On the Solar to Hydrogen Conversion Efficiency of Photoelectrodes for

Oct 2, 2014 - without the support of additional bias from an external power source such as a photovoltaic cell in tandem with the photoelectrode. An a...
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On the Solar to Hydrogen Conversion Efficiency of Photoelectrodes for Water Splitting

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photoelectrode potential (U) against a reference electrode such as the saturated calomel electrode (SCE)4 or the reversible hydrogen electrode (RHE).5 It is noteworthy that the STH conversion efficiency, as defined by eq 1, cannot be obtained from three-electrode measurements because the short-circuit condition is ill-defined. Although in principle one could measure the potential drop, that is the voltage (V), between the working and counter electrodes using an electrometer and thereby determine when short-circuit (V = 0) occurs, this would only bring it back to square one with the parasitic losses in the cell. Another problem arises from the fact that electrolysis is an endergonic reaction that requires work expenditure. The minimum voltage that must be exerted in order to split water is higher than the reversible potential of the water decomposition reaction, Urev = −ΔG/nF = 1.23 V (under standard conditions). In practice, a voltage of 1.6 V or above is typically applied in state-of-the-art electrolysis cells in order to drive the water splitting reaction at a sufficiently high rate.6 This voltage can be reduced by substituting metal electrodes with semiconductor photoelectrodes that produce photovoltage upon illumination. The photovoltage produced by the photoelectrode adds up to the external voltage from the power source such that the two of them together provide sufficient voltage to drive the water splitting reaction. Thus, the photoelectrode can save power from the power source. Ideally, the photovoltage produced by the photoelectrode would suffice to split water all alone without another power source. But in practice, there are hardly any photoelectrodes that can provide such a high photovoltage all by themselves, except for those made of wide band gap materials such as SrTiO3 and KTaO3 that absorb only a small fraction of the solar spectrum resulting in too low a STH conversion efficiency.7 Tandem cells overcome this problem by combining the water splitting photoelectrode in tandem with another photoelectrode or photovoltaic cell that provides the additional voltage that is needed to split water. The pursuit of such tandem cell devices is a rather complicated engineering exercise that requires careful integration and optimization of both the optical and electrical coupling between the two photoelectrodes or photocells.8,9 Moreover, because photovoltaics is already a mature technology, whereas water photoelectrolysis is not; the photoelectrodes impose the bottleneck, by far and large, toward efficient tandem cell devices. Under the circumstances, it does not make much sense to spend so much time and efforts in integrating low-efficiency photoelectrodes with high-end photovoltaic cells. It is rather premature given the wide gap in performance between the two components. Indeed, except for very few reports that have convincingly demonstrated that building such tandem cell devices is possible,10,11 most of the work is still at the single component photoelectrode level rather

he conventional definition of the solar to hydrogen conversion efficiency of photoelectrochemical solar cells for water splitting is difficult to relate with three-electrode “halfcell” measurements of photoelectrodes that cannot split water without the support of additional bias from an external power source such as a photovoltaic cell in tandem with the photoelectrode. An alternative definition is proposed that allows the estimation of the intrinsic (internal) solar to chemical conversion efficiency of the photoelectrode rather than the overall efficiency of the entire tandem cell. We show how the photocurrent and photovoltage produced by the photoelectrode can be extracted from three-electrode voltammograms measured in the dark and under illumination. The photocurrent × photovoltage product yields the internal photovoltaic power produced by the photoelectrode which, upon multiplication by its electrolysis efficiency, provides the intrinsic solar to chemical conversion efficiency. This analysis yields the current and voltage that should be provided to the photoelectrode by the external power source in order to achieve optimal solar to chemical conversion efficiency. The solar to hydrogen (STH) conversion efficiency of photoelectrochemical solar cells for water splitting is defined as1 ⎡ |J (mA/cm 2)| × 1.23(V) × η ⎤ F⎥ STH = ⎢ sc ⎢⎣ ⎥⎦ P(mW/cm 2) AM1.5G

(1)

where Jsc is the short-circuit photocurrent density, ηF the Faradaic efficiency for hydrogen evolution, and P the incident illumination power density. All these parameters have to be measured under standard solar illumination conditions (AM1.5G), without any sacrificial reagents and without any pH or electrical bias between the working and counter electrodes.1 It is noteworthy that eq 1 defines the STH conversion efficiency of the entire photoelectrochemical cell including all of its components: working and counter electrodes, electrolyte, separator, wires, and so forth. Each and every one of them contributes to the overall loss because of recombination, polarization, diffusion, internal resistance, or other losses.2 The cell efficiency, as defined by eq 1, accounts for all these losses altogether. So although it is an important benchmarking parameter of the photoelectrochemical cell,1 it is not necessarily an accurate measure of the intrinsic efficiency of the photoelectrode within the cell. In order to measure the intrinsic efficiency of the photoelectrode excluding the parasitic losses that arise from other components in the cell, the photoelectrode should be tested in a three-electrode configuration that measures the current between the working photoelectrode and a counter electrode as a function of the photoelectrode potential against a third electrode, that is the reference electrode.3 Indeed, threeelectrode “half-cell” measurements are frequently carried out and many reports present the current density (J), measured in the dark and under illumination, as a function of the © 2014 American Chemical Society

Published: October 2, 2014 3330

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In order to demonstrate this concept, we use an iron oxide (α-Fe2O3) thin film photoanode as a case study. This particular photoanode comprises of a 50 nm thick α-Fe2O3 layer (undoped) that was deposited by spray pyrolysis on an FTOcoated glass substrate using a standard recipe as reported elsewhere.14 Figure 1 shows current density (J) vs potential (U)

than at the multicomponent tandem cell device level. With the lack of the second photoelectrode or photovoltaic cell that would provide the additional voltage to drive the water splitting reaction in the tandem cell, the missing voltage is readily provided by a potentiostat. This is the most convenient way to test the photoelectrodes, and at present, this seems like the most reasonable strategy to expedite progress in this field. However, it carries a toll, like any compromise does. The toll is that the efficiency of the tandem cell cannot be measured without assembling it together in the first place. Therefore, the STH efficiency defined by eq 1 becomes irrelevant when the photoelectrode is measured alone, without the rest of the tandem cell. In an attempt to overcome this shortcoming, the so-called applied bias photon-to-current conversion efficiency has been evoked1 ⎡ |J | × (1.23 − V ) ⎤ photo ⎥ ABPE = ⎢ ⎢⎣ ⎥⎦ P AM1.5G

(2)

where V is the voltage that is applied to the cell from an external power source and Jphoto is the photocurrent measured at this voltage. Although this equation is correct from the thermodynamic perspective, it does not allow quantifying the intrinsic efficiency of the photoelectrode because of the very same reasons that were mentioned above. Furthermore, it is noteworthy that some of the champion photoanodes that have been reported to date reached high photocurrents at potentials above 1.23 VRHE (volts against the reversible hydrogen electrode).11−13 This means that they require a voltage V in excess of 1.23 V in order to achieve high photocurrent, and hence, according to eq 2, the ABPE is negative, implying that there is no net chemical power produced from sunlight. Does it mean that they should not be operated above 1.23 VRHE? So, we see that the conventional efficiency definitions are made for the entire tandem cell, but they are less helpful when it comes to examining the intrinsic efficiency of the photoelectrode alone, without the rest of the tandem cell. Without knowing the intrinsic power characteristics of the water splitting photoelectrode, namely, the photocurrent vs photovoltage relationship, it is difficult to design optimal coupling with the rest of the tandem cell. Here, we show how the intrinsic power characteristics of the photoelectrode can be obtained, very simply, from current vs potential voltammograms measured in three-electrode configuration in the dark and under illumination. From the intrinsic power characteristics we obtain the maximum power point, namely, the maximum photocurrent × photovoltage product wherein the photoelectrode converts solar power to electrical power at its highest internal power conversion efficiency, thereby saving maximum power from the external power source. The product of this internal power conversion efficiency with the electrolysis efficiency of the photoelectrode in converting electrical power to chemical bonds yields the intrinsic solar to chemical conversion efficiency of the photoelectrode. The latter reaches a maximum at some specific current and potential values that define the optimal conditions for achieving maximum internal solar to chemical conversion efficiency by the photoelectrode. These conditions are essential for designing optimal coupling between the photoelectrode and the other components of the tandem cell, especially the second photoelectrode or photovoltaic cell.

Figure 1. Current density (J) vs potential (U) voltammograms measured in 1 M NaOH aqueous solution in the dark (dashed black line) and under AM1.5G solar simulated illumination (full black line), obtained with a 50 nm thick α-Fe2O3 layer deposited on FTO-coated glass substrate. The difference between the light and dark currents, that is, the photocurrent, is shown by the blue dotted curve with the blue open triangles.

voltammograms that were measured in 1 M NaOH aqueous solution in the dark (Jdark, dashed line) and under AM1.5G solar simulated illumination (Jlight, full line). The difference between the light and dark currents is the photocurrent, Jphoto = Jlight − Jdark, shown by the blue curve with the triangles. Central to the construction of the internal power characteristics of the photoanode is the photovoltage, Vphoto. Provided that the internal resistance of the photoanode is small, as can be seen by the steep slope of the dark voltammogram, the photovoltage is simply the potential shift between the light current and the dark current, as shown by the horizontal red arrow in Figure 1. It arises from the photovoltaic effect at the electrified junction between the photoanode and the aqueous electrolyte solution.15 Consequently, the photoanode behaves like an anode connected in series to a photovoltaic cell,16 an internal photovoltaic cell that corresponds to the photovoltaic action of the photoanode. The latter produces a photovoltage that adds up to the applied potential from the potentiostat (U).17 Thus, under illumination, the photoanode is effectively at a surplus potential with respect to the applied potential, Ulight (the subscript light signifies that this is the potential applied to the photoanode under light). The effective potential that drives the water oxidation current is Ueff,light = Ulight + Vphoto, where Ulight is the applied potential and Vphoto is the photovoltage. Because the current is an injective function of the effective potential, J = f(Ueff), under light it reaches the same level as in the dark at a lower applied potential (Ulight) than in the dark (Udark). In other words, Jlight = Jdark = J requires that Ueff,light(J) = Ueff,dark(J) = Udark(J); therefore, we get Vphoto(J) = Udark(J) − Ulight(J). This is illustrated by the horizontal red arrow in Figure 1, whose length is commensurate with the photovoltage at the respective current density (J = 0.2 mA/cm2 in this case). The 3331

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that depart as anodic photocurrent. Having the photovoltaic power of the photoanode spent on splitting water at the photoanode/electrolyte interface, the next question is how efficient the water splitting process is. In other words, what the electrolysis efficiency, ηel, is. This question has a simple answer. It is the efficiency of the photoanode in converting electric power to chemical power. The latter is given by the product of the reaction rate, measured in terms of the Faradaic current of the reaction, times the reversible potential of the reaction, that is 1.23 VRHE for water oxidation, yielding ηel = ηF × 1.23(VRHE)/Ueff(VRHE) wherein Ueff is the effective potential which is the sum of the applied potential from the external power source plus the internal photovoltage of the photoanode. As noted earlier, Ueff(J) = Udark(J) at the same current (J). Although we did not measure the Faradaic efficiency of this particular photoanode, it is safe to assume that it is 100% (or very close to it) based on previous reports on α-Fe2O3 photoanodes.18,19 So now the intrinsic solar to chemical conversion efficiency of the photoanode can be defined as

vertical red arrow shows the photocurrent at the same current density. Thereby, the photocurrent and photovoltage can be extracted from the dark and light voltammograms, as illustrated by the red arrows in Figure 1 for a current of 0.2 mA/cm2. Now that we know what the photovoltage is and how to extract it from the current vs potential voltammograms, we can draw the photocurrent as a function of the photovoltage, as shown in Figure 2a. This curve describes the intrinsic

⎡ J (mA/cm 2) × Vphoto(V) ⎤ photo ⎢ ⎥ ISTC = ηel ⎢⎣ ⎥⎦ P(mW/cm 2) AM1.5G ≅

2 ⎡ ⎤ 1.23(VRHE) ⎢ Jphoto (mA/cm ) × Vphoto(V) ⎥ ⎥⎦ Udark(VRHE) ⎢⎣ 100(mW/cm 2)

AM1.5G

(3)

where Udark is the potential that must be applied to the photoanode in order to reach the respective current in the dark. Equation 3 defines the intrinsic solar to chemical (ISTC) conversion efficiency of the photoanode, not to be mistaken with the solar to hydrogen (STH) conversion efficiency of the entire tandem cell that is defined by eq 1. Using the ISTC definition in eq 3 and the results in Figure 2(a) we can finally obtain the ISTC efficiency of the photoanode as a function of any of the aforementioned parameters, namely the current (J), photocurrent (Jphoto), photovoltage (Vphoto), and applied potential in the dark (Udark) or under light (Ulight). Figure 3 shows the ISTC efficiency of the photoanode as a function of the photocurrent density. Also

Figure 2. Intrinsic photovoltaic power characteristics of the photoanode. (a) The photocurrent as a function of the photovoltage. (b) The intrinsic photovoltaic power as a function of the photovoltage. The secondary y axis on the right of the figures shows the potential (Ulight) that was applied to the photoanode by the potentiostat, under light, as a function of the photovoltage.

photovoltaic power characteristics of the photoanode, and the photocurrent × photovoltage product, shown in Figure 2b, is the light-induced electric power produced, internally, by the photoanode. In other words, this is the intrinsic photovoltaic power of the photoanode. Also shown on the secondary y axis of these figures is the potential, Ulight, that was applied to the photoanode, under light, as a function of the photovoltage. The intrinsic photovoltaic power reaches a maximum of 0.16 mW/ cm2 at a potential (Ulight) of 1.4 VRHE. This corresponds to a power conversion efficiency of 0.16% from solar-simulated light power of 100 mW/cm2 to electric power of 0.16 mW/cm2. The fill factor reaches 43% at the maximum power point. Now there is an important difference between the power produced by the photoanode and that of a conventional solar cell. The latter provides electric power to the load connected to it, where the power is spend on carrying out an electrical work. But the photoanode carries out a different sort of work, a chemical work that is spent on splitting water in order to produce oxygen and protons from water as well as electrons

Figure 3. ISTC efficiency of the photoanode, plotted by the full line curve with respect to the primary y axis on the left, as a function of the photocurrent density on the x-axis. Also shown is the photoanode potential, under light, plotted by the dashed line curve with respect to the secondary y axis on the right. 3332

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shown in the same graph is the applied potential under light (Ulight), plotted by the dashed line curve. The maximum ISTC efficiency reaches 0.11% at a photocurrent density of 0.4 mA/ cm2 and a potential (Ulight) of 1.4 VRHE. The way to understand the ISTC efficiency of the photoanode is as follows. When the photoanode is powered by a power source, any power source for that matter, to its maximum ISTC point, it produces, under solar-simulated light, a photocurrent density of 0.4 mA/cm2 at an applied potential of 1.4 VRHE. In the dark it would have to be biased to a higher potential of 1.8 VRHE in order to produce the same current (see Figure 1). This means that the solar-simulated light power (100 mW/cm2) saves 0.4 V from the external power source, or a power of 0.4(V) × 0.4(mA/cm2) = 0.16 mW/cm2. But the electric power saved from the power source is reduced by the conversion efficiency of the electrolysis reaction, ηel ≅ 1.23(VRHE)/Udark(VRHE), which is 68% at this current level because Udark = 1.8 VRHE. Thus, the light-induced contribution to the chemical power produced by the photoanode is 68% × 0.16 (mW/cm2) = 0.11 mW/cm2, which corresponds to an ISTC efficiency of 0.11%. Figure 3 shows that this is the maximum ISTC efficiency of the photoanode. Drawing either more, or less, current from the photoanode would necessarily reduce the ISTC efficiency. Therefore, the maximum ISTC point defines the optimal conditions for operating the photoanode. It is noted, again, that the ISTC efficiency of the photoanode (defined by eq 3) is not the STH efficiency of the tandem cell (defined by eq 1). Nevertheless, there is a link between the two of them. Suppose the photoanode is coupled in tandem with a photovoltaic cell that provides just the right voltage and current to produce a photovoltage Vphoto and photocurrent Jphoto by the photoanode, and suppose there are no additional losses by the other components in the tandem cell, including the counter electrode, that is, the cathode where hydrogen evolves. Then the ISTC efficiency of the photoanode plus the ISTC efficiency of the photovoltaic cell, calculated using eq 3 with Vphoto of the photoanode replaced by Vphoto of the photovoltaic cell (i.e., Vphoto = Ulight in this case), predicts what the STH efficiency of this tandem cell would be under these conditions. Hence, the ratio between the ISTC of the photoanode and the STH of the tandem cell, or interchangeably the ratio between the photovoltage produced by the photoanode and the total voltage of all the power sources in the tandem cell (including the photoanode itself), corresponds to the partial contribution of the photoanode to the chemical power produced by the tandem cell. Applying the same analysis to the mesoscopic α-Fe2O3 photoanode reported by Warren et al.,13 Figure 4 shows the ISTC efficiency of this photoanode as a function of the photocurrent that it draws. It reaches a maximum value of 0.94% at a photocurrent of 3.4 mA/cm2, which requires applying a potential of 1.4 VRHE from the potentiostat, under light. This means that this photoanode would be best coupled to a power source that sets the photoanode potential to 1.4 VRHE, yielding a current of 3.4 mA/cm2 generated by the photoanode under AM1.5G sunlight. The chemical power produced under these conditions is 3.4(mA/cm2) × 1.23(V) = 4.2 mW/cm2, out of which 0.94 mW/cm2, that is 22%, is produced by internal solar power conversion at the photoanode whereas the rest comes from the power source (which could be a photovoltaic cell in tandem with the photoanode).

Figure 4. ISTC efficiency of the mesoscopic α-Fe2O3 photoanode reported by Warren et al. in ref 13, plotted by the full line curve with respect to the primary y axis on the left, as a function of the photocurrent density on the x axis. Also shown is the photoanode potential, under light, plotted by the dashed line curve with respect to the secondary y axis on the right.

To sum up, we have demonstrated how to extract the internal photovoltaic power characteristics and the intrinsic solar to chemical conversion efficiency of iron oxide (hematite) photoanodes from current vs potential voltammograms measured in the dark and under illumination using the ubiquitous three-electrode “half-cell” test configuration. This analysis yields the current that can be drawn from the photoanode and the potential that should be applied to it in order to make it convert solar power to chemical power at its maximum internal conversion efficiency. These parameters are essential for optimization of tandem cells for conversion of solar power to hydrogen fuel. The analysis is self-consistent and simple to pursue, and it should apply not only to iron oxide photoanodes but also to other types of photoanodes−as long as their internal resistance is small. The extension to high internal resistances will be discussed in a follow-up paper.

Hen Dotan† Nripan Mathews‡,§ Takashi Hisatomi‡,¶ Michael Graẗ zel‡ Avner Rothschild*,† †

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Department of Materials Science and Engineering, TechnionIsrael Institute of Technology, 32000 Haifa, Israel ‡ Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland

AUTHOR INFORMATION

Corresponding Author

*A. Rothschild. E-mail: [email protected]. Tel.: +972-4829-4576. Fax: +972-4-829-5677. Present Addresses

§ (N.M.) School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798. ¶ (T.H.) Department of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Notes

Views expressed in this Commentary are those of the author and not necessarily the views of the ACS. 3333

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The authors declare no competing financial interests.

(18) Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem 2011, 4, 432−449. (19) Iandolo, B.; Wickman, B.; Seger, B.; Chorkendorff, I.; Zoric, I.; Hellman, A. Faradaic Efficiency of O2 Evolution on Metal Nanoparticle Sensitized Hematite Photoanodes. Phys. Chem. Chem. Phys. 2014, 16, 1271−1275.

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ACKNOWLEDGMENTS The authors acknowledge support of this research by the European Commission (Nanostructured Photoelectrodes for Energy Conversion, NanoPEC, contract number 227179), and by Europe’s Fuel Cell and Hydrogen Joint Undertaking (FCH JU) under Grant Agreement No. 621252 (PECDEMO Photoelectrochemical Demonstration Device for Solar Hydrogen Generation).

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REFERENCES

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