Electronic spin states in catalysts
Transition metal-nitrogen-carbon materials, especially Fe-N-Cs, have been found to accelerate the oxygen reduction reaction (ORR). Despite a large number of studies devoted to improving the species content, specific surface area, and electronic conductivity of Fe-N-Cs activity, the performance remains unsatisfactory. To date, limited studies have been conducted to improve the catalytic performance of Fe-N-Cs electrocatalysts by modulating the electronic spin state of the Fe center. Dr. Guobin Zhang from Tsinghua University & Prof. Juan Antonio Zapien’s group at City University of Hong Kong have jointly proposed to modulate the structure of electronic FeN₄ by introducing Ti₃C₂ MXene with sulfur termini, which significantly enhances the ORR catalytic activity. MXene with sulfur termini induces spin-state transitions and Fe 3d electron delocalization of FeN₄ species with an upward shift of the d-band center, which enables Fe(II) ions to bind oxygen in a terminal adsorption mode, which is conducive to the initiation of oxygen reduction, the promotion of oxygen-containing group adsorption on FeN₄ species, and the ORR kinetics, and possesses a catalytic performance comparable to the commercial Pt-C with comparable catalytic performance to that of commercial Pt-C. ZABs developed for wearable devices using FeN₄-Ti₃C₂Sₓ also exhibited fast kinetics and good stability.
Synthesis and structural characterization of catalysts
The SEM image of Ti₃C₂ MXene is shown in Fig. 1a, exhibiting a nanosheet structure with folds. The SEM and TEM images of FeN₄-Ti₃C₂ and FeN₄-Ti₃C₂Sₓ in Figs. 1b-e show that their nanosheet structures become thicker and rougher and few nanoparticles are clearly seen adhering on the surface. The XRD patterns in Fig. 1f show that the peaks of the FeN₄-Ti₃C₂ and FeN₄-Ti₃C₂Sₓ samples are similar to those of the undoped Ti₃C₂, which suggests that the introduction of dopant did not change the structure of the undoped Ti₃C₂; however, the increase of the anatase peaks suggests that the doping process resulted in the mild oxidation of flaky Ti₃C₂. In addition, the broad peak at ~25° indicates the formation of amorphous carbon due to the moderate oxidation of lamellar Ti₃C₂ during the doping process. However, the intensity of the carbon skeleton peaks (D/G bands) of the doped samples became more pronounced in the spectra compared to the undoped Ti₃C₂. This is due to the fact that more defects are created during the doping and carbonization process. Usually, the defects affect the catalytic performance by influencing the electronic conductivity and active sites of the samples. Figure 1h shows the N₂ adsorption and desorption isotherms for all three samples, showing type IV isotherms with hysteresis, indicating the presence of pores in the structure. The calculated BET specific surface areas of these samples follow the following trend: Ti₃C₂>FeN₄-Ti₃C₂>FeN₄-Ti₃C₂Sₓ and, as shown in Fig. 1i, the mesoporous structure is predominant in all three samples. The decrease in BET specific surface area and pore size with increasing dopant species may be due to the filling effect of dopants.
SEM images of pristine Ti₃C₂, (b) FeN₄-Ti₃C₂ and (c) FeN₄-Ti₃C₂Sₓ; (d) FeN₄ -Ti₃C₂ and (e) TEM images of FeN₄-Ti₃C₂Sₓ; (f) XRD plots and (g) Raman spectra; and (h) N₂ adsorption-desorption isotherms, and
Wearable solid-state ZAB properties
Since the sample FeN₄-Ti₃C₂Sₓ exhibited high ORR activity, the article further used the developed FeN₄-Ti₃C₂Sₓ catalysts with alkali-resistant dual network PANa and cellulose hydrogel (PANa-cellulose ) as stretchable solid electrolytes to jointly construct a stretchable and abrasion-resistant fibrous ZAB.Fig. 4a illustrates that it can be stretched over 1000% strain without any fracture and visible cracks, with excellent tensile properties. The structure of the fibrous ZAB is shown in Fig. 4 b. A hydrogel electrolyte was used to first wrap the Zn spring electrode and then stretched and wrapped with FeN₄-Ti₃C₂Sₓ-loaded carbon paper as the air electrode. The charging and
discharging curves and corresponding power densities of the fibrous ZAB in the initial and 800% stretched states are shown in Fig. 4c, d. The large power density of the ZAB in the initial state is 133.6 mW-cm-², and that in the stretched state at 800 ℃ is 182.3 mW-cm-², which indicates that the the cell is stretchable and has good electrochemical performance in the stretched state. In addition, the cell exhibits excellent cycling stability with a stable cycling of 110 h at 2 mA cm-², as shown in Fig. 4e. To demonstrate its wear resistance, as shown in Fig. 4f and g, two 10 cm long and 2 mm diameter fiber
