Metal–organic frameworks (MOFs) as electrocatalysts for oxygen evolution reaction (OER) typically suffer from fast degradation under harsh electrolyte conditions, impeding their practical use in industrial electrolyzers. Besides, the evolution of catalytic centers in MOFs and the related influence on their performance along the progress of reaction have rarely been studied. Here, we report a type of structurally stable bimetallic FeNi-MOF nanoarrays with self-optimized electrocatalytic activities in the oxygen production. Such a unique dynamic phenomenon is related with the gradual valence increments of Fe ions in MOFs, which trigger the continuous performance improvement before reaching an optimal steady state. Apart from the intact crystalline structures upon cycling, these FeNi-MOFs achieve low overpotentials of 239 and 308 mV at the current densities of 50 and 200 mA cm–2, respectively, and show durable operation for over 1033 h (>43 days) at 100 mA cm–2 and for another 200 h at 500 mA cm–2. A direct comparison of isostructural and single crystalline Fe-MOFs and Ni-MOFs resolves higher activities of Fe sites in the bimetallic MOFs, which are corroborated by theoretical calculations. The Fe–O bond covalency increment during Fe oxidation enhances the proton–electron transfers with the oxygen 2p-band closer to the Fermi level, thereby expediting the OER process. This work provides deep insights into the understanding of catalytic processes in heterometallic MOFs.

Gang Bian received his Ph.D. degree in Chemistry from Jiangnan University in 2018. Then he joined the research group of Prof. Jian Zhu as a postdoctoral fellow at Nankai University. He focuses on the development of 2D covalent organic frameworks using solution-based approaches for electronic devices.

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Monolayer graphene shows Seebeck coefficient several times and gas-flow-induced voltage twenty times higher than that of bulk graphite. Here we find that the Seebeck coefficient of multilayer graphene increases monotonically with increasing layer and reaches its peak value at hexa-layer ~77% higher than for monolayer and then decreases, although the electric resistance decreases monotonically with increasing layer. The flow-induced voltage is significantly higher in 2, 4, 5, 6, 7 layered graphene than in 1, 3, 8 layered one, against the prevailing view that it should be proportional to Seebeck coefficient. These thickness effects are also in sharp contrast to that in continuous aluminum nanofilms.

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We show by systemically experimental investigation that gas-flow-induced voltage in monolayer graphene is more than twenty times of that in bulk graphite. Examination over samples with sheet resistances ranging from 307 to 1600 Ω/sq shows that the induced voltage increase with the resistance and can be further improved by controlling the quality and doping level of graphene. The induced voltage is nearly independent of the substrate materials and can be well explained by the interplay of Bernoulli's principle and the carrier density dependent Seebeck coefficient. The results demonstrate that graphene has great potential for flow sensors and energy conversion devices.*

High electron mobility and low sheet resistance were achieved in lattice-matched AlInN/AlN/GaN/AlN/GaN double-channel (DC) heterostructure. Two-dimensional electron gas (2DEG) of the DC heterostructure was divided into the double channels and the room-temperature mobility was increased to 1430 cm2/V s by reducing the 2DEG density in each channel, compared with low electron mobility (1090 cm2/V s) for lattice-matched AlInN/AlN/GaN single-channel heterostructure. It was found that the 2DEG mobility was limited by thickness of the AlN interlayer between the double channels. After the structure optimization, the room temperature electron mobility of the DC heterostructure reached 1570 cm2/V s with sheet resistance of 222 Ω/◻.