Researchers at CICATA-Legaria, the National Laboratory for Energy Conversion and Storage at Mexico's Instituto Politécnico Nacional (IPN) in Mexico City, have developed a composite of pyridine-coordinated transition-metal nitroprussides and reduced graphene oxide (rGO) as a bifunctional pre-electrocatalyst for zinc-air batteries (ZABs). Testing cobalt, nickel, and copper versions of the material, the team found that each metal favors a different half of the battery's oxygen chemistry, with the cobalt variant striking the best overall balance.
Rechargeable zinc-air batteries are attractive for their high theoretical energy density, low cost, and inherent safety, but their air cathode has to drive both the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charging, reactions with sluggish kinetics that typically require different types of catalysts. The field's benchmark catalysts, platinum for ORR and iridium or ruthenium oxides for OER, are scarce and expensive, pushing research toward cheaper first-row transition metals such as cobalt, nickel, and copper. Nitroprussides, a family of cyanometallate coordination polymers with the general formula T[Fe(CN)5NO], offered the team a synthetically simple, structurally tunable starting point for that search.
In the composite, pyridine ligands bind to the outer metal center of the nitroprusside and convert its native three-dimensional bulk structure into two-dimensional layers, a structural rather than catalytic role that allows the material to disperse effectively across the conductive rGO support. The researchers built three versions, CoPyNP:rGO, NiPyNP:rGO, and CuPyNP:rGO, and characterized them with X-ray diffraction, electron microscopy, infrared and Raman spectroscopy, and X-ray photoelectron spectroscopy, confirming that the choice of metal shifts both the composite's particle size and its electronic structure.
Electrochemical testing showed a clear division of labor: NiPyNP:rGO had the lowest OER onset potential and reached 10 mA/cm² at an overpotential of about 398 mV, while CuPyNP:rGO showed the most favorable ORR kinetics, with the lowest onset potential and highest limiting current. Neither extreme translated into the best full-battery performance. When assembled as air cathodes in zinc-air cells, CoPyNP:rGO delivered the highest peak power density, 54 mW/cm², and after 24 hours of galvanostatic cycling maintained the smallest charge-discharge voltage gap, 1.18 V, versus 1.29 V for the copper composite and 1.36 V for the nickel composite. The cobalt-based cell also reached a specific capacity of 809.2 mAh/g, close to zinc's theoretical capacity of 820 mAh/g. The team attributes cobalt's balanced performance to its intermediate d-band center position, sitting between nickel's and copper's, which avoids binding oxygen intermediates either too weakly (limiting ORR) or too strongly (limiting OER).
Post-cycling analysis of the cobalt electrode showed the nitroprusside is not the catalyst itself but a pre-catalyst: its characteristic cyanide signal disappeared during operation and was replaced by hydroxide and metal-oxygen signatures, confirming an in-situ transformation into a cobalt hydroxide/oxyhydroxide active phase. X-ray photoelectron spectroscopy on the cycled electrode still detected a pyridinic nitrogen signal, indicating the ligand remains bound within the reconstructed material rather than washing out, which the researchers propose helps stabilize the active hydroxide sheets against agglomeration. AC magnetic susceptibility measurements on the cycled electrodes further revealed structural disorder and competing magnetic interactions within the hydroxide phases, pointing to a more complex local structure than a simple, well-ordered crystal.
The authors frame the work as establishing coordination polymers, and cobalt-nitroprusside/rGO composites in particular, as viable, precious-metal-free candidates for bifunctional oxygen electrocatalysis, with the balance of a catalyst's d-band center offered as a design principle for future zinc-air battery electrode development.
