Unique catalytic properties of a butoxy chain-containing ruthenated porphyrin towards oxidation of uric acid and reduction of dioxygen for visible light-enhanced fuel cells

Article abstract of DOI:10.1016/j.electacta.2016.06.147

This paper reports the photoelectrocatalytic activities of a ruthenated porphyrin [Ru(phen)₂(IP-C₄O-TPP)]²⁺ (denoted as Ru(II)PTPP, phen = 1,10-phenanthroline, IP = imidazo[4,5-f][1,10]phenanthroline and TPP = 5,10,15,20-tetraphenylporphyrin) containing a covalently-linked butoxy chain (-C₄O-) between IP and TPP moieties by means of various electrochemical techniques in combination with absorption spectroscopy and scanning electronic microscopy. Ru(II)PTPP is assembled on the surface of CdS nanoparticles, showing two Ru(II)-based peaks at 0.296 V and 0.830 V, where uric acid (UA) can be photoelectrocatalytically oxidized in a linear range of 0.01-10.0 mmol L^?1. The ?C₄O- chain endows the Ru(II)PTPP/carbon felt (CF) electrode with favorable dioxygen (O₂) binding sites to achieve a couple of new redox peaks at ?0.213 V, where O₂ involves electrocatalytic reduction reactions. While employing 5.0 mmol L^?1 UA as fuel, and 60 mL min^?1 O₂ as oxidant, the proposed photoelectrochemical fuel cell shows open-circuit photovoltage of 0.656 V, short-circuit photocurrent density of 0.136 mA cm^?2, and maximum power density of 31.50 μW cm^?2 at 0.497 V under visible-light illumination of 0.18 mW cm^?2. The present study provides an interesting platform for the utilization of renewable energy sources.

Full text of DOI:10.1016/j.electacta.2016.06.147

Electrochimica Acta 212 (2016) 113–121
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Unique catalytic properties of a butoxy chain-containing ruthenated
porphyrin towards oxidation of uric acid and reduction of dioxygen for
visible light-enhanced fuel cells
a
a
a
a
b
a,
Junchen Liu , Yi Wang , Qiang Deng , Licai Zhu , Hui Chao , Hong Li *
a
Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry and Environment, South China Normal University,
Guangzhou 510006, PR China
b
Department of Chemistry, Sun Yat-Sen University, Guangzhou 510275, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 4 May 2016
Received in revised form 27 June 2016
Accepted 28 June 2016
2 4
This paper reports the photoelectrocatalytic activities of a ruthenated porphyrin [Ru(phen) (IP-C O-
TPP)] (denoted as Ru(II)PTPP, phen = 1,10-phenanthroline, IP = imidazo[4,5-f][1,10]phenanthroline and
2
+
TPP = 5,10,15,20-tetraphenylporphyrin) containing a covalently-linked butoxy chain (-C O-) between IP
4
Available online 29 June 2016
and TPP moieties by means of various electrochemical techniques in combination with absorption
spectroscopy and scanning electronic microscopy. Ru(II)PTPP is assembled on the surface of CdS
Keywords:
Ruthenium(II) polypyridyl complex
Porphyrin
Uric acid
Dioxygen
nanoparticles, showing two Ru(II)-based peaks at 0.296 V and 0.830 V, where uric acid (UA) can be
ꢀ1
photoelectrocatalytically oxidized in a linear range of 0.01-10.0 mmol L . The ꢀC
4
O- chain endows the
2
Ru(II)PTPP/carbon felt (CF) electrode with favorable dioxygen (O ) binding sites to achieve a couple of
new redox peaks at ꢀ0.213 V, where O
2
involves electrocatalytic reduction reactions. While employing
ꢀ1
ꢀ1
5
.0 mmol L UA as fuel, and 60 mL min
2
O as oxidant, the proposed photoelectrochemical fuel cell
Photoelectrochemical fuel cell
ꢀ2
shows open-circuit photovoltage of 0.656 V, short-circuit photocurrent density of 0.136 mA cm , and
ꢀ2
ꢀ2
maximum power density of 31.50
mW cm at 0.497 V under visible-light illumination of 0.18 mW cm
.
The present study provides an interesting platform for the utilization of renewable energy sources.
ã 2016 Elsevier Ltd. All rights reserved.
1
. Introduction
paid to the fabrication of photoelectrochemical fuel cells using
various photosensitizers such as mono- and multi-nuclear
ruthenium(II) polypyridyl complexes [13,14], different shaped
porphyrin arrays [15,16], and ruthenated porphyrins [17]. Among
these photosensitizers, a series of cationic ruthenated porphyrins
has attracted considerable interest because they possess charac-
teristic metal-to-ligand charge transfer (MLCT) transitions of Ru(II)
polypyridyl complexes, and strong Soret and Q absorption bands of
porphyrin moieties in visible light regions [18]. The majority of the
ruthenated porphyrins are usually formed by the coordination or
covalent linking of Ru(II) polypyridyl complexes with porphyrin
moieties [19,20]. Some of them have been demonstrated to be
appropriate photosensitizers with good photocatalytic and elec-
trocatalytic activities [21,22]. The investigations into this kind of
ruthenated porphyrins and their applications in photoelectro-
chemical fuel cells are still ongoing [23].
The conversion of solar energy and/or biomass energy to
electricity is very important for the development and utilization of
renewable energy [1,2]. Sunlight is converted either into chemical
energy by photosynthesis pathways or into electricity by dye-
sensitized solar cells and other types of photovoltaic systems [3].
The biological fuels can be oxidized to produce electricity by a
microbial or enzymatic fuel cell [4–6]. Alternatively, a photo-
electrochemical fuel cell is considered as a combination of dye-
sensitized solar cell with enzymatic biofuel cell or non-enzymatic
fuel cell, exhibiting crucial advantages for the renewable energy
conversion [7–10]. The photosensitizers used in photoelectro-
chemical fuel cells are usually assembled on the surfaces of
2 2
traditional semiconductors (e.g. TiO , SnO , ZnO, CdS), and play an
important role in the photoelectrocatalytic oxidation process of
added fuels [11,12]. In the past decades, much attention has been
Uric acid (UA) is an enzymatic end product from purine
derivatives in human metabolism [24]. The abnormal level of UA in
serum and urinary excretion is known to reflect purine metabolism
disorders, leading to an acute attack of gout disease along with
increasing risk for the development of nephrolithiasis and
*
Corresponding author. Fax: +86 20 39310187.
E-mail address: lihong@scnu.edu.cn (H. Li).
http://dx.doi.org/10.1016/j.electacta.2016.06.147
013-4686/ã 2016 Elsevier Ltd. All rights reserved.
0

114
J. Liu et al. / Electrochimica Acta 212 (2016) 113–121
hyperuricemia [25], therefore the assay and reuse of UA become
very important [26]. So far, the electrochemical oxidation of UA has
been widely investigated using various electrodes modified with
uricase and peroxidase enzymes [27], conducting polymer [28],
metal nanoparticles and nanoalloys [29], carbon nanomaterials
found that the flexible butoxy chain endows Ru(II)PTPP with
multifunctional catalytic properties, which can be applied to the
fabrication of visible light-enhanced UA fuel cells.
2. Experimental section
[30], etc. However, the electrochemical oxidation of UA always
undergoes pronounced fouling effects, exhibiting poor reproduc-
ibility [31], thereby there is an increasing attempt to promote the
oxidation of UA [32].
2.1. Chemicals and materials
Tris(hydroxylmethyl)aminomethane (Tris) from Sigma was
used to prepare a buffer solution with doubly distilled water,
In our previous studies, a [Ru(tatp)
3
]2+ (tatp = 1,4,8,9-tetra-aza-
ꢀ1
ꢀ1
triphenylene) modified ZnO nanoparticle electrode has been used
to improve the oxidation of UA, showing good reproducibility
consisting of 0.01 mol L Tris/0.05 mol L NaCl (pH 7.2). Cadmi-
um chloride (CdCl ), sodium thiosulphate (Na ) and uric acid
2
2 2 3
S O
[
33,34]. However, the need for ultraviolet (UV) light limits its
(UA) were used as received. UA was dissolved with buffer solutions
(pH 12.4), in which singly charged urate ions were formed [42].
Prior to each measurement, the test solution was adjusted to pH
7.2, at which the predominant tautomer of UA was a triketo form
[43]. Ru(II)PTPP was synthesized following previously reported
procedures [44], and its structure was shown in Fig. 1.
practical applications in photoelectrochemical fuel cells. A great
deal of attention is being paid to anionic and cationic ruthenated
porphyrins because of their unique photophysical and electro-
chemical properties [35], DNA-photocleavage activities [36], and
electrocatalysis towards the oxidation of organic molecules [37].
Simultaneously, some of ruthenated metal porphyrins have been
found to have electrocatalytic activities towards the reduction of
2.2. Instruments and methods
2
dioxygen (O ) [38,39]. To date, despite the fact that spectral,
structural and electrochemical properties of many ruthenated
porphyrins and their applications have been documented [40,41],
there are no reports on the photoelectrocatalytic oxidation of
organic molecules and electrocatalytic reduction of dioxygen by
ruthenated porphyrins with a flexible covalently- linked alkoxy
chain, in particular, for visible light-enhanced fuel cells.
Voltammetric measurement was performed on a CHI660a
electrochemical system (Shanghai, China). Unless otherwise noted,
the modified electrode was employed as the working electrode,
while using a titanium sheet counter electrode and an Ag/AgCl
reference electrode with 0.05 mol L NaCl salt bridge. Ultraviolet
(UV)-visible absorption spectra were performed by a UV-1700
spectrophotometer (Shimadzu, Japan). A Zeiss Ultra55 field-
emission scanning electron microscope (SEM, Germany) was
employed to analyze the surface appearance of the modified
electrode.
ꢀ
1
In the current work, we expect to investigate the photo-
electrocatalytic oxidation of UA and electrocatalytic reduction of
O
2
by [Ru(phen)
butoxy chain (-C
phenanthroline, IP = imidazo[4,5-f][1,10]phenanthroline and TPP =
,10,15,20-tetraphenylporphyrin), denoted as Ru(II)PTPP. An im-
portant attempt is made to evaluate the photoelectrocatalytic
oxidation of UA in the absence and presence of O by Ru(II)PTPP
assembled on a CdS nanoparticle modified indium-tin oxide (ITO)
electrode, as well as the electrocatalytic reduction of O by Ru(II)
PTPP immobilized on a carbon felt (CF) electrode. It is interestingly
2
(IP-C
4
O-TPP)]2 containing a covalently-linked
+
4
O-) between IP and TPP moieties (phen = 1,10-
5
2.3. Preparation of Ru(II)PTPP/CdS/ITO and Ru(II)PTPP/CF electrodes
2
The CdS/ITO electrode was prepared following previously
reported electrochemical deposition procedures [45]. Briefly, an
ꢀ
1
2
indium-tin oxide (ITO) sheet (10 V sq , Nanbo Co., Shenzhen,
China) was employed as the working electrode to prepare a CdS
[(Fig._1)TD$FIG]
Fig. 1. Schematic illustration of visible light-enhanced UA/O
II)PTPP.
2
fuel cell employing Ru(II)PTPP/CdS/ITO photoanode and Ru(II)PTPP/CF cathode. Inset shows the structure of Ru
(

J. Liu et al. / Electrochimica Acta 212 (2016) 113–121
115
[(Fig._2)TD$FIG]
ꢀ1
6
0 mL min
O
2
, respectively. The non-conductive back side of
the photoanode faced a tungsten halogen lamp mostly located
ꢀ
2
between 400 and 1100 nm with power density of 0.18 mW cm
.
All the measurements were performed at room temperature
ꢁ
(
25 ꢃ 27 C).
3
. Results and discussion
3
.1. Ru(III)PTPP/Ru(II)PTPP-based electrochemical behavior
Cyclic voltammograms of Ru(II)PTPP/CdS/ITO electrode in
buffer solutions are given in Fig. 2a, showing a pair of Ru(III)/Ru
II)-based redox peaks (peak I) [48]. The anodic peak height
linearly increases with increasing scan rate as depicted by the inset
(
2
ꢀ1
ꢁ0
a1 of Fig. 2a (R = 0.990). At 0.1 V s , the formal potential (E ) of Ru
II)PTPP/CdS/ITO electrode is 0.886 V, taken as an average of anodic
peak potential (Ep,a, 0.898 V) and cathodic peak potential (Ep,c
.828 V), yielding a peak separation ( ) of 70 mV. The results
suggest a surface-controlled electrochemical process for the Ru
(
,
0
DE
p
(
[
III)/Ru(II)-based redox reactions of Ru(II)PTPP/CdS/ITO electrode
49], which did not show the peak height change by more than 3%
after repetitive cycling for up to 20 min in buffer solutions.
According to the Laviron equation for diffusionless voltammo-
grams [50], the corresponding scan rate-dependent data are
shown in the inset a2 of Fig. 2a. While the electron transfer
coefficient is assumed to be 0.5, the heterogeneous electron
ꢀ
1
ꢁ0
f
transfer rate constant (k ) is calculated to be 1.15 cm s at E ,
suggesting a fast electron transfer process [51]. In the absence of
CdS nanoparticles (Fig. 2b), the Ru(II)PTPP/ITO electrode shows the
basically consistent voltammograms with the Ru(II)PTPP/CdS/ITO
Fig. 2. Cyclic voltammograms of Ru(II)PTPP/CdS/ITO (a) and Ru(II)PTPP/ITO (b) in
ꢀ1
buffer solutions at different scan rates (V s ): (1) 0.6, (2) 0.5, (3) 0.4, (4) 0.3, (5) 0.2,
6) 0.1, (7) 0.05, (8) 0.02, (9) 0.01. Inset shows the oxidative peak current (Ip,a, a1, a2)
or peak potential (Ep,a b1, b2) as a function of scan rate ( ).
f
electrode. However, the k value from Ru(II)PTPP/ITO electrode is
(
[3_TD$IFF]
,
[
4
_
T
D
$
D
I
F
F
]
1
_
T
D
$
D
I
F
F
]
y
[F(ig._3)TD$FIG]
film by 100-cycle potential sweeping between ꢀ0.3 and ꢀ0.9 V
ꢀ1
with 0.05 V s scan rate in a homemade three-electrode cell filled
ꢀ1
ꢀ1
2 2 2 3
with 0.01 mol L CdCl /0.1 mol L Na S O (pH 2.0), while using a
titanium sheet as counter electrode, and a saturated calomel
electrode (SCE) as the reference electrode. The resulting CdS/ITO
ꢁ
electrode was kept at a constant temperature of 40 C for 2 h, and
its average size and thickness were identified to be 35 nm and
168 nm by SEM measurements, respectively. A carbon felt (CF)
sheet (Tianjin Kairuifeng Electrical Carbon Co., China) with a
thickness of 0.2 cm was employed as the electrode substrate, which
facilitated the oxygen permeation. Ru(II)PTPP was subsequently
ꢀ
1
immobilized by dropping 15 mL of 0.025 mmol L Ru(II)PTPP in
DMSO onto the surfaces of CdS/ITO and CF electrodes (0.7 cm
ꢂ 0.8 cm), and then evaporating the solvent at a constant
ꢁ
temperature of 40 C for 12 h, yielding Ru(II)PTPP/CdS/ITO and
Ru(II)PTPP/CF electrodes, respectively [46]. For comparison, the
2
+
identical operation was used to prepare a [Ru(bpy)
2
(IP)] /CdS/ITO
0
electrode (bpy = 2,2 -bipyridine and IP = imidazo[4,5-f][1,10]phe-
nanthroline) [47].
2.4. Assembly of visible light-enhanced UA/O
2
fuel cell
The photoelectrochemical UA/O fuel cell was schematically
2
shown in Fig. 1. Ru(II)PTPP/CdS/ITO and Ru(II)PTPP/CF electrodes
were employed as photoanode and cathode, both of which were
placed to two quartz compartments separated by a Nafion 117
proton exchange membrane (DuPont). The anodic and cathodic
compartments were filled with 2.0 mL of buffer solutions
ꢀ1
containing nitrogen-saturated 5.0 mmol L
UA and injecting
Fig. 3. SEM images of Ru(II)PTPP/CdS/ITO (a) and CdS/ITO (b) electrodes.

116
J. Liu et al. / Electrochimica Acta 212 (2016) 113–121
[(Fig._4)TD$FIG]
TPP is covalently linked to the [Ru(phen)
O- chain, thereby its electron-donating ability is weak [54].
Consequently, Ru(II)PTPP exhibits a couple of redox peaks at a
more positive electrode potential than that of [Ru(bpy) (M-
PyTMPP)Cl] (Fig. 4a). Another issue is the presence of peak II at
(IP-)]2+ moiety by a
2
ꢀ
C
4
2
+
2
+
a more negative potential than those of [Ru(tatp)
3
]
and [Ru
+
(
bpy) (MPyTMPP)Cl] . The IP-C O-TPP ligand contains a flexible
2
4
covalently-linked alkoxy chain, which provides a chance for its
folding [55,56], leading to an increase in the adsorption intensity of
a small amount of Ru(II)PTPP.
3
.2. Electrocatalytic oxidation of UA on Ru(II)PTPP/CdS/ITO electrode
ꢀ1
In the presence of 5.0 mmol L UA, differential pulse voltam-
mograms of Ru(II)PTPP/CdS/ITO electrode in a dark box show two
anodic peaks at 0.848 V and 0.306 V (curve 3 in Fig. 4a), for which
the peak heights are far higher than those without UA. In contrast,
differential pulse voltammograms of UA on CdS/ITO electrode only
exhibit an anodic peak at 0.852 V. A comparison of Ru(II)PTPP/CdS/
ITO and CdS/ITO electrodes in the absence and presence of UA
suggests the electrocatalytic oxidation of UA by Ru(II)PTPP. The
onset potential for UA oxidation on the Ru(II)PTPP/CdS/ITO
2
+
3
electrode is more negative than that on [Ru(tatp) ] /ZnO/ITO
electrode reported in our previous studies [33], exhibiting better
electrocatalytic activity for UA oxidation. While increasing UA
ꢀ
1
concentration (CUA) in the range from 0.005 to 10.0 mmol L , the
anodic peak current heights exhibit a linear increase (curve 1 in
Fig. 5), suggesting the electrocatalytic oxidation of UA by Ru(II)
PTPP in the dark as described by Eqs. (1)–(2).
ꢀ
Fig. 4. Differential pulse voltammograms of Ru(II)PTPP/CdS/ITO (a) and CdS/ITO (b)
RuðIIÞPTPP ꢀ e @RuðIIIÞPTPP
ð1Þ
ꢀ1
electrodes in the absence (1,3) and presence (2, 4) of 5.0 mmol L UA in a dark box
1,2) or visible light irradiation (3,4).
(
RuðIIIÞPTPP þ UA! RuðIIÞPTPP þ UAOx
ð2Þ
0
.34 cm sꢀ1, which is smaller than that from Ru(II)PTPP/CdS/ITO
[F(ig._5)TD$FIG]
electrode, implying that the addition of CdS nanoparticles
facilitates the electron transfer between Ru(II)PTPP and underlying
electrode. The results from the SEM images (Fig. 3) reveal that Ru
(
II)PTPP is uniformly distributed on the surfaces of CdS nano-
particles, which offer a three-dimensional support structure to
promote the electron transfer between Ru(II)PTPP and ITO
substrate via the CdS layer [46].
It is worthy of notice that differential pulse voltammograms of
Ru(II)PTPP/CdS/ITO electrode in contrast to CdS/ITO electrode in a
dark box show two anodic peaks (I and II) at 0.830 V and 0.296 V
(
Fig. 4), where the weak peak II has a negative shift of 0.534 V in
contrast to peak I. To explain the phenomenon, Ru(II)PTPP is
compared with the previously reported Ru(II) complexes such as
2
+
0
[
Ru(bpy)
phenanthroline) [47], [Ru(tatp)
nylene) [33], and [Ru(bpy) (MPyTMPP)Cl] (MPyTMPP = 5-(4-
2
(IP)] (bpy = 2,2 -bipyridine and IP = imidazo[4,5-f][1,10]
2+
3
]
(tatp = 1,4,8,9-tetra-aza-triphe-
+
2
pyridyl)-10,15,20-tris(4-metylphenyl) porphyrin) [52]. The finding
is as follows. (i) In the absence of TPP (TPP = 5,10,15,20-
(IP)]2 only showed redox peaks
+
tetraphenylporphyrin), [Ru(bpy)
2
controlled by diffusion processes at more than 1.0 V. (ii) [Ru
2
+
(
tatp)
3
]
can be electrochemically assembled on the ITO electrode,
showing a pair of redox peaks at 0.458 V controlled by surface
+
2
electrochemical processes. (iii) [Ru(bpy) (MPyTMPP)Cl] can be
directly adsorbed to the ITO surface, indicating a pair of surface-
controlled redox peaks at 0.790 V. According to the results, there
are two issues to be considered. One is that the immobilization of
Ru(II)PTPP is mainly based on the interactions of TPP with CdS
nanoparticles or ITO surface. TPP has potential to serve as electron
donors to increase the electron density of Ru(II) centers, allowing
the ruthenated porphyrins to involve the Ru(II)-based redox
reactions at a relatively low electrode potential [53]. However, the
Fig. 5. Electrocatalytic oxidation currents (
electrode for peaks I (a) and II (b) as a function of UA concentration (CUA) under
dark (1) or visible light irradiation (2) conditions.
D
I) of UA on Ru(II)PTPP/CdS/ITO

J. Liu et al. / Electrochimica Acta 212 (2016) 113–121
117
[F(ig._7)TD$FIG]
3.3. Photoelectrocatalytic oxidation of UA
While turning on the light, as depicted by Figs. 4 and 5, the
catalytic currents or linear slopes are enhanced, suggesting that
the visible light irradiation facilitates the oxidation of UA. In the
absence of Ru(II)PTPP, the anodic electric field extracts the excited
electrons from conducting band of CdS, yielding photocurrent
response from CdS/ITO electrode and visible light-enhanced
oxidation current response of UA (Fig. 4b). Similarly, the visible
irradiation can promote Ru(II)PTPP to generate Ru(III)PTPP, which
oxidizes UA to regenerate Ru(II)PTPP, leading to an increase in the
catalytic oxidation of UA. To further demonstrate the contribution
of the light irradiation to the oxidation of UA, Fig. 6 gives
absorption spectra of Ru(II)PTPP/CdS/ITO, Ru(II)PTPP/ITO and CdS/
ITO electrodes in visible wavelength regions, showing metal-to-
ligand charge transfer (MLCT) transition characteristics of [Ru
2
+
(
phen)
2
(IP-)] moiety at 455 nm, Soret and Q bands of TPP moiety
Fig. 7. Effects of increasing UA concentration on open-circuit photovoltage (VOCP) of
Ru(II)PTPP/CdS/ITO electrode vs. Ag/AgCl electrode upon visible light irradiation
under nitrogen (1) or oxygen (2) atmospheres. The line 3 was obtained by
subtracting line 2 with line 1.
at 430, 560 and 610 nm, as well as characteristic absorbance band
of CdS [54]. The result suggests that Ru(II)PTPP can absorb the
visible light to form [Ru(phen) (IP-)] and TPP-based excited
2
2+
*
*
states, i.e. Ru(II)P TPP and Ru(II)PTPP , which are expressed as
electron donors that sustain the integrity of the CdS and Ru(II)PTPP
sensitizers [7,59]. Consequently, UA is photoelectrocatalytically
oxidized on the Ru(II)PTPP/CdS/ITO anode as depicted by Eqs. (4)–
Eq. (3).
RuðIIÞPTPP@h RuðIIÞP TPP þ RuðIIÞPTPP
n
ꢄ
ꢄ
ð3Þ
(
5).
*
To illustrate the electron transfer among Ru(II)P TPP, Ru(II)
PTPP , and CdS conduction band, their energy levels vs. NHE are
compared. The previously reported results from voltammetric
measurements have shown that the excited MLCT state potential of
*
ꢄ
ꢀ
RuðIIÞP TPP þ CdS@RuðIIIÞPTPP þ CdSðe Þ
ð4Þ
ð5Þ
2
+
[Ru(phen)
2
(IP-)]
is ꢀ1.24 V [44], which is above the CdS
RuðIIIÞPTPP þ UA!RuðIIÞPTPP þ UAOx
conduction band edge (ꢀ0.95 V) [57], ensuring electron injection
into the CdS conduction band. The excited state potential of TPP
was estimated to be ꢀ0.87 V [44], which is lower than the energy
3
2
.4. Electrocatalytic reduction of O on Ru(II)PTPP/CF electrode
*
level of Ru(II)P TPP. However, the intramolecular photoinduced
A number of metal porphyrins have been considered as
potential electrocatalysts for the reduction of O [60,61]. In this
study, it is interesting to know whether Ru(II)PTPP could have the
electrocatalytic activity towards the reduction of O . To facilitate
the immobilization of Ru(II)PTPP and the mass transport of O , a
electron transfer is difficult to carry out because of weak electronic
interaction between TPP and Ru(II) polypyridyl moiety by the
2
*
ꢀ
C
4
O- covalent linking [54]. In addition, Ru(II)PTPP exhibits an
2
excited state potential of only about 0.08 V below the CdS
conduction band, leading to relatively slow electron transfer
kinetics [58].
The open-circuit photovoltage (VOCP) of Ru(II)PTPP/CdS/ITO
electrode vs. Ag/AgCl electrode is also measured by changing UA
concentration. As shown in Fig. 7, the VOCP vs. logCUA plot under a
nitrogen atmosphere exhibits a linear increase as described by
2
carbon felt (CF) sheet is chosen as the supporting substrate. While
adding a Ru(II)PTPP/CF electrode to a nitrogen-saturated buffer
solution, the cyclic voltammograms show a pair of distinct redox
ꢁ
0
peaks at E = –0.213 V (peak III), whereas no corresponding redox
response is observed for the bare CF electrode in a same potential
range (Fig. 8). While further purging with pure nitrogen or using Ru
ꢀ1
2
V
OCP,N2/V = 0.571 + 0.0289log(CUA/mmol L ) (R = 0.988), suggest-
(
II)PTPP/ITO instead of Ru(II)PTPP/CF electrode, the peak III
*
ing that Ru(II)PTPP absorbs the visible light to form Ru(II)P TPP,
which releases the electron via the CdS layer to produce Ru(III)
PTPP used to oxidize UA. Simultaneously, UA acts as sacrificial
disappears. According to the molecular structure of Ru(II)PTPP, it
is anticipated to have a large stacking among TPP, IP and CF surface,
which favors the strong binding to O
(peak III) may be ascribed to redox reactions of O
II)PTPP/CF electrode. While nitrogen is changed to the oxygen
atmosphere, the cathodic peak is largely enhanced, suggesting the
electrochemical reduction of dissolved O . It is worthy to note that
the redox peak current height of O on the Ru(II)PTPP/CF electrode
2
. Therefore, the redox peaks
[
(
F
i
g
.
_
6
)
T
D
$
F
I
G
]
2
bound to the Ru
(
2
2
is larger than that on the bare CF electrode, revealing the
electrocatalytic activity of Ru(II)PTPP towards the reduction of
O
2
. This proposition is further demonstrated by the open-circuit
voltage (VOCV) measurement. As depicted by Fig. 9, VOCV shows a
linearly positive shift in the O permeation flux rate ranged
2
ꢀ1
2
between 10 and 60 mL min (R = 0.998).
2
To throw light on the binding of Ru(II)PTPP to O , the nitrogen
atmosphere is changed to oxygen, and an interesting phenomenon
is observed. VOCP is found to linearly decrease with increasing CUA
(
Fig. 7), and the regression equation is described as VOCP,O2/
ꢀ1
2
V = 0.348–0.218(CUA/mmol L ) (R = 0.992). Combined with the
previous reported result [44], there exists the energy transfer
between photoexcited Ru(II)PTPP and molecular oxygen to
Fig. 6. Absorption spectra of Ru(II)PTPP/CdS/ITO (1), Ru(II)PTPP/ITO (2) and CdS/ITO
3) electrodes.
(

118
J. Liu et al. / Electrochimica Acta 212 (2016) 113–121
[
(
F
i
g
.
_
8
)
T
D
$
F
I
G
]
[(Fig._9)TD$FIG]
Fig. 8. Cyclic voltammograms of Ru(II)PTPP(O
nitrogen-saturated (dashed line 1) or oxygen-saturated (solid line 2) buffer
solutions.
2
)/CF (a) and CF (b) electrodes in
Fig. 9. (a) Open-circuit voltage (VOCV) of Ru(II)PTPP/CF electrode vs. Ag/AgCl in
buffer solutions upon increasing oxygen flux rate (JO2): (1) 0, (2) 10, (3) 20, (4) 30, (5)
40, (6) 50, (7) 60 mL min ; (b) VOCV vs. JO2 plot.
ꢀ
1
1
generate singlet oxygen ( O
2
) [54], which is further consumed by
UA [62], making the photoexcited Ru(II)PTPP quenched. After
subtracting the influence of the Ru(III)-assisted UA oxidation, the
ꢀ
þ
RuðIIÞPTPP=CFðH
2
O
2
Þþ 2e þ 2H @ RuðIIÞPTPP=CF þ 2H O ð8Þ
2
linearly changed plot is obtained as described by
OCP
DV /
ꢀ1
2
V = ꢀ0.212–0.050log(CUA/mmol L ) (R = 0.990), suggesting that
3.5. Performance of visible light-enhanced UA fuel cell
1
2
UA can act as a scavenger against O generated from the Ru(II)
PTPP/CdS/ITO photoanode. The result provides a new method for
investigating the anti-oxidative activity of reducing agents in
biological fluids. Additionally, although the photocurrent response
The results mentioned above have shown that the Ru(II)PTPP/
CdS/ITO electrode can absorb the characteristic light to generate
the photovoltage and photocurrent when irradiated. As illustrated
*
from the Ru(II)PTPP/CdS/ITO anode under a pure nitrogen
by Fig. 1 and Eqs. (3)–(8), the excited Ru(II)P TPP under a nitrogen
2
atmosphere is larger as expected than those of [Ru(bpy)
2
(IP)]
with air or O
more remarkable decrease in the
photocurrent response (Fig. 10), suggesting that there exists a
high affinity of Ru(II)PTPP to the dissolved O
On the other hand, to further verify the electrocatalytic
reduction product, O is replaced with H to measure the
differential pulse voltammograms (Fig. 11), which show a linearly
increasing peak current with increasing H concentration
R = 0.995), demonstrating that the intermediate products
) can be electrocatalytically reduced to H O. Thus, the
atmosphere can be transformed into Ru(III)PTPP while injecting
electrons to the CdS thin layer, and the resulting Ru(III)PTPP
oxidizes UA to regenerate Ru(II)PTPP. In the light of a series of light-
dependent Ru(III)-mediated reactions, which are responsible for
the oxidation of UA and the injection of electrons, the Ru(II)PTPP/
CdS/ITO electrode is proposed as photoanode, and UA as
appropriate fuels. The Ru(II)PTPP/CdS/ITO anode under a nitrogen
atmosphere upon light excitation could transfer the electrons of
UA to involve the reduction of oxygen on the Ru(II)PTPP/CF cathode
via a visible light-enhanced UA fuel cell as described by Eq. (9).
+
/
CdS/ITO and CdS/ITO anodes, the replacement of N
2
2
atmosphere leads to
a
2
.
2
2 2
O
2 2
O
2
(
(
H
2
O
2
2
RuðIIÞPTPP;h
n
electrocatalytic reduction process of O
2
on the Ru(II)PTPP/CF
UA þ O2
!
UAOxþ H Oð9Þ
2
electrode is expressed as Eqs. (6)–(8).
Fig. 12 shows the photocurrent-photovoltage characteristic
curve of the assembled visible light-enhanced fuel cell at room
temperature. It can be seen from the experiments that the short-
RuðIIÞPTPP=CF þ O @RuðIIÞPTPP=CFðO
2
Þ
ð6Þ
2
ꢀ
2
circuit photocurrent density (ISC) is 0.136 mA cm , which is
smaller than those for photoelectrochemical ethanol fuel cells in
alkaline media [8,63], whereas it is greater than that of the UA fuel
ꢀ
þ
RuðIIÞPTPP=CFðO
2
Þþ 2e þ 2H @RuðIIÞPTPP=CFðH
2
O
2
Þ
ð7Þ
2
+
3
cell using [Ru(tatp) ] /ZnO/ITO photoanode [33]. When the

[(Fig._11)TD$FIG]
J. Liu et al. / Electrochimica Acta 212 (2016) 113–121
119
[(Fig._10)TD$FIG]
Fig. 11. (a) Differential pulse voltammograms of Ru(II)PTPP/CF electrode in buffer
solutions in the absence (1) and presence of (2) 0.05, (3) 0.2,(4) 0.5, (5) 0.8, (6)
ꢀ
1
1.0 mmol L
2 2
H O ; (b) Electrocatalytic reduction peak current (DIp,c) as a function
of H concentration (CH2O2).
2 2
O
4
. Conclusions
In summary, Ru(II)PTPP/CdS/ITO and Ru(II)PTPP/CF electrodes
show unique catalytic activities for the oxidation of UA and the
reduction of oxygen. The Ru(II)PTPP/CdS/ITO electrode under
nitrogen and oxygen atmospheres exhibits good photoelectroca-
talytic activity towards the oxidation of UA. The butoxy chain
endows the Ru(II)PTPP/CF electrode with favorable O
to achieve a couple of new redox peaks at ꢀ0.213 V, in which O
involves electrocatalytic reduction reactions. visible
2
binding sites
2
A
[(iFg._12T)ID$F]G
Fig. 10. Photocurrent changes of Ru(II)PTPP/CdS/ITO (a), [Ru(bpy)
2
(IP)]2+/CdS/ITO
b) and CdS/ITO (c) electrodes vs. Ag/AgCl in buffer solutions containing
(
ꢀ
1
5
.00 mmol L UA due to visible light irradiation under nitrogen (1), air (2) or
oxygen (3) atmospheres.
photovoltage is 0.497 V, the assembled UA fuel cell yields a
ꢀ
2
maximum power density (Pmax) of 31.50
approximate to those of glucose/O fuel cell with TCPP/TiO
or Zn-mesoporphyrin IX/TiO /FTO photoanode (TCPP = meso-
tetrakis(4-carboxyphenyl)porphyrin and FTO = fluorine-doped tin
m
W cm , which is
2
2
/FTO
2
2
ꢀ
oxide) [64,65], and SO
3
/H
/ITO photoanode [66], and is far larger as expected than
those of photoelectrochemical glucose fuel cell employing TiO
ITO anode and Pt cathode [67], as well as some enzymatic glucose/
biofuel cells [68,69]. The results suggest that Ru(II)PTPP has
bifunctional catalysis towards the photoelectrocatalytic oxidation
of UA and electrocatalytic reduction of O , which are very feasible
to fabricate visible light-enhanced UA fuel cells.
2 2 2
O fuel cell with [Ru(bpy) (MPyTMPP)
+
Cl] /TiO
2
2
/
O
2
ꢀ1
ꢀ1
Fig. 12. Cell performances of nitrogen-saturated 5.0 mmol L UA/60 mL min
O
2
2
fuel cell employing Ru(II)PTPP/CdS/ITO anode and Ru(II)PTPP/CF cathode under
visible light irradiation.

120
J. Liu et al. / Electrochimica Acta 212 (2016) 113–121
ꢀ
1
light-enhanced fuel cell employing 5.0 mmol L UA as the fuel,
and 60 mL min
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-extended analogues. Synthesis, spectroelectronic, electrochemical, and
ꢀ1
p
2
O as the oxidant, shows open-circuit photo-
voltage of 0.656 V, short-circuit photocurrent density of 0.136 mA
cm , and maximum power density of 31.50 mW cm at 0.497 V.
The present results should be of value for better understanding the
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promising potential to extend into their applications in enzyme-
free photoelectrochemical fuel cells and sensors.
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