A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an iR-corrected current density of 10 mA cm(-2) at -0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm(2) in rotating disk experiments under a H2 atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group P63/mmc with lattice parameters of a = 2.85176(2) Å and c = 10.9862(3) Å and a formula of Co0.6Mo1.4N2. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N-H moieties are abundant.

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Hard carbon is the leading candidate anode for commercialization of Na-ion batteries. Hard carbon has a unique local atomic structure, which is composed of nanodomains of layered rumpled sheets that have short-range local order resembling graphene within each layer but complete disorder along the caxis between layers. A primary challenge holding back the development of Na-ion batteries is that a complete understanding of the structure-capacity correlations of Na-ion storage in hard carbon has remained elusive. This article presents two key discoveries: first that characteristics of hard carbon's structure can be modified systematically by heteroatom doping, and second, that these structural changes greatly affect Na-ion storage properties, which reveals the mechanisms for Na storage in hard carbon. Specifically, via P or S doping, the interlayer spacing is dilated, which extends the low-voltage plateau capacity, while increasing the defect concentrations with P or B doping leads to higher sloping sodiation capacity. Our combined experimental studies and first principles calculations reveal that it is the Na-ion-defect binding that corresponds to the sloping capacity, while the Na intercalation between graphenic layers causes the low-potential plateau capacity. The new understanding provides a new set of guiding principles to optimize hard carbon for Na-ion battery applications.

Aqueous electrolytes come with an intrinsic narrow electrochemical stability window (1.23 V). Expanding this window represents significant benefits in both fundamental science and practical battery applications. Recent breakthroughs made via super-concentration have resulted in >3.0 V windows, but fundamental understanding of the related mechanism is still absent. In the present work, we examined the widened window (2.55 V) of a super-concentrated (unsaturated) aqueous solution of LiNO 3 through both theoretical and spectral analyses and discovered that a local structure of intimate Li + -water interaction arises at super-concentration, generating (Li + (H 2 O) 2 ) n polymer-like chains to replace the ubiquitous hydrogen bonding between water molecules. Such structure is mainly responsible for the expanded electrochemical stability window. Further theoretical and experimental analyses quantitatively differentiate the contributions to this window, identifying the kinetic factor (desolvation) as the main contributor. Such molecular-level and quantitative understanding will further assist in tailor designing more effective approaches to stabilizing water electrochemically.

The alloy-design strategy of combining multiple elements in near-equimolar ratios has shown great potential for producing exceptional engineering materials, often known as 'high-entropy alloys'. Understanding the elemental distribution, and, thus, the evolution of the configurational entropy during solidification, is undertaken in the present study using the Al 1.3 CoCrCuFeNi model alloy. Here we show that, even when the material undergoes elemental segregation, precipitation, chemical ordering and spinodal decomposition, a significant amount of disorder remains, due to the distributions of multiple elements in the major phases. The results suggest that the high-entropy alloy-design strategy may be applied to a wide range of complex materials, and should not be limited to the goal of creating single-phase solid solutions.