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

Abstract

Monolithic solar water splitting devices consist of photovoltaic materials integrated with electrocatalysts and produce solar hydrogen by water splitting upon solar illumination in one device. Upscaling of monolithic solar water splitting devices is obstructed by high ohmic losses in the electrolyte due to long ionic transport distances. A new design overcomes the problem by introducing micron sized pores in a silicon wafer substrate coated with electrocatalysts. A porous solar hydrogen device was simulated by applying a current corresponding to ca. 10% solar-to-hydrogen efficiency. Porous monoliths of 550 μm thickness with varying pore size and spacing were fabricated by laser ablation and electrochemically characterized. Ohmic losses well below 100 mV were reached at 14.4% porosity with 77 μm pores spaced 250 μm apart in 0.25 M KOH electrolyte. In 1 M KOH, 100 mV was reached at 6% porosity with 1 mm pore spacing. Our results suggest ohmic losses below 50 mV can be achieved when using 10 μm thick substrates at 0.2% porosity. These findings make it possible for monolithic solar water splitting devices to be scaled without loss of efficiency. ■ INTRODUCTIONHydrogen is one of the promising energy vectors that will likely be part of the future sustainable energy portfolio. 1 Using sunlight to split water into hydrogen and oxygen is a viable option for solar hydrogen production and several technologies exist that achieve water splitting at high efficiency. 2 A direct way to produce solar hydrogen is by using a solar water splitting device which combines light absorption, charge separation and electrochemical reactions in an integrated device. 3 High solarto-hydrogen (STH) efficiencies are reached using high-end photovoltaics (PV) coupled to electrocatalysts submerged in concentrated aqueous electrolyte. 4−7 The use of robust earthabundant catalysts and stable light absorbing materials with suitable band gaps has also been demonstrated. 8−11 These approaches show great promise at lab scale and development of practical systems is ongoing. 12−16 Beside the performance of catalysts and light absorbers, cell design is important and ohmic losses need to be minimized to maximize overall efficiency. 17,18 There are two common design types. 19,20 In the "wired" design ( Figure 1A) planar anode and cathode have opposed surfaces at close proximity such that the interelectrode distance to be covered by ions in the liquid electrolyte is minimized. 9−11 The "wireless" or "monolithic" layout ( Figure 1B) simplifies cell design by eliminating electrical contacts and wires through integration of all components in a monolithic flat assembly. 10

A Stand‐Alone Si‐Based Porous Photoelectrochemical Cell

“…By introducing micropores into a Si triple cell, all three losses have been suppressed and kept within the limit of 250 mV. Together with the research of Bosserez et al and our performed simulation on pH gradient formation, we conclude that a micropore pitch <250 µm is required, and an overall porosity of ≈7% ensures low ionic losses of <100 mV in acidic electrolyte conditions . Therefore, our final device was constructed of micropores with a diameter of 50 µm and a pitch of 166 µm.…”

“…Bosserez et al investigated a porous monolith for reducing the ionic pathway of (not photoactive) silicon electrodes with micron‐sized pores, with the aim to provide ionic shortcuts and gain more insight into the potential losses due to the electrolyte. A loss of less than 100 mV was found at 7.84 mA cm −2 in 1 m KOH at a porosity of ≈7% and a micropore pitch of 250 µm . However, two problems evolved for this conceptual device.…”

Membrane-less photoelectrochemical cells: product separation by hydrodynamic control

“…The placement of the membrane within the cell is further constrained by the compromise between reducing the average path length of ions between electrodes, minimising current density distribution and not block-ing light from being absorbed. There have been a number of novel examples in order to get round these issues such as a louvereddesign 13 or perforated photo electrode 14,15 . Another typical compromise is in the thickness of the membrane, where the energy loss due to ionic conduction is balanced against the gas crossover 16 .…”

Introductory lecture: sunlight-driven water splitting and carbon dioxide reduction by heterogeneous semiconductor systems as key processes in artificial photosynthesis

“…Schematic of an electrolysis cell using matched projected portions consisting of triple-junction amorphous Si (tj-a-Si) and an associated membrane electrode assembly. It has also been reported by Bosserez et al that flat PV-electrocatalyst assemblies are not scalable because of the long ion transport distances that are associated with scaling up 35. .…”