Will Solar-Driven Water-Splitting Devices See the ... - ACS Publications

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Will Solar-Driven Water-Splitting Devices See the Light of Day? James R. McKone, Harry B. Gray, and Nathan S. Lewis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4021518 • Publication Date (Web): 27 Aug 2013 Downloaded from http://pubs.acs.org on August 31, 2013

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Chemistry of Materials

Will Solar-Driven Water-Splitting Devices See the Light of Day? James R. McKone, Nathan S. Lewis*, and Harry B. Gray* Division of Chemistry and Chemical Engineering and the Beckman Institute, California Institute of Technology, Pasadena, CA 91125 ABSTRACT Through decades of sustained effort, researchers have made substantial progress on developing technologies for solar-driven water splitting. Nevertheless, more basic research is needed before prototype devices with a chance for commercial success can be demonstrated. In this perspective review, we summarize the major design constraints that motivate continued research in the field of solar-driven water splitting. Additionally, we discuss key device components that are now available for use in demonstration systems and prototypes. Finally, we highlight research areas where breakthroughs will be critical for continued progress toward commercial viability for solar-driven water-splitting devices. Four decades after the initial claim of solar-driven electrolysis at n-type TiO2 electrodes,1 and seven decades since the first silicon solar cell,2,3 several attempts to commercialize solar fuels technologies have recently emerged.4 Basic research efforts focused on demonstrating artificial photosynthesis devices have also increased greatly in number and in prominence during the past decade. These are encouraging developments, but much more fundamental research will be required if functional prototypes are to be developed. Herein we outline key constraints that inform research efforts in the area of integrated systems for solar-driven water electrolysis. We then summarize components for solar-driven watersplitting devices that are already developed sufficiently for use in demonstration systems. Finally, we outline key challenges to be addressed by the scientific community to produce integrated devices with the potential to be deployed on a global scale. SYSTEM DESIGNS A solar-driven water-splitting device requires, at minimum, a light absorber, fuel-forming electrocatalysts, an electrolyte, and a means to separate the products. The majority of proposed 1

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device designs can be organized with respect to their technological maturity and projected manufacturing cost (Figure 1), with two limiting cases being aqueous photocatalyst colloids and commercial photovoltaic (PV) modules connected to discrete electrolyzers. Integrated photoelectrochemical (PEC) devices of various types lie intermediate between these two limits. Several assessments of the cost, efficiency, and eventual viability of various device designs are available in the recent literature.5-8 Additionally, several reviews have described the relevant operating principles of solar fuels generation,9-12 as well as guidelines for properly measuring device efficiency.13

Figure 1: Schematic of three device architectures available for solar-driven water splitting, ordered according to their relative technological maturity and projected costs to manufacture.

Regardless of the chosen solar fuels device design, certain constraints exist on the development and combination of components. These constraints arise from three key system requirements: efficiency, stability, and scalability (Figure 2). It is not sufficient to generate parts of an artificial photosynthetic system that satisfy all of these key requirements. Indeed, absorbers, catalysts, membranes exist that are individually efficient, robust, and scalable. However, such materials cannot be arbitrarily combined to form a viable integrated system, because the best components for each aspect of water splitting do not yet operate under mutually compatible conditions. Therefore, judicious choice of operating conditions, and characterization of new materials under a variety of conditions, are critical needs for continued progress in solar-driven water-splitting research.

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Figure 2: Venn diagram depicting the three key requirements for viable solar-driven water-splitting devices, highlighting material sets that satisfy two out of the three.

Constraints also arise from physically combining components into functional water-splitting assemblies. For example, the most efficient catalysts, when deposited directly onto the most efficient light absorbers, do not always give rise to the most efficient composite photoelectrodes. For example, Ye et al. observed that BiVO4 photoanodes yielded more efficient PEC oxygen evolution when coupled to Pt or Co oxide electrocatalysts compared to when coupled with Ir oxide, despite the fact that Ir oxide alone is much more active toward the OER than Pt or Co oxides.14 Additionally, the geometry chosen to implement a specific solar fuels system can have a major impact on the total system efficiency, due to losses associated with electrical resistances and chemical transport processes. Some of these geometry and design constraints have been addressed in detail.15 Another major set of constraints on the development of solar-driven water-splitting materials centers on scalability. Many researchers have interpreted the requirement of scalability to mean that all materials to be used for artificial photosynthesis must consist of earth-abundant elements. Although this is a useful guiding principle for scalable design, it is not sufficient for efforts intended to move towards industrial-scale solar-driven water splitting. Instead, researchers should also focus on the relationships between elemental abundance, raw materials costs, and capital costs. Several recent reports serve as models for these wider considerations of materials choices for scalable solar conversion technologies.16-18 3



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An interesting example of complexities in assessing scalability is the use of CdTe as an active material for photovoltaics. Although Te has comparable abundance to the noble metals, PV modules that use CdTe as the absorber are currently manufactured at comparable cost to crystalline silicon modules. Similar arguments can perhaps be made for the noble metals with regard to solar fuels device design; noble metals may well be appropriate for use in demonstration systems and early commercialization attempts, even if their use for multi-terawatt energy storage levels would stress the resource base. We encourage researchers in this field to take the long view regarding research efforts on individual aspects of artificial photosynthetic systems. In the remainder of this perspective, we therefore summarize first which aspects of artificial photosynthetic devices are well-developed and can be considered ready for incorporation into demonstration systems. Then we issue a series of challenges to investigators in the field, identifying basic research advances that are needed to assure that solar fuels technologies will eventually see the light of day. WHAT WE HAVE Many important innovations in solar absorbers, catalysts, and membranes have already emerged. Some are now at the point where they might be incorporated into demonstration systems for solar-driven water electrolysis. Other innovations serve as encouraging stepping stones, suggesting that efficient, stable, and scalable integrated solar fuels generators can be achieved in the near future. Herein we summarize some of the important advances that likely will contribute to the viability of integrated solar-driven water-splitting systems. Separators Fuel cells and electrolyzers that operate in acidic or alkaline electrolytes have been commercially available for decades. Both approaches use a semi-permeable separator between the anode and the cathode.

Separator development for acidic water electrolysis has largely centered on

perfluorinated sulfonic acid (PFSA) polymers such as DuPont’s Nafion. Membranes composed of these polymers are ubiquitous in modern proton-exchange membrane (PEM) fuel cells and electrolyzers.19-21 These membranes work well for such applications, but suffer from high cost per unit area. 4


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Alkaline electrolyzers traditionally use asbestos separators, although porous composites consisting of refractory oxides embedded in an inert polymer matrix have also been developed.22 These separators allow dissolved species to pass, but the pores are sufficiently small to prevent a significant crossover flux of the product gases relative to the rate at which the gases are evolved at the electrodes. The lack of selective ionic transport in alkaline electrolysis separators, however, necessitates the use of actively pumped electrolyte and relatively large electrode separation. These constraints limit the maximum attainable H2 production rates and efficiencies for alkaline electrolysis. Existing PEM and alkaline electrolysis technologies can be adapted for use in integrated solardriven water-splitting systems, particularly those based on device designs including, or resembling, standard electrolysis electrode assemblies. A complete solar water splitting device incorporating a separator has not yet been reported, although the Texas Instruments Corporation used a proton-exchange membrane in their hydrobromic acid solar fuel cell system.23 Additionally, recent efforts have been made to assess the design constraints for membraneintegrated solar water splitting systems.24 The solar fuels community will likely benefit from continued research in acidic and alkaline fuel cells and electrolyzers, as well as research in chlorine production using the highly developed and commercialized chlor-alkali process. Catalysts for Acidic Media Proton-exchange membrane electrolyzers use highly active platinum-group metal catalysts for the hydrogen-evolution reaction (HER) and the oxygen-evolution reaction (OER). Cathodes generally consist of Pt, or alloys of Pt with other noble metals, deposited onto high surface-area carbon black supports.25 Anodes are made from mixtures of Ru, Ir, and one or more refractory metal oxides.26 These so-called dimensionally stable anode catalysts are stable and active toward water oxidation, although they operate at somewhat larger overpotentials than platinum-group metals do for hydrogen evolution. Several non-noble catalysts exhibit high activity toward the HER. Molybdenum disulfide was initially identified as a promising HER catalyst on the basis of density functional theory (DFT) calculations.27 Subsequent experimental work confirmed the high catalytic activity, and showed 5



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that the coordinatively unsaturated edge sites of the lamellar structure of MoS2 are the predominant active sites for the HER.28 These results have spurred efforts to modify MoS2 materials to attain optimal activity by exposure of a high density of the catalytically active edge sites to the electrolyte,29-32 as well as to improve its fundamental activity and conductivity through control of atomic and nanoscale structure.33,34 Separate work has led to several more non-noble HER catalyst candidates. Cui and co-workers have shown that MoSe2 nanostructures exhibit nearly the same catalytic activity as that of the corresponding sulfides.35 Several researchers have also developed carbide, nitride, and boride catalysts incorporating Mo or Ni/Mo that are active and stable under acidic conditions.36-39 Additionally Ni2P was proposed to be an active HER catalyst on the basis of DFT,40 and subsequent experimental work confirmed its high activity and good stability toward the HER in aqueous sulfuric acid solutions.41 Catalysts for Alkaline Media Alkaline electrolysis is a mature process that has reached a high degree of industrial usage.22 Water splitting under alkaline conditions allows the use of non-noble metal catalysts and inexpensive steel system components. Viable catalysts include nickel composites and nickel oxide/hydroxide for the HER and OER, respectively. For example, in the 1980s, Ni-Mo and NiMo-Cd composites were vigorously researched and patented for use in alkaline hydrogen evolution.42-46 These catalysts exhibited very high activities and stabilities, and were incorporated into commercial alkaline electrolysis systems.47 Earth-abundant transition metal oxides have also been studied for use in water oxidation, with respect to OER activity as well as mechanism. Nocera and coworkers have focused on elucidating the catalytic mechanism of cobalt and nickel oxides for the OER under pH-neutral conditions.48-53 Ultra-thin film deposition techniques have also been used to characterize the high fundamental alkaline OER activities demonstrated by earth-abundant catalysts such as nickel and cobalt oxides.54 A series of substituted perovskite oxides, originally developed for solid oxide fuel cell cathodes,55,56 also exhibit high activity toward the OER under alkaline conditions near room temperature.57 6


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Oxide absorbers Early attempts at PEC water splitting used crystalline TiO2, a wide band gap metal oxide absorber.1 Subsequent work indicated that n-type TiO2 alone cannot supply the photovoltage required to facilitate net water splitting under standard conditions, but requires an additional applied bias.58 Wider band gap metal oxides based on refractory metals, such as Sr titanates, can facilitate unassisted solar-driven water splitting.59,60 However, such materials exhibit very low energy-conversion efficiency under sunlight, due to their very large band gaps ( >3.2 eV). Subsequent work on metal oxides for solar-driven water splitting has focused on the development of semiconductors that have band gaps in the visible range. Materials of this type include Fe2O3, WO3, and BiVO4. None of these metal oxides have band edges suitably positioned to facilitate water splitting without an applied bias, but each has been studied as an oxygen-evolving photoanode for use in tandem solar-driven water-splitting systems. Recently Fe2O3 has undergone a resurgence of interest as a water-splitting photoanode. Many researchers have worked to improve its light absorption, charge collection, and interfacial charge-transport properties.61 In spite of its high optical absorption coefficient, Fe2O3 has not yet reached its theoretical potential in terms of photovoltage or photocurrent. Several researchers have suggested that the key challenges for Fe2O3 are low charge-carrier lifetimes and slow electron-transfer kinetics. These challenges have been addressed with some, albeit limited, success to date.62-65 Tungsten oxide, unlike most metal oxides, is stable toward corrosion in acidic solution. Due to this characteristic, as well as its relatively small band gap, WO3 has been developed for PEC oxygen evolution under acidic conditions. When illuminated in aqueous electrolytes, WO3 photoelectrodes can generate large photovoltages and reasonable anodic photocurrents.66 Interestingly, bare WO3 surfaces in contact with aqueous inorganic acids facilitate oxidation of anions such as Cl- to Cl2 instead of oxygen evolution, despite the more negative formal potential of the latter reaction.67,68 When an active OER catalyst is deposited on the surface, however, WO3 exhibits an essentially quantitative faradaic yield for oxygen evolution and good stability over laboratory timescales.69 7



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Bismuth vanadate is also promising as a light absorber for water oxidation. The band gap of BiVO4 is similar to that of WO3, but BiVO4 has a higher absorption coefficient and better bandedge alignment with respect to the standard potential for water oxidation. Consequently, optimized BiVO4 photoelectrodes have achieved impressive photocurrents and photovoltages for water oxidation when paired with suitable OER catalysts.70,71 Several researchers have also improved on the performance of BiVO4 photoanodes by doping the oxide with other elements, such as Mo and W.72,73 Non-oxide absorbers Although many non-oxide semiconductors have been developed for use in photovoltaics, only a few have been used in the production of solar fuels. Silicon, gallium arsenide, and thin-film chalcogenides remain the most promising semiconductors for photovoltaic applications.74 The thin-film chalcogenides, however, have not been studied extensively for solar fuels, due to their susceptibility to corrosion under aqueous conditions.75 Photovoltaics based on III–V multijunction cells were adapted into solar fuels demonstration devices in the 1990s. Device designs relying on direct contact between semiconductor surfaces and aqueous electrolytes were very efficient, but suffered from low stability.76,77 Nevertheless, the most efficient, stable solar-driven water-splitting devices have been constructed from multijunction III–V photovoltaic materials protected from corrosion in aqueous solution by cell geometries that do not rely on direct contact between the absorber layers and the electrolyte.78 Indium phosphide (InP) is another III-V material that has been heavily studied for solar-driven water splitting. This semiconductor was shown to evolve hydrogen efficiently from acidic solution when coupled to noble-metal catalysts such as Pt and Rh.79,80 Furthermore, using careful electrodeposition conditions, Heller and coworkers showed that catalyst layers on p-type InP could be made essentially transparent, resulting in very high photocurrents for hydrogen evolution.81 The p-type layered transition-metal chalcogenides have also been used for solardriven hydrogen evolution.82-84 These materials have exhibited promising photoactivity and stability, but they are not nearly as well-developed as InP.

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Silicon has been studied extensively with respect to its hydrogen-evolution activity, but even when coupled to noble metal catalysts, p-type Si does not display the high energy-conversion efficiency of p-type InP.85 More recent work, involving generation of “buried” Si p-n junctions, has allowed for efficient hydrogen evolution from planar and microstructured Si absorbers coupled to Pt catalysts and operating under acidic conditions.86 Chen et al. also showed that ntype Si can be coated with thin protecting oxide layers using atomic layer deposition. The resulting structures proved useful for PEC water oxidation when coated with an Ir OER catalyst.87 The high photovoltages and good stabilities exhibited by these Si absorbers are promising, and represent a notable departure from the canonical tandem water-splitting device designs that involve metal oxides as photoanodes and non-oxides as photocathodes.10 Subsequent work has also shown that Si photoanodes and photocathodes can be made more efficient and stable using various thin surface oxide coatings.88,89 CHALLENGES FOR THE COMMUNITY The aforementioned innovations, and many others, lend credibility to the development of efficient, robust, and scalable solar-driven water-splitting systems. Nevertheless, more effort is needed in key areas before promising demonstrations can be transitioned to commercialization. Herein we lay out several innovations that are needed to realize functional solar-driven water splitting in

integrated systems. We challenge investigators in the materials and chemistry

research communities to aggressively pursue efforts in these key areas. Alkaline-exchange membranes A highly conductive and stable anion-exchange membrane would be a major innovation for conventional alkaline water electrolysis and solar-driven alkaline electrolysis. As noted previously, current alkaline separators suffer from lack of selective ionic conductivity. Such separators therefore must be made rather thick, which increases the series resistance and limits cell efficiencies. Additionally, alkaline electrolyzers must use pumped electrolyte and operate at relatively low pressures, which increases their size and design complexity compared to PEM electrolyzers.20

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Realization of a viable anion exchange membrane (AEM) would permit development of electrolysis assemblies that retain the best characteristics of alkaline (low cost) and PEM (high rate and efficiency) electrolysis. These innovations would also benefit the field of solar-driven water electrolysis, perhaps allowing for commercially viable demonstrations involving low-cost photovoltaics coupled to new alkaline AEM electrolyzers. Promising advances have emerged recently in the pursuit of efficient and stable alkaline AEM electrolysis. Leng et al. reported a functional electrolysis cell based on a commercially available AEM and noble-metal catalysts.90 Their system operated on a feed of pure water with reasonable stability for over 500 hours. Additionally, Noonan et al. reported a phosphonium-based AEM that exhibited stable ionic conductivity of >20 mS cm-1 even upon immersion for many days in room temperature 15 M KOH(aq) solution or hot 1 M KOH(aq) solution.91 Existing alkaline membranes may already be useful in systems that are optimized with respect to the lower operating current densities of solar-driven electrolysis (~10 mA cm-2) relative to those used in conventional electrolysis (~1000 mA cm-2). Clearly this is a rich area for further study. Electrolysis in buffered electrolyte Alkaline electrolyzers operate in highly caustic electrolytes, and although PEM electrolyzers operate with neutral water as a feedstock, the active layers are in a highly acidic environment due to the presence of protonated sulfonic acid groups. Conversely, work on light absorbers and water splitting electrocatalysts has also focused on their potential use under buffered, pH-neutral conditions.71,92 Efficient operation in electrolytes buffered at neutral pH is touted for low corrosivity toward active materials (absorbers and catalysts) and system components (containment, piping, etc.), but no electrolysis system has yet been demonstrated to operate safely and efficiently at scale with the active components in a sustained pH-neutral environment. The main challenge associated with buffered water electrolysis is effectively neutralizing the pH gradient that develops across a separator between the anode and cathode compartments.93 Singlecompartment electrolysis in buffered solutions would alleviate the problem of pH gradients, but major safety concerns involved in generating terajoules (terawatt-seconds) of stored energy in an explosive mixture make this approach unattractive for any systems larger than laboratory scale. 10


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Researchers who are interested in designing solar-driven water-splitting systems that operate under mild conditions should consider the aforementioned challenges carefully. Nevertheless, a viable method for operating such a solar-driven system under pH-neutral conditions would be an important innovation in the field. Semiconductors such as Si and BiVO4, for example, are wellsuited for use as cathode and anode light absorbers, respectively, in buffered pH-neutral electrolytes. However, these two semiconductors cannot be used together under alkaline or acidic conditions due to their low respective stabilities in these media. One possible design for pH-neutral solar-driven electrolysis might involve using the natural diurnal cycle to neutralize concentration gradients overnight when the solar-driven electrolysis is not operative, but such an approach would likely require a collection of active feedback control systems to ensure efficient and safe electrolysis at all times. Another potential design that allows use of absorber/catalyst materials with disparate stabilities involves the utilization of bipolar membranes to permit a sustained static pH difference to be maintained between the anode and cathode environments.94 However, implementation of such an approach for production of solar fuels remains to be demonstrated. Molecular catalysts Much effort has gone into designing and characterizing molecular electrocatalysts for the HER. Several molecular HER catalysts operate at high turnover frequency (TOF) in the presence of strong acids. For example, complex 1 in Figure 3 is fluoroborated CoII diglyoxime, which evolves hydrogen electrocatalytically in acetonitrile solutions in the presence of strong acids.95 Complex 2 has been named Ni[PPh2NPh]2, and it also evolves hydrogen electrocatalytically from strong acids in acetonitrile and exhibits an enhanced TOF with the addition of ~1 M water.96 Like these two examples, most molecular HER electrocatalysts are only stable and/or active in nonaqueous solvents. This lack of aqueous stability and activity diminishes their direct applicability in solar-driven water-splitting schemes.

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Figure 3. Structures of two well-studied molecular HER electrocatalysts.

Molecules are ideal for detailed chemical study because they are well-defined and are amenable to a variety of analytical tools. Molecular HER catalysts are therefore well-suited for elucidation of the operative catalytic mechanism(s) of hydrogen evolution, as in the case of the Co– diglyoxime and Co–triphos systems.97,98 Careful study of the relationships between structure and function has also allowed for the design of newer and better generations of molecular catalysts, as with the Ni-based systems from DuBois, Bullock, and coworkers.96,99,100 However, nearly all of these studies have been performed under nonaqueous electrocatalytic conditions (usually in acetonitrile) and involve the concomitant use of organic acids. Much of the work on molecular HER catalysts has been focused on systems that exhibit high turnover frequencies, often at the expense of overpotential. Even Pt, the best single-element heterogeneous HER catalyst, operates at a TOF of ~1000 s-1 (per surface Pt atom) under electrolyzer operating conditions (~100 mV overpotential).101 This TOF is an order of magnitude lower than those reported for hydrogenase enzymes.102 Non-noble metal catalysts operate with TOFs that are several orders of magnitude lower still, and are nevertheless viable for use in alkaline electrolyzers.103 Moving forward, researchers of molecular systems will need to focus less on demonstrating high TOFs and more on pushing the limits of low-overpotential, aqueous operation. The development of molecular catalysts that are abundant, stable in water (soluble or attached to an electrode), and catalytically active for the HER should be a high priority in the field. Limited success has been achieved in the development of catalysts that are stable in the presence of 12


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water,104 but no catalysts have yet been discovered that exhibit the key qualities of stable, lowoverpotential hydrogen evolution in contact with a purely aqueous electrolyte. We hope that researchers will build on the now-extensive literature correlating molecular and electronic structure with catalytic activity to develop aqueous HER catalysts that are viable for solar-driven water-splitting applications. Non-noble OER catalysts for acidic media Many non-noble OER catalysts are available for use under alkaline conditions.105 However, to date the only OER catalysts that are active and stable under acidic conditions are noble metal oxides. Dimensionally stable anode catalysts have been important for PEM electrolysis as well as for the chlor-alkali production of chlorine, chlorate, and hypochlorite.26 Although these processes are carried out industrially, scaling hydrogen production to terawatt levels, as would be necessary for solar storage to make a meaningful impact on world energy use, cannot rely on the extensive use of scarce metals such as Ir and Ru. The discovery of non-noble OER catalysts that are stable under acidic conditions is therefore an extremely important target for materials research. The development of non-noble, acid-stable OER catalysts is an enormous challenge. Most nonnoble metals and their oxides are unstable under acidic conditions, and corrosion is often accelerated by anodic polarization. Perhaps guidance can be obtained from the oxygen-reduction literature, wherein researchers have developed active and stable catalyst composites by treating non-noble metals with carbon, nitrogen, and sulfur species at high temperatures under inert atmosphere.106 Efforts to improve the activities of metal oxides that have otherwise marginal stability under aqueous acidic conditions, such as MnO2, might also prove fruitful.107 Any nonnoble OER catalyst shown to exhibit extended acid stability, even at the expense of diminished activity compared to the Ru/Ir oxide composites, would represent a major advance in the field. Transparent catalysts Heller et al. demonstrated that “transparent” catalyst coatings could be generated when noble metals were deposited at low loadings in porous morphologies on InP electrodes.81 However, to achieve comparable overpotential performance to the noble metals, non-noble metal 13



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electrocatalysts must be deposited at comparatively high mass loadings (mg cm-2 compared to µg cm-2 for noble metals).29,41,108 If the catalyst is deposited directly onto the surface of the light absorber, these high mass loadings generally result in large optical losses due to absorption and/or scattering by the catalyst. Similar problems exist for OER catalysts deposited directly onto anode light absorbers. Trotochaud et al. recently modeled the effects of parasitic light absorption by metal oxide materials deposited onto the surface of an idealized photoanode light absorber.109 The optical losses caused by films of opaque metal oxides were so severe that sub-nanometer catalyst films produced the highest overall photoanode device performance, in spite of low electrocatalytic activities. Similarly, planar Si hydrogen-evolving photocathodes deposited with uniform films of Ni–Mo HER catalysts did not achieve energy-conversion efficiencies equal to those of Si coated with Pt, due to the tradeoff between activity and transparency for the catalyst layer.110,111 Researchers interested in the development of integrated solar-driven water-splitting systems will certainly need to address the challenge of parasitic light blocking by electrocatalysts. One possible route involves the development of much more active non-noble heterogeneous HER catalysts, so that very thin films with low optical absorption can be used (Figure 4a). Active molecular catalysts tethered to light absorber surfaces belong in this category as well, because even multilayers of molecular species are likely to be essentially transparent to incident photons. These approaches are worthwhile to pursue, but they are likely to be challenging. Another approach to the mitigation of catalyst parasitic light absorption involves the selective deposition of catalysts onto structured solar absorbers. For example, a light absorber could be fabricated in a high aspect-ratio structure, and a porous catalyst could be deposited only at the base of the structure (Figure 4b). This geometry effectively “hides” the catalyst from incident solar photons, while still allowing reactant and product transport through the porous framework. However, the design is only amenable to light absorber materials that can be structured with high aspect ratios while preserving efficient light absorption and charge collection. Nevertheless, similar approaches that take advantage of spatially selective deposition or three-dimensional structuring might allow non-noble catalysts with relatively low activities to be used in integrated photoelectrodes. 14


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Figure 4. Modes by which parasitic catalyst light blocking can be minimized in solar photoelectrodes: (a) highly active catalyst deposited on a planar absorber in a thin, porous film; (b) larger mass of moderately active catalyst deposited selectively at the base of a porous absorber structure.

Stable absorbers Work on photovoltaics and photoelectrochemistry has yielded a variety of efficient semiconductor light absorbers. Unfortunately, only a very few of these absorbers are stable in contact with aqueous electrolytes. For example, Si, InP, and WO3 are stable under acidic conditions, whereas many titanates and Fe2O3 are stable under alkaline conditions. Light absorbers that are stable under neutral conditions, such as BiVO4, have also been developed. As noted previously, however, water electrolysis at neutral pH is not currently viable for deployment at scale. A functional, integrated solar-driven water-splitting device could perhaps be designed using Si and WO3 absorbers, acid-stable catalysts, and a PEM separator. This approach was recently demonstrated for net solar-driven water splitting without an applied bias, but only under 12-fold concentrated solar illumination.112 Analogous stable and compatible absorber combinations for alkaline operation do not yet exist, although Rocheleau et al. demonstrated a putatively stable alkaline photoelectrolysis cell using an amorphous silicon absorber that was effectively insulated from direct contact with the electrolyte.113 We suggest that expansion of the library of efficient and stable light absorbers for use in alkaline or acidic conditions is a worthwhile goal. 15



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At least two methods are available for enhancement of the aqueous stability of light absorbers: new materials that are inherently stable could be discovered, or otherwise unstable semiconductors could be stabilized by surface treatments. The former approach is likely to be challenging, given the impressive amount of prior research devoted to identification of suitable solar absorbers. Recent work on p-type WSe2,84 however, suggests that new or relatively unexplored stable absorbers can nevertheless be found. Another promising approach for the development of water-stable photoelectrodes involves the generation of thin, protective layers on otherwise unstable semiconductors. Efforts in this area might draw on the extensive literature on transparent conductors.114 Notably, the ubiquitous transparent conducting materials, such as fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO), are not indefinitely stable under alkaline or acidic conditions. Many refractory and transition metal oxides, however, are stable in aqueous acid or base. One route toward the stabilization of otherwise sensitive absorbers might therefore involve the deposition of ultra-thin oxide layers onto semiconductor surfaces, as has been done recently with TiO2 and MnO2 on ntype Si photoanodes.87,88 The oxides used for this purpose does not need to be as conductive as conventional transparent conducting oxides, because electron transport only needs to occur through the film thickness, rather than laterally to current collectors, as in PV cells. Simple device designs One of the major advantages proposed by advocates of integrated solar-driven water splitting is the possibility of simple device design and construction. Several of the early proposals for photoelectrochemical water splitting devices were as simple as one or two semiconductor electrodes directly submersed into an aqueous electrolyte, with the charge separation facilitated by the semiconductor–liquid junctions.9,115 The only successfully demonstrated devices of this type to date, however, utilize wide-band-gap oxides and consequently exhibit very low solar– energy conversion efficiencies.60 As noted at the outset, various solar-driven water-splitting device designs have been assessed for their technological and economic feasibility.6,7 One design that has garnered interest for its low projected total cost of hydrogen production consists of semiconductor particles suspended in a 16


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large, flexible plastic enclosure. Promising results on systems of this type have been achieved by Domen and coworkers using oxynitride absorbers and noble-metal catalysts, but the efficiencies remain low.116-118 This approach also entails major safety concerns involved with the cogeneration of stoichiometric mixtures of hydrogen and oxygen in a single compartment on an industrial scale. Highly efficient solar-driven water-splitting devices could be constructed with existing technologies, but only by the use of conventional photovoltaics and electrolyzers, or very similar components.10 These devices are not simple to construct, and as a result they are very costly. Consequently, a design intermediate between the extremes of a PV/electrolyzer system and a colloidal water splitting system may constitute an optimum compromise between simplicity and feasibility. Although we favor a so-called integrated PEC approach (Figure 1 center), demonstration of an efficient and safe solar-driven water-splitting system that exploits an even simpler design would constitute an important accomplishment. Device design and construction is clearly an area that warrants continued consideration, discussion, and research. CONCLUDING REMARKS Fossil fuels have been critical to the development of modern society, but concerns over pollution, environmental degradation, and climate change demand that humans must transition to renewable sources of energy. It is our hope that artificial photosynthesis devices will “see the light of day” by providing a dominant contribution to the energy needs of humanity before the end of this century. But achieving that outcome will require innovations in materials development closely coupled to device design and engineering. Addressing the critical research innovations outlined herein will help accelerate the transition to a sustainable energy future. Acknowledgments: NSL acknowledges support from the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

JRM and HBG acknowledge the National Science

Foundation for support through the Powering the Planet Center for Chemical Innovation, grant CHE-0802907. JRM acknowledges the DOE Office of Science for a graduate research 17



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fellowship. The authors thank Dr. Shane Ardo and Adam Nielander for helpful comments during the preparation of this manuscript. Author Biographical Information Harry Gray is the Arnold O. Beckman Professor of Chemistry and the founding Director of the Beckman Institute at the California Institute of Technology. In 1961, after study at Northwestern University and the University of Copenhagen, he joined the faculty at Columbia University. In 1966, he moved to Caltech, where for over 40 years he has been doing research in biological inorganic chemistry and inorganic photochemistry. A member of the National Academy of Sciences, he has received the National Medal of Science (1986); the Priestley Medal (1991); the Wolf Prize (2004); and the Welch Award (2009). He is the PI of the NSF Solar Fuels Center for Chemical Innovation. Nathan Lewis, the George L. Argyros Professor of Chemistry, has been on the faculty at the California Institute of Technology since 1988. He has served as the Principal Investigator of the Beckman Institute Molecular Materials Resource Center at Caltech since 1992, and is the Scientific Director of the Joint Center for Artificial Photosynthesis, the DOE's Energy Innovation Hub in Fuels from Sunlight. Dr. Lewis has received numerous research and teaching awards, and he is currently the Editor-in-Chief of the Royal Society of Chemistry journal, Energy & Environmental Science. James McKone completed his graduate work in Chemistry at Caltech in the Spring of 2013, working with Nate Lewis and Harry Gray as part of the NSF Solar Fuels Center for Chemical Innovation. He was a DOE Office of Science Graduate Research Fellow from 2010-2013, and he received the Demetriades-Tsafka-Kokkalis Prize and the Clauser Prize in 2013 for his thesis work on light absorbers and electrocatalysts for solar hydrogen evolution. He is currently pursuing postdoctoral research on batteries and fuel cells at Cornell University.

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