Solar Energy Conversion and Environmental Remediation Using

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Solar Energy Conversion and Environmental Remediation Using Inorganic Semiconductor
Liquid Interfaces: The Road Traveled and the Way Forward ABSTRACT: Unlike their solid-state counterparts, semiconductor
liquid junctions are versatile in that the incident solar energy can be stored in the form of chemical fuels. Another attractive application is the use of irradiated oxide semiconductor-coated surfaces for self-cleaning and antifogging. The theme of this history-tinged Guest Commentary centers on the question of what has been accomplished in the above “photoelectrochemical” schemes over the 35 year time frame from 1975 to 2010. Progress in this field was aided by the infusion of new concepts and contributions from the materials chemistry and physics communities. A related aspect of discussion is how the active semiconductor material has evolved both chemically and morphologically in these applications. It is shown that despite impressive research advances, only a handful of the above concepts (e.g., dye-sensitized solar photon conversion and self-cleaning and antifogging surfaces) have made the successful transition from the laboratory to the marketplace.

A

lphonse Lamartine, the French poet and writer, said that history teaches everything including the future. In the words of the English writer, G. K. Chesterton, “We can be almost certain of being wrong about the future, if we are wrong about the past.” Therefore, it seems profitable to take a look at where we started and how far we have come in the field of photoelectrochemistry and solar energy conversion. The discussion below pertains very largely to inorganic semiconductors, and solid-state solar devices are discussed only fleetingly or when comparisons are relevant to their liquid junction counterparts. The Early Years. These can be defined as the years leading up to ∼1970. Much of the early work on semiconductor
electrolyte interfaces during this period was driven by their relevance to microelectronic devices. Indeed, this body of work was largely performed at corporate research laboratories in the U.S. and in Europe in the 1950s. Thus, topics such as corrosion of the semiconductor surface, hydrogen evolution coupled with corrosion, surface state formation, and potential distribution across the semiconductor
electrolyte interface were intensely studied, as exemplified by papers on Ge, Si, ZnO, CdS, and other binary compound semiconductors.1
6 The first report of a photovoltaic effect at the semiconductor
electrolyte interface dates back to Becquerel’s paper in 1839.7 He showed that illumination of a thin layer of AgCl coated on a sheet of Pt resulted in an electric current. The recognition that semiconductor electrodes in contact with electrolytes could generate large photocurrents, relative to their well-studied metal electrode counterparts, was traced to the photogeneration of electron
hole (e
h) pairs within the semiconductor via band gap excitation and their subsequent generation by the built-in electric field at the semiconductor
electrolyte junction.8
12 Concomitant with these developments was the recognition of application possibilities in the field of photography using spectral sensitization of wide-band-gap semiconductors and visible-light absorbing-organic dyes.13 In a natural progression, fundamental understanding of charge-transfer processes involving semiconductor electrodes and the role of conduction and valence bands within the semiconductor became crucial to these applications.9
12 Thus, the late 1960s and early 1970s saw tremendous progress in these topics of research, as summarized in monographs and book chapters.11,14
22 r 2011 American Chemical Society

Probably, the first instance of the use of a light-responsive electrode for the photoassisted decomposition of water is the paper in 1960 on “Decomposition of Water by Light”.23 An anthracene single-crystal slab was placed in a cell with two identical NaCl solutions on either side acting as transparent electrical contacts to each face of the organic crystal. When the solutions were electrically connected by an external wire and one side of the crystal was illuminated with 360 nm light, a photocurrent was seen to flow. Concomitantly, hydrogen was seen to evolve on the illuminated crystal face and oxygen at the “dark” crystal side.23 However, this field literally exploded with a report on the use of a TiO2 photoanode in an electrochemical cell for the splitting of water into H2 and O2.24

The flurry of activity that followed on the heels of the discovery of the photosplitting of water was spectacular. The Boom Years. The flurry of activity that followed on the heels of the discovery of the photosplitting of water was spectacular. Both the pace of activity and progress continued to rise during the period approximately spanning 1975
1985 (Figure 1). The range of semiconductors that was examined included not only oxides but also other binary semiconductors in the Group 12
16 and Group 13
15 families, such as CdE (E = S, Se, Te) and GaY (Y = As, P) and even alloys thereof. Much of this body of work has been reviewed elsewhere.16
20,25
27 Worthy of note in the present context, however, is the fact that these semiconductor materials already had a rich history of study from electrochemical and microelectronic perspectives Received: March 23, 2011 Accepted: May 6, 2011 Published: May 13, 2011 1301

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Figure 1. The results from a literature search using the ISI Web of Knowledge database using the keywords photoelectrochemistry, electrochemistry and solar energy conversion, semiconductor quantum dots and solar energy conversion, and solar water splitting. The inset contains an exponential fit of the literature search results (see text).

(see above). Instead, the attention had now turned to the possibilities associated with using their interfaces with water for solar energy conversion and for environmental remediation. Tempering the initial enthusiasm, however, came the quick recognition that many of these narrow-band-gap semiconductors (that were more optimal in terms of better optical overlap with the solar spectrum) and even some of the wide-band-gap semiconductor counterparts (e.g., GaP, ZnO) were also inherently chemically and electrochemically unstable in contact with aqueous media and upon photoexcitation. The strategies to overcome these stability limitations have been impressively diverse and have enjoyed varying degrees of success. These are reviewed elsewhere and need not be repeated here.25,27 It is worth noting that in the quest to stabilize the semiconductor against photoelectrochemical (PEC) corrosion, the devices that resulted could only be used for converting the incident photons to electrical energy rather than for storing them as H2. The latter was a major attractive feature of liquid junction devices and one that differentiated them from the solid-state photovoltaic counterparts. An exception to this worrying trend was the observation that p-type semiconductor-based junctions were relatively immune from PEC corrosion. Indeed, even Si (a semiconductor prone to corrosion

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passivation in contact with water) in the p-doped state could efficiently generate H2 from aqueous media.28,29 Several novel concepts emerged during this very fertile period in the evolution of PEC solar energy conversion. Four, in particular, may be mentioned, (a) the combination of n-type and p-type semiconductors for water (or HX, X = halogen) photosplitting in a Z-type scheme29,30 (reminiscent of the plant photosynthesis system31) or even in direct electronic contact as in a p
n “PEC diode”,32 (b) the proposal that a much less appreciable lowering of efficiency would result from the use of a polycrystalline film for a given semiconductor (rather than its single-crystal form) in a liquid junction device than in a corresponding solid-state photovoltaic device case,28,29 (c) passivation of surface states and grain boundaries at the semiconductor
electrolyte interface and efficiency enhancement by deliberate chemisorption of certain metal ions (e.g., Ru3þ) on the semiconductor surface,28,33,34 and (d) the discovery that layertype chalcogenides (e.g., MoE2 or WE2) are endowed with electronic, bonding, and surface chemistry features that make them attractive candidates as electrodes in PEC cells.35,36 Related to point (b) above is the fact that the vast majority of studies that emerged during this period used single crystals of the semiconductor in the PEC cells (Table 1). While these studies afforded valuable fundamental insights into the PEC behavior of the interface, practical applications were obviously hampered by two (coupled) facts associated with the use of single crystals, (a) their prohibitively high cost and (b) the fact that solar conversion devices by their very nature rely on large photoactive areas that are difficult to achieve with single crystals. The optical handicaps associated with the use of TiO2 for solar energy conversion (notwithstanding its chemical inertness and excellent PEC stability) prompted researchers to explore ways of sensitizing this material to visible light absorption. This was accomplished either by doping (a strategy borrowed from the solid-state physics community)37 or by chemical modification of the oxide surface with organic dyes or metal complexes.38 The former approach parallels developments in the closely related field of heterogeneous photocatalysis, and the latter strategy laid the foundation for dye-sensitized solar cells (DSSCs). Both of these aspects are further discussed below. In closing this section, the single-crystal-based PEC devices that were actively studied during this period may be termed firstgeneration or 1G (Table 1). By the same token, the researchers who forged the initial breakthroughs may also be regarded as 1G (Table 2). However, toward the middle-to-end of this period (∼1978 onward) emerged a second wave of researchers (Table 2) sustaining the rapid progress made by their mentors in the preceding years. The Hiatus or Lost Decade in Photoelectrochemical Solar Energy Conversion. The frenetic activity during the first decade from 1975 to 1985 (Figure 1) was soon followed by a considerable slowing down of the progress (and loss of momentum for the field) during the period from 1985 to 1990. For example, the third bar in the activity histogram in Figure 1 and the corresponding data point in the inset fall below the exponential trend line. It is tempting to ascribe this trend to the concomitant precipitous drop in the oil prices during this time period. Indeed, a policy shift to a de-emphasis of the search for alternatives to fossil fuels paralleled a serious decline in research funding for PEC solar energy conversion and, indeed, for solar energy R&D in general during this tumultuous period for renewable energy technologies. This in turn had the unfortunate consequence of turning many researchers away from this field, and the PEC solar 1302

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Table 1. Three Generations of Photoelectrochemical Active Materials Generation first-generation or 1G

Materials semiconductor single crystals

second-generation or 2G polycrystalline thin films; metal-doped semiconductors for visible light sensitization; semiconductor powder suspensions for photocatalysis; semiconductor superlattice photoelectrodes third-generation or 3G

nanostructured semiconductors and composites; size-quantized semiconductors (or quantum dots); semiconductor nanotubes and nanowires; semiconductor-conducting polymer composites; biomimetic architectures

Table 2. Three Generations of Researchers in Photoelectrochemical Energy Conversion Generation

Representative Names

1G

Heinz Gerischer, Allen Bard, Rudi Memming, Mark Wrighton, Adam Heller, Barry Miller, Joost Manassen, John Albery, John Goodenough, Roy Morrison, Mike Butler, Art Nozik, Arnim Henglein, Kenichi Honda, Hiroshi Tsubomura, Hideo Tamura,

Figure 2. The results from a literature search using the ISI Web of Knowledge database using the keywords semiconductor preparation, semiconductor electrodeposition, and semiconductor chemical bath deposition. Every attempt was made in this compilation to “filter” out the vast body of literature on semiconductor fabrication (stemming mainly from the engineering and device physics communities) directed at the microelectronics subject area.

Walter Gomes, Pedro Salvador, Gabor Somorjai, Yuri Pleskov, 2G

Helmut Tributsch, Akira Fujishima, Mary Archer Michael Gr€atzel, David Ginley, Art Ellis, Nate Lewis, Bruce Parkinson, Carl Koval, Andrew Bocarsly, Prashant Kamat, Reshev Tenne, Andrew Hamnett, Gary Hodes, Laurie Peter, David Cahen, Krishnan Rajeshwar, Norma de Tacconi, Achim Lewerenz, Joseph Hupp, Tom Mallouk, Shozo Yanagida, Yoshihiro Nakato, Daniel Lincot, Claude-Levy Clement, Anders Hagfeldt, Jan Augustynski, Gerald Meyer, John Kelly, Kazunari Domen, John Turner, Kohei Uosaki, Hiroshi Yoneyama, Michio Matsumura

3G

Lou Brus, Paul Alivisatos, Moungi Bawendi, Patrik Schmuki, Craig Grimes, Juan Bisquert, Kyoung-Shin Choi, Daniel Nocera, Charles Lieber, Daniel Vanmaekelbergh, Gerko Oskam, Akihiko Kudo, Paul Maggard, Frank Osterloh, Gerrit Boschloo, Stuart Licht, Song Jin, Peidong Yang, Tsukasa Torimoto, Jin Zhang, Susumu Kuwabata, Harry Atwater

energy conversion community was reduced to only a handful of diehard research groups worldwide. Fortunately, the field was sustained (and nurtured) by the 2G researchers and 2G solar conversion devices (Tables 1 and 2). For example, the demonstration that very efficient semiconductor
liquid junctions (with photovoltaic conversion efficiency approaching corresponding solid-state devices) could be constructed by paying careful attention to semiconductor quality39 was a beacon (at least in the eyes of this author) during this rather bleak period for the field. Paralleling this progress was the discovery that electrochemical methods could be used to prepare thin films of the semiconductor material on a suitable substrate. Although isolated studies had appeared (particularly on the anodic deposition route) in the 1970s, the cathodic semiconductor electrodeposition strategy rapidly gained currency during the period from ∼1985 onward

(Figure 2).40 This development came on the heels of two pioneering papers on the Cd
Te binary compound system.41,42 This body of work has been reviewed elsewhere,40,43
47 although not all of the studies on the derived thin films (or epitaxial layers and superlattices) were directed toward solar energy conversion. Indeed, these studies resulted not only in binary compounds but also in ternary compound semiconductor counterparts (e.g., CuInSe2), and these developments had a favorable impact in terms of applicability in solid-state solar photovoltaic cells.46 Perhaps it would not be an exaggeration to state that research on “mild” or cost-effective alternatives to ultrahigh vacuum (UHV)based methods for semiconductor preparation (such as electrodeposition and chemical bath deposition) went a long way to indirectly fostering the health of the PEC energy conversion field during the period of 1985
1990. The literature search result shown in Figure 2 does show steady and significant activity in the area of semiconductor preparation by the chemistry and materials communities. Once again, it is noted that this progress was now being made in many of the laboratories of the 2G researchers (Table 2). Other significant conceptual developments during this period revolved around the notion that “hot” carriers could be invaluable for PEC solar energy conversion both in terms of efficiency enhancements (see below) and for driving photoreactions that otherwise would not be possible with their thermalized carrier counterparts.48,49 Associated with these ideas was the use of semiconductor electrodes (derived from UHV methods such as molecular beam epitaxy) based on superlattices.50,51 Such materials had only been examined hitherto from their applicability in opto-electronic devices such as solid-state diode lasers and lightemitting diodes. Heterogeneous Photocatalysis, Environmental Remediation, and Water Photosplitting Using Semiconductor Suspensions. Heterogeneous photocatalysis is very closely related to PEC energy conversion but mostly utilizes semiconductors as photocatalysts 1303

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Figure 3. The results from a literature search using the ISI Web of Knowledge database using the keywords photocatalysis and TiO2, photocatalysis and oxide semiconductor, and photocatalysis and pollutant degradation. The related literature on water splitting and dyesensitized solar cells was excluded from this database. The inset contains an exponential fit of the literature search results (see text).

instead of as electrodes in conventional electrochemical cells. (We do note the development of “photoelectrocatalysis”, an approach that uses photocatalysts in the form of electrodes instead suspensions; see ref 52.) As the term implies, these active materials are used to drive, under photoexcitation, thermodynamically “down-hill” processes that are kinetically sluggish in the dark. Examples of such processes include the oxidation of hydrocarbons and organic dyes and the reduction of organic nitro compounds. It must be noted, in this context, that both the splitting of water (into H2 and O2) as well as reactions such as the PEC reduction of CO2 are thermodynamically uphill processes. The notion that inorganic semiconductors (especially oxides) could be used for environmental photoremediation probably had its origins in the colloid and pigment chemistry communities, although environmental and atmospheric research communities very soon jumped on this bandwagon. Two review articles, spaced more than a decade apart, summarize these developments.52,53 The first articles describing the use of irradiated TiO2 (and other oxide semiconductor) suspensions for environmental remediation appeared in 1976 and 1977,54
57 although an earlier research report (from a water treatment center) that appeared in 1969 was also cited (see ref 57). These early works demonstrated that environmental pollutants and toxic substances such as phenol, cyanide, and polychlorinated biphenyls could be broken down to less harmful products.

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It is worth noting that the three articles cited above appeared very soon after the discovery of the use of TiO2 for UV- or solarassisted water splitting. The subsequent growth of this field can only be described as explosive (Figure 3), and this growth has been uninterrupted over the period from ∼1975 through the time of writing of this Guest Commentary (cf., Figures 1 and 3; compare also the quality of the corresponding model fits shown as the dashed lines in the insets of the two figures.). Unlike its PEC energy conversion counterpart, this field was not buffeted by the vagaries of the oil marketplace. Since the early work, TiO2 has emerged as a model workhorse for an inorganic photocatalyst, a trend no different from the PEC energy conversion case. However, unlike in the latter case, the range of other semiconductor candidates that have been successfully deployed has been more limited because of two limitations, (a) susceptibility to photocorrosion, a handicap that also besets PEC energy conversion (see above), and (b) the need for a very positive valence band location for the semiconductor such that the photogenerated holes have sufficient oxidizing power to generate species such as hydroxyl radicals. The same fundamental electrochemical principles underpin both PEC energy conversion and heterogeneous photocatalysis applications, and this fact has been recognized and discussed.58 An added impetus to the latter field came from the colloid chemistry community, and this trend especially manifested in the photocatalyst preparation aspects where techniques such as sol
gel synthesis (a staple of the colloid chemistry community) have played a key role. The recognition of the limitations of TiO2 in terms of incompatibility with the solar spectrum, as in the PEC energy conversion case, spurred much activity on visible light sensitization of this material. The primary strategy employed was doping of the host oxide lattice with either metal ions25,26,37 or nonmetallic elements such as C, S, or N.26,59 The progress made in this area and the challenges have been reviewed elsewhere.26,29 Suffice it to say here that the results have been mixed, and the observation of visible light response for TiO2 is not always accompanied by an improvement in the photocatalytic activity. A related development in PEC energy conversion, namely, the use of semiconductor suspensions either for water photosplitting or for CO2 reduction, undoubtedly derived inspiration from heterogeneous photocatalysis and colloid chemistry. This was a fertile area of activity that began in the 1980s and continues unabated to this day.16,26,29 The suspension-based approach does have practical advantages, including simplicity of photoreactor design, but suffers from two crucial handicaps; (a) the sites for H2 and O2 photoevolution are not spatially and physically separated, leading to potential explosion hazards, and (b) no electrical bias can be applied to a semiconductor suspension, unlike to its electrode counterpart, such that only very fast (and selective) interfacial reaction kinetics will prevent the photogenerated e
h pairs from simply recombining and decreasing product yields. The consequence is a poor quantum yield (only a few percent). Commercial realization of heterogeneous photocatalysis and PEC energy conversion, based on the use of semiconductor suspensions, would be clearly hampered because of the challenges outlined in the preceding two paragraphs. This is further elaborated in a concluding section below. However, it is worth pointing out that a related strategy of coating architectural and other surfaces with TiO2 films for developing materials that undergo self-cleaning or antifogging upon irradiation60 is a notable success story in this area (see below). 1304

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Indeed, the two areas of activity have coalesced with the advent of semiconductor Q-dots as visible light sensitizers.72,73

Much of the subsequent development in the growth of PEC energy conversion from 1990 onward can be traced to the nanotechnology revolution and the advent of DSSCs.

Figure 4. The results from a literature search using the ISI Web of Knowledge database using the keyword dye-sensitized solar cell.

The Nano Revolution or Deja Vu All Over Again. The period starting approximately in 1990 heralded the advent of the nanotechnology era. Perhaps not surprising, in retrospect, this also brought about a sea change in fortunes for the PEC energy conversion field. This manifests, for example, by the two data points that fall noticeably above the exponential trend line in the model fit (see inset, Figure 1). Two trends are worthy of note in the evolution of the 3G semiconductor materials and energy conversion devices (Table 1). First was the realization (undoubtedly aided by cross-pollination of the semiconductor physics and colloid chemistry communities) that shrinking of the semiconductor particle size and paying close attention to size dispersion can bring about dramatic advantages in the optical absorption profile. Specifically, size-quantization effects cause significant changes in the optical band gap of the semiconductor.61
63 This has led to the development of a family of 3G semiconductor nanocrystals called quantum dots (or simply Q-dots).61,64,65 Hot carriers have been observed with mixed semiconductor Q-dots (e.g, CdSe/ZnSe, PbSe/TiO2) including core/shell configurations, although liquid electrolytes were not utilized in these studies.48,66,67 The second crucial development that occurred in the area of PEC energy conversion around 1990 was tied to the recognition of the tremendous surface area enhancement that could be secured via nanostructuring of the semiconductor material. Thus, one of the key obstacles with the use of semiconductor single crystals (or even a flat polycrystalline thin film) for visible light sensitization via dye adsorption (see above) could be immediately circumvented by the use of a nanostructured (or “mesoscopic”; see ref 68) oxide film. Sensitized photocurrents could be amplified more than 1000fold by substituting the single crystal (or flat thin film) with a nanostructured TiO2 film (see also, for example, Figure 5 in ref 77) several micrometers thick. It is not an exaggeration to state that the DSSC based on such TiO268
71 provided a tremendous fillip to the field of PEC energy conversion just when it needed it most in 1990. Note the very significant jump in activity that occurred between the periods of 1986
1990 and 1991
1995 as exemplified by data points #3, #4, and #5 in Figure 1(inset). Much of the subsequent development in the growth of PEC energy conversion from 1990 onward can be traced to the nanotechnology revolution and the advent of DSSCs (Figure 4).

Other 3G photovoltaic concepts that may be mentioned include approaches to “break” the so-called Shockley
Quiesser efficiency threshold (Table 1).74 One such approach was discussed earlier based on the use of hot carriers. Another is a photovoltaic photon conversion mechanism based on multiple exciton generation (MEG).48 Experimental demonstrations of MEG effects have been a controversial topic; however, a very recent report uses a PEC framework to observe photocurrent amplification attributable to them.75 The 3G concept of manipulating the morphology of semiconductors on a nanoscale, which manifested first with nanostructured semiconductor films and Q-dots (see above), has been extended to the development of nanotubular arrays (NTAs) of TiO2.21,76
78 These NTAs have been applied to water photoelectrolysis,79 PEC CO2 reduction,80,81 and in DSSCs.82 Silicon nanowires have also been deployed in PEC cells.83,84 A major impetus in these developments was the notion of reducing the dimensionality of electron collection from 2 to 1, that is, the directions of light absorption and charge collection are orthogonalized. Time-resolved terahertz spectroscopy, however, has revealed that exciton-like trap states in TiO2 nanotubes limit electron mobility.85 Thus, while NTAs do not suffer from freecarrier loss due to grain boundary scattering or defect states, the deleterious influence of trap states must be ameliorated in these 3G materials before the advantages with the use of NTAs for PEC energy conversion devices are completely realized. From a researcher's perspective, we have also reached the third wave or 3G (Table 2). These include, interestingly enough, not only researchers mentored in the laboratories of their 2G predecessors but also senior investigators who migrated to PEC energy conversion after distinguished work in other areas such as spectroscopy, nanotechnology, polymer chemistry, inorganic chemistry, and the like. It is also worth noting that 1G and 2G researchers continue to make contributions to 3G concepts and devices such as MEG and DSSCs, a healthy sign of longevity and continued productivity. Where Are We Now? With the preceding historical perspective at hand, where are the related fields of PEC energy conversion and environmental remediation poised at present, and how far are they from commercial realization? It is worth noting at the outset that the three generations of PEC materials and/or concepts (Table 1) mirror the corresponding developments in the photovoltaic (PV) community.86 The exponential trend noted in the growth of publications (see insets, Figures 1 and 2) is also reminiscent of Raymond Kurzweil’s Law of Accelerating Returns, which has been widely noted in technological progress, which happens exponentially and not linearly.87 1305

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competitive here from an active materials cost perspective and for applications demanding flexible solar cell deployment.68,71 A key ongoing development extends this technology to also involve a storage (e.g., water photosplitting) component.68,89,90 It is worth noting a recent success story in the energy conversion arena, namely, lithium battery technology. This technology is more favorably placed for large-scale deployment in the transportation sector than polymer electrolyte membrane fuel cells (PEM FCs). In the former instance, the laboratory research developments were able to transition easily into the marketplace, having been already vetted by the consumer electronics sector. As with PEM FCs, this sort of transition also is yet to happen with PEC energy conversion. In both of these latter cases, efficiency and cost have been bottlenecks along with stability/corrosion issues.

On the positive side of the ledger, we have seen many fundamental advances in these research areas over the period from 1975 to 2010.

Figure 5. The results from a literature search using the ISI Web of Knowledge database using the keywords water photosplitting reactor, photoreactor, photocatalytic reactor, and photoelectrochemical reactor.

Notwithstanding the policy and economic realities (a discussion of which lies beyond the scope of this Guest Commentary), an arbitrary laboratory efficiency of 10% can be set as a benchmark before the particular solar device or process is considered to be ready for scale-up and commercialization. Against this efficiency bar, only the DSSC technology would fare well, where solar energy conversion efficiencies in excess of 10% have been routinely reported, at least for cells filled with ionic liquid-based electrolytes.68
71 Analogous solid-state or gel-based systems have to develop further along the efficiency trajectory.73 In the water solar photoelectrolysis case, the situation is more sobering, and despite a three-decade history, there are only isolated reports of solar conversion efficiencies higher than the benchmark.20,26 Even in the one study where high efficiency was reported, the photoelectrode material was comprised of several semiconductor layers (derived from an expensive fabrication method such as MBE) in a photobiased junction in tandem with the water splitting component.88 We have already noted above the roadblocks to efficient utilization of the solar spectrum and, more importantly, of the photogenerated e
h pairs for processes aimed at environmental remediation. Currently, the use of UV light (and concomitant electrical costs) coupled with the nontrivial costs associated with the photocatalyst itself and rather low quantum yields remain as roadblocks to commercialization. Possible exceptions to this trend are applications to air purification and self-cleaning surfaces (both in the commercial and residential sectors),60 where the heterogeneous photocatalysis technology is showing promise to be competitive. One challenge in the environmental remediation arena is that the competing technologies are very mature and cost-effective. Therefore, the barriers to commercialization for a new entrant are consequently quite steep. Photoelectrochemical reduction of CO2 is even further removed from being poised for commercialization. It is worth noting that solid-state photovoltaic technology is much further along the trajectory to commercialization because laboratory efficiencies have surpassed the 10% efficiency benchmark on a more routine basis. However, the DSSC technology is

Where Do We Go From Here? The preceding paragraphs may have painted a bleak picture for the immediate prospects of commercialization for PEC energy conversion and environmental remediation. However, on the positive side of the ledger, we have seen the many fundamental advances in these research areas over the period from 1975 to 2010. We have also seen how the infusion of new ideas from other disciplines (e.g., colloid chemistry, nanotechnology) has aided progress. There is every reason to be optimistic about further progress in the years to come. To facilitate the transition of these research advances to commercialization, however, many of the 3G concepts have to deliver on the promise of the much higher efficiency levels, theoretically possible beyond the Shockley
Quiesser limit.74 At the very least, practical efficiencies approaching the 10% benchmark have to be achieved on a more routine basis. This in turn will serve to attract the attention of the chemical and electrochemical engineering communities and prompt them to tackle the issues of photoreactor design and process scale-up. For example, many of the advances that have occurred in PEMbased water electrolyzer designs20 can be easily incorporated into the corresponding solar counterparts but with design features optimizing the incident light flux into the reactor. The literature search results in Figure 5 are symptomatic of the significant void that currently exists on this topic. Only a total of ∼350 papers were registered cumulatively over the period of 1975
2010. Compare also the number of publications appearing on the ordinate in Figure 5 vis-a-vis those in Figures 1
4. Finally, efforts from the 2G researchers will continue to serve as a bridge to the earlier generation, and in turn will inspire new generations of researchers (3G and beyond, drawn both from the junior ranks and from allied disciplines) (Table 2) to join the fray. The grand challenge is to continue to move science and engineering forward so that technologies beyond air cleaning and self-cleaning surfaces can be moved to the marketplace from the research laboratory. We began this Commentary with literary quotes and 1306

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The Journal of Physical Chemistry Letters end with one from Robert Frost: “But I have promises to keep, and miles to go before I sleep, and miles to go before I sleep.”

The grand challenge is to continue to move science and engineering forward so that technologies beyond air cleaning and self-cleaning surfaces can be moved to the marketplace from the research laboratory.

Krishnan Rajeshwar* Center for Renewable Energy Science & Technology (CREST), Department of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ BIOGRAPHY Krishnan Rajeshwar is a Distinguished University Professor and Associate Dean of the College of Science at the University of Texas at Arlington. He currently serves as the Editor of Electrochemical Society's Interface. His research interests include photoelectrochemistry, solar energy conversion, renewable energy, materials chemistry, semiconductor electrochemistry, and environmental chemistry. ’ ACKNOWLEDGMENT This article is dedicated to Prof. Adam Heller in recognition of his seminal contributions to this field and for inspiring new generations of researchers, including this author. Much of the research in my laboratory on PEC energy conversion and environmental remediation has been supported by the U.S. Department of Energy. On a personal note, I feel privileged to have been part of this research community since ∼1978. As a researcher belonging to the second generation, I have been able to gain a unique perspective on this field from its halcyon days to its current healthy state. In a sense, along with the other research colleagues from my generation (Table 2), I have served as a bridge connecting the first generation (that laid down the foundation for this field) and the 3G of researchers and beyond (that will carry the torch forward). I thank Tom Moore (Arizona State University) for alerting me to ref 31, Tom Mallouk (Pennsylvania State University), Achim Lewerenz (Helmholtz Centre for Materials and Energy, Berlin), and Norma de Tacconi (University of Texas, Arlington) for helpful comments on an initial version of this manuscript. Finally, Wilaiwan Chanmanee and Hari Krishna Timmaji are thanked for invaluable help with the literature search and the results that went into Figures 1
5.

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’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on May 13, 2011. The Acknowledgment was updated. The revised paper was reposted on May 20, 2011.

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