Photoelectrochemical System Studies

7/21/2019 Photoelectrochemical System Studies

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Photoelectrochemical System Studies

A. Bansal, O. Khaselev and J. A. TurnerNational Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401

ABSTRACT

The efficiency for any solar-to-hydrogen conversion process is particularly important because anarea of land must be covered to collect the sunlight. Any efficiency losses in the system must bemade up by an increase in the collector area. This will ultimately have a significant impact onthe overall cost and siting ability of such a system. Section one of this report will explore theefficiency gains from a direct conversion system as compared to a separated PV/electrolysissystem. Section two will discuss our results looking at the catalysis of hydrogen production onsemiconductor electrodes.

EFFICIENCY OF PEC SYSTEMS

The energy efficiency of electrolysis can generally be defined as the chemical potential ofhydrogen divided by the voltage required for electrolysis operation [1]. At the normal operatingcurrent density of commercial electrolyzers (~1 A/cm

2), this voltage ranges from 1.82.0 volts.

Thus, an electrolyzer operating at 1.9 volts operates with an energy efficiency of 65% (1.23/1.9).Combining this electrolysis efficiency with a 12% PV array efficiency gives a solar-to-hydrogenefficiency of 7.8%. To increase the overall solar-to-hydrogen efficiency, the energy efficiency ofelectrolysis must be increased; thus, one must operate at a lower voltage. This requires bettercatalysts (particularly on the oxygen side), or a decrease in the current density (which results ina decrease in the rate of hydrogen production). Maintaining the rate of hydrogen productionwhile reducing the current density could be done by increasing the area of the electrodes (oradding more electrolyzers), but costs would increase due to an increase in the amount ofmaterial used for the electrolyzer.

A photoelectrochemical (PEC) system combines the harvesting of solar energy with theelectrolysis of water [2]. Depending on the type of semiconductor material and the solarintensity, the current density for electrolysis with a PEC system is 10-30 mA/cm2. Figure 1presents a current-voltage curve for water splitting in an electrochemical cell consisting of two Ptelectrodes of identical area. At a current density similar to short-circuit photocurrent from a solarcell, hydrogen and oxygen generation is achieved at an effective applied voltage ofapproximately 1.35 V, giving rise to an electrolysis efficiency of 91% [3]. One of the major

advantages then of a direct conversion photoelectrochemical system is that the area availablefor electrolysis approximates that of the solar cell. A PEC hydrogen generation system not onlyeliminates most of the costs of the electrolyzer, but it also has the possibility of increasing theoverall efficiency of the process leading to a further decrease in costs. Note, although thisexperiment was done with Pt electrodes, at this current density, less expensive nickel and metaloxide electrodes could be used with similar results.

Proceedings of the 2000 DOE Hydrogen Program Review

NREL/CP-570-28890


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We have previously shown a high solar-to-hydrogen PEC conversion device [4], but problems ofsemiconductor stability, and interfacial kinetics, make it difficult to quantify the efficiency gainsfrom such a PEC system. For this work, we used an integrated PV/electrolysis design toemulate a PEC system. Unlike a direct-conversion PEC system, an integrated PV/electrolysissystem has separated PV and electrolyzer parts. However, for an integrated system, the area ofthe electrolyzer system is identical to the area of the PV collector. These integrated systems

allow us to study the issues involving the effects of current density, solar intensity variability, andlight concentration on the overall efficiency for hydrogen production. In this study, we utilized anintegrated PV/electrolysis design based on a low-cost amorphous silicon (a-Si). multijunctiondevice. This cell is capable of generating a voltage sufficient for water splitting, and we note thatits lower system cost, combined with the good efficiency, represents an important area ofresearch for integrated hydrogen production systems. It is demonstrated that this monolithicPV/electrolysis configuration can lead to a higher solar-to-hydrogen conversion efficiency thancoupled systems involving electrolyzers and photovoltaic solar cells alone.

For an integrated PV/electrolysis configuration to work properly, the solar cell must providesufficient voltage to drive the water redox reactions, and also provide any additional voltageneeded to overcome the overvoltage losses resulting from the hydrogen and oxygen evolution

reactions. The total output photovoltage must include the thermodynamics for water splitting(1.23 volts at 25C), polarization losses aand c, for the anodic and cathodic processesrespectively, and the IR potential drop in the bulk of electrolyte, which can be significant whengas evolution occurs. A multijunction integrated PV/electrolysis configuration comprises either atandem (or triple) pn-pn/metal/electrolyte cell, with water reduction occurring at the back-contact/electrolyte interface, and oxidation occurring at the electrolyte/counter electrode (anode)interface, or alternately, a tandem np-np/metal/electrolyte cell with water oxidation occurring atthe back-contact/electrolyte interface and reduction occurring at the electrolyte/counterelectrode (cathode) interface. Figure 2 shows the power curve for the triple junction p-i-n a-Sisolar cell under 100 mW/cm

2 illumination, clearly indicating its ability to provide sufficient

voltage for water splitting. Figure 3 shows the bias-voltage dependence of the photocurrent(hydrogen production) for a triple-junction a-Si(Pt)/2M KOH/Pt cell. The bias-voltage

characteristics are consistent with that expected from the current-voltage characteristics shownin Figures 1 and 2. For the triple-junction a-Si(Pt)/2MKOH/Pt cells, the light-limited current was

reached approximately at zero bias. Operated at zero bias, with shorted electrodes, the cellgenerated hydrogen and oxygen.

In operation, an ideal integrated PV/electrolysis system would operate under short-circuitconditions. For such configuration, the efficiency for hydrogen production can be calculatedusing the equation: efficiency = (power out)/(power in). The input power is the incident lightintensity of 100 mW/cm2. For the output power, assuming 100% photocurrent electrolysisefficiency, the hydrogen production photocurrent is multiplied by 1.23 volts, the ideal fuel cell

limit at 25 C (lower heating value [LHV] of hydrogen). Using this calculatation, our experimentalresults for the hydrogen production efficiency for the triple junction a-Si(Pt)/2M KOH/Pt it is:

1.23V6.4mA/cm2/100mW/cm

2=7.8%.

The value for the a-Si system is significant in that this efficiency is realized by a PV cell with onlya 9% solar-to-electrical efficiency. Because of the lower current density, this system isperforming water electrolysis with an equivalent efficiency of 86% (effective electrolysis voltageof 1.42 volts [LHV]). This means that the maximum operating voltage of the a-Si PV cell is closeto that required for electrolysis.


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To maximize solar-to-hydrogen efficiency of the PV/electrolysis system operating at short circuitcondition, a multijunction photovoltaic cell should be specifically designed to match electricalparameters for water splitting. For any direct solar-to-hydrogen conversion system, themaximum voltage that can be used in an efficiency calculation is fixed by the chemical potentialof hydrogen. In order to increase the conversion efficiency, the current (and therefore the rate ofproduction of hydrogen) must be increased. The ideal system would reach maximum power at

less than 1.6 V, and a current density more than 20 mA/cm2. Although the solar-to-electricalconversion efficiency of such cell would be less, the solar-to-hydrogen efficiency of theintegrated PV/electrolysis system (and an analogous PEC system) would benefit.

CATALYSIS FOR THE PHOTOELECTROCHEMICAL PRODUCTION OF HYDROGEN

For a practical, stable and efficient semiconductor based water splitting system, four criteriamust be simultaneously satisfied. The bandgap of the semiconductor must be greater than 1.7eV, the band edges of the semiconductor must overlap the H2/H2O and O2/H2O redox potentialsunder H2/O2 evolution condition and charge transfer across the semiconductor-liquid interfaceshould be fast (Fig. 4). In addition, the semiconductor surface should be chemically stable in

the aqueous media.

Although p-type GaInP2with a bandgap of 1.8-1.9 eV has been identified as a promisingphotocathode for water splitting, its band edges are 0.2-0.4 V too negative to effectphotoelectrolysis.[5-7] Electrochemical investigations of GaInP2 in aqueous media have foundthat the semiconductor surface is unstable and susceptible to corrosion [8]. Furthermore, it isnot known if the charge transfer rate from p-GaInP2 to water under illumination is sufficientlyhigh or if this rate can be further catalyzed.

At present, there is no information on the energetic position of the band edges or on the kineticsof charge transfer at the p-GaInP2/water interface under illumination i.e. under conditions of H2evolution. Several impedance measurements in the literature have previously reported that

band edges of semiconductors are not fixed and are prone to migration under illumination [9-19]. These changes in flatband potential under illumination have been interpreted to indicateevidence of surface effects such as charge trapping, adsorption or changes in the surfacechemistry. The band edge positions are also known to be strongly affected by interfacialkinetics under illumination [13,15]. Increased band edge migration as a result of illuminationhas been observed for interfaces with poor interfacial charge transfer ability and this migrationhas been suppressed by treating the semiconductor surfaces with transition metal catalysts[14,20].

In this report, we describe our work on evaluating the effect of interfacial kinetics on the bandedge positions of the p-GaInP2/water interface under illumination i.e. under conditions of H2evolution. In addition, we describe the results of C-V and I-V measurements on p-GaInP2

surfaces modified with transition metal ions to evaluate their effectiveness as reagents forinducing band edge shifts in the dark and as electrocatalysts for suppressing band edgemigration under illumination. In this study, the flow of photogenerated cathodic current at thesemiconductor surface under illumination corresponds to generation of hydrogen fromphotoelectrolysis of water.

Note: A semiconductor surface treatment such as the adsorption of a transition metal ion, canhave two distinct effects on the energetics of the interface. Due to change of net charge in theHelmholtz layer, adsorption of a transition metal as a charged species (ion) on the


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semiconductor surface can change the band edge position relative to the band edge position foran underivatized surface - in the dark. This effect is referred to as band edge "shift" or flatband"shift" in this paper. On the other hand, the measured band edge position vs. a solutionreference can also move as a result of illumination due to accumulation of photogeneratedcharges at the interface. This movement is not due to a change in the charge in the Helmholtzregion. Its extent is determined by the level of illumination at the semiconductor surface and the

interfacial charge transfer kinetics. This movement can be suppressed if the transition metalspecies acts as an electrocatalyst and catalyses charge transfer across the interface, therebyreducing the accumulation of charge at the interface. In this report, the movement of bandedges due to photoelectron accumulation is referred to as "migration". In measuring the"migration" effect, the band edge position of a surface under a given level of illumination iscompared with the band edge position for the same surface in the dark. Although both of theseeffects have been individually described in the literature as "shifts" of band edges, [9-19] wehave used the terms "shift" and "migration" in this paper to distinguish between the two.

Impedance Data Analysis - A detailed impedance analysis of the p-GaInP2/water system in thedark has been previously reported, in which capacitance data in the frequency range 500 < f 1.7 eV

Counter

Electrode

H2O/H 2

H2O/O 2

1.23 eV

a

c


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Figure 5. Mott-Schottky plots of p-GaInP2 electrodes immersed in pH 1, 7 and 13, measured in the dark

and under varying levels of illumination.

0

1 1016

2 1016

3 1016

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

pH 7

0.11 mA/cm 2

0.22 mA/cm 2

0.56 mA/cm 2

1.12 mA/cm 2

Dark

0

5 1014

1 10

15

1.5 1015

2 1015

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2

pH 13

0.07 mA/cm 2

0.17 mA/cm 2

0.35 mA/cm 2

Dark

pH 10

2 1015

4 1015

6 1015

8 1015

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

Applied Potential, V vs. SCE

1/C2(1/F2)

0.07 mA/cm 2

0.13 mA/cm 2

0.33 mA/cm 2

0.66 mA/cm 2

Dark


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Figure 6. a) Flatband potentials of etched p-GaInP2 electrodes, measured vs. SCE, at five different pHvalues for varying levels of illumination. b) Migration of flatband potentials vs. the Vfb values observedin the dark (Vfb, light - Vfb, dark) in solutions of 5 different pH

CurrentDensity,mA/c

m2

Flatband Potential, V vs. SCE

(a)

Flatband Potential Shift, V

CurrentDensity

,mA/cm2

(b)

pH 1

pH 4

pH 7

pH 10

pH 13Dark

Dark

pH 1

pH 4

pH 7

pH 10

pH 13

0.01

0.1

1

10

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6

0.01

0.1

1

10

-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0


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Figure 7. Mott-Schottky plots for a) an untreated and b) a RuIII-treated p-GaInP2 surface, measured inthe dark and under illumination in 3 M KOH solution.

Applied Potential. V vs. SCE

1/C2(1/F2)

0

2 10 15

4 10 15

6 10 15

8 10 15

1 10 16

1.2 10 16

0

5 10 14

1 10 15

1.5 10 15

2 1015

2.5 10 15

-1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2

0.05 mA/cm2

0.10 mA/cm2

0.24 mA/cm2

0.48 mA/cm2

Dark

(a)

(b)

0.08 mA/cm2

0.16 mA/cm2

0.39 mA/cm2

Dark

0.78 mA/cm2

Untreated

Ru IIItreated


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Figure 8. a) Flatband potentials of untreated (etched only) and metal-ion treated p-GaInP2 electrodesunder illumination in the 3 M KOH solution. b) Migration of flatband potentials vs. the Vfb valuesobserved in the dark (Vfb, light - Vfb, dark) in 3 M KOH solution.

(a)

Dark

0.01

0. 1

1

10

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

0.01

0. 1

1

10

-0.5 -0.4 -0.3 -0.2 -0.1 0

Flatband Potential, V vs. SCE

Flatband Potential Migration, V

CurrentD

ensity,mA/cm

2

CurrentDensity,m

A/cm

2

Dark

(b)

Untreated

OsIIItreated

CoIIItreated

RhIIItreated

RuIIItreated

Pt treated

Untreated

OsIIItreated

CoIIItreated

RhIIItreated

RuIIItreated

Pt treated


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Figure 9. Current-Voltage (I-V) curves observed for untreated and metal-ion treated p-GaInP2 electrodes in3 M KOH solution.

Untreated

Osmium treated

Untreated

Ruthenium treated

Untreated

Cobalt treated

Untreated

Rhodium treated

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

urren

ens

y,m

cm

Applied Potential, V vs. SCE

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Photoelectrochemical System Studies