Tailoring the electrochemical hydrogen evolution activity of Cu3P through oxophilic surface modification

Article abstract of DOI:10.1016/j.elecom.2020.106691

The hydrogen evolution reaction in alkaline medium can be promoted by the modification of platinum group metal with oxophilic metals. Herein, an earth abundant metal was first converted to copper phosphide Cu₃P and the latter was electrochemically activated by a short and simple procedure to yield an oxidized electroactive surface. Compared with the pristine Cu₃P catalyst, the electrochemical activated Cu₃P catalyst exhibits an HER overpotential of 155 mV at 10 mA/cm² (90 mV decrease from Cu₃P catalyst) in alkaline medium with a stability over 24 h). XRD, XPS and EDS reveal that the oxidation occurs only on the surface of Cu₃P which shows the presence of both copper phosphide and oxide species.

Full text of DOI:10.1016/j.elecom.2020.106691

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Tailoring the Electrochemical Hydrogen Evolution Activity of Cu3P through
Oxophilic Surface Modification
Thazhe Veettil Vineesh, Valeria Yarmiayev, David Zitoun
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S1388-2481(20)30042-4
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https://doi.org/10.1016/j.elecom.2020.106691
ELECOM 106691
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Electrochemistry Communications
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16 February 2020
16 February 2020
Please cite this article as: T. Veettil Vineesh, V. Yarmiayev, D. Zitoun, Tailoring the Electrochemical Hydrogen
Evolution Activity of Cu3P through Oxophilic Surface Modification, Electrochemistry Communications (2020),
doi: https://doi.org/10.1016/j.elecom.2020.106691
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Tailoring the Electrochemical Hydrogen Evolution Activity of Cu3P through
Oxophilic Surface Modification
Thazhe Veettil Vineesh, Valeria Yarmiayev, David Zitoun*
Department of chemistry and Bar Ilan Institute of Nanotechnology and Advanced Materials
(BINA), Bar Ilan University, Ramat Gan 5290002, Israel
Abstract
The hydrogen evolution reaction in alkaline medium can be promoted by the modification of
platinum group metal with oxophilic metals. Herein, an earth abundant metal was first converted
to copper phosphide Cu3P and the latter was electrochemically activated by a short and simple
procedure to yield an oxidized electroactive surface. Compared with the pristine Cu3P catalyst,
the electrochemical activated Cu3P catalyst exhibits an HER overpotential of 155 mV at 10
mA/cm2 (90 mV decrease from Cu3P catalyst) in alkaline medium with a stability over 24 hours).
XRD, XPS and EDS reveal that the oxidation occurs only on the surface of Cu3P which shows
the presence of both copper phosphide and oxide species.
1. Introduction
Hydrogen evolution reaction (HER) is a reaction of technological significance as it enables the
production of hydrogen, a clean energy carrier, from a water-splitting process using electrical
energy.[1,5] Platinum (Pt) is considered to be the benchmarked electrocatalyst for HER because
of its strong ability for the adsorption and recombination of reactive hydrogen intermediates.[3] In
case of Pt, the HER activity in alkaline medium is much weaker than in an acidic electrolyte,[4,5]
However, the HER activity of Pt in alkaline medium can be enhanced with a bifunctional Pt-

Ni(OH)2 composite.[6] Such an effect has been shown, with hybrid catalysts featuring a single
layer of Ni(OH)2 nanosheets on Pt[7] or an arrangement of nanometer-scale Ni(OH)2 clusters on
Pt[8] demonstrating an enhanced HER activity in alkaline media compared to the standard Pt/C
catalyst. The activity of nickel, silver, and copper catalysts for the HER in alkaline solutions
were also enhanced by modification of the metal surfaces with Ni(OH)2.[9] This demonstrates
that oxophilicity is an efficient handle for promoting HER catalysis in alkaline conditions.
The HER performance of a series of Cu−M bimetallics[10] and Cu- Metal oxides or hydroxides
catalysts were studied, revealing that the modification of Cu surface with oxophilic metals/metal
hydroxides improved the HER activity of pure Cu. In case of Cu the adsorption of water molecule
through the interaction of the proton group and eventual formation of Hads is rather difficult
because of its weak hydrogen binding sites. However, with the addition of an oxophilic site such
as hydroxides/oxides can help adsorb water to the surface through weak interactions with the
oxygen atom of the hydroxyl group while a nearby Cu site can facilitate the formation of Hads
.
[11]
.Even though such a chemical treatment important, too high coverage leads to a decrease in
performance due to the passivation of the Cu active sites Nevertheless, metallic copper is not a
good catalyst for HER and this study is having only fundamental interest. Recent studies have
demonstrated the enhanced HER activity of earth-abundant metal phosphides, which still have to
be rationalized except for some hydrogen binding calculations with more knowledge on the
nature of their surface chemistry. Among the various catalysts studied the phosphides of Ni, Co,
Cu and Fe got tremendous attraction due to its low cost and high performance.[12–14] Here in, we
propose that the HER performance of Cu3P (an active catalyst for HER) could further be
improved with the assistance of copper oxide/hydroxide species through a simple
electrochemical route. Cu hydroxide materials have previously not been associated with HER

activity but the change in oxophilicity of the Cu3P catalyst should give an improved HER
performance on the hybrid surface.
2. Materials and Methods
Copper phosphide (Cu3P) was synthesized in a two-zone furnace by vapor transport. A Cu mesh
(Nano-Nouvelle, Australia) was located in the cold zone of a quartz tube and a boat of 18 mg red
phosphorus (Sigma Aldrich, 97%) was placed in the hot zone. The system was placed under
vacuum and the cold and hot zones were respectively set to 160°C and 430 °C. The reaction was
continued for 30 min to obtain the final product Cu3P. The crystalline phases of the pristine
copper membrane, freshly prepared Cu3P and electrochemical activated Cu3P materials were
examined by X-ray diffraction (XRD, Bruker D8 advance). The morphology and elemental
compositions of copper phosphide catalysts reported in this work were visualized using scanning
electron microscopy (SEM, FEI Magellan 400) equipped with an energy-dispersive X-ray
spectroscopy attachment. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Nexsa) was
used to analyze the surface composition of the copper phosphide membrane before and after
electrochemical activation.
Linear sweep voltammetry (LSV) measurements were conducted in 0.1 M KOH aqueous
solution for the HER before and after electrochemical activation with a sweep rate of 5 mV s−1,
with a Bio-Logic VMP3 potentiostat. All polarization curves were corrected by current-
resistance (iR) compensation within the cell. All the experiments were carried out using Milli Q
water (18.2 MΩ cm-1) at room temperature. The electrochemical impedance spectroscopy (EIS)
was tested by using an alternating current voltage of 10 mV in the frequency range of 10 mHz to
100 kHz. The long-term stability test was performed using chronoamperometry with
overpotential of 0.2 V. Double layer capacitance measurements were done on both the electrodes

from 0.1V to 0.3V. The fresh prepared electrode was tested as hydrogen evolution reaction
catalyst in a 0.1M KOH solution. The electrochemical activity of the as prepared samples was
evaluated in a Teflon three electrode cell from, which consists of Cu3P as working electrode,
glassy carbon and Hg/HgO as counter and reference electrode respectively. All the reference
electrode potentials were converted to RHE using Nernst equation unless otherwise stated. After
the first linear sweep voltammetry (LSV) in the HER region, the electrode was activated at 1.7 V
for 2.5 hours in the same solution. This step was named electrochemical activation (EA) of the
catalyst and the sample named Cu3P-EA. After the activation step, the HER activity was
measured again.
3. Results and Discussion
The phosphorous vapor transport process yielded a black mesh (named Cu3P) as we mentioned
in previous report[15]. The sample was electrochemically activated for 2 hours at 1.7 V in 0.1M of
KOH solution, to yield electrochemically activated copper phosphide (Cu3P-EA). During this
process, the surface of copper phosphide was oxidized to copper oxides/hydroxides. The material
characterization by X-ray diffraction analysis was done on the three samples: starting material of
copper membrane, Cu3P and Cu3P-EA. The diffraction pattern of Cu3P (Fig. 1a) shows the
characteristic peaks of hexagonal Cu3P (JCPDS 071-2261) within the space group P63cm. The
diffraction peaks of metallic Cu were not detected, as evidenced from the comparison of the
Cu3P diffraction pattern to the Cu pattern (Fig. 1a), which confirms the complete conversion to
Cu3P. XRD analysis of Cu3P electrode after electrochemical activation (Cu3P-EA) showed
diffraction peaks of the hexagonal Cu3P (JCPDS 071-2261) along with Copper oxide (CuO,
monoclinic (tenorite), space group C12/c1). The XRD results clearly confirm that the
electrochemical activation resulted in the oxide formation on top of Cu3P ((Fig. 1a).

(c)
(e)
(d)
(f)
Figure 1. (a) XRD analysis of Cu3P as prepared – blue curve and Cu3P-EA – red curve (*
indicates the phase corresponds to CuO) (b) EDS results of Cu3P and Cu3P–EA at three
operating voltages – 5, 10 and 15 kV. (c-d) SEM images of Cu3P and Cu3P-EA (e-f) TEM and
HRTEM images of Cu3P-EA.

The EDS analysis (Fig. 1b) reveals that the Cu3P is composed of Cu and P elements in the along
with oxygen. Oxygen atoms are detected mainly on the surface while the composition is close to
stoichiometric Cu3P in the bulk of the sample. The amount of oxygen is low in Cu3P, even close
to the surface (9% atomic ratio at 5kV). EDS analysis of Cu3P-EA shows an increase in oxygen
content for all operating voltage values, especially on the surface (24.8 % atomic ratio at 5kV).
The morphology of Cu, Cu3P and Cu3P-EA are revealed using SEM. The surface of Cu3P is
evenly covered by plaques formed perpendicular to the surface (Fig. 1c). After electrochemical
activation, the SEM micrographs reveal the same plaques, but with more surface roughness due
to the bubble formation during the activation process (Fig. 1d). After electrochemical activation
of Cu3P, we expect the formation of oxide/hydroxide species on the surface of the electrode. The
thickness of the amorphous layer formed on the surface of Cu3P is found to be 8 nm using TEM
and HRTEM analysis (Fig. 1e and Fig. 1f). The amorphous layer is conformal on all the flakes
observed by HRTEM.
For detailed characterization of the chemical composition on the Cu3P-EA, we analyzed and
compared the XPS spectra of Cu3P-EA and in-situ ion-etched Cu3P-EA to gain knowledge on the
chemical nature of the Cu3P surface and sublayer after electrochemical activation. Fig. 2 shows
high-resolution Cu 2p and P 2p XPS spectra of the Cu3P-EA before and after the etching. As
shown in Fig.2a before and after etching, the Cu3P-EA displays two major peaks corresponding
to Cu 2p1/2 and Cu 2p3/2 at respectively 932.8 eV and 952.6 eV. The binding energy values of
933.7 eV, 934.9 eV and 954.1eV correspond to the oxidized copper at the surface of Cu3P. After
the etching process the percentage of oxidized copper at the surface of Cu3P-EA is drastically
reduced as shown in Fig. 2c. The deconvoluted P 2p spectrum of Cu3P- EA before etching shows

binding energy values only above 131 eV, which indicates that the phosphide ion present on the
surface of Cu3P- EA is also oxidized during the electrochemical activation process. After etching
the P2P spectra of Cu3P- EA shows a new set of peaks centered at 129.3eV and 130.1 eV, which
corresponds to 2p3/2 and 2p1/2 of unoxidized phosphorous respectively. The results clearly
confirm that only the surface of Cu3P is converted to an oxide or a hydroxide during the
electrochemical activation.
Figure 2: High resolution XPS spectra of (a) Cu 2p and (b) P 2p in Cu3P-EA before etching.
High-resolution XPS spectra of (a) Cu 2p and (b) P 2p in Cu3P-EA after etching.

In order to evaluate the hydrogen evolution reaction (HER) activity of the catalysts, the linear
sweep voltammetry (LSV) in alkaline conditions was collected for pristine Cu membrane,
activated Cu membrane (Cu-EA), Cu3P and Cu3P-EA (Fig. 3a). The pristine Cu membrane
displays a high overpotential (> 600 mV), even after electrochemical activation. The
overpotentials measured at a current density of 10 mAcm-2 for Cu3P and Cu3P-EA are
respectively 250 mV and 155 mV. The result clearly indicates that the electrochemical oxidation
at 1.7 V nets a gain of 95 mV at a current density of 10 mA/cm2. The overpotential to reach a
current density of 10 mAcm-2 is 50 mV for commercial Pt/C and the values are very close to
other reported recently reported Cu3P systems [16–20]. The overpotential reaches a plateau after
only 30 minutes of electrochemical activation. The Tafel plot for the corresponding LSV
measurements is shown in Fig. 3b. The Tafel slope decreases from 133 mV/dec to 125 mV/dec
after electrochemical activation. Ideally, the Tafel slopes should be derived from steady-state
measurements. For that purpose, the data extracted from the LSV measurements at 5 mVs-1 was
double-checked with chronoamperometric measurements. Fig. 3c shows the EIS spectra of the
electrodes under HER conditions. Among the two electrodes, Cu3P-EA shows the lower charge
transfer resistance value (7.5 Ω) in comparison to its Cu3P counterpart (10.5 Ω), which indicates
the faster electron transfer kinetics of Cu3P-EA in alkaline HER conditions. The stability of
Cu3P-EA electrode towards HER was performed at an over potential of 160 mV and 200 mV
(Fig. 3d and Fig. 3e). In both cases, the current density was found to be stable for more than 24
hours. The material is found to be leaching out from the surface at higher current density (100
mAcm-2). In order to stabilize the membrane, we coated nafion on the surface of Cu3P-EA and
performed the stability test and the material was found to be stable for more than 12 hours as
shown in Fig. 3f.

Figure 3: (a) LSV polarization curves of different materials, (b) Tafel plots of Cu3P ,Cu3P-EA
Nd Pt/C (c) Nyquist plots of EIS of Cu3P and EA-Cu3P samples in 0.1 M KOH at an applied
over potential of 100 mV vs. RHE , (d & e) chronoamperometric measurement at 160 mV and
200 mV overpotential, (f) chronopotentiometric measurement at 100 mAcm-2.
The electrochemical surface area (ECSA) can be used as an approximate guide for surface
roughness within an order-of-magnitude accuracy. To estimate the effective surface areas of
Cu3P and Cu3P-EA, we measured the capacitances of the double layer at the solid/liquid

interface. The cyclic voltammograms (CVs) were collected in the region of 0.10–0.30 V with
scan rate varying from 10 mV/s to 60 mV/s, where the current response should only be due to the
charging of the double layer (Fig. 4). The capacitances of Cu3P and Cu3P-EA are respectively
22.5 and 25.5 mF/cm2, thus indicating that the electrodes have a similar surface roughness.
Therefore, we can conclude that the increased HER activity of the Cu3P-EA is solely due to the
oxides/hydroxides formation and not to the increased ECSA of the Cu3P formed at high
potentials by bubbling.
Figure 4: (a) CV in 0.1 M KOH solution and (b) corresponding capacitive currents at 0.20 V as
a function of scan rate for Cu3P; (a) CV in 0.1 M KOH solution and (b) corresponding capacitive
currents at 0.20 V as a function of scan rate for Cu3P-EA.

The surface characterization of the oxidized catalyst by XPS and HRTEM, which consists of a
layer of oxide origin, provides some hints about the origin of the enhanced HER activity. This
oxide layer of a few nanometers is interfaced with the Cu3P underlayer. In our controlled
experiment on the Cu membrane, we found that metallic Cu is a poor HER catalyst, even after
surface oxidation. Therefore, the Cu3P/CuO interface seems to be instrumental to promote the
HER activity. Moreover, the sample did not show any change after the stability test as shown in
the SEM images (Fig. 5a and 5b).
Figure 5: (a) SEM images of Cu3P-EA before (a) and after (b) the stability test
3. Conclusion
Cu3P was synthesized by vapor transport from a Cu mesh and the Cu3P surface was partially
oxidized through a simple method of electrochemical activation. Compared with the pristine
Cu3P catalyst, the electrochemical activated Cu3P-EA catalyst exhibits an HER overpotential of
160 mV at 10 mA/cm2 (90 mV decrease from Cu3P catalyst) in alkaline medium with a stability
over 24 hours. The ECSA estimated from the double layer capacitance measurements show
similar values for the catalyst before and after electrochemical activation. The enhanced HER

catalytic activity is therefore attributed to the partial coverage of Cu3P with oxophilic species –
copper and phosphorous oxide/hydroxide on the surface. XRD, XPS and EDS reveal that the
oxidation occurs only on the surface of Cu3P-EA which shows the presence of both copper
phosphide and oxide species. Further we would like to extend our study using laser-induced
temperature jump technique to explain the mechanism of enhanced activity after activation.
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Graphical abstract
Thazhe Veettil Vineesh: conceptualization, methodology of the electrochemical study, Writing -
Original Draft, visualization. Valeria Yarmiayev: investigation of the synthesis. David Zitoun:
supervision and Writing - Review & Editing
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:

Highlights
1.
Electrochemically activated Copper Phosphide (Cu3P) demonstrates an overvoltage of
only 160 mV at a current density of 10 mAcm-2 for hydrogen evolution activity in alkaline
medium with stable activity over 24 hours.
2.
The surface oxophilicity of Cu3P is tuned through a simple electrochemical oxidation and
the materials were thoroughly characterised by various electrochemical, microscopic and
spectroscopic methods.
3.
The electrochemically activated Copper phosphide (Cu3P-EA) showed improved
performance in comparison with the pristine Cu3P counterpart.
4.
A net gain of 90mV was obtained at a current density of 10 mAcm-2, opens a new avenue
to further improve the HER activity of metal phosphides.