Dual-Functional Photocatalytic and Photoelectrocatalytic Systems for

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Dual Functional Photocatalytic and Photoelectrocatalytic Systems for Energy and Resource-Recovering Water Treatment Tae Hwa Jeon, Min Seok Koo, Hyejin Kim, and Wonyong Choi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03521 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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ACS Catalysis

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Dual Functional Photocatalytic and Photoelectrocatalytic Systems for

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Energy and Resource-Recovering Water Treatment

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Tae Hwa Jeon, Min Seok Koo, Hyejin Kim, and Wonyong Choi*

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Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea

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*To whom correspondence should be addressed:

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(W.C.) E-mail: [email protected] ; Tel: +82-54-279-2283

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Graphical Abstract

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1 Environment ACS Paragon Plus

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Abstract

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The solar-driven photo(electro)catalytic chemical process is a key technology for utilization

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of solar energy. It is being intensively investigated for the application to environmental

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remediation and solar fuel production. Although both environmental and energy applications

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operate on the basis of the same principle of photo-induced interfacial charge transfer, most

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previous studies have focused on either process only since one process requires catalyst

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properties and reaction conditions that are very different from the other. This perspective

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describes the dual functional photo(electro)catalytic process that enables the water treatment

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along with the simultaneous recovery of energy (e.g., H2 and H2O2) or resource (e.g., metal

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ions) and discusses the status and perspectives of this emerging technology. The essential

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feature of the process is to utilize the hole oxidation power for the degradation of water

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pollutants and the electron reduction power for the recovery of energy and resource from

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wastewaters at the same time. Various photocatalytic (PC), photoelectrochemical (PEC), and

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photovoltaic-driven electrochemical (PV-EC) processes with different dual functional

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purposes (e.g., pollutants removal combined with H2 or H2O2 production, heavy metal

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recovery, denitrification, fuel cell) are introduced and discussed. The reviewed technology

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should offer new chances for the development of next-generation water treatment processes

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based on water-solar energy nexus.

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Keywords

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Water-energy nexus; Solar fuel; Solar water treatment; Photocatalysis; Advanced oxidation

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processes


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ACS Catalysis

1. Introduction

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Photo(electro)catalysis has attracted great interests for various applications such as the

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production of solar fuels, water and air purification, organic synthesis, and recovery of

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resources

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thermodynamically either spontaneous or non-spontaneous can be achieved in the

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photo(electro)catalytic system by coupling photoactive materials (usually semiconductors)

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and suitable reactants and subsequently initiating interfacial charge transfer reactions on the

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semiconductor surface.9,10 Most research works in this area have focused on the development

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of photocatalysts and photoelectrodes having high efficiency, selectivity, and stability in

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specific catalytic reaction conditions.11,12 The application of photo(electro)catalytic process

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for water treatment has been extensively investigated as an advanced oxidation process

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(AOP),2,13 which is initiated by photo-induced charge transfers on semiconductor materials,

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and is subsequently followed by the generation of reactive oxygen species (ROS) such as

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superoxide radical ion (O2•-), hydrogen peroxide (H2O2), and hydroxyl radical (•OH).14-22 At

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the same time, photo-excited electrons and holes can directly participate in the redox

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transformation reactions of target pollutants,1 which include the degradation of various

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organic pollutants,23-25 conversion of toxic anions (e.g., chromate,26,27 arsenite,28 bromate,29,30

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nitrate31-33), heavy metal ions,34 and even inactivation of microorganisms.35-37 The concurrent

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reactions of electrons and holes on photoexcited semiconductor can achieve the

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transformation of a wide variety of aquatic pollutants into less toxic or non-toxic forms under

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ambient conditions as long as energetic photons are available.

utilizing

solar

photon

energy.1-8

Various

redox

processes

that

are

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On the other hand, solar fuel production via photo(electro)catalysis has also received

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intense attention as a viable method of solar energy storage since the early study of

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photoelectrocatalytic hydrogen production was reported.38-40 The reaction mechanism for fuel

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production by photo(electro)catalysis is basically the same as the above water treatment


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application in that the overall process is initiated by the interfacial photo-induced charge

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transfers. However, the characteristics of the interfacial charge transfers are very different.

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The environmental photo(electro)catalysis depends largely on the generation of ROS, which

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is achieved by single-electron transfer (because ROS is usually radical species) and in the

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presence of dissolved O2 (because O2 is both a common precursor of ROS and a reagent

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needed for mineralization). In contrast, the solar fuel photo(electro)catalysis requires multi-

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electron transfers because cheap and abundant precursors such as H2O, CO2, and N2 should

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be converted (or reduced) into energy-rich fuel molecules (e.g., H2, CH3OH, HCOOH, NH3)

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by a series of electron transfers.41-45 The solar energy storage process should be carried out in

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the absence of O2 because dioxygen molecule is a good scavenger of photogenerated

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electrons and hinders the overall fuel synthesis process. Therefore, the environmental

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photo(electro)catalysis and the solar fuel photo(electro)catalysis require very different

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reaction conditions and catalytic materials and are difficult to be achieved simultaneously

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using the same photocatalyst.

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The utilization of photoactive semiconductor materials in solar conversion has

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employed three different methods, which are classified into photocatalytic (PC),

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photovoltaic-driven electrochemical (PV-EC), and photoelectrochemical (PEC) systems. PC

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system is based on semiconductor particle suspension in which photocatalysts and target

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reactants are mixed together in a single reaction medium (usually aqueous phase).46 PC

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system has been widely investigated in laboratories over the past decades because it is easy to

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work with.47 PV-EC system is the device that couples a photovoltaic and an electrolytic

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cell.48,49 Since the catalytic reaction is separated from the light absorption part in this system,

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it offers the great advantage of easier optimization of both electrocatalysis (electrode) and

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light harvesting (photovoltaic) with higher efficiencies.50 In addition, both photovoltaic and

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electrolytic devices are commercially available in a large-scale, which makes this system


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ACS Catalysis

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highly practical.51 PEC system is a hybrid concept of PC and EC system that combines the

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light harvesting and electrocatalysis function simultaneously on the same electrode. It is

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composed of two electrodes immersed in electrolytes and at least one of them should be

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semiconductor material to be a photoanode or a photocathode. Common PEC system consists

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of a semiconductor photoanode and a metallic cathode.52 Since the photoelectrode is the key

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part for the overall conversion process, it is essential to develop proper semiconductor

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photoelectrodes for the desired purposes such as environmental remediation and fuel

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synthesis.53-55

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In this perspective, we focus on the integrated semiconductor photochemical systems

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that are designed for achieving water treatment and energy/resource recovery simultaneously.

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The basic concept is illustrated in Scheme 1. In most cases, the water treatment part utilizes

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the hole oxidizing power while the energy/resource recovery part is driven by the electron

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reducing power. Although each application alone (water treatment or energy/resource

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recovery) has been extensively investigated, the combined systems with dual purposes have

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been attracting attentions only recently. The published literature examples on this topic are

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listed in Table 1. In typical water splitting studies, the reducing power of electrons are

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converted to H2 energy but the hole oxidation power is wasted in producing useless O2.

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Similarly, in photocatalytic water treatments, the hole oxidation power drives the degradation

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of pollutants but the reducing power of electrons is largely wasted. In essence, the key

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concept in the dual functional process is to use both the hole oxidation power for the

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degradation of water pollutants and the electron reduction power for the recovery of energy

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and resource from wastewaters at the same time. The main challenges in dual-functional

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photo(electro)catalysis lie in that the desired photocatalyst properties are different between

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the two systems. It should make the solar-driven water treatment more viable and sustainable


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since the removal of aquatic pollutants concurs with the recovery of energy and resource

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form the polluted water, which is conceptually ideal for water-energy nexus.

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2. Hydrogen production coupled with water treatment

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2.1. Photocatalytic system

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The photocatalytic reactions are based on the interfacial charge transfers of excited

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electrons and holes in semiconductor. For highly efficient PC conversions, enhancing the

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light harvesting efficiency of semiconductor materials presents the biggest challenge. Since

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bare semiconductor materials have limited efficiencies in absorbing and utilizing solar light,

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the semiconductor materials have been modified or hybridized in various ways such as

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bandgap engineering, interfacial heterojunction formation, surface complexation, impurity

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doping, and sensitization.1,2,9 The high light absorption efficiency is necessary but not

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sufficient for high solar conversion efficiency that should strongly depend on the interfacial

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charge transfer characteristics. The interfacial charge transfer efficiency depends on the

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energy levels of the redox couples in the electrolyte: only the redox couples of which energy

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levels are located within the bandgap are thermodynamically allowed (e.g., redox couples

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shown in Scheme 2). In a typical mode of photocatalysis for water treatment, the oxidative

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destruction of water pollutants is initiated by the hole transfer, which is concurrently coupled

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with the electron transfer to dissolved O2. On the other hand, a typical water splitting

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photocatalysis utilizes hole transfer to oxidize water and electron transfer to produce H2. The

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dual functional photocatalysis for H2 production coupled with water treatment should utilize

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the hole transfer to degrade organic pollutants and the electron transfer to generate H2 at the

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same time. Scheme 1 illustrates the overall processes. The key problem is how to control the

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charge transfers selectively. The electrons should be consumed to reduce water (or protons)


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ACS Catalysis

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selectively for H2 production via 2-electron transfer (not to reduce O2) whereas the holes

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should be used to oxidize organics mainly via 1-hole transfer leading to the generation of a

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carbon-centered radical, not to oxidize water for producing O2 via multi-hole transfer. The

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one-electron oxidation of water to OH radical requires a highly positive potential (2.70 VNHE)

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but it can occur rapidly with little kinetic limitation.1 On the other hand, the four-electron

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oxidation of water to O2 needs a much lower potential (1.23 VNHE) but is kinetically hindered.

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The control over the single- vs. multi-electron transfer determines the overall process. The

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dual functional photocatalysts should enable the single-hole transfer and the multi-electron

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transfer at the same time, which differs from the charge transfer mechanism of typical

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environmental photocatalysts (single-hole and single-electron transfer) and water-splitting

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photocatalysts (multi-hole and multi-electron transfer).

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Since photocatalysis is a surface chemical process, the controlled charge transfer and

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catalysis should be related with the surface properties which can be modified in various ways.

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Kim et al. developed a simple method for the dual functional photocatalysis by modifying the

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surface of TiO2 with platinum nanoparticles and surface fluorides (denoted as F-TiO2/Pt).56

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Figure 1a compares bare TiO2, F-TiO2, Pt/TiO2, and F-TiO2/Pt for the photocatalytic

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degradation of 4-chlorophenol (4-CP) and the concurrent generation of H2 in the deaerated

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suspensions. With F-TiO2/Pt, the 4-CP degradation accompanied the notable production of

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H2 whereas the H2 production was insignificant with other catalysts. Using bisphenol A (BPA)

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instead of 4-CP as a substrate also exhibited the same behavior. The production of H2 on F-

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TiO2/Pt was much enhanced in the presence of organic compounds and the H2 production

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was gradually reduced as the organic compounds were depleted during the repeated cycles of

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dual functional photocatalysis (see Figure 1b). This indicates that the organic pollutants serve

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as a hole scavenger (or electron donor) for H2 generation, which was markedly reduced as the

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organic pollutants are degraded. The overall process demonstrates that the surface-modified


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F-TiO2/Pt makes holes oxidize organic pollutants and electrons reduce water to generate H2

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at the same time while other photocatalysts (bare TiO2, F-TiO2, Pt/TiO2) cannot. The

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proposed mechanism of the dual functional photocatalysis on F-TiO2/Pt is illustrated in

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Figure 1c.57 The photogenerated holes on bare TiO2 generate the surface-bound OH radicals,

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which are rapidly recombined with electrons in the absence of electron acceptors (e.g., O2).

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The Pt nanoparticles deposited on TiO2 form Schottky barrier at the interface that separates

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the electrons from holes by accumulating electrons on Pt and serve as a co-catalyst for

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hydrogen production.58-60 This electron trapping on Pt hinders the charge pair recombination

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and increases the lifetime of charge carriers. On the other hand, the fluorination treatment of

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the TiO2 surface reduces the concentration of surface OH groups which are the sites of hole

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trapping.61 It also hinders the adsorption of organic compounds on the TiO2 surface and

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subsequently retards the direct hole transfer to organic compounds.62 As a result, the surface

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fluorination decreases the efficiency of surface hole trapping and then the holes on the

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fluorinated TiO2 surface preferentially react with the adsorbed water molecules instead. The

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oxidation of adsorbed water molecule (not surface hydroxyl group) generates weakly bound

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(mobile) OH radicals which can easily desorb from the surface. This process essentially

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removes the holes out of the TiO2 surface with minimizing the charge recombination between

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the electrons trapped in Pt and the surface-trapped holes. Consequently, the trapped electrons

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on Pt are much longer lived to reduce water with producing more H2 in the presence of

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surface fluorides. As a result, both the production of H2 and the degradation of organic

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compounds are synergically enhanced on F-TiO2/Pt. Such a unique photocatalytic behavior

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was also observed when the phosphate ions were used instead of fluoride ions in preparing P-

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TiO2/Pt (see Figure 1d).63 The phosphate ions are adsorbed on the TiO2 surface by

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substituting the surface OH groups as fluoride ions do and the above discussed mechanism of

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F-TiO2/Pt can be also applied to the dual functional photocatalysis on P-TiO2/Pt. The dual-


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ACS Catalysis

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component surface modification (Pt deposition and surface fluorination) enables the dual

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functional photocatalysis by hindering the surface-mediated recombination and enhancing

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electron transfer for H2 production at the same time. Many other photocatalytic systems that

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investigated the H2 production coupled with the oxidation of various pollutants such as urea,

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dyes, biomass-derived products (e.g., glucose, glycerol), and H2S are listed in Table 1.

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Regardless of the kind of pollutants, the key mechanism should be related with the selective

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charge transfer: single hole transfer to initiate the degradation of pollutants and the multi-

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electron transfer for H2 evolution.

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Most photocatalytic studies of H2 production have been carried out in the deaerated

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condition since H2 is not evolved in the presence of dissolved O2.64 However, this dual

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functional photocatalyst system cannot remove the total organic carbon (TOC) because the

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mineralization of organic pollutants requires the presence of dioxygen.65 To overcome this

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problem, Cho et al. employed a hybrid catalyst of Cr2O3/Rh/SrTiO3 to enable the

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simultaneous H2 production with the mineralization of 4-CP under a deaerated condition.66

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They prepared rhodium nanoparticles covered with a thin chromium shell as a cocatalyst,

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which has been originally developed as a water splitting photocatalyst (see Figure 2a).67 The

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Cr2O3/Rh/SrTiO3 exhibited far higher activity than Rh/SrTiO3 in both 4-CP removal and

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TOC removal, which indicates that 4-CP could be mineralized in the suspension of

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Cr2O3/Rh/SrTiO3 even in the deaerated condition (Figure 2b). The evolution of H2 and O2

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concurrently occurred non-stoichiometrically on Cr2O3/Rh/SrTiO3, but not on Rh/SrTiO3

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(Figure 2c). The fact that O2 was generated along with the 4-CP degradation on

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Cr2O3/Rh/SrTiO3 implies that holes react with both 4-CP and water molecules at the same

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time. The in-situ generated O2 can be immediately used for the mineralization of 4-CP, which

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explains why 4-CP could be mineralized in a deaerated condition. The loading of Cr2O3 shell

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on the Rh nanoparticles changes the reaction mechanism by blocking the back reaction of H2


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and O2 (producing H2O) on Rh and allowing the selective electron transfer to water/protons

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only, not to O2. Figure 2d shows that Cr2O3/Rh/SrTiO3 is more efficient for H2 production

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than F-TiO2/Pt as a dual functional photocatalyst. It was proposed that 4-CP was degraded

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via a direct hole transfer and mineralized with utilizing in-situ generated O2. All the above-

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mentioned dual functional photocatalytic systems successfully demonstrated the simultaneous

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H2 generation and organics degradation, which should serve as a conceptual model for the

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dual functional water treatment technology. However, the practical realization should require

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extensive efforts beyond the conceptual demonstration.

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2.2. PEC and PV-EC systems

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The dual functional photocatalytic water treatment based on the slurry process cannot

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be suitable for practical applications because the post-photocatalysis separation and recovery

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processes of photocatalyst particles demand additional efforts and energy. Therefore, the

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more practical dual functional photocatalytic water treatment should employ immobilized

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photocatalysts on the electrode surface in a PEC device. The PEC system utilizes an external

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electrical bias under photo-irradiation to separate the electrons and holes to a cathode and an

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anode, respectively. This should be an ideal method to achieve the destructive removal of

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organic pollutants on the photoanode and the reduction of water to H2 on the cathode at the

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same time (see Scheme 1b). In a recent study, electrochromic titania nanotube arrays

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(denoted as Blue-TNTs) have been proposed as a photoanode, which exhibited higher charge

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carrier density and electrical conductivity for its PEC application to simultaneous water

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treatment and H2 production (Figure 3a).54 Electrochromic behavior is related with the

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surface defect generation which changes the Ti oxidation state from +4 to +3 during the

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cathodic polarization. The formation of Ti3+ in the TiO2 lattice accompanies the color change

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of the electrode and the red shift of the absorption spectrum (Figure 3b). The Blue-TNTs

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photoanode coupled with a stainless steel cathode markedly enhanced the efficiency of


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pollutant degradation and H2 production whereas intact TNTs or TiO2 nanoparticle film did

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not exhibit such dual functional PEC activities (Figure 3c).

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The photo-induced hole on the photoanode can be also utilized in generating reactive

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chlorine species (RCS) (via oxidizing chloride) as an oxidant for water treatment. Recently,

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Kim et al. reported a hybrid PEC system that achieved the desalination of saline water, RCS-

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mediated water treatment, and hydrogen production (Figure 4a) at the same time.68 A

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hydrogen-treated TiO2 nanorod (H-TNR) photoanode and a Pt foil cathode were placed in an

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anode and cathode cell, respectively, with a middle cell containing saline water facing these

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cells through the membranes. Under the irradiation, photogenerated charges initiates

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desalination of saline water by transporting chloride and sodium ions in the middle cell

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toward the anode and cathode cell, respectively. RCS is generated by photogenerated holes

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on H-TNR photoanode, which can oxidize urea (and other organic compounds), while

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hydrogen is produced on the cathode with a faradaic efficiency of ~80% (see Figure 4b and c).

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Cho et al. reported that a PV-powered electrolysis system employing a multilayer bismuth

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(Bi)-doped TiO2 anode and a stainless steel cathode can treat domestic wastewater with

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simultaneous H2 production (Figure 5a).69 Low-cost polycrystalline PV panel was employed

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to apply a DC potential across the anode and the cathode. The COD removal was achieved by

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the homogeneous reaction between the oxygen-demanding compounds and RCS under a

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static anodic potential (+2.2 or +3.0 VNHE) (Figure 5b and c). Therefore, over 95% removal of

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COD and ammonium ions was achieved within 6 h with the concurrent H2 production (the

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current efficiency ranging from 34% to 84%).

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While most studies on the simultaneous removal of organic pollutants and H2

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production employed only one or two organic pollutants as a test substrate, the successful

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application to real domestic wastewater demonstrated the practical feasibility of the prototype

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PV-powered wastewater electrolysis with simultaneous H2 production. Since chloride ions


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are ubiquitous in natural fresh waters, wastewaters and sea water, the activation of chloride

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into RCS under solar irradiation is an appealing method to achieve dual functional water

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treatment. The sustained production of RCS in PEC and (PV-)EC systems should serve many

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purposes such as chlorination disinfection, tertiary treatment of wastewaters, toilet water

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treatment, and ship ballast water treatment.70-73 For example, a small-scale PV-EC reactor

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(treatment capacity: 0.13 m3/d) based on the simultaneous RCS generation and H2 production

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has been proposed as a recycling system for toilet water.74

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3. In-situ production of H2O2 in water treatment

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Hydrogen peroxide (H2O2) is a green oxidant which is being widely used in water

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treatment, chemical synthesis, and bleaching.75 On the other hand, it can be considered as a

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green liquid fuel that can be utilized for electricity generation via a fuel cell.76 Since H2O2

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decomposes to generate water and dioxygen only, it is a clean fuel that does not leave carbon

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footprint. The photochemical synthesis of H2O2 as an alternative solar fuel using photoactive

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semiconductor materials is being actively investigated.77-79 Therefore, water treatment

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coupled with H2O2 production through dual functional photocatalysis is an attractive option.

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The in-situ generated H2O2 can be recovered as a fuel or be utilized as in-situ oxidant of

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water treatment (e.g., Fenton process).80 For example, a recent study reported a ternary-

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hybrid photocatalytic system that consisted of modified carbon nitride, WO3, and Fe3+ for

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As(III) oxidation under visible light (λ > 420 nm), which utilizes in-situ generated H2O2 as a

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Fenton reagent.81 The current industrial method of H2O2 synthesis (i.e., anthraquinone

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process) requires H2 gas, organic solvents, and high energy input, which is not eco-friendly.75

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The photocatalytic production of H2O2 can be an alternative green synthetic method that

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demands dioxygen, water and light only. H2O2 can be generated through proton-coupled


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electron transfers (PCET) to dioxygen molecule through the selective two-electron transfer

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(eq 1).82

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O2 + 2H+ + 2e− → H2O2

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P → Pox + ne−

(E° = 0.695 VNHE) (1)

(2)

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To maximize the photonic efficiency of H2O2 production, the number of electrons transferred

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to O2 should be limited to two and the competing electron transfers to protons (2H+ + 2e−

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→ H2) should be hindered. To serve this dual purpose photo(electro)catalysis, the ideal

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electron donors for H2O2 production should be the pollutants (P) themselves, which can be

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oxidatively converted or degraded in contaminated water (eq 2).

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Zong et al. developed a PEC system where the toxic pollutant H2S can be utilized as

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an electron donor for H2O2 production (eq 3) without an external electrical bias (Figure 6a).83

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H2S + O2 → H2O2

+ S

(3)

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In this PEC system, hazardous H2S gas (usually generated as a waste from coal and

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petroleum chemical processing) can be oxidized to elemental sulfur (S) on the photoanode,

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which can be coupled with the reduction of O2 to H2O2 on the cathode. The Carbon/p+n-Si

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coupled electrode was used for an unbiased system, and anthraquinone (AQ) and iodide were

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added in the cathode and anode cell, respectively. The use of carbon as the counter electrode

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generated higher photocurrent than Pt electrode, which reduces the cost of the system by

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eliminating the need of expensive Pt electrode (Figure 6b). The redox shuttle of I-/I3- was

303

employed for the oxidation of H2S in the photoanode compartment (to mediate the hole

304

transfer from the p+n-Si electrode to H2S) while the AQ/H2AQ redox shuttle was used for the

305

2-electron reduction of O2 in the cathode cell. These redox shuttle species are more favorably


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306

oxidized and reduced than water and protons and the PEC device exhibited highly enhanced

307

current and 1.1% of solar-to-chemical conversion efficiency. This combined PEC-chemical

308

reaction system achieved significant photocurrent density (8 mA cm-2 in an unbiased system),

309

leading to higher H2O2 generation rate compared to other PEC H2O2 production system.84-86

310

It is interesting to note that seawater can be also utilized instead of pure water for the

311

production of H2O2.87 Mase et al. reported a two-compartment PEC cell using m-WO3/FTO

312

photoanode and CoII(Ch)/Carbon paper cathode for the production of H2O2. The cobalt

313

complex fixed on the carbon paper (cathode) can generate H2O2 via O2 reduction with

314

prohibiting the coordination of chloride ion on the cobalt complex. The catalytic production

315

of H2O2 on cathode is much enhanced by the oxidation of chlorides (serving as an electron

316

donor) on photoanode in seawater and NaCl solution than in pure water (Figure 6c).

317

Compared with 4-electron oxidation of water molecule, 2-electron oxidation of chloride

318

(leading to Cl2 and HOCl) is much favored so that the overall PEC process is much improved

319

in seawater condition. The in-situ generated RCS might be further utilized as an oxidant for

320

sea water treatment (e.g., ship ballast water). H2O2 produced by this PEC reaction could be

321

utilized in a fuel cell to generate electrical energy. The H2O2 fuel cell with Ni mesh anode

322

and FeII3[CoIII(CN)6]2/carbon cloth cathode in one-compartment cell demonstrated 0.28%

323

solar-to-electricity conversion efficiency using H2O2 produced in the PEC reactor.

324

Shi et al. reported an interesting case of an unassisted PEC system that produced H2O2

325

on both BiVO4 photoanode and carbon cathode (Figure 6d).88 The optimized system achieved

326

the H2O2 production rate of 0.48 μmol min-1 cm-2, which is the highest value compared to

327

other oxide-semiconductor based PEC H2O2 production systems. The in-situ generated H2O2

328

can be utilized in water treatment through the reductive decomposition into OH radical,21,89

329

Since the PEC water splitting has efficiency limits due to various parameters such as

330

fractional solar light absorption,90 more efficient PEC system should utilize the most of


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charge carrier energy by converting both holes and electrons into useful products. For

332

example, Figure 6e illustrates an efficient PEC system for H2O2 production that utilizes not

333

only electrons but also holes. The in-situ generated H2O2 along with RCS can be utilized for

334

water treatment.

335 336

4. Redox-coupled conversion and recovery of toxic metal ions

337

Wastewater containing various heavy metals such as lead, copper, nickel, arsenic, and

338

chromium is a major environmental problem for which various remediation technologies are

339

being developed. Since heavy metal ions are non-degradable, it is very important to treat and

340

remove before the discharge into the environment. PC and PEC processes are suitable for the

341

recovery of heavy metal ions since the photo-induced reduction of metal ions leads to their

342

deposition onto the surface of catalyst or electrode. However, the actual process is more

343

complex than a simple reduction of metal ions. Toxic heavy metal species are often found as

344

stable complexes, which are recalcitrant and difficult to remove. Many advanced oxidation

345

processes (AOPs) have been widely investigated for the degradation of common chelating

346

agents such as ethylenediamine tetraacetic acid (EDTA).91,92 The photocatalytic degradation

347

of organometallic complexes release heavy metal ions which should be further treated for

348

removal or recovery.

349

Zhao et al. investigated the simultaneous PEC oxidation of Cu-EDTA on a TiO2-film

350

as a photoanode and reductive recovery of Cu2+ ions on a stainless-steel cathode (Figure

351

7a).93 Unlike the slurry photocatalytic system in which Cu2+ ions released from the

352

degradation of Cu-EDTA should be recovered from the slurry phase by additional separation

353

processes, the PEC system does not need the separation process because copper ions are

354

directly deposited on the cathode.94 The PEC process showed enhanced activities for not only


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the degradation of Cu-EDTA but also the recovery of Cu2+ ions compared to the PC and EC

356

systems (Figure 7b). However, because of the recalcitrant nature of metal-complexes, their

357

oxidative degradation proceeds slowly, which limits the efficient treatment. Zeng et al. added

358

persulfate (S2O82-) in the PEC reactor, which can be reductively decomposed into sulfate

359

radicals (SO4•-) on a cathode.95 Sulfate radicals with the high oxidation power can decompose

360

a wide range of recalcitrant pollutants. This persulfate-PEC system can enhance the

361

degradation efficiency of Cu-EDTA from 47.5% to 98.4% in 60 min and recovered the Cu2+

362

ions quantitatively under a wide range of pH condition compared with the PEC system

363

without persulfate (Figure 7c). The heavy metal ions complexed with inorganic anions

364

present a more difficult case for the metal recovery process. For example, the decomposition

365

of copper cyanides (Cu(CN)32-) releases toxic cyanide ions which could accompany the

366

generation of toxic HCN gas. Therefore, the treatment process should be carried out at higher

367

pH > 10 to avoid the protonation of the cyanide ions (pKa(HCN) = 9.2).96 Under such

368

alkaline condition, the liberated metal ions are not only reductively deposited on the cathode

369

but also easily precipitated. Zhao et al. reported the visible light-induced PEC degradation of

370

copper cyanides in the presence of EDTA or K4P2O7 with using a photoanode of Bi2MoO6.96

371

Cyanide ions were oxidized to cyanate ions (CNO-) by holes (Figure 7d) and the concurrent

372

oxidation of Cu+ to Cu2+ also occurred on the Bi2MoO6 photoanode. This resulted in the

373

deposition of CuO and CuOOH on the photoanode, which inhibited the further PEC reaction.

374

However, in the presence of EDTA or K4P2O7, liberated copper ions were immediately

375

complexed with EDTA or P2O74-, which could be reductively deposited on the cathode

376

(Figure 7e and f). With this strategy, cyanide could be effectively removed with the

377

simultaneous Cu recovery.

378

For practical applications, the decomposition of EDTA or CN- below the legal

379

emission levels and the recovery of Cu should be economically feasible. However, the power


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consumption is one of the major parameters that should be critically evaluated for the

381

application of this PEC process to water treatment. Most PEC processes require an additional

382

external bias although they are the photochemical process. Therefore, the electrical power

383

consumption is needed. In addition, the use of lamps instead of sunlight needs additional

384

energy for irradiation. Although the PEC process exhibited the superior performance for the

385

degradation of pollutants and metal recovery compared to the electrolytic system, the total

386

energy consumption of PEC system including the light source should be carefully compared

387

with other photocatalytic and electrolytic systems through technoeconomic analysis.97,98

388

Most of photocatalytic or PEC/EC water treatment applications involve the removal

389

of pollutants by oxidation or reduction alone. However, such processes have a limitation in

390

that the thermodynamic driving force of only one charge carrier is utilized while that of the

391

other charge carrier is wasted. For the better performance, the system should take advantage

392

of both the oxidation and reduction processes of pollutants simultaneously. Some examples

393

have been reported in the literature.

394

Choi et al. studied a sequential treatment of photocatalytic oxidation and dark

395

reduction using Ag/TiO2 (Figure 8a).99 In the sequential combination of photocatalysis-dark

396

thermal reaction, organic pollutants were first oxidized under UV irradiation with concurrent

397

storage of electrons in Ag. Then, the electrons stored in Ag/TiO2 and the intermediates

398

generated from the degradation of organic pollutants were utilized for the reduction of

399

hexavalent chromium (Cr(VI)) in the dark condition. Owing to the superior electron storage

400

capacity of Ag metal nanoparticles, Ag/TiO2 exhibited high electron storage compared to

401

Au/TiO2 and Pt/TiO2, which improved the reduction process on the metal nanoparticles and

402

the reactivity of remaining holes on TiO2 at the same time.100 The photocatalytic oxidation

403

efficiency of 4-CP decreased in the order of Pt/TiO2 > Ag/TiO2 > TiO2, however, Ag/TiO2

404

exhibited the higher activity for the removal of Cr(VI) than bare TiO2 and Pt/TiO2 in the post-


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405

irradiation dark period (Figure 8b). In Ag/TiO2 system, the stored electrons were long-lived

406

even in the presence of dissolved O2 and the Cr(VI) removal on Ag/TiO2 in the dark period

407

continued for hours. This was further confirmed by an open circuit potential measurement

408

which showed that a residual potential on the Ag/TiO2 electrode persisted over an hour in the

409

dark period (Figure 8c).

410

Simultaneous oxidation of As(III) to As(V) and reduction of Cr(VI) to Cr(III) is an

411

interesting treatment example for the redox-coupled removal of pollutants.101 Sun et al.

412

demonstrated that the synergistic redox conversion of Cr(VI) and As(III) could be enhanced

413

by the in-situ generated H2O2 in a three-dimensional electrocatalytic reactor employing

414

AuPd/CNT as an electrocatalyst (Figure 9a).102 Although the use of H2O2 increases the

415

treatment cost but the in-situ synthesis of H2O2 through O2 reduction is highly desirable as we

416

discussed in the previous section. In Figure 9b, AuPd/CNTs showed higher peak current

417

density and a more positive onset potential compared with other samples. This implies that

418

AuPd/CNTs electrode is more kinetically favored for O2 reduction and in-situ H2O2

419

generation compared to other electrode samples. Therefore, the enhancement of Cr(VI)/As(III)

420

redox conversion on AuPd/CNTs was attributed to the electrocatalytic in-situ generation of

421

H2O2 (Figure 9c). In addition, hydroxyl radicals (•OH) and superoxide radicals (O2•-) can be

422

also generated via the reduction of O2.103 The Cr(VI) reduction was little influenced by •OH

423

and O2•-, however, the role of •OH and O2•- influenced the As(III) oxidation reaction (Figure

424

9d).

425

The most outstanding advantage of PC- and PEC-based treatment technologies

426

compared with other AOPs is that they enable the reductive conversion of pollutants as well

427

as the oxidative degradation. The treatment of recalcitrant toxic metal complexes provides a

428

good example in which such merit makes the PC/PEC treatment versatile. The oxidative

429

degradation of ligands induces the decomplexation of the metal ions and the concurrent


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reductive conversion of metal ions makes the recovery of metal species facile. The oxidative

431

and reductive conversions in PC/PEC system can be also achieved sequentially as shown in

432

the example of Figure 8.101

433 434

5. Nitrogen Conversion Coupled with Organic Oxidation

435

Nitrogen is present in nature with various forms including nitrate, nitrite, nitrogen

436

oxides, dinitrogen, and ammonium (in the order of decreasing oxidation state). Among them,

437

nitrate is the most oxidized form (+5 oxidation state) of nitrogen species in the environment

438

and it can be found in agricultural or industrial wastewaters with high concentrations.104

439

Since nitrate is highly dissolved in water and non-adsorbing on metal oxides and most

440

adsorbents, the efficient conversion or removal of nitrate in water treatment has been a

441

crucial issue. The sequential reduction of nitrate should generate various products such as

442

nitrite, dinitrogen, and ammonium, which should be coupled with oxidation of organic

443

compounds (electron donors). The ideal scenario of the nitrate removal in water is to utilize

444

organic pollutants as in-situ electron donors to reduce nitrate selectively to dinitrogen, which

445

should complete the abiotic denitrification process in a single step. However, most reported

446

studies of nitrate conversion investigated the transformation of nitrate to ammonium ions. For

447

photocatalytic system for nitrate reduction, metal-modified photocatalysts were generally

448

investigated to use metallic catalyst for nitrate reduction in the presence of organic electron

449

donors.105,106 For example, Kominami et al. reported photocatalytic nitrate reduction to

450

ammonia using metal-loaded TiO2 (Pt, Pd, Ni, Au, Ag, Cu, etc) in the presence of oxalic

451

acid.107 Using these metal-loaded TiO2, photocatalytic activities for nitrate conversion to

452

ammonia, production of hydrogen, and oxidation of oxalic acid were investigated. Ag- or Cu-

453

loaded TiO2 samples showed the highest nitrate reduction with 96-99% selectivity to NH3.


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Ni- or Au-loaded TiO2 samples showed much lower NH3 selectivity (less than 22%) with

455

higher activities for H2 production, whereas Pt- or Pd-loaded TiO2 samples showed the

456

highest activities for H2 production with no activities for generation of NH3. The

457

photocatalytic reduction of nitrate should compete with photocatalytic hydrogen production

458

that should reduce the overall efficiency of nitrate conversion. Therefore, the control of

459

catalyst selectivity is critical. In general, metals with high hydrogen overvoltage (HOV)

460

favors nitrate reduction over H2 production. Similar to this study, most PC systems for nitrate

461

reduction has been investigated with various metallic catalysts, in particular silver (Ag),

462

copper (Cu) or Cu-based alloys.106,108-110 However, the complete conversion of nitrate to

463

dinitrogen gas has been rarely reported among the numerous studies of PC systems.111

464

For denitrification in water treatment, electrochemical (EC) system is one of the

465

emerging approaches.104 The direct reduction of nitrate to dinitrogen gas is also difficult in

466

EC system because complicated reaction pathways compete with nitrate reduction and

467

specific conditions are needed in each step. Therefore, a typical strategy for EC

468

denitrification is to combine the nitrate reduction to ammonia on the cathode and the

469

ammonia oxidation to dinitrogen on the anode in the same EC reactor.112,113 For ammonia

470

oxidation, the indirect oxidation process mediated by chlorine radical (Cl•) is an effective

471

approach.114 Y. Zhang et al. reported an advanced concept for PEC denitrification using

472

chlorination.115 In this study, they prepared WO3 nanoplate film as a photoanode for chlorine

473

oxidation under visible light irradiation and Pd-Cu alloy which was deposited on Ni foam

474

(NF) as a cathode for selective nitrate reduction to ammonia (Figure 10a). Pd-Cu alloy

475

electrode showed the highest electrocatalytic activity for nitrate reduction compared with

476

other electrodes (Pt, bare NF, Cu/NF, Pd/NF) (Figure 10b). They investigated nitrate removal

477

in a PEC-chlorine system and found that both nitrate and ammonia are effectively removed as


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478

chlorine ion concentration increased with demonstrating a high removal efficiency of total

479

nitrogen (~95%) (Figure 10c and d).

480 481 482

6. Photo(electro)catalytic fuel cell

483

Photo(electro)catalytic fuel cell (PEFC) is a promising technology that recovers

484

electricity along with the PEC oxidation of organic pollutants.116 This system consists of a

485

photoanode and a (photo)cathode in the solution containing organic substrates (such as

486

glucose, alcohol), and operates through the concurrent PEC oxidation of organic substrates on

487

the photoanode and dioxygen reduction on the cathode.117 The basic concept of PEFC is very

488

similar to conventional PEC system but PEFC should operate without an external bias, driven

489

by the Fermi level difference of two electrodes under irradiation. The Fermi level difference

490

generates an internal bias so that photo-excited electrons in photoanode can be transferred to

491

the cathode via an external circuit with generating electricity. Therefore, the net effect of

492

PEFC is to convert chemical energy contained in organic pollutants into electrical energy

493

with the assistance of photoenergy. Although simultaneous degradation of bio-organic fuels

494

and recovery of energy have been also achieved by microbial fuel cell (MFC), the current

495

efficiency of MFC is limited by the complex biochemical electron transfer process and

496

complicated operation processes involving bacteria cultivation.118 In this regards, the

497

combined systems of PEC cell or dye-sensitized solar cell (DSSC) with MFC have been

498

investigated to overcome the current limits of MFC. For the PEC-MFC combined system,

499

different configurations can be possible: i) microbial-modified electrode as a cathode in PEC

500

cell,119 ii) enzyme-mediated biochemical oxidation in PEC cell,120 and iii) assembled device

501

between PEC cell and MFC.121 Although all PEC-MFC combined systems are more efficient


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502

than conventional MFC, the system performance is still limited by the operating conditions

503

for microorganisms.

504

PEFC can overcome such limits of MFC-based systems. In a PEFC, rapid charge

505

transfer process can be achieved by oxide semiconductor-based photo(electro)catalysis so

506

that the system can be effectively operated even without external bias. Chen et al. reported

507

wastewater treatment system combined with PEFC to utilize chemical energy abstracted from

508

wastewater to generate electricity and hydrogen gas.122 In this study, they prepared WO3

509

photoanode and Cu2O photocathode and induced an internal bias by mismatched Fermi levels

510

between two electrodes (Figure 11a). In this way, electricity was generated without external

511

bias by transferring photogenerated electrons from WO3 photoanode to Cu2O photocathode

512

via an external circuit (Figure 11b and c). At the same time, organic compounds (phenol,

513

Rhodamine B, and Congo red) were simultaneously oxidized by photogenerated holes on

514

WO3 photoanode (Figure 11d). Zhou et al. reported a solar-charged PEFC with similar

515

electrode configurations (WO3 photoanode-Cu2O photocathode) for phenol oxidation and

516

simultaneous hydrogen production.123 Hydrogen production on Cu2O electrode was 3 times

517

higher when coupled with the phenol oxidation on WO3 electrode (compared with the case

518

coupled with water oxidation on WO3), which implies that phenol oxidation on WO3 provides

519

more electrons that are transferred though the external circuit. In addition, hydrogen could be

520

produced on Cu2O even under the dark after the irradiation period due to the stored electrons

521

in the WO3 electrode, which enabled simultaneous wastewater treatment, energy recovery,

522

and charge storage.

523

PEFC might be a good candidate technology for simultaneous wastewater treatment

524

and hydrogen production but there are obvious limits for choosing photoelectrode materials.

525

To drive effective internal electron flow between two electrodes, the Fermi level of

526

photoanode should be more negative than that of (photo)cathode and the Fermi level


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527

difference should be as high as possible. However, the suitable material combinations that

528

match such properties are not many and the development of new photo-active materials for

529

PEFC applications are highly encouraged.

530 531

7. Summary and outlook

532

The solar chemical conversion processes initiated by the bandgap excitation of

533

semiconductor materials have been extensively investigated for decades by employing the

534

photocatalysis of suspended semiconductor nanoparticles and the photoelectrocatalysis of

535

semiconductor electrodes. Both processes are based on the photoinduced interfacial charge

536

transfers, which utilize the reductive and oxidative power of electrons and holes, respectively.

537

The semiconductor-based PC and PEC processes have many advantages since they can be

538

environmental benign (no toxic chemicals needed), sustainable (driven by solar power), facile

539

and safe (working in the ambient condition), and economically feasible (when low cost and

540

earth-abundant semiconductor materials are employed). In most photocatalytic and PEC

541

processes, the main purposes of the chemical conversion processes are focused on either the

542

electron-drive reduction (e.g., H2 production, CO2 conversion) or the hole-driven oxidation

543

(e.g., oxidation of organic compounds). Although any PC or PEC process should involve the

544

reductive and oxidative conversions simultaneously to make the overall charge balance, most

545

studies focus on either process (target conversion reaction) and the other counterpart reaction

546

has received less attention and been little utilized for useful conversions.

547

This perspective article aims to introduce, analyze and discuss the dual functional PC,

548

PEC, and PV-EC processes that utilize both electron reducing power and hole oxidizing

549

power efficiently to achieve simultaneous water treatment (pollutant degradation) and the

550

recovery of energy (e.g., H2) and resources (e.g., heavy metal elements). Many studies have

551

tried to design and modify the photoactive materials for this purpose. The integration of water


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treatment with energy/resource recovery in a single PC/PEC system is an ideal example of

553

energy and environmental applications of solar energy but there is a long way to go beyond

554

the conceptual demonstration towards commercialization. The dual functional water

555

treatment is considered a suitable method that can solve the inextricable water-energy nexus

556

problem. The merits, demerits, and challenges in developing the dual functional PC, PEC,

557

and PV-EC processes are summarized in Table 2. The overall process can be limited by low

558

efficiency, instability, and high operation cost even at a laboratory scale. To solve these

559

limitations and establish practical solar-driven systems, we need to develop not only highly

560

efficient and durable photocatalytic materials but also economical and scalable solar reactors

561

for commercialization. Considering that there is little effort for this latter part despite the

562

intensive interests on the former part, the practical realization of the solar-powered dual

563

functional water treatment technology needs more balanced approaches and efforts in both

564

material development and the reactor design and engineering.

565 566

Acknowledgements

567

This work was financially supported by the Global Research Laboratory (GRL) Program

568

(NRF-2014K1A1A2041044), Basic Science Research Program (NRF-2017R1A2B2008952),

569

and the framework of international cooperation program (NRF-2017K2A9A2A11070417),

570

which were funded by the Korea Government (MSIP) through the National Research

571

Foundation (NRF).

572 573

References

574 575 576 577

(1) Park, H.; Kim, H.-i.; Moon, G.-h.; Choi, W. Photoinduced Charge Transfer Processes in Solar Photocatalysis Based on Modified TiO2. Energy Environ. Sci. 2016, 9, 411-433. (2) Park, H.; Park, Y.; Kim, W.; Choi, W. Surface Modification of TiO2 Photocatalyst for Environmental Applications. J. Photochem. Photobiol. C 2013, 15, 1-20.


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(174) Qi, F. J.; Yang, B.; Wang, Y. B.; Mao, R.; Zhao, X. H2O2 Assisted Photoelectrocatalytic Oxidation of Ag-Cyanide Complexes at Metal-free g-C3N4 Photoanode with Simultaneous Ag Recovery. ACS Sustainable Chem. Eng. 2017, 5, 5001-5007. (175) Paschoal, F. M. M.; Anderson, M. A.; Zanoni, M. V. B. Simultaneous Removal of Chromium and Leather Dye from Simulated Tannery Effluent by Photoelectrochemistry. J. Hazard. Mater. 2009, 166, 531-537. (176) Wu, Z. Y.; Zhao, G. H.; Zhang, Y. J.; Liu, J.; Zhang, Y. N.; Shi, H. J. A Solar-Driven Photocatalytic Fuel Cell with Dual Photoelectrode for Simultaneous Wastewater Treatment and Hydrogen Production. J. Mater. Chem. A 2015, 3, 3416-3424. (177) Zhang, Y.; Li, J. H.; Bai, J.; Li, L. S.; Xia, L. G.; Chen, S.; Zhou, B. X. Dramatic Enhancement of Organics Degradation and Electricity Generation via Strengthening Superoxide Radical by using a Novel 3D AQS/PPy-GF Cathode. Water Res. 2017, 125, 259269. (178) Zhou, Z. Y.; Wu, Z. Y.; Xu, Q. J.; Zhao, G. H. A Solar-Charged Photoelectrochemical Wastewater Fuel Cell for Efficient and Sustainable Hydrogen Production. J. Mater. Chem. A 2017, 5, 25450-25459. (179) Kim, J.; Choi, W. J. K.; Choi, J.; Hoffmann, M. R.; Park, H. Electrolysis of Urea and Urine for Solar Hydrogen. Catal. Today 2013, 199, 2-7. (180) Cho, K.; Hoffmann, M. R. Molecular Hydrogen Production from Wastewater Electrolysis Cell with Multi-Junction BiOx/TiO2 Anode and Stainless Steel Cathode: Current and Energy Efficiency. Appl. Catal. B 2017, 202, 671-682. (181) Liu, Y. B.; Xie, J. P.; Ong, C. N.; Vecitis, C. D.; Zhou, Z. Electrochemical Wastewater Treatment with Carbon Nanotube Filters coupled with In Situ Generated H2O2. Environ. Sci. Water Res. Technol. 2015, 1, 769-778. (182) Cho, K.; Kwon, D.; Hoffmann, M. R. Electrochemical Treatment of Human Waste Coupled with Molecular Hydrogen Production. RSC Adv. 2014, 4, 4596-4608. (183) Jiang, J. Y.; Chang, M.; Pan, P. Simultaneous Hydrogen Production and Electrochemical Oxidation of Organics using Boron-Doped Diamond Electrodes. Environ. Sci. Technol. 2008, 42, 3059-3063. (184) Juang, R. S.; Lin, L. C. Treatment of Complexed Copper(II) Solutions with Electrochemical Membrane Processes. Water Res. 2000, 34, 43-50. (185) Szpyrkowicz, L.; Zilio-Grandi, F.; Kaul, S. N.; Polcaro, A. M. Copper Electrodeposition and Oxidation of Complex Cyanide from Wastewater in an Electrochemical Reactor with a Ti/Pt Anode. Ind. Eng. Chem. Res. 2000, 39, 2132-2139. (186) Kim, J.; Kwon, D.; Kim, K.; Hoffmann, M. R. Electrochemical Production of Hydrogen Coupled with the Oxidation of Arsenite. Environ. Sci. Technol. 2014, 48, 20592066.

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Table 1. Dual functional water treatment systems for pollutants oxidation combined with simultaneous recovery of energy or resource (reduction process) Type of systema

Catalyst

Oxidation process (Pollutant degradation)

PC PC

F-TiO2/GO/Pt F-TiO2/Pt

PC PC PC PC PC

F(or P)-TiO2/Pt SrTiO3/Rh/Cr2O3 F-TiO2/Pt Er3+:Y3Al5O12/Pt-TiO2 TiO2/CdS/Pt

PC PC

TiO2/Pt TiO2/Pt

PC

TiO2/Agarose hydrogel

PC PC PC PC PC PC PC PC PC

TiO2/AgX P-TiO2/Pt Er3+:Y3Al5O12/MoS2–NaTaO3– PdS Cu2O cubooctahedrons Zinc oxysulfide (ZnO0.6S0.4) Pt/TiO2/Nf Ti3+-doped TiO2 TiO2/Pt MoS2/ZnIn2S4/RGO

4-Chlorophenol (4-CP) 4-CP, bisphenol A, 2,4dichlorophenoxyacetic acid 4-CP, urea 4-CP 4-CP, bisphenol A Phenol, glycerol Inorganic (S2-/SO32-) or organic (ethanol) Formaldehyde Alcohols(Methanol, Ethanol, 1propanol, 1-butanol), Organic acids(Formic acid, Acetic acid), Acetaldehyde Heavy metal (Cd2+)-containing wastewater Waste activated sludge Estrogenic activity Amaranth(dye)

PC PC PC

Ru-doped LaFeO3 F-TiO2/Pt CNT/TiO2/Pt

PC

Fe2O3 polymorphs

PC PC

TiO2/Pt TiO2/Pt

PC PC PC PC PC PC PC PC PC PC PC PC PC PC PC

N-doped ZnO N-TiO2/graphene CdS/TiO2 CdS QD/GeO2 glass CdxZn1-xS TiO2 TiO2 TiO2 TiO2 TiO2/Cu TiO2/Ag TiO2/Ag(or Pd) TiO2/Ag g-C3N4/MoS2 CdS/Sr(NbZn)O

Dyes (MO, RhB or MB) RV5 RhB MB, MO, RhB, 4-CP AO7 Dyes (RhB, EY, MB), Fulvic acid, p-nitrophenol Glucose Glucose Biomass-derived compounds (arabinose, fructose, glucose, cellobiose) Biomass-derived compounds (ethanol, glycerol, glucose) Glycerol Bio-ethanol, Glycerol, Alcohols, Saccharides, starch, cellulose H2S H2S H2S H2S H2S EDTA Cyanide Cu-EDTA, Glycerol NH3 Glycerol Formic acid Formic acid 4-CP RhB As(III)


Reduction process (Recovery of fuel & resource) H2 H2

Ref.

H2 H2 H2 H2 H2

63

H2 H2

127

H2

129

H2 H2 H2

129

H2 H2 H2 H2 H2 H2

132

H2 H2 H2

138

H2

141

H2 H2

142

H2 H2 H2 H2 H2 Fe, Hg, Ag, Cr Cu H2 NO2- to N2 NO3-, H2 NO3NO3Cr(VI) to Cr(III) Cr(VI) to Cr(III) H2

144

57 124

66 56 125 126

128

130 131

133 134 135 136 137

139 140

143

145 146 147 148 149 91 92 150 151 31 152 99 153 154

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ACS Catalysis

PC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC PEC

TiO2/Pt Photoanode : Electrochromic TiO2 nanotube Cathode : Stainless steel Photoanode : Multi-layered BiOx–TiO2/Ti electrodes Photoanode : WO3 Cathode : Cobalt chlorin complex Photoanode : (C,N)-doped TNT Cathode : Pt wire Photoanode : TiO2 nanorods Photocathode : Carbon coated Cu2O nanowire Photoanode : TiO2 film Cathode : Pt foil Photoanode : TiO2 nanofiber/Ag@AgCl Cathode : Pt film Anode : Cu2O/TiO2 nanotube Cathode : Pt wire Anode : TiO2 Cathode : Pt Photoanode : W-doped TiO2 nanotube Cathode : Pt foil Photoanode : WO3/TiO2/Ti Cathode : Pt mesh Photoanode : CdSe/TiO2 Nanotube Cathode : Pt foil Photoanode : TiO2 nanotube Cathode : Pt foil Photoanode : WO3 Cathode : Pt foil Photoanode : Fe2O3/Ni(OH)2 Cathode : Pt Photoanode : Metal-doped Fe2O3 Cathode : Carbon paste with Pt photoanode: WO3 Cathode: Si PVC Photoanode : n-Si Cathode : Pt Photoanode : n-Si Cathode : Carbon Photoanode : TiO2 film Cathode : Stainless steel Photoanode : IrOx-coated Ti mesh Cathode : Stainless steel Photoanode : TiO2/Ti Cathode : Stainless steel Photoanode : Bi2MoO6 Cathode : Ti Photoanode : TiO2 nanotube Cathode : Ti plate Photoanode : g-C3N4

As(III), 4-CP 4-CP

H2

155

Phenol

H2

156

Chlorine oxidized species in seawater

H2O2

87

Perfluorooctanoic acid (PFOA)

H2

157

Phenol, Toluene, p-Xylene, Mesitylene, Hydroquinone, Phloroglucinol Nitrogen containing water (Urea, Formamide, NH4+) Urban wastewater (17-βethynylestradiol)

H2

158

H2

159

H2

160

Ibuprofen

H2

161

Aniline, salicylic acid

H2O2

162

RhB

H2

163

RB5

H2

164

MO

H2

165

MB

H2

166

Biomass derived organic wastes (ethanol, glycerol or sorbitol) Glucose

H2

167

H2

168

Glucose

H2

169

H2S

H2

170

H2S

H2

171

H2S

H2O2

83

Cu-EDTA

Cu

93

EDTA

Cu

172

Cu-EDTA

Cu

95

Cu-Cyanide

Cu

96

Ofloxacin

Cu

173

Ag-Cyanide

Ag

174


54

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PEC PEFC

PEFC PEFC PEFC PV- EC PV- EC PV- EC EC

EC EC EC EC EC EC

Cathode : Ti plate Photoanode : TiO2 Cathode : Pt Photoanode: TiO2 nanotube(or nanorod) Photocathode : Carbon coated Cu2O nanowire Photoanode : TiO2 nanotube Cathode : anthraquinone/ polypyrrole/graphite felt Photoanode : WO3 Photocathode : Cu2O Photoanode : WO3 Photocathode : Cu2O Anode : BiOx-TiO2 Cathode : Stainless steel Anode : BiOx/TiO2 Cathode : Stainless steel Anode : BiOx/TiO2 Cathode : Stainless steel Anode : Active carbon nanotube Cathode : Active carbon nanotube Anode : BiOx/TiO2 Cathode : Stainless steel Anode : Boron-doped diamond Cathode : Stainless steel Anode : IrO2/Ti Cathode : Pt/Ti Anode : Pt/Ti plate Cathode : Copper cylinder Anode : BiOx-TiO2 Cathode : Stainless steel Anode : Ru-coated Ti Cathode : Ru-coated Ti AuPd/CNT (electrocatalyst)

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Acid dye, Surfactant

Cr(VI) to Cr(III)

175

Phenol

H2

176

MO

Electricity, H2O2

177

Phenol, RhB, Congo red

Electricity, H2

122

Phenol

Electricity

178

Urea in human urine

H2

179

Mixture of domestic wastewater & stored urine Domestic wastewater

H2

180

H2

69

Phenol

H2O2

181

Domestic wastewater & Human urine Cyanide-containing wastewater, 4-nitrophenol Cu-EDTA

H2

182

H2

183

Cu

184

Cyanide

Cu

185

As(III)

H2

186

As(III)

Cr(VI) to Cr(III)

102

a

PC: Photocatalytic, PEC: Photoelectrochemical, PEFC: Photoelectrochemical fuel cell, EC: Electrochemical, PV-EC: Photovoltaic-driven Electrochemical systems


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Table 2. Comparison of solar-driven technologies for dual-functional water treatment process Photocatalytic (PC)  Single phase reaction medium  Very simple design and setup (low capital cost)  No need of electrolytes  Good stability  Direct energy supply from solar light

Photoelectrochemical (PEC)  Redox potential controllable by an applied bias  Easier control of reaction parameters & conditions  Relatively simple design and setup  Separation of water treatment cell from energy/ resource recovery cell  Direct energy supply from solar light

Photovoltaic-driven EC (PV-EC)  Mature status of PV and EC technology  Most merits of PEC system  Separation of the light collecting part from the reactor part  Highest solar conversion efficiency  Easy scale-up and wide range of applications using commercialized PV

Demerits

 Low efficiency and small scale operation (difficult to scale up)  Redox potentials limited by SC bandgap and band positions  Difficulty to optimize reaction conditions for both oxidation and reduction simultaneously  Difficult to investigate the operating mechanisms  Need of separation of catalyst and products

 Medium efficiency and medium scale operation (difficult to scale up)  Highly caustic electrolyte needed  Lack of long-term stability of SC electrode due to fouling and material deterioration  Difficulty to construct large area SC electrode

 High capital cost  Need of electricity and the electricity loss during EC conversion  Highly caustic electrolyte needed  Limited by PV and EC efficiency & cost

Challenges & Future works

 To control and optimize the catalyst and reaction conditions for both oxidation and reduction simultaneously in a single phase medium  Catalyst engineering for higher efficiency and selectivity

 Development of SC photoelectrodes with improved efficiency and durability  Development of SC photoelectrodes that operate under mild electrolyte and circumneutral pH conditions

 System engineering to reduce the cost for PV and EC parts  Development of PV and EC technology

Merits

 Development of efficient & economical solar reactors for PC, PEC, and PV-EC  System & material engineering optimized for the characteristics of target water to be treated  Systematic investigation about the coupling between the kind of pollutants to be removed and the kind of energy/resource to be recovered  Antifouling strategy for practical water treatment applications


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions Scheme 1. (a) Common applications of semiconductor photocatalysis for environmental remediation and water splitting. Red dashed line represents the uncommon dual-functional photocatalysis for simultaneous water treatment and energy recovery. (b) Photoelectrocatalytic processes for simultaneous water treatment and energy/resource recovery. Scheme 2. Energy level diagram for a typical photocatalyst (TiO2) and various redox couples that should serve as either an electron acceptor (right side) or an electron donor (left side). Figure 1. (a) Simultaneous degradation of 4-CP and production of H2 in UV-irradiated suspensions of bare TiO2, F-TiO2, Pt/TiO2, and F-TiO2/Pt. (b) Time profiles of H2 production from the degradation of the organic substrate (4-CP and BPA) during repeated photocatalysis cycles in the same batch reactor. Reproduced with permission from ref 56. Copyright 2010 The Royal Society of Chemistry. (c) Schematic illustrations of interfacial charge transfer and recombination occurring on Pt/TiO2-F. Reproduced with permission from ref 57. Copyright 2015 Elsevier. (d) Production of H2 in the suspension of bare TiO2, F-TiO2, P-TiO2, Pt/TiO2, F-TiO2/Pt, and P-TiO2/Pt with 4-CP under λ > 320 nm irradiation. Reproduced with permission from ref 63. Copyright 2012 The Royal Society of Chemistry. Figure 2. (a) HR-TEM image of Cr2O3/Rh/SrTiO3 with a schematic illustration. (b) Photocatalytic degradation of 4-CP and concurrent chloride ion production in a de-aerated suspension of Rh/SrTiO3 and Cr2O3/Rh/SrTiO3. (c) Time profiles of H2 and O2 production in the presence of 4-CP in the irradiated suspension of Cr2O3/Rh/SrTiO3 and Rh/SrTiO3. (d) Comparison of the initial H2 production rate between Cr2O3/Rh/SrTiO3 and F-TiO2/Pt in the presence of 4-CP. Reproduced with permission from ref 66. Copyright 2016 The Royal Society of Chemistry. Figure 3. (a) Schematic illustration of electrochromic TiO2 nanotube arrays (Blue-TNTs) combined with a stainless steel electrode for PEC degradation of organic compounds with simultaneous H2 production. (b) DRS spectra of pristine TiO2 nanotube arrays (TNTs) and Blue-TNTs. (c) PEC degradation of 4-CP on TNTs and Blue-TNTs with the concurrent H2 production for five repeated runs. Reproduced with permission from ref 54. Copyright 2017 American Chemical Society. Figure 4. (a) Schematic illustration of a sunlight-driven hybrid PEC system that couples desalination, water treatment, and H2 production at the same time. (b) Time profiles of urea removal and TOC (upper panel) and concomitant production of nitrate and ammonia (lower panel). (c) H2 production in the cathode compartment and Faradaic efficiency. Reproduced with permission from ref 68. Copyright 2018 The Royal Society of Chemistry. Figure 5. (a) Schematic illustration of the PV-powered electrolysis system for wastewater treatment with simultaneous H2 production. (b), (c) Time profiles of COD removal in


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ACS Catalysis

domestic wastewater with different chloride concentration (0, 10, 30, 50 mM) and anodic potential (L: 2.2V, H: 3.0 VNHE). Reproduced with permission from ref 69. Copyright 2014 American Chemical Society. Figure 6. (a) Schematic illustration of the PEC cell with redox shuttles for the production of H2O2 and S from H2S. (b) Current-voltage curves in a two electrode system on p+n-Si with Carbon or Pt as the counter electrode under AM 1.5 irradiation (100 mW cm-2). Reproduced with permission from ref 83. Copyright 2014 The Royal Society of Chemistry. (c) Time profiles of PEC production of H2O2 in water (red circle), seawater (blue circle), and NaCl solution (blue square) at pH 1.3 under AM 1.5 irradiation (100 mW cm-2). Reproduced with permission from ref 87. Copyright 2016 Nature Publishing Group. (d) Time profiles of H2O2 production on BiVO4 photoanode and carbon cathode in a two-electrode system with O2 purging with an applied bias of 1.5 V. Reproduced with permission from ref 88. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Overall scheme for PEC production of H2O2 and reactive chlorine species (RCS) on photoanode coupled with the concurrent H2O2 production on the catalysts-loaded cathode. Figure 7. (a) Schematic illustration of PEC oxidation of Cu-EDTA and recovery of Cu. (b) Removal profiles of Cu-EDTA complexes and the concurrent profiles of Cu recovery with photocatalysis (PC), electrooxidation (EO), and PEC processes. Reproduced with permission from ref 93. Copyright 2013 American Chemical Society. (c) Variation of residual Cu complexes ratio and percentage of Cu recovery in UV/S2O82-(Persulfate), PEC, EC/Persulfate, and PEC/Persulfate processes. Reproduced with permission from ref 95. Copyright 2016 American Chemical Society. (d) Time profiles of intermediates produced by PEC oxidation of Cu(CN)32-. Effect of (e) EDTA and (f) K4P2O7 on total cyanide removal and Cu recovery. Reproduced with permission from ref 96. Copyright 2015 American Chemical Society. Figure 8. (a) Schematic illustration of photocatalytic degradation of 4-CP and the following reductive removal of Cr(VI) in the dark using Ag/TiO2. (b) Time profiles of the photocatalytic degradation of 4-CP under UV irradiation and the subsequent Cr(VI) reduction in the dark (after turning off the light). (c) Time profiles of the normalized open circuit potential (OCP) of the bare TiO2, Pt/TiO2, and Ag/TiO2 in 0.1 M NaClO4 (pH 3) with 300 M 4-CP under λ > 320 nm irradiation. Reproduced with permission from ref 99. Copyright 2017 American Chemical Society. Figure 9. (a) Schematic illustration of simultaneous transformation of Cr(VI) and As(III) using AuPd/CNTs as an electrocatalyst. (b) The rotating ring-disk electrode (RRDE) curves of CNTs, Pd/CNTs, Au/CNTs, and AuPd/CNTs in O2-saturated H2SO4 solution (pH 3). The inset shows the corresponding electron transfer number of AuPd/CNTs. (c) Redox conversion of Cr(VI) and As(III) in the presence of AuPd/CNTs, electrolysis, and electrolysis with AuPd/CNTs. (d) Effects of various free radical scavengers on redox conversion of Cr(VI) and As(III) after 15 min reaction. Reproduced with permission from ref 102. Copyright 2015 American Chemical Society.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10. (a) Schematic illustration of the inorganic nitrogen removal mechanism in PEC system based on WO3 photoanode and Cu-Pd loaded Ni foam cathode. (b) Electrocatalytic nitrate removal efficiencies on different cathode materials. Effect of chlorine concentrations on PEC (c) nitrate removal and (d) ammonia generation. Reproduced with permission from ref 115. Copyright 2018 American Chemical Society. Figure 11. (a) Energy level diagram of WO3/W-Cu2O/Cu photoelectrocatalytic wastewater fuel cell (PWFC) system for organic degradation and H2 production or electricity generation (VP, photovoltage). (b) The open-circuit voltage of PWFC system of WO3/W-Cu2O/Cu in the dark and under AM 1.5 irradiation (100 mW cm-2) in 0.1M KH2PO4 (pH 7). (c) Photocurrenttime profiles of PWFC system with no external bias in 0.1M KH2PO4 (pH 7). Photocurrent activities of other electrodes (WNA-Cu2O/Cu, WO3/W-Pt and Pt-Cu2O/Cu) were also compared. (d) Removal efficiency of phenol in photolysis and PWFC systems of WO3/WCu2O/Cu and WNA-Cu2O/Cu. Reproduced with permission from ref 122. Copyright 2012 American Chemical Society.


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ACS Catalysis

Scheme 1. (a) Common applications of semiconductor photocatalysis for environmental remediation and water splitting. Red dashed line represents the uncommon dual-functional photocatalysis for simultaneous water treatment and energy recovery. (b) Photoelectrocatalytic processes for simultaneous water treatment and energy/resource recovery.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 2. Energy level diagram for a typical photocatalyst (TiO2) and various redox couples that should serve as either an electron acceptor (right side) or an electron donor (left side).


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ACS Catalysis

Figure 1. (a) Simultaneous degradation of 4-CP and production of H2 in UV-irradiated suspensions of bare TiO2, F-TiO2, Pt/TiO2, and F-TiO2/Pt. (b) Time profiles of H2 production from the degradation of the organic substrate (4-CP and BPA) during repeated photocatalysis cycles in the same batch reactor. Reproduced with permission from ref 56. Copyright 2010 The Royal Society of Chemistry. (c) Schematic illustrations of interfacial charge transfer and recombination occurring on Pt/TiO2-F. Reproduced with permission from ref 57. Copyright 2015 Elsevier. (d) Production of H2 in the suspension of bare TiO2, F-TiO2, P-TiO2, Pt/TiO2, F-TiO2/Pt, and P-TiO2/Pt with 4-CP under λ > 320 nm irradiation. Reproduced with permission from ref 63. Copyright 2012 The Royal Society of Chemistry.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) HR-TEM image of Cr2O3/Rh/SrTiO3 with a schematic illustration. (b) Photocatalytic degradation of 4-CP and concurrent chloride ion production in a de-aerated suspension of Rh/SrTiO3 and Cr2O3/Rh/SrTiO3. (c) Time profiles of H2 and O2 production in the presence of 4-CP in the irradiated suspension of Cr2O3/Rh/SrTiO3 and Rh/SrTiO3. (d) Comparison of the initial H2 production rate between Cr2O3/Rh/SrTiO3 and F-TiO2/Pt in the presence of 4-CP. Reproduced with permission from ref 66. Copyright 2016 The Royal Society of Chemistry.


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ACS Catalysis

Figure 3. (a) Schematic illustration of electrochromic TiO2 nanotube arrays (Blue-TNTs) combined with a stainless steel electrode for PEC degradation of organic compounds with simultaneous H2 production. (b) DRS spectra of pristine TiO2 nanotube arrays (TNTs) and Blue-TNTs. (c) PEC degradation of 4-CP on TNTs and Blue-TNTs with the concurrent H2 production for five repeated runs. Reproduced with permission from ref 54. Copyright 2017 American Chemical Society.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. (a) Schematic illustration of a sunlight-driven hybrid PEC system that couples desalination, water treatment, and H2 production at the same time. (b) Time profiles of urea removal and TOC (upper panel) and concomitant production of nitrate and ammonia (lower panel). (c) H2 production in the cathode compartment and Faradaic efficiency. Reproduced with permission from ref 68. Copyright 2018 The Royal Society of Chemistry.


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ACS Catalysis

Figure 5. (a) Schematic illustration of the PV-powered electrolysis system for wastewater treatment with simultaneous H2 production. (b), (c) Time profiles of COD removal in domestic wastewater with different chloride concentration (0, 10, 30, 50 mM) and anodic potential (L: 2.2V, H: 3.0 VNHE). Reproduced with permission from ref 69. Copyright 2014 American Chemical Society.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (a) Schematic illustration of the PEC cell with redox shuttles for the production of H2O2 and S from H2S. (b) Current-voltage curves in a two electrode system on p+n-Si with Carbon or Pt as the counter electrode under AM 1.5 irradiation (100 mW cm-2). Reproduced with permission from ref 83. Copyright 2014 The Royal Society of Chemistry. (c) Time profiles of PEC production of H2O2 in water (red circle), seawater (blue circle), and NaCl solution (blue square) at pH 1.3 under AM 1.5 irradiation (100 mW cm-2). Reproduced with permission from ref 87. Copyright 2016 Nature Publishing Group. (d) Time profiles of H2O2 production on BiVO4 photoanode and carbon cathode in a two-electrode system with O2 purging with an applied bias of 1.5 V. Reproduced with permission from ref 88. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Overall scheme for PEC production of H2O2 and reactive chlorine species (RCS) on photoanode coupled with the concurrent H2O2 production on the catalysts-loaded cathode.


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ACS Catalysis

Figure 7. (a) Schematic illustration of PEC oxidation of Cu-EDTA and recovery of Cu. (b) Removal profiles of Cu-EDTA complexes and the concurrent profiles of Cu recovery with photocatalysis (PC), electrooxidation (EO), and PEC processes. Reproduced with permission from ref 93. Copyright 2013 American Chemical Society. (c) Variation of residual Cu complexes ratio and percentage of Cu recovery in UV/S2O82-(Persulfate), PEC, EC/Persulfate, and PEC/Persulfate processes. Reproduced with permission from ref 95. Copyright 2016 American Chemical Society. (d) Time profiles of intermediates produced by PEC oxidation of Cu(CN)32-. Effect of (e) EDTA and (f) K4P2O7 on total cyanide removal and Cu recovery. Reproduced with permission from ref 96. Copyright 2015 American Chemical Society.


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Figure 8. (a) Schematic illustration of photocatalytic degradation of 4-CP and the following reductive removal of Cr(VI) in the dark using Ag/TiO2. (b) Time profiles of the photocatalytic degradation of 4-CP under UV irradiation and the subsequent Cr(VI) reduction in the dark (after turning off the light). (c) Time profiles of the normalized open circuit potential (OCP) of the bare TiO2, Pt/TiO2, and Ag/TiO2 in 0.1 M NaClO4 (pH 3) with 300 M 4-CP under λ > 320 nm irradiation. Reproduced with permission from ref 99. Copyright 2017 American Chemical Society.


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ACS Catalysis

Figure 9. (a) Schematic illustration of simultaneous transformation of Cr(VI) and As(III) using AuPd/CNTs as an electrocatalyst. (b) The rotating ring-disk electrode (RRDE) curves of CNTs, Pd/CNTs, Au/CNTs, and AuPd/CNTs in O2-saturated H2SO4 solution (pH 3). The inset shows the corresponding electron transfer number of AuPd/CNTs. (c) Redox conversion of Cr(VI) and As(III) in the presence of AuPd/CNTs, electrolysis, and electrolysis with AuPd/CNTs. (d) Effects of various free radical scavengers on redox conversion of Cr(VI) and As(III) after 15 min reaction. Reproduced with permission from ref 102. Copyright 2015 American Chemical Society.


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Figure 10. (a) Schematic illustration of the inorganic nitrogen removal mechanism in PEC system based on WO3 photoanode and Cu-Pd loaded Ni foam cathode. (b) Electrocatalytic nitrate removal efficiencies on different cathode materials. Effect of chlorine concentrations on PEC (c) nitrate removal and (d) ammonia generation. Reproduced with permission from ref 115. Copyright 2018 American Chemical Society.


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ACS Catalysis

Figure 11. (a) Energy level diagram of WO3/W-Cu2O/Cu photoelectrocatalytic wastewater fuel cell (PWFC) system for organic degradation and H2 production or electricity generation (VP, photovoltage). (b) The open-circuit voltage of PWFC system of WO3/W-Cu2O/Cu in the dark and under AM 1.5 irradiation (100 mW cm-2) in 0.1M KH2PO4 (pH 7). (c) Photocurrenttime profiles of PWFC system with no external bias in 0.1M KH2PO4 (pH 7). Photocurrent activities of other electrodes (WNA-Cu2O/Cu, WO3/W-Pt and Pt-Cu2O/Cu) were also compared. (d) Removal efficiency of phenol in photolysis and PWFC systems of WO3/WCu2O/Cu and WNA-Cu2O/Cu. Reproduced with permission from ref 122. Copyright 2012 American Chemical Society.


Dual-Functional Photocatalytic and Photoelectrocatalytic Systems for

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