Facile synthesis of Fe-doped CoP nanosheet arrays wrapped by graphene for overall water splitting

Field-Effect Enhancement of Non-Faradaic Processes at Interfaces Governs Electrocatalytic Water Splitting Activity

Recognizing the essential factor governing interfacial hydrogen/oxygen evolution reactions (HER/OER) is central to electrocatalytic water-splitting. Traditional strategies aiming at enhancing electrocatalytic activities have mainly focused on manipulating active site valencies or coordination environments. Herein, the role of interfacial adsorption is probed and modulated by the topological construct of the electrocatalyst, a frequently underestimated non-Faradaic mechanism in the dynamics of electrocatalysis. The engineered Co0.75Fe0.25P nanorods, anchored with FeOx clusters, manifest a marked amplification of the surface electric field, thus delivering a substantially improved bifunctional electrocatalytic performance. In alkaline water splitting anion exchange membrane (AEM) electrolyzer, the current density of 1.0 A cm-2 can be achieved at a cell voltage of only 1.73 V for the FeOx@Co0.75Fe0.25P|| FeOx@Co0.75Fe0.25P pairs for 120 h of continuous operation at 1.0 A cm-2. Detailed investigations of electronic structures, combined with valence state and coordination geometry assessments, reveal that the enhancement of catalytic behavior in FeOx@Co0.75Fe0.25P is chiefly attributed to the strengthened adsorptive interactions prompted by the intensified electric field at the surface. The congruent effects observed in FeOx-cluster-decorated Co0.75Fe0.25P nanosheets underscore the ubiquity of this effect. The results put forth a compelling proposition for leveraging interfacial charge densification via deliberate cluster supplementation.

Tin-doped NiFe2O4 nanoblocks grown on an iron foil for efficient and stable water splitting at large current densities

Developing low-cost and self-supported bifunctional catalysts for highly efficient water splitting devices is of great significance. Herein, different from previously reported NiFe2O4-based electrocatalysts, we have grown nano-NiFe2O4 directly onto the iron foil (IF) surface and in situ introduced Sn4+ into NiFe2O4. The resulting experimental phenomena confirmed that the as-synthesized Sn-NiFe2O4/IF can deliver large-current densities (>1000 mA cm-2) during oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) processes at a low overpotential. The needed overpotentials at the current density of 10 and 1000 mA cm-2 are 231 and 368 mV for OER and 57 and 439 mV for HER, respectively. Additionally, when applied for the two-electrode water splitting, the corresponding needed voltage for Sn-NiFe2O4/IF at the current density of 10 mA cm-2 was only 1.56 V, which was comparable to the commercial Pt/C-RuO2/IF electrode. Thus, the introduced Sn4+ greatly enhanced the electrocatalytic property of Sn-NiFe2O4/IF, resulting in a superior bifunctional catalyst that can be applied for large-scale hydrogen production.

Constructing interface engineering and tailoring a nanoflower-like FeP/CoP heterostructure for enhanced oxygen evolution reaction

The inexpensive and highly efficient electrocatalysts toward oxygen evolution reaction (OER) in water splitting electrolysis have displayed promising practical applications to relieve energy crisis. Herein, we prepared a high-yield and structurally regulated bimetallic cobalt-iron phosphide electrocatalyst by a facile one-pot hydrothermal reaction and subsequent low-temperature phosphating treatment. The tailoring of nanoscale morphology was achieved by varying the input ratio and phosphating temperature. Thus, an optimized FeP/CoP-1-350 sample with the ultra-thin nanosheets assembled into a nanoflower-like structure was obtained. FeP/CoP-1-350 heterostructure displayed remarkable activity toward the OER with a low overpotential of 276 mV at a current density of 10 mA cm^-2, and a low Tafel slope of only 37.71 mV dec^-1. Long-lasting durability and stability were maintained with the current with almost no obvious fluctuation. The enhanced OER activity was attributed to the presence of copious active sites from the ultra-thin nanosheets, the interface between CoP and FeP components, and the synergistic effect of Fe-Co elements in the FeP/CoP heterostructure. This study provides a feasible strategy to fabricate highly efficient and cost-effective bimetallic phosphide electrocatalysts.

Fabrication of nanosheet-assembled hollow copper-nickel phosphide spheres embedded in reduced graphene oxide texture for hybrid supercapacitors

Owing to their metalloid characteristics with high electrical conductivity, transition metal phosphides (TMPs) have attracted considerable research attention as prospective cathodes for hybrid supercapacitors. Unfortunately, they usually exhibit low rate performance as well as poor longevity, which does not meet the demands of hybrid supercapacitors. The nanocomposite constructed from reduced graphene oxide (rGO) and TMPs with a highly porous nature can effectively overcome the above-mentioned issues, greatly widening their utilization. In this work, we fabricated nanosheet-assembled hollow copper-nickel phosphide spheres (NH-CNPSs) by the controllable phosphatizing of copper-nickel-ethylene glycol (CN-EG) precursors. Then, porous NH-CNPSs were embedded in rGO texture (NH-CNPS-rGO) to form a unique porous nanoarchitecture. The obtained NH-CNPS-rGO has several advantages benefiting as the cathode electrode, such as (i) the hollow structure as well as porous nanosheets are conducive to fast electrolyte diffusion, (ii) the electrical conductivity of NH-CNPS is further enhanced when coupled with the rGO texture, hence promoting electron transfer in the whole structure, (iii) wrapping NH-CNPSs within the rGO texture endows the nanocomposite with much better structural stability, resulting in longer durability of the electrode, And (iv) the porous structures generated in the nanocomposite provide a perfect space for reducing the mass transfer resistance and accessing the electrolyte, thereby boosting the reaction kinetics. The tests demonstrated that the optimal NH-CNPS-rGO electrode revealed a capacity of up to 1075 C g^-1, a superior rate capacity, and exceptional longevity of 94.7%. Moreover, a hybrid supercapacitor (NH-CNPS-rGO‖AC) equipped with the NH-CNPS-rGO-cathode electrode and activated carbon (AC)-anode electrode represented a satisfactory energy density of 64 W h kg^-1 at 801 W kg^-1 and amazing longevity (91.8% retention after 13 000 cycles), which endorses the promising potential of NH-CNPS-rGO for high-efficiency supercapacitors. This research showcases an appropriate method to engineer hollow TMP-rGO nanocomposites as effective materials for supercapacitors.

Modulating the electronic structure of ternary transition metal phosphide for enhanced hydrogen evolution activity

Rationally designing ternary transition-metal phosphides (TMPs) for the hydrogen evolution reaction (HER) is desirable but remains a significant challenge. Herein, ternary FeCoNiP encapsulated in a porous carbon shell, coupled with N-doped carbon nanotubes (FeCoNiP@NCNTs) are synthesized via a simple pyrolysis-phosphatization strategy derived from FeCoNi-MOF-100@dicyandiamide. Because Co/Ni enters the FeP lattice, FeCoNiP@NCNTs show a favorable catalytic performance towards the HER with low overpotential values of 86.7 and 233.5 mV at 10 mA cm^-2 in acidic and alkaline media, respectively, surpassing the HER performance of FeP@NCNTs, FeCoP@NCNTs, and FeNiP@NCNTs. Impressively, FeCoNiP@NCNTs display adequate acid-resistance capacity during the HER process, with nearly negligible decay due to the thin graphitized carbon shell structure with a thickness of 11.5-20.3 nm. The results of experiments, structural characterization, and density functional theory (DFT) calculations demonstrate that Co/Ni co-doping can modulate the adsorption and dissociation processes of H⁺ and downshift the d-band center of FeP. This work proposes a strategy for fabricating ternary TMP catalysts with heterogeneous structures for the HER.

Synergistic effect of Mn doping and hollow structure boosting Mn-CoP/Co2P nanotubes as efficient bifunctional electrocatalyst for overall water splitting

The sluggish kinetic of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) severely hampers the commercial application of electrochemical water splitting, promoting the urgent exploration of high-efficient bifunctional electrocatalysts. Heteroatom doping and structure engineering have been identified as the most effective strategies to boost the catalytic activity of electrocatalysts. Herein, Mn doping and hollow structure were integrated in the design of Co-based transition metal phosphide catalyst to prepare Mn-CoP/Co₂P nanotubes (denoted as Mn-CP NTs) by a facile template-free method. Confirmed by characterization analysis, the introduced Mn species were in high dispersion in the regular CoP/Co₂P hollow tubular framework. Such a favorable design in composition and structure effectively boosted the catalytic activity of Mn-CP NTs toward electrochemical water splitting. The Mn-CP NTs showed superior HER and OER activity demonstrated by the low overpotentials of 82 mV (vs HER) and 309 mV (vs OER) at the current density of 10 mA cm^-2, as well as the satisfactory durability. When used as both cathode and anode in electrolyzer for overall water splitting, only a low cell voltage of 1.67 V was required for the Mn-CP NTs to drive 10 mA cm^-2, accompanied with excellent stability confirmed by over 50 h test.

Heterostructured ZnCo2O4-CoOOH nanosheets on Ni foam for a high performance bifunctional alkaline water splitting catalyst

It is of utmost importance to explore bifunctional electrocatalysts for water splitting. Herein, unique ZnCo₂O₄-CoOOH heterostructured ultrathin nanosheets on Ni foam are reported that combines a two-step hydrothermal method. This catalyst exhibits excellent catalytic performances to achieve a current density of 10 mA cm^-2 with an ultralow overpotential of 115 mV for HER, attaining an overpotential of 238 mV at 20 mA cm^-2 for OER. Remarkably, ZnCo₂O₄-CoOOH/Ni shows a voltage of 1.494 V to drive a current density of 10 mA cm^-2. Such performances are due to the inter-penetrative pores present in the ultrathin nanosheets that provide large surface areas and expose massive active sites to enhance activities. In addition, the unique nanosheet structure and the 3D Ni foam substrate possess large specific surface areas, which can facilitate mass diffusion. This excellent performance is ascribed to the ZnCo₂O₄-CoOOH heterostructure that manipulates strong synergy to improve the electrochemical activity. This study offers new insight on an innovative approach for the exploitation of effective bifunctional electrocatalysts with a heterostructure.

Ni3N/Co4N nanosheet heterojunction electrocatalyst for hydrogen evolution reaction in alkaline fresh water/simulated seawater

The Ni3N/Co4N nanosheet heterojunction exhibits higher HER activity in alkaline fresh water and simulated seawater.