Surface-Oxidized Dicobalt Phosphide Nanoneedles as a Nonprecious

Subscriber access provided by UCL Library Services

Letter

Surface Oxidized Dicobalt Phosphide Nanoneedles as a Non-Precious, Durable and Efficient OER Catalyst Anirban Dutta, Aneeya K. Samantara, Sumit K Dutta, Bikash Kumar Jena, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00144 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 26, 2016

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 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Surface Oxidized Dicobalt Phosphide Nanoneedles as a NonPrecious, Durable and Efficient OER Catalyst Anirban Dutta,†# Aneeya K. Samantara,‡# Sumit K. Dutta,† Bikash Kumar Jena*‡ and Narayan Pradhan*† †

Department of Materials Science and Centre for Advanced Materials, 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: Needle shaped narrow hexagonal phase 1D nanostructures of dicobalt phosphide (Co2P) is reported as an efficient electrocatalyst for oxygen evolution reaction (OER). Without other metal incorporation, which were typically followed for enhancing the OER activity, the electrochemical performance was observed superior in comparison to all reported cobalt based nanostructured metal phosphides. For anodic metamorphosis, these nanostructures like all other metal phosphides, undergo surface oxidation, but remains more active and superior to pure cobalt oxides as well as surface oxidized different shaped monocobalt phosphides. Moreover, the synthesis was also followed by adopting a moderate synthetic protocol where PH3 gas was used as phosphorus source and also up scaled to gram level. In addition, the hydrogen evolution reaction (HER) performance of these phosphides were further studied and the performance observed comparable to best reports.

TOC:


Page 2 of 18

Page 3 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Electrochemical water splitting is one of the convenient and environmentally inoffensive routes for the possible hydrogen economy.1-8 The efficiency of this hydrogen generation is largely dependent on the anodic oxygen evolution reaction (OER).9-13 However, the OER faces very sluggish reaction kinetics as it involves multiproton-coupled electron transfer steps.14 Hence, the water splitting faces high activation energy barrier and require highly active OER catalysts to overcome.14 Recent developments suggest that different noble metal oxides and oxy-hydroxides of transition metals were shown their superior OER activities.9,

15-16

In addition, it has been

further revealed that metal phosphides are also emerged as active electrocatalysts for water splitting.7 Among these, Ni and Co phosphides remain in forefront.5-7, 12-13, 17-20 However, while these are widely studied for HER, 5-7, 17-18 their OER activities are less explored. It is established that, the metal phosphides during OER in alkaline medium undergo anodic metamorphosis and results oxide overlayer.12, 20-21 However, these in-situ transformed heterostructures were shown remarkable enhanced catalytic activity with outstanding long term stability. Further investigations suggest that for OER, surface oxidized nickel phosphide is optimized as superior catalyst.20 On contrary, cobalt phosphides even though studied extensively; but the OER performance is limited. Enormous efforts have been put forwarded in designing appropriate cobalt phosphide catalysts to get enhanced efficiency towards OER. Mixing with carbon12 and incorporating Fe inside the crystal lattice,22 the catalytic performances have been enhanced. Very recently, inserting Mn into the lattice of dicobalt phosphide, the activity is also boosted.14 However, still the performance remains under the activity of nickel phosphides irrespective of incorporation of several additives. Hence, exploring cobalt phosphide nanostructures catalysts for obtaining optimized OER activity remains challenging.


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

Since, the activity of a catalyst is typically a surface phenomenon, designing proper size, shape as well as phase of the material are critically important for obtaining the best optimized performance and the reaction chemistry in the colloidal synthesis can manipulate these parameters and lead to the required catalysts owing to higher performance. In this work, hexagonal phase narrow 1D nanostructures of dicobalt phosphide designed via PH3 gas mediated synthesis, is explored as an efficient OER catalyst. Similar to other metal phosphides, these are partially surface oxidized during electrocatalysis in presence of KOH; showing superior activity among all reported cobalt based nanostructured metal phosphides without having any other additives.12,

14, 20-22

The activity remains even better than all benchmarked oxide catalysts as

listed in reference 16 by McCrory et al.16 In 1 M KOH, the state-of-art current density of 10 mA/cm2 was found at an overvoltage of 310 mV which also remains significantly lower in comparison to the commercially available standardized IrO2/C under similar experimental condition. Moreover, the synthesis could lead to gram scale preparation of the materials. In addition, this was also observed as efficient HER catalyst comparable with the best reports making the 1D structure as bifunctional electrocatalyst.

For the synthesis of Co2P 1D nanostructures, ex-situ produced PH3 gas was used as phosphorous source (Figure S1 and experimental).23-25 In a typical reaction, required amount of CoCl2.6H2O and alkyl amine were loaded in a 200 ml three neck round bottom flask, deaerated and heated to 230 oC. PH3 gas was purged at this temperature and the reaction was further annealed at 200 oC. Figure 1a-1c (Figure S2) present transmission electron microscopic (TEM) images at different resolutions and Figure 1d shows the high-angle annular dark field (HAADF) image of obtained Co2P nanostructures. These 1D nanostructures has needle type shape whose lengths were varied within 70±10 nm and width (at centre) 5±2 nm. Widths at the centre of these 4 ACS Paragon Plus Environment

Page 4 of 18

Page 5 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

structures were the maximum and ends were tapered. These nanostructures are obtained in a gram scale reaction where in one set more than 1 gm purified Co2P was collected.

Figure 1. (a-c) TEM images of needle shaped Co2P nanostructures in different resolutions. (d) HAADF-STEM image of Co2P nanostructures. (e) Powder X-ray diffraction pattern of Co2P nanostructures. (f) HRTEM image of a tapered nanorod. (g) Selected area FFT pattern. Planes are labelled as per the viewing direction. (h) An atomic model of a typical 1D nanostructure of Co2P.

For understanding the crystal phase, powder X-ray diffraction (XRD) was carried out and the pattern presented in Figure 1e. The observed peak positions here resembled with those of bulk hexagonal Co2P (ICSD Code: 107550).26 High resolution TEM (HRTEM) image was also analysed for understanding the growth direction. Figure 1f presents a typical HRTEM image and the selected area FFT pattern is shown in Figure 1g. Crystal plane analysis showed that d-spacing of planes parallel to the major axis was 0.175 nm which resembled to (002) planes of wurtzite


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

Co2P. This confirms these needle structures were grown along the polar [002] direction. Accordingly, a typical 1D rod type atomic model Co2P was designed and depicted in Figure 1h.

Figure 2. (a) Linear sweep voltammograms for the OER by GCE, IrO2/C, Co3O4/C and Co2P/C in 1M KOH at sweep rate of 5 mV/s with 0.2 mgcm-2 loading. (b) The corresponding Tafel plot. (c) Long-term stability test for the OER by Co2P/C on glassy carbon plate in 1 M KOH at 10mA/cm2. (d) The LSV before and after stability test. Inset in (c) is the photograph of the modified electrode before and at the time of stability test.


Page 6 of 18

Page 7 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

These nanostructures were further explored as electrocatalyst and their activities were studied. For electrode fabrication, the polished glassy carbon rotating disk electrode (GC-RDE) was pre-treated by 20 repeated cycles from 0.5 to 1.9 V (vs. RHE) in 1 M KOH to increase the hydrophilicity and wettability of the surface20 and then an ink composed of these structures, carbon powder and nafion (details in SI) was prepared and drop-casted onto the GC-RDE. The measurements were carried out in a two compartment 3-electrode electrochemical cell. The linear sweep voltammograms (LSV) for OER were recorded at a sweep rate of 5 mV/s with the Co2P modified GC-RDE at a rotation of 1600 rpm. The final data were recorded after completing the first 5 cyclic voltammetric scans (1.20 - 1.65 V vs. RHE) to activate the catalyst and obtain a steady current density. Figure S3 shows the LSVs during the activation period and interestingly the first scan had a predominant broad peroxidation peak before the onset potential which diminished during the second and consecutive scans. Similar observation was also reported for CoP which signified the irreversible surface oxidation of the phosphide catalyst.21 Post catalysis sample also supported partial oxidation which discussed in later section. Figure 2a presence the LSV data of Co2P/C catalyst after the activation period. The control experiment on GC-RDE was also performed for verification and the state-of-art catalyst IrO2/C. Further, as the surface was oxidized, the activity was also compared with Co3O4/C (synthesis provided in supporting information). LSV data obtained using all these electrodes were presented after iR correction (Uncompensated data presented in Figure S4). Activities of these electrode materials were compared with respect to the overpotential, which defined as the potential require to reach 10mA/cm2 current density and for Co2P, it was observed 310 mV (five repeated experiments were carried out and the recorded LSVs before and after iR correction is presented in Figure S5 of the SI, the standard deviation 7 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

Page 8 of 18

calculated as 7 mV). While compared to the standard catalyst IrO2/C measured under similar conditions, it was observed 40 mV lower and even lowered than the Co3O4/C by 100mV indicating Co2P nanostructures are superior to IrO2 and Co3O4 as OER catalyst. To our best knowledge, this is noted as the lowest ever potential among all cobalt based phosphide materials and ranked among leading nonprecious metal based OER catalysts. Table 1. Electrochemical Catalytic Performance Values of Metal Phosphide Catalysts Materials

Tafel Slope mV/dec

Electrolyte (M, KOH)

Overpotential (V) at10 mA/cm2

Loading (mg/cm2)

Ni2P Nanowires

47

1

0.330

0.14

Ni2P Nanoparticles

59

1

0.290

0.14

Ultrafine CoP-CNT

50

0.1(NaOH)

0.330

0.28

(Co0.54Fe0.46)P2

-

0.1

0.37

0.2

CoP NRs/C

71

1

0.32

0.71

CoP NPs/C

99

1

0.34

0.71

CoMnP

61

1

0.33

0.28

Co2P

128

1

0.37

0.28

Co2P nanoneedles

50

1

0.31

0.2

References

20 20 17 22 12 12 14 14 This work

Further, the reaction kinetics involved in the OER was determined by examining the Tafel polarisation plot (plot of “ղ vs log j”) at lower overpotential (Figure 2b). From the linear fitting of the Tafel plot, the calculated slope was observed to be 50 mV/dec for Co2P/C. This value was observed to be close to that of IrO2/C (46 mV/dec). This low value of Tafel slope also supports superior OER activity of Co2P/C catalyst.21, 27 For comparison, activities of different phosphide based electrocatalyst were summarized in Table 1.Here, best reports of Ni2P are also incorporated.20 Further, these results also compared to different oxides, oxo-hydroxides and several more phosphides (Table S1). A movie recorded during the evolution of oxygen from the 8 ACS Paragon Plus Environment

Page 9 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Co2P modified GC plate electrode during electrolysis at 0.42 V is presented in Movie S1 in the supporting information. The stability of Co2P catalyst for OER was further analysed by Chronopotentiometric electrolysis at a current density of 10 mA/cm2 and presented in Figure 2c. After 10 hour continuous electrolysis, an increase of only 15 mV overpotential was observed indicating a standard level of stability for long term operation. LSV plots at the beginning and after 10 hours electrolysis was also obtained and shown in Figure 2d. Further, the post catalysis samples were analysed to understand any in-situ transformation of the catalyst. Figure S6 shows the TEM image of the electrode catalyst collected after the electrolysis and this reflects the retention of 1D nanostructure. Figure 3a presents the HAADFSTEM image and the elemental mapping for Co, P and O. EDS spectra of the sample is presented in Figure 3b. The result shows the presence of 46% Co, 18% P and 36% of O. This indicates that some percentage of Co2P, presumably on the surface of the nanostructures, is converted to cobalt oxides (CoOx). This is also observed from the powered XRD (Figure 3c). Further, X-ray photoelectron spectroscopy (XPS) study of the post catalysis samples were carried out and data presented in Figure 3d. The peaks at 778.2 (2p3/2) and 793.2 (2p1/2) eV were assigned to the binding energies of Co(II) in Co2P.The Co peaks at 781.6 (2p3/2) and 797.8 (2p1/2) eV as well as the two satellite peaks at 786.7 and 803.5 eV corresponds to Co(II) in cobalt oxide in the sample. The peaks at 129.7 and 133.9 eV is assigned to phosphide and phosphate respectively.12 These observations manifested that the surface of the catalyst is partially converted to form oxide or oxo-hydroxide over-layer.12 Moreover, the Co2P-CoOx interface helps for better carrier transportation from the core Co2P to the oxides as reported for several similar OER catalysts.12, 20 Further, higher valent P species which is formed during the catalysis 9 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

Page 10 of 18

as observed may act as a P ligand on the catalyst. Similar to the molecular catalyst, these higher valent P species can flip its coordination mode during the redox switching process of cobalt and facilitates the process.21 The surface oxidation phenomenon was also reflected during catalyst activation, as the first scan had a predominant broad peroxidation peak before the onset potential signifying the irreversible surface oxidation of the Co2P. However, the peak diminished even in the second scan with decrease in the onset potential and Tafel slope (Figure S3b). 21

Figure 3. (a) HAADF-STEM image and elemental mapping for Co, O and P of the post catalysis sample. (b) EDS spectra showing presence of Co, O and P. (c) Powder XRD pattern of post catalysis sample. (d) XPS spectra of Co and P obtained from post catalysis sample.

Being, the catalysis is a surface phenomenon, we further studied the role of shape and phase of these nanostructures to correlate the significant enhanced OER performance. The unique feature here is the needle shaped narrow 1D structure, which are reported for better charge transfer28 and high metallic content compared to mono cobalt phosphide enabling more metallic character29 and helped the same as reflected from low value of charge transfer resistance in Nyquist plot (Figure S4b). However these thin 1D nanostructures enhances the overall surface area suitable for better catalysis. This enhanced electrocatalytic performance of the as 10 ACS Paragon Plus Environment

Page 11 of 18

synthesized Co2P/C over the IrO2/C and Co3O4/C was further evaluated by calculating the double layer capacitance (Cdl) and roughness factor (Rf) of the samples (detail experimental details are discussed in supporting information). The calculated Cdl value found to be 3.71 mF with an Rf of 473.6 which remained much higher than that for IrO2/C (i.e. Cdl=1.79 with Rf=228.95) and Co3O4/C (i.e. Cdl=0.19 with Rf=25.43) (Figure S7). Since the Cdl and the Rf are directly proportional to the active surface area of the electrocatalyst; the results clearly demonstrated that the as synthesised 1D nanostructures of Co2P have higher electrocatalytically active surface area compared to the IrO2 and Co3O4 sample with same loading (0.2mg cm-2) on the electrode surface.30-31 So, the better exposure and utilization of electroactive sites on the higher surface area of Co2P/C indeed helped in enhancing the OER performance and addition of other metals into the phosphide matrix were not required.

a

b

40 30

Overpotential (V)

Co2P/C in KOH

j (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Co 2P/C in Fe Free KOH

20 10 0 1.2

1.4

1.6

1.8

0.4

0.3 KOH Fe Free KOH

0.2 0

1

2

log (j / mAcm-2)

E (V vs. RHE)

Figure 4.(a) Linear sweep voltammograms for the OER by Co2P/C in 1 M commercially available and Fe free KOH at sweep rate of 5 mV/s. (b)The corresponding Tafel plot.


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

Page 12 of 18

However, literature reports reveal that even trace amount of Fe presence in commercial KOH enhances the OER activity of the catalyst.20, 32 These have been reported for Co and Ni based oxides and (oxy)hydroxides.9, 33 Herein, as our catalyst Co2P partially surface oxidized, so it possess the possibility of incorporation of Fe which might be the reason for the superior activity.9,

34

To confirm this, we have carried out the electrocatalysis using commercial KOH

contaminated with Fe and the Fe-free KOH (see supplementary information for purification).34 Figure 4a presents the LSV of Co2P/C catalyst and Figure 4b shows corresponding Tafel plots in both commercial and Fe-free KOH. Interestingly, the current density at 10 mA/cm2 in both cases remained almost same and so also the Tafel slope. This suggests Fe does not contribute in the activity of Co2P catalyst. To state here that from ICP-OES measurement, 0.025% of Fe was detected in the post-catalysis sample using commercial KOH. Apart from OER, we also studied the other part reaction, HER of water splitting though these were established for both CoP6,

18

and Co2P35-36 in different shapes and phases but

hexagonal phase is explored here for the first time. For HER, the measurement was carried out in 0.5 M H2SO4 solutions and an increase in cathodic current was observed at an onset potential of 0.02 V (vs. RHE). It required 125 mV and 140 mV overpotential to produce 10 and 20 mA/cm2 respectively, compared to the benchmark HER catalyst Pt/C where require overpotentials were of 50 and 52 mV respectively (Figure 5a and iR compensated and uncompensated data provided in Figure S8). Though the activity here remained low in comparison to Pt/C but the catalytic activity is comparable to other best not-Pt based HER catalysts in acidic aqueous solution.5, 18-19, 37-39

Further, Tafel plot gave a slope of 50mV/dec and 32 mV/dec for Co2P and commercial Pt/C,

suggesting a Volmer-Heyrovsky and Volmer-Tafel mechanism might be followed by the Co2P and Pt/C modified GCE (Figure 5b).Further we have investigated the long term operational 12 ACS Paragon Plus Environment

Page 13 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

durability of the Co2P towards the catalytic activity for HER in 0.5 M H2SO4 by employing the chronopotentiometry (Figure S9). After 10 hour of electrolysis at 10mA/cm2 current density, no significant change in the overpotential was observed, indicating the higher stability of the Co2P catalyst for the long term HER operation. With this results, the Co2P 1D nanostructures reported here could be termed as a bifunctional electrocatalysts where are indeed in demand in current technological needs.

Figure 5. (a) Linear sweep voltammograms for the HER by Pt/C, Co2P/C and GCE in 0.5M H2SO4 at sweep rate of 5mV/s. (b) The corresponding Tafel plot.

In conclusion, needle shaped thin hexagonal Co2P nanostructures were synthesized and explored as an efficient OER catalyst. In basic medium during electrolysis these were partially surface oxidized and recorded current density of 10 mA/cm2 at overvoltage of 310 mV which was observed superior to all cobalt based phosphides. In addition, these were also observed superior than benchmark IrO2/C catalyst. The composition and the narrow needle shaped structure having wide area active surfaces were found responsible for this enhanced catalytic activity. In addition, these nanostructures also showed efficient hydrogen evolution reactions,


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

Page 14 of 18

turning these to a highly active bifunctional catalyst. The synthesis was also followed in a moderate approach and in gram scale. Results reported here suggests that with proper manipulation the synthetic reaction chemistry more appropriate and active nanostructures can be designed and used as highly efficient electrocatalyst for oxygen evolution reaction.

ASSOCIATED CONTENT Supporting Information Experimental, methods, supporting figures. AUTHOR INFORMATION Corresponding Author Email: (NP) [email protected], (BKJ) [email protected] #

The Authors Contribute Equally.

ACKNOWLEDGMENT DST of India (SR/NM/NS-1383/2014(G) and MNRE India (No. 102/87/2011-NT) are acknowledged for funding. AD, SKD and AKS acknowledge CSIR and NP to DST Swarnajayanti for fellowship.

REFERENCES

(1) Xiao, P.; Chen, W.; Wang, X. A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution. Adv. Energy Mater. 2015, 5, 1500985-1500998. (2) Morales-Guio Carlos, G.; Stern, L. A.; Hu, X. Nanostructured Hydrotreating Catalysts for Electrochemical Hydrogen Evolution. Chem. Soc. Rev. 2014, 43, , 6555-6569. (3) Yan, Y.; Thia, L.; Xia, B. Y.; Ge, X.; Liu, Z.; Fisher, A.; Wang, X. Construction of Efficient 3D Gas Evolution Electrocatalyst for Hydrogen Evolution: Porous FeP Nanowire Arrays on Graphene Sheets. Adv. Sci. 2015, 2, 1500120-1500128. 14 ACS Paragon Plus Environment

Page 15 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(4) Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. HighIndex Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14023-14026. (5) 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. (6) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem., Int. Ed. 2014, 53, 5427-5430. (7) Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 1529-1541. (8) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (9) 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. (10) Wang, Y.; Jiang, K.; Zhang, H.; Zhou, T.; Wang, J.; Wei, W.; Yang, Z.; Sun, X.; Cai, W.B.; Zheng, G. Bio-Inspired Leaf-Mimicking Nanosheet/Nanotube Heterostructure as a Highly Efficient Oxygen Evolution Catalyst. Adv. Sci. 2015, 2, 1500003-1500011. (11) 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. (12) 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. (13) 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. (14) Li, D.; Baydoun, H.; Verani, C. N.; Brock, S. L. Efficient Water Oxidation Using CoMnP Nanoparticles. J. Am. Chem. Soc. 2016, 138, 4006-4009.


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

Page 16 of 18

(15) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399-404. (16) McCrory Charles, C. L.; Jung, S.; Peters Jonas, C.; Jaramillo Thomas, F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. (17) Hou, C. C.; Cao, S.; Fu, W. F.; Chen, Y. Ultrafine CoP Nanoparticles Supported on Carbon Nanotubes as Highly Active Electrocatalyst for Both Oxygen and Hydrogen Evolution in Basic Media. ACS Appl. Mater. Interfaces 2015, 7, 28412-28419. (18) Popczun, E. J.; Roske, C. W.; Read, C. G.; Crompton, J. C.; McEnaney, J. M.; Callejas, J. F.; Lewis, N. S.; Schaak, R. E. Highly Branched Cobalt Phosphide Nanostructures for HydrogenEvolution Electrocatalysis. J. Mater. Chem. A 2015, 3, 5420-5425. (19) 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. (20) 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. (21) 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. (22) 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. (23) Dutta, A.; Dutta, S. K.; Mehetor, S. K.; Mondal, I.; Pal, U.; Pradhan, N. Oriented Attachments and Formation of Ring-on-Disk Heterostructure Au-Cu3P Photocatalysts. Chem. Mater. 2016, 28, 1872-1878. (24) Dutta, A.; Samantara, A. K.; Adhikari, S. D.; Jena, B. K.; Pradhan, N. Au Nanowire-Striped Cu3P Platelet Photoelectrocatalysts. J. Phys. Chem. Lett. 2016, 7, 1077-1082. (25) Manna, G.; Bose, R.; Pradhan, N. Semiconducting and Plasmonic Copper Phosphide Platelets. Angew. Chem., Int. Ed. 2013, 52, 6762-6766.


Page 17 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(26) Li, T.; Liao, J. P.; Wang, Y. F. Solvothermal Synthesis and Magnetic Properties of β-Co2P Nanorods. Mater. Sci.-Pol. 2015, 33, 312-316. (27) 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. (28) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. OneDimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater. 2003, 15, 353-389. (29) Blanchard, P. E. R.; Grosvenor, A. P.; Cavell, R. G.; Mar, A. X-ray Photoelectron and Absorption Spectroscopy of Metal-Rich Phosphides M2P and M3P (M = Cr-Ni). Chem. Mater. 2008, 20, 7081-7088. (30) Shen, L.; Che, Q.; Li, H.; Zhang, X. Mesoporous NiCo2O4 Nanowire Arrays Grown on Carbon Textiles as Binder-Free Flexible Electrodes for Energy Storage. Adv. Funct. Mater. 2014, 24, 2630-2637. (31) Jiang, Y.; Zhang, X.; Ge, Q. Q.; Yu, B. B.; Zou, Y. G.; Jiang, W. J.; Song, W. G.; Wan, L.J.; Hu, J. S. ITO@Cu2S Tunnel Junction Nanowire Arrays as Efficient Counter Electrode for Quantum-Dot-Sensitized Solar Cells. Nano Lett. 2014, 14, 365-372. (32) 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. (33) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W. Revised Oxygen Evolution Reaction Activity Trends for First-Row Transition-Metal (Oxy)hydroxides in Alkaline Media. J. Phys. Chem. Lett. 2015, 6, 3737-3742. (34) 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. (35) Callejas, J. F.; Read, C. G.; Popczun, E. J.; McEnaney, J. M.; Schaak, R. E. Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP. Chem. Mater. 2015, 27, 3769-3774.


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

Page 18 of 18

(36) Wang, P.; Song, F.; Amal, R.; Ng, Y. H.; Hu, X. Efficient Water Splitting Catalyzed by Cobalt Phosphide-Based Nanoneedle Arrays Supported on Carbon Cloth. ChemSusChem 2016, 9, 472-477. (37) McEnaney, J. M.; Chance Crompton, J.; Callejas, J. F.; Popczun, E. J.; Read, C. G.; Lewis, N. S.; Schaak, R. E. Electrocatalytic Hydrogen Evolution Using Amorphous Tungsten phosphide Nanoparticles. Chem. Comm. 2014, 50, 11026-11028. (38) Callejas, J. F.; McEnaney, J. M.; Read, C. G.; Crompton, J. C.; Biacchi, A. J.; Popczun, E. J.; Gordon, T. R.; Lewis, N. S.; Schaak, R. E. Electrocatalytic and Photocatalytic Hydrogen Production from Acidic and Neutral-pH Aqueous Solutions Using Iron Phosphide Nanoparticles. ACS Nano 2014, 8, 11101-11107. (39) 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.


Surface-Oxidized Dicobalt Phosphide Nanoneedles as a Nonprecious

Recommend Documents