energies Review Towards the Hydrogen Economy—A Review of the Parameters That Influence the Efficiency of Alkaline Water Electrolyzers Ana L. Santos 1,2, Maria-João Cebola 3,4,5 and Diogo M. F. Santos 2,* 1 TecnoVeritas—Serviços de Engenharia e Sistemas Tecnológicos, Lda, 2640-486 Mafra, Portugal; [email protected] 2 Center of Physics and Engineering of Advanced Materials (CeFEMA), Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal 3 CBIOS—Center for Research in Biosciences & Health Technologies, Universidade Lusófona de Humanidades e Tecnologias, Campo Grande 376, 1749-024 Lisbon, Portugal; [email protected] 4 CERENA—Centre for Natural Resources and the Environment, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal 5 Escola Superior Náutica Infante D. Henrique, 2770-058 Paço de Arcos, Portugal * Correspondence: [email protected] Abstract: Environmental issues make the quest for better and cleaner energy sources a priority. Worldwide, researchers and companies are continuously working on this matter, taking one of two approaches: either finding new energy sources or improving the efficiency of existing ones. Hydrogen is a well-known energy carrier due to its high energy content, but a somewhat elusive one for being a gas with low molecular weight. This review examines the current electrolysis processes for obtaining hydrogen, with an emphasis on alkaline water electrolysis. This process is far from being new, but research shows that there is still plenty of room for improvement. The efficiency of an electrolyzer mainly relates to the overpotential and resistances in the cell. This work shows that the path to better Citation: Santos, A.L.; Cebola, M.-J.; electrolyzer efficiency is through the optimization of the cell components and operating conditions. Santos, D.M.F. Towards the Hydrogen Following a brief introduction to the thermodynamics and kinetics of water electrolysis, the most Economy—A Review of the recent developments on several parameters (e.g., electrocatalysts, electrolyte composition, separator, Parameters That Influence the interelectrode distance) are highlighted. Efficiency of Alkaline Water Electrolyzers. Energies 2021, 14, 3193. Keywords: alkaline water electrolysis; cell components; efficiency; electrocatalysts; overpotential https://doi.org/10.3390/en14113193 Academic Editor: Jin-Soo Park 1. Introduction Received: 13 April 2021 Nowadays, energy is at the top of everybody’s needs and concerns. Never before has Accepted: 25 May 2021 Published: 29 May 2021 the consumption of energy been so high, and never before has the impact of the use of that energy been as intense and negative on the planet as a whole. Publisher’s Note: MDPI stays neutral Growing energy demand due to population increase poses a serious threat to the global with regard to jurisdictional claims in economy, environment, and consequent climate change [1]. Currently, fossil fuels are the published maps and institutional affil- principal resources used to obtain energy. However, these sources are non-renewable, and iations. they have been exploited on such a large scale for the past 200 years that their reserves are due to run out: in 50 to 60 years for oil and gas, and 150 to 200 years for coal, according to recent forecasts [2]. Furthermore, the pollution resulting from the consumption of fossil fuels is a major issue that must be taken into consideration. Consequently, renewable energy resources, such as solar energy, wind, or wave power, have attracted considerable Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. attention as an alternative response to increasing energy demand, without causing a major This article is an open access article environmental impact. Besides cost issues, there are, however, some technical problems, distributed under the terms and related to energy fluctuations produced by renewable sources that lead to low energy conditions of the Creative Commons delivery efficiencies and thereby limit their large-scale applications. To take advantage of Attribution (CC BY) license (https:// all the energy produced from these sources, it is crucial to find a solution to storing the creativecommons.org/licenses/by/ energy produced in excess when there is a peak of production, and to make it available to 4.0/). be consumed at a time when consumption exceeds production [3]. Energies 2021, 14, 3193. https://doi.org/10.3390/en14113193 https://www.mdpi.com/journal/energies Energies 2021, 14, 3193 2 of 39 storing the energy produced in excess when there is a peak of production, and to make it available to be consumed at a time when consumption exceeds production [3]. Hydrogen is seen as a key component to overcome this issue due to its high energy yield, high gravimetric energy density, relatively high abundance on earth, and zero emis- sions during consumption. Being an energy carrier, it is as clean as the method employed in its production, so producing hydrogen by splitting water using electricity generated Energiesfrom2021, 14 ,renewable 3193 sources has many advantages. Its use not only benefits from the abun-2 of 35 dance of water resources but also the zero pollutant emissions generated. In addition, the hydrogen obtained has a high level of purity of 99.99% [4,5]. However, only 4% of the total Hydrogen is seen as a key component to overcome this issue due to its high energy hydrogen currentlyyield, produced high gravimetric comes from energy the density,electrolysis relatively of water. high abundance Most hydrogen on earth, pro- and zero duced comes from fossilemissions fuels, during specifically consumption. from Beingthe steam an energy reforming carrier, of it isnatural as clean gas as (48%) the method followed by oil andemployed coal gasification in its production, (30% and so producing 18%, respectively) hydrogen by [6]. splitting This wateris because using electricitythe hydrogen producedgenerated from the from electrolysis renewable sourcesof water has must many advantages.compete with Its use the not relatively only benefits low from the abundance of water resources but also the zero pollutant emissions generated. In addition, prices of the hydrogenthe hydrogenproduced obtained from hasfossi a highl fuel-based level of purity techniques of 99.99%. Nevertheless, [4,5]. However, onlyhydro- 4% of the gen is seen as a promisingtotal hydrogen approach currently to compet producede with comes fossil from thefuels electrolysis and to ofend water. the Mostplanet’s hydrogen energy dependencyproduced on them, comes so that from it fossil has fuels,become specifically an item from on thethe steam political reforming agenda. of natural For gas instance, the European(48%) Commission followed by oil launched and coal gasification its Clean (30% Hydrogen and 18%, Alliance respectively) in July [6]. This 2020 is to because seek the commitmentthe hydrogenof European produced Union from coun the electrolysistries to large-scale of water must hydrogen compete with production. the relatively low prices of the hydrogen produced from fossil fuel-based techniques. Nevertheless, hydrogen The aim is for this productionis seen as a promising to be attained approach vi toa competeelectrolysis with fossilbut, fuelsgiven and that to endthis the process planet’s is energy still very expensive,dependency production on from them, sonatural that it hasgas become is also an allowed. item on the Thus, political large agenda. efforts For are instance, being put towards increasingthe European th Commissione efficiency launched of electrolysis. its Clean Hydrogen Alliance in July 2020 to seek the commitment of European Union countries to large-scale hydrogen production. The aim is for this production to be attained via electrolysis but, given that this process is still very 2. Water Electrolysis expensive, production from natural gas is also allowed. Thus, large efforts are being put In the electrolysistowards of water, increasing a direct the efficiency current of (DC) electrolysis. is applied and electrons flow from the negative terminal of the DC source to the cathode. There, the electrons are consumed 2. Water Electrolysis by hydrogen ions existing in the water, and hydrogen gas is formed. To keep the charge In the electrolysis of water, a direct current (DC) is applied and electrons flow from balance in the cell, thethe negativehydroxide terminal ions ofresulting the DC source from to the the cathodic cathode. There, reaction the electrons move towards are consumed the anode surface, whereby hydrogen they lose ions electrons existing in the(that water, return and to hydrogen the positive gas is formed.terminal To of keep the the DC charge source) and oxygenbalance is formed in the cell,[7,8]. the The hydroxide overal ionsl reaction resulting of from water the cathodic electrolysis reaction is move repre- towards sented by Equation the(1). anode surface, where they lose electrons (that return to the positive terminal of the DC source) and oxygen is formed [7,8]. The overall reaction of water electrolysis is represented by Equation2H (1).2O + energy → 2H2 (g) + O2 (g) (1) 2H2O + energy ! 2H2 (g) + O2 (g) (1) The main water electrolysisThe main water technologies electrolysis available technologies are available alkaline are water alkaline electrolysis water electrolysis (AWE), proton exchange(AWE), protonmembrane exchange (PEM) membrane electrolysis, (PEM) electrolysis, and the andsolid the oxide solid oxide electrolyzer electrolyzer cell