Growing global energy demands and climate change motivate the development of new renewable energy technologies. In this context, water splitting using sustainable energy sources has emerged as an attractive process for carbon-neutral fuel cycles. A key scientific challenge to achieving this overall goal is the invention of new catalysts for the reductive and oxidative conversions of water to hydrogen and oxygen, respectively. This review article will highlight progress in molecular electrochemical approaches for catalytic reduction of protons to hydrogen, focusing on complexes of earth-abundant metals that can function in pure aqueous or mixed aqueous-organic media. The use of water as a reaction medium has dual benefits of maintaining high substrate concentration as well as minimizing the environmental impact from organic additives and by-products.

Metal-organic frameworks (MOFs) are a class of porous materials with unprecedented chemical and structural tunability. Their synthetic versatility, long-range order, and rich host-guest chemistry make MOFs ideal platforms for identifying design features for advanced functional materials. This review addresses synthetic approaches to control MOF attributes for realizing material properties such as charge conductivity, stability, surface area, and flexibility. Along with an updated account on MOFs employed in batteries and supercapacitors, new directions are outlined for advancing MOF research in emergent technologies such as solid-state electrolytes and battery operation in extreme environments. G lobal demands for clean energy storage and delivery continue to push developing technology to its limits. Batteries and supercapacitors are among the most promising technologies for electrical energy storage owing to their portability and compact size for on-demand usage. Despite their promise, chemical and physical limitations of existing materials hinder performance and require new, creative solutions. For instance, polymers and conductive carbon materials are relatively inexpensive, scalable, and synthetically tunable but can lack physical and chemical stability for device implementation. On the other hand, solid inorganic materials, such as metal oxides and silicon, are used as electrode materials due to their robust structure and redox-active sites. However, sluggish ion diffusion of metal oxides limit charge/ discharge rate capabilities and large volumetric changes lead to mechanical instability. Drawbacks in these current platforms motivate the discovery and development of new materials for advanced energy storage devices. Metal-organic frameworks (MOFs) are attractive candidates to meet the needs of nextgeneration energy storage technologies. MOFs are a class of porous materials composed of metal nodes and organic linkers. Their modular nature allows for great synthetic tunability, affording both fine chemical and structural control. With creative synthetic design, properties such as porosity, stability, particle morphology, and conductivity can be tailored for specific applications. As the needs of each energy storage device are different, this synthetic versatility of MOFs provides a method to optimize materials properties to combat inherent electrochemical

TitleTowards a comprehensive understanding of visible-light photogeneration of hydrogen from water using cobalt(ii) polypyridyl catalysts Permalink https://escholarship.org/uc/item/9tw104fd Journal Energy and Environmental Science, 7(4) Homogeneous aqueous solutions of photocatalytic ensembles, consisting of [Ru(bpy) 3 ] 2+ as a photosensitizer, ascorbic acid/ascorbate as the electron source, and 10 distinct Co 2+ -based molecular catalysts, were evaluated for visible-light induced hydrogen evolution using high-throughput screening. The combined results demonstrate that Co 2+ complexes bearing tetradentate ligands yield more active photocatalytic compositions than their congeners with pentadentate ligands while operating with high catalyst stability. Additionally, molecular Co 2+ catalysts with cis open coordination sites appear to be significantly more active for hydrogen evolution than those with trans open sites. As evidenced by mass spectrometric analysis of the reactor headspace and associated deuteration experiments, the H 2 gas generated in all instances was derived from aqueous protons. One of the most promising cis-disposed Co 2+ species, [Co(bpyPY2Me)(CH 3 CN)(CF 3 SO 3 )](CF 3 SO 3 ) (1), engages in highly efficient hydrogen evolving photocatalysis, achieving a turnover number of 4200 (H 2 /Co) and a turnover frequency of 3200 (H 2 /Co per h) at pH 4 under simulated sunlight (AM 1.5G, 100 mW cm À2 ) at room temperature. At equimolar concentrations of photosensitizer and 1, the total hydrogen produced appears to be exclusively limited by the photostability of [Ru(bpy) 3 ] 2+ , which was observed to decompose into an Ru(bpy) 2 -ascorbate adduct, as evidenced by HPLC and ESI-MS experiments.Lowering the operating temperature from 27 to 5 C significantly attenuates bpy dissociation from the sensitizer, resulting in a net $two-fold increase in hydrogen production from this composition. The primary electron transfer steps of this photocatalytic ensemble were investigated by nanosecond transient absorption spectroscopy. Photoexcited [Ru(bpy) 3 ] 2+ undergoes reductive quenching by ascorbic acid/ascorbate (k q ¼ 2.6 Â 10 7 M À1 s À1 ), releasing [Ru(bpy) 3 ] + from the encounter solvent cage with an efficiency of 55 AE 5%. In the presence of catalyst 1, [Ru(bpy) 3 ] + generated in the initial flash-quench experiment transfers an electron (k et ¼ 2 Â 10 9 M À1 s À1 ) at an efficiency of 85 AE 10% to the catalyst, which is believed to enter the hydrogen evolution cycle subsequently. Using a combinatorial approach, all ten Co 2+ catalysts were evaluated for their potential to operate under neutral pH 7.0 conditions. Catalyst 7, [Co(PY4MeH 2 )(CH 3 CN)(CF 3 SO 3 )](CF 3 SO 3 ), was revealed to be most promising, as its performance metrics were only marginally affected by pH and turnover numbers greater than 1000 were easily obtained in photocatalytic hydrogen generation. These comprehensive findings provide guidelines for the development of molecular compositions capable of evolving hydrogen from purely aqueous ...