Spatial decoupling of light absorption and catalytic activity of Ni–Mo-loaded high-aspect-ratio silicon microwire photocathodes

Abstract

Substrates with arrays of silicon microwires (4 µ m diameter, 40 µ m length and 6 µ m pitch) with radial junctions (p-type base and 900 nm A solar-driven photoelectrochemical cell provides a promising approach to enable the large-scale conversion and storage of solar energy, but requires the use of Earth-abundant materials. Earth-abundant catalysts for the hydrogen evolution reaction, for example nickel-molybdenum (Ni-Mo), are generally opaque and require high mass loading to obtain high catalytic activity, which in turn leads to parasitic light absorption for the underlying photoabsorber (for example silicon), thus limiting production of hydrogen. Here, we show the fabrication of a highly efficient photocathode by spatially and functionally decoupling light absorption and catalytic activity. Varying the fraction of catalyst coverage over the microwires, and the pitch between the microwires, makes it possible to deconvolute the contributions of catalytic activity and light absorption to the overall device performance. This approach provided a silicon microwire photocathode that exhibited a near-ideal short-circuit photocurrent density of 35.5 mA cm −2 , a photovoltage of 495 mV and a fill factor of 62% under AM 1.5G illumination, resulting in an ideal regenerative cell efficiency of 10.8%.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar‐Driven Water Splitting

“…Another approach to reducing parasitic light absorption of electrocatalysts is to minimize the geometric footprint of the opaque electrocatalysts on the light‐absorbing surfaces of semiconductors . Constructing three‐dimensional (3D) electrocatalysts on the surface of semiconductor photoelectrodes can relax the constraint on the catalyst loading against catalyst footprint, and therefore decouple the light absorption of semiconductor and surface charge transfer.…”

“…d) Contour plot of the η IRC as a function of pitch and wire coverage for all tested combinations. Reproduced with permission . Copyright 2018, Springer Nature Publishing AG.…”

“…They used the ratio of the J sc of p‐Si/Co‐P to that of bare p‐Si to evaluate the parasitic light absorption, and concluded that the parasitic light absorption of Co‐P on p‐Si‐µW surfaces was reduced by a factor of 2 relative to that on planar Si surfaces . Lately, Huskens et al developed a method that allows for spatioselectively depositing Ni‐Mo HER catalysts on vertically aligned Si‐µW arrays, which enabled effective decoupling between the catalytic activity and light absorption of photocathode . They found that the Si‐µW with their upper 2 µm loaded with Ni‐Mo, Figure 1c has PEC performance superior to the Si‐µW with the entire length loaded with Ni‐Mo.…”

Electrochemical coating of micro-structured silicon for photoelectrochemical water splitting

“…[50] Vijselaar et al found that photovoltaic current densities of silicon microwires with a radial p/n junction increase with increasing microwire spacing. [45] Our results show predominantly the same trend ( Figure 6.9) which indicates that underperformance of devices with a low microwire spacing might be due to limitations in light absorption. This is supported by the study of Chakthranont et al who published a similar concept as discussed in this chapter, after completion of our study, and obtained higher photocurrents in sulfite oxidation on microwire-structured samples than on flat equivalents.…”

“…[45] Their results strengthen our hypothesis of diffusion limitation of redox species in the internal volume of the microwire array, since in the cited study light trapping properties of the microwire structure were utilized without suffering from mass transfer inside the microwire array because of the placement of the catalyst at the top only.…”