Improved kinetics of reduction of alkaline water on the g‐CN‐ supported transition metal oxide/boride hetero‐interface: A case study

Abstract

Summary The presence of multiple active sites and hetero‐junction on a multi‐component hetero‐structured catalyst would exhibit modest hydrogen binding strength and charge transfer, as well as redistribution during interaction with the intermediates during the water reduction process. In this study, we synthesized a ternary iron oxide/nickel boride/graphitic carbon nitride (Fe3O4/NixB/g‐CN) composite as the electrode material for the reduction of water under alkaline environment. Preformed NixB seed and Fe3O4 phases were co‐deposited on a g‐CN layer. Post‐catalytic characterization studies indicated facet selective reactive sites for NixB and Fe3O4 during the reduction of water. The composite required 130 mV overpotential for 10 mA cm−2 current density. The Tafel slope value of 99 mV dec−1 indicated that the adsorption step might determine the rate of reaction. The long‐term chronoamperometric study showed almost unaltered current density over ~50 h at 150 mV, only ~7.0% current loss over 50 h. The reactivity and stability can be attributed to the synergistic interaction within metal centers and the carbon support with a large surface area.

Rapid Printing of High-Temperature Polymer-Derived Ceramic Composite Thin-Film Thermistor with Laser Pyrolysis

“…The total XPS spectrum (Figure c) showed a significant increase in the content of oxygen and boron on the sample’s surface. From the B 1s spectrum (Figure d), B–B and Ti–B bonds disappeared from the sample’s surface after high-temperature oxidation, and only the B–O bonds (191–194 eV) , existed with high peak intensity. It indicates that the B powder and TiB 2 on the sample’s surface were entirely oxidized, and abundant B 2 O 3 was generated.…”

Role of Physicochemical Features toward Bifunctional Redox Activity of Transition-Metal Molybdates

“…In addition, the variation of current densities at definite overpotential showed that CoMo provided a higher current with the progress of reaction compared to others (Figure S6). The kinetics of the progressing reaction as well as the rate-determining step is reflected by the Tafel slope value, which is defined as the potential need to increase 10 units of current density . Generally, the HER proceeds through an adsorption step (Volmer step: H 2 O + Cat + e – → Cat–H + OH – and Tafel slope = 120 mV dec –1 ) followed by the desorption step, where the desorption step can be of two types: atom ion reaction (Heyrovskey reaction: Cat–H + H + + e – → H 2 + Cat + OH – , Tafel slope = 30 mV dec –1 ) and combination reaction (Tafel reaction: Cat–H + Cat–H → H 2 + 2Cat, Tafel slope = 40 mV dec –1 ).…”

“…OER involves the four-step adsorption/desorption process involving OH*, O* and OOH* intermediates, where the rate-determining step can be assumed from the electron-transfer coefficient (α) that signifies the lowering of free energy barrier through modification of the electrode–electrolyte interface potential. This α value can be calculated from the Tafel slope value following the relation: b = 2.303 R T F α , α = n normalf γ + n normalr β , where n f and n r are the numbers of electrons transferred before and each occurrence of RDS, respectively; γ is the stoichiometric number, typically 1, and β is the symmetrical potential barrier. , Now, the higher b values indicated that the adsorption step would be the slowest step during OER and the desorption would occur via proton hopping over the M–O interface. The progress of the reaction has further been understood from the post-OER XRD patterns of TMMos (Figure S7C–F), where the TMMo signed a weak odd ‘l’ reflection leading to a shift in the peak values showing that the cation–oxygen interaction has occurred strongly to which the Mo sites were distorted, although no structural changes were observed making it stable after the reaction as well and indicating Mo to be the active site for OER.…”

“…The kinetics of the progressing reaction as well as the rate-determining step is reflected by the Tafel slope value, which is defined as the potential need to increase 10 units of current density. 6 Generally, the HER proceeds through an adsorption step (Volmer step: Because the current appears to be linearly dependent with overpotential near the standard reduction potential region, Tafel slopes have been calculated at the higher overpotential region to avoid the redox term of the Butler−Volmer equation and to achieve bias free results. 37 The higher values of Tafel slopes showed that the slowest step of the reaction is the adsorption step with high surface coverage.…”

“…Noble metal (Pt, Ir, and Rh)-based electrocatalysts have been used as electrode materials to overcome this sluggishness because they fulfill the condition of the Sabatier principle to balance the adsorption–desorption of reaction intermediates with minimum limiting potential ( U lim ), calculated from the energy difference of the surface-adsorbed intermediates . However, the low availability, high production cost, low stability, and catalyst poisoning in an alkaline medium limited their wide range pertinence . Under this condition, the search for low-cost electrocatalysts with high efficiency and stability that can replace noble metal-based electrode materials has directed researchers toward abundant and budget friendly transition-metal (TM)-based materials like TM–oxide, sulfides, borides, carbides, selenides, molybdates, vanadates, ferrates, ,− TM/carbon composites, bimetallic/trimetallic TM alloys, etc.…”