Thermal conductivity of multiwalled carbon nanotubes ͑CNT's͒ prepared using a microwave plasma chemical vapor deposition system is investigated using a pulsed photothermal reflectance technique. We find that the average thermal conductivity of carbon nanotube films, with the film thickness from 10 to 50 m, is around 15 W/m K at room temperature and independent of the tube length. Taking a small volume filling fraction of CNT's into account, the effective nanotube thermal conductivity could be 2ϫ10 2 W/m K, which is smaller than the thermal conductivity of diamond and in-plane graphite by a factor of 9 and 7.5, respectively.

Water electrolysis offers a promising energy conversion and storage technology for mitigating the global energy and environmental crisis, but there still lack highly efficient and pHuniversal electrocatalysts to boost the sluggish kinetics for both cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). Herein, we report uniformly dispersed iridium nanoclusters embedded on nitrogen and sulfur co-doped graphene as an efficient and robust electrocatalyst for both HER and OER at all pH conditions, reaching a current density of 10 mA cm −2 with only 300, 190 and 220 mV overpotential for overall water splitting in neutral, acidic and alkaline electrolyte, respectively. Based on probing experiments, operando X-ray absorption spectroscopy and theoretical calculations, we attribute the high catalytic activities to the optimum bindings to hydrogen (for HER) and oxygenated intermediate species (for OER) derived from the tunable and favorable electronic state of the iridium sites coordinated with both nitrogen and sulfur.

Polymer electrolyte membrane water electrolysis (PEMWE) has been regarded as a promising technology for renewable hydrogen production. However, acidic oxygen evolution reaction (OER) catalysts with long‐term stability impose a grand challenge in its large‐scale industrialization. In this review, critical factors that may lead to catalyst's instability in couple with potential solutions are comprehensively discussed, including mechanical peeling, substrate corrosion, active‐site over‐oxidation/dissolution, reconstruction, oxide crystal structure collapse through the lattice oxygen‐participated reaction pathway, etc. Last but not least, personal prospects are provided in terms of rigorous stability evaluation criteria, in situ/operando characterizations, economic feasibility and practical electrolyzer consideration, highlighting the ternary relationship of structure evolution, industrial‐relevant activity and stability to serve as a roadmap towards the ultimate application of PEMWE.

concentration has increased from ≈277 ppm in 1750, prior to Industrial Revolution, to ≈410 ppm in 2019 and the average annual growth of carbon emission in 2018 and 2019 is greater than its 10-year average. [1] Thus, it is urgent to search for renewable and zero-carbon energy sources as alternatives to replace fossil fuels. [2] Although solar, wind and tidal energy are abundant and sustainable, their intermittent and weather-dependent limitations require development and deployment of highly efficient energy conversion and storage systems at large scale to bridge the time gap between supply and demand. [3] An attractive solution is to convert the electrical energy derived from the aforementioned renewable energy sources into chemical energy in the form of hydrogen. [4,5] Pure or mixed hydrogen is playing important roles in transportation, industrial sectors, and chemical transformation processes. [6] However, at present, 95% of hydrogen is still produced by reforming of fossil fuels that emits significant amount of CO 2 and only 4% is through water electrolysis, mainly owing to the much higher production cost of the latter. [7] Therefore, crucial to enable this sustainable vision is to improve the efficiency and scalability of water electrolysis technology.

Developing highly active and stable water oxidation catalysts with reduced cost in acidic media plays a critical role in clean energy technologies such as fuel cells and electrolyzers. Precious iridium-based oxides are still the only oxygen evolution reaction (OER) catalysts with reasonable activity and stability in acid. Herein, we design iridium–tungsten composites with a metallic tungsten-rich core and an iridium-rich surface by the sol–gel method followed by hydrogen reduction. The thus obtained iridium–tungsten catalyst shows much higher intrinsic water oxidation activity (100 mA/mgIr at an overpotential of 290 mV) and stability (100 h at 10 mA/cm2 geom) together with reduced iridium content (33 wt % only) as compared with pure iridium oxide. An operando method using H2O2 as a probe molecule is developed to determine the relative adsorption strength of the reaction intermediates (*OH and *OOH) in the OER process. Detailed characterization shows that the tungsten-incorporated surface not only modulates the adsorption energy of oxygen intermediates on iridium but also enhances the stability of iridium species in acid, while the metallic tungsten core exhibits high electrical conductivity, all of which collectively give rise to the much enhanced catalytic performance of iridium–tungsten composite in acidic water oxidation. A single-membrane electrode assembly is further prepared to demonstrate the advantages and potential application of iridium–tungsten composite in practical proton exchange membrane electrolyzers.