Facile Access to an Active γ‐NiOOH Electrocatalyst for Durable Water Oxidation Derived From an Intermetallic Nickel Germanide Precursor

Abstract

Identifying novel classes of precatalysts for the oxygen evolution reaction (OER by water oxidation) with enhanced catalytic activity and stability is a key strategy to enable chemical energy conversion. The vast chemical space of intermetallic phases offers plenty of opportunities to discover OER electrocatalysts with improved performance. Herein we report intermetallic nickel germanide (NiGe) acting as a superior activity and durable Ni‐based electro(pre)catalyst for OER. It is produced from a molecular bis(germylene)‐Ni precursor. The ultra‐small NiGe nanocrystals deposited on both nickel foam and fluorinated tin oxide (FTO) electrodes showed lower overpotentials and a durability of over three weeks (505 h) in comparison to the state‐of‐the‐art Ni‐, Co‐, Fe‐, and benchmark NiFe‐based electrocatalysts under identical alkaline OER conditions. In contrast to other Ni‐based intermetallic precatalysts under alkaline OER conditions, an unexpected electroconversion of NiGe into γ‐NiIIIOOH with intercalated OH−/CO32− transpired that served as a highly active structure as shown by various ex situ methods and quasi in situ Raman spectroscopy.

Evolving Highly Active Oxidic Iron(III) Phase from Corrosion of Intermetallic Iron Silicide to Master Efficient Electrocatalytic Water Oxidation and Selective Oxygenation of 5‐Hydroxymethylfurfural

“…[ 56 ] However, they closely resemble the Raman features observed for nickel‐, cobalt‐, and iron‐containing layered double hydroxides that form during the OER and can be assigned to the δ(FeO) (473 cm −1 ) and ν(FeO) (558 cm −1 ) vibration modes of edge sharing [FeO 6 ] octahedra (insets of Figure 2a). [ 30,57,58 ] The band at 240 cm −1 and the one at around 640 cm −1 are most likely caused by the frozen electrolyte and the FTO substrate, respectively. [ 59–61 ] The XANES spectrum of the pristine FeSi powder shows a slow rise in absorption and no edge maximum as expected due to the soft silicon ligands (Figure 2b).…”

“…[ 20,24 ] Recently, for the OER, non‐noble‐metal‐based intermetallics showed remarkable activities and it was revealed that, despite their inherent thermodynamic stability under ambient conditions, some undergo partial corrosion during the OER evolving active oxidized sites interconnected to the conductive precursor phase. [ 25–30 ] Also an intermetallic nickel silicide has been investigated which showed only moderate OER activities probably due to the absence of highly active iron sites. [ 31 ]…”

Perspective on intermetallics towards efficient electrocatalytic water-splitting

“…The nanoparticular intermetallic phases proved to be very efficient during alkaline OER exhibiting low overpotentials and prolonged stability of three weeks. 105 A chemical reduction approach was also utilized to derived phase pure FeSn 2 nanocrystals recorded one of the lowest η 10 of 197 mV (Tafel slope = 33 mV dec -1 ) with the stability of over 60 h. This system was found to be even superior to analogous Fe-, FeNi as well as noble-metal-based materials. 102 An up-todate table on the activity parameters of intermetallics has been provided in Table 2.…”

“…In comparison to pure Fe-based materials, the Fe 6 Ge 5 showed superior OER activity with η 10 of 221 mV and a Tafel slope of 32 mV dec -1 and was constantly mediated the reaction under high potential for more than a day. In continuation, we further developed synthetic strategies to obtain NiGe 105 and FeAs 149 from molecular SSPs. The nanoparticular intermetallic phases proved to be very efficient during alkaline OER exhibiting low overpotentials and prolonged stability of three weeks.…”

“…The activity was attributed to the structural flexibility of Ni sites triggered by the defected structure with ionic intercalation of OH − /CO 3 2− between the large interplanar spacing of γ-NiOOH, In both cases, the transformation resulted in higher ECSA values, and better electronic conductivities to promote favorable OER kinetics with improved charge transfer properties to facilitate the OER. 105 Of late, we reported an intermetallic Fe 6 Ge 5 as a novel alkaline OER precatalyst forming first an in-situ metastable core-shell structure that slowly collapses in prolonged OER conditions finally to form a Fe III O x H y . 98 We recently investigated CoSn 2 for overall water splitting where after alkaline OER, CoSn 2 underwent a slow but rather complete structural transformation (Figure 11).…”