Figure 1. a) In situ characterization and surface
reconstruction processes of transition metal phosphides. b) HRTEM-HAADF
and element distribution of the Ni-CoP. c) HRTEM images of Ni-CoP. d)
High resolution XPS-Co 2p and e) soft XAS-Co L-edge of CoP and Ni-CoP.
2.2. Alkaline water splitting properties
An important limiting factor in the water splitting reaction rate and
energy consumption is the anode OER[31-33].
Therefore, anode catalysts with excellent performance and stability are
required to achieve efficient green hydrogen production. Covalent-like
bonding mode between transition metal sites and non-metal sites in
CoP-based catalyst can lead to excellent thermodynamic behavior and
conductivity. The electrochemical OER properties of CoP-based catalysts
were studied in the electrolyte solution 1 M KOH, which exhibits lower
overpotentials of 236, 287, and 291 mV to reach 10, 150 and 300 mA
cm-2, respectively (Table S1, Figure 2a and Figure
S7a,b). Such overpotential is competitive among the other Co-based
phosphide or noble metal catalysts reported in the related reference
(Table S2 and Figure S7c). The Tafel slope and Rct of
Ni-CoP nanowire are fitted as 52.55 mV dec-1 and 1.80
Ω, which is much lower than that of other M-CoP catalyst and 6
mg-commercial catalysts (Figure 2b, Figure S7d-f). The above results
indicate that Ni-CoP nanowire has excellent reaction kinetics and charge
transport behavior. In addition, the double layer capacitor
(Cdl) of sample are calculated as 698.84 (CoP), 680.51
(Cr-CoP), 621.43 (Fe-CoP) and 538 mF cm-2 (Ni-CoP)
(Figure S8a-d). The linear scanning voltammetry (LSV) curves of all
samples are normalized by Cdl, which is purposed to
study the intrinsic catalytic activity. The normalized LSV curves
exhibit that OER performance of Ni-CoP nanowires is still better than
that of CoP and other M-CoP nanowires (Figure S8e,f). To explore the
relationship between heteroatom Ni content and catalytic performance,
the heteroatom Ni doping content is controlled by the molar ratio
between nickel nitrate and cobalt nitrate, which has been described in
the experimental part. Similar to Ni-CoP-0.1 mmol, CoP nanowires with
different heteroatom Ni content (0.15 mmol and 0.2 mmol) are
successfully prepared in CC, which was confirmed in SEM and XRD (Figure
S9). Compared with Ni-CoP-0.15 mmol and 0.2 mmol, Ni-CoP-0.1 mmol
nanowire still exhibit the smallest overpotential at 10, 150 and 300 mA
cm-2, which indicate most excellent OER performance
(Table S3 and Figure S10).
Similarly, three-electrode system was used to analyze the hydrogen
evolution reaction (HER) activity of CoP-based catalysts in 1 M KOH. The
Ni-CoP nanowire with superior HER performance still exhibits lower
overpotential at 100, 200 and 400 mA cm-2, which is
99, 163 and 298 mV lower than that of bare CoP, respectively (Table S4,
Figure 2c and Figure S11b). Specific current density can be achieved by
Ni-CoP at lower applied potentials, which are significantly better than
bare CC, Co(OH)F, CoP, other M-CoP nanowires, commercial Pt/C and other
TMPs-based catalysts (Table S5, Figure S11a,c). The Tafel slope of
Ni-CoP (40.54 mV dec-1) is much smaller than CoP
(63.17 mV dec-1), commercial Pt/C (67.82 mV
dec-1) and other M-CoP nanowires, revealing that the
HER kinetic are accelerated by heteroatom Ni (Figure S11d). The Ni-CoP
nanowires with lower Tafel slope and charge transfer resistance
(Rct = 1.837 Ω) exhibit ultrafast reaction kinetics
based on Volmer-Heyrovsky mechanism during HER (Figure S11e,f). The
Cdl of CoP, Cr-CoP, Fe-CoP and Ni-CoP nanowires are
calculated to be 35.34 mF cm-2, 29.38 mF
cm-2, 20.39 mF cm-2 and 19.94 mF
cm-2, respectively (Figure S12a-d). Besides, the
normalized LSV curves of Ni-CoP nanowire has the most excellent
catalytic activity, which originates from the regulation behavior of
heteroatom Ni on the electronic structure around Co sites (Figure
S12e,f). For relationship between different Ni doping amounts and HER
performance, Ni-CoP-0.1 mmol nanowire exhibits the smallest
overpotential (at 100, 200, 300 mA cm-2), which imply
excellent HER performance compared with other heteroatom contents (Table
S6 and Figure S13).
Considering the excellent activity and reaction kinetics exhibited by
Ni-CoP nanowires (even better than 6 mg-commercial Pt/C and
RuO2 catalysts), the Ni-CoP nanowire grown on CC were
used as cathode/anode in the electrolytic cell (two-electrode), which
can directly evaluate the feasibility of the electrolytic water
splitting system (Ni-CoP || Ni-CoP) in practical
applications. Compared with commercial
Pt/C||RuO2 catalytic system (1.62 V), the
Ni-CoP || Ni-CoP electrodes exhibit an excellent overall
water splitting performance, which can reach a current density of 10 mA
cm-2 at 1.59 V (Figure 2d). For other recently
reported Co-based bifunctional catalysts, the excellent water-splitting
performance exhibited by the Ni-CoP || Ni-CoP catalyst
with lower overpotential is still competitive (Table S7 and Figure S14).
The chronoamperometric curve collected at current density of 10 mA
cm-2 demonstrate that the corresponding potential of
Ni-CoP || Ni-CoP catalyst failed to show noticeable
change for at least 24 h (Figure S15a). In addition, compared with the
LSV curves of Ni-CoP nanowires before the stability test, the Ni-CoP
nanowire demonstrate superior durability with no obvious attenuation
after continuous operation 24 h (Figure S15b). Notably, the SEM images
exhibit that the Ni-CoP electrode retains the original nanowire
structure even after the HER test. It is worth noting that the Ni-CoP
electrode still maintained the pristine nanowire structure after the
long-time HER test, which is obviously different from the Ni-CoP
electrode after the long-time OER test (inset in Figure 2e). Considering
the deep reconstruction phenomenon under long-term working conditions,
high-resolution XPS were used to analyze the elemental composition and
chemical valence state after the OER stability test (current density =
10 mA cm-2, acquisition time = 24 h). The
high-resolution XPS Co 2p peak signals of the double-transition metal
active sites in Ni-CoP nanowire are not annihilated by the long-term
alkaline oxygen evolution environment, which reveal the fundamental
source of the excellent stability (Figure S16a). Subsequently, it can be
clearly seen that Co 2p in the high-resolution XPS image is composed of
spin-splitting signals introduced by Co3+ and
Co4+, including Co 2p3/2, Co
2p1/2 and satellite peaks. The high-resolution P 2p
spectrum exhibits a distinct typical peak introduced by the P-O bond,
which is attributed to the re-adsorption behavior of
PO43- on the catalyst surface (Figure
S16b). However, the signals introduced by Co-P bonds in pristine Ni-CoP
could not be observed, corresponding to the dissolution process of
Ni-CoP at low positive potential. For alkaline OER,
OH- adsorption, proton desorption and
electro-oxidation processes are index to accompanying phenomena of the
surface reconstruction behavior. Compared with pristine Ni-CoP, the
high-resolution O 1s of Ni-CoP nanowires after the stability test
exhibit obvious peaks at 529.59 eV, 530.86 eV, 531.53 eV, and 533.37 eV,
which are indexed by lattice oxygen (Co-O),
O22- species, OH-group (or Vo) and adsorbed H2O molecules
(Figure S16c). In particular, the signal peak provided by the
O22- and OH- group
represents the incomplete evolution stage of CoOOH to
CoO2 species, which implies that the OER active phase of
Ni-CoP is identified as Ni-CoOOH/CoO2 species after
stability test. Besides, chronopotentiometry curves under multicurrent
step and large current density were used to further study the stability
of Ni-CoP in water electrolysis reactions. The multicurrent
chronopotentiometry curve of Ni-CoP nanowire with the current increasing
from 10 to 190 mA cm-2 (increase at a rate of 10 mA
cm-2 per 10 min) (Figure S15c). The stable
overpotential can be clearly observed, which undoubtedly further
confirms the excellent stability and mass transfer properties (Figure
S15d). The long-term stability of Ni-CoP || Ni-CoP
system was examined by a constant current chronopotentiometry at the
current density of 10 and 100 mA cm-2 for 100 h.
Compared with previously reported noble metal-based catalysts, the
overpotential of the Ni-CoP system only decreased by 0.61% and 2.62%
after water splitting reaction for 100 h, which confirms the reference
value of the Ni-CoP system in the green hydrogen production process
(Figure 2e).