CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 57, 2017 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš, Laura Piazza, Serafim Bakalis 
Copyright © 2017, AIDIC Servizi S.r.l. 

ISBN 978-88-95608- 48-8; ISSN 2283-9216 

Nano-Engineered Electrodes for the Generation of Solar 
Fuels: Benefits and Drawbacks of Adopting a Photo-

Electrocatalytic (PECa) Approach 
Claudio Ampelli*a, Chiara Genovesea, Francesco Tavellaa, Siglinda Perathonera, 
Gabriele Centib 
aDepartment of Chemical, Biological, Pharmaceutical and Environmental Sciences – University of Messina, ERIC aisbl and 
CASPE/INSTM, v.le F. Stagno d'Alcontres, 31 – 98166 Messina, Italy 
bDepartment of Mathematical, Computer, Physical and Earth Sciences – University of Messina, ERIC aisbl and 
CASPE/INSTM, v.le F. Stagno d'Alcontres, 31 – 98166 Messina, Italy 
ampellic@unime.it 

In this contribution, the advantages of adopting a photo-electrocatalytic (PECa) approach for the production of 
solar fuels were discussed through the analysis of the factors that can influence the performances of PECa 
cells. Particularly, there are some issues, conventionally not studied, that should be taken into account already 
at the initial stage of cell design, such as: i) light harvesting and charge separation, ii) electron conductivity 
and mass diffusion, iii) productivity and type of products formed. The choice of the electrodic materials and the 
preparation of nanostructured electrodes are of great importance to improve the above aspects, by enhancing 
the photo-catalytic activity and limiting the overpotential of the cell. The experimental data were obtained by 
preparing and testing two different types of electrodes, one based on highly ordered TiO2 nanotube arrays (the 
photo-anode) and the other based on Cu nanoparticles deposited on nano-carbons (the electro-cathode). The 
electrodes were assembled together and located in a compact device with two separate compartments for 
each half-reaction. Finally, they were tested in the process of water photo-electrolysis and/or photo-reforming 
of organic wastes for the production of H2, as well as in the process of CO2 reduction to liquid fuels. Results, 
given in terms of H2 evolution, solar-to-hydrogen (STH) efficiency and liquid fuel productivity, were very 
promising. Particularly, 195 mol of H2 were produced in 4 h of light irradiation without adding sacrificial 
donors, being over 16 % of this production due to only solar irradiation. This attractive result (if considering 
that non-doped TiO2 was employed as the photo-anode) is to ascribe to the enhanced visible light absorption 
of TiO2 nanotube arrays (due to their structural resonance effect) and to the reduced charge recombination 
(for their improved electron transfer). On the other hand, CO2 electrocatalytic reduction allowed obtaining not 
only formic acid (as usually obtained in conventional cells) but also methanol, ethanol, acetic acid, isopropanol 
and acetone. The formation of these products is due to the catalytic sites located in the interface between the 
small copper particles and the carbon surface, favouring electron transfer and C-C bond formation. 

1. Introduction 

The photo-electrocatalytic (PECa) approach for the production of solar fuels is an attractive technology for the 
tomorrow’s sustainable production of energy (Armaroli and Balzani, 2016). PECa systems consist of compact 
devices, similar to fuel cells, with separated zones for the reactions of oxidation (photo-anode) and reduction 
(electro-cathode) (Hisatomi et al., 2014). The small volume and the separation of the two reaction 
environments are both aspects of fundamental importance to enhance the photo-catalytic activity, as they i) 
reduce the charge recombination effects, ii) limit overpotential due to mass transport and proton diffusion, and 
iii) avoid safety problems and costs for the separation of products (Ampelli et al., 2016a). One of the important 
challenges for the future commercialization of PECa cells is the preparation of active, robust and low-cost 
catalysts (Li and Wu, 2015), as well as a proper design of the configuration of the cell and the related catalytic 
electrodes (Zhao et al., 2016). However, there are some issues that currently limit the efficiency of solar PECa 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1757267

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Ampelli C., Genovese C., Tavella F., Perathoner S., Centi G., 2017, Nano-engineered electrodes for the generation 
of solar fuels: benefits and drawbacks of adopting a photo-electrocatalytic (peca) approach, Chemical Engineering Transactions, 57, 1597-
1602  DOI: 10.3303/CET1757267 

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cells and that should be taken into account in designing the electrodes: i) light harvesting and band gap 
structure of the photo-catalysts, ii) charge separation efficiency, iii) good collection of the charges iv) catalytic 
activity and selectivity of the redox half-reactions and v) overall efficiency and productivity. The preparation of 
3D nanostructured electrodes can highly favour all these factors, by increasing the active area and limiting the 
overpotential phenomena (Ampelli et al., 2016b). Recently, hierarchical tree-like TiO2 nanostructures, showing 
improved electron transfer, were successfully proposed as photoanodes for photoelectrochemical water 
splitting (Matarrese et al., 2014). 
In this context, we discuss here about our recent findings on the preparation of the electrode materials and 
assembling of the electrodes in our novel design PECa cells. Particularly, the electrodes are based on highly 
ordered TiO2 nanotube arrays and metal nanoparticles (NPs) deposited on nano-carbons, and they were 
tested in the process of water photo-electrolysis and/or photo-reforming of waste organic solutions for the 
production of H2, as well as in the process of CO2 reduction to liquid fuels. The production of higher C-chain 
compounds (C2-C9 hydrocarbons, alcohols, oxygenates, etc.) should be preferable with respect to H2, as 
liquid fuels can easily be stored and transported by using the current energy infrastructure (Ampelli et al., 
2015a). 

2. Experimental 

2.1 Photo-anode 

TiO2 nanotube array (TNTs) photo-electrodes were prepared by controlled anodic oxidation of Ti foils. The 
essence of the method can be described as a reconstruction of a thin TiO2 layer (formed initially by oxidation 
of a Ti foil) which occurs under the application of a constant voltage in presence of fluoride-based electrolytes 
(Macak et al., 2007). 
The starting titanium disc (Alfa Aesar, 0.025 mm of thickness, 3.5 cm of diameter) was anodized using a two-
electrode electrochemical cell working at room temperature and constant voltage (in the range 15-60 V) for 
times until 5 h (Ampelli et al., 2017a). The reaction bath consisted of a solution of ethylene glycol with H2O (2 
wt. %) and NaF (0.3 wt. %). Details about the geometry of the cell and the synthetic procedure were described 
elsewhere (Ampelli et al., 2015b). 
After the preparation, the nanostructured amorphous substrates were annealed at 450 °C in air for 3 h in order 
to induce crystallization into the anatase phase. The thin metallic Ti layer (remained non-oxidized after the 
anodization) can act as the electron-collective layer during the photo-catalytic process. 

2.2 Electro-cathode 

Metal Cu nanoparticles (NPs) were deposited onto a nanocarbon substrate by incipient wetness impregnation 
method using an ethanolic solution containing the metal precursor [Cu(NO3)2•3H2O]. The total amount of 
copper loaded onto the carbon substrate was finally 10 wt. %. The nanocarbon substrate consists of 
commercial carbon nanotubes (CNTs, PR-24-XT-PS Pyrograf®), having an average diameter of about 100 
nanometers. They were pyrolyzed at 750 °C to remove polyaromatic hydrocarbons from their surface. Due to 
their turbographic structure, these CNTs offer a large number of sites for functionalization of the external 
surface (Genovese et al., 2013). CNTs were activated by direct oxidative treatment in concentrated HNO3, 
thus introducing oxygen functionalities on the carbon surface. Details about the pre-treatment of CNTs and 
metal deposition were reported elsewhere (Ampelli et al., 2015c). The as-prepared electrocatalyst was then 
deposited on a gas diffusion layer (GDL 25-BC Sigracet®). 

2.3 Assembling the electrodes in PECa cell 

The two electrodes (TNTs and Cu/CNTs/GDL layers) were assembled together by hot pressing with a Nafion® 
membrane to form a Membrane Electrode Assembly (MEA), as it is shown in Figure 1. The MEA was then 
located within the PECa device and tested in different processes for the production of solar fuels. 
The mechanism of reaction can be described as follows: i) the irradiated light, coming from an external solar 
simulator (Xe-arc lamp of 300 W, Lot Oriel), reaches the photo-anode surface creating a charge separation 
(electrons e- and holes h+), ii) the holes oxidize the water to oxygen (or the waste organics to CO2 in the case 
of photo-reforming), iii) protons and electrons are transferred to the cathode compartment (the protons 
crossing the Nafion® membrane and the electrons through an external circuit) iv) where they combine to 
produce H2 (or combine with CO2 to form liquid fuels). 
The products were finally analysed by gaschromatography (GC) and mass spectrometry (MS). 
 
 
 

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Figure 1: Schematic depiction of the Membrane Electrode Assembly (MEA) in a solar PECa cell. The insert in 

layer 1 shows a TEM image of Cu/CNTs, while the insert in layer 5 shows a SEM top image of TiO2 nanotube 

arrays prepared by anodic oxidation. 

3. Results and discussion 

3.1 Benefits and drawbacks of PECa cells 

There are many benefits in adopting the PECa approach for the production of solar fuels. In general, even if 
other technologies can be used to convert solar energy into chemical energy (i.e. high temperature 
thermochemical approach), PECa systems are potentially more advantageous as: i) they work at room 
temperature (or at temperatures slightly higher than r.t., i.e. 50-60 °C, taking into account that temperature 
may increase during exposition to sunlight); ii) the electrolyte can fully be recycled, but we demonstrated that 
electrolyte-less configuration is also possible (Ampelli et al., 2016a); iii) the energy required for the process 
comes solely from sunlight; iv) the production of liquid fuels is performed by converting back CO2, thus 
diminishing its level in the environment and closing its cycle of production/consumption (Goeppert et al., 
2014). 
The particular benefits of using a PECa device, instead, refer to: i) the separation of the products of oxidation 
(O2) and reduction (H2, or the products of CO2 reduction), avoiding safety problems, costs of separation, 
quenching effects, ii) the continuous operation, with easy recovery of the reaction products, and iii) the 
compact design, easily scalable. These aspects should all be taken into account in designing the cell, because 
they determine the choice of the materials constituting the electrodes and their needs (Ampelli et al., 2017b). 
However, there are many constrains, not accounted in general, which limit the performances of PECa cells. 
They are mainly related to problems of light harvesting, charge recombination, electron transport, proton 
diffusion, which increase the overpotential of the cell. The development of nanostructured electrodes may 
strongly improve the PECa performances, by limiting the above overpotential phenomena and enhancing the 
quantum efficiency and productivity of the cell. 

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Figure 2: Hydrogen evolution as a function of time from the cathode of a PECa cell in water photo-electrolysis. 

3.2 Photo-anodes for H2 production 

TNTs prepared by anodic oxidation were tested in the process of water photo-electrolysis. Figure 2 shows the 
H2 production detected in the gaseous stream leaving the cathode. After an initial induction period of 10-15 
min, due to establish the equilibrium in the cell, the H2 evolution increased linearly with a quite constant 
production rate, at least in the time range investigated. We also detected O2 evolution in the anode side of the 
PECa cell and the H2:O2 ratio was the expected for stoichiometric water photo-electrolysis. No H2 was instead 
detected in the anode part. When a full spectrum of irradiated light was used, ~195 mol of hydrogen were 
measured after five-hour time of irradiation. 
It is well known that one of the main drawbacks of TiO2 is that it works only if irradiated in the ultraviolet (UV) 
region. Being the UV part only a short percentage of the solar irradiation, the challenge is to shift the TiO2 
band gap to the visible region, thus exploiting better the entire solar spectrum. Creating a nano-architecture in 
TiO2 can improve its visible light absorption, due to the structural resonance effect for light diffraction (Ampelli 
et al., 2017b). 
By cutting the light irradiation using a filter AM 1.5G (this filter simulates the standard terrestrial solar 
irradiance distribution), the quantity of H2 at the cathode decreased to ~32 mol after five-hour time of 
irradiation, which is over 16 % with respect to H2 productivity obtained at open spectrum. Considering that the 
percentage of UV light in solar spectrum is only 4 %, the result obtained by AM 1.5 G filter evidences an 
improved performance of TNTs in the visible region with respect to standard TiO2. 
If ethanol (10 vol. %) was added into the photo-anode, H2 productivity increased even to ∼355 μmol (not 
shown in the figure) with a side production of acetaldehyde (ethanol dehydrogenation). This may open the 
route for a new application, the treatment of waste organic solutions containing alcohols, i.e. coming from 
biorefineries, which are too dilute to be processed by conventional catalytic routes. 
Table 1 reports the solar-to-hydrogen (STH) efficiency calculated following the Eq.(1) (Varghese and Grimes, 
(2008): 

STH =
RH2  ∗  ∆G

0

Ptot ∗ A 
 (1) 

where RH2 is the rate of hydrogen production (mol·s
-1), G0 is the standard Gibbs energy at standard 

conditions (2.372 x 105 J·mol-1), Ptot is the light irradiance (W cm
-2) and A is the irradiated area of the 

photocatalyst (cm2). 
 
 

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Table 1: Solar-to-hydrogen (STH) efficiency (%) for TNTs-based photoanode. 

Process Irradiated light STH (%) 
Water photo-electrolysis Full spectrum 0.83 
Water photo-electrolysis AM1.5G filter 0.23 
Ethanol photo-reforming Full spectrum 1.43 
Ethanol photo-reforming AM1.5G filter 0.24 
 

3.3 Electro-cathodes for CO2 reduction 

The Cu/CNTs/GDL electrodes were tested in the process of CO2 electrocatalytic reduction, in presence of 
aqueous KHCO3 (0.5 M) electrolyte and 10 mL pure CO2 flow. In these experiments, solar light was simulated 
by applying a small bias (-1 V) between the two electrodes. 
Table 2 summarizes the productivity of the liquid products obtained in the reduction process. Under these 
conditions, formic acid and acetic acid formed as the main products with similar productivity (4.25 and 4.17 
mol in 4 h, respectively), while 0.27 mol of methanol was also produced.  

Table 2: Products obtained from the electrocatalytic reduction of CO2 in 4 h of applied potential (-1 V). 

Electro-cathode Formic acid, mol Acetic acid, mol Methanol, mol 
Cu/CNTs/GDL 4.25 4.17 0.27 
CNTs/GDL 1.29 0.88 - 
 
The analysis of the outlet gaseous stream leaving the cathode revealed the formation of H2, with a rate of 
production of 0.32 mol min-1, while no CO and CH4 were detected. The observed rates of reaction remained 
quite constant until 24 h, evidencing a good stability on a laboratory scale. The data obtained by the 
electrocatalytic test with only CNTs/GDL (the CNTs were functionalized by oxidative pre-treatment but no Cu 
particles were deposited) are also reported in Table 2, in order to evaluate the catalytic influence of the CNTs 
on the whole activity. 
As it can be observed, CNTs-based electrode itself is able to convert CO2 towards formic acid and acetic acid 
with a production rate of 1.29 and 0.88 mol in 4 h, respectively, but no methanol was produced. The activity 
of the oxidized CNTs can be ascribed to the presence of oxygen functional groups of different kinds (ketone, 
carboxylic and carbonyl groups) created by the oxidative pre-treatment near defects and edges on the surface 
of CNTs (Ampelli et al., 2014). Their presence is necessary to anchor strongly the Cu NPs on CNTs. 
Consequently, these oxygen functional groups may act as electron trapping sites favouring the reduction of 
the adsorbed species (Wang and Su, 2014). However, Cu/CNTs/GDL electrode allowed obtaining also traces 
of ethanol, acetone and isopropanol. The formation of these products can be ascribed to the adsorption of the 
reactant species on the catalytic sites located in the interface between the small copper particles and the CNT 
surface, favouring electron transfer and C-C bond formation (Genovese et al., 2017). 

4. Conclusions 

In this work, different electrodes based on highly ordered TiO2 nanotube arrays and Cu nanoparticles 
deposited on nano-carbons were tested in the process of water photo-electrolysis and/or photo-reforming of 
organic wastes for the production of H2, as well as in the process of CO2 reduction to liquid fuels. The results, 
which have been given in terms of H2 production, solar-to-hydrogen (STH) efficiency and liquid productivity, 
are very promising. 
We also discussed about the importance of the choice of materials to be used for the catalytic electrodes, 
which depends on several constraints such as light harvesting, charge separation, electron conductivity, 
proton and mass diffusion, productivity and selectivity. These issues are not usually taken into account in the 
engineering design of PECa cells, but are strictly related to the catalytic materials used. In general, the most 
important factors to consider refer to i) the need of having a compact design, ii) the separation of the anodic 
and cathodic zones, iii) the robustness of the cell and iv) the continuous flow operation (for an easy recovery 
of products). The development of nanostructured electrodes can highly favour these factors, by increasing the 
active area and limiting the overpotential of the cell. 
In conclusion, the photo-electrocatalytic (PECa) approach has been demonstrated as an attractive technology 
for a future sustainable production of energy, fuels and chemicals. The PECa approach may be adopted not 
only for the production of solar fuels, but also for other applications, i.e the activation of small molecules, such 

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as the synthesis of ammonia starting from N2 and H2O, as we recently demonstrated (Chen et al., 2017) as a 
novel process using C-based electrodes without the need of doping with noble metals. 

Acknowledgments 

The authors acknowledge the PRIN 2015 Project (Solar driven chemistry: new materials for photo- and 
electro-catalysis - 2015K7FZLH), which has partially supported this work. 

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