As a promising substitute for fossil fuels, hydrogen has emerged as a clean and renewable energy. A key challenge is the efficient production of hydrogen to meet the commercial-scale demand of hydrogen. Water splitting electrolysis is a promising pathway to achieve the efficient hydrogen production in terms of energy conversion and storage in which catalysis or electrocatalysis plays a critical role. The development of active, stable, and low-cost catalysts or electrocatalysts is an essential prerequisite for achieving the desired electrocatalytic hydrogen production from water splitting for practical use, which constitutes the central focus of this review. It will start with an introduction of the water splitting performance evaluation of various electrocatalysts in terms of activity, stability, and efficiency. This will be followed by outlining current knowledge on the two half-cell reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), in terms of reaction mechanisms in alkaline and acidic media. Recent advances in the design and preparation of nanostructured noble-metal and non-noble metal-based electrocatalysts will be discussed. New strategies and insights in exploring the synergistic structure, morphology, composition, and active sites of the nanostructured electrocatalysts for increasing the electrocatalytic activity and stability in HER and OER will be highlighted. Finally, future challenges and perspectives in the design of active and robust electrocatalysts for HER and OER towards efficient production of hydrogen from water splitting electrolysis will also be outlined.

kinetics of ORR is around five orders of magnitude slower than that of HOR, thereby requiring a much higher Pt loading in the cathode along with more active and durable ORR electrocatalysts than pure Pt catalysts. [1] This requirement presents challenges for the development of advanced cathode catalysts with lower cost, higher activity and higher durability than Pt. Meanwhile, traditional alkaline fuel cells (AFCs) working on concentrated 30−45% KOH electrolytes gained little attention for decades mainly due to their high sensitivity to atmospheric CO 2 . [2,3] The OH − ions in the electrolyte react with CO 2 and form K 2 CO 3 , which can precipitate out as solid crystals, blocking pores in the electrode and gas diffusion layer. In addition, the consumption of OH − reduces the conductivity of the electrolyte. This issue is addressed by replacing KOH solution with a solid anion exchange membrane (AEM) without mobile cations. An AMFC offers several important advantages over PEMFCs, including: 1) low dissolution rates of catalysts, allowing the use of less expensive Pt-free electrocatalysts; 2) wide selections of materials and components that are stable at high pH; and 3) inexpensive solid electrolytes that do not need fluorinated ionomers. Despite their promise, AMFCs are still in the early development stage and have not been systematically investigated due to the lack of highly conductive and durable AEMs. The recent development of highly conductive The rapid progress of proton exchange membrane fuel cells (PEMFCs) and alkaline exchange membrane fuel cells (AMFCs) has boosted the hydrogen economy concept via diverse energy applications in the past decades. For a holistic understanding of the development status of PEMFCs and AMFCs, recent advancements in electrocatalyst design and catalyst layer optimization, along with cell performance in terms of activity and durability in PEMFCs and AMFCs, are summarized here. The activity, stability, and fuel cell performance of different types of electrocatalysts for both oxygen reduction reaction and hydrogen oxidation reaction are discussed and compared. Research directions on the further development of active, stable, and low-cost electrocatalysts to meet the ultimate commercialization of PEMFCs and AMFCs are also discussed.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202006292.

The preparation of noble metal-semiconductor hybrid nanocrystals with controlled morphologies has received intensive interest in recent years. In this study, facile one-pot reactions have been developed for the synthesis of Au-ZnO hybrid nanocrystals with different interesting morphologies, including petal-like and urchin-like nanoflowers, nanomultipods and nanopyramids. In the synthesis strategy, oleylamine-containing solution serves as the reaction medium, and the in situ generated Au seeds play an important role in the subsequently induced growth of ZnO nanocrystals. With the aid of several surfactants, hybrid nanocrystals with different morphologies that have considerable influences on their optical and photocatalytic activities are readily achieved. Through high-resolution transmission electron microscopy measurements, an observed common orientation relationship between ZnO and Au is that ZnO nanocrystals prefer to grow with their polar {001} facets on the {111} facets of Au nanocrystals, and well-defined interfaces are evident. Surface plasmon resonance bands of Au with different positions are observed in the UV-vis spectra, and the UV and visible emissions of ZnO are found to be dramatically reduced. Finally, the as-prepared Au-ZnO nanocrystals exhibit excellent photocatalytic activity for the photodegradation of rhodamine B compared with pure ZnO nanocrystals. The Au-ZnO hybrid nanopyramids show the highest catalytic efficiency, which is correlated with the exposed crystal facets, crystallinity and the formation of hybrid nanostructures. The as-prepared Au-ZnO hybrid nanocrystals are expected to find diverse potential applications in the fields such as photocatalysis, solar energy conversion, sensing and biological detection.

The ability to tune the surface partial charge of noble metal catalysts at the nanoscale size dimension is essential for harnessing the activity of nanocatalysts in many important environmental catalytic reactions, such as hydrocarbon oxidations. We report herein a synthetic pathway to control the catalyst−support interactions and enable a surface partial-charge-tuned enhancement of the nanocatalytic activity using titania-supported Pt nanoparticles for oxidation of toluene as a model system. This pathway involved the onepot wet-chemical synthesis of TiO 2 -supported ultrasmall Pt nanoparticles (1−5 nm) with a controlled morphology and size. The catalysts showed greatly enhanced activities for toluene oxidation, exhibiting a strong dependence on the support morphology (e.g., nanopowders, nanowires, nanotubes, and mesoporous structures). The tunable partial negative charges on Pt were achieved by suppressing roughening of the support surface, favoring O 2 spillover from TiO 2 to Pt. In situ DRIFT data further reveal two parallel pathways responsible for oxidation of the methyl group and opening of the aromatic ring, the latter of which is favored by the partial negatively charged Pt, leading to a high activity. These findings provided insights into tuning nanoscale catalytic properties, which has significant implications for the design of supported noble metal nanocatalysts for environmental catalysis.