Bi‐Functional Iron‐Only Electrodes for Efficient Water Splitting with Enhanced Stability through In Situ Electrochemical Regeneration

Abstract

are the indirect conversion of solar into chemical energy using a photovoltaic (PV)-electrolyzer [ 9,10 ] system and its direct conversion with a photoelectrochemical (PEC) cell. [11][12][13] The International Energy Agency identifi ed a key obstacle to the widespread implementation of water splitting technologies to be the development of cheap and active materials to catalyze both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which operate under identical and mild conditions with high efficiency and stability. [ 14 ] The most active catalysts of the HER and OER contain precious metals: Pt and RuO 2 /IrO 2 , respectively, [15][16][17][18] but there is no real possibility to scale up these materials for global demand due to their scarcity. A sustainable and global process will require the use of inexpensive, widely abundant and nontoxic materials. Figure S1 (Supporting Information) shows the recent cost of transition metal resources and their relative crustal abundance. Rare, high cost elements are suitable for use in luxury items such as catalytic converters in automobiles. Even medium cost-abundance catalysts such as the much investigated HER catalysts, Ni-Mo alloys, [ 19,20 ] as well as Ni 2 P, [ 21 ] CoP, [ 22 ] and MoS 2 , [23][24][25][26] and the OER catalysts, cobaltphosphate (CoP i ), [ 27 ] nickel-borate (NiB i ), 25 and (mixed) metal oxide-hydroxides [29][30][31][32] can still suffer from scalability issues. For example, cobalt constitutes the largest cost in most Li-ion rechargeable batteries (LiCoO 2 ), which has contributed to significant research into its replacement with iron (LiFePO 4 ) and manganese (LiMnO 2 ) analogs. [ 33 ] Approaches to overcome the use of more expensive metals in catalysis include the use of ultra-low concentrations of the active catalyst embedded in a lower-cost matrix [ 34 ] or in this case the identifi cation of active species which contain only Earth-abundant elements. [ 30 ] Only three transition metals (titanium, manganese, and iron) are truly abundant in Earth's crust (the primary source of metal ores) and within this group, iron is by far the most plentiful. Hence iron is arguably the most attractive element on which to base sustainable catalysts, due to its effectively inexhaustible supply, low toxicity, and negligible environmental impact. Despite this, iron-based catalysts have been relatively underreported and underused, probably due to their notoriety for corrosion.Iron-based electrodes for either the HER or the OER have been reported, [35][36][37][38][39][40] but bi-functionally active iron-only materials in water splitting are unknown. For example, FeP is described as a promising high activity noble-metal-free material Scalable and robust electrocatalysts are required for the implementation of water splitting technologies as a globally applicable means of producing affordable renewable hydrogen. It is demonstrated that iron-only electrode materials prove to be active for catalyzing both proton reduction and water oxidation in alkali...

Electro‐Oxidation of Ni42 Steel: A Highly Active Bifunctional Electrocatalyst

“…Manganese oxides based OER electrocatalysts as a model system for the oxygen evolving complex of photosystem II were frequently studied and based on head space measurements the Faradaic efficiency amounted to 78% after 90 min of OER in pH controlled Na 2 SO 4 [60] . The charge to oxygen conversion rates of recently developed full water splitting electrocatalysts determined in alkaline solution ranged between 95.8 and 100% [109,110,112] . A comparison of the OER properties of Ni42-300 at pH 7 with the corresponding characteristics of noble metal containing catalysts is a sine qua for an in-depth evaluation of the electrocatalytic characteristics of our material.…”

From a Molecular 2Fe‐2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting

“…[2] Thec urrent state-of-the-art catalysts that perform both the OER and HER are based on noble metals,which are unsuitable for practical large-scale applications owing to their scarcity and high cost. [5] Among these are cheap transitionmetal-based materials,including oxides, [2a,c-f] chalcogenides, [6] and pnictides, [6b,7] which have been explored as alternative OER, HER, and overall water-splitting catalysts. [5] Among these are cheap transitionmetal-based materials,including oxides, [2a,c-f] chalcogenides, [6] and pnictides, [6b,7] which have been explored as alternative OER, HER, and overall water-splitting catalysts.…”

“…[6] In early 2017, Gao et al reported using FeSe 2 for the OER with ac onsiderably low overpotential of 330 mV at ac urrent density of 10 mA cm À2 . [6] In early 2017, Gao et al reported using FeSe 2 for the OER with ac onsiderably low overpotential of 330 mV at ac urrent density of 10 mA cm À2 .…”

Highly Active Three‐Dimensional NiFe/Cu2O Nanowires/Cu Foam Electrode for Water Oxidation

“…In 2015, Shi and co-workers [17] reported aN i-Fe layered double hydroxide (LDH) on three-dimensional electrochemically reduced graphene oxide that exhibited as mall overpotential of h = 259 mV at 10 mA cm À2 and alow Tafel slope of 39 mV decade À1 . [51,52] Herein we report that Cu 2 On anowires (NWs),f abricated on aC F( denoted as Cu 2 ON Ws/CF) by an eco-friendly and facile electrochemical method, were loaded with an advanced NiFe electrocatalyst to form ah ighly efficient three-dimensional anode (NiFe/Cu 2 ON Ws/CF;S cheme 1). In particular,m acroporous Cu foam (CF) has great advantages owing to its low cost and high electrical conductivity.…”