Structure evolution and durability of Metal-Nitrogen-Carbon (M = Co, Ru, Rh, Pd, Ir) based oxygen evolution reaction electrocatalyst: A theoretical study.

Developing low-cost, high activity and stability oxygen evolution reaction (OER) catalysts is significantly important but still challenging for water electrolyzers. In this work, we calculated the OER activity and stability of Metal-Nitrogen-Carbon (MNC, M = Co, Ru, Rh, Pd, Ir) based electrocatalyst with different structures (MN4C8, MN4C10, MN4C12) using density functional theory (DFT) method. These electrocatalysts were divided into three groups based on the value of ΔG*OH, that is ΔG*OH > 1.53 eV (PdN4C8, PdN4C10, PdN4C12), ΔG*OH < 1.23 eV (RuN4C8, RuN4C10, RuN4C12, CoN4C8, CoN4C10) and 1.23 eV < ΔG*OH < 1.53 eV (RhN4C8, RhN4C10, RhN4C12, IrN4C8, IrN4C10, IrN4C12, CoN4C12), and ΔG*OH determine whether the structure evolution will appear. The results proved that MNC (M = Rh, Ir) with 1.23 eV < ΔG*OH < 1.53 eV shows higher OER activity due to moderate binding energy between reaction intermediates and MNC. Furthermore, these catalysts could maintain MNC structure without further oxidation and structural evolution under working conditions (high temperature, dynamic condition, local electric field and strong specific adsorption), therefore show excellent stability. However, MNC electrocatalyst with ΔG*OH > 1.53 eV or ΔG*OH < 1.23 eV revealed less stability under working conditions, due to their low intrinsic stability or structural evolution under working conditions, respectively. In conclusion, we proposed a comprehensive evaluation method for MNC electrocatalysts by taking ΔG*OH as the screening criterion for OER activity and stability, as well as ΔEb under working condition as descriptor of stability. This is of great significance for the design and screening of ORR, OER and HER electrocatalysts under working conditions.

Uneven phosphoric acid interfaces with enhanced electrochemical performance for high-temperature polymer electrolyte fuel cells.

Ultrahigh mass transport resistance and excessive coverage of the active sites introduced by phosphoric acid (PA) are among the major obstacles that limit the performance of high-temperature polymer fuel cells, especially compared to their low-temperature counterparts. Here, an alternative strategy of electrode design with fibrous networks is developed to optimize the redistribution of acid within the electrode. Via structural tailoring with varied electrospinning parameters, uneven migration of PA with dispersed droplets is observed, subverting the immersion model of conventional porous electrode. Combining with experimental and calculation results, the microscaled uneven PA interfaces could not only provide extra diffusion pathways for oxygen but also minimize the thickness of PA layers. This electrode architecture demonstrates enhanced electrochemical performance of oxygen reduction within the PA phase, resulting in a 28% enhancement of the maximum power density for the optimally designed electrode as cathode compared to that of a conventional one.

Effect of an external electric field, aqueous solution and specific adsorption on segregation of PtML/MML/Pt(111) (M = Cu, Pd, Au): a DFT study.

The oxygen reduction reaction (ORR) that occurs on the outermost layer of electrocatalysts is significantly affected by the composition and structure of the electrocatalysts. During the preparation of PtM alloy electrocatalysts, high-temperature annealing in an inert or reducing atmosphere could promote the segregation of M toward the core, forming a highly active Pt-skin structure. However, under fuel cell operating conditions, the adsorption of oxygen-containing groups could stimulate the easily dissolved M to segregate to the surface, reducing the activity and stability of the electrocatalysts. In this work, we conducted segregation energy calculation of PtM (M = Cu, Pd, Au) electrocatalysts under specific adsorption (SA), aqueous solution (AS) and an external electric field (EEF) with a density functional theory method. It was found that different factors have different effects on the segregation energy: ΔΔESA ≫ ΔΔEEEF > ΔΔEAS. The coupling effects have also been considered and compared: ΔΔESA+EEF > ΔΔESA+AS > ΔΔEEEF+AS. When including all three factors, the change of segregation energy could reach 1.63 eV. Therefore, operating conditions have a noteworthy influence on the segregation behavior of PtM ORR electrocatalysts, which should be considered in the further design of PtM ORR electrocatalysts.

Size-dependence of the electrochemical performance of Fe-N-C catalysts for the oxygen reduction reaction and cathodes of direct methanol fuel cells.

Comprehension of the structure-activity relationship is of great importance for the rational design of electrocatalysts for the oxygen reduction reaction (ORR). Herein, Fe-N-C catalysts obtained from zeolitic imidazolate framework-8 (ZIF-8) with a tunable size ranging from 30 to 400 nm are precisely synthesized. Structural investigation indicates that the catalyst with smaller size possesses a higher proportion of mesopores originating from particle stacking, which leads to enhanced catalyst utilization and accelerated mass transport. The size effect of the catalyst on ORR activity is systematically investigated by rotation disk electrode (RDE) and direct methanol fuel cell (DMFC) tests. The electrochemical performance of the Fe-N-C catalyst is found to be increased with the reduction of its particle size. The correlation among size, mesoporosity and catalyst performance is discussed, giving new inspiration for the development of rational design strategies of non-precious metal catalysts.

A novel isophorone wastewater treatment technology-wet electrocatalytic oxidation and its degradation mechanism study.

How to solve the poisoning and loss of catalysts in catalytic wet air oxidation (CWAO) process remains a great challenge. In this work, an electric field was introduced into wet air oxidation (WAO) process for the efficient degradation of isophorone (IP) wastewater for the first time, named as wet electrocatalytic oxidation (WEO) process. Different composite electrodes including Ti/PbO2, Ti/Pt, Ti/Ru-Ir and Ti/Ir-Ta electrode were selected as the anodes of WEO technique and the results showed that the total organic carbon (TOC) removal via WEO process with PbO2 anode (89.56 %) was much higher than CWAO equipped with noble metal catalyst (Ru/TiZrO4, 75.0 %). Additionally, the current efficiency of WEO process was 85.6 %, which was significantly better than that of EO process (12.1 %). A response surface methodology was applied to elucidate the effects of reaction conditions on IP degradation. Analysis of response surface model showed TOC removal were markedly affected (p ≤ 0.01) by the reaction time (t), temperature (T), current density (ID), T2 and ID2, and also determined (p ≤ 0.05) by the interactions of T with t and ID respectively. In addition, a synergistic effect was proved to take place in WEO process with synergistic effect factor f of 1.2 at optimized conditions. As an advanced wastewater treatment technology, WEO integrates the advantages of both electro-catalytic oxidation (EO) and WAO.

The mechanism and activity of oxygen reduction reaction on single atom doped graphene: a DFT method.

Heteroatom doped graphene as a single-atom catalyst for oxygen reduction reaction (ORR) has received extensive attention in recent years. In this paper, the ORR activity of defective graphene anchoring single heteroatom (IIIA, IVA, VA, VIA and VIIA) was systematically investigated using a dispersion-corrected density functional theory method. For all of the 34 catalysts, 14 of which were further analyzed, and the Gibbs free energy of each elementary reaction was calculated. According to the scaling relationship between ΔG OOH* and ΔG OH*, we further analyzed the rate-determining step of the remaining 20 catalysts. The results show that when the ORR reaction proceeds in the path O2 → OOH → O → OH → H2O, the reaction energy barriers are lower than 0.8 eV for Te-SV, Sb-DV, Pb-SV, Pb-DV, As-SV, As-DV, B-SV, Sn-SV and N-SV. Our result provides a theoretical basis for further exploration of carbon-based single-atom catalysts for ORR.

Monomorphic platinum octapod and tripod nanocrystals synthesized by an iron nitrate modified polyol process.

Monomorphic Pt octapod and tripod nanocrystals have been successfully synthesized by an iron nitrate modified polyol process, in which iron nitrate has been proven to be vitally important for slowing down the reduction rate of Pt precursors.

Size-controllable synthesis of monodispersed SnO2 nanoparticles and application in electrocatalysts.

Size-controllable tin oxide nanoparticles are prepared by heating ethylene glycol solutions containing SnCl(2) at atmospheric pressure. The particles were characterized by means of transmission electron microscopic (TEM), X-ray diffraction (XRD) studies. TEM micrographs show that the obtained material are spherical nanoparticles, the size and size distribution of which depends on the initial experimental conditions of pH value, reaction time, water concentration, and tin precursor concentration. The XRD pattern result shows that the obtained powder is SnO(2) with tetragonal crystalline structure. On the basis of UV/vis and FTIR characterization, the formation mechanism of SnO(2) nanoparticles is deduced. Moreover, the SnO(2) nanoparticles were employed to synthesize carbon-supported PtSnO(2) catalyst, and it exhibits surprisingly high promoting catalytic activity for ethanol electrooxidation.

An improved palladium-based DMFCs cathode catalyst.

A novel carbon-supported palladium-rich Pd3Pt1/C catalyst prepared by a modified polyol process showed a better cell performance than Pt/C in direct methanol fuel cells, which may be attributed to palladium's inactivity to methanol electro-oxidation while exhibiting good performance to oxygen reduction reaction.