tang, Y. H.; Zheng, R. K.; Qiao, S. Z. Three-Dimensional Smart Catalyst Electrode for Oxygen Evolution Reaction. Adv. Energy Mater. 2015, 5, 1500936. (37) Liu, W.; Hu, E.; Jiang, H.; Xiang, Y.; Weng, Z.; Li, M.; Fan, Q.; Yu, X.; Altman, E. I.; Wang, H. A Highly Active and Stable Hydrogen Evolution Catalyst Based on Pyrite-Structured Cobalt Phosphosulfide. Nat. Commun. 2016, 7, 10771. (38) Pradhan, B.; Kumar, G. S.; Dalui, A.; Khan, A. H.; Satpati, B.; Ji, Q.; Shrestha, L. K.; Ariga, K.; Acharya, S. Shape-Controlled Cobalt Phosphide Nanoparticles as Volatile Organic Solvent Sensor. J. Mater. Chem. C 2016, 4, 4967−4977. (39) Yan, X.; Tian, L.; He, M.; Chen, X. Three-Dimensional Crystalline/Amorphous Co/Co3O4 Core/Shell Nanosheets as Efficient Electrocatalysts for the Hydrogen Evolution Reaction. Nano Lett. 2015, 15, 6015−6021. (40) Shinagawa, T.; Garcia-Esparza, A. T.; Takanabe, K. Insight on Tafel Slopes from a Microkinetic Analysis of Aqueous Electrocatalysis for Energy Conversion. Sci. Rep. 2015, 5, 13801. (41) You, B.; Jiang, N.; Sheng, M.; Bhushan, M. W.; Sun, Y. Hierarchically Porous Urchin-Like Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2016, 6, 714−721. (42) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. P. Closely Interconnected Network of Molybdenum Phosphide Nanoparticles: A Highly Efficient Electrocatalyst for Generating Hydrogen from Water. Adv. Mater. 2014, 26, 5702−5707.

(43) Tung, C. W.; Hsu, Y. Y.; Shen, Y. P.; Zheng, Y.; Chan, T. S.; Sheu, H. S.; Cheng, Y. C.; Chen, H. M. Reversible Adapting Layer Produces Robust Single-Crystal Electrocatalyst for Oxygen Evolution. Nat. Commun. 2015, 6, 8106. (44) Song, F.; Hu, X. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477. (45) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661−4672. (46) Xiao, P.; Chen, W.; Wang, X. A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution. Adv. Energy Mater. 2015, 5, 1500985. (47) Xiao, P.; Sk, M. A.; Thia, L.; Ge, X.; Lim, R. J.; Wang, J. Y.; Lim, K. H.; Wang, X. Molybdenum Phosphide as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 2624−2629.

8745

DOI: 10.1021/acsnano.6b04252 ACS Nano 2016, 10, 8738−8745