Mass transport aspects of electrochemical solar-hydrogen generation

Abstract

The conception of practical solar-hydrogen generators requires the implementation of engineering design principles that allow photo-electrochemical material systems to operate efficiently, continuously and stably over their lifetime. At the heart of these engineering aspects lie the mass transport of reactants, intermediates and products throughout the device. This review comprehensively covers these aspects and ties together all of the processes required for the efficient production of pure streams of solar-hydrogen. In order to do so, the article describes the fundamental physical processes that occur at different locations of a generalized device topology and presents the state-of-the-art advances in materials and engineering approaches to mitigate masstransport challenges. Processes that take place in the light absorber and electrocatalyst components are only briefly described, while the main focus is given to mass transport processes in the boundary-layer and bulk liquid or solid electrolytes. Lastly, a perspective on how engineering approaches can enable more efficient solar-fuel generators is presented. Broader contextSolar-hydrogen generators have the potential to trigger the incorporation of a much larger share of solar energy sources in our current energy landscape. These technologies can directly capture and store energy from the sun in the form of hydrogen molecules. As hydrogen can be transported, and stored for long periods of time, solar-hydrogen devices allow for the spacio-temporal decoupling of the energy generation and consumption processes, and in this way provide a larger flexibility to the electrical grid. While significant focus has been given to the development of photoelectrochemical materials for solar water-splitting, equally important are engineering aspects related to the materials' integration into devices that can operate stably, and produce pure hydrogen streams continuously over long lifetimes. This review provides a broad view on these engineering challenges and approaches to conceive practical solar-hydrogen generators.

Mitigating voltage losses in photoelectrochemical cell scale-up

“…21 The signicant loss of efficiency has been attributed not to the scale-up of the light absorber material (i.e., BiVO 4 ) itself, but more to the ohmic losses and mass-transport (proton/hydroxyl ions) limitations. Several modeling studies have also investigated some of these aspects (substrate losses, ionic drop, mass transfer), [24][25][26][27][28] but the overall quantication of the various loss mechanisms related to scale-up has not been reported. This is important in order to propose and implement appropriate (photo)electrochemical engineering and design strategies in order to overcome the scale-up related losses.…”

Membrane-less photoelectrochemical cells: product separation by hydrodynamic control

“…Product separation is most commonly achieved using an ion permeable membrane between electrodes. However this has significant disadvantages such as appreciable ohmic losses 9 (particularly in alkaline systems, which have a low conductivity 10 ) and significant costs 11,12 . 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.…”

“…Mass-transfer control through manipulation of the hydrodynamics might be more facile for PEC devices than electrolysers due to typical operational current densities being an order or multiple orders of magnitude lower 9 . This is due to the fact that the current density in a PEC device is proportional to the power intensity of light and this is limited by the solar resource.…”

Perspectives on the photoelectrochemical storage of solar energy

“…When scaling up to sizes beyond 10 cm 2 , mass and ion transport limitations in the solution phase start to severely affect the efficiencies. 44 Optimal membrane size and positioning can be used to minimize diffusion distances while still maintaining good H 2 and O 2 separation, but they introduce additional system complexity and costs. Moreover, one would need to ensure that the membrane does not block the incoming light.…”