The Role of Oxygenic Groups and sp3 Carbon Hybridization in Activated Graphite Electrodes for Vanadium Redox Flow Batteries

Hydrogen Sulfide Splitting into Hydrogen and Sulfur through Off-Field Electrocatalysis

Hydrogen sulfide (H2S), a toxic gas abundant in natural gas fields and refineries, is currently being removed mainly via the Claus process. However, the emission of sulfur-containing pollutants is hard to be prevented and the hydrogen element is combined to water. Herein, we report an electron-mediated off-field electrocatalysis approach (OFEC) for complete splitting of H2S into H2 and S under ambient conditions. Fe(III)/Fe(II) and V(II)/V(III) redox mediators are used to fulfill the cycles for H2S oxidation and H2 production, respectively. Fe(III) effectively removes H2S with almost 100% conversion during its oxidation process. The H+ ions are reduced by V(II) on a nonprecious metal catalyst, tungsten carbide. The mediators are regenerated in an electrolyzer at a cell voltage of 1.05 V, close to the theoretical potential difference (1.02 V) between Fe(III)/Fe(II) and V(II)/V(III). In a laboratory bench-scale plant, the energy consumption for the production of H2 from H2S is estimated to be 2.8 kWh Nm-3 H2 using Fe(III)/Fe(II) and V(II)/V(III) mediators and further reduced to about 0.5 kWh Nm-3 H2 when employing well-designed heteropolyacid/quinone mediators. OFEC presents a cost-effective approach for the simultaneous production of H2 and elemental sulfur from H2S, along with the complete removal of H2S from industrial processes. It also provides a practical platform for electrochemical reactions involving solid precipitation and organic synthesis.

Chemical Doping and O-Functionalization of Carbon-Based Electrode to Improve Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) holds promise for large-scale energy storage applications, despite its lower energy and power densities compared to advanced secondary batteries available today. Carbon materials are considered suitable catalyst electrodes for improving many aspects of the VRFB. However, pristine graphite structures in carbon materials are catalytically inert and require modification to activate their catalytic activity. Among the various strategies developed so far, O-functionalization and chemical doping of carbon materials are considered some of the most promising pathways to regulate their electronic structures. Building on the catalytic mechanisms involved in the VRFB, this concise review discusses recent advancements in the O-functionalization and chemical doping of carbon materials. Furthermore, it explores how these materials can be tailored and highlights future directions for developing more promising VRFBs to guide future research.

Computational Quantum Chemistry Insights into the Mechanism of VO2 + Reduction on Graphene-Based Electrodes

The identity of active sites for redox reactions within vanadium redox flow batteries (VRFBs) isstill controversial despite decades of research into the matter. Here, we use density functional theory to examine the premise of selected surface functional groups as active sites and provide mechanistic insights into the reaction pathway for the positive electrode reaction. The adsorption of electroactive species on phenol and carbene-like edge carbon sites was compared using model aromatic clusters. Phenol groups were not favorable sites for the chemisorption of VO2 + in either V-down or O-down approach In contrast, carbene-like edge carbon sites readily adsorbed VO2 + via an oxygen-down approach, mimicking gas-phase CO2 adsorption mechanisms. Subsequent steps to complete the reaction pathway are a series of proton adsorptions and reaction products desorption. The rate-determining step for a reaction pathway using an edge site is VO2+ desorption step with a Gibbs energy of activation of +84 kcal mol-1 .

Structure-activity correlation of thermally activated graphite electrodes for vanadium flow batteries

Thermal activation of graphite felts has proven to be a valuable technique for electrodes in vanadium flow batteries to improve their sluggish reaction kinetics. In the underlying work, a novel approach is presented to describe the morphological, microstructural, and chemical changes that occur as a result of the activation process. All surface properties were monitored at different stages of thermal activation and correlated with the electrocatalytic activity. The subsequently developed model consists of a combined ablation and damaging process observed by Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. Initially, the outermost layer of adventitious carbon is removed and sp² layers of graphite are damaged in the oxidative atmosphere, which enhances the electrocatalytic activity by introducing small pores with sharp edges. In later stages, the concentration of reaction sites does not increase further, but the defect geometry changes significantly, leading to lower activity. This new perspective on thermal activation allows several correlations between structural and functional properties of graphite for the vanadium redox couple, describing the importance of structural defects over surface chemistry.

Structural changes on the surface of graphite felts after thermal activation were monitored. Fundamental correlations led to a new model to explain the morphological evolution and its effects on the electrocatalytic activity in vanadium flow batteries.

Review of the Development of First-Generation Redox Flow Batteries: Iron-Chromium System

The iron-chromium redox flow battery (ICRFB) is considered the first true RFB and utilizes low-cost, abundant iron and chromium chlorides as redox-active materials, making it one of the most cost-effective energy storage systems. ICRFBs were pioneered and studied extensively by NASA and Mitsui in Japan in the 1970-1980s, and extensive studies on ICRFBs have been carried out over the past few decades. In addition, ICRFB is considered to be one of the most promising directions for cost-effective and large-scale energy storage applications, as its cost can theoretically be lower than that of zinc-bromine and all-vanadium RFBs, giving it the potential for large-scale promotion. With the resolution of problems such as hydrogen evolution and electrolyte intermixing, the ICRFB technology is moving out of the laboratory and striving for greater power and more stable industrialization requirements. This Review summarizes the history, development, and research status of key components (carbon-based electrode, electrolyte, and membranes) in the ICRFB system, aiming to give a brief guide to researchers who are involved in the related subject.

Rapid wet-chemical oxidative activation of graphite felt electrodes for vanadium redox flow batteries

To boost the performance of vanadium redox flow batteries, modification of the classically used felt electrodes is required to enable higher cycling performance and longer life cycles. Alternative approaches to the standard thermal oxidation procedure such as wet chemical oxidation are promising to reduce the thermal budget and thus the cost of the activation procedure. In this work we report a rapid 1 hour activation procedure in an acidified KMnO₄ solution. We show that the reported modification process of the felt electrodes results in an increase in surface area, density of oxygenated surface functionalities as well as electrolyte wettability, as demonstrated by N₂-physisorption, XPS, Raman spectroscopy as well as contact angle measurements. The activation process enables battery cycling at remarkably high current densities up to 400 mA cm⁻². Stable cycling at 400 mA cm⁻² over 30 cycles confirms promising stability of the reported activation procedure.

Schematic diagram of the K-GF fabrication process. Step 1: deposition of MnO_x layers onto the P-GF electrode surface using acidified KMnO₄ solutions. Step 2: removal of MnO_x layers using an acidified H₂O₂ solution to produce the K-GF electrode.