RESUMEN
Anion exchange membrane fuel cells (AEMFCs) that operate at high pH, offer the advantage of enabling the use of abundant 3d-transition metal-based electrocatalysts. While they have shown remarkable improvement in performance, their long-term durability remains insufficient for practical applications with the alkaline polymer electrolytes (APEs) being the limiting factor. The stability of APEs is generally evaluated in concentrated alkaline solutions, which overlooks/oversimplifies the complex electrochemical environment of the catalyst layer in membrane electrode assembly (MEA) devices. Herein, we report a study of the degradation of the membrane and ionomer independently under realistic H2-air (CO2 free) fuel cell operation, using proton nuclear magnetic resonance (1H-NMR), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). While the membrane degradation was minimal after the AEMFC stability test, the ionomer in the catalyst layers degraded approximately 20% to 30% with the cathode being more severely affected than the anode. The ionomer degradation decreased the catalyst utilization and significantly increased the ionic resistance, leading to significant performance degradation in the AEMFC stability test. These findings emphasize the importance of ionomer stability and the need to consider the electrochemical environments of MEAs when evaluating the stability of APEs.
RESUMEN
Precisely tailoring the electronic structures of electrocatalysts to achieve an optimum hydroxide binding energy (OHBE) is vital to the alkaline hydrogen oxidation reaction (HOR). As a promising alternative to the Pt-group metals, considerable efforts have been devoted to exploring highly efficient Ni-based catalysts for alkaline HOR. However, their performances still lack practical competitiveness. Herein, based on insights from the molecular orbital theory and the Hammer-Nørskov d-band model, we propose an ingenious surface oxygen insertion strategy to precisely tailor the electronic structures of Ni electrocatalysts, simultaneously increasing the degree of energy-level alignment between the adsorbed hydroxide (*OH) states and surface Ni d-band and decreasing the degree of anti-bonding filling, which leads to an optimal OHBE. Through the pyrolysis procedure mediated by a metal-organic framework at a low temperature under a reducing atmosphere, the obtained oxygen-inserted two atomic-layer Ni shell-modified Ni metal core nanoparticle (Ni@Oi-Ni) exhibits a remarkable alkaline HOR performance with a record mass activity of 85.63 mA mg-1, which is 40-fold higher than that of the freshly synthesized Ni catalyst. Combining CO stripping experiments with ab initio calculations, we further reveal a linear relationship between the OHBE and the content of inserted oxygen, which thus results in a volcano-type correlation between the OH binding strength and alkaline HOR activity. This work indicates that the oxygen insertion into the top-surface layers is an efficient strategy to regulate the coordination environment and electronic structure of Ni catalysts and identifies the dominate role of OH binding strength in alkaline HOR.
RESUMEN
The alkaline polymer electrolyte fuel cells (APEFCs) hold great promise for using nonnoble metal-based electrocatalysts toward the cathodic oxygen reduction reaction (ORR), but are hindered by the sluggish anodic hydrogen oxidation reaction (HOR) in alkaline electrolytes. Here, a strategy is reported to promote the alkaline HOR performance of Ru by incorporating 3d-transition metals (V, Fe, Co, and Ni), where the conduction band minimum (CBM) level of Ru can be rationally tailored through strong d-d orbital coupling. As expected, the obtained RuFe nanosheet exhibits outstanding HOR performance with the mass activity of 233.46 A gPGM -1 and 23-fold higher than the Ru catalyst, even threefold higher than the commercial Pt/C. APEFC employing this RuFe as anodic catalyst gives a peak power density of 1.2 W cm-2 , outperforming the documented Pt-free anodic catalyst-based APEFCs. Experimental results and density functional theory calculations suggest the enhanced OH-binding energy and reduced formation energy of water derived from the downshifted CBM level of Ru contribute to the enhanced HOR activity.
RESUMEN
SignificanceWe present a groundbreaking advance in completely nonprecious hydrogen fuel cell technologies achieving a record power density of 200 mW/cm2 with Ni@CNx anode and Co-Mn cathode. The 2-nm CNx coating weakens the O-binding energy, which effectively mitigates the undesirable surface oxidation during hydrogen oxidation reaction (HOR) polarization, leading to a stable fuel cell operation for Ni@CNx over 100 h at 200 mA/cm2, superior to a Ni nanoparticle counterpart. Ni@CNx exhibited a dramatically enhanced tolerance to CO relative to Pt/C, enabling the use of hydrogen gas with trace amounts of CO, critical for practical applications. The complete removal of precious metals in fuel cells lowers the catalyst cost to virtually negligible levels and marks a milestone for practical alkaline fuel cells.
