Nowadays, the overuse of fossil fuels has caused a serious energy and environmental crisis. Hydrogen is a promising energy carrier in a sustainable society because of its high energy density and clean combustion products. Water electrolysis technology making use of water to produce green hydrogen shows the advantages of high energy efficiency and a clean reaction process. However, the shortage of freshwater resources limits the development of water electrolysis technology, especially in areas where fresh water is limited. In this regard, seawater occupies more than 96% of the water resource on earth, and seawater electrolysis is a promising method for sustainable hydrogen production. However, chloride ions (Cl-) in seawater cause challenges of side reactions and corrosion, resulting in the low selectivity and poor stability of electrocatalysts, which hinders the application of seawater electrolysis technology.
Recently, Professor Bilu LIU’s team at Tsinghua SIGS has developed a corrosion-resistant RuMoNi catalyst for seawater electrolysis. The catalyst is prepared by the hydrothermal method followed by electrochemical activation. RuMoNi electrocatalyst is an array of nanorods with porous surfaces. Transmission electron microscopy showed that the catalyst was composed of a conductive Ni4Mo substrate, RuO2/NiOOH active phase, and NiMoO4 corrosion-resistant layer.
Figure 1. Design principle and microscopic characterization of the RuMoNi electrocatalyst
The researchers investigated the OER performance of the prepared catalyst in a three-electrode cell. In an electrolyte composed of 1.0 M KOH and seawater, the RuMoNi electrocatalyst only needs an overpotential of 245 mV to deliver a current density of 100 mA/cm2 and the electrocatalyst reaches a current density of 1000 mA/cm2 at 1.7 V vs. RHE. The RuMoNi electrocatalyst operates well in alkaline, saline, and seawater electrolytes, showing its universality in different electrolytes. Results show that the RuMoNi electrocatalyst has near 100% OER selectivity in seawater electrolytes and it has a lower charge transfer resistance than nickel foam (Ni foam) and RuO2. Based on Δη/Δlog|j| (Rη/j), a reliable criterion for kinetic study of electrocatalysis under a wide current range, RuMoNi electrocatalyst shows faster reaction kinetics than RuO2.
Figure 2. OER performance in alkaline seawater at high current densities
Seawater is a highly corrosive environment which poses a significant challenge to the durability of electrocatalysts. Researchers study the long-term stability of the catalyst, the RuMoNi catalyst can operate stably for over 3000 h at a current density of 500 mA/cm2. The voltage decay rate (DV) of the catalyst defined by is 0.64 μV/h, which meets the standard set by the US Department of Energy (1 μV/h). In the electrolyte of high temperature and high salt concentration, no obvious performance decay occurred during the stability test. The above results show that the RuMoNi catalyst has excellent corrosion resistance and stability.
Figure 3. Durability tests of the RuMoNi electrocatalyst in different practical conditions
The researchers also studied the mechanism for stability and selectivity of the RuMoNi electrocatalyst. The contact angle tests indicate that RuMoNi electrocatalyst has super-hydrophilic properties, which decreases the bubble adhesion force. Tafel plots show that the corrosion potential of RuMoNi electrocatalyst is higher than Ni foam. Analysis of the electrolyte component shows that the atomic concentrations of Ru and Mo remain constant during the 150-h stability test at a 500 mA/cm2, indicating the excellent corrosion resistance of RuMoNi electrode at high current densities. The Researchers found that Mo on the electrode surface is +6 valence and exists in NiMoO4. The MoO42- absorbs on the electrode surface while the potential Increases and repels Cl- through electrostatic repulsion, thus inhibiting the electrode corrosion.
Figure 4. Corrosion-resistant mechanism of the RuMoNi electrocatalyst
Using the RuMoNi electrocatalyst as the anode and cathode, the researchers assembled an alkaline seawater anion exchange membrane (AEM) electrolyzer. The AEM electrolyzer can achieve a current density of 1 A/cm2 at a voltage of 1.72 V, and it can operate stably in an alkaline seawater electrolyte for 240 h at a current density of 0.5 A/cm2. The RuMoNi AEM electrolyzer is of high activity, H2 production rate, and stability, and is superior to the state-of-the-art AEM seawater electrolyzers.This improved performance suggests the possibility of industrialized AEM alkaline seawater electrolysis.
Figure 5. AEM electrolyzer performance in alkaline seawater
The research has recently been published in Nature Communications in an article entitled “A corrosion-resistant RuMoNi catalyst for efficient and long-lasting seawater oxidation and anion exchange membrane electrolyzer.” The corresponding authors are Bilu LIU and Qiangmin YU from Tsinghua SIGS. The first author of this paper is Xin KANG, PhD candidate at SIGS. Hui-Ming CHENG, Wencai REN, Zhibo LIU, and Chenghua SUN are co-authors. The research was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology of People’s Republic of China, the Guangdong Science and Technology Department, and the Science, Technology and Innovation Commission of Shenzhen Municipality.
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Written by Tianghao Zhang & Xin Kang
Edited by Alena Shish & Yuan Yang