Operando unraveling photothermal-promoted dynamic active-sites generation in NiFe2O4 for markedly enhanced oxygen evolution

The ability to develop highly active and low-cost electrocatalysts represents an important endeavor toward accelerating sluggish water-oxidation kinetics. Herein, we report the implementation and unraveling of the photothermal effect of spinel nanoparticles (NPs) on promoting dynamic active-sites generation to markedly enhance their oxygen evolution reaction (OER) activity via an integrated operando Raman and density functional theory (DFT) study. Specifically, NiFe₂O₄ (NFO) NPs are first synthesized by capitalizing on amphiphilic star-like diblock copolymers as nanoreactors. Upon the near-infrared light irradiation, the photothermal heating of the NFO-based electrode progressively raises the temperature, accompanied by a marked decrease of overpotential. Accordingly, only an overpotential of 309 mV is required to yield a high current density of 100 mA cm⁻², greatly lower than recently reported earth-abundant electrocatalysts. More importantly, the photothermal effect of NFO NPs facilitates surface reconstruction into high-active oxyhydroxides at lower potential (1.36 V) under OER conditions, as revealed by operando Raman spectroelectrochemistry. The DFT calculation corroborates that these reconstructed (Ni,Fe)oxyhydroxides are electrocatalytically active sites as the kinetics barrier is largely reduced over pure NFO without surface reconstruction. Given the diversity of materials (metal oxides, sulfides, phosphides, etc.) possessing the photo-to-thermal conversion, this effect may thus provide a unique and robust platform to boost highly active surface species in nanomaterials for a fundamental understanding of enhanced performance that may underpin future advances in electrocatalysis, photocatalysis, solar-energy conversion, and renewable-energy production.

Motivated by the need for accelerating sluggish reaction kinetics at the anode (1, 2), the focus of water electrolysis has been centered heavily on oxygen evolution reaction (OER) toward sustainable hydrogen fuel production. To date, there has been much effort in developing low-cost yet high-performance earth-abundant transition-metal alternatives to commonly used noble metals for OER. Intriguingly, many Ni-, Co-, Fe-, and Mn-based oxides experience a dynamic surface-reconstruction process to form more active oxyhydroxides, which are recognized as true catalytically active species for OER in alkaline media (3, 4). Among various transition metal-based OER catalysts, bimetal spinel-structured oxides in the form of AB₂O₄ (A and B are different metal ions) have garnered much attention due to their rich compositions, electron configurations, and valence states (5). Interestingly, inverse spinel NiFe₂O₄ (NFO), in principle, exhibits enhanced catalytic activity toward OER because of the presence of multivalent elements (i.e., Ni³⁺/Ni²⁺ and Fe³⁺/Fe²⁺) (6). It is important to note that studies on facilitating the surface reconstruction of NFO to achieve high-performance OER are relatively few, and a fundamental understanding as to what makes the derived OER catalysts perform well remains elusive.Recently, introducing thermal energy to promote electrocatalytic conversion has attracted significant interest (7–10). The thermal energy could accelerate the motion of reactant molecules, promote their effective collisions during the reaction, and thus make it easier to overcome the activation barrier (11, 12). Moreover, the use of thermal energy could also promote the surface reconstruction of catalysts into highly active species and accelerate the electrocatalytic kinetics, thereby leading to improved efficiency (13). Electrocatalysts with photothermal effect (referred to as photothermal electrocatalysts) enable in situ heating due to photo-to-thermal conversion under illumination with visible or near-infrared (NIR) light, thereby dispensing with the need for extra devices required to provide thermal energy. More importantly, in sharp contrast to common approaches where the entire solution is heated (14), the photothermal effect is localized on electrocatalysts themselves (15), thus effectively enabling heat modulation to a defined region (i.e., the working electrode). Despite recent impressive advances in transition metal oxides (e.g., Fe₃O₄ and Co₃O₄) as photothermal agents for cancer therapy (15), their implementations for photothermal-assisted OER, in particular spinel oxides, are comparatively few and limited in scope. Moreover, it has been reported that Ni- and Co-based OER catalysts are prone to surface reconstruction into highly active oxyhydroxides (16, 17). Surprisingly, the photothermal effect on promoting surface reconstruction in spinel oxide catalysts has yet to be explored.Herein, we report an integrated operando Raman and density functional theory (DFT) plus Hubbard U (DFT + U) study to exercise and unveil the photo-to-thermal conversion of inverse spinel oxide nanoparticles (NPs) in promoting the generation of dynamic active sites via surface reconstruction into oxyhydroxides and thus greatly enhancing their OER activity. First, a series of amphiphilic star-like poly(acrylic acid)-block-poly(styrene-co-acrylonitrile) (denoted PAA-b-PSAN) diblock copolymers with a well-defined molecular weight (MW) and low polydispersity index (PDI) are exploited as nanoreactors to synthesize a set of PSAN-ligated NFO NPs with different sizes and PSAN chain lengths. The effects of the NFO NP sizes and the outer PSAN chain lengths on catalytic activity of NFO NPs are then scrutinized. Interestingly, NFO NPs of the largest size (∼12 nm) ligated with the shortest PSAN chains (M_n = 7K) display the best OER reactivity on a glassy carbon (GC) electrode in alkaline media as a result of the high fraction of the exposed electrochemically active surface area and the fastest electrocatalytic kinetics. Subsequently, the photothermal effect of PSAN-ligated NFO NPs is exploited to promote their surface reconstruction and thus boost OER. More importantly, an operando Raman spectroelectrochemistry study is performed to unveil the mechanism of photothermal-assisted enhancement in OER reaction, revealing the emergence of electrocatalytically active γ-NiOOH at a lower potential (1.36 V) during the surface-reconstruction process with a photothermal effect. Finally, the first-principle calculations substantiate that the reconstructed surface, i.e., (Ni,Fe)oxyhydroxides, plays a pivotal role as the active site for electrocatalytic reaction. As such, photothermal electrocatalysts (e.g., metal oxides, sulfides, phosphides, etc.) may render significantly low overpotential and fast OER kinetics, representing an array of important materials that couple the localized heating with electrochemistry for effective renewable-energy production.