Preparation of Asymmetric Single-Atom Electrocatalysts for

High-Performance Oxygen Reduction Reaction

Qiyue Cui

Denison University, U.S.A.

Keywords: Metal-Organic Frameworks, Single-Atom Catalysts, Oxygen Reduction Reaction, Electrocatalysis, Fuel

Cells.

Abstract: Metal-organic frameworks (MOFs) have been regarded as a kind of supramolecular non-noble metal-organic

hybrids via the strong coordination bonds, which have a highly tunable porous structures, high surface area,

and fully exposed and uniformly dispersed metal centers, facilitating mass transport and highly-efficient

electron transfer. In this study, we explore the synthesis strategy to prepare hierarchical single-atom

electrocatalysts with porous and conductive carbon supports based on a serial of MOFs templates. The various

MOF templates were prepared by room-temperature self-assembly or hydrothermal processes. The as-

synthesized MOFs were well-designed for the construction of hierarchical nanostructures. Subsequently, a

facile and controlled high-temperature pyrolysis treatment was applied for the as-synthesized MOF templates,

which allowed the organic ligands to reduce metal centers by releasing hydrogen by changing themselves to

porous and conductive carbon materials. Finally, the nanostructured morphology and electrical activity of the

as-synthesized single-atom catalysts were investigated by the X-ray diffraction, X-ray photoelectron

spectroscopy, Raman spectrum, Spherical aberration electron microscopy, and synchrotron radiation

characterization, electrochemical impedance, and cyclic voltammetry. Density Functional Theory (DFT)

calculations suggest that the designed asymmetric planar four-ligand structure may be the most favorable

catalytic sites. Previous studies were restricted by using Cu elements as the metal active centers and S

the ligands, but broadening the catalyst selection areas, e.g., using monoatomic metal centers such as Fe, Mn,

and introducing P element in the ligand, may improve the electronic structure properties of the catalyst, which

can effectively reduce the energy barrier in the ORR, a key rate-limiting step in fuel cells. Based on this idea,

we investigated six asymmetric monoatomic electrocatalysts, namely, Fe-S

, Cu-S

, Mn-S

, Cu-P

Fe-P

, and Mn-P

, respectively. The synthesized catalysts have abundant and fully exposed active sites,

which can be applied in the cathode reaction of fuel cells and help human species to cope with the global

energy crisis and the energy transition in fields such as electric vehicles.

1 INTRODUCTION

Metal-organic frameworks (MOFs) combine

inorganic and organic chemistry, which is popular in

the recent nanostructured materials fields. Metal

atoms coordinated by organic ligands can form one-,

two- or three-dimensional structures (crystalline

materials). MOFs can be used in multiple fields such

as adsorption, separation, storage, catalysis, and

energy conversion. Common single-atom catalysts

using the MOFs as templates are Fe, Co, Ni, Cu, Mn,

and W, which have excellent performances in

corresponding applications via a well-designed

preparation. (Sun, 2019)

Metal-organic framework-derived materials

mainly include porous carbon, metal oxides, metal

porous carbon composites, metal oxides, and porous

carbon composites. The main synthesis strategies are

chemical etching, high temperature pyrolysis,

oxidation, etc. Considering some disadvantages of

metal-organic frameworks in practical applications,

such as low crystal stability, poor performance of

electrical conductivity, low catalytic site activity,

limited mass transfer and diffusion in the

microporous structure, etc. There are many synthetic

strategies for high-temperature pyrolysis, such as

direct pyrolysis of templates, pyrolysis of

encapsulated guests. The template of the bulk, the co-

pyrolysis of the template carrier, the first solvent

100

Cui, Q.

Preparation of Asymmetric Single-Atom Electrocatalysts for High-Performance Oxygen Reduction Reaction.

DOI: 10.5220/0012003100003625

In Proceedings of the 1st International Conference on Food Science and Biotechnology (FSB 2022), pages 100-105

ISBN: 978-989-758-638-5

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

etching and then the pyrolysis, etc. During the

pyrolysis process, the heating rate, holding time and

gas atmosphere are the key points of pyrolysis

strategy research, which directly determine the

synthesized metal-organic framework derivatives

properties.

Oxygen reduction reaction (ORR) is one of the

common energy conversion reactions, and the

common products of ORR reactions are H

O, which

is a four-electron reaction product of oxygen

reduction reaction, and H

, which is a two-electron

product of oxygen reduction reaction. At present, the

most widely used oxygen reduction catalysts in

cathode reaction of fuel cells are precious Pt/C

materials, usually 20% or higher content, which are

expensive and scarce. (Tang, 2016) Therefore, the

three worthwhile goals to design high-performance

ORR catalyst are low cost, high activity, and high

stability. The biggest advantage of single-atom

catalysts is that they can achieve the maximum

utilization of atoms. (Sun, 2019)

Single-atom catalysts has many advantages, such

as high efficiency, maximum atomic utilization,

efficient and unique electronic structures, and unique

geometric construction. To prepare highly active

single-atom catalysts, the homo-dispersedly and

catalytically active sites are necessary. (Sun, 2019)

To improve the transfer of mass and electron as well

as the stability of catalytic active sites, and the tight

interactions of catalytically active site and carrier are

also required and necessary. Many literatures have

reported obvious improvements in the single-atom

catalysts for ORR.

(Shang, 2020; Xie, 2021; Sun,

2019) Typically, Metal-N

, such as FeN

, is usually

considered as an ideal catalyst, and the design concept

of this configuration has been demonstrated by

density functional theory and electrochemical testing

with excellent results. (Xie, 2021)

Usually, in addition to metal atoms, single-atom

catalysts are N and C elements. In our research, we

want to introduce S and P elements, and the key issue

is how to design an effective synthesis method. At

least one N elements around the metal element needs

to be removed and replaced with a sulfur or

phosphorus elements. In recent days, a few studies

have pointed out that the symmetrical configuration

of FeN

is not conducive to stable intermediate

products. Inspired by this clue, we can try to

synthesize single-atom catalysts with asymmetric

coordination. In addition, we would use DFT to

explain the underlying catalytic mechanism for the

enhanced ORR performances. (Xie, 2021) DFT

calculations are a method of quantum chemistry for

studying the electronic structures of multi-electron

systems. The primary research object of DFT focuses

on small molecules or isolated cluster structures, and

can calculate transition state energy, bond and

reaction energy, molecular orbital, thermodynamic

properties, reaction path, etc.

2 EXPERIMENTAL

2.1 Preparation of Materials

2.1.1 Chemicals

The chemicals are usually sourced from Alfa Aesar

and Sigma Aldrich, and the purchased reagents were

not further purified. The synthesis method uses Cu,

Fe, and Mn-ZIF-8 template, the source of S is sulfur

powder, and the source of P is phytic acid.

(Shang,

2020; Benítez, 2020)

2.1.2 Hydrothermal Synthesis of

Metal-Organic Framework Templates

The preparation of MOF templates went through

dissolving and recrystallizing the powder. Heating

and pressurizing in a sealed pressure vessel, using

water as the solvent.

First, mix the solutions of metal ions and organic

ligands. Then, transfer the solution to a hydrothermal

kettle and heat it in a vacuum oven to keep it warm,

allowing the metal ions and organic ligands to

coordinate. After that, cooling the solution to room

temperature, centrifuged and separated from the

MOF, washing several times with methanol solutions,

then dried in a vacuum oven.

The key synthesis step is to first hold the

temperature at 450 ℃ for 2 hours in the environment

of inert Ar. In this process, the sulfur powder is

volatilized into sulfur vapor and embedded in the

MOF framework. After incubation at 950 ℃ for four

hours, the template was completely carbonized to

synthesize single-atom catalysts with porous and

conductive carbon supports. The accurate reaction

Figure 1: Schematic preparation taking Cu, Fe, and Mn-ZIF-8 as the template.

Preparation of Asymmetric Single-Atom Electrocatalysts for High-Performance Oxygen Reduction Reaction

101

temperatures of high-temperature pyrolysis can be

determined by the thermogravimetric analysis

(TGA). (Benítez, 2020) TGA is a method to explore

the physical and chemical properties of a substance

with an increase in temperature over time. TGA can

provide abundant information about phase

transitions, such as evaporation, sublimation,

absorption, and desorption.

2.2 Electrochemical Measurements

Cyclic voltammetry is the commonly used

experimental method in electrochemistry. A three-

electrode system was used for electrochemical

measurements, with Ag/AgCl in saturated potassium

chloride solution as the reference electrode, graphite

rod as the counter electrode, glassy carbon as the

working electrode, and 0.1 M KOH solution as the

electrolyte solution. The catalyst was dispersed in a 5

wt% Nafion solution and dried by a heat lamp before

the testing. (Zhou, 2020)

2.2.1 Grind and Disperse Nanomaterials

Grind the nanomaterials into fine particles, and then

disperse the nanomaterials in the Nafion solution.

Usually, ultrasonic treatments are required for around

0.5 hours to 2 hours. The dispersed solution is usually

water and ethanol, the volume ratio is 1:1. (Zhou,

2020)

2.2.2 Preparation of Nanomaterial Working

Electrode by the Drop Coating Method

Take 5 microliters of nanomaterial dispersion liquid

and drop it on the 3 mm glassy carbon working

electrode, and then usually need to volatilize the

solvent through an electric heating lamp.

2.2.3 Assembling the Three-Electrode

System

The nanomaterial-modified electrode is the working

electrode and usually uses the Ag/AgCl electrode or

saturated calomel electrode (SCE) as the reference

electrode. (Shang, 2020)

2.3

Materials Characterizations

Material characterizations were divided into three

parts: (1) Crystal Structure Analysis; (2) Morphology

analysis by electron microscope; and (3) Material

surface element and valence analysis. The

performance characterization of ORR materials is

mainly performed by voltammetry.

3 RESULTS AND DISCUSSION

3.1

Novelties

Our study provides a general approach for

synthesizing and tuning the activity of single-atom

catalysts and applying in the fields of energy

conversion. The synthesis of Cu and S

coordinated single-atom catalysts based on metal-

organic framework templates has been reported,

which is a Cu-S

coordination form, namely,

asymmetrically Cu-S

. (Shang, 2020) Inspired by

this report, we explored high-performance ORR

catalysts using Fe, Cu, and Mn as single-atom metal

centers coordinated with S

, and P

, namely, Fe-

, Cu-S

, Mn-S

, Cu-P

, Fe-P

and Mn-

, respectively. We hope the oxygen reduction

activity is higher than that of Cu-S

while

introducing multiple mental atoms and phosphorus.

Figure 2: Schematic atomic interface model of Fe-S

Cu-S

, Mn-S

, Cu-P

, Fe-P

and Mn-P

3.2

Materials Characterizations

Tuning atomic interfaces is an important method to

tune the activity of single-atom catalysts. Two

strategies to improve catalyst activity are to improve

the number of active sites and improve the activity of

a single catalytic active site, while the metal single-

atom is the catalytic center.

Mesoporous metal-organic framework materials

are good templates to increase the number of exposed

active sites and improve catalytic performance.

Pyrolysis of metal-organic framework materials is an

effective way to increase the number of catalysts, the

key lies in the formation of mesoporous structure,

which is beneficial to improve electron transfer and

material structure.

FSB 2022 - The International Conference on Food Science and Biotechnology

102

Figure 3: Schematic illustration of the preparation of mesoporous metal-organic framework materials.

The currently reported strategy is to select

suitable MOFs. The organic ligand of the template

contains amino groups and the metal center is Al

After high-temperature pyrolysis, the carbon support

of the MOFs can be obtained. The aluminum catalyst

carrier should be pyrolyzed into a carbon skeleton at

high temperature, immersed in Fe (II)-phenanthroline

solution, then removed and dried after adsorbing iron

atoms, and then pyrolyzed again to obtain zero-valent

iron atoms.

3.3

Electrochemical Properties

First, we used cyclic voltammetry to evaluate the

electrochemical performances of Fe-S

, Cu-S

Mn-S

, Cu-P

, Fe-P

and Mn-P

. The onset

potential, half-wave potential, and maximum limited

current are three important indicators for ORR. We

expect that we could select the best electrochemical

performances of ORR catalysts from the small library

of Fe-S

, Cu-S

, Mn-S

, Cu-P

, Fe-P

and Mn-P

single-atom solid catalysts based on the

indicators of onset potential, half-wave potential, and

maximum limited current. We would pay much

attention to the overpotential of ORR. This indicator

is highly related with the voltage efficiency of fuel

cells. To overcome the potential differences of ORR

thermodynamically controlled values and

experimentally measured values, the single-atom

solid electrocatalysts were decorated on the cathode

electrode. The as-synthesized novel Fe-S

, Cu-

, Mn-S

, Cu-P

, Fe-P

and Mn-P

single-atom solid catalysts could reduce the Gibbs

free energy greatly. As a result, the onset potential

and half-wave potential would be reduced as well as

maximum limited current would be increased. More

heat loss would be avoided due to the excellent ORR

performance in the fuel cells.

To explore the electrical properties of Fe-S

Cu-S

, Mn-S

, Cu-P

, Fe-P

and Mn-P

single-atom solid catalysts, we also plan to use

electrochemical impedance measurements. The

proposed equivalent circuits would support the

analysis of nanostructured materials and be expected

to be in line with a series of materials charac-

terizations. (Small, 2017) We would pay much

attention to the value of charge-transfer resistance. If

the Fe-S

, Cu-S

, Mn-S

, Cu-P

, Fe-P

and Mn-P

single-atom solid catalysts have an

excellent electrochemical performance, the value of

charge-transfer resistance fitting from a proposed

equivalent circuit would be reduced greatly. Since the

oxygen has a faster diffusion rate, the diffusion-

related parameters would be analyzed in our research.

The schematic Zinc air fuel cells are exhibited in

the Fig. 4. The zinc-air fuel cells get the energy via

zinc oxidation of oxygen in the air. The oxygen

reduction reaction is the rate-limiting step. The as-

synthesized asymmetric single-atom electrocatalysts

would boost this reaction rate, as a result, the zinc-air

battery based on asymmetric single-atom electro-

catalysts is expected to get better performances.

(Chen, 2018) Many advanced studies have been

explored in the Zinc-air fuel cells. For example, in

recent Science paper, non-alkaline rechargeable zinc

air Batteries was reported. The stable circulation in

air could persist for 1600 hours amazingly. (Sun,

2021) For the first time, a new non-alkaline

rechargeable zinc-air battery was reported, and the

reaction mechanism of reversible generation and

degradation based on zinc peroxide (ZnO

) was

successfully analyzed.

There is no doubt that this breakthrough work not

only provides new understanding and research ideas

for the subsequent development of highly reversible

secondary metal-air batteries. However, in addition

to the excitement, there is a question worth

wondering: Has the problem of zinc-air batteries

really been solved? The answer is negative. In

addition to problems such as electrolyte evaporation

and dendrite growth, another issue that requires the

most attention is the charge-discharge rate. This work

takes up to 20 hours for a charge-discharge cycle. So,

we would select our as-synthesized Fe-S

, Cu-

, Mn-S

, Cu-P

, Fe-P

and Mn-P

single-atom solid catalysts based on the time of the

stable circulation in air. Since the interaction of single

atoms and carbon support is coordination bond. This

kind of chemical bond has a much better stability than

the metal bond that traditional nanoparticles have. In

Preparation of Asymmetric Single-Atom Electrocatalysts for High-Performance Oxygen Reduction Reaction

103

Figure 4: Zinc–air fuel cells.

the addition, the as-synthesized carbon support based

on MOF templates has the porous and conductive

nanostructures. The reliable stability of carbon

supports is also expected in the stability testing of

Zinc air fuel cells.

3.4 Applications and Comparisons

The search for high-performance ORR reaction

catalysts is the "Holy Grail" catalyst for

electrochemical energy conversion devices.

(Mohamed Fathi Sanad, 2021) In catalytic reactions,

it is generally believed that the electronic properties

of the catalyst determine how the reactants and

intermediates bind to the catalyst surface. Density

functional theory (DFT) calculations show that the

single-atom catalyst can effectively activate and

dissociate the O-O bond, reducing the energy barrier

of O-O bond breaking and improving the ORR

activity. In addition, the DFT calculations would

provide plenty of information about the energy of

adsorption and desorption. DFT calculations would

give us scientific guidance about the ORR catalysts

design and preparation. (Shang, 2020) Since the Fe-

, Cu-S

, Mn-S

, Cu-P

, Fe-P

and Mn-

single-atom solid catalysts have different metal

centers and ligands, make quantitative analysis of

oxygen adsorption and desorption would inspire us

the design of more variety of asymmetric single-atom

catalysts.

Single atom catalysts (SACs) have a high

utilization rate of metal active center and adjustable

coordination structure. However, because of the lack

of group sites connected by multiple atoms, single

atom sites are difficult to be used in complex and

multi-step catalytic reactions. (Xie, 2021) For

example, monatomic Pt catalyst in THE ORR

catalytic process, isolated Pt atom cannot destroy the

O-O bond through lateral adsorption, so it is difficult

for monatomic Pt catalyst to effectively catalyze

ORR by four-electron mechanism.

There are literature reports on the synthesis of Cu-

Co bimetallic ORR catalysts via Cu-Co MOFs.

(Mohamed Fathi Sanad, 2021) Through XPS analysis

and DFT calculations, we demonstrate the forceful

electronic coupling between Cu-Co, which triggers

an efficient electron transfer process, and it is the key

to achieving high-performance ORR-active catalysis.

The synthesis process of Co-Cu bimetallic metal-

organic framework is based on a low-temperature

hydrothermal method. (Mohamed Fathi Sanad, 2021)

The catalytic activity is superior to that of the noble

metal Pt in alkaline environments because of the

specific electronic collaboration existing in the Co-

Cu bimetallic center in the MOF. (Mohamed Fathi

Sanad, 2021)

In addition, the common products of ORR

reactions are H

O, which is a four-electron reaction

product of oxygen reduction reaction, and H

which is a two-electron product of oxygen reduction

reaction. So, we need to analyze the products of ORR

carefully. We should avoid the generation of H

the ORR based on the design and selection of Fe-

, Cu-S

, Mn-S

, Cu-P

, Fe-P

and Mn-

single-atom solid catalysts. Because the H

product would make the maximum limiting current

become much smaller and H

product has the

oxidative activity that would damage the fuel cells

devices if we want to pursue an actual application

with a long time duration.

4 CONCLUSIONS AND

PERSPECTIVES

There are main challenges in the synthesis of single-

atom catalysts and their application in energy: (1)

There are many kinds of MOFs, but only a few (ZIF-

FSB 2022 - The International Conference on Food Science and Biotechnology

104

8, ZIF-67, MIL-101-NH2, and UiO-66-NH

) can

synthesize SAC, and the catalytic active sites are

limited to Fe, Co, Ni, Cu, Mn, W, so looking forward

to exploring more types of MOFs with metal active

sites. (Shang, 2020) (2) Usually, the metal content of

SAC is very low because to prevent the formation of

metal agglomerates, it is necessary to increase the

metal content under the premise of preventing metal

agglomeration. (3) There is still a lack of methods for

regulating the shape of catalyst nanometers. (4) The

in-depth analysis based on DFT calculations for

asymmetrical single-atom catalysts still need to be

explored and provide more specific information for

asymmetrical single-atom catalysts design. (5) There

are many works should be explored about the

evaluation of fuel cells in real applications.

It's worth noting that in situ X-ray absorption

spectroscopy and situ electron microscopy provide

powerful tools for studying the catalytic mechanism

behind SAC in recent years. In one aspect, we expect

the commercial applications of fuel cells in the near

future if the low-cost, abundant, high active ORR

catalysts can be acquired easily. This would boost the

transitions of global energy structures from fossil

energy domination. Many key climate issues would

also be solved due to this advancement. In the other

aspect, exploration of many advanced instruments for

ORR catalysts analysis is highly desirable in the next

few years. This would help us know the underlying

scientific mechanism and ensure the sustainability of

technology developments.

REFERENCES

Benítez, A., Amaro-Gahete, J., Esquivel, D, et al. MIL-88A

Metal-organic Framework as a Stable Sulfur-host

Cathode for Long-cycle Li-s Batteries. Nanomaterials

2020, 10, 424.

Huishan Shang, Xiangyi Zhou, Juncai Dong, et al.

Engineering unsymmetrically coordinated Cu-S1N3

single atom sites with enhanced oxygen reduction

activity. Nature Communications 2020, 11, 1-11.

Haolin Tang, Shichang Cai, Shilei Xie, et al. Metal-

organic-framework-derived Dual Metal- and Nitrogen-

doped Carbon as Efficient and Robust Oxygen

Reduction Reaction Catalysts for Microbial Fuel Cells.

Advanced Science 2016, 3 1500265

Mohamed Fathi Sanad, Alain R. Puente Santiago, Sarah A.

Tolba, et al. Co–Cu bimetallic metal organic

framework catalyst outperforms the Pt/C benchmark

for oxygen reduction. Journal of the American

Chemical Society, 2021, 143, 4064-4073.

Small, Leo J, Tina M. Nenoff. Direct Electrical Detection

of Iodine Gas by a Novel Metal–Organic-Framework-

Based Sensor. ACS applied materials & interfaces,

2017, 9, 44649-44655.

Sun Wei, Fei Wang, Bao Zhang, et al. A rechargeable zinc-

air battery based on zinc peroxide chemistry. Science

2021, 371, 46-51.

Tingting Sun, Lianbin Xu, Dingsheng Wang, et al. Metal

organic frameworks derived single atom catalysts for

electrocatalytic energy conversion. Nano Research,

2019, 12, 2067-2080.

Xiaoying Xie, Lishan Peng, Hongzhou Yang, et al. MIL‐

101‐derived mesoporous carbon supporting highly

exposed Fe single‐atom sites as efficient oxygen

reduction reaction catalysts. Advanced Materials,

2021, 33, 2101038.

Yuanjun Chen, Shufang Ji, Shu Zhao, et al. Enhanced

oxygen reduction with single-atomic-site iron catalysts

for a zinc-air battery and hydrogen-air fuel cell. Nature

Communications 2018, 9, 5422.

Yazhou Zhou, Xiafang Tao, Guangbo Chen, et al.

Multilayer Stabilization for Fabricating High-loading

Single-atom Catalysts. Nature Communications 2020,

11.

Preparation of Asymmetric Single-Atom Electrocatalysts for High-Performance Oxygen Reduction Reaction

105