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Electrodeposited Sn–Cu@Sn dendrites have enhanced selectivity for CO2 reduction to formic acid on a gas diffusion electrode.

Much effort has been devoted to developing electrocatalysts applicable to anion exchange membrane water electrolyzers (AEMWEs). Among many candidates for oxygen evolution reaction, 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 anodes has detrimental effects on the stability of AEMWEs 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. The monolayer NiFe-LDH deposited on Ni foam showed much better AEMWE performance than conventional bulk NiFe-LDH due to better electrical conductivity and higher hydrophilicity. A high energy conversion efficiency of 72.6% and 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 as key factors for high-performance AEMWEs.

The electrochemical CO2 reduction reaction (CO2RR) has undergone great advances recently. The gas diffusion electrode (GDE) has been introduced to enhance productivity. However, the studies on catalysts on the GDE are scarce, especially for the production of liquid products. Here, we deposited Sn electrocatalysts electrochemically with dense tips (SnDT) on a gas permeable carbon cloth electrode. The prepared cathode had a uniform surface covered with SnDT catalysts. We achieved a high specific production rate of the CO2RR towards formate, 1 mmol m−2 s−1, at only −0.76 VRHE where partial current density and faradaic efficiency toward formate were −18.7 mA cm−2 and 62.5%, respectively, in neutral electrolyte. Formate productivity of 65 mg h−1 was achieved using a customized cell with 4 cm2 of the SnDT GDE. Formate was stably produced for 72 h without changes in faradaic efficiency and current density. The product distribution was affected by local pH. The effect of catalyst shape was also investigated by comparing with commercial Sn nanoparticles deposited on a carbon cloth electrode.

Minimizing the use of Pt catalysts in proton exchange membrane fuel cells (PEMFCs) is important, considering its high price and scarcity. Herein, we demonstrate novel catalysts for PEMFCs with exceptionally high mass activity and durability, in which block copolymer-based carbon supports are loaded with an ultra-small amount of Pt. The mass activity measured after 30 000 cycles of single cell tests was 0.81 A mgPt−1 at 0.9 V, which is the highest performance reported to date. The newly developed catalyst yielded nearly the same power density as that of the commercial Pt/C, even with 1/20 of the Pt usage. The carbon supports were prepared by carbonizing crosslinked domains of block copolymer particles selectively, resulting in mesoporous carbon particles with ∼25 nm pores. When Pt was deposited, thin carbon shells were formed encapsulating PtFe nanoparticles, catalyzing the oxygen reduction reaction efficiently with high durability.

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.