this presentation is just an introduction to the vast field of photoelectrochemical cells and it's constituents.

1. Designing synthetic
photosynthetic systems
Submitted by
Roshen Reji Idiculla
ID MS 2014/11

2. “The general struggle for existence of living beings is therefore not a fight for energy, which is
plentiful in the form of heat, unfortunately untransformably, in every body. Rather, it is a
struggle for entropy that becomes available through the flow of energy from the
hot Sun to the cold Earth. To make the fullest use of this energy, the
plants spread out the immeasurable areas of their leaves and harness the
Sun's energy by a process as yet unexplored, before it sinks down to the temperature
level of our Earth, to drive chemical syntheses of which one has no inkling
as yet in our laboratories.” Boltzmann 1886
E. Broda, Ludwig Boltzmann, Oxbow Press, Woodbridge, 1983.

3. Evolution of photosynthesis
• Primitive coloured cells evolved
mechanisms to use light energy
absorbed by their pigments to
initiate useful cell reactions.
• Green plants, with their ability to
use light energy to convert carbon
dioxide and water to carbohydrates
and oxygen, are the culmination of
this evolutionary process

4. Solar energy conversion
0.015% of the
world’s electricity
demand.
11% 0.3%
Phys. Today 60, 3, 37 (2007); http://dx.doi.org/10.1063/1.2718755

5. Fossil fuels and solar energy
conversion
• Fossil resources are stored sunlight, which was emitted by our sun millions
of years ago and converted by plants into high energy chemicals.
• Upon anaerobic fermentation, these substances were converted into the
fossil fuel resources used today.
o The principle idea of
storing sunlight in
chemical bonds can serve
as a great inspiration in
the search for alternative
energy carriers

6. Phys. Today 60, 3, 37 (2007); http://dx.doi.org/10.1063/1.2718755

7. SOLAR CONVERSION PROCESS
Reza Razeghifard-Natural and artificial photosynthesis - John Wiley and Sons

8. Photovoltaics
1. Absorption of light near the surface of the
semiconductor creates electron-hole pairs.
2. Holes (minority carriers) drift to the
surface of the semiconductor (the photo
anode) where they react with water to
produce oxygen:
2H+ + H2O ½ O2 (g) + 2H+
3. Electrons (majority carriers) are conducted
to a metal electrode (typically Pt) where
they combine with H+ ions in the electrolyte
solution to make H2 :
2e- + 2H+ H2 (g)
4. Transport of H+ from the anode to the
cathode through the electrolyte completes
the electrochemical circuit.
The overall reaction :
hn + H2O H2(g) + ½ O2 (g)
Elena A. Rozhkova, Katsuhiko Ariga (eds.)-From Molecules to Materials- Pathways to Artificial
Photosynthesis-Springer International Publishing (2015)

9. Photovoltaic Energy Efficiency
• Intrinsic efficiency limit for a solar cell
using a single semiconducting material is
31%.
– Light with energy below the bandgap
of the semiconductor will not be
absorbed
– The excess photon energy above the
bandgap is lost in the form of heat.
• Multijunction (MJ) tandem cell
– Maximum thermodynamically
achievable efficiencies are increased
to 50%, 56%, and 72% for stacks of 2,
3, and 36 junctions with
appropriately optimized energy gaps
AM1.5solarflux
(10
21
photons/sec/m
2
/m)
1 2 3 4
Energy (eV)
1
2
3
4
5
Cell 1 (Eg1)
Cell 2 (Eg2)
Cell 3 (Eg3)
Eg1 > Eg2 > Eg3
Phys. Today 60, 3, 37 (2007); http://dx.doi.org/10.1063/1.2718755

10. Photovoltaic Energy Efficiency
Single junction 31%
Silicon (crystalline) 25%
Silicon (nanocrystalline) 10%
Gallium arsenide 25%
Dye sensitized 10%
Organic 3%
Multijunction 32% 66%
Concentrated sunlight
(single junction)
28% 41%
Carrier multiplication 42%
Phys. Today 60, 3, 37 (2007); http://dx.doi.org/10.1063/1.2718755
Photovoltaic conversion efficiencies of 12-15% represent the process: sunlight to
electric power, without including any energy storage.
Laboratory best Thermodynamic limit

11. Natural photosynthesis
E m volts
-1.5
-1.0
-0.5
0
+0.5
+1.0
P680
P680*
Pheo
QA
QB
H+
OUT
P.Q(pool)
2 Fe.S
2 Cyt b6
cyt f
H+
OUT
H+
IN
Z
OEC
(Mn)
Cyt b559
H2O
PC
P700
Cyclic
pathway
Fd
FNR
Chl a1*
Chl a
Fe-S centres
O2
e
NADP
Light reactions
Proton transport across thylakoid membrane
Electron transfer
O2
e transport to O2 at reducing side of PS2

12. Possibility of formation of different
NADPH isomers during the reduction
of NADP+
Joliot,P.; Kok,B.; Bioenergetics of Photosynthesis 1975, 387-412

13. Photosynthetic efficiency and energy
losses
Available
light
energy
At sea level 100%
50% loss ,as 400nm-700nm is
photosynthetically usable
50%
20% loss as loss in reflection,
absorption and transmission of leaves
40%
77% loss representing quantum
efficiency requirement for CO2
fixation in 680nm light and
9.2%
40% loss due to respiration 5.5%
Overall
PS
efficiency
Krassen, Henning; Ott, Sascha; Physical Chemistry Chemical Physics. 2011, 13, (1): 47–57

14. Challenges in designing efficient photosynthetic systems:
Critical Issues in Research
Photon absorption
and harvesting
How do we control light
harvesting to utilize all
of the photons?
-Need to know how to
design and control
exciton transfer in
molecular systems
-Need absorbers to
harvest the bulk of the
solar spectrum
Charge separation
and transport
How do we avoid
recombination of photo-
generated charge
carriers?
-Need to overcome
geminate
recombination in
organic systems
-Need to design
transport to reduce
non-geminate
recombination in all
systems
Photocatalysis
How do we produce
fuels with the energy
provided by visible light
absorption?
-Need hetero/homo -
geneous catalytic
systems for water
splitting
-Need to couple light
absorption to catalytic
processes for C-C
bond formation

15. Perfect Light Harvesting
Systems
• high extinction coefficient
• broad spectral absorption
• long excited-state lifetime
• favourable oxidation and reduction potentials
• high photochemical stability
• synthetic tractability.

16. Where there is life, there are
porphyrins
• Tetrapyrrolic macrocycles, eg. Porphyrins, chlorins, and bacteriochlorins are
vital for life .
• Without the light harvesting and trapping activities of reduced porphyrins such
as chlorophylls and bacteriochlorophylls, there could be no photosynthesis.
• The partially reduced porphyrins, the chlorins, in conjugation with non-
transition metal magnesium form chlorophylls.
• Porphyrins enable oxygen storage (myoglobin) and transport (haemoglobin) ,
electron transport (cytochromes) and catalysis (oxygenases, peroxidases, etc.)

17. Macrocycle biosynthesis
• Porphinoid family is unique amoong range of cofactors used by nature,
uroporphyrinogen III, a wide diversity of biological functions is affected.1
• The partially reduced porphyrins, the chlorins, in conjugation with non-
transition metal magnesium form chlorophylls, whose light asorption
characterisitcs make them ideal for driving photosynthesis.
1. A.Eschenmoser, 1988, Engl. Angew.Chem.Int.Ed.,275.

18. Porphinoid family

19. Importance of magnesium
• By not being coupled to chlorin p system, excited state of chlorophyll
molecule lives long enough to perform it’s vital function of photosynthetic
charge separation.2
• Mg2+ serves to coordinate H2O and for holding special-pair chlorophyll dimers
3 that trap photonic energy.
2. P.Mathai, J.Breton, A.Vermeglio and M.Yates, 1996, FEBS Lett, 63, 171
2. I.B.Ganago, et.al. , 1982, FEBS Lett, 140, 127
3. J.E.Hunt,et.al. , 1984, J.Am.Chem.Soc, 106,2242

20. Spectroscopic
properties of chlorophylls
• The chlorophylls contain 2 major absorption
bands, one in the blue or near UV region and
one in the red or near IR region.
• The lowest excited state is relatively long-lived
(ns) and is the state that is used for electron
transfer and energy storage in photosynthesis.
• The lack of a significant absorption in the green
region gives the chlorophylls their characteristic
green or blue–green color.
• These absorption bands are 𝜋 → 𝜋∗ transitions,
involving the electrons in the conjugated 𝜋
system of the chlorin macrocycle.
Reza Razeghifard-Natural and artificial photosynthesis - John Wiley and Sons

21. Haem Biosynthesis

22. Enzymatic condensation of PBG

23. Metalloporphyrins are redox
active
• Mg porphyrinate – donate electrons, form cation radicals
• Sn(IV) porphyrinate – pick up electrons, form anion radicals
• The central ions are extremely electron rich and bind tightly to all kinds of N
ligands.
• The visible absorption and fluorescence spectra are sensitive to substitution
patterns , central metal ions and changes of environment.

24. • Acetone pyrrole belongs to class of tetrapyrrolic macrocycles in which pyrrole
units are connected by sp3 hybridized carbon bridges.
• Such compounds are porphyrinogens.
• This pyrrole is the precursor of almost all tetrapyrrole pigments in nature.
Mauzerall , 1960; fuhrhop, 1974

25. Synthesis of chlorins and
bacteriochlorins
• Porphyrin-type macrocycles have a rich pattern of absorption bands
particularly in NIR of spectra due to macrocycle aromaticity and symmetry.
• These used in light-energy conversion processes (mimicking of
photosynthetic processes4 or in electric energy production by dye-sensitized
solar cells.)
• A simpler way to produce chlorins or bacteriochlorins is through Diels–Alder
and 1,3-dipolar cycloadditions.
• Chlorins are formed as the main products and bacteriochlorins or
isobacteriochlorins are formed as side products.
4 a) M. R. Wasielewski, Acc. Chem. Res. 2009, 42, 1910–1921;
b) T. S. Balaban, H. Tamiaki, A. R. Holzwarth, Top. Curr. Chem. 2005, 258, 1–38;
c) M. Leslie, Science 2009, 323, 1286–1287;
d) V. Balzani, A. Credi, M. Venturi, ChemSusChem 2008, 1, 26– 58.

26. 1 - 2,2-dioxo-1H,3H-pyrazolo[1,5-c][1,3]thiazole
2- diazafulvenium methide
3- new 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorin derivatives
4- corresponding bacteriochlorins

27. no single chromophore is capable of efficiently powering artificial photosynthesis.

28. • Zn(II) tetraphenyl porphyrin
(ZnTPP) has a very high
extinction coefficient, but it
covers only a narrow region of
the solar spectrum
• Perylene bisimides (PBIs) have
much wider spectral coverage,
but the extinction coefficient is
lower than that of ZnTPP.
• Dye aggregates,5,6 have much broader absorption ranges.
• Broadening of absorption bands to cover the whole visible range has been
demonstrated for color pigments and applied for efficient organic photovoltaic
materials.7
5 Z. Chen, A. Lohr, C. R. Saha-Mo¨ller and F. Wu¨rthner, Chem. Soc. Rev., 2009, 38, 564–584.
6 J. L. McHale, J. Phys. Chem. Lett., 2012, 3, 587–597.
7 Z.-X. Xu, V. A. L. Roy, Z.-T. Liu and C. S. Lee, Appl. Phys. Lett., 2010, 97, 163301–163303

29. O’Regan and Gra¨tzel introduced concept of low cost, high efficiency solar cells in
1991 using a charge-transfer ruthenium dye adsorbed on a film of nanostructured
titanium dioxide (TiO2) semiconductor and an iodide/triiodide redox mediator 8.
8 B. O’Regan and M. Gra¨tzel, Nature, 1991, 353, 737–740.
TiO2 –P + hv TiO2-P* light absorption and initial charge separation by the
dye molecule
followed by
TiO2-P* TiO2(e-)-P+ charge stabilisation through rapid electron injection into
the TiO2 conduction band
2TiO2(e-)-P+ + 3I- 2 TiO2(e-)-P + I3
- dye regeneration by the iodide
Finally,
I3
- + 2e- 3 I- reduction of triiodide at the counter electrode completes the
cycle

30. Photosensitiser dyes (P)

31. • In 1997, Styring's group reported a molecule containing a sensitizer covalently
linked to a manganese complex .
• Ruthenium chromophore donate an electron to an external acceptor and oxidize a
coordinated manganese ion
• A Ru–Tyr molecular dyad could be used
to power the light driven oxidation of a
dinuclear Mn2III,III complex (Lomoth et
al., 2006).
Lomoth, R.; Magnuson, A.; Sjödin, M.; Huang, P.;
Styring, S.& Hammarström, L. (2006).
Mimicking the electron donor side of Photosystem II in
artificial photosynthesis,
Photosynth. Res., v.87, p.25–40

32. • When light is shined on [Ru(bpy)3]2+, the excited state generated,
[Ru(bpy)3]2+*, is capable of transferring an electron to a SEA such as
[Co(NH3)5Cl]2+ or Na2S2O8 and subsequently forming the strong oxidant,
[Ru(bpy)3]3+. 2,9,10
• When [Ru(bpy)3]2+ is employed in a homogeneous water reduction system,
the photogenerated [Ru(bpy)3]2+* can receive an electron from a SED
(triethanolamine) and form the strong reductant [Ru(bpy)3]+.
9 F. Puntoriero, G. La Ganga, A. Sartorel, M. Carraro, G. Scorrano, M. Bonchio and S. Campagna, Chem. Commun., 2010, 46, 4725–4727.
10 N. Kaveevivitchai, R. Chitta, R. Zong, M. El Ojaimi and R. P. Thummel, J. Am. Chem. Soc., 2012, 134, 10721–10724.
[Ru(bpy)3]2+ and its analogues -
photosensitisers for both water
oxidation and water reduction.

33. dyad molecules (D).

34. Perfect Water Oxidizing Complexes
• Metal centre easily accessible to stable higher oxidizing states
• Avoid highly charged intermediates
• Must have an available coordination site for aqua ligand
• Ligands must be oxidatively robust
• Difficult ligand substitution by water molecules

35. Mn compounds in WOC
• Usual water oxidation catalysts are based on mononuclear Ru, Ir, and Fe and
dinuclear Ru and Mn and tetranuclear Ru, Mn, and Co active sites
• Co, Ru and Ir compounds - effective catalysts for water oxidation, but are
expensive and often relate to potentially carcinogenic salts.
• Mn compounds found in OEC of PSII in nature -cheap and environmentally
friendly..

36. Mullins, C.S. & Pecoraro, V.L. (2008) Reflections on small molecule manganese models that seek to
mimicphotosynthetic water oxidation chemistry, Coord. Chem. Rev., v.252, p.416–443
Tetra nuclear manganese
complexes

37. The WOC and the localization of the
substrate water binding sites on the WOC
Umena, Y.; Kawakami, K.; Shen, J.R. & Kamiya, N. (2011) Crystal structure of
oxygenevolving photosystem II at a resolution of 1.9Ǻ. Nature, v.473, p.55-60
The WOC in PSII -a tetranuclear
manganese complex

38. o Busch and coworkers reported the first
structurally characterized example of a
mononuclear Mn (IV) complex with two
terminal hydroxo ligands (Yin et al.,
2006).
Yin, G.; McCormick, J.M.; Buchalova, M.;. (2006) Synthesis, Characterization, and Solution Properties of a Novel Cross-Bridged
Cyclam Manganese(IV) Complex Having Two Terminal Hydroxo Ligands, Inorg. Chem., v.45, p. 8052- 8061
o Since 1982, when the first well
characterised molecular WOC (the
so-called ‘‘blue dimer’’, WOC1 ) was
reported, a significant number of
WOCs have been synthesised,
including mononuclear and
polynuclear transition metal
complexes.

39. Water oxidation mechanisms -
water nucleophilic attack (WNA).
• A water molecule from the solvent attacks the oxo group from the M–O moiety.
• The M–O fragment is electrophilic enough to be attacked by a nucleophilic
water solvent molecule.
• The interaction between the HOMO of the water molecule and the LUMO of
the metal–oxo (M–O) complex creates O-O bond
• The cleavage of the M–O bond forms O2 and the reduced metal centre.20

40. Water oxidation mechanisms – interaction
between two M–O entities (I2M).
• Interaction between two M–O entities, can be a radical coupling or a reductive
elimination.
• It occurs both in an intra- and in an inter-molecular manner.
• Dinuclear complex WOC3 - the intra-molecular interaction of the two RuQO
moieties.
• Mononuclear ruthenium complex WOC6 - an inter-molecular interaction between
two complexes.

41. Dimeric tetraarylporphyrins linked
by 1,2-phenylene bridge as a
model for the WOC in PSII
Shimazaki et al. (2004) reported
dimanganese complexes of dimeric
tetraarylporphyrins linked by 1,2-phenylene
bridge
Shimazaki, Y.; Nagano T.; Takesue, H. ;Ye, B.H.; Tani F. & Naruta,Y.(2004) Characterization of a
dinuclear Mn(V)=O complex and its efficient evolution of O2 in the presence of water, Angew.
Chem. Int. Ed., v.43, p.98-100

42. Proton reduction
Redox couples with a more reductive potential than the couple E(H2O/H2) = 0.41 V
vs. NHE at pH 7 generates H2.
Transition metal complexes store electrons via multiple redox states.
The HECs have been divided into two major categories
• catalysts based on rhodium and platinum
More reactive towards protons to form metal hydrides.
• catalysts based on cobalt, nickel, iron or molybdenum
cheaper and more abundant metals.
11. V. S. Thoi, Y. Sun, J. R. Long and C. J. Chang, Chem. Soc. Rev., 2013, 42, 2388–2400.
12. M. Wang, L. Chen and L. Sun, Energy Environ. Sci., 2012, 5, 6763–6778.
13. K. Sakai and H. Ozawa, Coord. Chem. Rev., 2007, 251, 2753–2766.

43. Proton reduction mechanisms
key intermediate hydride H–Mn+ that can react in three different ways.
1. heterolytic pathway - involves protonation and hydrogen evolution
2. homolytic pathway - forms M(n-1)+ and releases hydrogen
3. further reduction to a low valent hydride H–M(n-1)+ for heterolytic or homolytic
pathways

44. Pt and Rh Catalysts
• In 1979 Lehn and Sauvage14 showed that a P2 photosensitised aqueous
solution of colloidal platinum, using HEC1 , produce H2 under visible light
irradiation.
• Fukuzumi32 developed first catalyst acting in purely aqueous medium . HEC2
catalyst was used together with P2 as a photosensitiser and sodium
ascorbate/ ascorbic acid buffer which acted as both the proton source and the
electron donor.
14 W. T. Eckenhoff and R. Eisenberg, Dalton Trans., 2012, 41, 13004–13021.

45. • The group of Sakai has developed numerous photocatalytic proton reduction
systems that are active in pure water.15
• They are mainly based on platinum complexes (mono- and binuclear) with a
nitrogen rich coordination sphere
15 K. Sakai and H. Ozawa, Coord. Chem. Rev., 2007, 251, 2753–2766.
16 S. Fukuzumi, T. Kobayashi and T. Suenobu, Angew. Chem., Int. Ed., 2011, 50, 728–731.
Binuclear Pt hydrogen evolving caalysts

46. Co catalysts
• Cobalt complexes stabilised by other nitrogen donor ligands have
 remarkable stability in water
 high activity in both photochemical and electrochemical systems.
• A large family of diimine/dioxime cobalt HECs have been designed.17,18
Cobaloxime-type complex [Co(dmgH)2] (dmgH2 = dimethylglyoxime) promotes
photocatalytic proton reduction using P2 as a photosensitiser.
• A successful homogenous
photocatalytic system in pure water
uses complex HEC6 together with
natural photosystem I (PSI) as a
photosensitiser and ascorbic acid as a
SED.3
17 W. T. Eckenhoff and R. Eisenberg, Dalton Trans., 2012, 41, 13004–13021.
18. M. Wang, L. Chen and L. Sun, Energy Environ. Sci., 2012, 5, 6763–6778.

47. • The heterogeneous approach, using catalysts attached on solid surfaces,
improves the performance of cobaloxime type complexes.
• Artero and coworkers fabricated a highly active cathode grafted with catalyst
HEC7 reaching up to 5.5 * 104 TON.
• In 2008 Artero, Fontecave, and co-workers first reported H2 evolution from dyads
1a–b in the presence of Et3N as the sacrificial reductant and Et3NH+ as the proton
source 19
19. V. Artero, M. Chavarot-Kerlidou and M. Fontecave, Angew. Chem., Int. Ed., 2011, 50, 7238–7266.
20. A. Fihri, V. Artero, M. Razavet, C. Baffert, W. Leibl and M. Fontecave, Angew. Chem., Int. Ed., 2008, 47,
564–567.

48. Mo catalysts
High-valent molybdenum catalysts HEC8 and HEC9 have polypyridyl-based
ligands These are among the best molecular electrocatalysts for hydrogen
production used perform in pure water.
21. V. S. Thoi, Y. Sun, J. R. Long and C. J. Chang, Chem. Soc. Rev., 2013, 42, 2388–2400.
HEC8 M= Mo, X=O2-
HEC9 M=Mo, X=S2
2-

49. Fe catalysts
Dimeric iron(I) complexes resembles natural enzyme [FeFe]-hydrogenase cofactor.
Low water solubility is resolved by
• encapsulating the catalyst inside micelles or cyclodextrins
• attaching the catalyst on a solid support
• using ligands with water affinity.3,28
In HEC10 , a ligand containing trimeric ethylene glycol chains is used.

50. Ni catalysts
Diphosphine nickel complexes HEC11 and HEC12 were first developed by
DuBois and coworkers .

51. CO2 reduction
Difficulties in CO2 reduction
• kinetic inertness of CO2 (eqn (1))
• even though multiple proton-coupled electron transfers to CO2 are thermodynamically
facile (eqn (2)–(6)), it require large overpotentials to occur.
• Reduction of CO2 may lead to CO, HCOOH, HCOH, CH3OH, CH4, or higher hydrocarbons.
(less selectivity)
• In protic solvents, hydrogen evolution (eqn (7)) is often favored over CO2 reduction.

52. Tetraalkylammonium salts as
mediators
• In 1983, Bockris and co-workers discovered role of electrolyte in electrochemical
reduction of CO2 at a p-type CdTe photocathode.22
• In a 0.1 M solution of tetrabutylammonium perchlorate (TBAP) , CO2 was reduced.
• Several tetraalkylammonium perchlorates (C2–C8, i.e., tetraethyl to tetraoctyl) and
NH4ClO4 promoted the photoelectrochemical CO2. reduction.23
• NH4
+ act as a redox mediator
22 I. Taniguchi, B. Aurianblajeni and J. O. Bockris, J. Electroanal. Chem., 1983, 157, 179–182.
23 I. Taniguchi, B. Aurianblajeni and J. O. Bockris, J. Electroanal. Chem., 1984, 161, 385–388.
CO2
*- is reduced to CO, using light energy.

53. Aromatic nitriles and esters as
catalysts
• The groups of Saveant and Vianello had studied the catalytic properties of radical anions
for CO2 reduction in an aprotic medium such as DMF.24,25
• They observed a marked difference in the product distribution between direct electrolysis
on Hg and electrolysis catalyzed by radical anions.
• Various mixtures of CO and oxalate were produced in direct electrolysis.
• Only oxalate was produced using radical anions of aromatic esters and nitriles, as
catalysts.
24 A. Gennaro, A. A. Isse, M. G. Severin, E. Vianello, I. Bhugun and J. M. Saveant, J. Chem. Soc. Faraday Trans.,
1996, 92, 3963–3968.
25 A. Gennaro, A. A. Isse, J. M. Saveant, M. G. Severin and E. Vianello, J. Am. Chem. Soc., 1996, 118, 7190–7196.

54. Mechanism of direct electrolysis at an
inert electrode (mercury)

55. Mechanism of reactions catalyzed by
aromatic ester and nitrile compounds
• reduction of ester or nitrile to an aromatic radical anion (eqn 20)
• The radical anion then transfers the electron to CO2 to form a CO2
*- anion (eqn 21)
• CO2
*- anion dimerizes to give oxalate (eqn 22)

56. Pyridinium derivatives as catalysts
Bocarsly and coworkers proposed the following mechanism
41 E. B. Cole, P. S. Lakkaraju, D. M. Rampulla, A. J. Morris, E. Abelev and A. B. Bocarsly, J. Am. Chem. Soc., 2010, 132,
11539–11551.

57. Photoactivated CO2 reduction.
• Lehn first published photocatalytic CO2 reduction 41 using [Ru(bpy)3]2+ (P2) as a
photosensitizer, CoCl2 as a catalyst, and TEOA (triethanolamine) as a SED in
aqueous solution.
• The same group presented the photocatalyst [Re(bpy)(CO)3Cl] (CRC1). The rhenium
complex plays a double role in the reaction by absorbing light and performing
catalytic CO2 reduction.
41 E. E. Benson, C. P. Kubiak, A. J. Sathrum and J. M. Smieja, Chem. Soc. Rev., 2009, 38, 89–99.

58. Photoelectrochemical cells
(PECs)
The conversion of solar energy into chemical fuels utilizes devices including the
assembly of suitable modules for light harvesting, water oxidation and proton
reduction in a single PEC, mimicking natural photosynthesis.
Water oxidation and proton reduction half reactions are in 2 separate compartments

59. Each compartment contains an electrode
 the anode, performing water oxidation
 the cathode, performing proton reduction
Electrodes are connected through an external circuit for electron flow.
• Semiconducting material constituting electrode – photosensitizer
• WOC and HEC are dissolved in the homogeneous phase
• PEC are anchored onto the electrode/photoelectrode
• a proton exchange membrane (PEM) physically separate the two compartments
to collect O2 and H2 and avoid their potentially hazardous reaction back to H2O.

60. Conclusion
• Inspired, but not constrained, by nature, artificial systems can be designed to capture
light and oxidize water and reduce protons or other organic compounds to generate
useful chemical fuels.
• Photocatalytic and photoelectrochemical water splitting under irradiation by sunlight
has received much attention for production of renewable hydrogen from water on a
large scale.
• Many challenges still remain in improving energy conversion efficiency, such as utilizing
longer-wavelength photons for hydrogen production, enhancing the reaction efficiency
at any given wavelength, and increasing the lifetime of the semiconductor materials.