High Performance Anion Exchange Membrane Water Electrolyzer Using Monolayer Nickel-Iron Layered Double Hydroxide As Anode Catalysts

Abstract

Much efforts have been devoted to developing electrocatalysts applicable to anion exchange membrane water electrolyzer (AEMWE). AEMWE operates in basic condition, which allows non-noble metal-based catalysts to be used, while its membrane electrode assembly (MEA) design allows higher current density compared to conventional alkaline water electrolyzer (AWE). Among many candidates for oxygen evolution reaction (OER), NiFe layered double hydroxide (LDH)-based electrocatalysts show the highest activity in an alkaline medium. Unfortunately, the poor electrical conductivity of NiFe-LDH limits its potential as an electrocatalyst, which was often solved by hybridization with conductive carbonaceous materials. However, we find that using carbonaceous materials for anode has detrimental effects on the stability of AEMWE at industrially relevant current densities.In this work, a facile monolayer structuring is suggested to overcome low electrical conductivity and improve mass transport without using carbonaceous materials. Bulk NiFe-LDH (B-NiFe-LDH) with multiple cationic layers was synthesized by following a conventional co-precipitation method, while monolayer NiFe-LDH (M-NiFe-LDH) was prepared using a similar method, but with formamide present in solvent. While Fe 2p and O 1s did not show noticeable difference, M-NiFe-LDH had larger Ni3+ peak than B-NiFe-LDH, indicating that M-NiFe-LDH has more Ni species in NiOOH environment rather than Ni(OH)2 environment. Here, more NiOOH phase in M-NiFe-LDH caused higher conductivity, leading to higher specific activity.The effect of electrical conductivity was further investigated by mixing the catalysts with various amounts of carbon materials. When carbon black (Vulcan XC-72) was loaded together with B-NiFe-LDH, the OER activity increased from the absence of carbon up to carbon-to-catalyst weight ratio of 0.1. Further addition of carbon did not increase the current density. On the other hand, the OER activity barely changed upon carbon addition for M-NiFe-LDH. As only Ni3+/4+ species are known to have OER activity, facile electron transfer from Ni2+ sites to Ni3+/4+ is important. Mixing carbon materials with B-NiFe-LDH provided conductive networks into previously “electron-unreachable” regions, which was also confirmed by the increase in the size of Ni oxidation peaks upon carbon addition. On the other hand, the contact between glassy carbon and M-NiFe-LDH catalyst was sufficient to have efficient electron transfer without carbon.The M-NiFe-LDH deposited on Ni foam (NF) showed much better AEMWE performance than B-NiFe-LDH, due to better electrical conductivity and higher hydrophilicity. When the M-NiFe-LDH was loaded on carbon paper (CP) instead of Ni foam, water electrolysis performance was enhanced, but stability was greatly lowered. Similar to the M-NiFe-LDH, the CP substrate showed the higher initial performance than the NF substrate for B-NiFe-LDH, but the B-NiFe-LDH/CP stopped operating eventually even at milder conditions. Overall, using M-NiFe-LDH/NF electrode, the high energy conversion efficiency of 72.6% and an outstanding stability at a current density of 1 A cm-2 over 50 h could be achieved without carbonaceous material.This work highlights electrical conductivity and hydrophilicity of catalysts in membrane-electrode-assembly (MEA) as key factors for high performance AEMWE.