DOI: 10.3303/CET25117122 

 

 

 

 

 
 
 

 
 

 
 
 

 
 

 
 
 

 
 

 

 

 
 
 

 
 
 

 
 
 

 

 

 
 
 

Paper Received: 15 January 2025; Revised: 20 February 2025; Accepted: 9 May 2025 
Please cite this article as: Giusi D., Ronsisvalle A.M., Costantino V., Amoroso V., Perricone S.I., Tavella F., Genovese C., Ampelli C., 2025, 
Tuning The CuO Catalyst Properties for an Optimal H2:CO Ratio , Chemical Engineering Transactions, 117, 727-732  DOI:10.3303/CET25117122 

CHEMICAL ENGINEERING TRANSACTIONS 

VOL. 117, 2025

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto 

Copyright © 2025, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-17-5; ISSN 2283-9216 

Tuning The CuO Catalyst Properties for an Optimal H2:CO 

Ratio 

Daniele Giusi*, Angela M. Ronsisvalle, Veronica Costantino, Viviana Amoroso

Sefora Isabella Perricone, Francesco Tavella, Chiara Genovese, Claudio Ampelli

Department of Chemical, Biological, Pharmaceutical and Environmental Sciences (ChiBioFarAm) – University of Messina,

ERIC aisbl and CASPE/INSTM, v.le F. Stagno d'Alcontres, 31 – 98166 Messina, Italy 

dgiusi@unime.it

This study reports the design and engineering of CuO-based electrocatalysts synthesized via a facile

hydrothermal approach to optimize the H2:CO syngas ratio in CO2 electrocatalytic reduction. This process offers

a sustainable alternative to traditional methods by operating under mild conditions, reducing greenhouse gas

emissions, and enabling the use of renewable energy. Three CuO-based electrocatalysts, synthesized by the

same hydrothermal method but with different precursors, were used to prepare innovative gas-diffusion 

electrodes and tested in an optimized compact electrochemical flow-by reactor. Unlike conventional devices

with liquid anode and cathode compartments separated by a membrane, this reactor features an additional gas

chamber adjacent to the cathode liquid compartment, forming a triple gas/solid/liquid interface that enhances

the local CO2 concentration at the electrode surface. The study demonstrates that introducing different salts 

(e.g., Li⁺ or Na⁺) during synthesis enables significant modifications to the structural properties of the CuO 

catalysts, leading to different product distributions in CO₂ electroreduction. The optimal H2:CO ratio, suitable for

Fischer-Tropsch or methanol synthesis, can be modulated introducing Li+ or of Na+ salt in the synthetic CuO 

route, as well as by fine-tuning the operating conditions, such as the cell potential and current density. The 

performance differences among the catalysts were elucidated through electrochemical characterizations, 

including measurements of capacitance, resistance and electrochemical active area (ECSA), as well as through 

the analysis of hydrophilicity properties of the electrode surface. 

1. Introduction

Syngas, a mixture composed of carbon monoxide and hydrogen in different stoichiometric ratios, can be used 

to produce pharmaceuticals, intermediates (e.g., ammonia, methanol), plastics, solvents, and fertilizers. 

Currently, the most efficient technologies for syngas synthesis include coal gasification and natural gas 

reforming through the mediation of a catalyst and the main disadvantage of these processes is the high

temperatures and pressures required (Gao et al., 2023). The possibility of varying the H2/CO mixture ratio during 

the electrochemical process, for example by tailoring the active sites of the catalyst or through engineering the 

electrochemical reactor (Ampelli et al., 2023b), as well as by choosing the best operating conditions, represents

an attractive route to perform Fischer-Tropsch or methanol processes. Methanol, a valuable intermediate in the

synthesis of several chemicals and e-fuels and used as energy carrier (Sarvestani et al., 2024), can be

synthesized through the main reaction (eq.1), followed by the secondary reaction (eq.2). 

CO + H2 → CH3OH      ΔH= −90.7 kJ/mol  (1) 

CO2 + 3H2 → CH3OH + H2O   ΔH= −49.4 kJ/mol    (2) 

CO2 utilization, as shown by equation n.2, can directly contribute to reducing the greenhouse effect (Marchese 

et al., 2021) and consequently, an electrochemical reduction approach can also be a valuable tool. This strategy, 

until now, has led to low faradaic efficiencies and furthermore, additional costs must also be accounted for 

downstream separation processes. 

727



Electrochemical CO2 reduction is currently considered a green approach for producing chemical feedstock and 

liquid fuels, making it a valuable tool for carbon recycling and energy storage, especially if driven by renewable 

sources, such as sunlight (Ampelli et al., 2023a). The CO2 molecule is one of the most stable carbon-based 

molecules, and its activation, via the formation of the CO2
·− radical anion intermediate, requires considerable 

effort to produce carbon-based chemicals.  

The selectivity and faradaic efficiency of these processes are often low because the formation of carbon-based 

compounds competes with the unavoidable parasitic hydrogen evolution reaction (HER). Among the various 

available catalysts, copper currently appears to be the only one capable of producing long-chain carbon 

compounds (C1-C3), although selectivity and yields remain low despite several efforts (Giusi et al., 2022a). 

Carbon monoxide (CO) production, for those materials that allow low HER yields, seems to be the most readily 

achievable product, as it requires only two-electron transfer process (Poon et al., 2022). Materials such as silver 

(Ag), and gold (Au), preferentially reduce CO2 to CO, while simultaneously exhibiting low yields in HER (Leonzio 

et al., 2024). By fine-tuning the material properties, it is possible to optimize the presence of active sites both 

for CO2 reduction reactions and HER, not limiting the search for materials solely to gold or silver. Palladium, for 

example, despite being known to be easily poisoned by CO (with which it bonds strongly), has been considered 

a promising candidate for the electrochemical production of syngas with a 1:1 H2/CO ratio (Sheng et al., 2017), 

but a sustainable approach should require the use of more available materials.  

In this paper, we propose the synthesis of two CuO-based electrocatalysts obtained through the same 

hydrothermal procedure but introducing two different cations, such as Li+ (from LiOH) and Na+ (from NaOH), 

and comparing their electrochemical performance in the CO2 reduction reaction. These two synthesized 

materials were compared with a commercial CuO catalyst used as a reference material.  

The synergy of the various chemical and physical properties of the materials can produce H2:CO mixtures with 

a different ratio, highlighting that engineering the surface properties is as important as the choice of the catalytic 

material itself, without the need to use expensive and critical raw elements (Tabassum et al., 2022). 

2. Materials and Methods

2.1 CuO-based catalysts 

The CuO (Li-modified) catalyst was synthesized via a facile hydrothermal treatment by dissolving 1 mmol of 

CuCl2 and 3 mmol of LiOH in 20 mL of H2O. The resulting solution, stirred for 30 minutes, was then transferred 

to a 50 mL Teflon autoclave and heated to 180°C for 2 hours with a ramp of 5°C min-1. The autoclave was then 

cooled to room temperature, and the black precipitate centrifuged and washed three times with H2O and ethanol 

to remove the impurities. The resulting solid was then dried overnight at 70°C in a vacuum oven. 

The CuO (Na-modified) catalyst was synthesized with the same synthetic strategy as the CuO (Li-modified) 

material but replacing the LiOH salt with 20 mL of 2M NaOH. 

The CuO (reference) sample was purchased from "Merck©" and its purity was ≥ 99% (ACS reagent). 

The working electrodes were prepared by depositing the catalyst (obtained as a solid) through spray-coating on 

a carbon gas-diffusion layer (GDL, Sigracet® 36BB, supplied by Ion Power). This technique consists of the 

deposition, using an airbrush, of the catalytic material (in the form of an ink) on the pre-heated carbon substrate 

to allow the solvent to evaporate and fix the catalytic material on its surface.  

The ink was obtained by mixing a certain amount of the catalysts with 50 µL of 5% Nafion® perfluorinated solution 

and 10 mL of 2M 2-propanol for 20 minutes under sonication, until a stable suspension was obtained. The 

catalysts were deposited on GDL until a loading of 0.5 mg cm-2 was achieved. 

2.2 X-Ray diffraction (XRD) analysis 

The purity of the electrocathodes and their crystallographic structure was analysed by X-ray diffraction 

crystallography (XRD). The crystallinity of all materials was demonstrated by the presence of well-defined peaks 

(Figure 1). Our synthesized samples show more defined and narrow peaks than the commercial sample, thus 

evidencing a higher crystallinity degree. The main detected peaks corresponding to (110), (1̅11), (111), (1̅12), 

(2̅02), (020), (202), (1̅13), (3̅11), (113), (2̅20), and (311) crystallographic planes were observed at 32.8°, 35.7°, 

39.0°, 46.5°, 48.9°, 53.6°, 58.6°, 61.7°, 66.6°, 68.2°, 72.7°, and 75.2°, proving that CuO was the only phase 

present in all materials. 

728



 

Figure 1: XRD patterns for CuO (Li-modified), CuO (Na-modified) and CuO (reference). 

 

2.3 Morphology analysis 

The morphological analysis of the three materials was conducted through the scanning-electron microscopy 

(SEM) and the images are presented in Figure 2. While the CuO (reference) material presents spherical particles 

with a diameter range between 0.40 and 1.7 µm, the introduction of two different salts (Li+ and Na+) via the same 

synthetic process in the two other samples CuO (Li-modified) and CuO (Na-modified), did not lead to significant 

changes in the morphology, which in both cases correspond to interconnected nanoplates. The latter are known 

to exhibit a high density of active sites compared to different particle shapes (Hecker et al., 2023). The sample 

substituted with Li+ presents nanoplates with maximum length of about 5 µm, slightly longer compared to the 

sample substituted with Na+ (3 µm). 

 

Figure 2: Scanning electron microscopy for a) CuO (reference); b) CuO (Na-modified) and c) CuO (Li-modified). 

2.4 Electrochemical device 

Electrochemical tests were conducted in an innovative homemade flow-by electrochemical reactor, made of 

Plexiglas to facilitate internal inspection. This reactor consists of two liquid chambers, an anode and a cathode, 

and a gas chamber adjacent to the cathode chamber, where CO2 is continuously fluxed. This configuration 

allows for the formation of a triple gas/solid/liquid interphase, which has the following advantages: i) improved 

diffusion of CO2 to the active sites of the catalytic material; ii) increased local availability of CO2, which would 

otherwise be limited by its maximum concentration in the aqueous electrolyte (Giusi et al. 2023b); iii) separation 

of gaseous and liquid products, with the former collected from the gas chamber outlet and the latter taken from 

the liquid reservoirs at the end of the tests. The two liquid chambers were separated by a proton-exchange 

membrane (Nafion® NR324), which serves to separate the reaction environments (allowing operation with 

different electrolytes and pH), preventing mixing of formed products, improving safety issues, and facilitating 

proton (H+) transport from the anode to the cathode chamber, where they participate in the CO2 reduction 

reactions (in addition to HER). Two solutions of 0.1 M KHCO3 and 1 M KOH were employed as catholyte and 

anolyte, respectively, and were recirculated continuously in the chambers with a peristaltic pump at a flow rate 

of 10 mL min-1 for the entire test duration. The 0.1 M KHCO3 catholyte was pre-saturated for 30 minutes with 

CO2 before all tests to achieve a final pH of 6.8, while an additional CO2 flow (20 mL min-1) was maintained in 

25 30 35 40 45 50 55 60 65 70 75 80

CuO (reference)

CuO (Na-modified)

In
te

n
s
it
y
, 

a
.u

.

2θ, degree

(1
1
0
)

(1
1
1
)

(1
1
1
)

(2
0
2
)

(0
2
0
)

(2
0
2
)

(1
1
3
)

(3
1
1
)

(1
1
3
)

(2
2
0
)

(3
1
1
)

(1
1
2
)

Copper oxide

CuO (Li-modified)

729



the cathode reservoir (in addition to the gas chamber) to limit pH variation during the test due to proton 

consumption and to keep the solution saturated with CO2. The electrodes used as anode and cathode had an 

active area of about 5 cm2. During the tests, the produced gases were sampled periodically (every 5 minutes) 

and analysed using Gas Chromatography (MicroGC GCX Pollution Analytic Equipment), while the liquid 

samples were taken at the end of the tests and analysed using Ion Chromatography (IC, column organic acids) 

to detect organic acids and Gas Chromatography-Mass Spectrometry (GC-MS, equipped with a Stabilwax 

column and He as the carrier gas) to check for the formation of alcohols such as methanol, ethanol, and other 

organic compounds. 

3. Results and Discussion 

3.1 CO2 Reduction tests 

The CO2 reduction tests were conducted through the application of the following electrochemical protocol: i) 

capacitance measurement through double layer technique; ii) running of chrono voltammetry cycles (CVs) up 

to the potential set for the test (see point iv), for the stabilization of the material and evaluation of the onset-

potential. iii) resistance measurement through electrochemical impedance spectroscopy (EIS); iv) running of 

chrono-amperometry tests (CAs) at increasing potentials.  

3.1.1 Chronoamperometry tests 

The CAs were conducted from -0.2 up to -1 V vs RHE, depending on the current densities achieved for the 

various catalysts (which exhibited different conductivities), and each test lasted 1 hour. For example, for the 

CuO (Li-modified) sample, the maximum potential reached was -0.6 V vs RHE, since the current densities 

achieved were already comparable to those observed at lower potentials for the other two electrodes. While for 

the CuO (reference) and CuO (Na-modified), the H2:CO ratio varied from 3.5 to 4 for the first and from 4 to 5 for 

the second (depending on the applied potential), the CuO (Li-modified) sample presents significantly lower 

values, ranging from a minimum of 1.3:1 to 1.8:1 for the highest potential. In industrial methanol synthesis, a 

mixture of CO, CO₂, and H₂ is typically used, characterized by the M module, defined as (H₂-CO₂)/(CO+CO₂), 

whose optimal value is equal to 2. In practice, slightly higher ratios are often employed to enhance process 

efficiency, shifting the equilibrium toward the methanol production, improving reaction kinetics, and preventing 

the catalyst deactivation due to coke formation (Hankin et al., 2017). This last step is the real test for evaluating 

performance in the CO2 reduction, in terms of current density, productivity and faradaic efficiency (Figures 3a, 

b and c). 

 
 

Figure 3: H2:CO ratio and current density profiles vs. applied voltage for a) CuO (reference); b) CuO (Na-

modified) and c) CuO (Li-modified). 

3.1.2 Electrochemical and catalytic properties  

The investigated chemical and physical properties of our materials are reported in Table 1. The electrochemical 

active surface area (ECSA) was obtained by dividing the double layer capacitance (Cdl) by the specific 

capacitance (Cs). The former was obtained through an electrochemical method (double layer) consisting of 

several chrono voltammetry cycles performed around the open circuit potential (OCP) value (to provide only 

non-faradaic charge transfer processes) at different scan rates. The CuO (Li-modified) sample showed a higher 

ECSA than the two other samples, equal to 60 cm2, compared to 31 cm2 and 45 cm2 for the CuO (Na-modified) 

and CuO (reference), respectively. Contact angle measurements with water droplets were also carried out, 

highlighting how the modification with Li+ resulted in a higher contact angle than the Na+ modified material (which 

could not be measured, due to its high hydrophilicity), but lower than CuO (reference). The combination of the 

higher ECSA, increased capacitance and lower hydrophilicity can explain the remarkable difference in terms of 

-0.6 -0.8 -1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

H
2
:C

O
 

Voltage vs RHE, V

 CuO (reference)

-0.6 -0.8 -1

-1

0

1

2

3

4

5

6

7

8

9

C
u
rr

e
n
t 
d
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n
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y
, 
m

A
 c

m
-2

-0.6 -0.8

0

1

2

3

4

5

6

H
2
:C

O
 

Voltage vs RHE, V

 CuO (Na-modified)

-0.6 -0.8

0

1

2

3

4

5

6

C
u
rr

e
n
t 
d
e
n
s
it
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, 
m

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 c

m
-2

-0.2 -0.4 -0.6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

H
2
:C

O
 

Voltage vs RHE, V

 CuO (Li-modified)

0

2

4

6

8

10

C
u
rr

e
n
t 
d
e
n
s
it
y
, 
m

A
 c

m
-2a) b) c) 

730



H2:CO ratio among the various samples. The onset potential measurements (from the CVs curves) also 

confirmed the results from the wettability tests, indicating that the hydrophilicity follows the following order: 1) 

CuO (Na-reference); 2) CuO (Li-reference), and 3) CuO (reference). The higher hydrophilicity, related to the 

presence of different functional groups (for example OH-) can be associated with a superior capacity to bind 

water molecules, activating it and promoting the HER reaction more than the CO2 reduction reactions. 

Additionally, the contribution of the sample morphology must also be taken in account, since for example the 

lower hydrophilicity of the (CuO-Li modified) sample may be the key to explaining the lower H2:CO ratio, as the 

reduction of CO2 to CO can be favoured compared to HER, due to the reduced interaction with water molecules. 

The titration of OH- surface groups with nitric acid instead highlighted a higher concentration of acidic sites in 

the sample modified with Li+ compared to the others (Figure 4, a). The capacitance profile (normalized to ECSA) 

and the H2:CO ratio, as a function of the various materials, are reported in Figure 4, b. The CuO (Li-reference), 

which provides the lowest H2:CO ratio, exhibits lower normalized capacitance than the CuO sample modified 

with Na+ but comparable to the CuO (reference). While this data may explain the differences in performance 

between the two Na and Li samples, i.e. the greater hydrophilicity observed in CuO (Na-reference) influencing 

interfacial water absorption and double layer capacitance, these factors may not fully explain the differences 

relative to the CuO (commercial) sample. In the latter case, the morphology (different from the two synthesized 

samples, which are similar) and, therefore, the concentration of active sites could be more decisive.  

Table 1: ECSA, capacitance and contact angle measurements for the three investigated CuO electrodes. 

Catalysts ECSA, cm2 Capacitance, µF Contact angle, °C  

CuO (reference) 45 3802 130  

CuO (Na-modified) 31 3751 Not measurable  

CuO (Li-modified) 60 4950 111  

       

Figure 4: a) Titration with nitric acid; b) Normalized capacitance (to ECSA) and H2:CO ratio for the different 

materials. 

 

It was demonstrated that the introduction of Li+ (instead of Na+) resulted to a significant change in the material 

physical properties, specifically by increasing its hydrophobicity, leading to a lower H2:CO ratio ranging from 

1.3:1 to 1.8:1 (depending on the applied potential), suitable for the second step production of liquid fuels via 

Fischer-Tropsch process. On the contrary, a higher H2/CO ratio can be obtained with Na+, thus more suitable 

for methanol production. The electrochemical tests were supported by various chemicals analysis and 

electrochemical characterizations, which revealed significant changes among the three materials in terms of 

capacitance, electrochemically active surface area (ECSA) and resistivity. An analysis of hydrophilic properties 

and wettability was conducted through the titration of the surface hydroxyl groups with nitric acid and through 

contact angle measurements, indicating that the CuO (Li-modified) material was more hydrophobic than the Na-

modified CuO (and slightly less than the CuO-commercial one). This evidence, in combination with higher ECSA 

and double-layer capacitance, can explain the different results. 

4. Conclusions 

This study has demonstrated that optimising the chemical and physical properties of the electrocatalyst is more 

critical for enhancing performance in CO2 electrocatalytic reduction than the choice of the material itself. By 

varying the properties of the same material (CuO), such as hydrophilicity, capacitance, and conductivity, it was 

possible to modulate the H2:CO ratio, which is important for subsequent processes like methanol synthesis or 

Fischer-Tropsch process, involving higher temperatures and pressures and using and iron and cobalt-based 

CuO

(Li-reference)

CuO

(Na-reference)

CuO

(reference)

0

1

2

3

4

5

6

7

8

p
H

 Initial

 Final

CuO

(Li-modified)

CuO

(Na-modified)

CuO

(reference)

0

20

40

60

80

100

120

140

N
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4.0

4.5

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2
:C

O

a) b) 

731



catalysts (Jeske et al., 2021). The introduction of Li+ during hydrothermal synthesis resulted in a H2:CO ratio 

(about 1.8:1) close to the stochiometric ratio required for the methanol synthesis (2:1), whereas a higher ratio 

was observed for the sample synthesized with Na+ and for the commercial CuO sample, thus more suitable for 

direct industrial application. Analysis of the chemical and physical properties, using both chemical and 

electrochemical methods, confirmed that their combination accounts for the significant difference in product 

distribution. The ability to vary the H2:CO ratio by adjusting only the synthesis parameters or operating conditions 

is an easy and sustainable way to avoid the use of expensive materials with a high environmental impact, making 

the overall process scalable and economically efficient. 

Acknowledgments 

This work is funded by the European Union through the DECADE (ID: 862030) and SUPERVAL (ID: 101115456) 
H2020 projects, which are gratefully acknowledged.  

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