Enhanced oxygen evolution reaction kinetics through biochar-based nickel-iron phosphides nanocages in water electrolysis for hydrogen production

Highly-efficient and stable non-noble metal electrocatalysts for overcoming the sluggish kinetics of oxygen evolution reaction (OER) is urgent for water electrolysis. Biomass-derived biochar has been considered as promising carbon material because of its advantages such as low-cost, renewable, simple preparation, rich structure, and easy to obtain heteroatom by in-situ doping. Herein, Ni₂P–Fe₂P bimetallic phosphide spherical nanocages encapsulated in N/P-doped pine needles biochar is prepared via a simple two-step pyrolysis method. Benefiting from the maximum synergistic effects of bimetallic phosphide and biochar, high conductivity of biochar encapsulation, highly exposed active sites of Ni₂P–Fe₂P spherical nanocages, rapid mass transfer in porous channels with large specific surface area, and the promotion in adsorption of reaction intermediates by high-level heteroatom doping, the (Ni_0.75Fe_0.25)₂P@NP/C demonstrates excellent OER activity with an overpotential of 250 mV and a Tafel slope of 48 mV/dec at 10 mA/cm² in 1 M KOH. Also it exhibits a long-term durability in 10 h electrolysis and its activity even improves during the electrocatalytic process. The present work provides a favorable strategy for the inexpensive synthesis of biochar-based transition metal electrocatalysts toward OER, and improves the water electrolysis for hydrogen production.     

Two-dimensional numerical pore-scale investigation of oxygen evolution in proton exchange membrane electrolysis cells

Oxygen blocking the porous transport layer (PTL) increases the mass transport loss, and then limits the high current density condition of proton exchange membrane electrolysis cells (PEMEC). In this paper, a two-dimensional transient mathematical model of anode two-phase flow in PEMEC is established by the fluid volume method (VOF) method. The transport mechanism of oxygen in porous layer is analyzed in details. The effects of liquid water flow velocity, porosity, fiber diameter and contact angle on oxygen pressure and saturation are studied. The results show that the oxygen bubble transport in the porous layer is mainly affected by capillary pressure and follows the transport mechanism of ‘pressurization breakthrough depressurization’. The oxygen bubble goes through three stages of growth, migration and separation in the channel, and then be carried out of the electrolysis cell by liquid water. When oxygen breaks through the porous layer and enters the flow channel, there is a phenomenon that the branch flow is merged into the main stream, and the last limiting throat affects the maximum pressure and oxygen saturation during stable condition. In addition, increasing the liquid water velocity is helpful to bubble separation; changing the porosity and fiber diameter directly affects the width of pore throat and the correlative capillary pressure; increasing porosity, reducing fiber diameter and contact angle can promote oxygen breakthrough and reduce the stable saturation of oxygen.     

Microscopic high-speed video observation of oxygen bubble generation behavior and effects of anode electrode shape on OER performance in alkaline water electrolysis

An important difficulty associated with alkaline water electrolysis is the rise in anode overpotential attributable to bubble coverage of the electrode surface. For this study, a system with a high-speed video camera was developed, achieving in-situ observation of bubble generation on an electrode surface, monitoring an area of 1.02 mm² at 6000 frames per second. The relation between polarization curve (current density up to 3.0 A cm^?2) and oxygen bubble generation behavior on nickel electrodes having cylindrical wires and rectangular wires of different sizes (100–300 μm) was clarified. The generated bubbles slide upward, contacting the electrode surface and detaching at the top edge. Observations indicate that small electrodes have short bubble residence time and thin bubble covering layer on the electrode. As a result, the small electrode diameter contributes to smaller overpotential at high current density.     

三维催化电解法处理丙烯腈废水的探讨

丙烯腈是重要的化工原料,在合成纤维、塑料等领域有着广阔的应用前景。但其生产过程污染严重,污水有毒有害,治理难度高。采用传统污水处理方式处理此类污水效果不佳。以三维催化电解法为主要思路,研究替代传统生化污水处理方式处理丙烯腈废水的可行性及相关技术参数,探索了处理此类石油化工污水的新途径。     

浅谈不同给液方式在铜电解车间的应用

不同给液方式对铜电解过程中有重要的影响,不同的循环方式会影响槽内温度分布、电解液成分及阳极泥沉降等,因此,根据铜电解生产不同情况的需要,分析对比了多种给液方式在贵冶电解车间的应用,总结了这几种给液方式的优缺点和适用条件。     

Clean hydrogen production in a full biological microbial electrolysis cell

The recent interest in microbial electrolysis cell (MEC) technology has led the research platform to develop full biological MECs (bioanode-biocathode, FB-MEC). This study focused on biohydrogen production from a biologically catalyzed MEC. A bioanode and a biocathode were initially enriched in a half biological MFC (bioanode-abiocathode, HB-MFC) and a half biological MEC (abioanode-biocathode, HB-MEC), respectively. The FB-MEC was established by transferring the biocathode of the HB-MEC and the bioanode of the HB-MFC to a two-chamber MEC. The FB-MEC was operated under batch (FB-MEC-B) and recirculation batch (FB-MEC-RB) modes of operation in the anodic chamber. The FB-MEC-B reached a maximum current density of 1.5 A/m² and the FB-MEC-RB reached a maximum current density of 2.5 A/m² at a similar applied voltage while the abiotic control system showed the maximum of 0.2 A/m². Hydrogen production rate decreased in the FB-MEC compared to that of the HB-MEC. However, the cathodic hydrogen recovery increased from 42% obtained in the HB-MEC to 56% in the FB-MEC-B and 65% in the FB-MEC-RB, suggesting the efficient oxidation and reduction rates in the FB-MEC compared to the HB-MEC. The onset potential for hydrogen evolution reaction detected by linear sweep voltammetry analysis were −0.780 and −0.860 V vs Ag/AgCl for the FB-MEC-RB and the FB-MEC-B (−1.26 for the abiotic control MEC), respectively. Moreover, the results suggested that the FB-MEC worked more efficiently when the biocathode and the bioanode were enriched initially in half biological systems before transferring to the FB-MEC compared to that of the simultaneously enriched in one system.     

Three-dimensional coupling numerical simulation of two-phase flow and electrochemical phenomena in alkaline water electrolysis

Alkaline water electrolysis has the advantage of scalability for industrial-scale mass production of hydrogen; however, it is operated under a lower current density than other methods of water electrolysis because a high overpotential resulting from ion transport limitations will occur at high current density. Bubble dynamics can both prevent ion transport by its existence and accelerate it by bubble-induced flow. In this study, we conduct three-dimensional coupling numerical simulations of two-phase flow and electrochemical phenomena to elucidate the mechanisms by which microscale bubble dynamics influence ion transport and the cell overpotential. We find that the flow induced by rising microbubbles enhances ion transport to the anode and suppresses the cell overpotential. Moreover, bubble atomization further suppresses the overpotential because smaller bubbles approach the anode more closely than larger ones and accelerate ion transport to the anode surface.     

Facile synthesis of Ni doped CoWO4 nanoarrays grown on nickel foam substrates for efficient urea oxidation

Urea electrolysis is a promising technology for hydrogen production, which can alleviate environmental pollution of urea-rich wastewater. It's worth noting that electrochemistry activity can be significantly improved by reasonably regulating the electron configuration around the active site for the doped materials. In this work, a series of well-tuned Ni doped CoWO₄ nanoarrays on Ni foam supports have been prepared through a typical hydrothermal approach for the first time. Moreover, the resulting Ni–CoWO₄-2 material significantly promotes urea oxidation performance with an applied potential of 1.35 V at 50 mA cm^?2, which is lower than that of water oxidation reaction (1.60 V). Density functional theory results suggest that the Ni doped CoWO₄ has larger urea adsorption energy compared with CoWO₄ and the CO(NH₂)₂ molecule is strongly adsorbed on surface of Ni doped CoWO₄, which is beneficial to accelerate the kinetics of the reaction and improve the electrocatalytic activity of the urea electrolysis.     

Discriminating between the effect of pulse width and duty cycle on the hydrogen generation performance of 3-D electrodes during pulsed water electrolysis

The present study investigates the effect of applying voltage and current pulses during alkaline water electrolysis using 3-D Ni-based electrodes. The pulses had a square shape and alternated hydrogen production and resting time. When voltage pulses were applied, it was observed that the current at on-time was systematically higher than the current during DC electrolysis. However, during off-time, a change in polarization was observed, which decreased the overall voltage pulse performance. For pulses with a 50% duty cycle and a pulse width of 1 ms, the current response was mainly capacitive and almost no hydrogen production occurred. Current pulses on the other hand were proven to be much more promising in improving the energetic process efficiency. In that case, a pulse period of 2 ms resulted in an overpotential reduction of 17% for a 50% duty cycle. This reduction further increased to 28% when decreasing the duty cycle to 20%. Finally, in all cases where faradaic processes were dominant, applying a forced electrolyte flow was shown to be beneficial.