By creating a hydrophobic electrode surface, the competing hydrogen evolution reaction can be drastically reduced, which can dramatically enhance the performance of the electrochemical CO2 reduction.

1. Literature Presentation
Debabrata Bagchi
Ph. D. Student
03/09/2019

3. CO2 reduction as a source of sustainable fuel
CO2
C2 & C3
products
ethylene, ethanol & n-propanol
Difficulties
 low solubility of CO2 (33 mM)
 proton reduction to H2 (HER)
 formation of C2 & C3 products has
various thermodynamic and kinetic
demands and requires multiple
proton and electron transfers

5. Plastron Effect:
 These plastrons are composed of hydrophobic
hairs that trap air and thereby allow the spider to
respire under water.
 The gas-trapping phenomenon occurs when
hydrophobicity is simultaneously presented on
microscale and nanoscale surface structuration
The use of a hydrophobic surface to trap a layer of gas
between the solution–solid interface.

6. Hydrophobic Cu-surface as CO2-Catalyst
Hydrophobic surface through modification of hierarchically structured dendritic
Cu with a monolayer of waxy alkanethiol
diving bell spider
for subaquatic
breathing
Nature
hydrophobic
dendritic Cu
surface for CO2-RR
Nature-Inspired

7. Preparation of Hydrophobic Cu dendrite
Aqueous electrodeposition procedures : hierarchical architectures with both
micro- & nanoscale
0.2 M CuSO4
+
1.5 M H2SO4
Cu foam electrode was
immersed into the solution
current of -0.5 A (80 s) was
applied using a galvanostat
Under intense H2 bubbling
 The porous structure of this material, resulting from the combination of copper
dendritic growth and the concomitant generation of a foam due to intense hydrogen
bubbling
Angew. Chem. Int. Ed. 2017, 56, 4792–4796.

8. Hydrophobic Treatment of dendritic Cu
1-octadecanethiol
submersing the dendritic Cu
into liquid 1-octadecanethiol
at 60 °C for 15 minutes to
form an alkanethiol layer.
Hydrophobic Treatment
1-octadecanethiol-treated Cu dendrite

9. XPS spectra in the Cu & S regions
 The alkanethiolation removes Cu oxide from the surface to leave Cu–S bonds

10. Auger spectra of the dendritic Cu electrode
 Analysis of the Cu LMM Auger showed no evidence of metallic Cu0 at
the surface

11. ATR-FTIR spectroscopy study
 ATR-FTIR difference spectrum of a
Cu-coated Si prism with and without
1-octadecanethiol treatment.
 Sample was submerged in 0.1 M
CsHCO3 electrolyte (CO2- saturated,
pH 6.8, room temperature),
 Spectra shows the presence of CH2
and CH3 groups.
 The presence of the alkanethiol layer was further confirmed through
attenuated total reflectance-Fourier transform infrared

12. Contact angle & ECSA measurements
Cu dendrite surface is hydrophilic Alkanethiol-treated Cu electrode
is super hydrophobic
 Initial characterization of the dendrites’ electrochemical properties revealed a
significant decrease in ECSA on the introduction of hydrophobicity

13.  BET surface area analysis
through Kr-adsorption :
Wettable dendrite: 90 cm2 cm–2 &
Hydrophobic dendrite: 92 cm2 cm–2
 The decrease in ECSA is therefore induced by gas trapping at the
interface between the hydrophobic dendrite and the solution
ECSA disparity is not from a
loss in geometric surface area!
 On the application of a reducing potential over 60 minutes in an aqueous electrolyte,
 Increase is assigned to the loss of 1% of alkanethiol when labile Cu0 oxidation
states are reached
Change of ECSA upon applied potential

14. Change of catalyst surface during electrolysis
SEM image of the hydrophobic
dendrite after 2.5 h of an
applied cathodic potential
 show brighter Cu regions at the tips of
the dendrite
Application of -E
 The hydrophobic dendrite requires an initial
application of potential to generate a stable
liquid–electrode–gas triple-phase boundary
at the top of the dendrite where
electrochemical reactions take place
This activation was monitored through
one day of repeated LSV scans,

15. Stability of hydrophobic surface during electrolysis
 Contact angle measurements of the hydrophobic dendrite before
and after passing –15 mA cm–2
 After 720 minutes electrolysis  Ref. Spectra of 1-octadecanethiol

16. Catalytic activity of hydrophobic and wettable Cu dendrites
 The lowered current at a given potential for hydrophobic Cu,
̶ significantly lower ECSA of the hydrophobic dendrite,
̶ lack of proton reduction activity exhibited by this electrode.
 CA study confirmed that even at highly cathodic potentials (-1.6V) the
hydrophobic dendrite had a vastly lowered H2 evolution activity

17.  at –1.2 V versus RHE, at which point the current was too low to
detect C2 products
CA study at different potential

18. A schematic of the cathodic
chamber of the H-type
Potential dependent product analysis
 The optimal CO2 reduction selectivity on
the wettable dendrite was not attained at
lower potentials
 Needs higher potential to get C1 and C2
products
 Attention: positioning of the electrode to
ensure effective interaction with incoming
bubbles
 During electrolysis, CO2 was introduced as
a stream of gas from the bottom of the cell

19.  With the hydrophobic dendrite, the capture
and retention of the gaseous CO2 stream
was observed, which caused a bubble to
engulf the entire electrode surface
 If the gas flow was not incident to the
hydrophobic dendrite to constantly refill
this bubble, the formation of C1 and C2
products was severely reduced
Better capture and retention of CO2
 At lower partial pressures of CO2, E-CO2 reduction rate dropped

20. Controlled current electrolysis (CCE)
 The hydrophobic dendrite
 The wettable dendrite required
̶ required a higher cathodic applied potential to reach
30 mA cm–2 (E= –1.1 to –1.5 V vs. RHE)
̶ had much higher FEs for CO2 reduction
̶ a less cathodic potential to reach
30 mA cm–2 (E= –0.8 to –1.0 V vs. RHE)
̶ carried out mostly H2 evolution

21. C1/H2
C2/C3/H2
Extended CO2 at a of –30 mA cm–2
 The dotted lines indicate when the electrode was adjusted to realign with
incident CO2 bubbles
 C2-product formation is sensitive to interaction with inbound CO2: drops in C2
production when the CO2 flow fell out of line with the electrode surface
 a gradual decrease in C2-production activity because of the destruction of
regions of the dendrite surface due to mechanical stress by collision of bubbles
mechanical removal
of the dendrite

22. Role of hydrophobicity in CO2-RR over HER
 As highly potentials,
both the dendrites either form
Cu–H* or Cu–COOH* intermediates,
 Selectivity is controlled by the mass
transport of the two substrates.
 Wettable dendrite has a large liquid–
electrode interface & therefore only
the aqueous H+/CO2 are substrates,
more HER is expected
 Electrolyte is pushed away from the
hydrophobic dendrite Cu surface
 form an electrolyte–solid–gas triple-
phase boundary at the electrode
 CO2 mass transport is then
omnidirectional, whereas H+ comes
unilaterally from the bulk solution,
which drastically increases the local
CO2 concentration. promotes C–C
coupling
Wettable
Hydrophobic

23.  A hydrophobic coating of long-chain alkanethiols on dendritic Cu,
with no further modification, led to a drastic increase in the CO2
reduction selectivity
 The hydrophobic dendrite’s selectivity for C2 products (74% total)
rivals that of state-of-the-art gas-diffusion electrode systems in
alkaline conditions (66%)
 The difference is a result of a plastron effect; a gaseous layer
trapped at the surface of the electrode that increases the local
CO2 concentration
 It is hypothesized that the combination of nanostructured surfaces
with hydrophobic Cu2O forms similar voids that trap CO2 to create
an electrolyte–electrode–gas triple-phase boundary and promotes
C2 products formation.
Summary and Conclusion