Design And Performance Of MXene-based Electrocatalyst For Energy-saving Hydrogen Production From Low-grade Water

Hydrogen represents the most promising green and efficient secondary energy to replace traditional fossil fuels.It has been included in the national development strategy track of China and developed countries globally.Water electrolysis coupling with renewables such as solar and wind is regarded as one of the low-carbon and clean ways for hydrogen production,which is highly desirable to meet the goal of"carbon peaking and carbon neutrality".It becomes increasingly appealing due to significant advantages of environmentally friendly,carbon neutrality and grid-scale storage and sustainable utilization of intermittent renewable energy for yielding high-purity hydrogen.But its large-scale application is greatly restricted by the high energy consumption and heavy reliance on scarce freshwater resources.However,the theoretical voltage of water electrolysis is as high as 1.23 V.It is challenging to reduce the energy input and hydrogen cost by optimizing the catalyst and electrolysis process.Low-grade water（e.g.,seawater,brackish water and industrial wastewater）reserves nearly infinite hydrogen in>97%of the planet’s total water resources.Moreover,the electrolysis of low-grade water with complex ionic chemistry faces extra challenges of electrode contamination,deactivation and chlorine corrosion,seriously limiting electrolysis efficiency and sustainability.Based on the decoupling and reorganization of electrochemical water splitting reaction,this study reports to developing energy-saving,low-emission and cost-effective low-grade water（e.g.,seawater,acid/base wastewater）electrolysis for hydrogen production,as well as highly active electrodes targeting direct use of low-grade water in the electrolysis processes.The main results are summarized as follows:Seawater electrolysis is challenged by notorious anode corrosion and detrimental chlorine chemistry in complex chemical environments.Herein,the sustainable alkaline seawater electrolysis strategy is proposed leveraging highly active oxygen evolution reaction（OER）catalysts to reduce anodic potential and avoid notorious chlorine electro-oxidation reactions.By taking advantage of synergistically chemical coupling of Ni Fe-BDC metal-organic framework and two-dimensional transition metal carbides（MXene）with excellent hydrophilic surface and metallic conductivity,a new type of highly active and stable OER catalyst is fabricated with high conductivity,aggregation resistance and large reactive surface area.An overall enhancement in water adsorption,mass/charge transport and electrochemical active surface area is achieved to promote OER activity and kinetics.As a result,the Ni Fe-BDC/MXene catalyst could even outperform the noble-metal Ru O₂ for OER in alkaline seawater in terms of activity,ionic contamination resistance and long-term durability.An alkaline seawater electrolyzer is assembled by using the Ni Fe-BDC/MXene anode and commercial Pt/C cathode for seawater splitting.Stable electrolysis can proceed at～1.7 V for over 110 h with～100%Faraday efficiency.Benefited from low cell voltage,the notorious problems of chlorine electro-oxidation reactions and anode corrosion are avoided in complex chemical environments of seawater.The seawater electrolysis efficiency is fundamentally restricted by anodic OER with a large potential barrier.Replacing the sluggish OER by thermodynamically more favorable hydrazine oxidation reaction（Hz OR）with lower potential offers a ground-breaking strategy for energy-saving and chlorine-free hydrogen production.The basic cell voltage and electricity expense of hybrid seawater electrolyzer are even lower than the theoretical limit of overall water splitting(1.23 V and 2.94 k Wh m^-3 H₂).A bifunctional electrode is designed by assembling Ni Co-decorated porous nanoarray of carbon nanosheets with 3D conductive MXene-wrapped frame.It exhibits superaerophobic-hydrophilic and hydrazine-friendly electrocatalytic interface for boosting hydrogen evolution reaction（HER）and Hz OR.For alkaline seawater electrolysis at a high current density of 500 m A cm^-2,the hybrid seawater electrolyzer can work below 1.15V to yield hydrogen at a rate of 9.2 mol h^-1 g_cat^-1 with a low electricity expense of 2.75 k Wh per m³ H₂.The electricity expense is largely reduced by 40-50%relative to commercial alkaline water electrolysis.It also exhibits 90%lower CO₂ equivalent emission to natural gas steam reforming for hydrogen production.Meanwhile,it enables fast hydrazine degradation to～3 ppb residual,falling below the allowable value（10 ppb）set by U.S.Environmental Protection Agency.To realize energy-saving hydrogen production,a strategy is proposed for on-demand hydrogen and electricity generation with the additional function of water desalination enabled by electrochemical neutralization chemistry.It allows efficient use of not only chemical energy from acid-base neutralization reaction but also the low-grade heat from the surroundings to perform hydrogen and/or electricity generation without external electricity supply.A low-Pt cathode with 3.97 wt.%Pt is fabricated by chemically coupling Pt nanoparticles with MXene-wrapped 3D conductive framework.The resultant catalyst（Pt/MXene/CF）exceeds the commercial 20 wt.%Pt/C in mass activity for catalyzing acidic hydrogen evolution.An electrochemical neutralization cell is assembled by using the Pt/MXene/CF cathode and Ni Co@C/MXene/CF anode developed above.It can output 0.81 k Wh electricity upon production of 1 m³ hydrogen with the highest peak power density up to 85.5 m W cm^-2.When completely working for hydrogen production,the electrochemical neutralization cell could yield hydrogen at a rapid rate up to 70.1 mol h^-1 m^-2 with～100%Faradaic efficiency.Fast removal of salt from the saline water at a high rate of 56.1 mol h^-1 m^-2 is simultaneously enabled for efficient water desalination.The function of hydrogen production or electricity generation can be readily switched as desired without breaking the cell operation or configuration.A rough techno-economic analysis suggests that the hydrogen cost can be cut by 70-80%relative to alkaline water electrolysis powered by renewables when feeding acidic and alkaline wastes,hydrazine sewage and industrial brine stream.