+ PhotoFuelCell Wastewater Electricity Solar Harnessing solar - PowerPoint PPT Presentation

+ PhotoFuelCell Wastewater Electricity Solar 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

+ Semiconductor photocatalysis O 2 CB e -  - O 2 hv R + ,  OH VB h + R, OH - 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 • Teoh et al. J. Phys. Chem. Lett. 2012 , 3 , 629 (Invited Perspective)

+ Photocatalysis in wastewater treatment O 2 e - CB - O 2 hv R + , OH VB h + R, OH - Not really thinking outside the box! 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

+ Photoelectrochemical system Let’s make a photoelectrode n -type Counter semiconductor electrode e O 2 , H + V OC e O 2  - , H 2 O 2 , R + ,  OH H 2 O Kho, Iwase, Teoh, Mädler, Kudo, Amal, J. Phys. Chem. C 2010 , R, OH - h 114 , 2821 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 

Photoelectrodrodes design Ti Area I Ti Area Cd S II C d • Wang, Teoh et al. J. Phys. Chem. C 2013 , 117 , 1857 • Yang, Teoh et al. Adv. Funct. Mater. 2012 , 22 , 2821 • 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 • Hu, Teoh et al. AIChE J. 2016 , Special issue

Anodisation synthesis Potentiostat Teflon cell Electrolyte ( − ) (+) Pt counter electrode Stirrer Viton O-ring Metallic foil Ohmic contact  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! • Yang, Teoh et al. Adv. Funct. Mater. 2012 , 22 , 2821 • Hu, Teoh et al. ChemSusChem 2015 , 8 , 4005

Characterization of photoelectrodes n -type Counter semiconductor electrode (-) e H + e (+) H 2 O 2 OH - h  PEC characterization carried out in the absence of organics interactions – pure water splitting Onset potential reflects the quasi-Fermi level of the n-  type semiconductor  Onset potential: Nb 2 O 5 > TiO 2 > WO 3 Photocurrent reflects the turnover frequency  Saturation photocurrent:  TiO 2 > WO 3 ≈ Nb 2 O 5 (absorption, excitation, carrier diffusion, surface charge transfer) of the photoelectrode • Hu, Teoh et al. ChemSusChem 2015 , 8 , 4005

Efficiencies of photoelectrodes n -type Counter semiconductor electrode (-) e H + e (+) H 2 h  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  organics 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 (a) At the anode: EPR intensity (a.u.) e - HCHO e - e - H + • CH 2 OH H 2 CH 3 OH 3440 3460 3480 3500 3520 Magnetic field (Gauss) • Photocatalytic oxidation of methanol produces (b) hydroxymethyl radicals N • Highly reducing species (-0.95 V vs NHE), but sluggish CH OH 2 O direct reduction of water m/ z = 144 • Detection of hydroxymethyl radicals by DMPO (5,5- Dimethyl-1-pyrroline N-oxide) spin trapping molecule 100 110 120 130 140 150 m/z Kho, Iwase, Teoh, Mädler, Kudo, Amal, J. Phys. Chem. C 2010 , 114 , 2821 9 Teoh, Mädler, Amal, J. Catal. 2007 , 251 , 271

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 of the pure adsorbed species

PhotoFuelCell (PFC) efficiencies PFCs can be assembled by inducing O 2 reduction on the cathode n -type Counter semiconductor electrode e O 2 , H + V OC e O 2 - , H 2 O 2 , R + , OH H 2 O R, OH - h Pt counter electrode reduced O 2 through various paths:  One-electron reduction: O 2 + e -  O 2  - (E ° = -0.284 V vs RHE) O 2 +H + + e -  HO 2  (E ° = -0.046 V vs RHE) Two-electron reduction: O 2 + 2H + + 2e -  H 2 O 2 (E ° = +0.682 V vs RHE) Four-electron reduction: O 2 + 4H + + 4e -  2H 2 O (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 • Hu, Teoh et al. ChemSusChem 2015 , 8 , 4005

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

Tandem PhotoFuelCell? Barrier Pt p -NiO layer e H 2 O H 2 h O 2 e H 2 O h e e h We developed highly efficient and stable p -NiO   The development of p -type PFC is currently underway as precursor to a tandem PFC cell • 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