High-Efficiency Photoelectrochemical Hydrogen Production Using Multijunction Amorphous Silicon Photoelectrodes

Abstract

Photoelectrochemical solar-to-hydrogen conversion efficiencies as high as 7.8% (based on the lower heating value of hydrogen) have been demonstrated in outdoor testing using a photocathode fabricated from triple junction amorphous silicon-solar cells and a separate catalytic anode. The tests were conducted in a specially designed Teflon-sealed reactor in 1 N KOH with a photoactive area of 0.27 cm2 and anode and cathode areas of 1 cm2. The hydrogen production rates, inferred from direct measurement of the anodic/cathodic currents and confirmed by independent volumetric and gas chromatographic measurements of the evolved hydrogen, were in excellent agreement with the rates expected from the measured solid-state JV behavior of the solar cell and the overpotentials of the thin-film catalysts. The thin-film catalysts, CoMo hydrogen catalysts deposited by sputtering from a compound target and NiFe y O x oxygen catalysts deposited from nickel−iron Permalloy target by reactive sputtering, have, in separate tests, shown no degradation after over 7200 h of operation in 1 N KOH electrolyte. During outdoor testing, the solar-to-hydrogen conversion efficiency decreased in the late afternoon as the blue portion of the spectrum decreased, a result of the spectral sensitivity of the solar cell used to construct the photoelectrode. Detailed modeling of the multijunction amorphous silicon cells is being conducted to identify structures that are better load-matched to the catalyst performance and that could yield higher hydrogen production efficiencies. Future work to advance this technology includes development of improved thin-film catalysts and development of transparent protective coatings that will allow complete immersion of the active electrode into the electrolyte.

An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems

“…achieved. 12,13,15,16 In this case, improving the catalyst performance, increasing the catalyst loading, or decreasing the solution resistances will not improve the STH efficiency further.…”

“…14,15,[18][19][20] The operating photovoltage produced by the light absorbers should exceed the sum of the thermodynamically required potential difference, the resistance losses of the electrolytes, and any overpotentials that are required to drive water splitting at a given current density. When the photovoltage constraint is met, the operating photocurrent density in an integrated PEC system therefore determines the STH efficiency.…”

Minimization of Ionic Transport Resistance in Porous Monoliths for Application in Integrated Solar Water Splitting Devices

“…19,21 Ohmic losses are often minimized by using concentrated acid or alkaline electrolytes such as H 2 SO 4 , HClO 4 , or KOH at concentrations of 0.5 M or higher. 4,5,14,22,23 In wireless systems with flat monoliths consisting of dense PV and electrocatalysts mounted on either side, ions need to travel around the dense monolith to cover the distance between anode and cathode. Already at the centimeter scale unacceptable ohmic losses of hundreds of millivolts and more are to be expected.…”

Interplay of oxygen-evolution kinetics and photovoltaic power curves on the construction of artificial leaves

“…Direct solarto-fuels conversion can be achieved by interfacing suitable catalysts that carry out two separate half-reactions of water splittingthe four electron-four proton oxidation of water to O 2 and the two electron two-proton reduction of the produced protons to H 2 -to a photovoltaic (PV) material. Numerous device configurations have been proposed for photoelectrochemical (PEC) water splitting (4)(5)(6)(7)(8)(9)(10), and they can be broadly categorized into those devices wherein the photovoltaic material makes a rectifying junction with solution as opposed to those in which the rectifying junctions are protected from solution or "buried." Fig.…”