Ammonia is a hydrogen-dense, carbon-neutral energy carrier, but sluggish oxidation kinetics and catalyst toxicity limit its large-scale use in low-temperature alkaline direct ammonia fuel cells (DAFCs). Inspired by the “Grotthuss hopping” theory, we designed a hydroxyl group-modified membrane electrode catalyst system through the modification of interface key groups. This system can interact with adsorbed OH– (hereafter, OH–ad), thus being similar to a “relay” that allows OH–ad to jump from one active site to another and then react with ammonia intermediates, injecting more impetus into the kinetic process of the orderly release of OH–ad. The results of molecular dynamics (MD) simulations and operando Fourier transform infrared spectroscopy (operando-FTIR) further demonstrated and confirmed that this process follows the Gerischer–Mauerer (G–M) mechanism. Consequently, the mass activity of the best membrane electrode catalyst in this series, PtNiNC@OH0.05 (373 A g–1Pt), has been significantly enhanced in alkaline media, which is 3.2 times higher than that of the commercial 20% Pt/C (116 A g–1Pt) catalyst. Even more surprisingly, the DAFC of the membrane electrode catalyst as an anode achieves a peak power density of 17.4 mW cm–2 at 60 °C, which is 9.89-fold higher than that of 20% Pt/C (1.76 mW cm–2). The interface modification method based on the “relay” jumps proposed by us provides a way to fabricate DAFC membrane electrode catalysts.
Dai Yanmei,Teng Jiayu,Zhang Qingyu,Sun Dongchang,Tao Zijuan,Huang Changshun,Luo Liangli,Liu Bo,Ou Zhimin
9α-Hydroxyandrost-4-ene-3,17-dione (9α-OH-AD) is a valuable intermediate for the manufacture of steroid drugs such as hydrocortisone. Its bioproduction from 4-androstene-3,17-dione (4-AD), however, is limited by the low efficiency of the two-component steroid 9α-hydroxylase (KSH) system and insufficient cofactor regeneration during whole-cell catalysis. In this work, RvKshA and RvKshB were mined and heterologously expressed, and plasmid screening identified E. coli BL21/pET28a + -RvKshA/pETDuet-1-RvKshB as the best-performing strain. Structural analyses based on molecular docking and molecular dynamics simulations clarified the cooperative catalytic mechanism of KshA and KshB, providing guidance for rational system construction. More importantly, to address the NADH supply bottleneck, we established a dual-enzyme coupling strategy by introducing formate dehydrogenase (FDH) for in situ cofactor regeneration. This design enabled efficient steroid hydroxylation, giving a 98.3% conversion at 20 mM 4-AD. When combined with high-density fermentation and fed-batch feeding, the engineered strain achieved a substrate processing capacity of 80 mM and a spatiotemporal yield of 42.8 mM/(L·day). This study provides an integrated enzymatic and bioprocess solution for improving whole-cell steroid hydroxylation efficiency and offers a practical basis for the scalable biosynthesis of 9α-OH-AD.
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