SHAO Yiqun, YUE Xin

(School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, Guangdong, China)

Extended abstract:

[Background and purposes] Due to climate change, protecting our earth has become increasingly important. In order to address this issue, developing sustainable and clean energy is particularly necessary. Among various new energy sources, hydrogen energy has garnered significant attention due to its clean emissions, high calorific value and renewable availability. Among various methods, water electrolysis for hydrogen production is more favored, due to its high efficiency and environmental friendliness. However, hydrogen production through water electrolysis is constrained by the multi-electron step oxygen evolution reaction (OER) at anode. Moreover, although noble metal catalysts exhibit excellent catalytic performance, they are prohibitively expensive. Therefore, developing efficient non-noble metal OER catalysts is of particular importance. Spinel cobalt-manganese oxide (MnCo₂O₄), exhibiting advantages of abundant resources, low cost and tunable electronic structure, has emerged as a highly promising catalyst. However, its intrinsic activity is limited, due to the restricted conductivity and active sites. This study was aimed to propose an electrochemical strategy that achieves dual synergistic enhancement of oxygen evolution performance and stability through controlled discharge-recharge cycles. It is found that Zn²⁺ intercalation during discharge induces tensile strain in the MnCo₂O₄, which modifies the electronic overlap intensity and potentially influences the eg orbital occupancy of Co, thereby contributing to the optimization of the electronic structure. The recharging process facilitates Zn²⁺ deintercalation from catalyst surfaces, exposing more active sites and preserving residual strain and electronic structure tuning effects formed during discharge. Through dual synergies of electronic structure optimization and surface modification, recharged samples demonstrate superior catalytic activity and stability, as compared with both the pristine and discharged samples. A novel "moderate reduction followed by partial oxidation" pretreatment paradigm is established, simultaneously optimizing intrinsic catalytic activity, conductivity and structural stability. The simple yet efficient strategy holds promise for application in other embedded catalyst materials, providing innovative approaches for resource utilization of spent battery materials and design of high-performance oxygen evolution catalysts.

[Methods] MnCo₂O₄ spinel was synthesized by using thermal decomposition method, followed by annealing at 350 ℃ in air. Electrochemical synthesis was conducted on the CR2032 platform, using zinc foil as the anode, the catalytic materials as the cathode, and zinc perchlorate as the electrolyte. The reaction was conducted at a current density of 10 μA·cm⁻², with MCO-S and MCO-C prepared, through constant-current discharge for 10 h, followed by 10 h discharge and 2 h charging. Structural evolution and surface changes were characterized using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). OER performance of the catalysts was tested at 25 ℃ in 1 M KOH solution w a scanning rate of 1 mV·s⁻¹, while its stability was evaluated at a current density of 100 mA·cm⁻². All electrochemical data were corrected for IR effects.

[Results] XRD results indicate that MCO-S exhibits more significant tensile strain than MCO, while MCO-C demonstrates compressive strain. Raman spectroscopy and FTIR spectra collectively demonstrate how Zn²⁺ intercalation/deintercalation regulates the octahedral sites, inducing tensile and compressive strains respectively. SEM and TEM results reveal no significant changes in sample morphology and grain size, with no microcracks. HR-TEM results further confirm the presence of reversible strain on specific crystal planes. XPS analysis result indicates higher defect oxygen content in both MCO-S and MCO-C, as compared with the original MCO, with MCO-S showing the highest levels. Post-recharge, defect oxygen exhibited slight rebound. The Zn/Co ratio in MCO-C was significantly lower than that in MCO-S, suggesting surface Zn²⁺ deintercalation during recharge and subsequent exposure of more active sites. Electrochemical test results demonstrated that MCO-C exhibited a current density of 127.6 mA·cm⁻² at 470 mV overpotential, significantly outperforming MCO (72.1 mA·cm⁻²) and MCO-S (34.9 mA·cm⁻²). MCO-C exhibited 10 mA·cm⁻² current density with only 400 mV overpotential, surpassing MCO (420 mV) and MCO-S (440 mV). Tafel slope measurements revealed MCO-C's slope value of 52.26 mV·dec⁻¹, lower than those of MCO (88.02 mV·dec⁻¹) and MCO-S (92.25 mV·dec⁻¹), indicating superior oxygen evolution reaction kinetics.

[Conclusions] An electrochemical approach was employed, which significantly enhances oxygen evolution performance and stability of spinel MnCo₂O₄ through coordinated charge-discharge cycling. XRD, Raman, and infrared spectroscopy analyses results reveal that Zn²⁺ intercalation induces lattice stretching strain during discharge, while Zn²⁺ deintercalation causes lattice compression strain during charging, with all three crystal structures remaining intact. XPS analysis results demonstrate that post-charging Zn²⁺ deintercalation exposes previously covered active sites, while lattice strain synergistically modulates cobalt's d-band center and eg orbital occupation. Electrochemical resuls indicate that the MCO-C samples subjected to charge-discharge-recharge cycling exhibit optimal performance, surpassing both original MCO samples and uncharged MCO-S samples in all metrics. In summary, this electrochemical method leverages reversible Zn²⁺ intercalation/deintercalation to synergistically tune lattice strain, electronic structure and surface active sites, thus providing novel insights for performance optimization of spinel oxide electrocatalysts.

Key words: spinel oxide; oxygen evolution reaction; lattice strain; electrochemical method