The electrochemical oxygen evolution reaction (OER) plays a fundamental role in several energy technologies, which performance and cost-effectiveness are in large part related to the used OER electrocatalyst. Herein, we detail the synthesis of cobalt-iron oxide nanosheets containing controlled amounts of well-anchored SO 4 2− anionic groups (CoFe x O y -SO 4 ). We use a cobalt-based zeolitic imidazolate framework (ZIF-67) as the structural template and a cobalt source and Mohr's salt ((NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O) as the source of iron and sulfate. When combining the ZIF-67 with ammonium iron sulfate, the protons produced by the ammonium ion hydrolysis (NH 4 + + H 2 O = NH 3 •H 2 O + H + ) etch the ZIF-67, dissociating its polyhedron structure, and form porous assemblies of two-dimensional nanostructures through a diffusion-controlled process. At the same time, iron ions partially replace cobalt within the structure, and SO 4 2− ions are anchored on the material surface by exchange with organic ligands. As a result, ultrathin CoFe x O y -SO 4 nanosheets are obtained. The proposed synthetic procedure enables controlling the amount of Fe and SO 4 ions and analyzing the effect of each element on the electrocatalytic activity. The optimized CoFe x O y -SO 4 material displays outstanding OER activity with a 10 mA cm −2 overpotential of 268 mV, a Tafel slope of 46.5 mV dec −1 , and excellent stability during 62 h. This excellent performance is correlated to the material's structural and chemical parameters. The assembled nanosheet structure is characterized by a large electrochemically active surface area, a high density of reaction sites, and fast electron transportation. Meanwhile, the introduction of iron increases the electrical conductivity of the catalysts and provides fast reaction sites with optimum bond energy and spin state for the adsorption of OER intermediates. The presence of sulfate ions at the catalyst surface modifies the electronic energy level of active sites, regulates the adsorption of intermediates to reduce the OER overpotential, and promotes the surface charge transfer, which accelerates the formation of oxygenated intermediates. Overall, the present work details the synthesis of a high-efficiency OER electrocatalyst and demonstrates the introduction of nonmetallic anionic groups as an excellent strategy to promote electrocatalytic activity in energy conversion technologies.

The use of non-metal charge carriers such as ammonium (NH 4 + ) in electrochemical energy storage devices offers advantages in terms of weight, element abundance, and compatibility with aqueous electrolytes. However, the development of suitable electrodes for such carriers lags behind other technologies. Herein, we present a high-performance anode material for ammonium-ion supercapacitors based on a MoO 3 /carbon (MoO 3 @C) composite. The NH 4 + storage performance of such composites and their practical application prospects are evaluated both in a three-electrode configuration and as symmetric supercapacitors. The optimized material reaches an unprecedented specific capacitance of 473 F·g −1 (158 mAh·g −1 ; 1592 mF·cm −2 ) at a current density of 1 A·g −1 , and 92.7% capacitance retention after 5000 cycles in a three-electrode set-up. This outstanding performance is related to the presence of oxygen vacancies that enhance the composites' ionic/ electronic transportation and electrochemical reaction site, while at the same time facilitating the formation of hydrogen bonds between NH 4 + and the host material. Using the optimized composite, symmetric supercapacitors based on an (NH 4 ) 2 SO 4 gel electrolyte are fabricated and demonstrated to provide unmatched energy densities above 78 Wh·kg −1 at a power density of 929 W·kg −1 . Besides, such devices are characterized by extraordinary capacitance retention of 97.6% after 10,000 cycles.

Novel hierarchically structured microporous biocarbons with exceptionally high capacities for CO capture have been synthesized from the abundant agricultural waste of rice husk (RH), using a facile methodology that effectively integrated carbonization, activation, and potassium intercalation into a one-step process. Textural characterization demonstrates that the synthesized biocarbons exhibit exceedingly high ultra-microporosity accounting for up to 95% of total porosity mainly as a result of the naturally occurring silicon compounds within the RH molecular framework structures. With a modest surface area of up to 1035 m/g and a total pore volume of 0.43 cm/g, the best performing RH carbon has shown exceptionally high and fully reversible CO uptake capacity of 2.0 mmol/g at 25 °C and a CO partial pressure of 0.15 bar, which represents one of the highest uptakes ever reported for both carbon and MOF materials usually prepared from using cost-prohibitive precursor materials with cumbersome methodologies. It has been found that up to 50% of the total CO uptake is attributable to the unique surface chemistry of the RH carbons, which appears to be dominated by the enhanced formation of extra-framework potassium cations owing to the exceedingly high levels of ultra-microporosity and the presence of zeolitic structures incorporated within the carbon matrices. Characterizations by EDX element mapping, XPS, and heat of adsorption measurements confirm the existence of a range of zeolitic structures, which essentially transforms the RH carbons into a kind of zeolite-carbon nanocomposite material with strong surface affinity for CO.

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