Alternative Oxidation Reactions for Solar-driven Fuel Production - ACS

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Alternative Oxidation Reactions for Solar-driven Fuel Production Charles Lhermitte, and Kevin Sivula ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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ACS Catalysis

Alternative Oxidation Reactions for Solar-Driven Fuel Production Charles R. Lhermitte, Kevin Sivula*

Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, 1015 Lausanne, Switzerland

ABSTRACT: For nearly half a century, water oxidation has been extensively investigated as the electron source for solar powered H2 fuel production from water. However, despite a thermodynamic potential of only 1.23 V required at standard conditions, driving the oxygen evolution reaction (OER) typically requires 1.5 - 1.8 V resulting in a significant loss. Over the last decade, numerous researchers have begun to re-explore the idea of replacing water oxidation with more kinetically facile oxidation reactions in photoelectrochemical and photocatalytic solar H2 production systems. Alternate photooxidation reactions can be employed as a means of chemical valorization, in

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addition to providing electrons for H2 production from water while reducing the losses associated with the OER. In this perspective, we discuss other possible oxidation reactions, and in particular we highlight recent progress in the investigation of organic based photo-oxidation reactions. We focus on oxidation reactions that have potential applications as a form of chemical valorization, and that can take place in aqueous solutions to allow concurrent H2 production via water reduction at a (photo)cathode. A critical assessment and an outlook towards the prospective large scale implementation of this technology is finally considered.

KEYWORDS:

Solar

Fuels,

artificial

photosynthesis,

photoelectrochemistry,

photocatalysis, photoanode, oxygen evolution reaction.

Introduction

Coal, oil, and natural gas have literally fueled the technological development of our society, however, due to their limited supply and the polluting nature of their combustion products, the use of fossil fuel does not represent a sustainable nor environmentally


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friendly means of energy consumption. Furthermore, as the world population increases and the reserves of fossil fuels disappear, the challenges associated with their use are further exacerbated.1,2 As a result of the limitations associated with fossil fuels, the effort to develop sustainable and environmentally safe energy technologies has steadily intensified over the last few decades. Of the possible methods, solar energy represents the most promising renewable energy source for a sustainable society because it can meet the energy needs of our entire planet (ca. 20 TW) even if only a fraction of the available 89,000 TW absorbed by the land and oceans is converted to a useable form.117

However, despite its promise, solar energy has a few limitations. First, it is not homogenously dispersed across the planet; some places receive a much higher flux of solar irradiation than others due to geography, the tilt of the Earth’s rotational axis, and weather. Furthermore, solar energy is inherently periodic with diurnal variations, and the demand for power consumption during the diurnal cycle does not always match the availability. This poses a challenge when using traditional photovoltaic devices to convert energy from the Sun to electricity. Moreover, the current methods for large scale power


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distribution, namely electricity grids, have not been designed for the large variations inherent to photovoltaic power. Because of this, there is the potential for grid overload; on particularly sunny days, a photovoltaic farm can overload the energy grid if the demand is not sufficient. Indeed, this has been observed with Californian solar farms, where they have occasionally needed to pay consumers to take on their excess solar energy, in order to avoid overloading their power grid.3

These challenges highlight the need to find a way to store solar energy. One of the most promising means of solar energy storage is in the chemical bonds of liquid/gaseous fuels. Indeed, the infrastructure for the storage, transportation and distribution of fluid fuels is already in place from the use of fossil fuels. Hydrogen, H2, production from H2O via electrolysis, is a leading candidate clean fuel given the abundance of water, and the possibility to convert solar energy into an electric potential to drive its electrolysis. In this process, water is split into its constituent elements via the following redox reactions:

Eqn. 1

2 H 2O

4H+ + O2 + 4e–

E = 1.23 V vs. RHE

Eqn. 2

4e– + 4H+

2H2

E = 0 V vs. RHE


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Where E represents the electrochemical potential of the reaction (in Volts versus the reversible hydrogen electrode, RHE). Of the two half reactions, only the hydrogen evolution reaction (HER) produces a viable chemical fuel. The water oxidation step, also called the oxygen evolution reaction (OER), only serves to supply electrons and protons to feed the HER; the produced O2 is of little or no value considering its abundance in the atmosphere. Traditionally, the water electrolysis reaction can be accomplished with metallic electrodes (with the HER occurring at the cathode and the OER taking place on the anode) driven by a standard photovoltaic cell (e.g. based on semiconducting silicon). However, the process can alternatively be accomplished using semiconducting photoelectrodes—which directly use sunlight to create a photopotential to drive the reactions. In either case, the overall result is the effective storage of solar energy in the chemical bonds of H2.1,4

It should be noted that although H2 is promising target as a solar fuel, there are some potential difficulties with its global implementation as an energy vector, mainly due to its low volumetric energy density. As such, the sustainable (photo)electrocatalytic production


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carbon based fuels, such as methane, and methanol, has also attracted considerable interest. A large body of research has demonstrated that CO2 reduction to either methane, or methanol is possible using either photo- or electrocatalyst materials.5–11 Importantly, these demonstrations reveal that photocatalyst materials need not be limited to water splitting, and are very capable of carrying out a wide variety of chemistries on the cathode side. Likewise, we need not be limited on the anode side. Indeed, when considering the water splitting process to produce H2 or to produce fuels via CO2 reduction, it is not necessary to conduct the oxygen evolution reaction (OER) at the anode side, in order to allow the reduction reaction to occur. Furthermore, considering the two half reactions of water splitting, the OER is the one with the greatest kinetic barriers. Chemically this is to be expected, since to turn over 1 molecule of O2, the coordination of 2 molecules of water is required, as well as up to four proton coupled electron transfer steps. These large energy barriers are revealed when considering that considerable portion of energy is wasted as an overpotential of about 0.3 - 0.5 V for a current density of about 10 mA cm– 2.12–15

Given that only 1.23 V is thermodynamically required to split water, the

overpotential losses of the OER can reach up to 30%.


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Recently, researchers have begun to explore alternatives to the OER in photo-driven solar fuel production systems. The motivation behind this work is to not only find a way to provide the electrons and protons for the HER or CO2 reduction reactions with lower anodic overpotential, but to also identify new means of producing valorized chemical products via the oxidation reaction.16–20 While there are a few excellent reviews that broach the subject of alternative electrochemical reactions, the discussion concerning the direct photooxidations of alternative substrates by photoanodes/photocatalysts has not been closely discussed, and could benefit from further examination.17,19 Therefore, in this perspective we discuss possible replacements for the OER and, in particular, we highlight recent

progress

in

the

investigation

of

organic

based

photochemical

and

photoelectrochemical oxidation reactions. We focus on oxidation reactions that have potential applications as a form of chemical valorization, and that can take place in aqueous solutions to allow concurrent H2 production at the cathode.

Motivation for Alternate Oxidation Reactions to Replace the OER


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The prospective ability to create additional value-added chemicals instead of O2 is already quite attractive, however a few significant advantages arise when considering the implications of developing new oxidative photosynthetic reactions. First, if we consider coupling such processes to reductive H2 generation from water, by carefully controlling the type of reactions occurring at the anode, this can avoid the generation an explosive O2/H2 gas mixture.21 This is significant since it could eliminate the need for a gas separation membrane, which would lower the cost, improve the safety, and simplify the design of a photocatalytic, or photoelectrochemical, reactor that would drive such processes.

Additionally, using photo-driven chemical processes as a means to replace homogeneous molecular oxidants in traditional (non-electrochemical) oxidation reactions can be imagined. This type of strategy has already been successfully adopted in the closely related field of synthetic electrochemistry. For example, it has been shown that the oxidation of furans to form 2,5-dimethoxy-2,5-dihydrofurans—which typically is carried out in a Br2 containing methanol solution along with a weak base—can be


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successfully accomplished using an electrically polarized anode instead.19,22 This is an important example that demonstrates how heterogeneous polarized surfaces may be employed as replacements for homogeneous molecular oxidants. Since most electrochemical reactions can take place at a semiconducting electrode/photocatalyst— provided that the electronic band structure is appropriate—this suggests that illuminated semiconducting materials could also serve as powerful replacements to molecular oxidants. This would provide a few key advantages in a photo-driven reactor. First, handling the “oxidant” becomes much less complicated due to its heterogeneous nature. In addition, this method eliminates the need to remove excess oxidant from the reaction mixture, which facilitates the final purification, and potentially lowers the overall cost of the reaction. Next, if the semiconducting photoactive material is functionalized with a specific cocatalyst to drive the reaction, the separation of the catalyst from the reaction mixture is greatly simplified since the solid material can simply be removed and rinsed. Additionally, this also makes these catalysts easier to reuse.


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Finally, a large number of known alternative oxidation reactions involve simple 1 or 2 e– net oxidations. This, in general, simplifies the kinetics—which could reasonably reduce the overpotentials required, as mentioned above—and thus improves the conversion efficiency. This represents a significant advantage over OER given its aforementioned large overpotential.2,12,13,15

Considerations when Selecting Replacement Reactions for OER

Solvent System

The solvent system and supporting electrolyte are vital to the performance of both organic and inorganic oxidation reactions. In general, more polarizable, or ionic, substrates will demonstrate a higher reactivity at electrodes. This is due to the Lorentz forces that charged particles feel under the influence of an electric field. Thus, non-polar substrates, such as hexane, will demonstrate little to no reactivity at polarized electrodes, and should be avoided. As a result, polar solvent systems are the most successful for carrying out photoelectrosynthetic reactions since they are well suited for dissolving polar substrates.17 Additionally, since photoelectrolysis requires passing a current in solution,


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this limits what solvents may be used, since the solvent must also be able to dissolve a supporting electrolyte for the purpose of improving solution conductivity. Thus, solvents with a high (e.g. H2O), or intermediate (MeCN, MeOH, MeNO2) polarity are primarily used in photoelectrosynthesis. When considering concurrent H2 production in a photoreactor, water is the ideal choice to ensure that H2 evolution is the primary reaction at the cathode. However, using H2O as the solvent can affect the reactions taking place at the anode. In addition to possible competition with the OER, one must also consider the potential for nucleophilic attack from H2O, which could result in the formation of undesired oxygenated products. This adverse reaction can become particularly problematic if carbocations are produced during the oxidation. For example, when performing the electrolysis of carboxylate salts, this can lead to a variety of oxygenated organic products, such as alcohols and esters, due to the interference of water.23 However, despite these possible limitations in aqueous solution, a few groups have demonstrated success when exploring alternates to the OER in this solvent system. One strategy to avoid complications with water is to use a photoanode material that poorly


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interacts with water. For example, it has been demonstrated that WO3 prefers to oxidize more kinetically accessible substrates, as opposed to water, suggesting that this type of reactivity may be exploited to yield high selectivities for different substrates in an aqueous solution.14,24,25 In addition, with BiVO4-based photoanodes K.S. Choi and coworkers have shown that it is possible to carry out 2,5 hydroxymethyl furfural (HMF) oxidation to 2,5 difuran carboxylic acid (FDCA) in pH 9.2 sodium borate (NaBi) aqueous solutions.16 Similarly, Sun and coworkers demonstrated the operation of an HMF oxidation / HER electrochemical cell using dark Ni2P electrodes as the cathode and anode.21,26 The solvent system employed for this work was aqueous 1M KOH. Furthermore, they also have shown the successful oxidation of benzyl alcohol, furfuryl alcohol, and furfural in 1M KOH. Additionally, Reisner and coworkers, have also demonstrated sugar oxidation, as well as lignin decomposition, in basic aqueous electrolytes.18,20 These examples demonstrate that alternate oxidation reactions can still dominate over OER, even in aqueous solution. Of course issues with using water as the solvent can be eliminated by using nonaqueous systems where MeCN and MeOH are the primary solvents of choice. A wide


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ACS Catalysis

variety of organic electrochemical transformations have been demonstrated in MeCN, and indeed, given its wide redox window, this solvent is the standard for reporting redox potentials associated with organic electrochemical reactions.27 Methanol is also a useful solvent since it has been shown that carbon radicals formed during electrolysis are more stable in MeOH compared to water and thus higher concentration of alkane products is produced during certain anodic oxidation reactions using carbon-based substrates.23,28

Semiconductor Catalyst Selection Over the last few decades, metal oxide materials have emerged as promising candidates to drive photooxidation reactions. In particular, these materials have been thoroughly investigated as photocatalysts/photoanodes to drive the OER. Figure 1 depicts the band structure of a few common n-type metal oxide photoanodes. We observe that for most of these materials, the valence band, which is primarily composed of O 2p orbitals, lies in the powerfully oxidizing range of 2-3 V vs. RHE, which coincides with the standard potential, Eo, for a variety of oxidation reactions. Furthermore, when considering the VB of these materials relative to the OER—which lies at 1.23 V vs. RHE—it can be seen that photoexcited holes in these materials possess overpotentials of up to 0.77-1.77


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V when only 0.3-0.5 V is required. Even though the maximum photovoltage under one sun conditions is only up to about ~75% of the band gap for an ideal case, there still remains a large potential loss due to the considerable difference in energy between the photoexcited holes and the potential needed to drive the OER.29,30 Indeed, the highly oxidizing valence band present in these classic oxide semiconductors could be used more efficiently to drive chemical reactions whose oxidation potential is closer to the valence band maximum. In this fashion, more solar energy could be stored in the resulting chemical products. Thus n-type metal oxides such as WO3, and BiVO4 should prove to be optimal candidates to drive a wide variety of oxidative chemistries photocatalytically because they possess a highly oxidizing valence band, as well as relatively small band gaps (2.5-2.7 eV) which allow them to absorb a significant portion of visible light. We note that even if these materials were coupled with co-catalysts to improve the surface kinetics, ultimately it is the electronic band structure of the light absorbing metal oxide that will determine what redox reactions are thermodynamically possible at the surface of the catalyst. Indeed, a co-catalyst only reduces an activation energy barrier, that is, it only affects the kinetics of the reaction; this implies that even if the underlying


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metal oxide light absorber does not directly conduct the reaction at the surface, the energetic nature of the photo excited electron-hole pairs that it will generate under illumination will control the thermodynamics. This is because the photo-excited electron and hole energy levels are ideally dictated by the conduction band minimum and valence band maximum, respectively, which effectively limit what chemical reactions can be driven by the semiconductor. Finally, metal oxides have demonstrated the ability to react with substrates in aqueous solution which are thermodynamically challenging to oxidize. That is, they have demonstrated the ability to perform oxidation reactions with very high standard potentials (< 2 V vs. RHE). An example of this phenomenon is the competition between anion oxidation and OER for photogenerated holes in aqueous solution on WO3 electrodes.24,25 It has been demonstrated that anion oxidation will compete with OER in aqueous solution, which can reduce the Faradaic efficiency for OER to 0%.24 Indeed, Choi and coworkers reported 0% Faradaic efficiency for OER in pH 1-5 aqueous solutions that contained either NaCl or CH3COONa salts. In this work, they attributed the suppression of OER to anion oxidation effectively outcompeting water at the electrode surface. Furthermore,


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similar behavior has been reported by Lewis and coworkers, where they also observed that anion oxidation was the predominant process in acidic solutions containing HCl, H2SO4, and HClO4.25 These types of reports are promising when searching for kinetically faster replacements for OER that can be performed alongside HER in aqueous solution, since they demonstrate that alternate reactions can be performed in water without the concern for OER behaving as a significant competitor for photoexcited holes. Thus, metal oxide materials are poised to be particularly intriguing candidates to apply towards the oxidation of alternate substrates.

Figure 1. Band structures of oxide semiconductors for PEC applications vs. HER, OER, and some common organic oxidation reactions. Organic oxidation reactions are depicted


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over potential ranges, since they will vary depending on the other substituents present on the molecule. Reproduced with permission from Reference 27.

Photosynthetic Cell Design A promising design architecture for a light powered solar fuels reactor is that of a photoelectrochemical (PEC) cell. Figure 2a depicts a schematic of such a device. In this configuration, two photoelectrodes—a photocathode and a photoanode—separately carry out the reduction and oxidation reactions, respectively. The two photoelectrodes are wired together so that together they can generate sufficient photo potential to drive the reduction and oxidation reactions.

The PEC cell design is a promising technology

because it presents two distinct advantages versus conventional photovoltaic-electrolyzer technologies. First, since the current density passing though the photoelectrodes is small (ca. 10 mA cm–2) compared to the typical optimal operation of an electrolyzer (1000 mA cm–2) losses due to Ohmic resistance are reduced and a wider variety of catalysts can be considered as their performance at high current density is not as critical. Next, because


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the light absorption and the catalysis are carried out by the same electrode, this allows for a more compact design that optimizes the amount of surface area taken up by the device compared to a PV+electrolzer combination. However, we note that the photoexcited carrier generation and catalysis does not all need to take place on the same material, and there are numerous examples of metal oxide absorber/co-catalyst composite electrodes which demonstrate high efficiency and Faradaic efficiency for photoelectrochemical water oxidation.14,31–35

Figure 2. a) A PEC cell design. In this configuration, the oxidation and reduction half reactions take place at different sites, notably the photoanode for the former, and the photocathode for the latter. b) A simple photoreactor. In this configuration, a semiconducting particulate suspension carries out both the oxidation and reduction half reactions on its surface.


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On the other hand, a simple heterogeneous photoreactor (Figure 2b) may also be employed for solar fuel generation. This type of reactor is composed of a nanoparticulate suspension of a photocatalyst in a solution that contains the chemical substrate to be converted. In the prototypical realization, one type of photocatalytic nanoparticles is responsible for carrying out both the oxidation and reduction reactions on its surface. However, one of the key challenges associated with this type of reactor is the need for a photocatalyst that can absorb a considerable portion of visible light, is stable under the reaction conditions employed, and has semiconducting band energy potentials that are well suited to drive both the HER and the accompanying oxidation reaction. Since this has proved to be a major difficulty, one approach is to employ a Z-Scheme system with two complimentary photocatalysts that electronically communicate using a redox mediator.36 From a technoeconomic perspective heterogeneous photoreactors are the most feasible devices to implement on a large scale due to their simplicity.37 Indeed, since these reactors require no external circuitry, and are composed of a simple nanoparticulate suspension, this makes them considerably less expensive to implement than a PEC design or a PV+electrolyzer. Unfortunately, to date, the performance of these systems


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lags considerably behind that of the PEC cells. This is in part due to the ability to electronically study the individual photoelectrodes in a PEC cell, which presents a significant advantage towards materials development and optimization. Thus currently the PEC device remains widely employed in the advancement of solar fuel systems.

Alternate Reactions as Replacements for OER in a PEC H2 Cell Given the aforementioned limitation of the OER, it seems clear that an alternative reaction is of interest to enhance solar fuel production in the PEC cell or a photocatalytic (PC) reactor. However, which reaction should be chosen to maximize the performance and output of the reactor? In this section we highlight a variety of key reactions that are well suited as potential replacements for the OER in a solar powered PEC cell or PC reactor. Broadly, these types of reactions can be classified by chemical substrate. Both carbon based organic substrates and halide substrates, in particular Cl-, have been investigated. Indeed, the subject of organic based electrochemical oxidations (on nonphoto electrodes) has been investigated since the mid-19th century.38 The initial research in this field was driven by a motivation to develop new synthetic methods geared towards


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the production of organic small molecules. Since the first reports on electrochemical oxidation of organic molecules, a larger body of research has grown around this topic in the ensuing decades.19 More recently, the subject of photo/photoelectrochemical oxidations of these substrates has been investigated on a variety of different organic substrates. Specifically, over the last decade interest has piqued in the development of new means for biomass valorization. Indeed, when considering the availability of lignin and other forms of waste biomass, this would be an important resource to tap into if it could be transformed into high value commodity chemicals. Recently, researchers have succeeded in converting sugars, and other biomass derived small molecules, into higher value starting materials such as HMF.

Oxidation of HMF Over the last few years, a significant amount of attention has been drawn to the oxidation of 5-hydroxymethylfuran-2-carbaldehyde (HMF). This reaction has attracted a great deal of interest because the 2,5-Furandicarboxaldehyde (DFF) that results from the partial oxidation of the hydroxyl group on HMF to an aldehyde can be used as a monomer in the synthesis of a variety of furan based polymers.39,40 Furthermore, the fully oxidized


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derivative, 2,5-Furandicarboxylic acid (FDCA), has also been proposed as a promising biomass derived monomer that may be used to replace petrol based monomers for the synthesis of polyethylene terephthalate, PET, plastics.16 Thus, performing this reaction with a photoelectrode, or photocatalyst, is particularly attractive since the traditional (nonelectro)catalytic process to transform HMF into DFF typically employs the use of organic solvents, with H2O2 serving as the oxidant. Despite these processes demonstrating a high selectivity

for

DFF,

they

are

not

very

environmentally

friendly.41

Using

a

photoelectrode/photocatalyst is advantageous since it can replace the H2O2 by providing the necessary photogenerated holes for the oxidation of the HMF. Furthermore, one of the key challenges in HMF conversion is being able to fully oxidize the starting material selectively to FDCA. Many different homogeneous catalysts that have been reported to drive this reaction will often do so with poor selectivity for the FDCA product, or will require higher temperatures, as well as, high pressures of O2.39 As a result, the oxidation of HMF using (photo)electrochemical means has been explored since the 1990s. There are a number of ways that this reaction has been performed using (photo)electrochemical means; directly using an anode, indirectly by


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using an anode and a (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) redox mediator and photoelectrochemically using a TEMPO redox mediator and a visible light absorber such as BiVO4.16,42,43 Furthermore, many of these reports demonstrate the oxidation of HMF in aqueous solution, which illustrates that this reaction is a viable replacement for OER, and can be conducted alongside HER. Figure 3 depicts the possible oxidation pathways, as well as, some products that may be formed during the course of HMF oxidation.

B O A HO O

O

O

D O

O C O

O

O

O

O OH

HO

E O

O OH

OH

HO

Figure 3. Two possible oxidation pathways for HMF to form FDCA. A: 5(Hydroxymethyl)furan-2-carbaldehyde (HMF), B: 2,5-Furandicarboxaldehyde (DFF), C: 5-hydroxymethyl-2-furancarboxylic acid (HMFA), D: 5-Formyl-2-furancarboxylic acid (FFCA), and E: Furan-2,5-dicarboxylic acid (FDCA).


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One of the earlier reports on the oxidation of HMF to FDCA using electrocatalysts was reported by Skowronski and coworkers in 1995, where they used the radical co-catalyst TEMPO to improve the aldehyde yield from HMF.42 In 2012, Strasser and coworkers reported the electrochemical oxidation of biomass derived HMF in a 0.3M NaClO4 solution at pH 10. They performed the oxidation of HMF using Pt electrodes, however they obtained low conversion efficiency and observed DFF with an 18% yield as the primary product. No detectable amount of FDCA was observed, so the HMF oxidation could not go to completion under these conditions.44 In 2015, K.S. Choi and coworkers reported the photoelectrochemical oxidation of HMF to FDCA with near 100% Faradaic efficiency (FE) in aqueous solution. By using a similar strategy to that reported previously, they applied the TEMPO radical as a co-catalyst. They also found that the FE and onset potential of a gold working electrode could be improved for the oxidation of HMF to FDCA. Interestingly, during the course of electrolysis, little FDCA is detected at the beginning, and, it is not until the latter half of their electrolysis experiment that they begin to detect larger concentrations of FDCA. This


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suggests that 5-formyl-2-furancarboxylic acid (FFCA) may play an important role as an intermediate during HMF oxidation on Au in the presence of TEMPO. They applied a similar procedure when observing the effects of TEMPO on BiVO4’s ability to photocatalyze the oxidation of HMF. The dark electro-oxidation and the PEC cell behavior is summarized in Figure 4. They report that without the TEMPO mediator, no reactivity is seen between the illuminated BiVO4 and the HMF substrate, however, upon the addition of TEMPO in the presence of HMF, a negative shift in the onset potential is observed (see Figure 4 a,b). In the case of BiVO4, they also found a similar result to that of the Au electrodes, where FDCA did not begin to form until halfway through the electrolysis. They attributed this to the FFCA oxidation step being the slowest in the overall cycle. This hypothesis was supported by cyclic voltammograms (CVs) in solutions containing a mixture of TEMPO and either DFF, FFCA, HMF, and FDCA (Figure 4 c, d). From these data, it was observed that the initial HMF oxidation is the fastest step, while the oxidation of the FFCA was the slowest.


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Figure 4. Photoelectrochemical TEMPO-mediated HMF oxidation. a) linear scanning voltammetry (LSVs) of a BiVO4 photoanode obtained under 1 sun (orange line) and a Au electrode in the dark (black line) in a 0.5 M borate buffer solution (pH 9.2) containing 5 mM HMF and 7.5 mM TEMPO. b) Conversion and yield (%) changes of HMF and its oxidation products at 1.04 V versus RHE in a 0.5 M borate buffer solution containing 5 mM HMF and 7.5 mM TEMPO under 1 Sun. c) LSVs of an Au electrode obtained in a 0.5 M borate buffer solution (pH 9.2) (black line), a 0.5 M borate buffer solution containing 5 mM HMF (blue line) and a 0.5M borate buffer solution containing 5 mM HMF and 7.5 mM TEMPO (orange line). d) Cyclic voltammetry (CVs) obtained in a 0.5 M borate buffer solution containing 7.5 mM TEMPO (black line) and in the same solution containing 5 mM


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HMF (orange line), DFF (blue line) or FFCA (green line). Reproduced with permission from Reference 16.

To advance the concept of HMF oxidation, in 2016 Sun and coworkers demonstrated the electrochemical conversion of HMF without the need to use an expensive TEMPO mediator nor an expensive noble metal catalyst, such as Au or Pt. Instead, they demonstrated the application of a 3D Ni2P nanoparticle array on nickel foam (Ni2P NPA/NF) as an effective working electrode to drive HMF oxidation.26 Their full EC cell demonstrated a low overpotential to achieve their operating current densities of 10 and 50 mA cm–2 (1.44 and 1.58 V vs. RHE, respectively), and it was able to run for an extended period of time. While it was not demonstrated on a photoelectrode, the removal of the TEMPO reagent, at least in the dark electro-oxidation reaction provides a practical advantage.

Oxidation of Sugars and lignocellulose


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The photooxidation of sugars derived from lignocellulose, has recently become an emerging topic in the field.18,20 This system is particularly intriguing since the starting materials may be derived from biomass, and the energetics of coupling these reactions with H2 evolution are very favorable, despite these reactions involving multiple electron transfer steps. This concept is clarified when considering the potential required to drive glucose oxidation alongside the HER: Eqn. 3

2H+ + 2e–

Eqn. 4

C6H12O5

H2

E = 0 V vs. RHE

+

6

6 CO2 + 24 H+ + 24 e–

E = –0.001V vs. RHE

+

6

12 H2 + 6 CO2

E = +0.001V vs. RHE

H 2O Eqn. 5

C6H12O5 H 2O

From Eqn. 5, it is observed that coupling glucose oxidation with HER is nearly energy neutral, and thus the only energy required to drive this overall process would primarily be used to overcome activation energy barriers. This provides a significant advantage versus water splitting, which requires a minimum of 1.23 V, in addition to the extra energy to account for the barrier potential (overpotential). Furthermore, coupling glucose oxidation with the HER in a PEC cell would no longer limit the photoanode materials to oxides with


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deep valence bands, and thus, other semiconductors with smaller bandgaps could be used. As a result, coupling sugar oxidation to the HER in a PEC cell/photoreactor, could provide a more energy efficient way of making H2 fuel—provided that the glucose is derived from a waste stream—since the overall thermodynamic energy barrier would be considerably lower than that for water splitting. Indeed, a few groups have already reported on this process.18,20,45–47 Reisner and coworkers have demonstrated that this process can be accomplished purely photocatalytically on a single CdS particle (See Figure 5). They have reported lignocellulose oxidation, with concurrent H2 production, using CdS nanoparticles in aqueous 10 M KOH. In this report, they achieved 600 mmolH2 gCdS–1 over a period of six days. Despite the low turnover frequency for HER (~0.001 mmolH2 s–1 gCdS–1), this represents an important achievement considering the authors were able to achieve similar reaction rates when using grass, paper, and wood, as substrates. Since these types of substrates are primarily composed of very stable lignin and hemicellulose polymers, their breakdown is particularly challenging.


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Figure 5. Photoreforming of lignocellulose to H2 on CdS/CdOx. a) Lignocellulose is a component of plant cell walls and is comprised of cellulose surrounded by hemicellulose and lignin. b) Depiction of the photoreformation of lignocellulose into H2 using semiconducting CdS coated with CdOx. Photoexcited electrons and holes that are produced within the particle can travel to the surface to carry out the oxidation of lignocellulose along with the reduction of H2O to H2. Light absorption by CdS generates electrons and holes, which travel to the CdOx surface and execute proton reduction and lignocellulose oxidation, respectively. c) Photographs of CdS/CdOx nanoparticulate


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suspensions reacting with various plant base substrates under illumination. Reproduced with permission from Reference 18.

Additionally, this process has also been achieved in a photoelectrochemical cell using WO3 phtoanodes.47 Chen and coworkers demonstrated that WO3 can efficiently oxidize glucose to a mixture of CO and CO2. By adding 100 mM glucose to a 0.33M H2SO4 aqueous solution, they observed an increase of 50% (from ~1 to ~1.5 mA cm–2) in photocurrent at 1.5 V vs. the saturated calomel electrode (SCE). They were even able to couple the WO3 photoanode to a CdTe solar cell, along with a tungsten carbide cathode, to achieve a PEC cell that produced a Short-circuit current density, JSC, of 1.5 mA cm–2 under real world testing conditions (see Figure 6).


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Figure 6. Short-circuit current density (Jsc) recorded for a WO3|CdTe|WC tandem device under outdoor illumination in (i) 0.33M H2SO4 and (ii) 0.33M H2SO4 + 0.1M glucose. Reproduced with permission from Reference 47.

Other groups have also considered glucose oxidation in PEC cells used to produce solely electricity. Zhang and coworkers have demonstrated a PEC cell using BiVO4 and polyterthiophene as the photoanode and photocathode, respectively.45 In this work, glucose oxidation at the photoanode was performed alongside oxygen reduction at the photocathode. They were able to obtain an open circuit voltage, VOC, of 0.62 V, and a JSC of 0.775 mA cm-2 which represent significant activity for a photoelectrode fuel cell. However, despite demonstrating an interesting configuration, the performance is not relevant for implementing on an industrial scale. Furthermore, in this system no real valorized chemical products are formed since the only products are CO2 at the anode and H2O at the cathode.


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Oxidation of Carboxylates The oxidation of carboxylate species could also provide a viable alternative to the OER for a H2 producing PEC cell. This reaction was first discovered on metal electrodes in the mid-1800s by Hermann Kolbe.38 Carboxylate oxidation can undergo two reaction pathways, depending on the conditions in which the electrolysis is carried out. These reactions are described using a generic carboxylic acid as an example: Eqn. 6

2 RCOO–

R2 + 2 CO2 + 2e–

E = 2 - 2.5 V vs. RHE

Eqn. 7

RCOO– + OH–

ROH + CO2 + 2 e–

E = 2 - 2.5 V vs. RHE

Eqn. 6 represents the Kolbe pathway, which produces an alkane, while eqn. 7 represents the Hofer-Moest reaction, which yields an alcohol. The redox potential of these reactions is listed as a range since it will vary as a function of the R group on the carboxylic acid.23,28 It is clear from the reaction equations that Kolbe products should dominate at low pH, while Hofer-Moest products will be dominate at higher pH values. Since their discovery, these reactions have primarily been investigated using dark electrocatalysts, such as Pt, and carbon felt.23,28 In the case of dark electrocatalysts, it has been reported that the largest yield of alkane Kolbe products are obtained at high current densities, using


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Pt electrodes, in a mixed methanol/H2O solution. To optimize the Hofer-Moest products, it is preferable to apply lower current densities and employ the use of a pyridine/H2O mixed solvent system.28,28 There have also been a few investigations into performing this reaction using photoelectrodes. Bard and coworkers reported on the “photo-Kolbe” reaction on TiO2 photoanodes.48–50 In their report, they demonstrated that ethane could be

evolved

photoelectrochemically

from

a

solution

of

acetonitrile

containing

tetrabutylammonium acetate (Figure 7). They found that using TiO2 electrodes they could obtain a FE of 64% for ethane production under these conditions. Furthermore, in a later report they also demonstrated that a suspension of TiO2 particles in an acetic acid/H2O solution could effectively oxidize the acetate to form methane. This is unusual, since the expected Kolbe product from this reaction would be ethane. However, they also reported no production of H2 during this process, which suggests that both photocatalyzed redox processes on the TiO2 involved acetic acid to eventually yield CH4. Furthermore, involvement of H2O in the protonation of potential methyl radicals was suspected since isotopic labeling experiments indicated that H was abstracted from free acidic protons in solution.49,50


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Figure 7. Cyclic voltammograms of single crystalline TiO2 under 450 W xenon lamp. (a), in acetonitrile + 0.1M tetrabutyl ammonium perchlorate (TBAP), (b), in acetonitrile + 0.1M TBAP + tetrabutyl ammonium acetate (TBAAc), (c), in acetonitrile + 0.1M TBAP + 0.08M TBAAc + 0.1M acetic acid. Reproduced with permission from Reference 50.

Halide Oxidations When considering possible inorganic oxidation reactions that may be coupled to H2 evolution, halide oxidation emerges as a promising candidate. Halide oxidations typically adhere to the following general chemical equation: Eqn. 8

2 X–

X2 + 2e–


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Being a 2-electron redox reaction, halide oxidation is a kinetically simpler reaction compared to the OER. This has been confirmed experimentally since Cl– oxidation will dominate at a Pt electrode in aqueous solutions at high bias.51 Chloride is probably one of the most interesting halide substrates. Indeed, the chloralkali industry, which is responsible for the production of NaOH and Cl2 gas, produces around 56 million tons of chlorine a year. Chlorine is a crucial element for the synthesis of polymers and the productions of disinfectants. The primary means for the production of this element is the electrochemical oxidation of NaCl brine solutions in what is known as the chloralakli process. Despite being currently the most efficient means of producing this element, this process still consumes enormous amounts of energy. When considering chlorine as a replacement for OER in a hydrogen producing cell, it is not a good choice for an electrochemical cell, since the cell voltage would amount to 1.36 V when Cl– oxidation is coupled to HER if kinetic overpotentials are not considered. As a result, this process offers no significant advantages for dark electrolysis cells. However, when considering how this reaction may take place in a PEC cell, a few noticeable advantages arise. First, the Cl– anion does not absorb visible light, so it will not behave


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as a competitive absorber in solution. Next, in the case of a metal oxide photoanode, the oxidative power of a photoexcited hole is governed by the valence band maximum. From Figure 1, we observe that the valence band maximum (VBM) of many metal oxides is between 2-3 V vs. RHE. This means that a photoexcited hole in most metal oxide semiconductors should possess sufficient energy to drive Cl– oxidation (ECl-/Cl2 = 1.36 V vs. RHE). Furthermore, since these holes have significantly more energy than the oxidation potentials of water and Cl– oxidation, then Cl– oxidation should be the dominant process in aqueous brine containing solution, due to its simpler kinetics. Indeed, this is what is observed in the case of WO3 photoanodes, which demonstrate higher current densities, and a complete suppression of OER in NaCl solution.24,25 There have also been other reports on the PEC oxidation of chloride containing solutions using Si and metal chalcogenides as the photoanode. Wrighton and coworkers demonstrated improvements in Cl– and Br– oxidation using platinized n-MoS2 and n-WS2 electrodes.52 They obtained energy conversion efficiencies of 9.8% and 13.4% for nMoS2, and n-WS2, respectively for the oxidation of Cl- in 15M LiCl solution using 632.8 nm illumination. However, they also found that the stability of these electrodes was limited


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due to the gradual oxidative dissolution of the Pt co-catalyst. Similarly, Rajeshwar and coworkers demonstrated stable Cl– oxidation also in 15M LiCl using SnO2 coated Si electrodes that were decorated with a RuO2 co-catalyst.53 They found that they were able to obtain a conversion efficiency of 3.1%, but unfortunately did not comment on the stability of the electrodes during the photoelectrochemical oxidation. Finally, there has also been some more recent work on this topic presented by Meyer and coworkers.54 In their report, they present chloride oxidation catalyzed by a homogeneous silver ion catalyst. They demonstrated that in solutions with a sufficiently high concentration of Cl–, this prevents the precipitation of AgCl2, and allows them to form the complex ions AgCl2– and AgCl32. The formation of these species, in turn, allows access to higher oxidation states of silver by delocalizing the oxidative charge over the Cl– ligands. The result is a homogeneous Ag(I) catalyst that is able to catalyze Cl– oxidation at small overpotentials (~0.01 V). We do note that Cl– oxidation and its subsequent Cl2 gas production still possesses the disadvantage of requiring a gas separation strategy when coupled to HER, which could impact the cost of a reactor that conducts these processes. However, since performing


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the OER also requires a means of gas separation, Cl– oxidation is arguably a more attractive choice. Moreover, the OER is still a less efficient process due to high activation barriers to drive the reaction. Finally, O2 has little to no economic value, especially when compared to Cl2. Outlook When considering the theoretical limit for solar to H2 conversion for a water splitting PEC device, in the ideal case, with a dual band gap configuration (1.7 and 1.1 eV), the chemical conversion efficiency would amount to ~27% when accounting for losses due to the fraction of unused energy per absorbed photon.55 However, to date, the highest performing systems demonstrate solar to H2 (STH) conversion efficiency values of 1819%.56 While these results are promising, these champion devices employ semiconductors that are expensive and unstable over the long term. Thus, to date, even in these best cases, solar water splitting is unable to compete with current H2 production technologies such as natural gas reforming. In the above sections we suggested that replacing the OER with a more facile reaction, or a reaction that produces a more valuable product, can enhance the performance of the


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overall photoelectrochemical or photocatalytic solar to H2 conversion. We examined a few key reactions that may serve as potential replacements for the OER. Overall, these alternate reactions demonstrate either more favorable kinetics (simple 1-2 e– transfer reactions), or thermodynamics (Eo < 1.23V vs. RHE), which allow them to take place more rapidly on a semiconductor photocatalyst. Thus, in general, the alternate oxidation reactions discussed above demonstrate much higher rates on semiconductor surfaces compared to the OER. However, when considering which alternate reaction would be best suited to replace the OER, a few important criteria must be considered. First, a replacement for the OER must take place at a much higher rate than the OER, or demonstrate more favorable energetics when coupled to HER (E < 1.23V). This would decrease the energy requirement, and increase the process efficiency, by either lowering the kinetic overpotentials required, or decreasing the net thermodynamic barrier for the overall process (H2 reduction + substrate oxidation). For the reactions discussed in this perspective, they meet either one or both of these criteria. In the case of HMF, carboxylate, and halide oxidation, all those reactions demonstrate faster kinetics versus OER, despite having slightly higher redox potential, which allows them to dominate at a


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photoelectrode/photocatalyst under illumination. In the case of glucose oxidation, despite it being a more complicated reaction—requiring 24 e– oxidation per molecule of glucose— because the potential for its oxidation is much lower than that for H2O, this lowers the thermodynamic driving force required to drive its oxidation alongside HER which, in turn, allows more rapid H2 production in aqueous solution. Third, ideally a replacement for OER would yield a high value product. This would dramatically increase the usefulness of an H2 generating PEC cell since now both the cathode and anode would generate valorized chemical products. Furthermore, the types of substrates used could be derived from biomass processing, since many products of biomass conversion still require further oxidative processing, such as HMF. Fourth, the substrate must demonstrate high solubility in aqueous solution in order to facilitate concurrent H2 production. Finally, the starting material used for the anodic half reaction must not be too costly, after all, part of interest in pursuing overall water splitting is the fact that water represents an abundant, and inexpensive, starting material. Glucose/lignin oxidation, while promising, does not yield any valorized chemical products. Instead, the primary product of this reaction is CO2. Furthermore, current


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demonstrations of lignin oxidation still need to employ the use of 10 M concentration of KOH base in water to break down the lignocellulose. This type of constraint introduces a higher cost for the development of this type of system. Furthermore, it also limits the type of photocatalyst materials that can be employed due to the extremely alkaline conditions. HMF oxidation on the other hand does take place under milder conditions, and does indeed yield a valorized chemical product. Furthermore, Choi and coworkers have demonstrated that it is possible to extract the FDCA that is produced when oxidizing the HMF, by acidifying the solution and filtering out the precipitate.16 However, HMF is not very soluble in H2O, and can only be dissolved in millimolar concentrations. This, in turn, introduces mass transport limitations which could add to the overpotentials required to drive this reaction. Additionally, the related carboxylate oxidation reactions, while promising, have not been developed to a convincing level. There still remain only a few demonstrations of this process, and insight obtained from dark carboxylate electrolysis suggests that product selectivity will most likely be the primary challenge for this type of chemistry.


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However, when considering Cl– and carboxylate oxidations, these stand out as particularly attractive candidates to pursue as replacement for the OER. Both the starting materials for these types of reactions are low cost; Cl– can be obtained from seawater, and carboxylate salts can easily be obtained via the alkaline breakdown of waste fatty acids. Furthermore, in both cases, the obtained products represent valorized commodity chemicals (Cl2 and alkanes), and the starting materials demonstrate excellent water solubility. Finally, both these reactions are known to outcompete water oxidation at photoelectrode surfaces under anodic oxidation conditions.24,25 As a result, these two systems represent the most promising solutions as replacements for OER in an H2 PEC cell. Of the two, carboxylate oxidation may demonstrate a broader use; since by changing the carboxylate salt employed in the Kolbe reaction, you can change the products, this presents a potential advantage since it introduces a broader range of tunability for this type of chemistry. On the other hand, this also makes carboxylate oxidation trickier, since more precise control of the reaction conditions is required in order to optimize the production of the desired products. However, we note that this technology is still young, and there is plenty of room left to further explore it.


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Finally, when considering semiconducting catalyst materials to drive these alternate reactions, metal oxide materials stand out as ideal candidates. These materials are inexpensive, possess relatively small band gap energy (Eg = 2.0-2.7 eV) which allow them to absorb visible light, are easy to synthesize, and demonstrate long lasting durability in most aqueous solutions. Furthermore, one distinct advantage of the metal oxide semiconductor is the powerfully oxidizing O2P valence band. This allows these materials to carry out reactions whose reduction potential values are >1.23V vs. RHE. This distinct advantage allows them to drive reactions that form valorized chemical products such as Cl– oxidation, and carboxylate oxidation, using sunlight. Although, it often comes with the added disadvantage of having a valence band with a potential above 0V vs. NHE, which means that they must usually be coupled with an HER photocathode in a PEC in order to drive an overall redox process. However, this latter disadvantage is not new, and recent progress in our lab has demonstrated promising STH efficiencies of 0.5%, and about 1 mA cm–2 unassisted photocurrent density using a combination of a Cu2O photocathode, and BiVO4 photoanode.57 Furthermore, it has also been demonstrated that a Z-scheme approach using a redox mediator may also be employed to yield efficient photoreactors


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(>10% apparent quantum yield at 420 nm) containing a mixture of different metal oxide semiconductors.36 While these numbers are behind those of similar PEC systems using III-V semiconductors, we note that the overall STH is still severely limited by the OER. By replacing the OER with a kinetically simpler reaction—such as Cl– or carboxylate oxidation—the photocurrent onset at the anode would shift negatively which would improve the overlap between the photocathode and the photoanode. Furthermore, this could potentially even be performed without the use of a co-catalyst on the photoanode, which has been demonstrated as being a source of instability in metal oxide PEC cells since it can detach during device operation.57

Conclusions In this perspective, we have discussed possible alternatives to replace the OER in order to yield more efficient H2 production at the cathode. When considering some of the alternatives to OER in a PEC cell, it is observed that there is a wide range of reactions which demonstrate favorable kinetics/thermodynamics which allow them to take place at lower potentials, and even to dominate at the anode in aqueous solution.


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In the case of organic reactions, most of these reactions can take place at lower potentials on metal oxide photoanodes. Despite, in some cases, requiring a higher E (such as in carboxylate oxidation), because the valence band in most metal oxide semiconductors is so oxidizing, then the kinetically fastest reaction is what will dominate at the electrode surface. Thus, reactions such as carboxylate oxidation, and HMF oxidation, are able to dominate at metal oxide photoanodes in aqueous solution. Furthermore, these reactions possess the added benefit of turning over much more rapidly, which can improve the net output of the PEC H2 producing cell, or photoreactor. For inorganic reactions, halide, and in particular chloride, oxidation presents itself as an attractive replacement for OER. Since chloride oxidation is already well established using dark

electrodes

in

brine

solutions,

this

suggests

that

a

photooelectrochemical/photochemical approach to chloride oxidation might also be very efficient. Indeed, this reaction has been demonstrated to outcompete OER on semiconductor surfaces in aqueous solution, which demonstrates its promise as an OER replacement.


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In looking at both inorganic and organic chemistries, there are a broad range of reactions which can take place in aqueous solution, and replace OER. Of these, Cl– and carboxylate oxidation present the most promising candidates due to their speedy kinetics, valorized reaction products, and excellent solubility of the starting materials in aqueous solution. Recent advancements in this field have demonstrated promising results. However, there are still a few limitations which need to be overcome. Notably, photocurrent densities for these reactions must be improved; thus it will be imperative to develop catalysts that demonstrate a higher turnover frequency for these reactions. Furthermore, selectivity for a specific product can be challenging, for example, in the case of carboxylate oxidations, the involvement of H2O can lead to a number of oxygenated organic products. This, in turn, provides an important research challenge, since a broad product distribution might make purification rather costly. Finally, mass transport limitations due to the limited concentration of the starting material in solution may also be problematic, although this issue could potentially be overcome by using a flow reactor type cell.


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Despite these challenges, significant progress has been made and will continue. Research into potential replacements for OER is still in its infancy; by comparison, the development of OER chemistry at photoelectrodes, took a few decades to develop. Thus, we believe that there is a bright future ahead for the development of new solar fuel chemistries.

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT


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ACS Catalysis

The authors thank the EPFL for support, and Dr. Nestor Guijarro, and Dr. Florent Boudoire for helpful discussions.

ABBREVIATIONS CB, conduction band; CV, cyclic voltammogram; DFF, 2,5-Furandicarboxaldehyde; FDCA, 2,5-Furandicarboxylic acid; FFCA, 5-formyl-2furancarboxylic acid; HER, hydrogen evolution reaction; HMF, 5-hydroxymethylfuran-2 carbaldehyde; MeCN, acetonitrile; MeNO2, nitromethane; MeOH, methanol; OER, oxygen evolution reaction; PC, photochemical; PEC, photoelectrochemical; PET, polyethylene terephthalate polymer; PR, photoreactor; TBAAc, tetrabutylammonium acetate; TBAP, tetrabutylammonium perchlorate; TEMPO, (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl; VB, Valence band.

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