48
S. Li and R. Jin
2 H 2 O → 4 H
+
+ O 2 + 4 e
−
(acidic)
(2.6)
4 OH
−
→ 2 H 2 O + O 2 + 4 e
−
(alkaline)
(2.7)
In OER, the formation of oxygen requires a four-electron transfer, and the reaction kinetically favors single electron transfer at each step [32]. Therefore, catalysts
are required to overcome the energy barrier and lower the high overpotential in
the sluggish OER [33, 34]. To reduce the cost of catalyst materials, cheaper and
efficient alternative catalytic materials are extensively studied to replace the current
Ir-based materials. In previous reports, anchoring a small amount of gold onto cobaltbased materials can enhance the OER activity [35, 36]. However, the mechanism for
the improvement was not well understood due to the variability and complicacy of
the gold-loaded composites. In this section, the synergetic effects between Au NCs
and CoSe 2 nanosheets are introduced. This unique composite may provide valuable insights into the mechanistic study by taking advantage of the precise atomic
structures of Au NCs [1].
2.4.1 Au n NCs Promote OER at the Nanocluster/CoSe 2
Interface
The OER performance of metal NCs was first reported by Zhao et al. in 2017 [37]. In
this work, composites of Au 25 and ultra-thin CoSe 2 nanosheets were synthesized and
tested as OER catalysts. TEM images clearly show the ultra-thin nanosheet structure
of CoSe 2 (Fig. 2.6a–c). In the high-angle annular dark field scanning transmission
electron microscopy (HAADF-STEM), it can be observed that Au nanoclusters are
homogeneously dispersed on the surface of CoSe 2 nanosheets.
In the electrochemical test (Fig. 2.6d–f), the Au 25 /CoSe 2 composites show much
smaller onset potential (1.406 V vs. RHE) and higher current density than CoSe 2
nanosheets and Au 25 -loaded carbon. At 1.68 V, Au 25 /CoSe 2 composites achieve
a current density of 11.78 mA cm
−2 , which is 2.4 times that of CoSe 2 nanosheets
(4.92 mA cm
−2 ) and 20.7 times that of Au 25 -loaded carbon (0.57 mA cm
−2 ). Also, the
composites exhibit higher current density and smaller overpotential than commercial
Pt/C catalysts. In the stability test, the polarization curve and UV-vis spectra of
the Au 25 /CoSe 2 composites exhibit the same features before and after 1000 cycles,
indicating excellent stability of the composites as OER catalysts (Fig. 2.6g).
The XPS and Raman analysis of CoSe 2 and composites were conducted to explain
the enhanced OER activity of Au 25 /CoSe 2 composites (Fig. 2.7). The binding energy
of Co 2p in the composites shows a ~1 eV decrease compared with CoSe 2 , indicating
electronic interaction between the Au 25 and CoSe 2 nanosheet. Also, the Raman peak
at ca. 657 cm
−1 exhibits a shift toward higher wavenumber, suggesting the electronic
interaction. It is believed that such an electronic interaction is a key factor that
stabilizes the hydroperoxyl intermediates and optimizes interaction between CoSe 2
and oxygen.
S. Li and R. Jin
2 H 2 O → 4 H
+
+ O 2 + 4 e
−
(acidic)
(2.6)
4 OH
−
→ 2 H 2 O + O 2 + 4 e
−
(alkaline)
(2.7)
In OER, the formation of oxygen requires a four-electron transfer, and the reaction kinetically favors single electron transfer at each step [32]. Therefore, catalysts
are required to overcome the energy barrier and lower the high overpotential in
the sluggish OER [33, 34]. To reduce the cost of catalyst materials, cheaper and
efficient alternative catalytic materials are extensively studied to replace the current
Ir-based materials. In previous reports, anchoring a small amount of gold onto cobaltbased materials can enhance the OER activity [35, 36]. However, the mechanism for
the improvement was not well understood due to the variability and complicacy of
the gold-loaded composites. In this section, the synergetic effects between Au NCs
and CoSe 2 nanosheets are introduced. This unique composite may provide valuable insights into the mechanistic study by taking advantage of the precise atomic
structures of Au NCs [1].
2.4.1 Au n NCs Promote OER at the Nanocluster/CoSe 2
Interface
The OER performance of metal NCs was first reported by Zhao et al. in 2017 [37]. In
this work, composites of Au 25 and ultra-thin CoSe 2 nanosheets were synthesized and
tested as OER catalysts. TEM images clearly show the ultra-thin nanosheet structure
of CoSe 2 (Fig. 2.6a–c). In the high-angle annular dark field scanning transmission
electron microscopy (HAADF-STEM), it can be observed that Au nanoclusters are
homogeneously dispersed on the surface of CoSe 2 nanosheets.
In the electrochemical test (Fig. 2.6d–f), the Au 25 /CoSe 2 composites show much
smaller onset potential (1.406 V vs. RHE) and higher current density than CoSe 2
nanosheets and Au 25 -loaded carbon. At 1.68 V, Au 25 /CoSe 2 composites achieve
a current density of 11.78 mA cm
−2 , which is 2.4 times that of CoSe 2 nanosheets
(4.92 mA cm
−2 ) and 20.7 times that of Au 25 -loaded carbon (0.57 mA cm
−2 ). Also, the
composites exhibit higher current density and smaller overpotential than commercial
Pt/C catalysts. In the stability test, the polarization curve and UV-vis spectra of
the Au 25 /CoSe 2 composites exhibit the same features before and after 1000 cycles,
indicating excellent stability of the composites as OER catalysts (Fig. 2.6g).
The XPS and Raman analysis of CoSe 2 and composites were conducted to explain
the enhanced OER activity of Au 25 /CoSe 2 composites (Fig. 2.7). The binding energy
of Co 2p in the composites shows a ~1 eV decrease compared with CoSe 2 , indicating
electronic interaction between the Au 25 and CoSe 2 nanosheet. Also, the Raman peak
at ca. 657 cm
−1 exhibits a shift toward higher wavenumber, suggesting the electronic
interaction. It is believed that such an electronic interaction is a key factor that
stabilizes the hydroperoxyl intermediates and optimizes interaction between CoSe 2
and oxygen.