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Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions

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Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions catalysts Review Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions Sami M. Ibn Shamsah Department of Mechanical Engineering, College of Engineering, University of Hafr Al Batin, P.O. Box 1803, Hafr Al Batin 31991, Saudi Arabia; [email protected] Abstract: Of all the available resources given to mankind, the sunlight is perhaps the most abundant renewable energy resource, providing more than enough energy on earth to satisfy all the needs of humanity for several hundred years. Therefore, it is transient and sporadic that poses issues with how the energy can be harvested and processed when the sun does not shine. Scientists assume that electro/photoelectrochemical devices used for water splitting into hydrogen and oxygen may have one solution to solve this hindrance. Water electrolysis-generated hydrogen is an optimal energy carrier to store these forms of energy on scalable levels because the energy density is high, and no air pollution or toxic gas is released into the environment after combustion. However, in order to adopt these devices for readily use, they have to be low-cost for manufacturing and operation. It is thus crucial to develop electrocatalysts for water splitting based on low-cost and land-rich elements. In this review, I will summarize current advances in the synthesis of low-cost earth-abundant electrocatalysts for overall water splitting, with a particular focus on how to be linked with photoelectrocatalytic water splitting devices. The major obstacles that persist in designing these devices. The potential future developments in the production of efficient electrocatalysts for water electrolysis are also described. Keywords: earth-abundant electrocatalysts; water splitting; hydrogen evolution reaction (HER); Citation: Ibn Shamsah, S.M. oxygen evolution reaction (OER); renewable energy Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions. Catalysts 2021, 11, 429. https://doi.org/10.3390/catal11040429 1. Introduction Academic Editor: Rahat Javaid Solar and other renewable energy sources are abundant and undependable. If our appetite for green energy increases, we will see a growing market for alternative energy Received: 23 February 2021 storage devices in the future. The best way of using this energy is to use solar renewable Accepted: 24 March 2021 energy to break water into hydrogen and oxygen, as shown in Equation (1) [1]. It is possible Published: 27 March 2021 to store the hydrogen as a result of this reaction and work as green fuels. This would allow carbon-free energy and environmental protection. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 2H2O ! O2 + 2H2. (1) published maps and institutional affil- −1 iations. Equation (1) requires 286.0 kJ mol of energy for a reaction at room temperature and pressure. Electrochemical methods tend to be very effective in producing hydrogen by only using electricity and water. Solar energy is the most effective renewable energy source (our Earths’ surface received approximately ~1.2 × 1014 kJ energy in every second) [2]. Based on the advances in the electrolytic process, there is a growing interest in coupling Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. solar energy to the electrochemical water splitting process [3]. Thermodynamically, there This article is an open access article are two ways of producing fuel from solar energy; the consumption of ordinary solar distributed under the terms and cells to operate conventional electrolyzers (‘indirect” solar-to-hydrogen generation) or conditions of the Creative Commons the creation of technologies that can absorb sunlight and split water at the same time Attribution (CC BY) license (https:// (“direct” solar-to-hydrogen generation). Indirect techniques benefit from conventional creativecommons.org/licenses/by/ methods by trail-and-tested technologies but are less successful due to additional steps 4.0/). needed. Electricity was first produced in the solar photovoltaic cell and then consumed by Catalysts 2021, 11, 429. https://doi.org/10.3390/catal11040429 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 22 Catalysts 2021, 11, 429 2 of 21 needed. Electricity was first produced in the solar photovoltaic cell and then consumed by electrolysis in an independent cell [4]. There has been a growing interest in developing electrolysisand commercializing in an independent methods cell to [4generate]. There hassolar- beento fuels a growing efficiently interest in inrecent developing years [5,6]. and commercializingThis discovery of methods solar fuels to generate would solar-tointroduc fuelse the efficientlyidea of “artificial in recent photosynthesis,” years [5,6]. This discoverywhere solar of energy solar fuels is absorbed would introduce and processed the idea as of a collection “artificial photosynthesis,”of chemical bonds where converted solar energyinto a solar is absorbed fuel [7]. and processed as a collection of chemical bonds converted into a solar fuel [7Figure]. 1 describes two separate solar-to-hydrogen photosynthesis mechanisms, in whichFigure the electrically1 describes active two separatematerial solar-to-hydrogenis buried under the photosynthesis electrode and used mechanisms, as a cathode. in whichAny realistic the electrically and scalable active artificial material photosynthesis is buried under systems the electrode would have and usedstrict as material a cathode. use Anycriteria. realistic The andcurrent scalable densities artificial from photosynthesis these devices systemswould be would restricted have strictby the material strength use of criteria.the sunlight The (0.1 current Wcm densities−2), meaning from that these factual, devices current would densities be restricted are in bythe the sequence strength of of10 −2 themA·cm sunlight−2 for (0.1artificial Wcm photosynthesis), meaning that syst factual,ems in currentcomparison densities with are~0.5–2 in the Acm sequence−2 for com- of −2 −2 10mercial mA·cm electrolyzersfor artificial [8,9]. photosynthesis In order to genera systemste the in same comparison quantity with of gas ~0.5–2 per unit Acm time,for a commercialsolar energy electrolyzers to hydrogen [ 8photochemical,9]. In order to smart generate device the samewould quantity need 50–200 of gas times per unit the time,elec- atrode solar surface energy area to hydrogen of traditional photochemical electrical glass, smart rendering device wouldthe use need of precious 50–200 metal times elec- the electrodetrocatalysts surface not economically area of traditional feasible electrical in such artificial glass, rendering photosynthesis the use systems. of precious For metalartifi- electrocatalystscial photosynthesis not economicallyto work efficiently, feasible cata inlysts such must artificial be cheaper photosynthesis and easily systems. accessible For to artificiallimit our photosynthesis choice to transition to work metals efficiently, and th catalystseir compounds must be in cheaper the first and row easily [10]. accessible Catalysts toare limit also our used choice for various to transition applications metals andsuch their as renewable compounds energy, in the biomass first row conversion, [10]. Catalysts hy- aredrogen also production, used for various efficient applications reactions in such open as and renewable microreactor energy, systems, biomass etc. conversion,[11–36]. hydrogen production, efficient reactions in open and microreactor systems, etc. [11–35]. A B Figure 1. Artificial photosynthesis technology for solar energy into hydrogen processes: (A) wireless presentation and (B) Figure 1. Artificial photosynthesis technology for solar energy into hydrogen processes: (A) wireless presentation and (B) wired configuration. Thus, it is preferable to use thinner catalyst layers in both situations. Adapted from [36]. wired configuration. Thus, it is preferable to use thinner catalyst layers in both situations. Adapted from [37]. Electrolysis works the best at a specific pH range, e.g., in acidic solutions or more alka- line media.Electrolysis However, works the the overwhelming best at a specific majority pH ofrange, the semiconductors e.g., in acidic solutions suitable foror solarmore cellsalkaline degrade media. at harshHowever, pH values the overwhelming suggesting that ma developersjority of the are semiconductors finding neutral electrolytessuitable for forsolar artificial cells degrade photosynthesis. at harsh pH In artificial values suggesting photosynthesis that developers systems, milder are finding pH conditions neutral elec- can delaytrolytes the for rate artificial of degradation photosynthesis. of other In cell arti components,ficial photosynthesis which can systems, be essential milder for pH more con- extendedditions can stability delay the and rate total of system degradation cost [37 of]. other In contrast, cell components, electrolyte solutionswhich can with be essential neutral pHfor removemore extended various safetystability hazards and total associated system withcost utilizing[38]. 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