Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation

Abstract

In-depth insights into the structure–activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived Od-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni4+ and Fe4+ species, which generate oxygen-bridged NiIV-O-FeIV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free Od-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-NiIV-O-FeIV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

“…To obtain in-depth insight into the role of sulfur heteroatom incorporation in the OER kinetics, our group performed operando XAS experiments with newly designed NiFe-based coordination polymer-based OER catalysts that were doped with S atoms (S-R-NiFe-CPs). 647 Our results indicated that engineering of sulfur heteroatom incorporation promotes the generation of catalytically active moieties of high-valent S−Ni IV In addition to sulfur, chlorine incorporation has also been reported to trigger the restructuring of various materials. In a representative example, by means of operando XAS characterization and DFT calculations, Song et al probed the structural optimization of a new class of Co 2 (OH) 3 Cl OER precatalysts.…”

“…For example, a recent operando XAS study showed that the thickness of the prepared IrO 2 OER electrodes and the underlying catalyst loss through dissolution or delamination can drastically influence the apparent oxidation state of Ir during the OER . To avoid such artifacts, a catalyst mass loading range of 1–2 mg cm –2 (such as from our practical experience of operando XAS on CoP and NiFe-CPs) and a low X-ray flux (≤0.5 × 10 12 photons mm –2 s –1 ) are recommended for conducting a proper in situ / operando XAS experiment. Another important aspect to consider during in situ / operando X-ray or microscopy experiments is that the interaction between the electrolyte and beam might produce a significant number of radicals due to water radiolysis .…”

“…In contrast to pure Co–Fe–PB, the stabilized Fe centers in CoFeS x @Co–Fe–PB brought forward a robust and high-performance (Co, Fe)­OOH OER catalyst with a low overpotential of 286 mV at 10 mA/cm 2 , outperforming the pure Co–Fe–PB (417 mV at 10 mA/cm 2 ). To obtain in-depth insight into the role of sulfur heteroatom incorporation in the OER kinetics, our group performed operando XAS experiments with newly designed NiFe-based coordination polymer-based OER catalysts that were doped with S atoms (S-R-NiFe-CPs) . Our results indicated that engineering of sulfur heteroatom incorporation promotes the generation of catalytically active moieties of high-valent S–Ni IV –O–Fe IV –O during the OER.…”

Transformation mechanism of high-valence metal sites for the optimization of Co- and Ni-based OER catalysts in an alkaline environment: recent progress and perspectives

“…4d–g). 82 Greta R. Patzke et al 83 reported a related study on sulfide reduction of the NiFe coordination polymer ( S-R -NiFe-CP). The Ni K-edge XANES spectra of S-R -NiFe CPs at applied potentials between 0.8 and 1.45 V vs. RHE were recorded.…”

Efficient O–O Coupling at Catalytic Interface to Assist Kinetics Optimization on Concerted and Sequential Proton–Electron Transfer for Water Oxidation

“…The high-quality energy conversion effect and efficiency of energy utilization pursued by electrocatalysis are regarded as one of the important ways to solve the energy crisis. − With the rapid development of electrochemical reaction (hydrogen generation, CO 2 and N 2 reduction), oxygen evolution reaction (OER), which provides electrons and protons for them, cannot be ignored. − Meanwhile, the reaction environment of the OER application is also more complex, which induces greater challenges for the corresponding research mechanism and catalyst design. − …”