An efficient bifunctional two-component catalyst for oxygen reduction and oxygen evolution in reversible fuel cells, electrolyzers and rechargeable air electrodes

Abstract

We report on a non-precious, two-phase bifunctional oxygen reduction and evolution (ORR and OER) electrocatalyst with previously unachieved combined roundtrip catalytic reactivity and stability for use in oxygen electrodes of unitized reversible fuel cell/electrolyzers or rechargeable metal-air batteries. The combined OER and ORR overpotential, total, at 10 mA cm À2 was a record low value of 0.747 V. Rotating Ring Disk Electrode (RRDE) measurements revealed a high faradaic selectivity for the 4 electron pathways, while subsequent continuous MEA tests in reversible electrolyzer cells confirmed the excellent catalyst reactivity rivaling the state-of-the-art combination of iridium (OER) and platinum (ORR).Electrochemical energy storage based on the interconversion of renewable electricity and molecular fuels (solar fuels) and solid state structures (aqueous metal-air cells) invariably involves the oxygen/water redox system supplying and consuming water, protons, electrons and oxygen. This is why efficient catalysts for the oxygen evolution reaction (OER:are critical. [1][2][3][4] Combining the two functionalities in one single bifunctional oxygen redox electrode would greatly simplify the design of energy conversion devices or enhance the mobility and power-to-weight ratio. This plays an important role in spacecraft, aircraft, and ground transportation applications. Active oxygen redox catalysts such as IrO 2 or Pt are rare and expensive, which is why the development of efficient non-precious oxygen catalysts is of interest. 5-10 The layered double hydroxide of Ni and Fe (''NiFe-LDH'') is known to be one of the most active non-noble OER catalysts in alkaline solution. 5,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] In contrast, nitrogendoped carbon materials are promising non-precious candidates for the ORR. [27][28][29][30] Rather than exploring suitable bifunctional catalytic surface sites, or designing two distinct active sites on the same substrate, we propose the facile heterogeneous mixing of either material to obtain a two-phase bifunctional catalyst. This was shown for noble metal catalysts of iridium and platinum. 31,32 Recently, non-precious metal mixtures of Mn-Co oxides and carbon nanotubes have been tested. 33 Realizing that a twocomponent surface is necessary for highly active bifunctional catalysts, 34,35 in this contribution, we designed two-component NiFe-LDH -Fe-N-C catalysts resulting in today's most efficient bifunctional oxygen electrodes in 0.1 M KOH. A mutual improving effect between the two components in the two-phase structure with distinct neighbouring active sites appears key to the observed performance. Using a fast microwave-assisted solvothermal one-pot synthesis route (Fig. S1, ESI †), we prepared a carbon-supported crystalline NiFe-LDH catalyst material in a Ni/Fe ratio of B3.6 (Ni 0.78 Fe 0.22 (OH) x ) and a metal loading of B37 wt%.The X-ray diffraction (XRD) pattern (Fig. 1) is consistent with the data-based reflections of layered double hydroxides (JCPDS: 00-01...

NiCoFe‐Layered Double Hydroxides/N‐Doped Graphene Oxide Array Colloid Composite as an Efficient Bifunctional Catalyst for Oxygen Electrocatalytic Reactions

“…This phenomenon, also known as "house of cards" structure, was widely proposed in 2D nanosheets with distinguished electrical property between laminates and edges. [34] Therefore, the array-like Ni 2 Co I-II Fe-LDH/N-GO can be beneficial to the gas evolution reaction of OER process [11] and gas consumption reaction like ORR process. The morphologies in Figure 2c,d confirmed that Ni 2 Co III Fe-LDH/N-GO was in analogy to nanoarray material, where N-GO behaved as substrate and the vertically grown Ni 2 Co III Fe-LDH played the crucial role in electrocatalysis.…”

From Food Waste to Efficient Bifunctional Nonprecious Electrocatalyst

“…The Tafel analysis depicted in Figure a reveals a slope of 60–62 mV/dec, indicating a good control over O 2 mass transport and excluding possible artefacts often observed in thick film electrodes . Very interesting results have been reported for Fe‐based electrocatalysts based on either bulk or electrodeposited Fe obtained by pyrolysis of appropriate iron precursors or solvothermal approaches also using different C‐based supports, and for mixtures of Fe and other transition metals . However, such a high activity displayed by an iron‐based catalyst Fe(np)/nCs is unprecedented.…”

NiFe‐Based (Oxy)hydroxide Catalysts for Oxygen Evolution Reaction in Non‐Acidic Electrolytes

“…[16,51,58,[64][65][66][67][68] This method requires a certain temperature to start the hydrolysis process and is performed in autoclaves. Urea, dimethylformamide (DMF) and hexamethylenetetramine (HMT) are typical hydrolysis agents used.…”