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Article

Direct Synthesis of CuP2 and Cu3P and Their Performance as Electrocatalysts for Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions

by
Xiao Ma
1,2,3,
Xueni Huang
4 and
Abdessadek Lachgar
1,2,3,*
1
Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA
2
Center for Energy, Environment and Sustainability (CEES), Wake Forest University, Winston-Salem, NC 27109, USA
3
Center for Functional Materials (CFM), Wake Forest University, Winston-Salem, NC 27109, USA
4
Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA
*
Author to whom correspondence should be addressed.
Solids 2024, 5(1), 140-150; https://doi.org/10.3390/solids5010010
Submission received: 18 October 2023 / Revised: 19 February 2024 / Accepted: 23 February 2024 / Published: 7 March 2024
Browse Figures
Versions Notes

Abstract

:
Copper phosphides are promising materials for energy conversion applications because of their unique electronic structure and controllable composition. Two stoichiometric copper phosphides, CuP2 and Cu3P, were prepared by direct wet-chemical synthesis using red phosphorus. They were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron spectroscopy. The precursor selection, reaction temperature, time and solvent composition were also studied. CuP2 is the thermodynamically more stable product, but Cu3P is more commonly obtained. This work demonstrated that higher temperature helps in CuP2 formation. More importantly, using more trioctylphosphine oxide helps control the morphology leading to crystal growth along the crystallographic a-axis. CuP2 and Cu3P were tested for hydrogen evolution, oxygen evolution, and oxygen reduction reactions. CuP2 works better for HER in acidic conditions and OER in general, and Cu3P showed better activity than CuP2 for HER and ORR in an alkaline medium. This study has led to a simple approach to the synthesis of CuP2 nanowires.

Graphical Abstract

1. Introduction

Electrocatalytic energy conversion offers an efficient and environmentally friendly route for the conversion and storage of electrical energy. Significant advances have been achieved in understanding the intricacies of essential electrocatalytic reactions. Groundbreaking advancements have been achieved in understanding hydrogen evolution (HER), oxygen evolution (OER), and oxygen reduction (ORR) [1,2]. Electrocatalysts such as platinum-based materials [3,4] are well-known for their exceptional performance in terms of efficiency and durability for both water electrolysis and fuel cells, but it is highly desirable yet challenging to develop efficient non-platinum group metal (non-PGM) electrocatalysts [5,6]. Compared with PGM catalysts, non-PGM catalysts usually suffer from low efficiency and poor durability, but some transition metal-based materials such as transition metal phosphides have demonstrated potential as electrocatalysts, for the hydrogen evolution reaction (HER) [7,8]. Likewise, Ni-Fe-layered double hydroxides have been shown to act as good electrocatalysts for the oxygen evolution reaction (OER) [9], and iron-nitrogen-carbon active structures have been studied as electrocatalysts for oxygen reduction (ORR) [10,11]. Unlike Fe-based materials, Cu-based materials have been less studied, despite the fact that they have shown potential as an oxygen evolution catalyst [12]. In fact, some homogeneous ORR catalysts with copper (enzyme laccase [13,14] for instance) have been reported to have the highest onset potential, which suggest their potential as efficient electrocatalysts. Thus, Cu-based materials can be important in developing non-PGM electrocatalysts in energy conversion and other applications.
Phosphides have been reported as robust HER and ORR catalysts, and incorporating phosphorus can increase the catalyst robustness compared to metal alone [15,16]. To study the role of phosphorus in electrochemical reactions and gain a better understanding of the structure–performance relationship, it is important to study different forms of copper. CuP2 and Cu3P are the most common stoichiometric copper phosphides. CuP2 is considered to be the more thermodynamically stable binary, but has been less studied than Cu3P [17]. It crystallizes in the monoclinic space group P21/c, where Cu(I) is coordinated by four P atoms forming a distorted tetrahedral [18]. In terms of valency, half of the P atoms are formally neutral, and half can be assigned the oxidation state −1 [19]. Although CuP2 is the more thermodynamically stable phase [17], common wet-chemical synthesis (with phosphine) always generates Cu3P [20]. CuP2 can be synthesized by ball-milling [21,22], solvothermal [23], and solid–gas reaction [24], which usually requires high temperature or vacuum annealing. Barry et al. reported that the synthesis of amorphous CuP2 by mixing copper chloride and white and red phosphorus as a solid pallet can lower the annealing temperature to 500 °C [25]. This can also be achieved with tin flux at similar temperature [26]. Wolff et al. also used red phosphorus to synthesize copper-deficient Cu3P in ionic liquids [27].
In this work, direct synthesis of CuP2 and Cu3P was achieved by mixing Cu salt and red phosphorous in a mixture of trioctylphosphine oxide (TOPO) and 1-octadecene (ODE) directly at relatively low temperatures without further heat treatment. The use of inexpensive and stable red phosphorus as a reactant for the synthesis of Cu phosphides at lower temperatures by wet synthesis is not only a simple process but is also beneficial for mass production. By using high boiling point solvents, such as TOPO (408 °C) and ODE (317 °C), CuCl2 and red phosphorus resulted in well-crystallized CuP2 without further annealing. Excess TOPO seems to help in morphology control, leading to the formation of nanowires with a diameter of ~43 nm. In this paper, the direct synthesis of Cu3P and CuP2 and how the different phosphorus content affects their electrochemical performance is investigated.

2. Materials and Methods

2.1. Chemicals

Amorphous red phosphorus (−100 mesh, Alfa Aesar, Ward Hill, MA, USA, 98.9%), trioctylphosphine oxide (TOPO, STREM Chemicals, Newburyport, MA, USA, 99%), anhydrous copper (II) chloride (STREM Chemicals, Newburyport, MA, USA, min 98%), and 1-Octadecene (ODE, Aldrich, St. Louis, MO, USA, 90%) were used without further purification.
The crystal structure data of Cu3P and CuP2 were summarized in Table 1.

2.2. Synthesis of CuP2

In a 3-neck round bottom flask, 50 mg (0.37 mmol) of CuCl2, 30 mg (0.96 mmol) of red phosphorus, 8 mL of solvents, and a stir bar were added before connecting the flask to a Schlenk line. The set-up was degassed at 120 °C and purged with nitrogen. The temperature was increased to 320 °C and maintained for 48 h.

2.3. Synthesis of Cu3P

The setup for the synthesis of Cu3P was similar to that of CuP2, with 133 mg (0.99 mmol) of CuCl2, 30 mg (0.96 mmol) of red phosphorus, and 8 mL of solvents. The mixture was degassed, refilled with nitrogen, and refluxed at 250 °C for 48 h.
The as-synthesized samples were washed 3 times with hexane and acetone and redispersed in hexane for future use.
The proposed reactions can be represented by the following chemical equations:
3 CuCl2 + 8 Pred → 3 CuP2 + 2 PCl3
(Reflux at 320 °C, 48 h)
3 CuCl2 + 3 Pred → Cu3P + 2 PCl3
(Reflux at 250 °C, 48 h)

2.4. Characterization

Powder X-ray diffraction (PXRD) patterns were collected using a Bruker D2 Phaser (Bruker, Billerica, MA, USA) with 30 kV/10 mA Cu Kα X-ray radiation (1.5406 Å). Scanning electron microscopy (SEM) images were taken on a Zeiss Gemini 300 (Carl Zeiss, Oberkochen, Germany) and the energy dispersive spectroscopy analysis was performed using Bruker XFlash 6-30 (Bruker, Billerica, MA, USA) at 15 kV acceleration voltage. Transmission electron microscopy (TEM) images were acquired with the Thermo Fisher Talos F200i S/TEM (Thermo Fisher Scientific, Waltham, MA, USA) and Ceta camera. X-ray Photoelectron Spectroscopy (XPS) was performed using Thermo Scientific ESCALAB™ Xi+ (Thermo Fisher Scientific, Waltham, MA, USA). The samples were prepared by drop-casting the washed CuP2 and Cu3P onto a Si wafer and ion etching for 90 s.

2.5. Electrochemical Performance Study

Electrochemical measurements were acquired using a Gamry electrochemical workstation in a three-electrode system with 0.1 M NaOH as the electrolyte. In a typical experiment, the nanocrystals of CuP2 and Cu3P were drop-cast onto glassy carbon (for HER) or carbon cloth (for OER). Specifically, ~10 mg of catalyst was mixed with 10 mg of Vulcan black, dispersed in a mixture of isopropanol and water, and sonicated for 1 h. For the catalyst loading, 10 μL of the catalyst inks were drop-casted on the substrate. The reference electrode used was Ag/AgCl in saturated KCl solution and the counter electrode was a graphite rod. Oxygen reduction reaction measurements were performed on a rotating disk electrode, and 1600 rpm rotating rates were performed to achieve a well-defined oxygen transport condition.

3. Results

3.1. Material Synthesis and Characterization

Common solvents including hexadecane, 1-octadecene (ODE), and trioctylphosphine oxide (TOPO) were tested for the synthesis. Increasing the reaction’s temperature will increase the crystallinity of CuP2. TOPO has the highest boiling point and provides the best results in terms of the crystallinity and morphology control for CuP2 synthesis at a reaction temperature of 320 °C. Thermodynamic parameter is the predominant factor in synthesis; by altering the reaction temperature with the same recipe, Cu3P can also be obtained. For the solvents, 10% TOPO and 90% ODE (0.7 g TOPO and 7.2 mL ODE) were used and run at 320, 280, and 250 °C for 48 h (Figure 1a). PXRD data indicate that Cu3P was the dominant phase at 250 °C, and a mixture of Cu3P and CuP2 was obtained at 280 °C. The broad peak in Cu3P was possibly due to the lower crystallinity at lower reaction temperatures. Despite CuP2 being the thermodynamically favored product [17], Cu3P is often favored with common phosphorous precursors such as tris(trimethylsilyl)phosphine or trioctylphosphine in wet-chemical synthesis [20,28]. Therefore, Cu3P can be obtained at lower reaction temperatures possibly due to a lower energy barrier needed for its formation.
PXRD patterns of washed aliquots for CuP2 synthesis that were run at 320 °C showed the transition from amorphous to crystalline over time, suggesting there is no Cu3P intermediate for CuP2 synthesis. Mixing Cu or Cu3P nanocrystals with red P under similar conditions did not produce CuP2 either. The time evolution suggests a slow reaction process because of the solid–solid diffusions, and that better crystallinity can be obtained after 24 h. EDS was taken and normalized using the Cu peaks (Figure 1c). The Cl ratio consistently decreases over the reaction time, suggesting the reaction is driven by the removal of PCl3 species, which is consistent with the literature [25,29].
Besides the effects of different solvents and solvent mixtures, other copper salts were studied (Figure 2). By comparing the relative intensities at 45 degrees (where Cu3P has the strongest peaks), a higher temperature and a larger TOPO/ODE ratio can help in forming CuP2. Cu precursors, such as CuCl, copper acetylacetonate (Cu(acac)2), Cu(NO3)2·2.5H2O, and CuSO4·5H2O were also tested, and all reactions resulted in the predominant formation of CuP2. Cu (II) chloride and nitrate worked the best, but the coordination precursor Cu(acac)2, results in more Cu3P. Commonly used ligands such as oleic acid and oleylamine did not assist in the phosphidation nor the shape control; instead, they hindered the phosphidation process as more Cu3P products were observed in PXRD. Furthermore, the hot injection did not help in the CuP2 formation or the shape control. Therefore, although this wet-chemical approach lowered the reaction temperature required for CuP2 synthesis compared with vacuum annealing, the reaction itself is still similar to a solid-state reaction. TOPO helps the reaction process and temperature is the dominant factor for the conversion.
A larger TOPO/ODE ratio not only provides better crystallinity, but also tends to result in wire-shaped structures as shown in the electron microscopy images shown above. For Cu3P, the morphology is quite random, where small particles aggregate to form large particles. For CuP2, a larger TOPO/ODE ratio showed a strong tendency to form wire morphology. This might be due to TOPO binding to (0 1 2) crystal faces, allowing the nanocrystals to grow along the crystallographic a-axis, resulting in the observed wire-like shape [30]. The diameter of CuP2 wires ranges from 25 to 70 nm with the average diameter being 43 nm (Figure S1).
Due to the lower reaction temperature, Cu3P is formed and less crystallized than CuP2, and this observation is also consistent with the PXRD results. In the TEM images, 0.20 nm d-spacing can be measured for Cu3P particles (Figure 3a), which corresponds to (3 0 0) for Cu3P. In Figure 3b, 0.26 nm lattice spacing can be identified for CuP2 wires and corresponds to the (2 0 0) plane. For STEM elemental mapping, Cu3P and CuP2 both show a homogeneous distribution of Cu and P elements. There is an amorphous shell over CuP2 as observed in HRTEM and STEM, which is probably due a to solid-state reaction-like mechanism as reported before [31].
To further characterize the phases formed, electron diffraction studies were conducted for both Cu3P and CuP2 samples. Fourier transform was performed with the squared regions and CrysTBox [32] was used to verify the miller indices and zone axis. For Cu3P (Figure 4a), the diffraction spots were diffused due to small crystallite sizes and random orientations, but consistent with [001] as the zone axis in general. Two neighboring spots on the edge of a hexagon shape can be indexed as (3 0 0) and (3 −3 0), consistent with the hexagonal lattice symmetry of Cu3P. For CuP2 (Figure 4b), the blue and red diffraction spots can be attributed to (2 0 0) and (0 1 2) with a zone axis of [0 −2 1], and a calculated interplane angle of 77.83°, both matching the values measured from the electron diffraction pattern. This also confirms that the growth direction is along the crystallographic a-axis (or [1 0 0] direction).
The survey XPS scans of the as-prepared CuP2 and Cu3P are shown in Figure 5a. The C, O, and F peaks can be attributed to the microporous layers (carbon particles and fluoropolymer) of the gas diffusion electrode. The Cu and P contributions are highlighted in blue and red. In the high-resolution scans of P 2p (Figure 5b), the 2p 3/2 spin-orbit peak of CuP2 is approximately 0.5 eV lower than that observed for Cu3P, due to the fact that half of the P atoms have a formal valence of zero. The peak at 134.2 eV can be attributed to P-O bonding. The stronger P-O signal observed for Cu3P is possibly due to the inhomogeneous distribution of P, where a higher P concentration on the surface leads to more surface oxidation, which could also explain the broad peak observed around 30° for the Cu3P in PXRD. For the Cu 2p (Figure 5c), the binding energies of Cu3P (952.88 eV for 2p1/2 and 932.78 eV for 2p3/2) are almost the same as those observed for CuP2 (952.98 eV for 2p1/2 and 932.88 eV for 2p3/2). The absence of satellite peaks further proves the closed-shell d[10]-configuration of Cu(I).

3.2. Electrochemical Measurements

CuP2 and Cu3P were tested as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline medium. For HER, a three-electrode setup was used, with graphite as the counter electrode and Ag/AgCl as the reference electrode. The catalysts were mixed with carbon black and loaded on glassy carbon for HER. For OER, due to the unstable nature of glassy carbon at high applied potential [35], catalyst inks were drop-cast onto carbon cloth as working electrodes. As shown in Figure 6, both CuP2 and Cu3P are better for HER compared to Cu/C. From the LSV, 355 and 460 mV overpotential are required to reach a current density of 1 mA/cm2 in 0.1 M HCl, and 514 and 388 mV in 0.1 M NaOH, respectively. No iR correction was performed for any of the results presented here. Their Tafel slopes were also calculated and showed similar behavior as Cu. CuP2 showed better activity than Cu3P in an acidic environment, while in the alkaline medium, Cu3P showed a similar slope with Pt and outperformed CuP2 and Cu. The electrochemical active surface areas of 50 wt% Cu3P/C and CuP2/C on glassy carbon were measured in 0.1 M NaOH (Figure S3). The double-layer capacitance for CuP2/C on glassy carbon was 0.55 mF/cm2, and 43 mF/cm2 for Cu3P/C. Higher double-layer capacitance of CuP2 compared with Cu3P demonstrates the advantages of wire-shaped morphology.
For OER, it is known that copper-based material can be oxidized, so the measurements were recorded with cyclic voltammetry until the curves were stable for the acid and alkaline medium. CuP2 and Cu3P both showed OER activity for water oxidation (Figure 7). The onset potential of Cu3P/C and CuP2/C in 0.1 M HCl is ~1.58 V vs. RHE for both, which is almost the same as 50 wt% RuO2/C. In 0.1 M NaOH, CuP2 performed better than Cu3P, but both are inferior to RuO2. From electrochemical impedance spectroscopy (EIS), the series resistance (Rs, x-intercepts in Nyquist plot) from the electrode resistance for both Cu3P and CuP2 are smaller than that of the bare carbon cloth and remains the same after oxygen evolution reactions. The charge-transfer resistance (RCT) increased dramatically for both compounds after oxygen evolution reactions due to surface oxidation, but the bulk of the nanocrystals remain the same as indicated by the Rs. The long-term stability and robustness of OER for both materials still need to be improved.
For the oxygen reduction reaction, it is important to compare how different P content affects the electrocatalyst activity and durability. As shown in Figure 8, 25 wt% Cu3P/C outperformed CuP2/C in onset potential (E0, 0.72 vs. 0.7 V) and half-wave potential (E1/2, 0.60 vs. 0.58 V). The slope from Koutecky–Levich analysis suggests that both Cu3P and CuP2 prefer to perform the 2 e− transfer that generates H2O2 instead of the 4 e− transfer reactions to generate H2O, and the y-intercept suggests that Cu3P/C demonstrated a larger kinetic current. Both Cu3P and CuP2 showed similar Tafel slopes that were close to that of Ag (90 mV/dec) [36], which also showed 2 e− transfer behavior. The durability test was performed at 400 rpm for 1000 cycles, and the polarization curves showed a minimal drop for both. To study the catalyst stability in ORR, the catalyst inks were drop-cast onto the gas diffusion electrode and cyclic voltammetry (CV) experiments were performed between 0.4 and 0.8 V (500 mV/s) vs. RHE for 1000 cycles. The XPS spectra for Cu3P and CuP2 before and after CV are shown in Figure S4. For Cu3P, the peaks’ positions for P and Cu were the same before and after the 1000 cycles, while CuP2 had an obvious downshift, especially for the peak corresponding to P-O, which might indicate structural changes after ORR.

4. Conclusions

In summary, highly crystalline CuP2 was successfully synthesized by the wet chemistry route without additional annealing. The effects of precursors, ligands, solvents and temperature were studied. CuP2 was obtained with inexpensive red phosphorus as a precursor at a rather low temperature (320 °C). TOPO was found to assist in forming CuP2 with wire-like morphology, meanwhile, Cu3P could also be obtained at lower temperatures (250 °C). It was found that, unlike other ligand-assisted colloidal syntheses, the conversion rate is slow for CuP2 synthesis, and the reaction mechanism is similar to solid-state reactions. Because red phosphorus has less reactivity compared with TOP or TMS3P, it will allow the generation of more thermodynamically stable products such as CuP2. This wet-chemical synthesis lowered the temperature required for solid-state synthesis such as vacuum annealing. Also, the reactant can move more freely, allowing TOPO to cap (0 1 2) crystal faces and form nanowires that grow along the [1 0 0] directions.
Furthermore, CuP2 and Cu3P have different crystal and electronic structures which affect their electrochemical behavior. CuP2 works better for HER in acidic conditions and OER in general, while Cu3P showed better activity than CuP2 for HER and ORR in an alkaline medium. Both compounds demonstrate similar stability in both alkaline and acidic environments. This synthesis, which controls the stoichiometry and morphology, has the potential to produce CuP2 on a large scale for electrochemical and other energy-related studies, such as CO2 reduction [37] or batteries [38].

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/solids5010010/s1, Figure S1. SEM results of morphology differences by increasing TOPO/ODE ratio at the same temperature for CuP2 synthesis, specifically, 10% for (a), 30% for (b) and 50% for (c). (d) Corresponding diameter distribution histogram of CuP2 of (c). Figure S2. Typical EDS results for (a) Cu3P and (b) CuP2. Figure S3. ECSA comparison of (a,b) 50 wt% CuP2/C and (b) 50 wt% Cu3P/C on glassy carbon in 0.1 M NaOH. Figure S4. XPS results for CuP2 and Cu3P on gas diffusion electrode after scanned between 0.4 and 0.8 V vs. RHE for 1000 cycles, the results for pristine materials were shown in grey for easier comparison. Figure S5. (a) XRD and (b) EDS results for CuP2 on carbon cloth (cc) before HER, after HER and after ORR.

Author Contributions

Investigation; data curation and writing—original draft preparation, X.M.; TEM and STEM-EDS, X.H.; writing—review and editing, supervision and funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript.

Funding

X.M. and A.L. would like to acknowledge support from the Sabin Center for the Environment and Sustainability (CEES) and the Center for Functional Materials (CFM) from Wake Forest University. A.L. would like to thank the Fulbright U.S. Scholar Program (13044-WA) for the support.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

The authors would like to thank Chaochao Dun from Molecular Foundry for the XPS data measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Powder X-ray diffraction patterns of the products were obtained by varying the (a) reflux temperature and (b) time. (c) Normalized energy dispersive spectrum and elemental ratio of Cu:Cl:P (as inset) at different times during CuP2 synthesis.
Figure 1. Powder X-ray diffraction patterns of the products were obtained by varying the (a) reflux temperature and (b) time. (c) Normalized energy dispersive spectrum and elemental ratio of Cu:Cl:P (as inset) at different times during CuP2 synthesis.
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Figure 2. Powder X-ray diffraction results for CuP2 synthesis by varying the (a) temperature and solvents (hexadecane at 280 °C, ODE, 10% TOPO + 90% ODE, and 50% TOPO + 50% ODE at 320 °C) and (b) Cu precursor. The reference patterns of Cu3P and CuP2 correspond to JCPDS 02-1263 and 18-0452, respectively.
Figure 2. Powder X-ray diffraction results for CuP2 synthesis by varying the (a) temperature and solvents (hexadecane at 280 °C, ODE, 10% TOPO + 90% ODE, and 50% TOPO + 50% ODE at 320 °C) and (b) Cu precursor. The reference patterns of Cu3P and CuP2 correspond to JCPDS 02-1263 and 18-0452, respectively.
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Figure 3. SEM, HR-TEM, and STEM elemental mapping for (a) Cu3P and (b) CuP2.
Figure 3. SEM, HR-TEM, and STEM elemental mapping for (a) Cu3P and (b) CuP2.
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Figure 4. Selected area electron diffraction for (a) Cu3P and (b) CuP2. Experimental diffraction patterns (top right) were achieved by Fourier transform of the squared regions, and the calculated patterns (bottom right) were generated with CrysTBox [32] with the cif files from Materials Project [33,34].
Figure 4. Selected area electron diffraction for (a) Cu3P and (b) CuP2. Experimental diffraction patterns (top right) were achieved by Fourier transform of the squared regions, and the calculated patterns (bottom right) were generated with CrysTBox [32] with the cif files from Materials Project [33,34].
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Figure 5. (a) Survey scan and high-resolution scan of X-ray photoelectron spectra for (b) P 2p and (c) Cu 2p; samples were prepared by drop-casting CuP2 (red) and Cu3P (black) on gas diffusion electrode.
Figure 5. (a) Survey scan and high-resolution scan of X-ray photoelectron spectra for (b) P 2p and (c) Cu 2p; samples were prepared by drop-casting CuP2 (red) and Cu3P (black) on gas diffusion electrode.
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Figure 6. CuP2 and Cu3P on glassy carbon for HER and corresponding Tafel analysis in (a,b) 0.1 M HCl and (c,d) 0.1 M NaOH.
Figure 6. CuP2 and Cu3P on glassy carbon for HER and corresponding Tafel analysis in (a,b) 0.1 M HCl and (c,d) 0.1 M NaOH.
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Figure 7. OER and EIS analysis of CuP2 and Cu3P on carbon cloth in (a,b) 0.1 M HCl and (c,d) 0.1 M NaOH.
Figure 7. OER and EIS analysis of CuP2 and Cu3P on carbon cloth in (a,b) 0.1 M HCl and (c,d) 0.1 M NaOH.
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Figure 8. ORR measurements of (a) CuP2/C and (b) Cu3P/C in oxygen-saturated 0.1 M NaOH, and corresponding (c) Koutecky–Levich (K-L) analysis. (d) Tafel analysis for Cu3P and CuP2, and (e) durability test for CuP2/C after 1000 cycles of scans.
Figure 8. ORR measurements of (a) CuP2/C and (b) Cu3P/C in oxygen-saturated 0.1 M NaOH, and corresponding (c) Koutecky–Levich (K-L) analysis. (d) Tafel analysis for Cu3P and CuP2, and (e) durability test for CuP2/C after 1000 cycles of scans.
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Table 1. Crystal structures for Cu3P and CuP2.
Table 1. Crystal structures for Cu3P and CuP2.
Chemical CompositionCu3P *CuP2
Space Group P 3 ¯ c1P21/c
Unit cellSolids 05 00010 i001Solids 05 00010 i002
a b ca = 6.91 Å
b = 6.91 Å
c = 7.13 Å		a = 5.77 Å
b =4.78 Å
c = 7.50 Å
α β γα = 90.00°
β = 90.00°
γ = 120.00°		α = 90°
β = 112.63°
γ = 90.00°
# JCPDS02-126318-0452
ICSD code2677535282
# Materials Projectmp-14012mp-927
* There are other phases for Cu3P, including P 3 ¯ P 3 ¯ m1 and P63cm. The latter one has similar symmetry and unit cell, making it hard to differentiate with P 3 ¯ c1. All the analysis was based on P 3 ¯ c1 in this paper.
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Ma, X.; Huang, X.; Lachgar, A. Direct Synthesis of CuP2 and Cu3P and Their Performance as Electrocatalysts for Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions. Solids 2024, 5, 140-150. https://doi.org/10.3390/solids5010010

AMA Style

Ma X, Huang X, Lachgar A. Direct Synthesis of CuP2 and Cu3P and Their Performance as Electrocatalysts for Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions. Solids. 2024; 5(1):140-150. https://doi.org/10.3390/solids5010010

Chicago/Turabian Style

Ma, Xiao, Xueni Huang, and Abdessadek Lachgar. 2024. "Direct Synthesis of CuP2 and Cu3P and Their Performance as Electrocatalysts for Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions" Solids 5, no. 1: 140-150. https://doi.org/10.3390/solids5010010

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