+ PhotoFuelCell Wastewater Electricity Solar Harnessing solar - - PowerPoint PPT Presentation

+

Harnessing solar electricity from wastewater through photocatalytic fuel cells

Chenyan Hu, Wey Yang Teoh* (wyteoh@cityu.edu.hk)

Joint Laboratory for Energy and Environmental Catalysis Clean Energy and Nanotechnology (CLEAN) Laboratory School of Energy and Environment, City University of Hong Kong

Wastewater PhotoFuelCell Electricity Solar

+ Semiconductor photocatalysis

e- R, OH- R+, OH h

+

O2

-

O2

• A semiconductor consists of conduction band (CB) and valence band (VB), and

separated by the bandgap

• The semiconductor can absorb solar photons of energy equal to higher than the

bandgap to generate electron-hole pairs

• The net separated charge carriers diffuse to the surface to catalyse redox reactions, for

example the degradation of organic pollutants

hv

CB VB

• Teoh et al. J. Phys. Chem. Lett. 2012, 3, 629 (Invited Perspective)

+ Photocatalysis in wastewater treatment

• Photocatalysis is an established advanced oxidation process for wastewater treatment
• Solar energy (or artificial light) is input to remove the pollutants
• Engineers are brainwashed since University days to think that one man’s waste is

another man’s wealth

Not really thinking

• utside the box!

e- R, OH- R+, OH h+ O2

• O2

hv

CB VB

+ Photoelectrochemical system

• When the semiconductor is fabricated as photoelectrode and connected to the counter

electrode, a photoelectrochemical (PEC) system is formed

• Oxidation of organic pollutants remain the same as in any photocatalytic treatment
• In the process, electrons are extracted and forced to the counter electrode through the

external circuit to undergo reduction reactions

• Voila! Solar electricity is generated

e

R, OH- R+, OH n-type semiconductor

h e

O2

-, H2O2,

H2O O2, H+ Counter electrode VOC

Let’s make a photoelectrode

Kho, Iwase, Teoh, Mädler, Kudo, Amal, J. Phys. Chem. C 2010, 114 , 2821

Area I Area II

Ti Ti Cd C d S

Photoelectrodrodes design

• Wang, Teoh et al. J. Phys. Chem. C 2013, 117, 1857
• Yang, Teoh et al. J. Phys. Chem. C 2013, 117, 20406
• Wang, Teoh et al. Adv. Funct. Mater. 2013, 23, 4847
• Wang, Teoh et al. Nanoscale 2014, 6, 6084
• Yang, Teoh et al. Adv. Funct. Mater. 2012, 22, 2821
• Hu, Teoh et al. AIChE J. 2016, Special issue

Teflon cell Potentiostat Ohmic contact Metallic foil Stirrer Pt counter electrode Viton O-ring Electrolyte (+) (−)

• Anodisation synthesis is optimised to

produce nanostructured photoelectrodes with high photocatalytic efficiencies

• A readily scalable technique – this is the

same technology to produce your iphone cover!

Anodisation synthesis

• Yang, Teoh et al. Adv. Funct. Mater. 2012, 22, 2821
• Hu, Teoh et al. ChemSusChem 2015, 8, 4005

e

OH- O2 n-type semiconductor

h e

H2 H+ Counter electrode (+) (-)

Characterization of photoelectrodes

• PEC characterization carried out in the absence of
• rganics interactions – pure water splitting
• Onset potential reflects the quasi-Fermi level of the n-

type semiconductor

• Photocurrent reflects the turnover frequency

(absorption, excitation, carrier diffusion, surface charge transfer) of the photoelectrode

• Onset potential: Nb2O5 > TiO2 > WO3
• Saturation photocurrent:

TiO2 > WO3 ≈ Nb2O5

• Hu, Teoh et al. ChemSusChem 2015, 8, 4005

Efficiencies of photoelectrodes

e

n-type semiconductor

h e

H2 H+ Counter electrode (+) (-)

• The presence of organic electron donors increases the net

charge separation by efficiently scavenging the photoholes  decomposition of organic pollutants

• Photocurrent is generally increased with the increase in
• rganics concentration up to saturation
• Faster decomposition rate for shorter chain molecules
• Aromatics are capable of forming surface complex on the

photoelectrode surface that acts as charge recombination centre

• Hu, Teoh et al. ChemSusChem 2015, 8, 4005

Current doubling effect

• Photocatalytic oxidation of methanol produces

hydroxymethyl radicals

• Highly reducing species (-0.95 V vs NHE), but sluggish

direct reduction of water

• Detection of hydroxymethyl radicals by DMPO (5,5-

Dimethyl-1-pyrroline N-oxide) spin trapping molecule

100 110 120 130 140 150 m/z (b)

N O CH

2

OH

m/ z = 144

3440 3460 3480 3500 3520 Magnetic field (Gauss) EPR intensity (a.u.) (a)

CH3OH

• CH2OH

HCHO H+ H2

e- e- e-

Kho, Iwase, Teoh, Mädler, Kudo, Amal, J. Phys. Chem. C 2010, 114 , 2821 Teoh, Mädler, Amal, J. Catal. 2007, 251, 271

9 At the anode:

Efficiencies of photoelectrodes

• Since wastewater contains more than one type of

pollutants, it is important to assess and elucidate the presence of organic mixture

• The photoelectrode efficiency is related to the

adsorption of organic species on the photoelectrode surface

• At subsaturation, contribution of other organic

species contribute to the increased photocurrent

• When surface is saturated with the most highly

adsorbed species, the photocurrent reflects that

• f the pure adsorbed species

PFCs can be assembled by inducing O2 reduction on the cathode

• Pt counter electrode reduced O2 through various paths:

One-electron reduction: O2 + e-  O2

- (E° = -0.284 V vs RHE)

O2 +H+ + e-  HO2

 (E° = -0.046 V vs RHE)

Two-electron reduction: O2 + 2H+ + 2e-  H2O2 (E° = +0.682 V vs RHE) Four-electron reduction: O2 + 4H+ + 4e-  2H2O (E° = +1.23 V vs RHE)

• For efficient cathode, the oxygen reduction reaction ought to take place at more positive

potential than the quasi-Fermi level of the photoelectrodes

PhotoFuelCell (PFC) efficiencies

e

R, OH- R+, OH n-type semiconductor

h e

O2

• , H2O2,

H2O O2, H+ Counter electrode VOC

• Hu, Teoh et al. ChemSusChem 2015, 8, 4005

Application of PFCs can reduce the organic load in wastewater while generating solar electricity

• The measured photocurrents of the PFCs agree

well with the expected trend based on the photoelectrochemical characterisation

• Voc trend agrees well with the expected values

based on the quasi-Fermi levels of the photoelectrodes

PhotoFuelCell efficiencies

• Hu, Teoh et al. ChemSusChem 2015, 8, 4005

h e e H2O O2 H2 H2O e h h e p-NiO Pt

Barrier layer

• We developed highly efficient and stable p-NiO
• The development of p-type PFC is currently underway as precursor to a tandem PFC cell

Tandem PhotoFuelCell?

• Hu, Teoh et al. ACS Appl. Mater. Interfaces 2014, 6, 18558

+ Conclusions

• Design of efficient PhotoFuelCell based on rational approach
• Much higher efficiency PFCs can now be achieved in our lab based on

composite photoelectrodes

• Prediction models required to predict the PFC efficiencies based on

mixed pollutants in wastewater

• PFCs can be implemented in wastewater treatment plants to draw solar

electricity and to reduce organic loadings