Deciphering the <i>In Situ</i> Surface Reconstruction of Supercapacitive Bimetallic Ni-Co Oxyphosphide during Electrochemical Activation Using Multivariate Statistical Analyses

Abstract

Electrochemical energy storage (EES) technologies are playing a leading role in the global effort to address the energy challenges. Current EES systems are limited by their energy density, capacity, and cycling stability. Some of those limitations arise from nanoscale phenomena, which are not fully understood or accounted for. Electrochemical activation (ECA), an often-overlooked process, creates more active sites on the electrode material and boosts the activity of the system to achieve a higher storage capacity. Herein, the ECA of bimetallic Ni–Co oxyphosphides is investigated via a plethora of spectroscopic techniques, including transmission electron microscopy enhanced by multivariate statistical analysis as a tool to better analyze the obtained spectra. Interestingly, ECA induces an in situ reconstruction of the pre-electrode via phosphorus leaching, together with accelerated surface segregation of the reconstructed Ni and Co species. The electrodes with reconstructed composition showed 110% higher supercapacitive performance than their pre-electrode counterparts. Thanks to the electrochemical optimization approach, a hybrid device has been assembled with a superb performance. The device exhibits energy density values comparable with batteries: 89 W h kg–1 at a power density of 848 W kg–1 with an excellent stability over 10,000 galvanic charge–discharge cycles as manifested by the steady capacitive retention (94.2–100.9%) even during the last 1000 cycles.