Transition Metals and Stainless Steel for Low Cost Direct Water Splitting

The research on materials for the water splitting reactions is in focus for several years...<br/>the Oxygen evolution reaction (OER) and the Hydrogen evolution reaction (HER),<br/>We review the recent process of the global research on transition metal phosphide (TMP) based heterostructures for efficient water splitting.<br/><br/>To reduce the cost of electrochemical water splitting, two strategies to prepare robust and abundant 3D electrodes:<br/>a) the design and preparation of highly active catalysts, and<br/>b) the use of a cheaper substrate stainless steel for the preparation of 3D electrodes.<br/><br/>Both strategies have promising potential for the further design and preparation of abundant and robust 3D electrodes for efficient electrochemical water splitting as well as for other electrochemical areas.<br/><br/>For the first strategy, we prepared a heterostructure by combining two different catalysts, namely NiFe LDH (<i>layered double</i> hydroxides) nanosheets and NiCoP nanowires.<br/>The resulting 3D NiFe LDH/NiCoP@NF (nanofoam) electrodes demonstrated a better bifunctional activity for both the HER and the OER when compared to its counterparts.<br/>A new recipe for preparing TMP by the ALD process was also explored and optimized for the first time. The CoP ultrathin films prepared with our ALD recipe show a higher HER activity than CoP films prepared by the traditional post-phosphorization method.<br/><br/>The heterostructure with a strong synergistic effect is an effective approach to improve the catalytic activity of prepared catalysts and a novel strategy to design and prepare bifunctional electrocatalysts for efficient water splitting. However, the related reaction mechanisms happening on the surface of the heterostructure are still not understood.<br/>For the second strategy, we used carbon nano tubes (CNT) as the interface material to combine a stainless steel (SS) mesh with highly active catalysts to improve the activity of SS-based 3D electrodes for efficient water splitting. The CVD process for growing CNTs on the surface of SS was optimized, and the as-prepared CNT/SS electrodes were successfully modified with the state-of-the-art catalysts Pt and RuO2. The resulting 3D Pt/OxCNT/SS and RuO2/OxCNT/SS exhibited a better HER and OER activity, respectively, than the reference of 20 wt% Pt/C/SS and RuO2/SS.<br/><br/>Employing CNT as an interface material can make extremely stable SS become more suitable to the decoration with highly active electrocatalysts for the preparation of SS-based 3D electrodes towards efficient water splitting. The possibility of modifying CNT/SS with abundant and efficient catalysts to further enhance the performance of 3D SS-based electrodes is still needed to be investigated further.<br/><br/>The TMP-based catalysts deposited by ALD show a promising application potential for designing and preparing 3D electrodes for (photo)electrochemical water splitting as well as other electrochemical areas by depositing TMP on various 3D structures. Our work explored and improved the ALD process to obtain the controllable deposition of highly pure and stable CoP, but other TMP, such as Ni-based, Fe-based and Mo-based phosphides as well as their bi-/trimestral phosphides can also be prepared by the ALD approach.