Functional integration of Ni–Mo electrocatalysts with Si microwire array photocathodes to simultaneously achieve high fill factors and light-limited photocurrent densities for solar-driven hydrogen evolution

Abstract

An n + p-Si microwire array coupled with a two-layer catalyst film consisting of Ni-Mo nanopowder and TiO 2 light-scattering nanoparticles has been used to simultaneously achieve high fill factors and lightlimited photocurrent densities from photocathodes that produce H 2 (g) directly from sunlight and water.The TiO 2 layer scattered light back into the Si microwire array, while optically obscuring the underlying Ni-Mo catalyst film. In turn, the Ni-Mo film had a mass loading sufficient to produce high catalytic activity, on a geometric area basis, for the hydrogen-evolution reaction. The best-performing microwire array devices prepared in this work exhibited short-circuit photocurrent densities of À14.3 mA cm À2 , photovoltages of 420 mV, and a fill factor of 0.48 under 1 Sun of simulated solar illumination, whereas the equivalent planar Ni-Mo-coated Si device, without TiO 2 scatterers, exhibited negligible photocurrent due to complete light blocking by the Ni-Mo catalyst layer. Broader contextSolar-driven photoelectrochemical water splitting is a promising approach to enable the large-scale conversion and storage of solar energy. Few integrated systems have been realized that use earth-abundant semiconductor and catalyst materials for the half-reactions involved in solar-driven water-splitting, i.e. hydrogen evolution and oxygen evolution, while also achieving high energy-conversion efficiencies. We describe herein a hydrogen-evolving Si-based photoelectrode that exhibits high light-limited photocurrent densities, as well as high catalytic activities, while using a high mass loading of an earth-abundant electrocatalyst. The design is reminiscent of a membrane-electrode assembly as used for stand-alone fuel cell and electrolysis systems. The approach exploits the high aspect ratio of the absorber layer to avoid parasitic optical absorption normally associated with a thick catalyst layer on the surface of an illuminated photocathode.

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

“…High FF values in Si microwire photocathodes have been obtained by covering the bottom area between the microwires with Ni-Mo/TiO 2 (ref. 2 ) or cobalt phosphide nanoparticles 2,3 . The bestperforming Si homojunction microwire array photocathodes from these studies demonstrated an ideal regenerative cell efficiency, η irc , of 2.9 and 2.8% respectively [2][3][4] .…”

“…2 ) or cobalt phosphide nanoparticles 2,3 . The bestperforming Si homojunction microwire array photocathodes from these studies demonstrated an ideal regenerative cell efficiency, η irc , of 2.9 and 2.8% respectively [2][3][4] . These efficiencies, obtained in designs without surface passivation, were restricted by light absorption, evidenced by a limited photocurrent (< 15 mA cm −2 ), whereas passivated Si PV cells easily generate about 40 mA cm −2 (refs 5,6 ).…”

Mass transport aspects of electrochemical solar-hydrogen generation

“…Apart from these notable examples, most of the demonstrated solar-hydrogen devices place electrodes vertically in an electrolyte and do not investigate the limitations that bubble transport imposes on these systems. [54][55][56][57][58][59][60][61][62][63][64] In these cases, the devices rely on the upward movement of bubbles due to buoyant forces towards collection sites. In a deployable integrated solar-fuel generator, this configuration will pose significant challenges, as the photoabsorber will need to be placed normal to solar irradiation direction.…”

“…To mitigate these limitations, photoelectrodes can be designed in the microscale (e.g. microwires embedded into ion-conducting membranes 54,196,197 ) so that the lateral migration of ionic species is mainly dictated by distance between the reaction sites at opposite ends of the microstructures. The generalized discussion presented above summarizes some of the design principles for three prominent architectures of solarhydrogen generators.…”

Bright ways to utilize the sun

“…71 In the past decade, significant progress has been made in the exploration of new earth-abundant, inexpensive, and nontoxic catalysts for HER, including molybdenum sulfide, 116,117 firstrow transition metal dichalcogenides, 118,119 molybdenum carbide and boride, 120,121 tungsten carbide, 122 nickel phosphide (Ni2P), 123 cobalt phosphide (CoP), [124][125][126] , and nickel molybdenum (NiMo). 106,115,[127][128][129][130] The edge sites of molybdenum sulfide (MoSx) are analogous to the active center of the nitrogenase enzyme, while some surface cations of the first-row transition metal dichalcogenides resemble the ligand number and symmetry of active centers in hydrogenases (NiFe-hydrogenase, FeFe-hydrogenase, or Fehydrogenase). 116,131 The electronic structures of group VI transition metal carbides are similar to those of Pt-group metals.…”