Unraveling CoNiP‒CoP2 3D‐on‐1D Hybrid Nanoarchitecture for Long‐Lasting Electrochemical Hybrid Cells and Oxygen Evolution Reaction

Abstract Evolving cost‐effective transition metal phosphides (TMPs) using general approaches for energy storage is pivotal but challenging. Besides, the absence of noble metals and high electrocatalytic activity of TMPs allow their applicability as catalysts in oxygen evolution reaction (OER). Herein, CoNiP‒CoP2 (CNP‒CP) composite is in situ deposited on carbon fabric by a one‐step hydrothermal technique. The CNP‒CP reveals hybrid nanoarchitecture (3D‐on‐1D HNA), i.e., cashew fruit‐like nanostructures and nanocones. The CNP‒CP HNA electrode delivers higher areal capacity (82.8 μAh cm–2) than the other electrodes. Furthermore, a hybrid cell assembled with CNP‒CP HNA shows maximum energy and power densities of 31 μWh cm–2 and 10.9 mW cm–2, respectively. Exclusively, the hybrid cell demonstrates remarkable durability over 30 000 cycles. In situ/operando X‐ray absorption near‐edge structure analysis confirms the reversible changes in valency of Co and Ni elements in CNP‒CP material during real‐time electrochemical reactions. Besides, a quasi‐solid‐state device unveils its practicability by powering electronic components. Meanwhile, the CNP‒CP HNA verifies its higher OER activity than the other catalysts by revealing lower overpotential (230 mV). Also, it exhibits relatively small Tafel slope (38 mV dec–1) and stable OER activity over 24 h. This preparation strategy may initiate the design of advanced TMP‐based materials for multifunctional applications.

Demineralized water (DMW) was produced from the Milli-Q water purification system in our lab. All the received chemicals were of analytical grade and used as those received. Carbon fabric substrate was purchased from CeTech Co., South Korea.

Equations
The electrochemical performance of the prepared electrodes was estimated in terms of areal capacity, areal capacitance, and areal energy/power densities in three-and two-electrode systems using the following formulae:S[1] P = (S6) where 'C A ' is the areal capacity (Ah cm -2 ), 'C AC ' is the areal capacitance (F cm -2 ), 'C S ' is the specific capacity (Ah g -1 ), 'ΔV' is the potential window (V) after excluding potential drop, '∫ ( ) ' is an integral area, 'E A ' and 'E S ' are areal and specific energy densities (Wh cm -2 or Wh kg -1 ), and 'P' is the power densities (W cm -2 or W kg -1 ), respectively. ' ' is the discharge time (s).
The mass-balancing equation to estimate the correct mass of AC material (negative electrode) for the fabrication of the hybrid cell is given below.S[2] Here, m -, C -, and ΔVare the mass (g), areal capacitance (F cm -2 ), and potential window (V) of the negative electrode, respectively, while m + , Q + , and ΔV + are the mass (g), areal capacity (Ah cm -2 ), and potential window (V) of the positive electrode, respectively.      Note: Figure S6(c)(i) and Figure S6(d)(i) do not represent the single crystalline nature of the prepared material. They seem to be like that since the Gatan DigitalMicrograph Software focused on tiny spots where the lattice planes of the corresponding phase are only presented.
The lattice fringes of the selected area in Figure S6(b) are extracted using the Gatan DigitalMicrograph Software at another spot as indicated in Figure S6(b) to further enquire its phase. From this image, the software calculated the fringe width of ~0.22 nm, which corresponds to the (111) lattice plane of the CoNiP phase. Therefore, it is once more confirmed that the prepared material consists of a dual-phase with a dominant CoP 2 crystalline structure. The same analysis is also performed on the lattice planes related to the CoP 2 at a marked place with green color in Figure S6(b). The resulting image is presented in Figure S6

Fabrication of negative electrode (AC@CF):
Among several carbon-related materials, activated carbon (AC) was captivated as a promising negative electrode material due to its high porosity, large specific surface area, good electrical conductivity, and high mechanical stability. Therefore, it was chosen as a negative electrode candidate in the present work. The preparation of AC@CF electrode follows as: at first, the AC powder, super P carbon black, and PVDF powders were taken in an agate mortar in the weight ratio of 80:10:10, respectively. This mixture was then ground well for about half an hour.
Then, a sufficient volume of NMP solvent was dropped in the above mixture and ground for 10 more minutes. Finally, the viscous slurry was loaded on the cleaned CF substrate within the active area of 1  1 cm 2 . After drying at 80 C overnight, the AC-loaded CF (AC@CF) substrate was pressed at ~3 MPa using a presser instrument for the strong contact of AC material to the CF substrate. The mass of AC material on the CF substrate was noted to be ~3.5 mg cm -2 .
The electrochemical properties of the AC@CF electrode were evaluated in a threeterminal system consisting of 2 M KOH electrolyte. The CV profiles of the AC@CF electrode recorded at different sweep rates ( Figure S13(a)) exhibited a pseudo-rectangle-like shape, unveiling its non-faradaic-type charge storage mechanism. The nearly linear charge and discharge curves of the AC@CF electrode obtained at different current densities ( Figure S13(b)) further endorsed its non-faradaic-type charge storage. At 2 mA cm -2 , the AC@CF electrode delivered the areal capacitance of 986 mF cm -2 , and it retained 653 mF cm -2 even at a high current density of 40 mA cm -2 , as shown in Figure S13(c).