Green hydrogen is a key future energy resource, and its generation through electrochemical water splitting is a prominent option for industrial applications. This calls for robust and low-cost electrocatalyst to facilitate the kinetically and thermodynamically demanding oxygen evolution reaction (OER) which remains a challenging bottleneck of water splitting. In order to use the full potential of abundant transition metal-based catalysts, their structural design is only one part of the development process. In addition, their dynamic behavior and restructuring over longer operational periods during water splitting needs to be understood.[1]

Over the past years, we have brought forward a broad spectrum of 3d transition metal-based OER catalysts covering a wide range of compound types: from single atom catalysts [2] over molecules and coordination polymers [3, 4] to nanostructured and chalkogenide/phosphide-derived catalysts.[5] Selected examples of these catalyst types will be presented and their operando monitoring through X-ray absorption spectroscopy (XAS) and complementary methods will be discussed to provide in-depth insights into the dynamics of their active sites. This information is vital for the rational design of high-performance and robust electrocatalysts.

We performed a series of studies on Ni/Fe-based electrocatalysts to elucidate the interaction and optimal coordination environments of this highly active metal combination. We first constructed dual-site NiFe single atom catalysts (SACs) via a convenient synthesis protocol using g-C₃N₄ and glucose as precursors.[2] Their OER performance was superior to single site Ni-, Fe- and Co-SACs and the underlying structural reconstruction of the dual-site NiFe catalysts was investigated with a combination of operando XAS, high resolution HAADF-STEM investigations and DFT calculations. These results revealed that the Ni environment mainly underwent reconstruction towards active Ni-O-Fe moieties wherein both metal centers participate on the *OH deprotonation process, resulting in the formation of bridging O₂ species. The improved OER performance of dual-site NiFe SACs is assisted by the formation of spin channels via the Ni-O-Fe bonds. We furthermore designed ultra-thin NiFe-based coordination polymer derivatives for high OER performance.[3] Upon mild reduction with NaBH₄, the emerging reduced R-Ni₈Fe₂-CPs with sub-2 nm layered morphologies excelled through optimal exposure of active sites and fast electron transfer. The performance or their Ni-O-Fe active sites was further enhanced through the targeted incorporation of O and Ni deficiencies. In our next study, these materials were strategically optimized through controlled introduction of sulfur into the oxygen deficiencies around the Ni centers to generate S-R-NiFe-CPs with superior OER performance.[4] Their investigation through operando XAS revealed that sulfur tuning facilitates the formation of active Ni^IV-O-Fe^IV moieties, while their reconstruction was hindered in the sulfur-free materials, thereby confirming the benefits of anionic engineering strategies for the OER performance.

For overall water splitting, we developed a Fe-doped cobalt phosphide (Co@CoFe–P) catalyst for both hydrogen evolution reaction (HER) and OER. Low HER overpotentials at 10 mA/cm² were observed over a wide pH range (0~14), such as 83 mV in 0.5 M H₂SO₄ or 104 mV in 1.0 M KOH and a good OER activity with a low overpotential of 266 mV for 10 mA/cm² was obtained in 1.0 M KOH.[5] Operando XAS investigations demonstrated that partial Fe doping promoted the formation of HER active P–Co–O–Fe–P configurations in Co@CoFe–P with a lower energy barrier for water dissociation and  H* intermediate adsorption. Under OER conditions, a reconstruction process into OER active intermediates with Co^IV–O–Fe^IV  moieties was observed, while the formation of these active unites was hindered in P-free reference Co-FeOOH, thereby underlining the efficiency of anionic substitution strategies.

The talk will be concluded with an outlook on molecular and related Co-based OER catalysts to bridge the design perspectives between molecular and heterogeneous catalysts.