In Situ Formation of Nanostructured CoreâShell Cu3NâCuO to

Subscriber access provided by WEBSTER UNIV

Letter

In-situ Formation of Nanostructured Core-Shell Cu3N-CuO to Promote Alkaline Water Electrolysis Chakadola Panda, Prashanth Wilfried Menezes, Min Zheng, Steven Orthmann, and Matthias Driess ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00091 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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 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 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.

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 24 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

In-situ Formation of Nanostructured Core-Shell Cu3N-CuO to Promote Alkaline Water Electrolysis Chakadola Panda,†§ Prashanth W. Menezes,†§* Min Zheng,†§ Steven Orthmann,† Matthias Driess†* †

Department of Chemistry: Metalorganics and Inorganic Materials, Technische Universität

Berlin, Straße des 17 Juni 135, Sekr. C2, 10623 Berlin, Germany § These

authors contributed equally

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ACS Paragon Plus Environment

1

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 2 of 24

ABSTRACT. Electrochemical splitting of water to oxygen and hydrogen using earth-abundant first-row transition metal-based catalysts is a promising approach for sustainable energy conversion. Herein, we present a new convenient synthesis of copper nitride (Cu3N) which acts as a bifunctional electro(pre)catalyst for oxygen evolution (OER), hydrogen evolution reaction (HER) and overall water electrolysis in alkaline media. Strikingly, the electrophoretically deposited Cu3N on nickel foam (NF) displayed extremely low overpotentials for both OER and HER and the overall water splitting cell potential was mere 1.62 V with a remarkable durability of over 10 days. Most notably, the coordinatively unsaturated Cu in Cu3N transformed in-situ under reducing and in an oxidative environment into a copper-rich shell that serves as the active site over an equally important electrically conductive Cu3N core to drive proficient catalysis. To the best of our knowledge, this is the first report on copper nitride for efficient and durable alkaline water electrolysis.

TOC GRAPHIC


2

Page 3 of 24 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


3


Page 4 of 24

Development of inexpensive and sustainable technologies that can split water electrochemically into hydrogen and oxygen in large scale is one of the most desirable routes to renewable energy production.1-6 Although the half-cell reaction, hydrogen evolution reaction (HER) is feasible thermodynamically,7 the major limitation remains in the other half-cell reaction, i.e. oxygen evolution reaction (OER), that hinders overall water splitting systems for practical application.811

The activation barrier for OER is very high owing to the sluggish four-proton-coupled electron

transfer of OER that involves high-energy intermediates to furnish O-O bond formation.8 Along this line, the most efficient catalytic materials derived from noble metals (Pt, RuO2, IrO2 etc.) have been used as electrode materials to facilitate hydrogen evolution at the cathode and oxygen evolution at the anode.12,13 However, the practical large-scale application of these catalysts has been limited because of their high cost and scarcity. Hence, it is highly desirable to design an efficient, cost-effective and long-lived electrolyzers based on earth-abundant metals. Substantial research efforts have been devoted in designing alternative electrocatalysts based on first row transition metal (mostly Mn, Fe, Co, and Ni) materials that include oxides/hydroxides,14-17 chalcogenides,18-20 pnictides,21-24 phosphates,25 phosphites,26 carbides,27 borides28 as well as intermetallics29,30 for efficient OER, HER, and overall water-splitting with suitable overpotential and current densities. Extensive variation of the metals and non-metals has led to derive conclusions over their determinant role in the net reactivity. Irrespective of the nature of the nonmetal, most of the catalysts undergo in-situ surface transformation to hold metal-oxy/hydroxy species that play the actual role in facilitating oxygen evolution.14,31 Recently, nitrides of firstrow transition metals have been explored as a highly active and stable electrocatalyst for OER and HER owing to their high electrical conductivity and facile structural transformation with vacancies and disorders.32-34 Although copper being one of the highly abundant inexpensive


4

Page 5 of 24 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

metals with strong biological relevance (numerous copper based metallo enzymes exist)35 was already used for water-splitting,36-42 its metallic conducting nitrides have never been examined for electrocatalytic water-splitting. Here we report that Cu3N shows a remarkable bifunctional activity for electrocatalytic water-splitting.

Scheme 1. Cu3N was synthesized by a novel approach using copper acetate and urea as a precursor to form cubic nanostructures. The cubic Cu3N is an efficient bifunctional catalyst for overall water-splitting.

Starting point of our investigation was the development a facilely scalable and reliable synthesis of nano-sized Cu3N by nitridation of copper acetate, based on the in-situ decomposition of urea to liberate ammonia (Scheme 1). The utilization of urea over the direct use of NH3 is very advantageous because of easy handling and controllability, that is, over-reduction to form metallic copper can be avoided. Various spectroscopic and microscopic techniques were employed to elucidate the structural/morphological features and electronic structure of the as-


5


Page 6 of 24

synthesized Cu3N materials. A powder X-ray diffraction (PXRD) analysis of the material (Figure 1a) displayed reflections corresponding to phase pure Cu3N (JCPDS 74-242).43 The morphological feature as determined by scanning electron microscopy (SEM) displayed cubical Cu3N particles with a varying dimension of 60-100 nm (see Supporting Information, Figure S1). Transmission electron microscopy (TEM) and high-resolution (HR-) TEM techniques were applied to determine the surface and the internal core structure of the nanocubes (Figure 1b-1c; Figure S2); the crystal lattice spacing of 0.38 nm was attained corresponding to the (100) plane of Cu3N.43 The selected area electron diffraction (SAED) pattern also revealed high crystallinity (inset Figure 1b-1c) and ring characteristics of Cu3N (Figure S2d). The chemical composition and the presence of Cu and N of Cu3N were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy dispersive X-ray (EDX) analysis, respectively (Figure S3 and Table S1). EDX mapping of Cu3N confirmed the homogenous elemental distribution of Cu and N in the product (Figure 1d-1f). The Brunauer–Emmett–Teller (BET) surface area of the as-synthesized Cu3N material was found to be 4.35 m2g-1. X-ray photoelectron spectroscopy (XPS) on the as-synthesized Cu3N materials was carried out to reveal the inherent electronic structure (for oxidation state of components Cu and N see Supporting Information). The Cu 2p3/2 and Cu 2p1/2, in Figure S4, showing peaks centered at 932.6 and 952.5 eV, respectively, and the N 1s displaying a peak at 398.2 eV represents characteristic binding energies for Cu and N in Cu3N.31,44 However, the additional peaks at 934.8 and 954.7 eV (Cu 2p) could be unambiguously assigned to Cu(II) that has been originated from the surface passivation due to exposure to air.45 Such atmospheric surface oxidation is well documented for chalcogenides and pnictides in the literature.18,21,22 Furthermore, the Cu3N is conductive with a resistivity of 7.2 Ω/sq (Table S2).


6

Page 7 of 24 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 1. (a) PXRD pattern (JCPDS 74-242) and the Miller indices (hkl) of Cu3N. (b, c) displays the HR-TEM and TEM images of Cu3N, where a lattice spacing of 0.38 nm corresponding to the crystallographic (100) plane resulted. The inset of (b, c) depicts the SAED pattern confirming high crystallinity of the product; (d-f) shows EDX elemental mapping images from the SEM confirming the homogenous elemental distribution. After successful assignment of the desired chemical composition and electronic structure of Cu3N, we explored their reactivity in electrochemical OER, HER and overall water electrolysis in alkaline media. Recently, taking into account its 3D porous structure, higher surface area, and electrical conductivity, nickel foam (NF) has been preferred as an electrode substrate to probe the electrocatalytic activity of Cu3N.26 Hence, the Cu3N nanocubes were electrophoretically deposited on NF. Strikingly, the Cu3N deposited NF (Cu3N/NF) electrodes furnished excellent OER efficiencies in alkaline media in terms of overpotential and current


7


Page 8 of 24

densities. A current density of 10 mAcm-2 corresponding to OER was attained at an overpotential of 286±04 mV (Figure 2a and Figure S5a). Although the attained overpotential at 10 mAcm-2 is close to that of the state-of-the-art noble-metal catalysts (IrO2/NF and RuO2/NF), at the higher potential range, the current density of >800 mAcm-2 was observed for Cu3N/NF which can be correlated to the better electron conduction during catalysis. The associated Tafel slope of 118.5±0.5 mVdec-1 for OER was attained which was lower than those of noble-metal-based catalysts rendering favorable kinetics of Cu3N (Figure 2b). When used for HER, the initial polarization curve revealed a reduction peak which is irreversible indicating an in-situ reduction of the oxidized species CuII and CuI to Cu0 during HER (Figure S6).46,47 Further, Cu3N/NF delivered a current density of -10 mAcm-2 at an overpotential of 118±03 mV (Figure 2c and Figure S5b) reflecting its superiority over noble metal-based electrocatalysts. A Tafel slope of 122.0±0.5 mVdec-1, that is comparable to the noble metal-based catalysts were also observed for Cu3N/NF indicating the HER reaction proceeds via the Volmer-Heyrovsky mechanism.48 Electrochemical impedance spectroscopy (EIS) measurements were conducted on Cu3N/NF and a low charge transfer resistance was achieved (Figure S7), which can be corroborated to more rapid charge transfer kinetics, suggesting the enhanced electrical conductivity.29 In addition, to estimate the electrochemical surface area (ECSA), the double-layer capacitance (Cdl) was measured (Figure S8) and from the Cdl values and the specific capacitance of the material (Cs) per unit area, the ECSA was calculated (Supporting information).12,49 For Cu3N/NF, a Cdl value of 0.8 mFcm-2 was attained that corresponds to the ECSA of 0.48 cm2. This provides the information on the catalytic active sites favoring the efficient adsorption and transfer of reactants to enhance the electrochemical reaction. To have a fair comparison of current densities responsible for OER and HER catalysis, the ECSA normalized current densities for Cu3N/NF


8

Page 9 of 24 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

were also plotted against applied potentials and compared with that of geometric ones. Significantly increased current densities at the higher potential range were indicative of its high ECSA (Figure S9). Furthermore, the stability of the Cu3N/NF for OER and HER were measured at a potential of 1.52 V and -0.118 V (vs RHE) in aqueous 1 M KOH exhibiting excellent stabilities over 14 hours (Figure S10). In addition to this, the Cu3N was also deposited on FTO (Cu3N/FTO) and investigated for alkaline HER and OER yielding similar results that of NF (Figure S11). Recently, low-cost mixed Fe-Ni oxides have shown to catalyze OER most effectively and have been considered as benchmark catalysts for OER. Keeping this in mind, we prepared a Fe-Ni mixed oxide (FexNi3-xO4) (see Supporting Information for detailed synthesis and characterizations, Figure S12) and their electrochemical activities were compared with that of Cu3N (Figure S13) on both NF and FTO electrodes. Although the attained OER overpotentials of FexNi3-xO4 were much lower to Cu3N, their HER overpotentials were substantially higher compared to Cu3N (Tables S4-S5). To have a direct structural comparison, we prepared the metallic Cu and CuO following very similar procedure (Figure S14) as that of Cu3N and the electrochemical activities (Figure S15) towards OER and HER were compared. Interestingly, Cu3N displayed much superior activity in comparison to Cu and CuO (Tables S4-S5). In addition to this, Cu3N was also heated under air to form CuO (Figure S16) but the attained electrocatalytic activities were far less compared to Cu3N suggesting the requirement of coreshell structure for the promotion of OER and HER (Figure S17). These observations can be easily rationalized by correlating their crystal structure, and bonding criteria (see discussion below) and the existence of ‘N’ in the crystal structure. As the metallic Cu is also HER active and a conductive material, the higher activity of Cu3N for HER was further verified by EIS where the charge transfer resistance of Cu3N was significantly lower than Cu suggesting faster


9


Page 10 of 24

charge transfer kinetics of Cu3N (Figure S18). Similarly, the attained ECSA for Cu (0.34 cm2) was also smaller compared to Cu3N indicating the higher density of active sites in Cu3N (Figure S19). Moreover, the copper-based materials in the literature, especially, elemental Cu, CuO and hybrid materials based on both, found to be less active than Cu3N for electrochemical OER and HER yielding higher overpotentials.50-52

Figure 2. (a) OER and (c) HER polarization curves of Cu3N along with the benchmark noble metal-based catalysts recorded on NF with a standard three-electrode electrochemical cell in aqueous 1 M KOH electrolyte with a sweep rate of 1 mVs-1. The Tafel plots of OER and HER are shown in (b) and (d).


10

Page 11 of 24 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

Following the successful utilization in electrochemical OER and HER, we wondered how surface structural transformations affect the net reactivity and the nature of the active centers. Thus extensive post-operando microscopic, spectroscopic and analytic investigations were undertaken to elucidate surface structure modifications and the formation of active sites. Cyclic voltammetry (CV) exhibited a redox peak at ~1.4 V (vs. RHE) confirming the formation of highvalent Cu species with concomitant formation of oxy/hydroxyl species at the surface which was also supported by XPS analysis after OER catalysis (Figure S20).44,45 Such a phenomenon is quite common for materials based on first-row transition metals (Fe, Co, Ni, Cu).53-55 TEM and HR-TEM revealed that a complete destruction of the nanocubes into smaller nanostructures was procured that contained Cu3N core and CuO shell which was further surrounded by amorphous overlayer (core-(double)shell structure with Cu-hydroxo on the outermost layer), which is most likely the active center for catalyzing the OER (Figure 3a-3b; Figure S21). This can be correlated to the vigorous leaching of ‘N’ and the concomitant replacement by oxygen species under catalytic conditions. From the HR-TEM, a lattice spacing of 0.22 and 0.23 nm corresponding to the Cu3N and CuO was attained post OER (Figure 3b).43,56 The reflections attained in the SAED patterns were also consistent with the crystal phases of Cu3N and CuO. In this way, Cu3N undergoes severe transformation to furnish quite active CuO species both in terms of overpotential and current densities. Interestingly, the formation CuO is often documented for metallic copper electrodes and have been correlated to corrosion of Cu(II) ions followed by formation of a Cu@CuO in the presence of oxidative electrode potential where the CuO layer on the surface not only protects further corrosion but also promotes catalysis in OER.37,57


11


Page 12 of 24

Figure 3. HR-TEM images and SAED patterns of Cu3N post (a, b) OER and (c, d) HER. From the HR-TEM, a surface amorphous overlayer along with a lattice spacing of 0.22 and 0.23 nm corresponding to the Cu3N (violet) and CuO (orange) was attained post OER while no overlayer was visible post HER. The SAED (b, d) was also consistent with the crystal phases of Cu3N (JCPDS 74-242) and CuO (JCPDS 72-629). The core-level (e) Cu 2p and (f) O 1s spectra of Cu3N post OER and HER catalysis. The oxidation states and the bonding situations are shown with the dashed lines.


12

Page 13 of 24 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

Similarly, the TEM and HR-TEM analyses of the Cu3N post HER also suggested the formation of smaller Cu3N/CuO core-shell crystallites but without the presence of any amorphous overlayer (Figure 3c-3d; Figure S22). The presence of a crystalline CuO shell on Cu3N could be corroborated to the generation of metallic copper on the surface of Cu3N under reductive conditions (Figure S6) and subsequent oxidation to CuO under strongly alkaline media or instantaneous aerial oxidation when detached from the electrolyte and has frequently been described for metal chalcogenides or phosphides.18,21 The EDX spectra in both OER and HER suggested the presence of Cu and N in the materials along with significant oxygen content (Figure S23). Further insights into the active structures were gathered by XPS analysis. In the XPS spectra (Figure 3e and Figure S24a), the Cu 2p3/2 and Cu 2p1/2 of post OER material showed peaks at 934.5 and 954.6 eV corresponding to CuII states which could be rationalized to the complete oxidation of surface CuI sites to CuII under applied oxidative potential (consistent with CV results).39 However, in the post HER material, the peaks centered at 932.5 and 952.4 eV are associated to CuI of the Cu3N while the peaks centered at 934.6 and 954.8 eV corroborated to CuII, that signified the possible aerial oxidation of metallic Cu in strongly alkaline conditions (Figure 3e and Figure S26a).

31,44

Interestingly, the peaks for N 1s in the region of ~398.2 eV were absent in

both OER and HER treated materials demonstrating an increased thickness of the CuO


13


Page 14 of 24

shell (Figure S25). The O 1s spectrum post OER film exhibited that the material is largely hydroxylated and the formation of Cu-O bonds (CuO) was clearly evident in the post HER films (Figure 3f).58 The detailed deconvolution of Cu 2p and O 1s has been described in Figures S24-S26. Finally, the powder X-ray diffraction measurements on FTO substrates before and post catalysis were also conducted, that substantiated the existence of Cu3N core and the formation of surface CuO shell (Figure S27). To account for the superior activity of the Cu3N catalytic material, we compared the structural features of Cu3N with that of CuO and Cu (Figure 4a and Figure S28). A closer look at the crystal structure of Cu3N suggests the existence of coordinatively unsaturated copper sites that can be easily attacked by water molecules followed by deprotonations in the presence of applied electrode potential to form oxo-bridged copper sites leading to final CuO materials. This observation has already been proved experimentally by HR-TEM and SAED studies (see discussion above). Although the same feature is expected in the case of metallic copper, however, the presence of nitridic N3- in Cu3N has a benefit of adjusting the electronic structure of the Cu. The facile removal of N3- (‘N’) through oxidative potentials forms vacancies and defected CuO(H) structure but the retention of the crystalline Cu3N core assist enhanced electron transport.32 On the contrary, the CuO materials are poorly active in comparison to the Cu3N owing to the absence of a highly conductive Cu3N core.36,39 Based on the aforementioned results, the outstanding OER activity of Cu3N nanocubes can be attributed to (i) the particular features of the Cu3N structure with coordinatively unsaturated Cu


14

Page 15 of 24 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

and easy-to-oxidize N3- which are prone for major structural and morphological alterations under strongly alkaline media, (ii) the synergistic effect of Cu-rich amorphous overlayer consisting of Cu-oxo/hydroxo active species supported by crystalline CuO shell and Cu3N core, (iii) the formation of vacancy induced defected structure allows facile adsorption of OH- anions followed by deprotonation steps under oxidative potential that accelerate O-O bond formation to enhance net OER reactivity, and (iv) the metallic character of Cu3N that actually acts as a highly conductive layer between the active surface of the catalyst and the electrode substrate, enhancing the charge mobility. On the other hand, the formation of a metallic Cu-enriched CuO surface seems crucial for the onset of HER under reducing conditions along with the presence of a conductive core. In addition to this, the formation of a higher amount of electrocatalytically active sites as determined by ECSA as well as lower charge transfer resistance, leading to efficient electron transfer between the crystalline Cu3N and CuO, is suggested by results of EIS.

Figure 4. (a) The crystal structure of Cu3N showing the coordinately unsaturated Cu sites and (b) the polarization curves of the overall water-splitting of Cu3N║Cu3N on NF with the


15


Page 16 of 24

two-electrode configuration in aqueous 1 M KOH. The inset of (b) presents the long-term stability curve which was conducted by applying a cell voltage of 1.62 V over 10 days. Encouraged by the exceptional activity and stability of Cu3N for OER and HER, we attempted to fabricate an alkaline water electrolyzer by applying Cu3N/NF as both anode and cathode material (Figure S29). To our surprise, an overall water-splitting current density of 10 mAcm-2 was attained at only 1.60±0.01 V cell potential (LSV), which, to the best of our knowledge, is one of the lowest for any copper-based materials reported to date (Figure 4b). Moreover, when a constant cell voltage of 1.62 V was applied across the anode and cathode to monitor the durability of the electrodes, almost no activity loss was observed over a period of ten days (Figure 4b inset); that renders an exceptional robustness of the Cu3N/NF electrodes in comparison to literature known copper-based materials.45,59 Further, to account for the evolved gases (H2 at cathode and O2 at the anode) and the observed current densities, we have calculated the Faradic efficiencies (FE) of each half-cell reactions (HER and OER respectively). Along this line, a closed electrochemical cell was used such that the evolved gases from the headspace could be estimated by gas chromatography and the related FE was calculated to be nearly 100 % rendering the efficient selectivity for each half-cell reaction (Table S3). In addition, the relative evolution of H2 to that of O2 was also found out by employing an inverted (graduated) electrochemical cell such that each gas can be collected separately at atmospheric pressure and quantified (Figure S30). An approximate measured ratio of 2:1 for H2 and O2 evolution rules out any competitive side reaction during electrolysis (Figure S31). In summary, we have reported a new convenient synthesis of Cu3N electrocatalytic material by following the thermal decomposition of urea. Cu3N deposited on NF delivered excellent activity and stability when applied as cathode and anode for HER and OER, and the attained activities


16

Page 17 of 24 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

surpass those of well-known state-of-the-art noble metal-based catalysts and most of the copperbased catalysts reported to-date. Detailed post operando analyses by microscopic and spectroscopic methods suggest that a facile formation of defect-rich Cu-oxy/hydroxide active species (from Cu3N after the loss of ‘N’) is responsible for accelerating OER whereas HER is enhanced by cooperative effects exerted by highly conducting Cu3N core and metallic Cu (CuO) under reducing conditions. Moreover, considering the unique structural and electronic features, the superior catalytic activity and stability of Cu3N over metallic Cu and CuO has also been exemplified. An alkaline water electrolyzer using Cu3N/NF as the anode and cathode has been fabricated to deliver outstanding efficiency and stability for ten days. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.xxxxxx. Detailed experimental procedures for Cu3N, the microscopic, spectroscopic and analytical characterization of the material, electrochemical and post electrochemical investigation of the materials, etc. (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Chakadola Panda: 0000-0003-3073-8025


17


Page 18 of 24

Prashanth W. Menezes: 0000-0002-0665-7690 Matthias Drieß: 0000‐0002‐9873‐4103 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

Financial support by the Deutsche Forschungsgemeinschaft (Cluster of Excellence UniCat, EXC 314-2) is greatly acknowledged. The authors are indebted to Dr. Vitaly Gutkin for XPS, Mr. Christoph Fahrenson for SEM and Jan Niklas Hausmann for TEM analyses. REFERENCES (1) Panda, C.; Menezes, P. W.; Driess, M. Nano-Sized Inorganic Energy-Materials by the Low-Temperature Molecular Precursor Approach. Angew. Chem. Int. Ed. 2018, 57 (35), 11130-11139; Angew. Chem. 2018, 130 (35) 11298-11308. (2) Chu, S.; Majumdar, A. Opportunities and Challenges for a Sustainable Energy Future. Nature 2012, 488, 294. (3) Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001,

414, 332. (4) Li, X.; Hao, X.; Abudula, A.; Guan, G. Nanostructured Catalysts for Electrochemical Water Splitting: Current State and Prospects. J. Mater. Chem. A 2016, 4 (31), 11973-12000. (5) Roger, I.; Shipman, M. A.; Symes, M. D. Earth-abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 0003. (6) Singh, A.; Spiccia, L. Water Oxidation Catalysts Based on Abundant 1st Row Transition Metals. Coord. Chem. Rev. 2013, 257 (17), 2607-2622.


18

Page 19 of 24 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

(7) Zeradjanin, A. R.; Grote, J.-P.; Polymeros, G.; Mayrhofer, K. J. J. A Critical Review on Hydrogen Evolution Electrocatalysis: Re-exploring the Volcanorelationship. Electroanalysis 2016, 28 (10), 2256-2269. (8) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114 (24), 1186312001. (9) Menezes, P. W.; Indra, A.; Gutkinb, V.; Driess, M. Boosting Electrochemical Water Oxidation Through Replacement of Oh Co Sites in Cobalt Oxide Spinel with Manganese. Chem. Commun. 2017, 53 (57), 8018-8021. (10)Menezes, P. W.; Indra, A.; Littlewood, P.; Schwarze, M.; Gobel, C.; Schomacker, R.; Driess, M. Nanostructured Manganese Oxides as Highly Active Water Oxidation Catalysts: A Boost from Manganese Precursor Chemistry. ChemSusChem 2014, 7 (8), 2202-2211. (11)Menezes, P. W.; Indra, A.; Sahraie, N. R.; Bergmann, A.; Strasser, P.; Driess, M. Cobalt-Manganese-Based Spinels as Multifunctional Materials that Unify Catalytic Water Oxidation and Oxygen Reduction Reactions. ChemSusChem 2015, 8 (1), 164171. (12)McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem.

Soc. 2013, 135 (45), 16977-16987. (13)Kwon, S. J.; Fan, F.-R. F.; Bard, A. J. Observing Iridium Oxide (IrOx) Single Nanoparticle Collisions at Ultramicroelectrodes. J. Am. Chem. Soc. 2010, 132 (38), 13165-13167. (14)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 (22), 7549-7558. (15)Menezes, P. W.; Indra, A.; Gonzalez-Flores, D.; Sahraie, N. R.; Zaharieva, I.; Schwarze, M.; Strasser, P.; Dau, H.; Driess, M. High-Performance Oxygen Redox Catalysis with Multifunctional Cobalt Oxide Nanochains: Morphology-Dependent Activity. ACS Catal. 2015, 5 (4), 2017-2027.


19


Page 20 of 24

(16)Wang, A.-L.; Xu, H.; Li, G.-R. NiCoFe Layered Triple Hydroxides with Porous Structures as High-Performance Electrocatalysts for Overall Water Splitting. ACS

Energy Lett. 2016, 1 (2), 445-453. (17)Roger, I.; Symes, M. D. First-row Transition Metal Catalysts for Solar-driven Water Oxidation Produced by Electrodeposition. J. Mater. Chem. A 2016, 4 (18), 6724-6741. (18)Panda, C.; Menezes, P. W.; Walter, C.; Yao, S. L.; Miehlich, M. E.; Gutkin, V.; Meyer, K.; Driess, M. From a Molecular 2Fe-2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting. Angew. Chem. Int. Ed. 2017,

56 (35), 10506-10510. (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 (12), 8069-8097. (20)Tran, P. D.; Tran, Thu V.; Orio, M.; Torelli, S.; Truong, Q. D.; Nayuki, K.; Sasaki, Y.; Chiam, Sing Y.; Yi, R.; Honma, I. et al. Coordination P structure and Revisited Hydrogen Evolution Catalytic Mechanism for Amorphous Molybdenum Sulfide. Nat.

Mater. 2016, 15, 640. (21)Menezes, P. W.; Indra, A.; Das, C.; Walter, C.; Gobel, C.; Gutkin, V.; Schmeisser, D.; Driess, M. Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in High-Performance Electrochemical Overall Water Splitting.

ACS Catal. 2017, 7 (1), 103-109. (22)Yao, S.; Forstner, V.; Menezes, P. W.; Panda, C.; Mebs, S.; Zolnhofer, E. M.; Miehlich, M. E.; Szilvási, T.; Ashok Kumar, N.; Haumann, M. et al. From an Fe2P3 Complex to FeP Nanoparticles as Efficient Electrocatalysts for Water-splitting. Chem.

Sci. 2018, 9, 8590-8597. (23)Tan, Y.; Wang, H.; Liu, P.; Cheng, C.; Zhu, F.; Hirata, A.; Chen, M. 3D Nanoporous Metal Phosphides toward High-Efficiency Electrochemical Hydrogen Production. Adv. Mater. 2016, 28 (15), 2951-2955. (24)Fu, Q.; Wu, T.; Fu, G.; Gao, T.; Han, J.; Yao, T.; Zhang, Y.; Zhong, W.; Wang, X.; Song, B. Skutterudite-Type Ternary Co1-xNixP3 Nanoneedle Array Electrocatalysts


20

Page 21 of 24 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

for Enhanced Hydrogen and Oxygen Evolution. ACS Energy Lett. 2018, 3 (7), 17441752. (25)Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321 (5892), 10721075. (26)Menezes, P. W.; Panda, C.; Loos, S.; Bunschei-Bruns, F.; Walter, C.; Schwarze, M.; Deng, X. H.; Dau, H.; Driess, M. A structurally Versatile Nickel Phosphite Acting as a Robust Bifunctional Electrocatalyst for Overall Water Splitting. Energy Environ.

Sci. 2018, 11 (5), 1287-1298. (27)Liu, Y.; Kelly, T. G.; Chen, J. G.; Mustain, W. E. Metal Carbides as Alternative Electrocatalyst Supports. Acs Catal. 2013, 3 (6), 1184-1194. (28)Li, H.; Wen, P.; Li, Q.; Dun, C.; Xing, J.; Lu, C.; Adhikari, S.; Jiang, L.; Carroll, D. L.; Geyer, S. M. Earth-Abundant Iron Diboride (FeB2) Nanoparticles as Highly Active Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Energy Mater. 2017, 7 (17), 1700513. (29)Menezes, P. W.; Panda, C.; Garai, S.; Walter, C.; Guiet, A.; Driess, M. Structurally

Ordered

Intermetallic

Cobalt

Stannide

Nanocrystals

for

High-

Performance Electrocatalytic Overall Water-Splitting. Angew. Chem. Int. Ed. 2018, 57 (46), 15237-15242. (30)Qin, F.; Zhao, Z.; Alam, M. K.; Ni, Y.; Robles-Hernandez, F.; Yu, L.; Chen, S.; Ren, Z.; Wang, Z.; Bao, J. Trimetallic NiFeMo for Overall Electrochemical Water Splitting with a Low Cell Voltage. ACS Energy Lett. 2018, 3 (3), 546-554. (31)Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2017, 2 (8), 1937-1938. (32)Walter, C.; Menezes, P. W.; Orthmann, S.; Schuch, J.; Connor, P.; Kaiser, B.; Lerch, M.; Driess, M. A Molecular Approach to Manganese Nitride Acting as a High Performance Electrocatalyst in the Oxygen Evolution Reaction. Angew. Chem. Int.

Ed. 2018, 57 (3), 698-702. (33)Xu, K.; Chen, P.; Li, X.; Tong, Y.; Ding, H.; Wu, X.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y. Metallic Nickel Nitride Nanosheets Realizing Enhanced Electrochemical Water Oxidation. J. Am. Chem. Soc. 2015, 137 (12), 4119-4125.


21


Page 22 of 24

(34)Han, N.; Liu, P.; Jiang, J.; Ai, L.; Shao, Z.; Liu, S. Recent Advances in Nanostructured Metal Nitrides for Water Splitting. J. Mater. Chem. A 2018, 6, 1991219933. (35)Lewis, E. A.; Tolman, W. B. Reactivity of Dioxygen−Copper Systems. Chem.

Rev. 2004, 104 (2), 1047-1076. (36)Huan, T. N.; Rousse, G.; Zanna, S.; Lucas, I. T.; Xu, X.; Menguy, N.; Mougel, V.; Fontecave, M. A Dendritic Nanostructured Copper Oxide Electrocatalyst for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2017, 56 (17), 4792-4796. (37)Du, J.; Chen, Z.; Ye, S.; Wiley, B. J.; Meyer, T. J. Copper as a Robust and Transparent Electrocatalyst for Water Oxidation. Angew. Chem. Int. Ed. 2015, 54 (7), 2073-2078. (38)Wang, R.; Dong, X.-Y.; Du, J.; Zhao, J.-Y.; Zang, S.-Q. MOF-Derived Bifunctional Cu3P Nanoparticles Coated by a N,P-Codoped Carbon Shell for Hydrogen Evolution and Oxygen Reduction. Adv. Mater. 2018, 30 (6), 1703711. (39)Zhang, B.; Li, C.; Yang, G.; Huang, K.; Wu, J.; Li, Z.; Cao, X.; Peng, D.; Hao, S.; Huang, Y. Nanostructured CuO/C Hollow Shell@3D Copper Dendrites as a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2018, 10 (28), 23807-23812. (40)Xiaoqian, Z.; Li, L.; Yan, Z.; Huijuan, Z.; Yu, W. Uniquely Confining Cu2S Nanoparticles in Graphitized Carbon Fibers for Enhanced Oxygen Evolution Reaction. Nanotechnology 2017, 28 (34), 345402. (41)Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Self-Supported Cu3P Nanowire Arrays as an Integrated High-Performance Three-Dimensional Cathode for Generating Hydrogen from Water. Angew. Chem. Int. Ed. 2014, 53 (36), 9577-9581. (42)Muthukumar, P.; Kumar, V. V.; Reddy, G. R. K.; Kumar, P. S.; Anthony, S. P. Fabrication of Strong Bifunctional Electrocatalytically Active Hybrid Cu–Cu2O Nanoparticles in a Carbon Matrix. Catal. Sci. Technol. 2018, 8 (5), 1414-1422. (43)Juza, R.; Hahn, H. Über die Kristallstrukturen von Cu3N, GaN und InN Metallamide und Metallnitride. Z. Anorg. Allg. Chem. 1938, 239, 282-287. (44)Deng, Y. L.; Handoko, A. D.; Du, Y. H.; Xi, S. B.; Yeo, B. S. In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen


22

Page 23 of 24 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

Evolution Reaction: Identification of Cu-III Oxides as Catalytically Active Species.

ACS Catal. 2016, 6 (4), 2473-2481. (45)Handoko, A. D.; Deng, S. Z.; Deng, Y. L.; Cheng, A. W. F.; Chan, K. W.; Tan, H. R.; Pan, Y. L.; Tok, E. S.; Sow, C. H.; Yeo, B. S. Enhanced Activity of H2O2-treated Copper(II) Oxide Nanostructures for the Electrochemical Evolution of Oxygen. Catal.

Sci. Technol. 2016, 6 (1), 269-274. (46)Menezes, P. W.; Indra, A.; Zaharieva, I.; Walter, C.; Loos, S.; Hoffmann, S.; Schlögl, R.; Dau, H.; Driess, M. Helical Cobalt Borophosphates to Master Durable Overall Water-splitting. Energy Environ. Sci. 2019, DOI: 10.1039/c1038ee01669k. (47)Wu, Z. S.; Gan, Q.; Li, X. L.; Zhong, Y. R.; Wang, H. L. Elucidating Surface Restructuring-Induced Catalytic Reactivity of Cobalt Phosphide Nanoparticles under Electrochemical Conditions. J. Phys. Chem. C 2018, 122 (5), 2848-2853. (48)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. (49)Yoon, Y.; Yan, B.; Surendranath, Y. Suppressing Ion Transfer Enables Versatile Measurements of Electrochemical Surface Area for Intrinsic Activity Comparisons. J.

Am. Chem. Soc. 2018, 140 (7), 2397-2400. (50)Liu, X.; Cui, S.; Sun, Z.; Ren, Y.; Zhang, X.; Du, P. Self-Supported Copper Oxide Electrocatalyst for Water Oxidation at Low Overpotential and Confirmation of Its Robustness by Cu K-Edge X-ray Absorption Spectroscopy. J. Phys. Chem. C 2016,

120 (2), 831-840. (51)Qian, M.; Liu, X.; Cui, S.; Jia, H.; Du, P. Copper Oxide Nanosheets Prepared by Molten Salt Method for Efficient Electrocatalytic Oxygen Evolution Reaction with Low Catalyst Loading. Electrochim. Acta 2018, 263, 318-327. (52)Liu, X.; Zheng, H.; Sun, Z.; Han, A.; Du, P. Earth-Abundant Copper-Based Bifunctional Electrocatalyst for Both Catalytic Hydrogen Production and Water Oxidation. ACS Catal. 2015, 5 (3), 1530-1538. (53)Menezes, P. W.; Indra, A.; Bergmann, A.; Chernev, P.; Walter, C.; Dau, H.; Strasser, P.; Driess, M. Uncovering the Prominent Role of Metal Ions in Octahedral


23


Page 24 of 24

versus Tetrahedral Sites of Cobalt-zinc Oxide cCatalysts for Efficient Oxidation of Water. J. Mater. Chem. A 2016, 4 (25), 10014-10022. (54)Indra, A.; Menezes, P. W.; Das, C.; Schmeisser, D.; Driess, M. Alkaline Electrochemical Water Oxidation with Multi-shelled Cobalt Manganese Oxide Hollow Spheres. Chem. Commun. 2017, 53 (62), 8641-8644. (55)Indra, A.; Menezes, P. W.; Sahraie, N. R.; Bergmann, A.; Das, C.; Tallarida, M.; Schmeisser, D.; Strasser, P.; Driess, M. Unification of Catalytic Water Oxidation and Oxygen Reduction Reactions: Amorphous Beat Crystalline Cobalt Iron Oxides. J.

Am. Chem. Soc. 2014, 136 (50), 17530-17536. (56)Asbrink, S.; Lorrby, L. J. A Refinement of the Crystal Structure of Copper(II) Oxide with a Discussion of Some Exceptional E.S.D.'s. Acta Crystallogr. B 1970, 26, 8-15. (57)Burke, L. D.; Ahern, M. J. G.; Ryan, T. G. An Investigation of the Anodic Behavior of Copper and Its Anodically Produced Oxides in Aqueous Solutions of High pH. J.

Electrochem. Soc. 1990, 137 (2), 553-561. (58)Cheng, N. Y.; Xue, Y. R.; Liu, Q.; Tian, J. Q.; Zhang, L. X.; Asiri, A. M.; Sun, X. P. Cu/(Cu(OH)2-CuO) Core/Shell Nanorods Array: In-situ Growth and Application as an Efficient 3D Oxygen Evolution Anode. Electrochim. Acta 2015, 163, 102-106. (59)Han, A.; Zhang, H. Y.; Yuan, R. H.; Ji, H. X.; Du, P. W. Crystalline Copper Phosphide Nanosheets as an Efficient Janus Catalyst for Overall Water Splitting.

ACS Appl. Mater. Interfaces 2017, 9 (3), 2240-2248.


24

In Situ Formation of Nanostructured CoreâShell Cu3NâCuO to

Recommend Documents

In Situ Formation of Nanostructured CoreâShell Cu3NâCuO to