Electrodeposited CopperâCobaltâPhosphide: A Stable Bifunctional

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Electrodeposited Copper-Cobalt-Phosphide: a Stable Bifunctional Catalyst for Both Hydrogen and Oxygen Evolution Reactions Ruwani Wasalathanthri, Samuel Jeffrey, Rasha A. Awni, Kai Sun, and Dean Mark Giolando ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04807 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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ACS Sustainable Chemistry & Engineering

Electrodeposited Copper-Cobalt-Phosphide: a Stable Bifunctional Catalyst for Both Hydrogen and Oxygen Evolution Reactions Ruwani N. Wasalathanthri†, Samuel Jeffrey†, Rasha A. Awniǂ, Kai Sun§ and Dean M. Giolando†* †Department

of Chemistry and Biochemistry, School of Green Chemistry and Engineering,

University of Toledo, Toledo, OH, 43606, USA ǂDepartment

of Physics and Astronomy, and Wright Center for Photovoltaics Innovation and

Commercialization (PVIC), University of Toledo, Toledo, OH 43606, USA §Department

of Materials Science and Engineering, University of Michigan, Ann Arbor, MI,

48109, USA *Corresponding Author: [email protected] All authors information: Dean M. Giolando, [email protected], 419-530-1511, 2801 West Bancroft St., Toledo, OH 43606, USA Ruwani N. Wasalathanthri, [email protected], 2801 West Bancroft St., Toledo, OH 43606, USA Samuel Jeffrey, [email protected], 2801 West Bancroft St., Toledo, OH 43606, USA Rasha A. Awni, [email protected], 2801 West Bancroft St., Toledo, OH

ACS Paragon Plus Environment

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

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43606, USA Kai Sun, [email protected], (734) 936-3353, Department of Materials Science and Engineering, University of Michigan, 2800 Plymouth Road, NCRC, Ann Arbor, MI, 48109, USA

KEYWORDS: bimetallic phosphide, electrochemistry, Janus electrocatalysts, sustainable chemistry, water splitting.


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INTRODUCTION Enhancing energy conversion and storage systems, such as water electrolysis, fuel cells and metal-air batteries, is needed for clean and efficient alternate energy sources,1-3 and to reduce pollution that is harmful to human health4. These systems involve either hydrogen evolution reaction (HER) or oxygen reduction reaction (ORR) at the cathode, and oxygen evolution reaction (OER) or oxidation of fuels at the anode.1 Among these, HER and OER are also the key reactions in water electrolysis and have attracted attention due to their sluggish kinetics.1, 5 Thus far, the best catalysts for these reactions include noble metals, such as Pt for HER, and IrO2 or RuO2 for OER, yet their low abundance and high cost limit widespread utilization.5-6 A variety of transition metal chalcognides7-13 and phosphides2-3, 5, 14-24 exhibit high HER activity. Owing to their higher affinity towards H2, transition metal phosphides (TMPs) are also excellent catalysts in other processes, such as hydrodesulfurization25, hydrodenitrification26, and CO hydrogenation,27 indicating a vast applicability of TMP HER catalysts.28

Moreover,

the

bimetallic

phosphides

Cu3P-CoP

hybrid

and

Cu0.3Co2.7P/nitrogen-doped carbon have shown improved HER activity due to synergistic effects caused by the atomic and electric coupling of bimetallic TMPs.29-30 In addition, many transition phosphides2, 14, 24, 31-35 have also exhibited OER activity. Noteworthy, many transition metal oxides/hydroxides6,

36-37

afford excellent OER activity, but suffer

degradation when employed as water splitting catalysts.38 Moreover, incompatibility of


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integrating two different catalysts on the anode and cathode leads to lower overall efficiency, thus, developing a single bifunctional electrocatalyst for both HER and OER has attracted attention.2, 12-13, 39 Even though, competent bifunctional TMP catalysts including Co-P2, 5, Cu-P16-17,

23

and Cu-Co-P30 have been reported for overall water splitting,

developing increasingly more stable and active bifunctional electrocatalysts derived from earth abundant materials is still a great challenge. Most of the methods for preparing TMPs generally involve complex high temperature synthetic procedures.17,

20, 22-23

Common methods include, wet chemical

synthesis using an organic solvent with trioctylphosphine (TOP) as the phosphorus source (at a temperature range of 220 °C – 385 °C), solid-state reaction using diammonium monohydrogenphosphate ((NH4)2HPO4) mixed with metal salts in a temperatureprogramed reduction in a H2 atmosphere (400 °C – 1000 °C), and gas-solid reduction using NaH2PO2 or red phosphorous.15,

40-42

However, these methods possess some

drawbacks such as, low yields, high temperature processing, production of highly toxic and flammable gases (PH3 or white phosphorous), and can also be highly demanding for TMP synthesis.40 Thus, it is important to simplify the catalysts preparation to avoid high energy consumption, environmental issues and also to lower the costs.14,

40

Electrodeposition is an ideal method for the preparation of TMP films onto electrode surfaces, which does not involve energy and environmental obstacles.3, 14 It is a simple, inexpensive and a rapid method that provides several benefits over high temperature


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synthetic methods. These benefits also include an ability to be carried out under normal laboratory conditions, easily controllable parameters to tune the morphology, thickness and composition, ability to obtain uniform coatings and excellent adhesion to the electrode substrate.3 Herein, we report a mixed copper-cobalt-phosphide (Cu-Co-P) electrodeposited onto highly conducting Cu foil using common chemical reagents, as a competent Janus type bifunctional electrocatalyst. As-deposited films consisted of ~30% of atomic percentage of P (P at%) with very strong adhesion to the Cu substrate and exhibit remarkable stability and activity for both HER (in 0.5 M H2SO4 and 1 M KOH) and OER (in 1 M KOH). In 1 M KOH, activity of Cu-Co-P is even better than Pt (for HER) and RuO2 (for OER). A water splitting electrolyzer consisting of Cu-Co-P electrodes provided superior stability and activity over a cell employing RuO2 and Pt black electrodes. RESULTS AND DISCUSSION

A

B Composition and Morphology of Cu-Co-P Films: Scanning electron microscopic (SEM)

C

D

images of as-deposited Cu-Co-P films show nanometer-sized

E

F

crystallites

distributed

throughout (Figures 1A-B), and film thickness is ~ 6 µm (Figure 1C). Elemental mapping by

Figure 1. SEM images of as-deposited Cu-Co-P films ACS(Inset: Paragon Plus Environment at magnifications of (A) 2500X, (B) 15,000X 85,000X) and (C) cross-section of the Cu-Co-P film and (D-F) corresponding EDX mapping.

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energy dispersive X-ray spectroscopy (EDX)

A

analysis of a cross-section confirmed the even distribution of Cu, Co and P (Figures 1D-F and Figures S3-4). The EDX quantification yielded

B

(31.1 ± 0.3) P at%, (51.8 ± 1.2) Co at% and (17.1 ± 1.2) Cu at%, and the mass loading of the electrode is circa 0.7 mg/cm2. The Cu-Co-P films exhibit strong adhesion to the Cu substrate, thus

C

it was not possible to remove Cu-Co-P without also removing Cu from the substrate, in order to perform inductively coupled plasma (ICP) mass

Figure 2. High resolution XPS spectra of (A) Co 2p, (B) Cu 2p and (C) P 2p regions of as-deposited CuCo-P films.

spectrometry or other elemental analyses. The crystalline phases were revealed in the HREM

images contained in the Supporting Information (Figure S5). Powder X-ray diffraction characterization (PXRD) of Cu-Co-P films showed the possibility of having Co2P and CoP3 crystalline aggregates (Figure S7), indicating Cu-Co-P is a mixed TMP composed with crystalline Co-P phases and Cu is possibly mixed in the Co-P lattice. In 2017, Lin, et al. showed the possibility of Cu substituting Co in Co-P, due to the similarity in electronegativity and ionic radius of Cu2+ and Co2+ ions.30 According to Lin, et al., the homogeneous substitution does not introduce any peak shifts nor additional peaks.


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Conceivably, there is a similar Cu ion substitution to the Co-P matrix in the electrodeposited Cu-Co-P films. The chemical states of Co, Cu and P on the surface of the Cu-Co-P films were characterized by XPS. Figure 2 shows the high resolution XPS spectra of the Co 2p, Cu 2p and P 2p regions of as-deposited Cu-Co-P films. In the Co 2p3/2 region, three peaks at 778.5, 779.6 and 781.7 eV can be assigned to Coδ+ in Co2P and CoP3 in the Cu-Co-P films, and the satellite, respectively. These binding energies are higher than that for zero valence Co (778.1 eV for 2p3/2) and are comparable with literature values, indicating the presence of small positive charge on Co centers.14, 20 The three peaks at higher binding energies of 793.5, 796.7 and 799.9 eV are assigned to Co 2p1/2 binding energies of Coδ+ in Co2P and CoP3 in the Cu-Co-P films, and the satellite of Co 2p1/2 peak, respectively.14, 20 The four peaks at 932.9, 934.8, 930.9 and 942.4 eV in the 2p3/2 region of the Cu 2p spectra can be assigned to Cuδ+, oxidized Cu(II) and Cu(I) in Cu-Co-P film, and the satellite of Cu 2p3/2 peak, respectively. The peaks at higher binding energies of 952.6, 954.5, 950.5 and 960.3 eV are the respective Cu 2p1/2 binding energies of Cuδ+, oxidized Cu(II) and Cu(I) in CuCo-P film, and the satellite of Cu 2p1/2 peak.17, 23, 43 In the P 2p spectra, the peaks at 129.4 eV (2p3/2) and 130.3 eV (2P1/2) are due to the Pδ- in Cu-Co-P. The binding energy 129.4 eV is smaller than that for zero valence P (130.2 eV) indicating the presence of partial negative charge on P centers. The binding energies for Pδ- in different metal phosphides are very


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similar, hence only one peak corresponding to Pδ- is observed in the P 2p spectra.20, 29 The peak at 133.2 eV is assigned to the oxidized P in the form of phosphate.3, 22-23 These results suggest the presence of partially positively charged Co and Cu centers and partially negatively charged P centers, indicating a charge transfer between Co/Cu and P. Due to this covalent character, the Co and Cu centers may act as hydrideacceptors, while P centers may act as proton acceptors, as in hydrogenases, and thus exhibits good catalytic activity.23 The presence of oxidized metal and P centers are common when the metal phosphides are exposed to air, and the metal phosphates are formed due to surface passivation.3 Interestingly, in Figure 2, only copper oxide peaks, but no cobalt oxide peaks are observed, indicating that Cu in the Cu-Co-P films may more easily oxidize than the Co sites under the 80 °C temperature deposition conditions. Noteworthy, low surface passivation is indicated by the low PO43- peak intensity in Figure 2C, which contrasts most TMPs where the PO43- peak intensity is greater than that of Pδ.3, 22, 29-30, 32, 44 According to the literature, the surface oxidation of TMPs can hinder the HER activity due to decrease in conductivity and blocking the active sites for HER.40 However, it is believed that the surface oxidation of TMPs does not lower the OER activity. Studies have shown that surface oxides/hydroxides and phosphates are active sites for OER catalysts, with core TMP acting as a conductive support for an effective electron transfer pathway.40,

45-47

Conceivably, the reduced surface oxidation on Cu-Co-P films

facilitates bifunctionality of the catalyst.


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Electrochemical activity and stability of Cu-Co-P Films towards HER: Linear sweep voltammetry is a common electrochemical technique used in evaluating different electrocatalysts, and the overpotential to reach a certain current density (ƞj) is used to evaluate the electrocatalytic activity. Generally, a wide range of TMPs as water splitting catalysts can be observed in the literature and their overpotentials (in 0.5 M H2SO4) to achieve 10 mA/cm2 current density vary in the range of 30 - 500 mV, and the Tafel slopes are mostly in the range of 30 – 150 mV/dec.40 However, the overpotential can also be highly dependent on many factors, such as the type of substrate and the porosity of the electrode, which alters the conductivity and the electroactive surface area, as well as on the electrochemical method used.40 One example is obtained on comparing CoP nanosheets grown on carbon cloth, which displayed a Tafel slope of 30.1 mV/dec and an overpotential of 49 mV to reach 10 mA/cm2,44 while the same on a Ti plate displayed a Tafel slope of 43 mV/dec and an overpotential of 90 mV for 10 mA/cm2.48 The CoP nanosheets on carbon cloth requires much lower overpotential compared to the CoP nanosheets on Ti plate, and that is due to the larger electroactive surface area on carbon cloth compared to a flat Ti plate. For these reasons, it is highly important to keep the morphology, the substrate, and other factors, such as the electrochemical parameters (scan rate, stirred or diffusion controlled, electrode assembly) similar, for an accurate


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evaluation and comparison of different catalysts.40 Therefore, in our work, we used electrodeposited Pt black on Cu substrates measured under the same conditions to compare the apparent activity of electrodeposited Cu-Co-P, in addition to commercial Pt/C catalyst. Copper foil was chosen as the substrate due to its superior conductivity and

A

B

no reactivity in acidic electrolyte. Choosing

a

metal

such

as

stainless steel as a substrate is not recommended, as it can

C

D

galvanically

react

in

acidic

electrolytes.3 Moreover, carbon electrodes undergo fast

E

F

degradation

under

oxidizing potentials, hence it is also not a desirable substrate for water splitting catalysts studies.

G

H Moreover, stability

test

of

during

the

continuous

electrolysis for 72 hours, a Pt mesh electrode was used as the

Figure 3. (A, C) Polarization plots and (B, D) corresponding Tafel plots (with linear fitted dotted line) comparing HER activities in (A, B) 0.5 M H2SO4 and (C, D) 1 M KOH. (E, F) auxiliary electrode (AE) Corresponding Nyquist plots of electrochemical impedance spectra (E) at -214 mV in 0.5 M H2SO4 and (F) at -165 mV in 1 M KOH. (G, H) Polarization plots (not iR compensated) of CuCo-P before and after continuous HER for 72 hours at 15 mA/cm2 ACS(Insets: Paragon Plus Environment in (G) 0.5 M H2SO4 and (H) in 1 M KOH; corresponding potential change).

instead of

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a carbon, due to the degradation of the carbon AE. Carbon as the AE produced a very similar response as the Pt mesh AE, for polarization curves with Cu-Co-P as the working electrode (Figure S11).49 The HER activity of Cu-Co-P films in both 0.5 M H2SO4 and 1 M KOH was evaluated, and corresponding polarization curves are contained in Figures 3A and 3C. In 0.5 M H2SO4 (Figure 3A), blank Cu did not show an appreciable current until the potential reached ~350 mV, while Cu-Co-P showed a rapid current increase beyond 200 mV. The ƞ10 and ƞ50 values were -262 and -281 mV for Cu-Co-P, -94 and -139 mV for Pt black, and -76 and 96 mV for Pt/C, respectively. In 1 M KOH, Cu-Co-P outperforms Pt black beyond -205 mV, demonstrating its superior HER activity in alkaline solution (Figure 3C). The ƞ10 and ƞ50 values were -231 and -288 mV for Cu-Co-P, and -247 and -317 mV for Pt black, respectively, and ƞ10 of Pt/C is -150 mV. The Pt/C catalyst exhibits a low starting overpotential for HER in 1 M KOH, however, at higher potentials the activity of Cu-Co-P approaches that of Pt/C. The corresponding Tafel plots are contained in Figures 3B and 3D, and linear portions were fitted to the Tafel equation (ƞ = b log | j | + a, where ƞ = overpotential, j = current density and b = Tafel slope). Tafel slope is an inherent property that indicates the rate limiting step of the electrochemical reaction at the electrode. The HER at an electrode surface can be explained by three possible reaction steps. First step is the discharge step


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(Volmer reaction), which is followed by either an electrochemical desorption step (Heyrovsky reaction) or a chemical recombination step (Tafel reaction). In acidic medium: Discharge step: H3O+ + e– → Hads + H2O (Volmer reaction, b=120 mV/dec) (1) Desorption step: Hads + H3O+ + e– → H2 + H2O (Heyrovsky reaction, b=40 mV/dec) (2) Recombination step: Hads + Hads → H2 (Tafel reaction, b=30 mV/dec) (3) In alkaline medium: Discharge step: H2O + e– → Hads + OH– (Volmer reaction, b=120 mV/dec) (4) Desorption step: Hads + H2O + e– → H2 + OH– (Heyrovsky reaction, b=40 mV/dec) (5) Recombination step: Hads + Hads → H2 (Tafel reaction, b=30 mV/dec)

(6)

In 0.5 M H2SO4, a small Tafel slope of 59 mV/dec was observed for Cu-Co-P, which is only slightly higher than that of Pt black (50 mV/dec) and Pt/C (33 mV/dec), indicating high activity of Cu-Co-P for HER in acidic electrolyte, and a Heyrovsky reaction as the rate determining step (RDS) for HER on both Cu-Co-P and Pt black electrode surfaces.50 The Tafel slope observed for Pt black is not similar to the value observed for Pt/C electrode, which is explained by difference in the mechanism for the different types of Pt-based electrodes.50 The Tafel slope of 20 wt% Pt/C is 33 mV/dec, which is consistent with


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literature reports, indicates a slow recombination step.51 In 1 M KOH, Cu-Co-P exhibits a Tafel slope of 86 mV/dec, which is less than 129 mV/dec of Pt black and only slightly higher than 59 mV/dec of Pt/C, indicating favourable kinetics of Cu-Co-P for HER in alakaline electrolytes. This also reveals a slow desorption step on both Cu-Co-P and Pt/C, and a slow Volmer reaction on Pt black. To further understand the electron transfer ability at the electrode/electrolyte surface under HER conditions, electrochemical impedance spectroscopy (EIS) was carried out at -214 mV in 0.5 M H2SO4 and at -165 mV in 1 M KOH, and the corresponding Nyquist Plots are given in Figure 3E and 3F, and Cu-Co-P displays a lower impedance in both acidic and alkaline electrolytes (Supporting Information Figure S22 and Table S4). As expected, in acidic electrolyte Pt black has the smallest diameter of the semicircle, indicating its very low impedance in acidic electrolytes. In alkaline medium, the semicircle of Cu-Co-P is smaller than that of Pt black, revealing faster kinetics on CuCo-P electrodes in alkaline medium. A superior long-term stability of Cu-Co-P for HER in both 0.5 M H2SO4 and 1 M KOH is displayed by maintaining a nearly constant activity during 72 hours of continuous electrolysis at 15 mA/cm2 cathodic current density (Figures 3G and 3H). In 0.5 M H2SO4, there is no loss in the activity, instead we see a slight enhancement after 72 hours, while in 1 M KOH the difference is negligible. The stability coupled with activity exhibited by Cu-Co-P for HER in both acidic and alkaline medium is promising as most metal phosphides exhibit long term stability either only in alkaline medium, or only ≤ 24 hours


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in acidic electrolytes.14, 30, 52-53. The chemical states of surface Cu, Co and P centers after long term electrolysis were analysed using XPS, and the presence of Cuδ+ centers and Coδ+ centers is observed to be similar to as-deposited films, and the peak for Cu(I) centers diminishes (further information is found in the Supporting Information: Figure S6 and Table S1). In addition, Cu-Co-P films possess 100% Faradaic efficiency for HER in both 0.5 M H2SO4 and 1 M KOH. Figures S20A and S20B indicates the measured volumes of H2 is agreeable with calculated volumes for 100% Faradaic yield. Electrochemical activity and stability of Cu-Co-P Films towards OER: Evaluation of OER activity of Cu-Co-P films was performed in 1 M KOH, and corresponding polarization curves are contained in Figure 4A. Initially, both RuO2 and Cu-Co-P show similar OER activity at ~1.5 V, with ƞ10 of 383 and 380 mV, respectively. However, after ~1.62 V, CuCo-P surpassed the activity of RuO2 with rapid evolution of O2, with ƞ50 of 513 and 425 mV for RuO2 and Cu-Co-P, respectively. Nevertheless, the OER activity of Pt black is less than both Cu-Co-P and RuO2 electrodes, which is also observed in the literature.54-56 Pt

A

C

B

Figure 4. (A) Polarization plots comparing OER activities in 1 M KOH. (B) Corresponding Tafel plots with linear fitted dotted line. (C) Polarization plots (not iR compensated) of Cu-Co-P before and after continuous OER for 72 hours at 15 mA/cm2 in 1 M KOH (Inset: corresponding potential change).


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black electrodes require an overpotential of 554 mV to reach 10 mA/cm2, which is significantly higher than that of Cu-Co-P. Linear fitting of Tafel plot for the Cu-Co-P electrode is contained in Figure 4B, which gave a Tafel slope of 66 mV/dec. This Tafel slope is even smaller than that of RuO2 (105 mV/dec), and Pt black (124 mV/dec) indicating more favorable OER kinetics on Cu-CoP.32,

56-57

Even though there are some discrepancies in the mechanisms proposed by

different research groups for OER in alkaline medium, most commonly proposed mechanisms are associated with the same surface adsorbed intermediates of MOH and MO (M denotes a catalytically active surface site).58-59 A simplified mechanism is explained by an initial discharge step of OH– ions on a catalytic site followed by other electrochemical and chemical reactions. Discharge step: M + OH– → MOH + e– (β=0.5, b=120 mV/dec)

(7)

MOH + OH– → MO– + H2O (β=1, b=60 mV/dec)

(8)

MO– → MO + e– (β=1.5, b=40 mV/dec)

(9)

2 MO → 2 M + O2

(10)

If the rate determining step is the first electron transfer reaction (7), the Tafel slope is equal to 120 mV/dec (assuming transfer coefficient β=0.5), and if it is the second electron transfer reaction (9), it gives a Tafel slope of 40 mV/dec (assuming β=1.5).


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However, if the rate determining step involves a chemical reaction as reaction (8), subsequent to the first electron transfer reaction, the Tafel slope will be 60 mV/dec (assuming β=1). Based on this, it can be inferred that the rate limiting step of the OER on electrodeposited Cu-Co-P electrode is a chemical reaction, as indicated in reaction (8), while those of both RuO2 and Pt black are the first discharge step (reaction (7)). A superior OER stability of Cu-Co-P films is displayed by maintaining a nearly constant activity for 72 hours of continuous OER electrocatalysis at 15 mA/cm2 anodic current density (Figure 4C). This high level of stability is strongly desired as many active noble OER catalysts lack long-term stability.6, 59-60 Considering these issues, the Cu-Co-P catalyst would be a better and a competitive candidate for efficient OER. There are no significant changes observed in the chemical states of surface Cu, Co and P centers after running 72 hours of OER as indicated by the XPS analysis, except the decrease in the intensity of Cu(I) peak (Figure S6 and Table S1). Moreover, Cu-Co-P displays 100% Faradaic yield as illustrated by Figure S20C, where the calculated and collected volumes of O2 are agreeable.

A

B

Electrolysis

cell

employing

Cu-Co-P

anode and cathode: Based on the above results, the Cu-Co-P Figure 5. (A) Polarization curves obtained in a two-electrode configuration (not iR compensated) for overall water splitting inACS 1 M KOH Plus with Environment different electrode Paragon combinations. (B) The comparison of total volume of H2 and O2 gases produced at different electrode couples in 1 M KOH at 1.9 V.

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electrodes are expected to act as an excellent bifunctional catalyst for water splitting in alkaline electrolytes. Thus, a two-electrode configuration was employed with different HER and OER electrode couples, in order to investigate the activity towards overall water splitting. These electrode couples (anode/cathode) include Cu-Co-P/Cu-Co-P, Cu-Co-P/Pt black, RuO2/Cu-Co-P and RuO2/Pt black, and the results are contained in Figure 5A. As expected, RuO2/Pt black showed the lowest overpotential for the initiation of overall water splitting, with ƞ10 of 1.80 V for Cu-Co-P/Cu-Co-P, 1.83 V for Cu-Co-P/Pt black, 1.86 V for RuO2/Cu-Co-P and 1.87 V for RuO2/Pt black (inset Figure 5A). However, at higher potentials, RuO2/Pt black showed much diminished catalytic activity, whereas Cu-CoP/Cu-Co-P surpassed the activities of all three other electrode couples, indicating its high activity for overall water splitting. Furthermore, we also compared the rate of H2 and O2 gas formation on different electrode couples in alkaline medium, at a constant applied potential of 1.9 V (a potential achievable using photovoltaic and other renewable energy sources). Figure 5B compares the total volume of gases produced on each electrode couple, with their initial rates obtained from the linear fit (dashed line). The Cu-Co-P/CuCo-P outperforms the other three electrode couples, and its initial rate of gas formation is ~2.5 times greater than that of the state-of-art electrode couple (RuO2/Pt black). This is a significantly higher improvement compared to the improvement shown by other Cu and Co based TMP Janus electrocatalysts.2, 23, 30 In contrast to Cu-Co-P/Cu-Co-P and CuCo-P/Pt black couples, RuO2/Cu-Co-P and RuO2/Pt black couples show a decrease in the


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rate of formation of gases after ~1 hour, due to the lower stability of RuO2 under the conditions of electrolysis. CONCLUSIONS In summary, a new Cu-Co-P catalyst was electrodeposited onto Cu foil to provide a highly stable and active electrocatalyst for overall water splitting. Strongly adherent films were obtained with low surface passivation that promotes both HER and OER. In alkaline medium, Cu-Co-P surpass the activity of the state-of-art catalysts for both HER (Pt black) and OER (RuO2). Our Janus electrocatalysts system produced ~2.5 times as much gaseous products than the system consisting of RuO2/Pt at 1.9 V. Thus, our Cu-Co-P is an ideal earth abundant materials candidate for excellent Janus electrocatalyst for overall water splitting and for other systems involving HER and OER. Finally, our study also offers new strategies to easily develop bimetallic phosphides, which are potential catalysts in many areas of chemistry. EXPERIMENTAL SECTION Materials: CoCl2 (Alfa-Aesar, lot #18650); H3PO2 (50% W/W, Alfa-Aesar, Lot #E01N05); H2SO4 (ACS, 95.0 - 98.0%, Alfa-Aesar, stock #33273); KOH (certified ACS, Fisher chemical, lot #035570); Pt/C (20 wt% Pt on Vulcan XC72, Sigma-Aldrich, lot# MKCG9262); RuO2 (Electronic grade, 99.95%, Alfa-Aesar, Lot #C21N11), Nafion perfluorinated ion-exchange powder (5 wt% solution in a mixture of lower aliphatic alcohols and water, Aldrich, lot


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#EN02015PG), K2PtCl6 (Matheson Coleman and Bell, PX1415); HClO4 (ACS reagent, 70%, Sigma-Aldrich, Batch #08496AP); Cu foils (Purchased from Revere Copper Products, Inc, lot #200453) and lab distilled water. All of the chemicals were used as received without further purification. All the solutions used in this work are aqueous unless otherwise specified. Preparation of electrodeposited Cu-Co-P and Pt films: Prior to electrodeposition, Cu foils were mechanically polished with P800 sandpapers, ultrasonically cleaned in a 1:1 ethanol:water mixture (SHARPERTEK ultrasonic cleaner SH80) for 20 minutes, followed by rinsing with de-ionized water. All of the electrodepositions were performed in a onecompartment two-electrode system with a Cu foil and a Pt mesh as the working electrode and the counter electrode, respectively. The working distance was 0.5 cm. Copper cobalt phosphide films were electrodeposited onto Cu foil from a plating bath containing CoCl2 (0.1 M) and H3PO2 (1 M) (pH ~1) at 80 °C (The incorporation of Cu into the film occurred as a result of Cu ion migration from the substrate Cu foil, which is discussed in the Supporting Information). A HY1802D DC power supply was used to provide 0.1 A/cm2 cathodic current density for 90 minutes. After the electrodeposition, the films were well washed with de-ionized water. To compare the catalytic activity thin films of Pt black were electrodeposited onto Cu substrates, based on methods reported in the literature, using a solution of 2 mM


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K2PtCl6 and 0.1 M HClO4.61-62 Electrodeposition was carried-out under a constant cathodic current density of 10 mA/cm2 for 10 min at room temperature. Preparation of RuO2 loaded electrodes: A mixture of RuO2 (16 mg), 5 wt% Nafion (120 µL), ethanol (1.08 mL) and water (800 µL) was sonicated for 30 minutes to obtain a homogeneous catalyst ink. The mixture was heated on a warm water bath (50 °C) until the volume is reduced to 0.5 mL, and was then cooled to room temperature. This ink was painted onto a polished Cu foil, and dried in air to give a loading of circa 1 mg/cm2.2 Preparation of Pt/C loaded electrodes: 16 mg of 20% Pt/C, 30 µL of 5 wt% Nafion, 800 µL of water and 1.18 mL of ethanol were ultrasonically mixed for 30 minutes until a homogeneous mixture is obtained. The prepared ink was then dropped on to a polished Cu foil and dried in air. The catalyst loading was circa 1 mg/cm2. 23 Characterization: The morphology and the average composition of samples were characterized using scanning electron microscopy (SEM) using a JEOL JSM-7500F scanning electron microscope (SEM) with a BRUKER XFlash 5010 series energy dispersive X-ray spectroscopy (EDX) detector, in the Instrumentation Center at the University of Toledo (UT), and JEOL 2010F analytical electron microscope (for HREM) at University of Michigan. Powder X-ray diffraction (PXRD) patterns were obtained using a PANalytical X'Pert Pro multi-purpose diffractometer a Xe Proportional detector to analyze the crystallites aggregates on Cu-Co-P films. The chemical states of surface Cu, Co and P on


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the electrodeposited Cu-Co-P films were characterized by X-ray photoelectron spectroscopy (XPS) using Kratos Axis Ultra X-ray photoelectron spectrometer with a monochromated Al Kα as the excitation source, at University of Michigan. X-ray photoelectron spectra of the Cu 2p, Co 2p and P 2p regions were used to examine the chemical sates of Cu, Co and P on the surfaces of as-deposited films and post HER and OER films. The spectra were calibrated relative to C 1s peak by setting it to be 285.00 eV. All of the 2p peaks were fitted using Shirley background model. Electrochemical measurements: Electrochemical measurements were conducted using a conventional one-compartment three-electrode system, with an O-ring apparatus on a Bioanalytical Systems (BAS, West Lafayette, IN) Epsilon-EC electrochemical analyzer.3 A Pt mesh and a Ag/AgCl (3 M NaCl) electrode (BASi, MF-2020) (0.209 V) were used as the auxiliary (AE) and reference electrode, respectively. When not in use, the reference electrode was stored in 3 M NaCl solution. Hydrogen evolution reaction catalysis measurements were performed using 0.5 M H2SO4 or 1 M KOH as the electrolyte solution. All of the measurements were performed under normal atmospheric conditions, and in a diffusion-controlled system. Before obtaining the polarization data for HER, the films were activated by cycling through -50 to -550 mV vs Ag/AgCl (3M NaCl) (in 0.5 M H2SO4) or from -800 to -1300 mV vs Ag/AgCl (3M NaCl) (in 1 M KOH) for 40 cycles at a scan rate of 100 mV/s, until a stable voltammogram was obtained. The HER activity of electrocatalysts were measured by linearly scanning from -200 to -700 mV vs Ag/AgCl (3 M NaCl) (in 0.5


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M H2SO4) or -900 to -1400 mV vs Ag/AgCl (3 M NaCl) (in 1 M KOH), at a scan rate of 5 mV/s. Oxygen evolution reaction catalysis experiments were performed using a 1 M KOH as the electrolyte solution. Prior to obtaining the polarization data for OER, the films were activated by running 40 cycles from -100 to +600 mV vs Ag/AgCl (3 M NaCl), at a scan rate of 100 mV/s. The OER activity of the electrocatalysts were measured by linearly scanning from +200 to +600 mV vs Ag/AgCl (3 M NaCl) at a scan rate of 2 mV/s. All polarization plots in this work are 60% iR compensated, unless otherwise noted, and are calibrated to Reversible Hydrogen Electrode (RHE), by converting the measured potentials according to the equation (11).3, 30, 63 E(RHE) = Emeasured (vs Ag/AgCl (3 M NaCl)) + 0.059 pH + 0.209 V

(11)

In order to show that the use of Pt electrode as the AE did not cause any altered electrochemical performance, the activity of Cu-Co-P films were evaluated for both HER and OER with a graphite rod as the AE, while the apparatus and other electrochemical parameters were kept the same. The long-term stability of the Cu-Co-P films were evaluated by performing chronopotentiometry at a constant cathodic (for HER) and anodic (for OER) current density of 15 mA/cm2 for 72 hours and recording the change in potential using a conventional two-electrode system equipped with a HY1802D DC power supply. The


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polarization plots of as-deposited films and after running HER/OER electrolysis were also compared to evaluate the stability of Cu-Co-P films. The electrochemical impedance spectroscopy (EIS) measurements were performed using a Solartron Modulab potentiostat equipped with a frequency response analyzer (Ametek Inc.), under a constant applied potential of -165 mV in 1 M KOH and -214 mV in 0.5 M H2SO4. The frequency was swept from 10 kHz to 1 Hz. The equivalent circuit model fitting of the complex impedance (Z’, Z”) spectra was performed using RelaxIS impedance spectrum analysis software. Faradaic efficiency of the Cu-Co-P films towards HER and OER was investigated by collecting the generated gases using a custom-built U-shaped apparatus.3 Dihydrogen and O2 gases were collected by passing a current of 50 mA through the working electrode for 20 minutes. Theoretical volume of H2 and O2 were calculated based on cumulative charge assuming 100% Faradaic efficiency for H2 and O2 production. Another experiment was performed in order to compare the amount of gaseous products (H2 and O2) generated at different electrode couples at a constant applied potential for a certain period of time. For this, a two-electrode configuration was used, with as-deposited Cu-Co-P films as both anode (OER) and cathode (HER) electrocatalysts (Cu-Co-P/Cu-Co-P couple), with a working distance of 6 mm, and 1 M KOH as the electrolyte (Figure S21). A constant potential of 1.9 V was applied for 2 hours and the total volume of gases generated were recorded. For comparison, several other electrode couples were also analyzed for the amount of total gaseous products, under the same


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conditions. These electrode couples include Cu-Co-P anode and Pt black cathode (CuCo-P/Pt black), RuO2 anode and Cu-Co-P cathode (RuO2/Cu-Co-P) and RuO2 anode and Pt black cathode couple (RuO2/Pt black). ASSOCIATED CONTENT Supporting

Information.

AAS

experimental

section,

detailed

explanation

on

incorporation of Cu into the film, Faradaic efficiency of Cu-Co-P films, PXRD characterization, HREM data, Pt black films characterization, EIS equivalent circuit fitting. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT We acknowledge Dr. Jon R. Kirchhoff, Dr. Yanfa Yan (for electrochemical resources) and Mr. Steven D. Moder (for custom glassware) at the University of Toledo (UT). We also thank the University of Michigan College of Engineering and NSF grant #DMR-0420785 for XPS analysis, Instrumentation Center in college of natural science and mathematics at UT for PXRD, and their NSF grant #0840474 for SEM. AUTHOR CONTRIBUTIONS


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Ms. Wasalathanthri (lead investigator) wrote the draft publication, and maintained the lead position in developing ideas, setting up appropriate experiments and, finally, in writing of the manuscript. Mr. Samuel Jeffrey is a University of Toledo undergraduate student who worked directly under Ms. Wasalathanthri supervision. She worked with him and discussed the research theme and what experiments needed to be done. She combined all of the results and included her data interpretation into her evolving manuscript. Ms. Awni is a graduate student in University of Toledo, and she conducted and analyzed the EIS experiments with Ms. Wasalathanthri. Dr. Sun, a high resolution XPS expert from the University of Michigan, collected and intrerpreted the XPS data, but Ms. Wasalathanthri was present to assist and observed the experiments. She also compared the XPS results to comparible results in the literature. Dr. Dean M. Giolando supervised the project helping Ms. Wasalathanthri develop ideas and the setting up appropriate experiments. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. CONFLICT OF INTEREST The authors declare no conflict of interest


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ABBREVIATIONS HER, hydrogen evolution reaction; OER, oxygen evolution reaction; TMP, transition metal phosphide. REFERENCES (1) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44 (15), 5148-5180, DOI 10.1039/C4CS00448E. (2) 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 (21), 6251-6254, DOI 10.1002/anie.201501616. (3) Wasalathanthri, R. N.; Jeffrey, S.; Su, N.; Sun, K.; Giolando, D. M. Stoichiometric Control of Electrocatalytic Amorphous Nickel Phosphide to Increase Hydrogen Evolution Reaction Activity and Stability in Acidic Medium. ChemistrySelect 2017, 2 (26), 8020-8027, DOI 10.1002/slct.201701755. (4) Landrigan, P. J.; Fuller, R.; Acosta, N. J. R.; Adeyi, O.; Arnold, R.; Basu, N.; Baldé, A. B.; Bertollini, R.; Bose-O'Reilly, S.; Boufford, J. I.; Breysse, P. N.; Chiles, T.; Mahidol, C.; CollSeck, A. M.; Cropper, M. L.; Fobil, J.; Fuster, V.; Greenstone, M.; Haines, A.; Hanrahan, D.; Hunter, D.; Khare, M.; Krupnick, A.; Lanphear, B.; Lohani, B.; Martin, K.; Mathiasen, K. V.; McTeer, M. A.; Murray, C. J. L.; Ndahimananjara, J. D.; Perera, F.; Potočnik, J.; Preker, A. S.; Ramesh, J.; Rockström, J.; Salinas, C.; Samson, L. D.; Sandilya, K.; Sly, P. D.; Smith, K. R.; Steiner, A.; Stewart, R. B.; Suk, W. A.; van Schayck, O. C. P.; Yadama, G. N.; Yumkella, K.; Zhong, M. The Lancet Commission on Pollution and Health. Lancet 2018, 391 (10119), 462-512, DOI 10.1016/S0140-6736(17)32345-0. (5) Libin, Y.; Honglan, Q.; Chengxiao, Z.; Xuping, S. An Efficient Bifunctional Electrocatalyst for Water Splitting Based on Cobalt Phosphide. Nanotechnology 2016, 27 (23), 23LT01, DOI 10.1088/0957-4484/27/23/23LT01. (6) Tung, C.-W.; Hsu, Y.-Y.; Shen, Y.-P.; Zheng, Y.; Chan, T.-S.; Sheu, H.-S.; Cheng, Y.-C.; Chen, H. M. Reversible Adapting Layer Produces Robust Single-Crystal Electrocatalyst for Oxygen Evolution. Nat. Commun. 2015, 6, 8106, DOI 10.1038/ncomms9106


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Synopsis: Strongly adherent Cu-Co-P films on copper substrate exhibit enhanced stability and are active bifunctional catalysts for both the hydrogen and oxygen evolution reactions.


Electrodeposited CopperâCobaltâPhosphide: A Stable Bifunctional

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Electrodeposited CopperâCobaltâPhosphide: A Stable Bifunctional