Polyoxometalates (POMs) are polyatomic ions with closed three-dimensional frameworks. Their unique structure contains a large number of redox active sites, making them promising electrocatalysts for electrochemical energy conversion and storage applications. Thus, this paper presents an overview of the use of POMs as electrocatalysts for electrochemical energy conversion and storage devices, such as batteries, supercapacitors, fuel cells, or water electrolyzers. A discussion of the viability of these materials as alternatives to noble metal-based electrocatalysts is made. The current status of these materials to respond to the challenges of converting modern energy systems into more sustainable ones is also envisaged.

Polyoxometalates (POMs) with transition metals (Co, Cu, Fe, Mn, Ni) of Keggin structure and lamellar-stacked multi-layer morphology were synthesized. They were subsequently explored as bifunctional electrocatalysts for oxygen electrodes, i.e., oxygen reduction (ORR) and evolution (OER) reaction, for aqueous rechargeable metal-air batteries in alkaline media. The lowest Tafel slope (85 mV dec⁻¹) value and the highest OER current density of 93.8 mA cm⁻² were obtained for the Fe-POM electrocatalyst. Similar OER electrochemical catalytic activity was noticed for the Co-POM electrocatalyst. This behavior was confirmed by electrochemical impedance spectroscopy, where Fe-POM gave the lowest charge transfer resistance of 3.35 Ω, followed by Co-POM with R_ct of 15.04 Ω, during the OER. Additionally, Tafel slope values of 85 and 109 mV dec⁻¹ were calculated for Fe-POM and Co-POM, respectively, during the ORR. The ORR at Fe-POM proceeded by mixed two- and four-electron pathways, while ORR at Co-POM proceeded exclusively by the four-electron pathway. Finally, capacitance studies were conducted on the synthesized POMs.

We present the synthesis and characterization of two monometallic coordination polymers, [Co(L)₂(H₂O)₂]_n (Co-CP) and [Ni(L)₂(H₂O)₂]_n (Ni-CP), alongside a heterobimetallic counterpart, CoNi-CP, derived from an amide-based multifunctional pro-ligand 4-(pyrimidin-5-ylcarbamoyl)benzoic acid (HL), and discussed their electrocatalytic activity in the oxygen evolution reaction (OER). The CPs were characterized using various techniques, including elemental analysis, IR spectroscopy, X-ray diffraction, and thermal and powder XRD analyses. The low-cost amide-functionalized transition metal pristine coordination polymers Co-CP and Ni-CP were demonstrated to catalyze the OER in alkaline media, surpassing the benchmark IrO₂ electrocatalyst performance. The heterometallic coordination polymer (CoNi-CP) displayed a lower Tafel slope value (and thus, faster kinetics) and higher long-term durability compared to its monometallic counterparts, Co-CP and Ni-CP. The results obtained show a pristine transition metal heterobimetallic coordination polymer as a low-cost electrocatalyst of great promise and high performance for OER catalysis in alkaline media.

Oxygen evolution reaction (OER) electrocatalysts are frequently made from noble metal-based oxides like ruthenium/iridium oxides. However, because of their scarcity and high price, researchers are now focusing on creating innovative OER catalysts based on affordable transition metals that have improved electrical conductivity and accessibility to active sites. Metal–organic frameworks (MOFs), a unique class of inorganic materials with excellent physical and chemical properties, have witnessed significant progress in promising green energy systems. In this work, a novel mixed-ligand metal–organic framework [Co(μ-1κN,2κN′-BDP)(μ₃-1κoo′,2κo′′2κo′′′-BTC)]_n·nH₂O (BDP = boron-dipyrromethene or BODIPY; BTC = benzene tricarboxylate) denoted as CoBDPMOF has been synthesized, and its composites with different carbon materials have been designed. Compared to the pristine MOF, the composites showed enhanced electrocatalytic activity toward the oxygen evolution reaction (OER) in alkaline media. In addition, the CoBDPMOF with activated carbon showed the highest OER performance with a low Tafel slope (82 mV dec⁻¹) and the highest j₆₀₀ (59.8 mA cm⁻²), outperforming noble metal IrO₂, the OER benchmark electrocatalyst. This study presents new insights into the design and application of CoBDPMOF-based materials for energy conversion and suggests promising avenues for further research and development in electrocatalysis.

A series of metal–organic framework-based materials of the MIL-101 family was prepared for potential application as anodic electrocatalysts in the direct borohydride fuel cell. The MIL-101-based materials were tested for borohydride oxidation reaction using cyclic voltammetry and chronoamperometry in alkaline media, with Au@MIL-101-NH₂ showing high responsiveness. The obtained data allow for the determination of kinetic parameters that characterize the borohydride oxidation on the prepared electrocatalysts. The activation energy for borohydride oxidation using an Au@MIL-101-NH₂ electrocatalyst was as low as 13.6 kJ mol⁻¹ with a reaction order of 0.4. The anodic charge transfer coefficient was 0.85, and the number of transferred electrons was 7.97, matching the theoretical maximum value of 8 electrons transferred during the borohydride oxidation reaction. The promising performance of Au@MIL-101-NH₂ suggests its potential application as an anode for direct borohydride fuel cells.