Abstract:
high-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XANES) and resonant inelastic X-ray scattering (RIXS) provides a powerful platform for resolving the electronic structures and reaction intermediates that govern electrocatalytic performance. Here, we highlight HERFD-XANES and RIXS enable mechanistic understanding of electrocatalysis beyond conventional XAS, by directly probing orbital regulation, spin/electronic-state evolution, and adsorbate-induced spectral responses under reaction-relevant conditions.
In sulfur-containing nickel porphyrin complexes for tandem CO₂ reduction,[1] RIXS and XAS, supported by theoretical analysis, reveal that thiophene-induced ligand-hole formation regulates the Ni 3d orbitals, increases the oxidation state of low-spin Ni centers, lowers the Ni d-band center, and reduces the CO₂-to-CO reaction barrier. Operando spectroscopy further verifies CO-intermediate formation on molecular Ni sites, accompanied by changes in the symmetry of the central Ni ion while preserving the molecular framework. When coupled with Cu catalysts, these molecularly generated CO intermediates enrich the Cu surface and promote C–C coupling, achieving a C₂ Faradaic efficiency of 74.3% and a C₂ partial current density of 445.8 mA cm⁻² in neutral electrolyte.
In Fe-doped CoFeP for water electrolysis,[2] HERFD-XANES and 1s3p RIXS directly resolve the Fe-induced downward shift and delocalization of Co 3d orbitals, thereby clarifying the electronic origin of enhanced bifunctional catalytic activity. When implemented in a membrane-electrode assembly, CoFeP delivers cell voltages of 1.51, 1.65, and 2.11 V at 10, 100, and 500 mA cm⁻², respectively, without iR correction, together with a hydrogen Faradaic efficiency above 97% and stable operation for 350 h.
Beyond orbital regulation, in situ HERFD-XANES also enables sensitive tracking of interfacial intermediate migration.[3] In the Pd/Ag model catalyst, potential-dependent Pd and Ag K-edge HERFD-XANES monitors white-line intensity variations associated with Pd/Ag 5p–O 2p hybridization. The weakened Pd white-line intensity and simultaneously enhanced Ag white-line intensity at 0.7 V versus RHE indicate OH* depletion on Pd and oxygen-intermediate accumulation on Ag, providing spectroscopic evidence for OH* spillover from OH*-rich Pd sites to OH*-poor Ag sites across the Pd/Ag interface.
Collectively, these studies establish HERFD-XANES and RIXS as complementary, high-resolution probes for mechanistic electrocatalysis. While RIXS provides orbital- and electronic-state sensitivity for identifying active-site regulation, HERFD-XANES resolves subtle XANES features and adsorbate-induced spectral changes during operation. This HERFD-XANES/RIXS framework connects orbital structure, intermediate dynamics, and catalytic performance, offering general design principles for next-generation electrocatalysts based on orbital engineering, tandem catalysis, and interfacial intermediate relay.
Keywords – operando, X-ray absorption spectroscopy, Raman, flow-based device, photocatalysis