Synergistic Effect of Inactive Iron Oxide Core on Active Nickel

Subscriber access provided by READING UNIV

Letter

Synergistic Effect of Inactive Iron Oxide Core on Active Nickel Phosphide Shell for Significant Enhancement in the OER Activity Anirban Dutta, Sankararao Mutyala, Aneeya K. Samantara, Suman Bera, Bikash Kumar Jena, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01141 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Energy Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 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

ACS Energy Letters

Synergistic Effect of Inactive Iron Oxide Core on Active Nickel Phosphide Shell for Significant Enhancement in the OER Activity Anirban Dutta, Sankararao Mutyala,Aneeya K. Samantara,¥ Suman Bera, Bikash Kumar Jena¥* and Narayan Pradhan*

Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India 700032 ¥

Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India 751013

1 ACS Paragon Plus Environment

ACS Energy Letters 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

Abstract: A unique core/shell nanostructured OER catalyst composed of electrochemically inactive crystalline Iron oxide core and active amorphous Nickel phosphide shell is presented which records superior OER activity. Even activators enhancing the activity of OER catalyst by promoting the redox reactions are reported; but here the exclusive position of Iron in the nanostructures indeed boosted the efficiency for its ideal placement. Moreover, these nanostructures are also prepared in a sophisticated mechanistic approach where selectively one metal is phosphidated and other did not. Interestingly, in absence of Iron, Nickel phosphide crystallized in different shape; but in presence of Iron, this specifically formed amorphous NixP which became more efficient for promoting OER. Detail of the formation of this active catalyst is studied, the electrochemical reactions are investigated and the OER activity is compared with different leading metal phosphides. TOC


Page 2 of 22

Page 3 of 22 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

ACS Energy Letters

Electrocatalytic oxygen evolution reaction (OER) is a technological need of storing electrical energy in chemical bonds. This is the half reaction for the overall water splitting process and remains the primary kinetic bottleneck of the overall process. This can be overcome by designing highly active OER electrocatalysts.1-15 Apart from developed metal oxides, chalcogenides and nitrides, recently, metal phosphides are emerged as a new class of competing efficient energy materials for catalyzing water oxidation.16-30 These are typically known as OER pre-catalysts which undergo insitu surface oxidation during electrochemical process and transformed to their corresponding oxides and oxy-hydroxides.18, 31 However, in most cases, the obtained oxidized products were observed even more active in comparison to respective metal oxides.17,

31-37

This indeed motivated designing more effective metal

phosphide catalysts for OER. Literature reports reveal that the leading phosphides in this category are the iron tried.9, 16-17, 19, 38-41 Apart from their pure phase materials, following cation and anion modulations, and tuning the phase and morphology, the catalytic performances were further studied to boost the OER performance.15-18, 33-36, 42-52 However, the underlying chemistry in designing such modulated materials and their electrochemical OER are still limited and need further exploration. One of the convenient approaches for enhancing the OER activity is inducing favorable synergistic effects by modulating phosphides with activating ions for promoting the electrochemical redox process.35-37, 42, 45 However, as these materials undergo oxidation which is a primary process for generating more effective electrocatalyst, the mode of introduction of the activators is critically important. Their presence in close proximity would essentially support for the efficient charge transportation and boosting the electrochemical redox process. Hence, 3 ACS Paragon Plus Environment


designing composed metal phosphides along with the activators in appropriate matrix is critically important. To address this, herein, a unique structure having crystalline Fe3O4 core and amorphous NixP shell is reported where the exclusive placement of Fe at core boosted the OER activity of Nickel phosphides. Such core/shell nanostructures are fabricated by selective phosphidations which allowed Ni to form shell of nickel phosphide on Iron oxides without encouraging any cross nucleations. These structures while explored as OER catalysts, superior catalytic activity was observed in comparison to nickel phosphides obtained in control reactions. On the contrary, while the activator Fe was introduced to the electrode by using commercial KOH or using Iron salt (even excess) or introducing iron oxide nanostructures, the enhanced activity was not observed. Hence, the placement of Fe in the phosphide OER catalyst matters to boost the catalytic activity and to our best knowledge this remained as one of the best OER catalyst among all nanostructured metal phosphides till date. Details of the chemistry of synthesis of the exclusive core/shell structures and through study of the electrochemical process for such high efficiency are carried out and reported in this communication. Phosphidation processes for the mixed metal phosphides were typically carried out via co-nucleation approach at elevated temperature. However, for the mixture of Ni and Fe acetylacetonates, using TPP as a phosphidating agent, selectively Ni was phosphidated and core/shell Fe3O4/NixP nanostructures were formed. Figure 1a shows the schematic presentation of the reaction where hexadecylamine, TPP, Fe(acac)2 and Ni(acac)2 were all loaded together in reaction flask and heated to 270 oC for obtaining the core/shell structures. Figure 1b-1e and Figure S1 show TEM images of these nanostructures in different resolutions obtained in a typical synthetic approach. These were observed nearly monodisperse and all particles were 4 ACS Paragon Plus Environment

Page 4 of 22

Page 5 of 22 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

ACS Energy Letters

uniformly shelled. Interestingly, core has more contrast than shell and the core/shell configurations were visible in all cases. The image in Figure 1e shows the core is possibly crystalline and shell is amorphous.

Figure 1. (a) Schematic presentation of the reaction for formation of core/shell nanostructures. (b-e) TEM images of core/shell nanostructures in different resolutions. (f) Powder X-ray diffraction pattern and (g) EDS pattern of core/shell nanostructures. (h) XPS of Fe in core crystalline Fe3O4. (i) HRTEM and (j) Selected area FFT pattern of a single core/shell nanostructure of Fe3O4/NixP. Further, identifying the phase of the core, powder X-ray diffraction was carried out and the obtained pattern presented in Figure 1f. Interestingly, the pattern resembles to both cubic Fe3O4 and tetragonal γ-Fe2O3, but they differ in the XRD pattern in the range 10-20 degree. In this case no peak in this region was observed and suggested that this peaks were from cubic Fe3O4 (JCPDS card no. 85-1436).53-55 However, energy dispersive spectrum (EDS) (Figure 1g)



showed presence of Ni, Fe, O and P in these nanostructures. Figure 1h depicts the X-ray photoelectron spectra where peak at 710.2 eV and 723.3 eV corresponds to Fe 2p3/2 and 2p1/2 respectively resembling to Fe3O4.56 Presence of phosphide, oxide and Nickel are also supported with XPS (Figure S2).57 From all these observations, the crystalline core confirmed as Fe3O4. However, as EDS showing presence of Ni and P, it would be concluded here that the shell could only be NixP. HRTEM image of a single core/shell structure is shown in Figure 1i and the selected area FFT pattern obtained from this image is presented in Figure 1j. Analysis suggests that the planes (113), (115) and (022) having d-spacing 2.53 Å, 1.61 Å and 2.96 Å of the nanorod are viewed along the zonal axis. The planner distance and inter-planner angle further confirmed the core as Fe3O4. The phosphidation of mixture of Fe(acac)2 and Ni(acac)2 led only Ni to Nickel phosphide and it was formed on Fe3O4 seeds. Hence, for understanding details of the selectivity and shell growth, several controlled reactions were performed. Figure S3 shows schematically the reaction conditions by varying different parameters. Interestingly, it was observed that in absence of Ni(acac)2, Fe(acac)2 also formed Fe3O4 as in the core/shell structures. Hence, it can be concluded here that, thermal decomposition of Fe(acac)2 formed Fe3O4 nanocrystals and it was independent of the presence or absence of phosphidating agent. Typical TEM image of Fe3O4 and powder XRD pattern in presence of TPP is shown in Figure 2a and Figure 2c respectively. Figure S4 presents EDS data showing no P, indicating Fe did not undergo


Page 6 of 22

Page 7 of 22 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

ACS Energy Letters

phosphidation. TEM image and Powder XRD of the Fe3O4nanocrystals prepared without TPP is provided in Figure S5.

Figure 2. (a) TEM image of Fe3O4 nanocrystals synthesized in absence of Ni(acac)2. (b-c) TEM image of Ni12P5 synthesized in absence of Fe(acac)2. (d) Power XRD pattern of respective samples of Fe3O4 and Ni12P5. Interestingly, in absence of Fe(acac)2, Ni(acac)2 formed crystalline highly monodisperse hollow nickel phosphide (TEM presented in Figure 2b-C and) particles. Power XRD pattern for both Iron oxide and Nickel phosphide nanostructures are shown in Figure 2c. From their peak positions, these resembled to cubic Fe3O4 (JCPDF card no. 85-14360) and tetragonal Ni12P5 (JCPDF card no. 22-1190) respectively. It is known that Ni(acac)2 on thermal decomposition in presence of alkylamines leads to Ni(0)(Figure S6),58-60 even mixture of Fe and Ni acetylacetonates without TPP resulted a mixture of Fe3O4 and Ni(0) nanoparticles (Figure S7). Hence, the unique finding here is the phosphidation of Ni on Fe3O4 without having any cross nucleations or formation of separate Ni-phosphide. Further to understand the core/shell nanostructures, the shelling process is decoupled from Fe3O4 formation. First, Fe3O4 nanocrystals were prepared in absence of Ni precursor and 7 ACS Paragon Plus Environment


then Ni(acac)2 was introduced for observing the shell growth. Very interestingly, it was observed that similar core/shell nanostructures were obtained (Figure S8). This suggests that phosphidation of Ni on Fe3O4 is energetically more feasible rather than formation of crystalline Ni-phosphide separately. This might be due to the Fe3O4 seed surface bonded energetically active TPP which are prone to decompose, triggers the nucleation of Nickel phosphide on the seed Fe3O4 nanocrystals. However, the size distribution was broad, in comparison to the reaction having both metal precursors taking together. As these structures contain unique configuration having two iron triad elements, these were further explored as OER electrocatalysts for observing their synergistic effects. Details of the electrochemical measurements are provided in experimental section. Figure 3a presents iR corrected LSVs of all these materials. Among these materials, the core/shell Fe3O4/NixP required only 260 mV for achieving the current density 10 mAcm-2 and observed the lowest in comparison to all controlled and even benchmarked IrO2 electrocatalyst (Figure S9). Reaction kinetics in these processes was investigated by examining the Tafel polarization plots (plot of “η vs log j”) at lower overpotential region. Figure3b presents the Tafel slopes of Fe3O4/NixP and Ni12P5 where the slopes of the linear fitting were observed 43 mV/dec and 128mV/dec respectively. The low overpotential along with the low Tafel slope in the core/shell nanostructures indicated the superiority of this catalyst than pure nickel phosphide. Further, a comparison table has been provided in the supporting information (Table T1). The comparison reveals the activity remains superior to all other nanostructured metal phosphide materials.1617, 31, 36, 47, 61-67


Page 8 of 22

Page 9 of 22 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

ACS Energy Letters

Figure 3. (a) Anodic polarization curves (LSV) of core/shell, Ni12P5 and Fe3O4 materials synthesized under identical reaction condition. (b) Tafel plots of core/shell and Ni12P5. (c) Stability plot for the core/shell material. Inset shows digital image of the electrode before and during the electrolysis. (d) LSVs of initial and after stability test of the core/shell materials. As Catalysis is a surface phenomenon, the electrochemically active surface area (ECSA) of synthesized catalyst materials was estimated from the electrochemical double-layer capacitance (CdL) of the catalytic surface. The CdL was calculated based on the plot of cathodic current (ic) as a function of the scan rate (ν) which appeared as straight line with a slope equal to CdL More discussions are provided in the supporting information. The calculated Cdl value for core/shell materials found to be 4.1 mF with Rf 1464.28 and for nickel phosphide Cdl was 5.6 mF and Rf 2001.58. On the basis of the slopes, the ECSA of nickel phosphide and core-shell was 9 ACS Paragon Plus Environment


observed 140.11 and 102.5 respectively (Figure S10). This implies that the enhanced electrochemically active surface area for the core-shell nanostructures helped for higher catalytic efficiency. The activity is also evidenced by measuring Faradaic efficiency by collecting the evolved gas using water displacement method (details provided in the supporting information). The estimated faradaic efficiency was 95.3% suggesting nearly all charge is consumed by OER without any side reactions (Figure S11). Further, the durability of the electrocatalyst was demonstrated measuring the stability through the chronopotentiometry technique under a static current density of 10 mAcm-2 for 25 hour. Figure 3c presents the chronopotentiometry plot and the inset shows the images of the core-shell modified electrode before and during the electrocatalysis. The LSVs obtained before and after the stability test also plotted (Figure 3d) and it showed almost no change in overpotential taken over 25 hour of continuous electrolysis, suggesting high durability of the Fe3O4/NixP in the long term electrochemical process.


Page 10 of 22

Page 11 of 22 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

ACS Energy Letters

Figure 4. Initial LSVs of (a) core/shell and (b) Ni12P5.(c) Comparison of LSVs of core-shell and Ni12P5. CVs of the Nickel phosphide nanostructures in (d) commercial KOH, (e) in presence of excess FeSO4, (f) along with the mixture of Fe3O4 nanostructures (1:1 by weight) in commercial KOH and (g) CVs of core/shell nanostructures. All the CVs were recorded with GC-RDE at a rotation of 1600 rpm and with scan rate of 5mV/sec. The sharp difference in the activity of Fe3O4/NixP and Ni12P5 further prompted us to investigate the insight mechanism in the electrochemical process. Interestingly, while initial CV scans were monitored, in both cases drastic increase in activities were observed, typically termed as catalyst activation.17 Figure 4a and Figure 4b present the initial LSVs of the core-shell Fe3O4/NixP and Ni12P5 modified electrodes respectively. Apart from the catalyst activation, broad peaks were also observed in both cases. For Ni12P5 (Figure 4b), the peak at 1.39 V signified for oxidation of Ni2+ to Ni3+.17 Intriguingly, for the core/shell structures similar peak was noticed at higher potential (~80 mV shifted) (Figure 4c). Literature reports reveal that the peak at 1.47 V obtained for core/shell corresponds to Ni3+ to Ni4+ and expected to be more redox active for OER.68 On comparison, this shifting could only be attributed due to presence of Iron core only. Interestingly, only the core Fe3O4 almost remained OER inactive in 1.1-1.6 vs RHE potential window; but its presence with NixP, boosted the activity (Figure 3a and Figure S12). Being there was no peak at ~1.39 V which was for Ni2+/Ni3+, it could be concluded that Iron suppressed this Ni2+/Ni3+ redox process and opened up new active species Ni3+/Ni4+ for OER. The role of Fe in enhancing the OER activity is still under debate; but in several cases especially the incorporation iron in oxides and oxy-hydroxides, it is stated that Fe enhances the conductivity of the system.68-69 This is also reflected from the low charge transfer resistance of 11 ACS Paragon Plus Environment


core-shell material compared to nickel phosphide (Figure S13). Apart from the enhancement in conductivity Fe also activate Ni by partial charge transfer to promote more active redox window,68,70 which is also consistent with our results and remain the sole reason for boosting the efficiency. However, the activator Fe in these nanostructures remains in the core and it became important to understand if such configured structures have any important role for the OER enhancement. For investigating this, CVs were performed for nickel phosphide nanostructures in different conditions and compared with our specially designed core/shell structures. Since, the commercial KOH contains trace amount of Iron, and it is reported that the even trace amount of Iron present in the solution is enough to activate the nickel phosphide catalyst,14 continuous CV scans were performed for Ni12P5 in KOH (Figure 4d). These results suggest light increase in activity with ~3% Iron incorporation (in comparison to Nickel and measurement done through ICP-AES) without altering the redox process of Ni. However, when the CVs were performed in presence of excess Fe in KOH, the activity increases more than the commercial KOH(Figure 4e). Though, the peak position of Ni remains same with ~6% Iron incorporation. Interestingly, even when the electrode was prepared with a mixture of Fe3O4 and Ni12P5nanostructures, the observation remained almost same as Ni12P5 in commercial KOH (Figure 4f) and no prominent shift in the peak position was noticed. But, in case of core/shell nanostructure the peak was shifted to around 1.47 V (Figure 4g), and this concluded that the Iron source should be present in the domain of same nanostructure enhancing the activity. Hence, the designed core/shell structure is important here for the metal phosphides for getting


Page 12 of 22

Page 13 of 22 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

ACS Energy Letters

superior catalyst. This is possibly because of the close vicinity Fe provides better opportunity for the redox relay process with Ni during the OER process.

Figure 5. XPS spectra of post catalysis sample; (a) for Fe 2p, (b) for Ni 2p and (c) for P 2p. Further, co-relating these oxidation processes post-catalysis samples were analyzed. Figure 5a shows the XPS of the final sample obtained after electrolysis. Peaks at 712.3 eV and 725.5 eV correspond to 2p3/2 and 2p1/2 of Fe respectively, indicating formation of Fe oxyhydroxides.71 The Ni 2p shows wide difference from the initial peak and hence these are fitted for obtaining the model spectrum for quantitative analysis. The resolved spectrum reveals peak at 853.8 eV, 855.6 eV and 857.3 eV corresponds to NiO, Ni(OH)2 and NiOOH respectively (Figure 5b).72 However, the P 2p spectra shifted drastically and their resolved peaks at 133.8 eV and 135.9 eV corresponds to 2p3/2 and 2p1/2 respectively of higher valent P species (mostly phosphate, Figure 5c).32 The post catalysis XRD of the sample also analyzed and these resembled with hydrated oxides and oxy-hydroxides of nickel (Figure S14) along with FeOOH indicating the core/shell material undergo oxidation and form oxidized products like all phosphide materials.73 TEM images of these nanostructured materials were carried out at different time intervals of electrolysis and results suggests the core/shell materials were 13 ACS Paragon Plus Environment


disintegrated with the course of time(Figure S15). These results suggest the shell is oxidized to oxides and oxy-hydroxides, the core Fe3O4 turned to respective oxy-hydroxides and also phosphide transformed to phosphates. These also support our previous discussion on the redox processes observed during the electrolysis process. In conclusion, a core/shell nanostructure for efficient OER catalysis was reported having crystalline Fe3O4 core and amorphous NixP shell. Selective phosphidation approach was adopted which exclusively phosphidated Ni but remained inactive to Fe leading to the desired nanostructures without encouraging cross nucleation. While explored for OER, interestingly, Fe3O4 remained electrochemically OER inactive, but boosted the OER efficiency of NixP catalysts. Using nickel phosphides in different conditions and introducing Fe salts and oxides intentionally in the electrode, did not show the synergistically such enhanced OER activity, confirming the composition and configuration of the catalyst matters. The presence of Iron at the center in same matrix indeed helped for electrochemical talks with Ni for enhancing the redox process. This suggests that the designing with particular combination of heterostructured core/shell materials would indeed help in developing efficient catalyst for water oxidation. ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx Experimental section, TEM, XPS, EDS, EDLC, Faradiac Efficiency and comparison table (pdf)


Page 14 of 22

Page 15 of 22 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

ACS Energy Letters

AUTHOR INFORMATION Corresponding Author (NP) [email protected], (BKJ) [email protected] ACKNOWLEDGMENT DST of India (SR/NM/NS-1383/2014(G), PDF/2016/003700) and MNRE India (No. 102/87/2011NT) are acknowledged for funding. AD and AKS acknowledge CSIR, SM to SERB NPDF and NP to DST Swarnajayanti for fellowship.

REFERENCES (1) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the Rational Design of Non-precious Transition Metal Oxides for Oxygen Electrocatalysis. Energy Environ. Sci. 2015,8, 1404-1427. (2) Hong, W. T.; Welsch, R. E.; Shao-Horn, Y. Descriptors of Oxygen-Evolution Activity for Oxides: A Statistical Evaluation. J. Phys. Chem. C 2016,20, 78-86. (3) Guo, Y.; Tong, Y.; Chen, P.; Xu, K.; Zhao, J.; Lin, Y.; Wu, C.; Xie, Y.; Chu, W.; Peng, Z. Engineering the Electronic State of a Perovskite Electrocatalyst for Synergistically Enhanced Oxygen Evolution Reaction. Adv Mater 2015, 27, 5989-94. (4) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27, 7549-7558. (5) Iandolo, B.; Hellman, A. The Role of Surface States in the Oxygen Evolution Reaction on Hematite. Angew. Chem., Int. Ed. 2014, 53, 13404-13408. (6) Xu, Y.; Duan, S.; Li, H.; Yang, M.; Wang, S.; Wang, X.; Wang, R. Au/Ni12P5 Core/Shell Single-Crystal Nanoparticles as Oxygen Evolution Reaction Catalyst. Nano Res. 2017, 10, 3103-3112.



(7) Ahn, S. H.; Manthiram, A. Cobalt Phosphide Coupled with Heteroatom-Doped Nanocarbon Hybrid Electroctalysts for Efficient, Long-Life Rechargeable Zinc-Air Batteries. Small 2017, 13,1702068-1702079 (8) Li, F.; Bu, Y.; Lv, Z.; Mahmood, J.; Han, G.-F.; Ahmad, I.; Kim, G.; Zhong, Q.; Baek, J.-B. Porous Cobalt Phosphide Polyhedrons with Iron Doping as an Efficient Bifunctional Electrocatalyst. Small 2017, 13, 1701167-1701173. (9) Wang, P.; Pu, Z.; Li, Y.; Wu, L.; Tu, Z.; Jiang, M.; Kou, Z.; Amiinu, I. S.; Mu, S. Iron-Doped Nickel Phosphide Nanosheet Arrays: An Efficient Bifunctional Electrocatalyst for Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 26001-26007. (10) Zhang, B.; Lui, Y. H.; Ni, H.; Hu, S. Bimetallic (FexNi1-x)2P Nanoarrays as Exceptionally Efficient Electrocatalysts for Oxygen Evolution in Alkaline and Neutral Media. Nano Energy 2017, 38, 553-560. (11) Zhao, Z.; Schipper, D. E.; Leitner, A. P.; Thirumalai, H.; Chen, J.-H.; Xie, L.; Qin, F.; Alam, M. K.; Grabow, L. C.; Chen, S. et al. Bifunctional Metal Phosphide FeMnP Films from Single Source Metal Organic Chemical Vapor Deposition for Efficient Overall Water Splitting. Nano Energy 2017, 39, 444-453. (12) Zou, H.-H.; Yuan, C.-Z.; Zou, H.-Y.; Cheang, T.-Y.; Zhao, S.-J.; Qazi, U. Y.; Zhong, S.-L.; Wang, L.; Xu, A.-W. Bimetallic Phosphide Hollow Nanocubes Derived from a Prussian-blueanalog Used as High-Performance Catalysts for the Oxygen Evolution Reaction. Catal. Sci. Technol. 2017, 7, 1549-1555. (13) Zhu, K.; Wu, T.; Zhu, Y.; Li, X.; Li, M.; Lu, R.; Wang, J.; Zhu, X.; Yang, W. Layered FeSubstituted LiNiO2 Electrocatalysts for High-Efficiency Oxygen Evolution Reaction. ACS Energy Lett. 2017, 2, 1654-1660. (14) Stoerzinger, K. A.; Diaz-Morales, O.; Kolb, M.; Rao, R. R.; Frydendal, R.; Qiao, L.; Wang, X. R.; Halck, N. B.; Rossmeisl, J.; Hansen, H. A.; Vegge, T. et al. Orientation-Dependent Oxygen Evolution on RuO2 without Lattice Exchange. ACS Energy Lett. 2017, 2, 876-881. (15) Masa, J.; Barwe, S.; Andronescu, C.; Sinev, I.; Ruff, A.; Jayaramulu, K.; Elumeeva, K.; Konkena, B.; Roldan Cuenya, B.; Schuhmann, W. Low Overpotential Water Splitting Using


Page 16 of 22

Page 17 of 22 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

ACS Energy Letters

Cobalt-Cobalt Phosphide Nanoparticles Supported on Nickel Foam. ACS Energy Lett. 2016, 1, 1192-1198. (16) Chang, J.; Xiao, Y.; Xiao, M.; Ge, J.; Liu, C.; Xing, W. Surface Oxidized Cobalt-Phosphide Nanorods As an Advanced Oxygen Evolution Catalyst in Alkaline Solution. ACS Catal. 2015, 5, 6874-6878. (17) Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Ni2P as a Janus Catalyst for Water Splitting: the Oxygen Evolution Activity of Ni2P Nanoparticles. Energy Environ. Sci. 2015, 8, 2347-2351. (18) Dutta, A.; Pradhan, N. Developments of Metal Phosphides as Efficient OER Precatalysts. J. Phys. Chem. Lett. 2017, 8, 144-152. (19) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6, 8069-8097. (20) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oygen Evolution Reaction. J. Am. Chem. Soc. 2013,135, 16977-16987. (21) Jia, X.; Zhao, Y.; Chen, G.; Shang, L.; Shi, R.; Kang, X.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Ni3FeN Nanoparticles Derived from Ultrathin NiFe-Layered Double Hydroxide Nanosheets: An Efficient Overall Water Splitting Electrocatalyst. Adv. Energy Mater. 2016, 6, 1502585-1502591. (22) Zhong, X.; Jiang, Y.; Chen, X.; Wang, L.; Zhuang, G.; Li, X.; Wang, J.-g. Integrating Cobalt Phosphide and Cobalt Nitride-embedded Nitrogen-rich Nanocarbons: High-performance Bifunctional Electrocatalysts for Oxygen Reduction and Evolution. J. Mater. Chem. A 2016, 4, 10575-10584. (23) Masud, J.; Swesi, A. T.; Liyanage, W. P. R.; Nath, M. Cobalt Selenide Nanostructures: An Efficient Bifunctional Catalyst with High Current Density at Low Coverage. ACS Appl. Mater. Interfaces 2016, 8, 17292-17302. (24) Mabayoje, O.; Shoola, A.; Wygant, B. R.; Mullins, C. B. The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as "Pre-catalysts". ACS Energy Lett. 2016, 1, 195-201.



(25) Chen, P.; Xu, K.; Fang, Z.; Tong, Y.; Wu, J.; Lu, X.; Peng, X.; Ding, H.; Wu, C.; Xie, Y. Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2015, 54, 14710-14714. (26) Xu, K.; Ding, H.; Jia, K.; Lu, X.; Chen, P.; Zhou, T.; Cheng, H.; Liu, S.; Wu, C.; Xie, Y. Solution-Liquid-Solid Synthesis of Hexagonal Nickel Selenide Nanowire Arrays with a Nonmetal Catalyst. Angew. Chem., Int. Ed. 2016, 55, 1710-1713. (27) Masud, J.; Umapathi, S.; Ashokaan, N.; Nath, M. Iron Phosphide Nanoparticles as an Efficient Electrocatalyst for the OER in Alkaline Solution. J. Mater. Chem. A 2016, 4, 97509754. (28) Masud, J.; Nath, M. Co7Se8 Nanostructures as Catalysts for Oxygen Reduction Reaction with High Methanol Tolerance. ACS Energy Lett. 2016, 1, 27-31. (29) Schipper, D. E.; Zhao, Z.; Leitner, A. P.; Xie, L.; Qin, F.; Alam, M. K.; Chen, S.; Wang, D.; Ren, Z.; Wang, Z.; Bao, J.; Whitmire, K. H., A TiO2/FeMnP Core/Shell Nanorod Array Photoanode for Efficient Photoelectrochemical Oxygen Evolution. ACS Nano 2017,11, 40514059. (30) Pu, Z.; Zhang, C.; Amiinu, I. S.; Li, W.; Wu, L.; Mu, S. General Strategy for the Synthesis of Transition-Metal Phosphide/N-Doped Carbon Frameworks for Hydrogen and Oxygen Evolution. ACS Appl. Mater. Interfaces 2017, 9, 16187-16193. (31) Ryu, J.; Jung, N.; Jang, J. H.; Kim, H.-J.; Yoo, S. J. In Situ Transformation of HydrogenEvolving CoP Nanoparticles: Toward Efficient Oxygen Evolution Catalysts Bearing Dispersed Morphologies with Co-oxo/hydroxo Molecular Units. ACS Catal. 2015, 5, 4066-4074. (32) Dutta, A.; Samantara, A. K.; Dutta, S. K.; Jena, B. K.; Pradhan, N. Surface-Oxidized Dicobalt Phosphide Nanoneedles as a Nonprecious, Durable, and Efficient OER Catalyst. ACS Energy Lett. 2016, 1, 169-174. (33) Liu, M.; Li, J. Cobalt Phosphide Hollow Polyhedron as Efficient Bifunctional Electrocatalysts for the Evolution Reaction of Hydrogen and Oxygen. ACS Appl. Mater. Interfaces 2016, 8, 2158-2165.


Page 18 of 22

Page 19 of 22 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

ACS Energy Letters

(34) Mendoza-Garcia, A.; Zhu, H.; Yu, Y.; Li, Q.; Zhou, L.; Su, D.; Kramer, M. J.; Sun, S. Controlled Anisotropic Growth of Co-Fe-P from Co-Fe-O Nanoparticles. Angew. Chem., Int. Ed. 2015, 54, 9642-9645. (35) Li, Z.; Dou, X.; Zhao, Y.; Wu, C., Enhanced Oxygen Evolution Reaction of Metallic Nickel Phosphide Nanosheets by Surface Modification. Inorg. Chem. Front. 2016, 3, 1021-1027. (36) Zhao, Y.; Chang, C.; Teng, F.; Zhao, Y.; Chen, G.; Shi, R.; Waterhouse, G. I. N.; Huang, W.; Zhang, T. Water Splitting: Defect-Engineered Ultrathin δ-MnO2 Nanosheet Arrays as Bifunctional Electrodes for Efficient Overall Water Splitting, Adv. Energy Mater. 2017, 7, 1700005 (1-7). (37) Zhao, Y.; Jia, X.; Chen, G.; Shang, L.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; O'Hare, D.; Zhang, T. Ultrafine NiO Nanosheets Stabilized by TiO2 from Monolayer NiTi-LDH Precursors: An Active Water Oxidation Electrocatalyst. J. Am. Chem. Soc. 2016, 138, 6517−6524. (38) Li, D.; Baydoun, H.; Verani, C. N.; Brock, S. L. Efficient Water Oxidation Using CoMnP Nanoparticles. J. Am. Chem. Soc. 2016, 138, 4006-4009. (39) Yu, J.; Li, Q.; Li, Y.; Xu, C.-Y.; Zhen, L.; Dravid, V. P.; Wu, J. Ternary Metal Phosphide with Triple-Layered Structure as a Low-Cost and Efficient Electrocatalyst for Bifunctional Water Splitting. Adv. Funct. Mater. 2016, 26, 7644-7651. (40) Li, J.; 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, 26, 6785–6796. (41) Zhang, Z.; Hao, J.; Yang, W.; Tang, J. Iron Triad (Fe, Co, Ni) Trinary Phosphide Nanosheet Arrays as High-performance Bifunctional Electrodes for Full Water Splitting in Basic and Neutral Conditions. RSC Adv. 2016, 6, 9647-9655. (42) Duan, J.; Chen, S.; Vasileff, A.; Qiao, S. Z. Anion and Cation Modulation in Metal Compounds for Bifunctional Overall Water Splitting. ACS Nano 2016, 10, 8738-8745. (43) Jin, Z.; Li, P.; Xiao, D. Metallic Co2P Ultrathin Nanowires Distinguished from CoP as Robust Electrocatalysts for Overall Water-splitting. Green Chem. 2016, 18, 1459-1464.



Page 20 of 22

(44) Chang, J.; Liang, L.; Li, C.; Wang, M.; Ge, J.; Liu, C.; Xing, W. Ultrathin Cobalt Phosphide Nanosheets as Efficient Bifunctional Catalysts for a Water Electrolysis Cell and the Origin for Cell Performance Degradation. Green Chem. 2016, 18, 2287-2295. (45) Li, Y.; Zhang, H.; Jiang, M.; Kuang, Y.; Sun, X.; Duan, X. Ternary NiCoP Nanosheet Arrays: An Excellent Bifunctional Catalyst for Alkaline Overall Water Splitting. Nano Res. 2016, 9, 2251-2259. (46) Yan, Y.; Zhao, B.; Yi, S. C.; Wang, X. Assembling Pore-rich FeP Nanorods on the CNT Backbone as an Advanced Electrocatalyst for Oxygen Evolution. J. Mater. Chem. A 2016, 4, 13005-13010. (47) 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. (48) Yu, X.-Y.; Feng, Y.; Guan, B.; Lou, X. W.; Paik, U. Carbon Coated Porous Nickel Phosphides Nanoplates for Highly Efficient Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 12461250. (49) Liang, H.; Gandi, A. N.; Xia, C.; Hedhili, M. N.; Anjum, D. H.; Schwingenschlogl, U.; Alshareef, H. N. Amorphous NiFe-OH/NiFeP Electrocatalyst Fabricated at Low Temperature for Water Oxidation Applications. ACS Energy Lett. 2017, 2, 1035–1042. (50) Mukherjee,

D.;

Austeria,

P.

M.;

Sampath,

S.

Two-Dimensional,

Few-Layer

Phosphochalcogenide, FePS3: A New Catalyst for Electrochemical Hydrogen Evolution over Wide pH Range. ACS Energy Lett. 2016,1, 367-372. (51) Fu, S.; Zhu, C.; Song, J.; Engelhard, M. H.; Li, X.; Du, D.; Lin, Y. Highly Ordered Mesoporous Bimetallic Phosphides as Efficient Oxygen Evolution Electrocatalysts. ACS Energy Lett. 2016, 1, 792-796. (52) Liu, P. F.; Li, X.; Yang, S.; Zu, M. Y.; Liu, P.; Zhang, B.; Zheng, L. R.; Zhao, H.; Yang, H. G. Ni2P(O)/Fe2P(O) Interface Can Boost Oxygen Evolution Electrocatalysis. ACS Energy Lett. 2017, 2, 2257-2263. (53) Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Gozzo, F.; Garcia-Hernandez, M.; Garcia, M. A.; Cingolani, R.; Cozzoli, P. D., Architectural Control of Seeded-Grown Magnetic-


Page 21 of 22 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

ACS Energy Letters

Semiconductor Iron Oxide-TiO2 Nanorod Heterostructures: The Role of Seeds in Topology Selection. J. Am. Chem. Soc. 2010, 132, 2437-2464. (54) Scarfiello, R.; Nobile, C.; Cozzoli, P. D. Colloidal Magnetic Heterostructured Nanocrystals with Asymmetric Topologies: Seeded-Growth Synthetic Routes and Formation Mechanisms. Frontiers in Materials 2016, 3, art 56. (55) Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Curri, M. L.; Innocenti, C.; Sangregorio, C.; Achterhold, K.; Parak, F. G.; Agostiano, A.; Cozzoli, P. D. Seeded Growth of Asymmetric Binary Nanocrystals Made of a Semiconductor TiO2 Rodlike Section and a Magnetic γ-Fe2O3 Spherical Domain. J. Am. Chem. Soc. 2006, 128, 16953-16970. (56) Luo, Z.; Marti-Sanchez, S.; Nafria, R.; Joshua, G.; de la Mata, M.; Guardia, P.; Flox, C.; Martinez-Boubeta, C.; Simeonidis, K.; Llorca, J. et al. Fe3O4@NiFexOy Nanoparticles with Enhanced Electrocatalytic Properties for Oxygen Evolution in Carbonate Electrolyte. ACS Appl. Mater. Interfaces 2016, 8, 29461-29469. (57) Guan, Q.; Cheng, X.; Li, R.; Li, W. A feasible Approach to the Synthesis of Nickel Phosphide for Hydrodesulfurization. J. Catal. 2013, 299, 1-9. (58) Moreau, L. M.; Ha, D.-H.; Zhang, H.; Hovden, R.; Muller, D. A.; Robinson, R. D. Defining Crystalline/Amorphous Phases of Nanoparticles through X-ray Absorption Spectroscopy and X-ray Diffraction: The Case of Nickel Phosphide. Chem. Mater. 2013, 25, 2394-2403. (59) Zheng, X.; Yuan, S.; Tian, Z.; Yin, S.; He, J.; Liu, K.; Liu, L. Nickel/Nickel Phosphide CoreShell Structured Nanoparticles: Synthesis, Chemical, and Magnetic Architecture. Chem. Mater. 2009, 21, 4839-4845. (60) Li, D.; Senevirathne, K.; Aquilina, L.; Brock, S. L. Effect of Synthetic Levers on Nickel Phosphide Nanoparticle Formation: Ni5P4 and NiP2. Inorg. Chem. 2015, 54, 7968-7975. (61) Huang, H.; Yu, C.; Yang, J.; Zhao, C.; Han, X.; Liu, Z.; Qiu, J. Strongly Coupled Architectures of Cobalt Phosphide Nanoparticles Assembled on Graphene as Bifunctional Electrocatalysts for Water Splitting. ChemElectroChem 2016, 3, 719-725. (62) Zhu, Y.-P.; Liu, Y.-P.; Ren, T.-Z.; Yuan, Z.-Y. Self-Supported Cobalt Phosphide Mesoporous Nanorod Arrays: A Flexible and Bifunctional Electrode for Highly Active Electrocatalytic Water Reduction and Oxidation. Adv. Funct. Mater. 2015, 25, 7337-7347.



(63) Ledendecker, M.; Krick Calderon, S.; Papp, C.; Steinrueck, H.-P.; Antonietti, M.; Shalom, M. The Synthesis of Nanostructured Ni5P4 Films and their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 12361-12365. (64) Bai, Y.; Zhang, H.; Feng, Y.; Fang, L.; Wang, Y. Sandwich-like CoP/C Nanocomposites as Efficient and Stable Oxygen Evolution Catalysts. J. Mater. Chem. A 2016, 4, 9072-9079. (65) Jiang, N.; You, B.; Sheng, M.; Sun, Y. Electrodeposited Cobalt-Phosphorous-Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 6251-6254. (66) Zhang, Z.; Liu, S.; Xiao, J.; Wang, S. Fiber-based Multifunctional Nickel Phosphide Electrodes for Flexible Energy Conversion and Storage. J. Mater. Chem. A 2016, 4, 9691-9699. (67) Han, A.; Chen, H.; Sun, Z.; Xu, J.; Du, P. High Catalytic Activity for Water Oxidation Based on Nanostructured Nickel Phosphide Precursors. Chem. Commun. 2015, 51, 11626-11629. (68) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136, 6744-6753. (69) 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. (70) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni–Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2013, 135, 12329-12337. (71) Zou, S.; Burke, M. S.; Kast, M. G.; Fan, J.; Danilovic, N.; Boettcher, S. W. Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution. Chem. Mater. 2015, 27, 8011-8020. (72) Shen, S. Y.; Zhao, T. S.; Xu, J. B.; Li, Y. S. Synthesis of PdNi Catalysts for the Oxidation of Ethanol in Alkaline Direct Ethanol Fuel Cells. J. Power Sources 2010, 195, 1001-1006. (73) Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2017, 2, 1937–1938.


Page 22 of 22

Synergistic Effect of Inactive Iron Oxide Core on Active Nickel

Recommend Documents