SigmaXi Research Showcase

1. Atomic Force Microscopic Study
of Carbon Catalysts for
Rechargeable Metal-Air Battery
Alan P. Chen
11th Grade, Santa Cruz High School

2. Introduction
 Rechargeable metal-air battery is a unique
power device with a high energy density. The
overall efficiency is determined by two major
reactions at the cathode, oxygen reduction
reaction (ORR) and oxygen evolution reaction
(OER),
 Although precious metals, such as platinum (Pt), ruthenium (Ru), and
iridium (Ir), possess excellent catalytic activity for either ORR or OER, none
of these noble metal catalysts displays a satisfactory performance for
both, and their low natural abundance and high costs greatly hinder their
practical applications.
 Development of bifunctional catalysts with a low cost and high activity is
of both fundamental and technological significance, but remains a great
challenge.
𝑂2 + 2𝐻2 𝑂 + 4𝑒−
4𝑂𝐻−ORR
OER


3. Motivation
• Recent research has shown that porous carbon doped
with metal and nitrogen is excellent catalysts toward
both ORR and OER in alkaline solutions
• Metal-nitrogen (M-N) bonds are the catalytic active sites
• Porosity: enhanced surface area, easy transport of reactants and
products, and accessibility to catalytic active sites
• Carbon powders are prepared by heating organic
precursors and metal salts in an oxygen-free atmosphere
(similar to charcoal preparation). The material structure is
complex.
• Where are the M-N active sites located within the carbon?
• Atomic force microscopy (AFM) will be an effective tool to probe
the structure at the atomic scale.

4. Sample Preparation
lotus root starch (250 mg)
SiO2 nanoparticles (10-20 nm, 100
mg)
FeCl2 (0.2 M, 450 mL)
melamine solution (4 mg/mL, 6 mL)
Hot
water
hydrogel
Pyrolysis at 850 ºC
in argon
porous
carbon

5. Porous Carbon-Based Zinc-Air Battery
a) b) c)
 The obtained porous carbon displays excellent activity as the air-cathode for a Zn-air
battery, in comparison with that using commercial Pt/C-RuO2 mixture, along with a Zn
plate as the anode. It can easily light up a LED.
 (a) The porous carbon Zn-air battery shows an open circuit voltage (OCV) of 1.50 V
and a maximum power density of 231 mW cm−2, higher than those of the Pt/C-RuO2
counterpart.
 (b) By normalizing the energy output to the weight of dissipated Zn, the calculated
specific capacity and energy density (966 mAh g−1 and 1188 Wh kg−1) are markedly
higher than those of Pt/C-RuO2.
 (c) After 1100 charge-discharge cycles (400 s for each cycle), the porous carbon Zn
battery still affords a high round-trip efficiency of 59% and a narrow discharge-recharge
voltage gap of 0.79 V, much better than those of Pt/C-RuO2.
 Taken together, these results demonstrate that the porous carbon aerogels can be used
as high-performance bifunctional oxygen electrodes for Zn-air batteries, thanks to its
high open-circuit voltage, large power density, and superb durability.
FeNC
Pt/C-RuO2
FeNC
Pt/C-RuO2
FeNC
Pt/C-RuO2

6. Electron Microscopy
Hydrogel consists of
interconnected
microcavities of 20–50
μm in size, forming a
honeycomb-like 3D
framework.
hydrogel Porous
carbon
Aerogel displays a hierarchical porous 3D
carbon skeleton, rich in pores of about 10 nm,
and embedded with Fe atoms.
FeN4

7. Atomic Force Microscopy
● Oxford Instruments Asylum Cypher S AFM housed in an Ar
gas filled glove box (Velasco Lab, UCSC Physics)
● Resolution < 1 nm (significantly better than the most
powerful optical microscope)
Deposited
solute
100 mm
AFM
Cantilever
SiO2

8. Sample Preparation
In fast force mapping measurements, four
quantities are extracted simultaneously from the
deflection curve of the AFM cantilever: (i) the z-
position at which the cantilever begins deflection
(topography); (ii) the maximum deflection of the
cantilever as it is pressed into the sample (max
force); (iii) the negative deflection incurred by the
tip as it is retracted from the sample (adhesion
force); and (iv) the maximum current flowing
through the sample (current).
Topography Current
SiO2
graphite
Tip Pokes every pixel, and
measures current
Clean substrate
Sample deposition by dropcasting

9. Topography and Max Force Images
• Panel (a) is a 150 nm2 scan of the topographic height of porous
carbon. Panel (b) depicts the corresponding max force scan.
• Both data exhibit a ~10 nm variation in the mechanical and
electrical properties of the porous carbon, confirming the
formation of nanowrinkled carbon.

10. Adhesion Force and Current Images
• Panel (c) portrays the adhesion force of the same region in (a,b). White
regions form an intricate network of edges through this scan, connecting
at nodes with typically three edges forming trigonally symmetric
vertices (pink arrows). Additionally, darker areas appear to be bordered
by these high adhesion or “sticky” nodes.
• Panel (d) shows the maximum current flowing through the porous
carbon sample when the tip is brought into contact. The regions of high
or low current exhibit a structure that varies on the order of ~ 10 nm.

11. Mechanical and Electrical properties
 The adhesion force, which represents the bulk
modulus or stiffness of the sample, indicates that
these round regions are stiffer in the center and
softer around the edges.
 Typically, sp2-hybridized carbon exhibits
hydrophobic characters, whereas defective
carbons are more hydrophilic. With an AFM tip
that consists of a hydrophilic silicon oxide layer, a
high adhesion force corresponds to a hydrophilic
domain. This implies that the FeN4 metal centers
are most likely situated within the high adhesion
force areas. Interestingly, it can be seen that the
soft nodes correspond to high electrical
conductance.
 Taken together, these results suggest that the
FeN4 metal centers are mostly located in the high
adhesion and high conductivity area of the porous
carbon aerogel. Both features are conducive to
oxygen electrocatalysis.
side view
top view
sp2 C
FeN4

12. Conclusions
• Porous carbon doped with Fe and N serves as effective catalysts
for rechargeable zinc-air battery
• AFM studies show that the FeN4 active centers are mostly
located in the high adhesion force and high conductivity areas
of the porous carbon, two unique features conducive to the
catalytic activity.
• Fast force mapping studies have also been carried out with
other materials, such as graphene quantum dots.
• Further studies: AFM studies of the active sites under
electrochemical control

13. Acknowledgment
• I would like to thank my mentors, Prof. Jairo
Velasco Jr and Mr. Johnny Davenport in the
Department of Physics, University of California
Santa Cruz, without whom this work would not be
possible.