Is Photocatalysis the Next Technology to Produce Green Hydrogen to Enable the Net Zero Emissions Goal?

Abstract

Today, humankind is confronted by a series of global challenges: energy/environmental crises, population growth, pandemics, and geopolitical wars, that simultaneously promote the depletion of natural resources and accelerate the contamination Energy security concerns require novel greener and more sustainable processes, and Paris Agreement goals have put in motion several measures aligned with the 2050 roadmap strategies and net zero emission goals. Renewable energies are a promising alternative to existing infrastructures, with solar energy one of the most appealing due to its use of the overabundant natural source of energy. Photocatalysis as a simple heterogeneous surface catalytic reaction is well placed to enter the realm of scaling up processes for wide scale implementation. Inspired by natural photosynthesis, artificial water splitting's beauty lies in its simplicity, requiring only light, a catalyst, and water. The bottlenecks to producing a high volume of hydrogen are several: Reactors with efficient photonic/mass/heat profiles, multifunctional efficient solar-driven catalysts, and proliferation of pilot devices. Three case studies, developed in Japan, Spain, and France are showcased to emphasize efforts on a pilot and large-scale examples. In order for solar-assisted photocatalytic H 2 to mature as a solution, the aforementioned bottlenecks must be overcome for the field to advance its technology readiness level, assess the capital expenditure, and enter the market.

Do potential dependent kinetics play a role in photocatalytic rate trends?

“…The full derivation of eqn ( 7) can be found in ref. 5 and 27, with the main term in large brackets accounting for situations where the depletion (space charge) region width W does not fully capture all incident photons [modifying eqn (6)].…”

“…4b where V int rises around onset of the photocurrent under 3 Suns of illumination and then similarly levels off as the relation in eqn ( 6) is satisfied. The argument to this effect is more subtle for nanostructured systems, where enhanced light absorption and an increased total active surface area (relative to the illumination flux) can potentially reduce the need for a Tafel term to meet the saturation relation in eqn (6).…”

“…7 Hence, a deeper understanding of photoanode kinetics directly addresses the efficiency of oxidative reactions in a wide range of pollutant remediation and energy applications. 2,[5][6][7] Most recently, a series of intriguing measurements have shed important new light on the fundamental mechanisms driving rate limiting oxidation reactions at the surfaces of photoanodes. [10][11][12][13][14][15] The most dramatic of these findings determined that the relation between the reaction rate (observable as a photocurrent) and surface concentration of photogenerated holes (p s ), can undergo an apparent reaction transition from first-order to higher order power dependence on p s as the light illumination intensity in increased in photosynthetic reactions.…”

“…4,5 Though fundamental in nature, a deeper understanding of such reaction rates could possibly lead to vast improvements in photocatalytic efficiency and broaden its impact within the nascent green technology transition. 6 Likewise, systematic improvements in the rate performance of photocatalytic reactions could have broad implications in the development of water treatment technologies and an associated wide range of environmental remediation applications. This includes the environmental decontamination of a range of pollutants including dyes, pesticides, herbicides, phenols and antibiotics.…”

Estimating Nonradiative Excited-State Lifetimes in Photoactive Semiconducting Nanostructures

“…Heterogeneous photocatalysis constitutes an appealing and sustainable strategy to simultaneously solve energy and environmental problems. − Photoactive semiconducting materials such as TiO 2 , ZnO or WO 3 and light harvesting are the ingredients to carry out photocatalytic processes activated heterogeneously. − Unfortunately, photocatalysis has nowadays significant limitations derived from a poor absorption of visible light by the photocatalyst, resulting in a low catalytic redox performance . In principle, the light absorption problem can be partially overcome by the so-called band gap engineering strategies implying the design of the atomic structure of the photocatalyst ad hoc through tailoring shape, size composition, and dimension. , This allows one adjusting the physical and chemical properties eventually promoting an improved photocatalytic performance. − On the other hand, the photocatalytic activity is directly linked to the generation of the so-called exciton.…”